Defects/imperfections in welds - porosity The characteristic features and principal causes of porosity imperfections are described. Best practice guidelines are given so welders can minimise porosity risk during fabrication.

Identification

Porosity is the presence of cavities in the weld metal caused by the freezing in of gas released from the weld pool as it solidifies. The porosity can take several forms:

• Distributed • Surface breaking pores • Wormhole • Crater pipes

Cause and prevention

Distributed porosity and surface pores

Distributed porosity (Fig. 1) is normally found as fine pores throughout the weld bead. Surface breaking pores (Fig. 2) usually indicate a large amount of distributed porosity

Fig. 1. Uniformly distributed porosity

Fig. 2. Surface breaking pores (T fillet weld in primed plate)

Cause Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld pool, which is then released on solidification to become trapped in the weld metal.

Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding. As little as 1% air entrainment in the shielding gas will cause distributed porosity and greater

than 1.5% results in gross surface breaking pores. Leaks in the gas line, too high a gas flow rate, draughts and excessive turbulence in the weld pool are frequent causes of porosity.

Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes or the workpiece surface. Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen.

Surface coatings like primer paints and surface treatments such as zinc coatings, may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides (see Fig 2). Special mention should be made of the so-called weldable (low zinc) primers. It should not be necessary to remove the primers but if the primer thickness exceeds the manufacturer's recommendation, porosity is likely to result especially when using welding processes other than MMA.

Prevention

The gas source should be identified and removed as follows:

Air entrainment

- Seal any air leak - Avoid weld pool turbulence - Use filler with adequate level of deoxidants - Reduce excessively high gas flow - Avoid draughts

Hydrogen - Dry the electrode and flux - Clean and degrease the workpiece surface

Surface coatings - Clean the joint edges immediately before welding - Check that the weldable primer is below the recommended maximum thickness

Wormholes

Characteristically, wormholes are elongated pores (Fig. 3), which produce a herring bone appearance on the radiograph.

Cause Wormholes are indicative of a large amount of gas being formed, which is then trapped in the solidifying weld metal. Excessive gas will be formed from gross surface contamination or very thick paint or primer coatings. Entrapment is more likely in crevices such as the gap beneath the vertical member of a horizontal-vertical, T joint which is fillet welded on both sides.

When welding T joints in primed plates it is essential that the coating thickness on the edge of the vertical member is not above the manufacturer's recommended maximum, typically 20µ, through over-spraying.

Prevention

Elongated pores or wormholes

Eliminating the gas and cavities prevents wormholes.

Gas generation

- Clean the workpiece surfaces - Remove any coatings from the joint area - Check the primer thickness is below the manufacturer's maximum

Joint geometry - Avoid a joint geometry, which creates a cavity

Crater pipe

A crater pipe forms during the final solidified weld pool and is often associated with some gas porosity.

Cause This imperfection results from shrinkage on weld pool solidification. Consequently, conditions, which exaggerate the liquid to solid volume change, will promote its formation. Switching off the welding current will result in the rapid solidification of a large weld pool.

In TIG welding, autogenous techniques, or stopping the wire before switching off the welding current, will cause crater formation and the pipe imperfection.

Prevention

Crater pipe imperfection can be prevented by removing the stop or by welder technique.

Removal of stop

- Use run-off tag in butt joints - Grind out the stop before continuing with the next electrode or depositing the subsequent weld run

Welder technique - Progressively reduce the welding current to reduce the weld pool size - Add filler (TIG) to compensate for the weld pool shrinkage

Porosity susceptibility of materials

Gases likely to cause porosity in the commonly used range of materials are listed in the Table.

Principal gases causing porosity and recommended cleaning methods

Material Gas Cleaning

C Mn steel Hydrogen, Nitrogen and Oxygen

Grind to remove scale coatings

Stainless steel Hydrogen Degrease + wire brush + degrease

Aluminium and alloys

Hydrogen Chemical clean + wire brush + degrease + scrape

Copper and alloys Hydrogen, Nitrogen Degrease + wire brush + degrease

Nickel and alloys Nitrogen Degrease + wire brush + degrease

Detection and remedial action

If the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub surface imperfections, detection is by radiography or ultrasonic inspection. Radiography is normally more effective in detecting and characterising porosity imperfections. However, detection of small pores is difficult especially in thick sections.

Remedial action normally needs removal by localised gouging or grinding but if the porosity is widespread, the entire weld should be removed. The joint should be re-prepared and re-welded as specified in the agreed procedure.

Weld defects / imperfections - incomplete root fusion or penetration The characteristic features and principal causes of incomplete root fusion are described. General guidelines on 'best practice' are given so welders can minimise the risk of introducing imperfections during fabrication.

Fabrication and service defects and imperfections

As the presence of imperfections in a welded joint may not render the component defective in the sense of being unsuitable for the intended application, the preferred term is imperfection rather than defect. For this reason, production quality for a component is defined in terms of a quality level in which the limits for the imperfections are clearly defined, for example Level B, C or D in accordance with the requirements of EN 25817. For the American standards ASME X1 and AWS D1.1, the acceptance levels are contained in the standards.

The SS Schenectady, an all welded tanker, broke in two whilst lying in dock in 1943. Principal causes of this failure were poor design and bad workmanship

The application code will specify the quality levels, which must be achieved for the various joints.

Imperfections can be broadly classified into those produced on fabrication of the component or structure and those formed as result of adverse conditions during service. The principal types of imperfections are:

Fabrication:

• Lack of fusion • Cracks • Porosity • Inclusions • Incorrect weld shape and size

Service:

• Brittle fracture • Stress corrosion cracking • Fatigue failure

Welding procedure and welder technique will have a direct effect on fabrication imperfections. Incorrect procedure or poor technique may produce imperfections leading to premature failure in service.

Incomplete root fusion or penetration

Identification

Incomplete root fusion is when the weld fails to fuse one side of the joint in the root. Incomplete root penetration occurs when both sides of the joint are unfused. Typical imperfections can arise in the following situations:

• An excessively thick root face in a butt weld (Fig. 1a) • Too small a root gap (Fig. 1b) • Misplaced welds (Fig. 1c) • Failure to remove sufficient metal in cutting back to sound metal in a double sided

weld (Fig. 1d) • Incomplete root fusion when using too low an arc energy (heat) input (Fig. 1e) • Too small a bevel angle, • Too large an electrode in MMA welding (Fig 2)

Fig. 1 Causes of incomplete root fusion

a) b)

c) d)

e)

a) Excessively thick root face b) Too small a root gap c) Misplaced welds d) Power input too low e) Arc (heat) input too low

Fig. 2 Effect of electrode size on root fusion

a)

b)

a) Large diameter electrodeb) Small diameter electrode

Causes

These types of imperfection are more likely in consumable electrode processes (MIG, MMA and submerged arc welding) where the weld metal is 'automatically' deposited as the arc consumes the electrode wire or rod. The welder has limited control of weld pool penetration independent of depositing weld metal. Thus, the non-consumable electrode TIG process in which the welder controls the amount of filler material independent of penetration is less prone to this type of defect.

In MMA welding, the risk of incomplete root fusion can be reduced by using the correct welding parameters and electrode size to give adequate arc energy input and deep penetration. Electrode size is also important in that it should be small enough to give adequate access to the root, especially when using a small bevel angle (Fig 2). It is common practice to use a 4mm diameter electrode for the root so the welder can manipulate the electrode for penetration and control of the weld pool. However, for the fill passes where penetration requirements are less critical, a 5mm diameter electrode is used to achieve higher deposition rates.

In MIG welding, the correct welding parameters for the material thickness, and a short arc length, should give adequate weld bead penetration. Too low a current level for the size of root face will give inadequate weld penetration. Too high a level, causing the welder to move too quickly, will result in the weld pool bridging the root without achieving adequate penetration.

It is also essential that the correct root face size and bevel angles are used and that the joint gap is set accurately. To prevent the gap from closing, adequate tacking will be required.

Best practice in prevention

The following techniques can be used to prevent lack of root fusion:

• In TIG welding, do not use too large a root face and ensure the welding current is sufficient for the weld pool to penetrate fully the root

• In MMA welding, use the correct current level and not too large an electrode size for the root

• In MIG welding, use a sufficiently high welding current level but adjust the arc voltage to keep a short arc length

• When using a joint configuration with a joint gap, make sure it is of adequate size and does not close up during welding

• Do not use too high a current level causing the weld pool to bridge the gap without fully penetrating the root.

Acceptance standards

The limits for lack of penetration are specified in BS EN 25817 (ISO 5817) for the three quality levels.

Lack of root penetration is not permitted for Quality Level B (stringent). For Quality Levels C (intermediate) and D (moderate) long lack of penetration imperfections are not permitted but short imperfections are permitted.

Incomplete root penetration is not permitted in the manufacture of pressure vessels but is allowable in the manufacture of pipework depending on material and wall thickness.

Remedial actions

If the root cannot be directly inspected, for example using a penetrant or magnetic particle inspection technique, detection is by radiography or ultrasonic inspection. Remedial action will normally require removal by gouging or grinding to sound metal, followed by re-welding in conformity with the original procedure.

Relevant standards

EN 25817:1992 (ISO 5817) Arc welded joints in steel - Guidance on quality levels for imperfections.

EN 30042: 1994 Arc welded joints in aluminium and its weldable alloys - Guidance on quality levels for imperfections.

Defects/imperfections in welds - slag inclusions

Prevention of slag inclusions by grinding between runs

The characteristic features and principal causes of slag imperfections are described.

Identification

Slag is normally seen as elongated lines either continuous or discontinuous along the length of the weld. This is readily identified in a radiograph, Fig 1. Slag inclusions are usually associated with the flux

processes, i.e. MMA, FCA and submerged arc, but they can also occur in MIG welding.

Causes

As slag is the residue of the flux coating, it is principally a deoxidation product from the reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. When the next layer is deposited, the entrapped slag is not melted out. Slag may also become entrapped in cavities in multi-pass welds through excessive undercut in the weld toe or the uneven surface profile of the preceding weld runs, Fig 2.

As they both have an effect on the ease of slag removal, the risk of slag imperfections is influenced by

• Type of flux • Welder technique

The type and configuration of the joint, welding position and access restrictions all have an influence on the risk of slag imperfections.

Fig. 1. Radiograph of a butt weld showing two slag lines in the weld root

Fig. 2. The influence of welder technique on the risk of slag inclusions when welding with a basic MMA (7018) electrode

a) Poor (convex) weld bead profile resulted in pockets of slag being trapped between the weld runs

b) Smooth weld bead profile allows the slag to be readily removed between runs

Type of flux

One of the main functions of the flux coating in welding is to produce a slag, which will flow freely over the surface of the weld pool to protect it from oxidation. As the slag affects the handling characteristics of the MMA electrode, its surface tension and freezing rate can be equally important properties. For welding in the flat and horizontal/vertical positions, a relatively viscous slag is preferred, as it will produce a smooth weld bead profile, is less likely to be trapped and, on solidifying, is normally more easily removed. For vertical welding, the slag must be more fluid to flow out to the weld pool surface but have a higher surface tension to provide support to the weld pool and be fast freezing.

The composition of the flux coating also plays an important role in the risk of slag inclusions through its effect on the weld bead shape and the ease with which the slag can be removed. A weld pool with low oxygen content will have a high surface tension producing a convex weld bead with poor parent metal wetting. Thus, an oxidising flux, containing for example iron oxide, produces a low surface tension weld pool with a more concave weld bead profile, and promotes wetting into the parent metal. High silicate flux produces a glass-like slag, often self-detaching. Fluxes with lime content produce an adherent slag, which is difficult to remove.

The ease of slag removal for the principal flux types are:

• Rutile or acid fluxes - large amounts of titanium oxide (rutile) with some silicates. The oxygen level of the weld pool is high enough to give flat or slightly convex weld

bead. The fluidity of the slag is determined by the calcium fluoride content. Fluoride-free coatings designed for welding in the flat position produce smooth bead profiles and an easily removed slag. The more fluid fluoride slag designed for positional welding is less easily removed.

• Basic fluxes - the high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the flux reduces the oxygen content of the weld pool and therefore its surface tension. The slag is more fluid than that produced with the rutile coating. Fast freezing also assists welding in the vertical and overhead positions but the slag coating is more difficult to remove.

Consequently, the risk of slag inclusions is significantly greater with basic fluxes due to the inherent convex weld bead profile and the difficulty in removing the slag from the weld toes especially in multi-pass welds.

Welder technique

Welding technique has an important role to play in preventing slag inclusions. Electrode manipulation should ensure adequate shape and degree of overlap of the weld beads to avoid forming pockets, which can trap the slag. Thus, the correct size of electrode for the joint preparation, the correct angles to the workpiece for good penetration and a smooth weld bead profile are all essential to prevent slag entrainment.

In multi-pass vertical welding, especially with basic electrodes, care must be taken to fuse out any remaining minor slag pockets and minimise undercut. When using a weave, a slight dwell at the extreme edges of the weave will assist sidewall fusion and produce a flatter weld bead profile.

Too high a current together with a high welding speed will also cause sidewall undercutting which makes slag removal difficult.

It is crucial to remove all slag before depositing the next run. This can be done between runs by grinding, light chipping or wire brushing. Cleaning tools must be identified for different materials e.g. steels or stainless steels, and segregated.

When welding with difficult electrodes, in narrow vee butt joints or when the slag is trapped through undercutting, it may be necessary to grind the surface of the weld between layers to ensure complete slag removal.

Best practice

The following techniques can be used to prevent slag inclusions:

• Use welding techniques to produce smooth weld beads and adequate inter-run fusion to avoid forming pockets to trap the slag

• Use the correct current and travel speed to avoid undercutting the sidewall which will make the slag difficult to remove

• Remove slag between runs paying particular attention to removing any slag trapped in crevices

• Use grinding when welding difficult butt joints otherwise wire brushing or light chipping may be sufficient to remove the slag.

Acceptance standards

Slag and flux inclusions are linear defects but because they do not have sharp edges compared with cracks, they may be permitted by specific standards and codes. The limits in steel are specified in BE EN 25817 (ISO 5817) for the three quality levels. Long slag imperfections are not permitted in both butt and fillet welds for Quality Level B (stringent) and C (moderate). For Quality Level D, butt welds can have imperfections providing their size is less than half the nominal weld thickness. Short slag related imperfections are permitted in all three-quality levels with limits placed on their size relative to the butt weld thickness or nominal fillet weld throat thickness.

Job knowledge for welders

Standards - application standards, codes of practice and quality levels

Production at Dennis vehicle manufacturers

Application standards and codes of practice ensure that a structure or component will have an acceptable level of quality and be fit for the intended purpose.

In this document, the requirements for standards on welding procedure and welder approval are explained together with the quality levels for imperfections. It should be noted that the term approval is used in European standards in the context of both testing and documentation. The equivalent term in the ASME standard is qualification.

Application standards and codes

There are essentially three types of standards, which can be referenced in fabrication:

• Application and design • Specification and approval of welding procedures • Approval of welders

There are also specific standards covering material specifications, consumables, welding equipment and health and safety. British Standards are used to specify the requirements, for example, in approving a welding procedure, they are not a legal requirement but may be cited by the Regulatory Authority as a means of satisfying the law. Health and Safety guidance documents and codes of practice may also recommend standards.

Codes of practice differ from standards in that they are intended to give recommendations and guidance, for example, on the validation of power sources for welding. It is not intended that should be used as a mandatory, or contractual, document.

Most fabricators will be working to one of the following:

• Company or industry specific standards • National BS (British Standard) • European BS EN (British Standard European Standard) • US AWS (American Welding Society) and ASME (American Society of Mechanical

Engineers) • International ISO (International Standards Organisation)

Examples of application codes and standards and related welding procedure and welder approval standards are listed in Table 1.

Table 1 Examples of application codes and standards and related welding procedure and welder approval standards

Welding standard

Application Application code/standard Procedure approval Welder approval

Pressure Vessels BS 5500 ASME VIII

BS EN 288 ASME IX

BS EN 287 ASME IX

Process Pipework BS 2633 BS 4677 ANSI/ASME B311 ANSI/ASME B31.3 BS 2971

BS EN 288 (Part 3) BS EN 288 (Part 4) ASME IX ASME IX BS EN 288 (Part 3) (if required)

BS EN 287 (Part 1)BS EN 287 (Part 2)ASME IX ASME IX BS 4872/BS EN 287

Structural Fabrication

AWS D1.1 AWS D1.2 BS 5135 BS 8118

AWS D1.1 AWS D1.2 BS EN 288 (Part 3) BS EN 288 (Part 4)

AWS D1.1 AWS D1.2 BS EN 287 BS EN 287 BS 4872

Storage Tanks BS 2654 BS 2594 API 620/650

BS EN 288 (Parts 3 & 4) BS EN 288 (Parts 3 & 4) ASME IX

BS EN 287 BS EN 287 ASME IX

Note 1: Reference should be made to the application codes/standards for any additional requirements to those specified in BS EN 287, BS EN 288 and ASME IX.

Note 2: Some BS Standards have not been revised to include the new BS EN standards: BS EN 287 and BS EN 288 should be substituted, as appropriate, for BS 4871 and BS 4870, respectively, which have been with drawn.

In European countries, national standards are being replaced by EN standards. However, when there is no equivalent EN standard, the National standard can be used. For example, BS EN 287 replaces BS 4871 but BS 4872 remains as a valid standard.

Approval of welding procedures and welders

An application standard or code of practice will include requirements or guidelines on material, design of joint, welding process, welding procedure, welder qualification and inspection or may invoke other standards for example for welding procedure and welder approval tests. The manufacturer will normally be required to approve the welding procedure and welder qualification. The difference between a welding procedure approval and a welder qualification test is as follows:

• The welding procedure approval test is carried out by a competent welder and the quality of the weld is assessed using non-destructive and mechanical testing techniques. The intention is to demonstrate that the proposed welding procedure will produce a welded joint, which will satisfy the specified requirements of weld quality and mechanical properties.

• The welder approval test examines a welder's skill and ability in producing a satisfactory test weld. The test may be performed with or without a qualified welding procedure (note, without an approved welding procedure the welding parameters must be recorded).

The requirements for approvals are determined by the relevant application standard or as a condition of contract (Table 1).

EN 287 and ASME IX would be appropriate for welders on high quality work such as pressure vessels, pressure vessel piping and off-shore structures and other products where the consequences of failure, stress levels and complexity mean that a high level of welded joint integrity is essential. In less demanding situations, such as small to medium building frames and general light structural and non- structural work, an approved welding procedure may not be necessary. However, to ensure an adequate level of skill, it is recommended that the welder be approved to a less stringent standard e.g. BS 4872.

'Coded welder' is often used to denote an approved welder but the term is not recognised in any of the standards. However, it is used in the workplace to describe those welders whose skill and technical competence have been approved to the requirements of an appropriate standard.

Quality Acceptance Levels for Welding Procedure and Welder Approval Tests

When welding to application standards and codes, consideration must be given to the imperfection acceptance criteria, which must be satisfied. Some standards contain an appropriate section relating to the acceptance levels while others make use of a separate standard. For example, in welding procedure and welder approval tests to EN 288 Pt3 and EN 287 Pt1, respectively, reference is made to EN 25817 (ISO 5817). It is important to note that the application standard may specify more stringent imperfection acceptance levels and/or require additional tests to be carried out as part of the welding procedure approval test. For example, for joints, which must operate at high temperatures, elevated temperature tensile test may be required whereas for low temperature applications, impact or CTOD tests may be specified.

Guidance on permissible levels of imperfections in arc-welded joints in steel (thickness range, 3 to 63mm) is given in EN 25817. Production quality, but not fitness-for-purpose, is defined in terms of three levels of quality for imperfections:

• Moderate - Level D • Intermediate- Level C • Stringent - Level B

The standard applies to most arc welding processes and covers imperfections such as cracks, porosity, inclusions, poor bead geometry, lack of penetration and misalignment.

As the quality levels are related to the types of welded joint and not to a particular component, they can be applied to most applications for procedure and welder approval. The quality levels which are the most appropriate for production joints will be determined by the relevant application standard which may cover design considerations, mode of stressing (e.g. static, dynamic), service conditions (e.g. temperature, environment) and consequences of failure.

When working to the European Standards, the welding procedure, or the welder, will be qualified if the imperfections in the test piece are within the specified limits of Level B except for excess weld metal, excess convexity, excess throat thickness and excess penetration type imperfections when Level C will apply.

Guidance levels for aluminium joints are given in EN 30042.

For the American standards ASME IX and AWS D1.1, the acceptance levels are contained in the standard. Application codes may specify more stringent imperfection acceptance levels and/or additional tests.

Relevant Standards

• American Welding Society, Structural Welding Code, AWS D1.1 • ASME Boiler and Pressure Vessel Code, Section IX: Welding Qualifications • BS 4872 Approval Testing of Welders when Welding Procedure Approval is not

Required • EN 287:1997 Approval Testing of welders for fusion welding • EN 288: Specification and approval of welding procedures for metallic materials • EN 25817:1992 (ISO 5817) Arc welded joints in steel - Guidance on quality levels

for imperfections. • EN 26520 Classification of imperfections in metallic fusion welds, with explanations. • EN 30042:1994 Arc-welded joints in aluminium and its weldable alloys. Guidance on

quality levels for imperfections.

Weld defects/imperfections in welds - lack of sidewall and inter-run fusion Demagnetising a pipe

This article describes the characteristic features and principal causes of lack of sidewall and inter-run fusion. General guidelines on best practice are given so that welders can minimise the risk of imperfections during fabrication.

Identification Lack of fusion imperfections can occur when the weld metal fails

• To fuse completely with the sidewall of the joint (Fig. 1) • To penetrate adequately the previous weld bead (Fig. 2).

Fig. 1. Lack of side wall fusion

Fig. 2. Lack of inter-run fusion

Causes The principal causes are too narrow a joint preparation, incorrect welding parameter settings, poor welder technique and magnetic arc blow. Insufficient cleaning of oily or

scaled surfaces can also contribute to lack of fusion. These types of imperfection are more likely to happen when welding in the vertical position.

Joint preparation Too narrow a joint preparation often causes the arc to be attracted to one of the side walls causing lack of side wall fusion on the other side of the joint or inadequate penetration into the previously deposited weld bead. Too great an arc length may also increase the risk of preferential melting along one side of the joint and cause shallow penetration. In addition, a narrow joint preparation may prevent adequate access into the joint. For example, this happens in MMA welding when using a large diameter electrode, or in MIG welding where an allowance should be made for the size of the nozzle.

Welding parameters It is important to use a sufficiently high current for the arc to penetrate into the joint sidewall. Consequently, too high a welding speed for the welding current will increase the risk of these imperfections. However, too high a current or too low a welding speed will cause weld pool flooding ahead of the arc resulting in poor or non-uniform penetration.

Welder technique Poor welder technique such as incorrect angle or manipulation of the electrode/welding gun, will prevent adequate fusion of the joint sidewall. Weaving, especially dwelling at the joint sidewall, will enable the weld pool to wash into the parent metal, greatly improving sidewall fusion. It should be noted that the amount of weaving might be restricted by the welding procedure specification limiting the arc energy input, particularly when welding alloy or high notch toughness steels.

Magnetic arc blow When welding ferromagnetic steels lack of fusion imperfections can be caused through uncontrolled deflection of the arc, usually termed arc blow. Arc deflection can be caused by distortion of the magnetic field produced by the arc current (Fig. 3), through:

• Residual magnetism in the material through using magnets for handling • Earth�s magnetic field, for example in pipeline welding • Position of the current return

The effect of welding past the current return cable, which is bolted to the centre of the place, is shown in Fig. 4. The interaction of the magnetic field surrounding the arc and that generated by the current flow in the plate to the current return cable is sufficient to deflect the weld bead. Distortion of the arc current magnetic field can be minimised by positioning the current return so that welding is always towards or away from the

clamp and, in MMA welding, by using AC instead of DC. Often the only effective means is to demagnetise the steel before welding.

Fig. 3. Interaction of magnetic forces causing arc deflection

Fig. 4. Weld bead deflection in DC MMA welding caused by welding past the current return connection

Best practice in prevention The following fabrication techniques can be used to prevent formation of lack of sidewall fusion imperfections:

• Use a sufficiently wide joint preparation • Select welding parameters (high current level, short arc length, not too high a

welding speed) to promote penetration into the joint side wall without causing flooding

• Ensure the electrode/gun angle and manipulation technique will give adequate side wall fusion

• Use weaving and dwell to improve side wall fusion providing there are no heat input restrictions

• If arc blow occurs, reposition the current return, use AC (in MMA welding) or demagnetise the steel

Acceptance standards The limits for incomplete fusion imperfections in arc-welded joints in steel are specified in BS EN 25817 (ISO 5817) for the three quality levels (see Table). These types of imperfection are not permitted for Quality Level B (stringent) and C (intermediate). For Quality level D (moderate) they are only permitted providing they are intermittent and not surface breaking.

For arc-welded joints in aluminium, long imperfections are not permitted for all three-quality levels. However, for quality levels C and D, short imperfections are permitted but the total length of the imperfections is limited depending on the butt weld or the fillet weld throat thickness.

Acceptance limits for specific codes and application standards

Application Code/Standard Acceptance limit

Steel ISO 5817:1992 Level B and C not permitted. Level D intermittent and not surface breaking.

Aluminium ISO 10042:1992

Levels B, C, D. Long imperfections not permitted. Levels C and D. Short imperfections permitted.

Pressure vessels BS5500: 1997 Not permitted Storage tanks BS2654: 1989 Not permitted

Pipe work BS2633: 1987 'L' not greater than 15mm (depending on wall thickness)

Line pipe API 1104:1983 'L' not greater than 25mm (less when weld length <300mm)

Detection and remedial action If the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub-surface imperfections, detection is by radiography or ultrasonic inspection. Ultrasonic inspection is normally more effective than radiography in detecting lack of inter-run fusion imperfections.

Remedial action will normally require their removal by localised gouging, or grinding, followed by re-welding as specified in the agreed procedure.

If lack of fusion is a persistent problem, and is not caused by magnetic arc blow, the welding procedures should be amended or the welders retrained.

This information was prepared by Bill Lucas with help from Gene Mathers.

Copies of other articles in the 'Job knowledge for welders' series can be found under Practical Joining Knowledge or by using the search engine.

Defects - solidification cracking

Weld repair on a cast iron exhaust manifold

A crack may be defined as a local discontinuity produced by a fracture, which can arise from the stresses, generated on cooling or acting on the structure. It is the most serious type of imperfection found in a weld and should be removed. Cracks not only reduce the strength of the weld through the reduction in the cross section thickness but also can readily propagate through stress concentration at the tip, especially under impact loading or during service at low temperature.

Identification

Visual appearance

Solidification cracks are normally readily distinguished from other types of cracks due to the following characteristic factors:

• They occur only in the weld metal • They normally appear as straight lines along the centreline of the weld bead,

as shown in Fig. 1, but may occasionally appear as transverse cracking depending on the solidification structure

• Solidification cracks in the final crater may have a branching appearance • As the cracks are 'open', they are easily visible with the naked eye

Fig. 1 Solidification crack along the centre line of the weld

On breaking open the weld, the crack surface in steel and nickel alloys may have a blue oxidised appearance, showing that they were formed while the weld metal was still hot.

Metallography

The cracks form at the solidification boundaries and are characteristically inter dendritic. The morphology reflects the weld solidification structure and there may be evidence of segregation associated with the solidification boundary.

Causes The overriding cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors that increase the risk include:

• Insufficient weld bead size or shape • Welding under high restraint • Material properties such as high impurity content or a relatively large amount

of shrinkage on solidification.

Joint design can have a significant influence on the level of residual stresses. Large gaps between component parts will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Therefore, weld beads with a small depth-to-width ratio, such as formed in bridging a large gap with a wide, thin bead, will be more susceptible to solidification cracking, as shown in Fig. 2. In this case, the centre of the weld, which is the last part to solidify, is a narrow zone with negligible cracking resistance.

Fig. 2 Weld bead penetration too small

Segregation of impurities to the centre of the weld also encourages cracking. Concentration of impurities ahead of the solidifying front weld forms a liquid film of low freezing point that, on solidification, produces a weak zone. As solidification proceeds, the zone is likely to crack as the stresses through normal thermal

contraction build up. An elliptically shaped weld pool is preferable to a teardrop shape. Welding with contaminants such as cutting oils on the surface of the parent metal will also increase the build up of impurities in the weld pool and the risk of cracking.

As the compositions of the plate and the filler determine the weld metal composition they will, therefore, have a substantial influence on the susceptibility of the material to cracking.

Steels

Cracking is associated with impurities, particularly sulphur and phosphorus, and is promoted by carbon whereas manganese and silicon can help to reduce the risk. To minimise the risk of cracking, fillers with low carbon and impurity levels and relatively high manganese content are preferred. As a general rule, for carbon-manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%.

Weld metal composition is dominated by the consumable and as the filler is normally cleaner than the metal being welded, cracking is less likely with low dilution processes such as MMA and MIG. Plate composition assumes greater importance in high dilution situations such as when welding the root in butt welds, using an autogenous welding technique like TIG, or a high dilution process such as submerged arc welding.

In submerged arc welds, as described in BS 5135 (Appendix F), the cracking risk may be assessed by calculating the Units of Crack Susceptibility (UCS) from the weld metal chemical composition (weight %):

UCS = 230C* + 190S + 75P + 45Nb - 12.3Si - 5.4Mn - 1 C* = carbon content or 0.08 whichever is higher

Although arbitrary units, a value of <10 indicates high cracking resistance whereas >30 indicates a low resistance. Within this range, the risk will be higher in a weld run with a high depth to width ratio, made at high welding speeds or where the fit-up is poor. For fillet welds, runs having a depth to width ratio of about one, UCS values of 20 and above will indicate a risk of cracking. For a butt weld, values of about 25 UCS are critical. If the depth to width ratio is decreased from 1 to 0.8, the allowable UCS is increased by about nine. However, very low depth to width ratios, such as obtained when penetration into the root is not achieved, also promote cracking.

Aluminium

The high thermal expansion (approximately twice that of steel) and substantial contraction on solidification (typically 5% more than in an equivalent steel weld) means that aluminium alloys are more prone to cracking. The risk can be reduced by using a crack resistant filler (usually from the 4xxx and 5xxx series alloys) but the disadvantage is that the resulting weld metal is likely to have non-matching properties such as a lower strength than the parent metal.

Austenitic Stainless Steel

A fully austenitic stainless steel weld is more prone to cracking than one containing between 5-10% of ferrite. The beneficial effect of ferrite has been attributed to its capacity to dissolve harmful impurities that would otherwise form low melting point segregates and consequently interdendritic cracks. Therefore the choice of filler material is important to suppress cracking so type 308 filler is used to weld type 304 stainless steel.

Best practice in avoiding solidification cracking

Apart from the choice of material and filler, the principal techniques for minimising the risk of welding solidification cracking are:

• Control joint fit-up to reduce gaps. • Before welding, clean off all contaminants from the material • Ensure that the welding sequence will not lead to a build-up of thermally

induced stresses. • Select welding parameters and technique to produce a weld bead with an

adequate depth to width ratio, or with sufficient throat thickness (fillet weld), to ensure the weld bead has sufficient resistance to the solidification stresses (recommend a depth to width ratio of at least 0.5:1).

• Avoid producing too large a depth to width ratio that will encourage segregation and excessive transverse strains in restrained joints. As a general rule, weld beads whose depth to weld ratio exceeds 2:1 will be prone to solidification cracking.

• Avoid high welding speeds (at high current levels), which increase the amount of segregation and the stress level across the weld bead.

• At the run stop, ensure adequate filling of the crater to avoid an unfavourable concave shape.

Acceptance standards As solidification cracks are linear imperfections with sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817). Crater cracks are permitted for quality level D.

Detection and remedial action Surface breaking solidification cracks can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic or radiographic examination techniques.

Most codes will specify that all cracks should be removed. A cracked component should be repaired by removing the cracks with a safety margin of approximately

5mm beyond the visible ends of the crack. The excavation is then re-welded using filler that will not produce a crack sensitive deposit.

Defects - hydrogen cracks in steels - identification

Preheating to avoid hydrogen cracking

Hydrogen cracking may also be called cold cracking or delayed cracking. The principal distinguishing feature of this type of crack is that it occurs in ferritic steels, most often immediately on welding or after a short time after welding.

In this issue, the characteristic features and principal causes of hydrogen cracks are described.

Identification

Visual appearance

Hydrogen cracks can be usually be distinguished due to the following characteristics: • In C-Mn steels, the crack will normally originate in the heat-affected zone

(HAZ) but may extend into the weld metal (Fig 1). • Cracks can also occur in the weld bead, normally transverse to the welding

direction at an angle of 45° to the weld surface. They are essentially straight, follow a jagged path but may be non-branching.

• In low alloy steels, the cracks can be transverse to the weld, perpendicular to the weld surface, but are non-branching and essentially planar.

Fig. 1 Hydrogen cracks originating in the HAZ (note, the type of cracks shown would not be expected to form in the same weldment)

On breaking open the weld (prior to any heat treatment), the surface of the cracks will normally not be oxidised, even if they are surface breaking, indicating they were formed when the weld was at or near ambient temperature. A slight blue tinge may be seen from the effects of preheating or welding heat.

Metallography

Cracks that originate in the HAZ are usually associated with the coarse grain region, (Fig 2). The cracks can be intergranular, transgranular or a mixture. Intergranular cracks are more likely to occur in the harder HAZ structures formed in low alloy and high carbon steels. Transgranular cracking is more often found in C-Mn steel structures.

In fillet welds, cracks in the HAZ are usually associated with the weld root and parallel to the weld. In butt welds, the HAZ cracks are normally oriented parallel to the weld bead.

Fig. 2 Crack along the coarse grain structure in the HAZ

Causes There are three factors that combine to cause cracking:

• Hydrogen generated by the welding process • A hard brittle structure which is susceptible to cracking • Residual tensile stresses acting on the welded joint

Cracking is caused by the diffusion of hydrogen to the highly stressed, hardened part of the weldment.

In C-Mn steels, because there is a greater risk of forming a brittle microstructure in the HAZ, most of the hydrogen cracks are to be found in the parent metal. With the correct choice of electrodes, the weld metal will have a lower carbon content than the parent metal and, hence, a lower carbon equivalent (CE). However, transverse weld metal cracks can occur especially when welding thick section components.

In low alloy steels, as the weld metal structure is more susceptible than the HAZ, cracking may be found in the weld bead.

The effects of specific factors on the risk of cracking are:

• Weld metal hydrogen • Parent material composition • Parent material thickness • Stresses acting on the weld • Heat input

Weld metal hydrogen content

The principal source of hydrogen is the moisture contained in the flux i.e. the coating of MMA electrodes, the flux in cored wires and the flux used in submerged arc welding. Mainly the electrode type determines the amount of hydrogen generated. Basic electrodes normally generate less hydrogen than rutile and cellulosic electrodes.

It is important to note that there can be other significant sources of hydrogen e.g. moisture from the atmosphere or from the material where processing or service history has left the steel with a significant level of hydrogen. Hydrogen may also be derived from the surface of the material or the consumable.

Sources of hydrogen will include:

• Oil, grease and dirt • Rust • Paint and coatings • Cleaning fluids

Parent metal composition

This will have a major influence on hardenability and, with high cooling rates, the risk of forming a hard brittle structure in the HAZ. The hardenability of a material is usually expressed in terms of its carbon content or, when other elements are

taken into account, its carbon equivalent (CE) value.

The higher the CE value, the greater the risk of hydrogen cracking. Generally, steels with a CE value of <0.4 are not susceptible to HAZ hydrogen cracking as long as low hydrogen welding consumables or processes are used.

Parent material thickness

Material thickness will influence the cooling rate and therefore the hardness level, microstructure produced in the HAZ and the level of hydrogen retained in the weld.

The 'combined thickness' of the joint, i.e. the sum of the thicknesses of material meeting at the joint line, will determine, together with the joint geometry, the cooling rate of the HAZ and its hardness. Consequently, as shown in Fig. 3, a fillet weld will have a greater risk than a butt weld in the same material thickness.

Fig.3 Combined thickness measurements for butt and fillet joints

Stresses acting on the weld

The stresses generated across the welded joint as it contracts will be greatly influenced by external restraint, material thickness, joint geometry and fit-up. Areas

of stress concentration are more likely to initiate a crack at the toe and root of the weld.

Poor fit-up in fillet welds markedly increases the risk of cracking. The degree of restraint acting on a joint will generally increase as welding progresses due to the increase in stiffness of the fabrication.

Heat input

The heat input to the material from the welding process, together with the material thickness and preheat temperature, will determine the thermal cycle and the resulting microstructure and hardness of both the HAZ and weld metal.

A high heat input will reduce the hardness level.

Heat input per unit length is calculated by multiplying the arc energy by an arc efficiency factor according to the following formula:

V = arc voltage (V) A = welding current (A) S = welding speed (mm/min) k = thermal efficiency factor

In calculating heat input, the arc efficiency must be taken into consideration. The arc efficiency factors given in BS EN 1011-1: 1998 for the principal arc welding processes are:

Submerged arc (single wire)

1.0

MMA 0.8MIG/MAG and flux cored wire 0.8TIG and plasma 0.6

In MMA welding, heat input is normally controlled by means of the run-out length from each electrode that is proportional to the heat input. As the run-out length is the length of weld deposited from one electrode, it will depend upon the welding technique e.g. weave width /dwell.

Defects - hydrogen cracks in steels - prevention and best practice

Preheating of a jacket structure to prevent hydrogen cracking

In this issue, techniques and practical guidance on the avoidance of hydrogen cracks are described.

Preheating, interpass and post heating to prevent hydrogen cracking There are three factors that combine to cause cracking in arc welding:

• Hydrogen generated by the welding process • A hard brittle structure which is susceptible to cracking • Residual tensile stresses acting on the welded joint

In practice, for a given situation (material composition, material thickness, joint type, electrode composition and heat input), the risk of hydrogen cracking is reduced by heating the joint.

Preheat

Preheat, which slows the cooling rate, allows some hydrogen to diffuse away and prevents a hard, crack-sensitive structure being formed. The recommended levels of preheat for carbon and carbon manganese steel are detailed in BS 5135. (Nb a draft European standard Pr EN 1011-2 is expected to be introduced in 2000). The preheat level may be as high as 200°C for example, when welding thick section steels with a high carbon equivalent (CE) value.

Interpass and post heating

As cracking rarely occurs at temperatures above ambient, maintaining the temperature of the weldment during fabrication is equally important. For susceptible steels, it is

usually appropriate to maintain the preheat temperature for a given period, typically between 2 to 3 hours, to enable the hydrogen to diffuse away from the weld area. In crack sensitive situations such as welding higher CE steels or under high restraint conditions, the temperature and heating period should be increased, typically 250-300°C for three to four hours.

Post weld heat treatment (PWHT) may be used immediately on completion of welding i.e. without allowing the preheat temperature to fall. However, in practice, as inspection can only be carried out at ambient temperature, there is the risk that 'rejectable,' defects will only be found after PWHT. Also, for highly hardenable steels, a second heat treatment may be required to temper the hard microstructure present after the first PWHT.

Under certain conditions, more stringent procedures are needed to avoid cracking than those derived from the nomograms for estimating preheat in BS 5135. Appendix E of this standard mentions the following conditions:

a. High restraint

b. Thick sections ( approximately 50mm)

c. Low carbon equivalent steels (CMn steels with C 0.1% and CE approximately 0.42)

d. 'Clean� or low sulphur steels (S approximately 0.008%), as a low sulphur and low oxygen content will increase the hardenability of steel.

e. Alloyed weld metal where preheat levels to avoid HAZ cracking may be insufficient to protect the weld metal. Low hydrogen processes and consumables should be used. Schemes for predicting the preheat requirements to avoid weld metal cracking generally require the weld metal diffusible hydrogen level and the weld metal tensile strength as input.

Use of austenitic and nickel alloy weld metal to prevent cracking In situations where preheating is impractical, or does not prevent cracking, it will be necessary to use an austenitic consumable. Austenitic stainless steel and nickel electrodes will produce a weld metal, which at ambient temperature has a higher solubility for hydrogen than ferritic steel. Thus, any hydrogen formed during welding becomes locked in the weld metal with very little diffusing to the HAZ on cooling to ambient.

A commonly used austenitic MMA electrode is 23Cr: 12Ni (e.g. from BS 2926:1984). However, as nickel alloys have a lower coefficient of thermal expansion than stainless steel, nickel austenitic electrodes are preferred when welding highly restrained joints to reduce the shrinkage strain. Figure 1 is a general guide on the levels of preheat when using austenitic electrodes. When welding steels with up to 0.2%C, a preheat would not normally be required. However, above 0.4%C a minimum temperature of

150°C will be needed to prevent HAZ cracking. The influence of hydrogen level and the degree of restraint are also illustrated in the figure.

Fig.1 Guide to preheat temperature when using austenitic MMA electrodes at 1-2kJ/mm a) low restraint (e.g. material thickness <30mm) b) high restraint (e.g. material thickness >30mm)

Best practice in avoiding hydrogen cracking

Reduction in weld metal hydrogen

The most effective means of avoiding hydrogen cracking is to reduce the amount of hydrogen generated by the consumable, i.e. by using a low hydrogen process or low hydrogen electrodes.

Welding processes can be classified as very low, low, medium or high depending on the amount of weld metal hydrogen produced:

Very low <5ml/100g Low 5 - 10ml/100g Medium 10 - 15ml/100g High >15ml/100g

Figure 2 illustrates the relative amounts of weld metal hydrogen produced by the major welding processes. MMA, in particular, has the potential to generate a wide range of hydrogen levels. Thus, to achieve the lower values, it is essential that basic electrodes are used and they are baked in accordance with the manufacturer's recommendations. For the MIG process, cleaner wires will be required to achieve very low hydrogen levels.

Fig.2 General relationships between potential hydrogen and weld metal hydrogen levels for arc welding processes

General guidelines

The following general guidelines are recommended for the various types of steel but requirements for specific steels should be checked according to BS 5135 or BS EN 1011:

Mild steel (CE <0.4) - Readily weldable, preheat generally not required if low hydrogen processes or electrodes are used - Preheat may be required when welding thick section material, high restraint and with higher levels of hydrogen being generated

C-Mn, medium carbon, low alloy steels (CE 0.4 to 0.5) - Thin sections can be welded without preheat but thicker sections will require low preheat levels and low hydrogen processes or electrodes should be used

Higher carbon and alloyed steels (CE >0.5) - Preheat, low hydrogen processes or electrodes, post weld heating and slow cooling required.

More detailed guidance on the avoidance of hydrogen cracking is described in BS 5135.

Practical Techniques

The following practical techniques are recommended to avoid hydrogen cracking:

• Clean the joint faces and remove contaminants such as paint, cutting oils, grease

• Use a low hydrogen process if possible • Dry the electrodes (MMA) or the flux (submerged arc) in accordance with the

manufacturer's recommendations • Reduce stresses on the weld by avoiding large root gaps and high restraint • If preheating is specified in the welding procedure, it should also be applied

when tacking or using temporary attachments • Preheat the joint to a distance of at least 75mm from the joint line ensuring

uniform heating through the thickness of the material • Measure the preheat temperature on the face opposite that being heated.

Where this is impractical, allow time for the equalisation of temperature after removing the preheating before the temperature is measured

• Adhere to the heat input requirements • Maintain heat for approximately two to four hours after welding depending on

crack sensitivity • In situations where adequate preheating is impracticable, or cracking cannot

be avoided, austenitic electrodes may be used

Acceptance standards As hydrogen cracks are linear imperfections, which have sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817).

Detection and remedial action As hydrogen cracks are often very fine and may be sub-surface, they can be difficult to detect. Surface-breaking hydrogen cracks can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic or radiographic examination techniques. Ultrasonic examination is preferred as radiography is restricted to detecting relatively wide cracks parallel to the beam.

Most codes will specify that all cracks should be removed. A cracked component should be repaired by removing the cracks with a safety margin of approximately 5mm beyond the visible ends of the crack. The excavation is then re-welded.

To make sure that cracking does not re-occur, welding should be carried out with the correct procedure, i.e. preheat and an adequate heat input level for the material type and thickness. However, as the level of restraint will be greater and the interpass time shorter when welding within an excavation compared to welding the original joint, it is recommended that a higher level of preheat is used (typically by 50°C).

References

BS 5135:1984 Arc Welding of Carbon and Carbon Manganese Steels

Pr EN 1011-1:1998 Welding - Recommendations for Welding of Metallic Materials Part 1- General Guidance for Arc Welding Part 2- Arc Welding of Ferritic Steels

BS EN ISO 13916: 1997 Welding - Guidance on the Measurement of Preheating Temperature, Interpass Temperature and Preheat Maintenance Temperature

N Bailey et al, Welding steels without hydrogen cracking, Woodhead Publishing, 1993

Defects - lamellar tearing

BP Forties platform lamellar tears were produced when attempting the repair of lack of root penetration in a brace weld

Lamellar tearing can occur beneath the weld especially in rolled steel plate which has poor through-thickness ductility. The characteristic features, principal causes and best practice in minimising the risk of lamellar tearing are described.

Identification

Visual appearance

The principal distinguishing feature of lamellar tearing is that it occurs in T-butt and fillet welds normally observed in the parent metal parallel to the weld fusion boundary and the plate surface , (Fig 1). The cracks can appear at the toe or root of the weld but are always associated with points of high stress concentration.

Fracture face

The surface of the fracture is fibrous and 'woody' with long parallel sections which are indicative of low parent metal ductility in the through-thickness direction, (Fig 2).

Fig. 1. Lamellar tearing in T butt weld

Fig. 2. Appearance of fracture face of lamellar tear

Metallography

As lamellar tearing is associated with a high concentration of elongated inclusions oriented parallel to the surface of the plate, tearing will be transgranular with a stepped appearance.

Causes It is generally recognised that there are three conditions which must be satisfied for lamellar tearing to occur:

1. Transverse strain - the shrinkage strains on welding must act in the short direction of the plate ie through the plate thickness

2. Weld orientation - the fusion boundary will be roughly parallel to the plane of the inclusions

3. Material susceptibility - the plate must have poor ductility in the through-thickness direction

Thus, the risk of lamellar tearing will be greater if the stresses generated on welding act in the through-thickness direction. The risk will also increase the higher the level of weld metal hydrogen

Factors to be considered to reduce the risk of tearing The choice of material, joint design, welding process, consumables, preheating and buttering can all help reduce the risk of tearing.

Material

Fig. 3. Relationship between the STRA and sulphur content for 12.5 to 50mm thick plate

Tearing is only encountered in rolled steel plate and not forgings and castings. There is no one grade of steel that is more prone to lamellar tearing but steels with a low Short Transverse Reduction in Area (STRA) will be susceptible. As a general rule, steels with STRA over 20% are essentially resistant to tearing whereas steels with below 10 to 15% STRA should only be used in lightly restrained joints (Fig. 3).

Steels with a higher strength have a greater risk especially when the thickness is greater than 25mm. Aluminium treated steels with low sulphur contents (<0.005%) will have a low risk.

Steel suppliers can provide plate which has been through-thickness tested with a guaranteed STRA value of over 20%.

Joint Design Lamellar tearing occurs in joints producing high through-thickness strain, eg T joints or corner joints. In T or cruciform joints, full penetration butt welds will be particularly susceptible. The cruciform structures in which the susceptible plate cannot bend during welding will also greatly increase the risk of tearing.

In butt joints, as the stresses on welding do not act through the thickness of the plate, there is little risk of lamellar tearing.

As angular distortion can increase the strain in the weld root and or toe, tearing may also occur in thick section joints where the bending restraint is high.

Several examples of good practice in the design of welded joints are illustrated in Fig. 4.

• As tearing is more likely to occur in full penetration T butt joints, if possible, use two fillet welds, Fig. 4a.

• Double-sided welds are less susceptible than large single-sided welds and balanced welding to reduce the stresses will further reduce the risk of tearing especially in the root, Fig. 4b

• Large single-side fillet welds should be replaced with smaller double-sided fillet welds, Fig. 4c

• Redesigning the joint configuration so that the fusion boundary is more normal to the susceptible plate surface will be particularly effective in reducing the risk, Fig. 4d

Fig. 4 Recommended joint configurations to reduce the risk of lamellar tearing

Fig. 4a

Fig. 4b

Fig. 4c

Fig. 4d

Weld size Lamellar tearing is more likely to occur in large welds typically when the leg length in fillet and T butt joints is greater than 20mm. As restraint will contribute to the problem, thinner section plate which is less susceptible to tearing, may still be at risk in high restraint situations.

Welding process As the material and joint design are the primary causes of tearing, the choice of welding process has only a relatively small influence on the risk. However, higher heat input processes which generate lower stresses through the larger HAZ and deeper weld penetration can be beneficial.

As weld metal hydrogen will increase the risk of tearing, a low hydrogen process should be used when welding susceptible steels.

Consumable Where possible, the choice of a lower strength consumable can often reduce the risk by accommodating more of the strain in the weld metal. A smaller diameter electrode which can be used to produce a smaller leg length, has been used to prevent tearing.

A low hydrogen consumable will reduce the risk by reducing the level of weld metal diffusible hydrogen. The consumables must be dried in accordance with the manufacturer's recommendations.

Preheating Preheating will have a beneficial effect in reducing the level of weld metal diffusible hydrogen. However, it should be noted that in a restrained joint, excessive preheating could have a detrimental effect by increasing the level the level of restraint produced by the contraction across the weld on cooling.

Preheating should, therefore, be used to reduce the hydrogen level but it should be applied so that it will not increase the amount of contraction across the weld.

Buttering Buttering the surface of the susceptible plate with a low strength weld metal has been widely employed. As shown for the example of a T butt weld (Fig. 5) the surface of the plate may be grooved so that the buttered layer will extend 15 to 25mm beyond each weld toe and be about 5 to 10mm thick.

Fig. 5. Buttering with low strength weld metal

a) general deposit on the surface of the susceptible plate

b) in-situ buttering

In-situ buttering ie where the low strength weld metal is deposited first on the susceptible plate before filling the joint, has also been successfully applied. However, before adopting this technique, design calculations should be carried out to ensure that the overall weld strength will be acceptable.

Acceptance standards As lamellar tears are linear imperfections which have sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817).

Detection and remedial action If surface-breaking, lamellar tears can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic examination techniques but there may be problems in distinguishing lamellar tears from inclusion bands. The orientation of the tears normally makes them almost impossible to detect by radiography.

Defects/imperfections in welds - reheat cracking

Brittle fracture in CrMoV steel pressure vessel probably caused through poor toughness, high residual stresses and hydrogen cracking

The characteristic features and principal causes of reheat cracking are described. General guidelines on 'best practice' are given so that welders can minimise the risk of reheat cracking in welded fabrications.

Identification

Visual appearance

Reheat cracking may occur in low alloy steels containing alloying additions of chromium, vanadium and molybdenum when the welded component is being subjected to post weld heat treatment, such as stress relief heat treatment, or has been subjected to high temperature service (typically 350 to 550°C).

Cracking is almost exclusively found in the coarse grained regions of the heat affected zone (HAZ) beneath the weld, or cladding, and in the coarse grained regions within the weld metal. The cracks can often be seen visually, usually associated with areas of stress concentration such as the weld toe.

Cracking may be in the form of coarse macro-cracks or colonies of micro-cracks.

A macro-crack will appear as a 'rough' crack, often with branching, following the coarse grain region, (Fig. 1a). Cracking is always intergranular along the prior austenite grain boundaries (Fig. 1b). Macro-cracks in the weld metal can be oriented either longitudinal or transverse to the direction of welding. Cracks in the HAZ, however, are always parallel to the direction of welding.

Fig.1a. Cracking associated with the coarse grained heat affected zone

Fig.1b. Intergranular morphology of reheat cracks

Micro-cracking can also be found both in the HAZ and within the weld metal. Micro-cracks in multipass welds will be found associated with the grain coarsened regions which have not been refined by subsequent passes.

Causes The principal cause is that when heat treating susceptible steels, the grain interior becomes strengthened by carbide precipitation forcing the relaxation of residual stresses by creep deformation at the grain boundaries.

The presence of impurities which segregate to the grain boundaries and promote temper embrittlement eg sulphur, arsenic, tin and phosphorus, will increase the susceptibility to reheat cracking.

The joint design can increase the risk of cracking. For example, joints likely to contain stress concentration, such as partial penetration welds, are more liable to initiate cracks.

The welding procedure also has an influence. Large weld beads are undesirable as they produce a coarse grained HAZ which is less likely to be refined by the subsequent pass and therefore will be more susceptible to reheat cracking.

Best practice in prevention The risk of reheat cracking can be reduced through the choice of steel, specifying the maximum impurity level and by adopting a more tolerant welding procedure / technique.

Steel choice If possible, avoid welding steels known to be susceptible to reheat cracking. For example, A 508 Class 2 is known to be particularly susceptible to reheat cracking whereas cracking associated with welding and cladding in A508 Class 3 is largely unknown. The two steels have similar mechanical properties but A508 Class 3 has a lower Cr content and a higher manganese content.

Similarly, in the higher strength, creep resistant steels, an approximate ranking of their crack susceptibility is as follows:

5 Cr 1Mo lower risk 2.25Cr 1 Mo 0.5Mo B 0.5Cr 0.5Mo 0.25V higher risk

Thus, in selecting a creep resistant, chromium molybdenum steel, 0.5Cr 0.5Mo 0.25V steel is known to be susceptible to reheat cracking but the 2.25Cr 1Mo which has a similar creep resistance, is significantly less susceptible.

Unfortunately, although some knowledge has been gained on the susceptibility of certain steels, the risk of cracking cannot be reliably predicted from the chemical composition. Various indices, including G1, PSR and Rs, have been used to indicate the susceptibility of steel to reheat cracking. Steels which have a value of G of less than 2, PSR less than zero or Rs less than 0.03, are less susceptible to reheat cracking

G1 = 10C + Cr + 3.3Mo + 8.1V - 2 PSR = Cr +Cu + 2Mo + 10V +7Nb + 5Ti - 2 Rs = 0.12Cu +0.19S +0.10As + P +1.18Sn + 1.49Sb

Impurity level Irrespective of the steel type, it is important to purchase steels specified to have low levels of trace elements (antimony, arsenic, tin and phosphorus). It is generally accepted that the total level of impurities in the steel should not exceed 0.01% to minimise the risk of temper embrittlement.

Welding procedure and technique

The welding procedure can be used to minimise the risk of reheat cracking by • Producing the maximum refinement of the coarse grain HAZ • Limiting the degree of austenite grain growth • Eliminating stress concentrations

The procedure should aim to refine the coarse grained HAZ by subsequent passes. In butt welds, maximum refinement can be achieved by using a steep sided joint preparation with a low angle of attack to minimise penetration into the sidewall, (Fig 2a). In comparison, a larger angle V preparation produces a wider HAZ limiting the amount of refinement achieved by subsequent passes, (Fig 2b). Narrow joint preparations, however, are more difficult to weld due to the increased risk of lack of sidewall fusion.

Fig.2a. Welding in the flat position - high degree of HAZ refinement

Fig.2b. Welding in the horizontal/vertical position - low degree of HAZ refinement

Refinement of the HAZ can be promoted by first buttering the surface of the susceptible plate with a thin weld metal layer using a small diameter (3.2mm) electrode. The joint is then completed using a larger diameter (4 - 4.8mm) electrode which is intended to generate sufficient heat to refine any remaining coarse grained HAZ under the buttered layer.

The degree of austenite grain growth can be restricted by using a low heat input. However, precautionary measures may be necessary to avoid the risk of hydrogen assisted cracking and lack-of-fusion defects. For example, reducing the heat input will

almost certainly require a higher preheat temperature to avoid hydrogen assisted cracking.

The joint design and welding technique adopted should ensure that the weld is free from localised stress concentrations which can arise from the presence of notches. Stress concentrations may be produced in the following situations:

• welding with a backing bar • a partial penetration weld leaving a root imperfection • internal weld imperfections such as lack of sidewall fusion • the weld has a poor surface profile, especially sharp weld toes

The weld toes of the capping pass are particularly vulnerable as the coarse grained HAZ may not have been refined by subsequent passes. In susceptible steel, the last pass should never be deposited on the parent material but always on the weld metal so that it will refine the HAZ.

Grinding the weld toes with the preheat maintained has been successfully used to reduce the risk of cracking in 0.5Cr 0.5Mo 0.25V steels.

Gouging processes

Gouging operations can be carried out using the following thermal processes:

• Oxyfuel gas flame • Manual metal arc • Air carbon arc • Plasma arc

Thermal Gouging Thermal gouging is an essential part of welding fabrication. Used for rapid removal of unwanted metal, the material is locally heated and molten metal ejected - usually by blowing it away. Normal oxyfuel gas or arc processes can be used to produce rapid melting and metal removal. However, to produce a groove of specific dimensions, particularly regarding depth and width, the welder must exercise careful control of the gouging operation. If this does not happen, an erratic and badly serrated groove will result.

Thermal processes, operations and metals which may be gouged or otherwise shaped:

Process operations Thermal process Primary Secondary

Metals

Oxyfuel gas flame Gouging

Grooving Washing Chamfering

Low carbon steels, carbon manganese steels (structural), pressure vessel steels (carbon not over 0.35%), low alloy steels (less than 5%Cr) cast iron (if preheated to 400-450 deg.C)

Manual metal arc Gouging Grooving

Chamfering

Low carbon steels carbon manganese steels (structural), pressure vessel steels, low alloy steels, stainless steels, cast iron, nickel-based alloys

Air carbon arc Gouging Grooving

Chamfering

Low carbon steels carbon manganese steels (structural), pressure vessel steels, low and high alloy steels, cast iron, nickel-based alloys, copper and copper alloys, copper/nickel alloys, aluminium

Plasma arc Gouging ChamferingGrooving Washing

Aluminium, stainless steels

Note: All processes are capable of cutting/severing operations. Preheat may or may not be required on some metals prior to gouging

Safety

It should be emphasised that because gouging relies on molten metal being forcibly ejected, often over quite large distances, the welder must take appropriate precautions to protect himself, other workers and his equipment. Sensible precautions include protective clothing for the welder, shielding inside a specially enclosed booth or screens, adequate fume extraction, and removal of all combustible material from the immediate area.

Industrial applications

Thermal gouging was developed primarily for removal of metal from the reverse side of welded joints, removal of tack welds, temporary welds, and weld imperfections. Figure 1 illustrates the value of typical back-gouging applications carried out on arc welded joints., while Fig. 2 shows imperfection removal in preparation for weld repair.

Fig.1 Typical back-gouging applications carried out on arc-welded joints

Fig. 2 Imperfection removal in preparation for weld repair

The gouging process has proved to be so successful that it is used for a wide spectrum of applications in engineering industries:

• Repair and maintenance of structures - bridges, earth-moving equipment, mining machinery, railway rolling stock, ships, offshore rigs, piping and storage tanks

• Removal of cracks and imperfections - blow holes and sand traps in both ferrous and non-ferrous forgings and castings

• Preparation of plate edges for welding

• Removal of surplus metal - strongbacks, lifting lugs and riser pads and fins on castings, excess weld bead profiles, temporary backing strips, rivet washing and shaping operations demolition of welded and unwelded structures - site work

Thermal gouging is also suitable for efficient removal of temporary welded attachments such as brackets, strongbacks, lifting lugs and redundant tack welds, during various stages of fabrication and construction work.

Oxygen-fuel Gas Flame Gouging

Oxygen-fuel (oxyfuel) flame gouging offers fabricators a quick and efficient method of removing metal. It can be at least four times quicker than cold chipping operations. The process is

particularly attractive because of its low noise, ease of handling, and ability to be used in all positions.

Process description

Flame gouging is a variant of conventional oxyfuel gas welding. Oxygen and a fuel gas are used to produce a high temperature flame for melting the steel. When gouging, the steel is locally heated to a temperature above the 'ignition' temperature (typically 900deg.C) and a jet of oxygen is used to melt the metal - a chemical reaction between pure oxygen and hot metal. This jet is also used to blow away molten metal and slag. It should be noted that compared with oxyfuel cutting, slag is not blown through the material, but remains on the top surface of the workpiece.

The gouging nozzle is designed to supply a relatively large volume of oxygen through the gouging jet. This can be as much as 300 litre/min through a 6mm orifice nozzle. In oxyacetylene gouging, equal quantities of oxygen and acetylene are used to set a neutral preheating flame. The oxygen jet flow rate determines the depth and width of the gouge. Typical operating parameters (gas pressures and flow rates) for achieving a range of gouge sizes (depth and width) can be seen in the Table.

Typical operating data for manual oxyacetylene flame gouging

Gouge dimensions Gas pressure Gas consumption Nozzle

orifice dia.(mm) Width

(mm) Depth (mm)

Acetylene (Bar)

Oxygen (Bar)

Acetylene (Litre/min)

Preheat (Litre/min)

Oxygen (Litre/min)

Travel speed

(mm/min)

3 6-8 3-9 0.48 4.2 15 22 62 600 5 8-10 6-12 0.48 5.2 29 31 158 1000 6.5 10-13 10-13 0.55 5.5 36 43 276 1200

When the preheating flame and oxygen jet are correctly set, the gouge has a uniform profile and its surfaces are smooth with a dull blue colour.

Operating techniques

The depth of the gouge is determined principally by the speed and angle of the torch. To cut a deep groove the angle of the torch is stepped up (this increases the impingement angle of the oxygen jet) and gouging speed is reduced. To produce a shallow groove, the torch is less steeply angled, see above, and speed is increased. Wide grooves can be produced by weaving the torch. The contour of the groove is dependent upon the size of the nozzle and the operating parameters. If the cutting oxygen pressure is too low, gouging progresses with a washing action, leaving smooth ripples in the bottom of the groove. If the cutting oxygen pressure is too high, the cut advances ahead of the molten pool - this will disrupt the gouging operation especially when making shallow grooves.

There are four basic flame-gouging techniques, which are used in the following types of application.

Progressive gouging

This technique is used to produce uniform grooves. Gouging is conducted in either a continuous or progressive manner. Applications include removal of an unfused root area on the reverse side of a welded joint, part-shaping a steel forging, complete removal of a weld deposit and preparing plate edges for welding.

Spot gouging

Spot gouging produces a deep narrow U-shaped groove over a relatively short length. The process is ideally suited to removal of localised areas such as isolated weld imperfections. Experienced operators are able to observe any imperfections during gouging. These appear as dark or light spots/streaks within the molten pool (reaction zone).

Back-step gouging

Once the material has reached ignition temperature, the oxygen stream is introduced and the torch moved in a backward movement for a distance of 15-20mm. The oxygen is shut off and the torch moved forward a distance of 25-30mm before restarting the gouging operation. This technique is favoured for removal of local imperfections, which may be deeply embedded, in the base plate.

Deep gouging

It is sometimes necessary to produce a long deep gouge. Such operations are completed using the deep gouging technique, which is basically a combination of progressive and spot gouging.

Manual Metal Arc Gouging The main advantage of manual metal arc (MMA) gouging is that it allows the operator to switch easily from welding to gouging, or cutting, simply by changing the type of electrode.

Process description

As in conventional MMA welding, the arc is formed between the tip of the electrode and the workpiece. MMA gouging differs because it requires special purpose electrodes with thick flux coatings to generate a strong arc force and gas stream. Unlike MMA welding where a stable weld pool must be maintained, this process forces the molten metal away from the arc zone to leave a clean cut surface.

The gouging process is characterised by the large amount of gas, which is generated to eject the molten metal. However, because the arc/gas stream is not as powerful as a gas or a separate air jet, the surface of the gouge is not really as smooth as an oxyfuel gouge or air carbon arc gouge.

Electrode

According to the size of gouge specified, there is a wide range of electrode diameters available to choose from. These grooving electrodes are also not just restricted to steels, and the same electrode composition may be used for gouging stainless steel and non-ferrous alloys.

Power source

MMA gouging can be carried out using conventional DC and AC power sources. In DC gouging, electrode polarity is normally negative but electrode manufacturers may well recommend electrode polarity for their brand of electrodes and for gouging specific materials. When using an AC power source, a minimum of 7OV open circuit (OCV) is required to stabilise the arc.

Although most MMA welding power sources can be used for gouging, the current rating and OCV must be capable of accommodating current surges and longer arc lengths.

Typical operating data for MMA gouging Gouging dimensions

Electrode diameter (mm)

Current (A) Depth

(mm) Width (mm)

Gouging speed (mm/min)

3.2 210 2 6 1200

4.0 300 3 8 1000 4.8 350 4 10 800

Operational characteristics

The arc is struck with an electrode, which is held at a normal angle to the workpiece (15 degrees backwards from the vertical plane in line with proposed direction of gouging). Once the arc is established, the electrode is immediately inclined in one smooth and continuous movement to an angle of around 15-20 degrees to the plate surface. With the arc pointing in the direction of travel, the electrode is pushed forward slightly to melt the metal. It should then be pulled back to allow the gas jet to displace the molten metal and slag. This forward and backward motion is repeated as the electrode is guided along the line to complete the gouge.

To produce a consistent depth and width of gouge, a uniform rate of travel must be maintained, together with the angle of electrode: 10-20 degrees. If the electrode angle becomes too steep, in excess of about 20 degrees, the amount of slag and molten metal will increase. This is a result of the arc penetrating too deeply. Digging the electrode into the metal causes problems in controlling the gouging operation and will produce a rough surface profile. For gouging in positions other than vertical, the electrode is always pushed forward. With vertical surfaces, the electrode is directed and pushed vertically downwards.

Application

MMA gouging is used for localised gouging operations, removal of defects for example, and where it is more convenient to switch from a welding electrode to a gouging electrode rather than use specialised equipment. Compared with alternative gouging processes, metal removal rates are low and the quality of the gouged surface is inferior. When correctly applied, MMA gouging can produce relatively clean gouged surfaces. For general applications welding can be carried out without the need to dress by grinding. However when gouging stainless steel, a thin layer of higher carbon content material will be produced - this should be removed by grinding.

Plasma Arc Gouging The use of the plasma arc as a gouging tool dates back to the 1960s when the process was developed for welding. Compared with the alternative oxyfuel and MMA gouging techniques, plasma arc has a

needle-like jet, which can produce a very precise groove, suitable for application on almost all ferrous and non- ferrous materials.

Process description

Plasma arc gouging is a variant of the plasma arc process. The arc is formed between a refractory (usually tungsten) electrode and the workpiece. Intense plasma is achieved by constricting the arc using a fine bore copper nozzle. By locating the electrode behind the nozzle, the plasma-forming gas can be

separated from the general gas supply used to cool the torch/assist the plasma gas to blow away molten metal (dross) from the groove.

The temperature and force of the constricted plasma arc is determined by the current level and plasma gas flow rate. Thus, the plasma can be varied to produce a hot gas stream or a high power, deeply penetrating jet. This ability to control quite precisely the size and shape of a groove is very useful for removing unwanted defects from a workpiece surface.

Whilst gouging, normal precautions should be taken to protect the operator and other workers in the immediate area from the effects of intense are light and hot metal spray. Unlike the oxyfuel and MMA processes, the plasma arc's high velocity jet will propel fume and hot metal dross some considerable distance from the operator. When using a deeply penetrating arc, noise protection is an essential requirement.

Equipment

The power source for sustaining this gouging arc must have a high open circuit voltage, usually well in excess of 100V. The torch is connected to the negative polarity of the power source and the workpiece must be connected to the positive. The plasma torch is the same as the one used for cutting; it will be either gas or water-cooled and have the facility for single and dual gas operation.

Electrodes are normally tungsten for argon and argon-based gases. However, when using air as the plasma gas, special purpose, for example hafnium tipped copper, electrodes must be used to withstand the more aggressive, oxidising arc.

Plasma and cooling gases

Plasma gas can be argon, helium, argon - H2, nitrogen or air. Argon - 35%H2 is normally recommended as a general- purpose plasma gas for cutting most materials. Alternative plasma gases are argon and helium. Argon, a colder gas, will reduce metal removal rates. Helium, which generates a hot but less intense arc than argon - H2, can produce a wider and shallower groove. Nitrogen and air are also used as plasma gases, especially for gouging C-Mn steels. Although gas costs will be substantially reduced, the groove surface profile will be inferior to that which can be achieved with argon - H2 gas. Air is not recommended for gouging aluminium as this requires an inert or reducing gas. Argon, nitrogen or air are all used as cooling gases. Use of argon will normally produce the best quality of gouge, but nitrogen or air will reduce operating costs.

Operating techniques

Gouging is effected by moving the torch forward at a steady controlled rate. It is carried out in a progressive manner to remove metal over a distance of 200 to 250mm. The jet can then be repositioned, either to deepen or widen the groove, or to continue gouging for a further 200 to 250mm. Principal process parameters are current level, gas flow rate, and speed of gouging. These settings determine groove size and metal removal rate. In a typical gouging operation on C-Mn steel, metal is removed at about 100 kg/hr at a speed of 0.5 m/min, and groove size will be around 12mm wide and 5mm deep.

The torch stand-off and its angle to the surface of the workpiece have a major influence on speed of travel, groove profile and quality of surface. The torch is normally held at a distance of 20mm from the

workpiece and inclined backwards to the direction of gouging at an angle of 40 to 45 degrees. Gouging will remove up to approximately 6mm depth of metal in a single pass.

The torch stand-off should not be reduced to less than 12mm, to avoid spatter build-up on the nozzle from the molten particles ejected from the groove. At standoff distances greater than 25mm, arc/gas forces are reduced and this lessens the depth of penetration of the jet. By reducing the torch angle to the workpiece surface, the plasma jet can be encouraged to 'skate' along the surface of the workpiece; this produces a shallower and wider groove. By increasing the angle of the torch the plasma jet is directed into the workpiece surface, resulting in a deeper and narrower groove.

Air Carbon Arc Gouging The main difference between this gouging technique and the others is that a separate air jet is used to eject molten metal to form the groove.

Process description

Air carbon arc gouging works as follows. An electric arc is generated between the tip of a carbon electrode and the workpiece. The metal becomes molten and a high velocity air jet streams down the electrode to blow it away, thus leaving a clean groove. The process is simple to apply (using the same equipment as MMA welding), has a high metal removal rate, and gouge profile can be closely controlled. Disadvantages are that the air jet causes the molten metal to be ejected over quite a large distance and, because of high currents (up to 2000A) and high air pressures (80 to 100 psi), it can be very noisy.

Application

As air carbon arc gouging does not rely on oxidation it can be applied to a wide range of metals. DC (electrode positive) is normally preferred for steel and stainless steel but AC is more effective for cast iron, copper and nickel alloys. Typical applications include back gouging, removal of surface and internal defects, removal of excess weld metal and preparation of bevel edges for welding.

Electrode

The electrode is a non-consumable graphite (carbon) rod, which has a copper coating to reduce electrode erosion. Electrode diameter is selected according to required depth and width of gouge. Cutting can be precisely controlled and molten metal/dross is kept to a minimum.

Power source

A DC power supply with electrode positive polarity is most suitable. AC power sources which are also constant current can be used but with special AC type electrodes. The power source must have a constant current output characteristic. If it does not, inadvertant touching of the electrode to the workpiece will cause a high current surge sufficient to 'explode' the electrode tip. This will disrupt the operation and cause carbon pick-up. As arc voltage can be quite high (up to 50V), open circuit voltage of the power source should be over 60V.

Air supply

The gouging torch is normally operated with either a compressed air line or separate bottled gas supply. Air supply pressure will be up to 100psi from the airline but restricted to about 35psi from a bottled supply. Providing there is sufficient airflow to remove molten metal, there are no advantages in using higher pressure and flow rates.

Carbon pickup

Although the molten metal picks up carbon, the air stream will remove carbon-rich metal from the groove to leave only minimal contamination of the sidewalls. Poor gouging technique or insufficient airflow will result in carbon pick-up with the risk of metallurgical problems, e.g. high hardness and even cracking.

Typical operating data for air carbon arc gouging:

Gouging dimensions Electrode

diameter (mm)

Current A Note: DC electrode Depth

(mm) Width (mm)

Carbon electrode consumed (mm/min)

Gouging speed

(mm/min)

6.4 275 6-7 9-10 120 609

8.0 350 7-8 10-11 114 711 9.5 425 9-10 12-13 100 660

Manual

13.0 550 12-13 18-19 76 508 8.0 300-400 2-9 3-8 100 1650-840

9.5 500 3-12 3-10 142 1650-635 13.0 850 3-15 3-13 82 1830-610

Automatic

16.0 1250 3-19 3-16 63 1830-710

Operation

Striking the electrode tip on to the workpiece surface to initiate the arc commences gouging. Unlike manual metal arc (MMA) welding the electrode tip is not withdrawn to establish arc length. Molten metal directly under the electrode tip (arc) is immediately blown away by the air stream. For effective metal removal, it is important that the air stream is directed at the arc from behind the electrode and sweeps under the tip of the electrode. The width of groove is determined by the diameter of electrode, but depth is dictated by the angle of electrode to the workpiece and rate of travel. Relatively high travel speeds are possible when a low electrode angle is used. This produces a shallow groove: a steep angle results in a deep groove and requires slower travel speed. Note, a steeply angled electrode may give rise to carbon contamination.

Oscillating the electrode in a circular or restricted weave motion during gouging can greatly increase gouging width. This is useful for removal of a weld or plate imperfection that is wider than the electrode itself. It is important, however, that weave width should not exceed four times the diameter of the electrode. The groove surface should be relatively free of oxidised metal and can be considered ready for welding without further preparation. Dressing by grinding the sidewalls of the gouge should

be carried out if a carbon rich layer has been formed. Also, dressing by grinding or another approved method will be necessary if working on crack-sensitive material such as high strength, low alloy steel.

Equipment for Oxyacetylene Welding Essential equipment components

Torch

The basic oxyacetylene torch comprises:

• Torch body (or handle) • Two separate gas tubes (through the handle connected to the hoses) • Separate control valves • Mixer chamber • Flame tube • Welding tip

NB The cutting torch requires two oxygen supplies to the nozzle, one mixed with fuel gas for preheating and a separate oxygen flow for cutting.

Hoses

Hoses are colour-coded red for acetylene and blue (UK) or green (US) for oxygen. Oxygen fittings on the hose have a right-hand thread while acetylene is left-handed.

Gas regulators

The primary function of a gas regulator is to control gas pressure. It reduces the high pressure of the bottle-stored gas to the working pressure of the torch, and this will be maintained during welding.

The regulator has two separate gauges: a high pressure gauge for gas in the cylinder and a low pressure gauge for pressure of gas fed to the torch. The amount of gas remaining in the cylinder can be judged from the high pressure gauge. The regulator, which has a pressure adjusting screw, is used to control gas flow rate to the torch by setting the outlet gas pressure. Note Acetylene is supplied in cylinders under a pressure of about 15 bars psi but welding is carried out with torch gas pressures typically up to 2 bars.

Flame traps

Flame traps (also called flashback arresters) must be fitted into both oxygen and acetylene gas lines to prevent a flashback flame from reaching the regulators. Non-return spring-loaded valves can be fitted in the hoses to detect/stop reverse gas flow. Thus, the valves can be used to prevent conditions leading to flashback, but should always be used in conjunction with flashback arresters.

A flashback is where the flame burns in the torch body, accompanied by a whistling sound. It will occur when flame speed exceeds gas flow rate and the flame can pass back through the mixing chamber into the hoses. Most likely causes are: incorrect gas pressures giving too low a gas velocity, hose leaks, loose connections, or welder techniques which disturb gas flow.

Identification of gas cylinders

An oxygen cylinder is colour-coded black and the acetylene cylinder is maroon. Oxygen and acetylene are stored in cylinders at high pressure. Oxygen pressure can be as high as 230 bars. Acetylene, which

is dissolved in acetone contained in a porous material, is stored at a much lower pressure, approximately 15 bars.

The appropriate regulator must be fitted to the cylinders to accommodate cylinder pressures. To avoid confusion, oxygen cylinders and regulators have right-hand threads and acetylene cylinders and regulators have left-hand ones.

Typical gas pressures and flow rates for C-Mn steel:

Acetylene Oxygen Steel thickness

(mm)

Nozzle size Pressure

(bar) Consumption

(l/min) Pressure

(bar) Consumption

(l/min)

0.90 1 0.14 0.50 0.14 0.50 1.20 2 0.14 0.90 0.14 0.90

2.00 3 0.14 1.40 0.14 1.40 2.60 5 0.14 2.40 0.14 2.40

3.20 7 0.14 3.30 0.14 3.30 4.00 10 0.21 4.70 0.21 4.70

5.00 13 0.28 6.00 0.28 6.00 6.50 18 0.28 8.50 0.28 8.50

8.20 25 0.42 12.00 0.42 12.00 10.00 35 0.63 17.00 0.63 17.00

13.00 45 0.35 22.00 0.35 22.00 25.00 90 0.63 42.00 0.63 42.00

Selection of correct nozzles

Welding torches are generally rated according to thickness of material to be welded. They range from light duty (for sheet steel up to 2mm in thickness) to heavy duty (for steel plate greater than 25mm in thickness). Each torch can be fitted with a range of nozzles with a bore diameter selected according to material thickness. Gas pressures are set to give correct flow rate for nozzle bore diameter. Proportions of oxygen and acetylene in the mixture can be adjusted to give a neutral, oxidising or carburising flame. (See the description of oxyacetylene processes) Welding is normally carried out using a neutral flame with equal quantities of oxygen and acetylene.

Equipment safety checks

Before commencing welding it is wise to inspect the condition and operation of all equipment. As well as normal equipment and workplace safety checks, there are specific procedures for oxyacetylene. Operators should verify that:

• flashback arresters are present in each gas line • hoses are the correct colour, with no sign of wear, as short as possible and not taped together • regulators are the correct type for the gas • a bottle key is in each bottle (unless the bottle has an adjusting screw)

It is recommended that oxyacetylene equipment is checked at least annually - regulators should be taken out of service after five years. Flashback arresters should be checked regularly according to manufacturer's instructions and, with specific designs, it may be necessary to replace if flashback has occurred.

For more detailed information the following legislation and codes of practice should be consulted:

• UK Health and Safety at Work Act 1974 • Pressure Systems and Transportable Gas Containers Regulations • British Compressed Gases Association, Codes of Practice • BOC Handbook

Equipment for MMA Welding Although the manual metal arc (MMA) process has relatively basic equipment requirements, it is important that the welder has a knowledge of operating features and performance to comply with welding procedures for the job and, of course, for safety reasons.

Essential equipment

The main components of the equipment required for welding are:

• Power source • Electrode holder and cables • Welder protection • Fume extraction

Tools required include: a wire brush to clean the joint area adjacent to the weld (and the weld itself after slag removal); a chipping hammer to remove slag from the weld deposit; and, when removing slag, a pair of clear lens goggles or a face shield to protect the eyes (lenses should be shatter-proof and noninflammable).

Power source

The primary function of a welding power source is to provide sufficient power to melt the joint. However with MMA the power source must also provide current for melting the end of the electrode to produce weld metal, and it must have a sufficiently high voltage to stabilise the arc.

MMA electrodes are designed to be operated with alternating current (AC) and direct current (DC) power sources. Although AC electrodes can be used on DC, not all DC electrodes can be used with AC power sources.

As MMA requires a high current (50-30OA) but a relatively low voltage (10-50V), high voltage mains supply (240 or 440V) must be reduced by a transformer. To produce DC, the output from the transformer must be further rectified. To reduce the hazard of electrical shock, the power source must function with a maximum no-load

voltage, that is, when the external (output) circuit is open (power leads connected and live) but no arc is present. The no-load voltage rating of the power source is as defined in BS 638 and must be in accordance with the type of welding environment or hazard of electrical shock. The power source may have an internal or external hazard-reducing device to reduce the no-load voltage; the main welding current is delivered as soon as the electrode touches the workpiece. For welding in confined spaces, you should use a low voltage safety device to limit the voltage available at the holder to approximately 25V.

There are four basic types of power source:

• AC transformer • DC rectifier • AC/DC transformer-rectifier • DC generator

AC electrodes are frequently operated with the simple, single-phase transformer with current adjusted by means of tappings or sliding core control. DC rectifiers and AC/DC transformer-rectifiers are controlled electronically, for example by thyristors. A new generation of power sources called inverters is available. These use transistors to convert mains AC (50Hz) to a high frequency AC (over 500 Hz) before transforming down to a voltage suitable for welding and then rectifying to DC. Because high frequency transformers can be relatively small, principal advantages of inverter power sources are undoubtedly their size and weight when the source must be portable.

Electrode holder and cables

The electrode holder clamps the end of the electrode with copper contact shoes built into its head. The shoes are actuated by either a twist grip or spring-loaded mechanism. The clamping mechanism allows for quick release of the stub end. For efficiency the electrode has to be firmly clamped into the holder, otherwise poor electrical contact may cause arc instability through voltage fluctuations. Welding cable connecting the holder to the power source is mechanically crimped or soldered.

It is essential that good electrical connections are maintained between electrode, holder and cable. With poor connections, resistance heating and, in severe cases, minor arcing with the torch body will cause the holder to overheat. Two cables are connected to the output of the power source, the welding lead goes to the electrode holder and the current return lead is clamped to the workpiece. The latter is often wrongly referred to as the earth lead. A separate earth lead is normally required to provide protection from faults in the power source. The earth cable should therefore be capable of carrying the maximum output current of the power source.

Cables are covered in a smooth and hard-wearing protective rubberised flexible sheath. This oil and water resistant coating provides electrical insulation at voltages to earth not exceeding 100V DC and AC (rms value). Cable diameter is generally selected on the basis of welding current level, As these electrode types are When welding, the welder air movement should be from duty cycle and distance of the work from the power source. The higher the current and duty cycle, the larger the diameter

of the cable to ensure that it does not overheat (see BS 638 Pt 4). If welding is carried out some distance from the power source, it may be necessary to increase cable diameter to reduce voltage drop.

Care of electrodes

The quality of weld relies upon consistent performance of the electrode. The flux coating should not be chipped, cracked or, more importantly, allowed to become damp.

Storage

Electrodes should always be kept in a dry and well-ventilated store. It is good practice to stack packets of electrodes on wooden pallets or racks well clear of the floor. Also, all unused electrodes, which are to be returned, should be stored so they are not exposed to damp conditions to regain moisture. Good storage conditions are 10 degrees C above external air temperature. As the storage conditions are to prevent moisture from condensing on the electrodes, the electrode stores should be dry rather that warm. Under these conditions and in original packaging, electrode storage time is practically unlimited. It should be noted that electrodes are now available in hermetically sealed packs that obviate the need for drying. However, if necessary, any unused electrodes must be redried according to manufacturer's instructions.

Drying of electrodes

Drying is usually carried out following the manufacturer's recommendations and requirements will be determined by the type of electrode.

Cellulosic coatings

As these electrode coatings are designed to operate with a definite amount of moisture in the coating, they are less sensitive to moisture pick-up and do not generally require a drying operation. However, in cases where ambient relative humidity has been very high, drying may be necessary.

Rutile coatings

These can tolerate a limited amount of moisture and coatings may deteriorate if they are overdried. Particular brands may need to be dried before use.

Basic and basic/rutile coatings

Because of the greater need for hydrogen control, moisture pick-up is rapid on exposure to air. These electrodes should be thoroughly dried in a controlled temperature drying oven. Typical drying time is one hour at a temperature of approximately 150 to 300 degrees C but instructions should be adhered to.

After controlled drying, basic and basic/rutile electrodes must be held at a temperature between 100 and 150 degrees C to help protect them from re-absorbing moisture into

the coating. These conditions can be obtained by transferring the electrodes from the main drying oven to a holding oven or a heated quiver at the workplace.

Protective clothing

When welding, the welder must be protected from heat and light radiation emitted from the arc, spatter ejected from the weld pool, and from welding fume.

Hand and head shield

For most operations a hand-held or head shield constructed of lightweight insulating and non-reflecting material is used. The shield is fitted with a protective filter glass, sufficiently dark in colour and capable of absorbing the harmful infrared and ultraviolet rays. The filter glasses conform to the strict requirements of BS 679 and are graded according to a shade number which specifies the amount of visible light allowed to pass through - the lower the number, the lighter the filter. The correct shade number must be used according to the welding current level, for example:

• Shade 9 - up to 40A • Shade 10 - 40 to 80A • Shade 11 - 80 to 175A • Shade 12 - 175 to 300A • Shade 13 - 300 to 500A

Clothing

For protection against sparks, hot spatter, slag and burns, a leather apron and leather gloves should be worn. Various types of leather gloves are available, such as short or elbow length, full fingered or part mitten.

Fume extraction

When welding within a welding shop, ventilation must dispose harmlessly of the welding fume. Particular attention should be paid to ventilation when welding in a confined space such as inside a boiler, tank or compartment of a ship.

Fume removal should be by some form of mechanical ventilation, which will produce a current of fresh air in the immediate area. Direction of the air movement should be from the welder's face towards the work. This is best achieved by localised exhaust ventilation using a suitably designed hood near to the welding area.

Further information

Please refer to: BS 638 Arc welding power sources, equipment and accessories BS 679 Filters, cover lenses and backing lenses for use during welding and similar operations.

Equipment for MIG Welding The MIG process is a versatile welding technique, which is suitable for both thin sheet and thick section components. It is capable of high productivity but the quality of welds can be called into question. To achieve satisfactory welds, welders must have a good knowledge of equipment requirements and should also recognise fully the importance of setting up and maintaining component parts correctly.

Essential equipment

In MIG the arc is formed between the end of a small diameter wire electrode fed from a spool, and the workpiece. Main equipment components are:

• Power source • Wire feed system • Conduit • Gun

The arc and weldpool are protected from the atmosphere by a gas shield. This enables bare wire to be used without a flux coating (required by MMA). However, the absence of flux to 'mop up' surface oxide places greater demand on the welder to ensure that the joint area is cleaned immediately before welding. This can be done using either a wire brush for relatively clean parts, or a hand grinder to remove rust and scale. The other essential piece of equipment is a wire cutter to trim the end of the electrode wire.

Power source

MIG is operated exclusively with a DC power source. The source is termed a flat, or constant current, characteristic power source, which refers to the voltage/welding current relationship. In MIG, welding current is determined by wire feed speed, and arc length is determined by power source voltage level (open circuit voltage). Wire burn-off rate is automatically adjusted for any slight variation in the gun to workpiece distance, wire feed speed, or current pick-up in the contact tip. For example, if the arc momentarily shortens, arc voltage will decrease and welding current will be momentarily increased to burn back the wire and maintain pre-set arc length. The reverse will occur to counteract a momentary lengthening of the arc.

There is a wide range of power sources available, mode of metal transfer can be:

• Dip • Spray • Pulsed

A low welding current is used for thin-section material, or welding in the vertical position. The molten metal is transferred to the workpiece by the wire dipping into the weldpool. As welding parameters will vary from around 100A \ 17V to 200A \ 22V (for a 1.2mm diameter wire), power sources normally have a current rating of up to 350A. Circuit inductance is used to control the surge in current when the wire dips into the weldpool (this is the main cause of spatter). Modern electronic power sources automatically set the inductance to give a smooth arc and metal transfer.

In spray metal transfer, metal transfers as a spray of fine droplets without the wire touching the weldpool. The welding current level needed to maintain the non short-circuiting arc must be above a minimum threshold level; the arc voltage is higher to ensure that the wire tip does not touch the weldpool. Typical welding parameters for a 1.2mm diameter wire are within 250A \ 28V to 400A \ 35V. For high deposition rates the power source must have a much higher current capacity: up to 500A.

The pulsed mode provides a means of achieving a spray type metal transfer at current levels below threshold level. High current pulses between 25 and 100Hz are used to detach droplets as an alternative to dip transfer. As control of the arc and metal transfer requires careful setting of pulse and background parameters, a more sophisticated power source is required. Synergic-pulsed MIG power sources, which are advanced transistor-controlled power sources, are preprogrammed so that the correct pulse parameters are delivered automatically as the welder varies wire feed speed.

Welding current and arc voltage ranges for selected wire diameters operating with dip and spray metal transfer:

Dip transfer Spray transfer Wire diameter (mm) Current (A) Voltage (V) Current (A) Voltage (V)

0.6 30 - 80 15 - 18

0.8 45 - 180 16 - 21 150 - 250 25 - 33 1.0 70 - 180 17 - 22 230 - 300 26 - 35

1.2 100 - 200 17 - 22 250 - 400 27 - 35 1.6 120 - 200 18 - 22 250 - 500 30 - 40

Wire feed system

The performance of the wire feed system can be crucial to the stability and reproducibility of MIG welding. As the system must be capable of feeding the wire smoothly, attention should be paid to the feed rolls and liners. There are three types of feeding systems:

• Pinch rolls • Push-pull • Spool on gun

The conventional wire feeding system normally has a set of rolls where one is grooved and the other has a flat surface. Roll pressure must not be too high otherwise the wire will deform and cause poor current pick up in the contact tip. With copper coated wires, too high a roll pressure or use of knurled rolls increases the risk of flaking of the coating (resulting in copper build up in the contact tip). For feeding soft wires such as aluminium dual-drive systems should be used to avoid deforming the soft wire.

Small diameter aluminium wires, 1mm and smaller, are more reliably fed using a push-pull system. Here, a second set of rolls is located in the welding gun - this greatly assists in drawing the wire through the conduit. The disadvantage of this system is increased size of gun. Small wires can also be fed using a small spool mounted directly on the gun. The disadvantages with this are increased size, awkwardness of the gun, and higher wire cost.

Conduit

The conduit can measure up to 5m in length, and to facilitate feeding, should be kept as short and straight as possible. (For longer lengths of conduit, an intermediate push-pull system can be inserted). It has an internal liner made either of spirally wound steel for hard wires (steel, stainless steel, titanium, nickel) or PTFE for soft wires (aluminium, copper).

Gun

In addition to directing the wire to the joint, the welding gun fulfils two important functions - it transfers the welding current to the wire and provides the gas for shielding the arc and weldpool.

There are two types of welding guns: 'air' cooled and water-cooled. The 'air' cooled guns rely on the shielding gas passing through the body to cool the nozzle and have a limited current-carrying capacity. These are suited to light duty work. Although 'air' cooled guns are available with current ratings up to 500A, water cooled guns are preferred for high current levels, especially at high duty cycles.

Welding current is transferred to the wire through the contact tip whose bore is slightly greater than the wire diameter. The contact tip bore diameter for a 1.2mm diameter wire is between 1.4 and 1.5mm. As too large a bore diameter affects current pick up, tips must be inspected regularly and changed as soon as excessive wear is noted. Copper alloy (chromium and zirconium additions) contact tips, harder than pure copper, have a longer life, especially when using spray and pulsed modes.

Gas flow rate is set according to nozzle diameter and gun to workpiece distance, but is typically between 10 and 30 l/min. The nozzle must be cleaned regularly to prevent excessive spatter build-up, which creates porosity. Anti-spatter spray can be particularly effective in automatic and robotic welding to limit the amount of spatter adhering to the nozzle.

Protective equipment

A darker glass than that used for MMA welding at the same current level should be used in hand or head shields.

Recommended shade number of filter for MIG/MAG welding:

Welding current A Shade number MIG Heavy metal MIG Light metal MAG

10 Under 100 Under 100 Under 8011 1001 - 175 100 - 175 80 - 125

12 175 - 300 175 - 250 125 - 17513 300 - 500 250 - 350 175 - 300

14 Over 500 350 - 500 300 - 50015 Over 500 Over 450

Equipment for Submerged-arc Welding The submerged-arc welding (SAW) process is similar to MIG where the arc is formed between a continuously fed wire electrode and the workpiece, and the weld is formed by the arc melting the workpiece and the wire. However, in SAW a shielding gas is not required as the layer of flux generates the gases and slag to protect the weld pool and hot weld metal from contamination. Flux plays an additional role in adding alloying elements to the weld pool.

Essential equipment

Essential equipment components for SAW are:

• Power source • Wire gun • Flux handling • Protective equipment

As SAW is a high current welding process, the equipment is designed to produce high deposition rates.

Power source

SAW can be operated using either a DC or an AC power source. DC is supplied by a transformer-rectifier and a transformer supplies AC. Current for a single wire ranges

from as low as 200A (1.6mm diameter wire) to as high as 1000A (6.0mm diameter wire). In practice, most welding is carried out on thick plate where a single wire (4.0mm diameter) is normally used over a more limited range of 600 to 900A, with a twin wire system operating between 800 and 1200A.

In DC operation, the electrode is normally connected to the positive terminal. Electrode negative (DCEN) polarity can be used to increase deposition rate but depth of penetration is reduced by between 20 and 25%. For this reason, DCEN is used for surfacing applications where parent metal dilution is important. The DC power source has a 'constant voltage' output characteristic, which produces a self-regulating arc. For a given diameter of wire, welding current is controlled by wire feed speed and arc length is determined by voltage setting.

AC power sources usually have a constant-current output characteristic and are therefore not self-regulating. The arc with this type of power source is controlled by sensing the arc voltage and using the signal to control wire feed speed. In practice, for a given welding current level, arc length is determined by wire burnoff rate, i.e. the balance between the welding current setting and wire feed speed, which is under feedback control.

Square wave AC square wave power sources have a constant voltage output current characteristic. Advantages are easier arc ignition and constant wire feed speed control.

Welding gun

SAW can be carried out using both manual and mechanised techniques. Mechanised welding, which can exploit the potential for extremely high deposition rates, accounts for the majority of applications.

Manual welding

For manual welding, the welding gun is similar to a MIG gun, with the flux, which is fed concentrically around the electrode, replacing the shielding gas. Flux is fed by air pressure through the handle of the gun or from a small hopper mounted on the gun. The equipment is relatively portable and, as the operator guides the gun along the joint, little manipulative skill is required. However, because the operator has limited control over the welding operation (apart from adjusting travel speed to maintain the bead profile) it is best used for short runs and simple filling operations.

Mechanised welding - single wire

As SAW is often used for welding large components, the gun, wire feeder and flux delivery feed can be mounted on a rail, tractor or boom manipulator. Single wire welding is mostly practised using DCEP even though AC will produce a higher deposition rate for the same welding current. AC is used to overcome problems with arc blow, caused by residual magnetism in the workpiece, jigging or welding machine.

Wire stickout, or electrode extension - the distance the wire protrudes from the end of the contact tip - is an important control parameter in SAW. As the current flowing between the contact tip and the arc will preheat the wire, wire burnoff rate will increase with increase in wire stickout. For example, the deposition rate for a 4mm diameter wire at a welding current of 700A can be increased from approximately 9 kg/hr at the normal 32mm stickout, to 14 kg/hr at a stickout length of 178mm. In practice, because of the reduction in penetration and greater risk of arc wander, a long stickout is normally only used in cladding and surfacing applications where there is greater emphasis on deposition rate and control of penetration, rather than accurate positioning of the wire.

For most applications, electrode stickout is set so that the contact tube is slightly proud of the flux layer. The depth of flux is normally just sufficient to cover the arc whose light can be seen through the flux.

Recommended and maximum stickout lengths:

Wire stickout Wire diameter mm Current range ANormal mm Maximum mm

0.8 100 to 200 12 -

1.2 150 to 300 20 -

1.6 200 to 500 20 -

2.0 250 to 600 25 63 3.2 350 to 800 30 76

4.0 400 to 900 32 128 4.75 450 to 1000 35 165

Mechanised welding - twin wire

Tandem arc connections SAW can be operated with more than one wire. Although up to five wires are used for high deposition rates, e.g. in pipe mills, the most common multi-wire systems have two wires in a tandem arrangement. The leading wire is run on DCEP to produce deep penetration. The trailing wire is operated on AC, which spreads the weld pool, which is ideal for filling the joint. AC also minimises: interaction between the arcs, and the risk of lack of fusion defects and porosity through the deflection of the arcs (arc blow). The wires are normally spaced 20mm apart so that the second wire feeds into the rear of the weld pool.

Gun angle

In manual welding, the gun is operated with a trailing angle, i.e. with the gun at an angle of 45 degrees (backwards) from the vertical. In single wire mechanised welding operations, the gun is perpendicular to the workpiece. However, in twin wire operations the leading gun is normal to the workpiece, with the trailing gun angled slightly forwards between an angle of 60 and 80 degrees. This reduces disturbance of the weld pool and produces a smooth weld bead profile.

Flux handling

Flux should be stored in unopened packages under dry conditions. Open packages should be stored in a humidity-controlled store. While flux from a newly opened package is ready for immediate use, flux, which has been opened and held in a store, should first be dried according to manufacturer's instructions. In small welding systems, flux is usually held in a small hopper above the welding gun. It is fed automatically (by gravity or mechanised feed) ahead of the arc. In larger installations the flux is stored in large hoppers and is fed with compressed air. Unused flux is collected using a vacuum hose and returned to the hopper.

Note: Care must be taken in recycling unused flux, particularly regarding the removal of slag and metal dust particles. The presence of slag will change the composition of the flux, which, together with the wire, determines the composition of the weld metal. The presence of fine particles can cause blockages in the feeding system.

Protective equipment

Unlike other arc welding processes, SAW is a clean process, which produces minimum fume and spatter when welding steels. (Some noxious emissions can be produced when welding special materials.) For normal applications, general workshop extraction should be adequate.

Protective equipment such as a head shield and a leather apron are not necessary. Normal protective equipment (goggles, heavy gloves and protective shoes) are required for ancillary operations such as slag removal by chipping or grinding. Special precautions should be taken when handling flux - a dust respirator and gloves are needed when loading the storage hoppers.

Equipment for TIG Welding Job Knowledge for Welders No. 6 describes the TIG welding process. Using an inert gas shield instead of a slag to protect the weldpool, this technology is a highly attractive alternative to gas and manual metal arc welding and has played a major role in the acceptance of high quality welding in critical applications.

Essential equipment

In TIG, the arc is formed between the end of a small diameter tungsten electrode and the workpiece. The main equipment components are:

• Power source • Torch • Backing system • Protective equipment

Power source

The power source for TIG welding can be either DC or AC but in both the output is termed a drooping, or constant current, characteristic; the arc voltage / welding current relationship delivers a constant current for a given power source setting. If the arc voltage is slightly increased or decreased, there will be very little change in welding current. In manual welding, it can accommodate the welder's natural variations in arc length and, in the event of the electrode touching the work, an excessively high current will not be drawn which could fuse the electrode to the workpiece.

The arc is usually started by HF (High Frequency) sparks which ionise the gap between the electrode and the workpiece. HF generates airborne and line transmitted interference, so care must be taken to avoid interference with control systems and instruments near welding equipment. When welding is carried out in sensitive areas, a non-HF technique, touch starting or 'lift arc', can be used. The electrode can be short circuited to the workpiece, but the current will only flow

when the electrode is lifted off the surface. There is, therefore, little risk of the electrode fusing to the workpiece surface and forming tungsten inclusions in the weld metal. For high quality applications, using HF is preferred.

DC power source

DC power produces a concentrated arc with most of the heat in the workpiece, so this power source is generally used for welding. However, the arc with its cathode roots on the electrode (DC electrode negative polarity), results in little cleaning of the workpiece surface. Care must be taken to clean the surface prior to welding and to ensure that there is an efficient gas shield.

Transistor and inverter power sources are being used increasingly for TIG welding. The advantages are:

• The smaller size makes them easily transported • Arc ignition is easier • Special operating features, e.g. current pulsing, are readily included • The output can be pre-programmed for mechanised operations

The greater stability of these power sources allows very low currents to be used particularly for micro-TIG welding and largely replaced the plasma process for micro-welding operations.

AC power source

For materials such as aluminium, which has a tenacious oxide film on the surface, AC power must be employed. By switching between positive and negative polarity, the periods of electrode positive will remove the oxide and clean the surface. The figure shows current and voltage waveforms for (sine wave) AC TIG welding.

Disadvantages of conventional, sine wave AC compared with DC are:

• The arc is more diffuse • HF is required to reignite the arc at each current reversal • Excessive heating of the electrode makes it impossible to maintain a tapered

point and the end becomes balled

Square wave AC, or switched DC, power sources are particularly attractive for welding aluminium. By switching between polarities, arc reignition is made easier so that the HF can be reduced or eliminated. The ability to imbalance the waveform to vary the proportion

of positive to negative polarity is important by determining the relative amount of heat generated in the workpiece and the electrode.

To weld the root run, the power source is operated with the greater amount of positive polarity to put the maximum heat into the workpiece. For filler runs a greater proportion of negative polarity should be used to minimise heating of the electrode. By using 90% negative polarity, it is possible to maintain a pointed electrode. A balanced position (50% electrode positive and negative polarities) is preferable for welding heavily oxidised aluminium.

Torch

There is a wide range of torch designs for welding, according to the application. Designs, which have the on/off switch and current control in the handle, are often preferred to foot controls. Specialised torches are available for mechanised applications, e.g. orbital and bore welding of pipes.

Electrode

For DC current, the electrode is tungsten with between 2 and 5% thoria to aid arc initiation. The electrode tip is ground to an angle of 600 to 900 for manual welding, irrespective of the electrode diameter. For mechanised applications as the tip angle determines the shape of the arc and influences the penetration profile of the weld pool, attention must be paid to consistency in grinding the tip and checking its condition between welds.

For AC current, the electrode is either pure tungsten or tungsten with a small amount (up to 0.5%) of zirconia to aid arc reignition and to reduce electrode erosion. The tip normally assumes a spherical profile due to the heat generated in the electrode during the electrode positive half cycle.

Gas shielding

A gas lens should be fitted within the torch nozzle, to ensure laminar gas flow. This will improve gas protection for sensitive welding operations like welding vertical, corner and edge joints and on curved surfaces.

Backing system

When welding high integrity components, a shielding gas is used to protect the underside of the weld pool and weld bead from oxidation. To reduce the amount of gas consumed, a localised gas shroud for sheet, dams or plugs for tubular components is used. As little as 5% air can result in a poor weld bead profile and may reduce corrosion resistance in materials like stainless steel. With gas backing systems in pipe welding, pre-weld purge time depends on the diameter and length of the pipe. The flow rate/purge time is set to ensure at least five volume changes before welding.

Stick on tapes and ceramic backing bars are also used to protect and support the weld bead. In manual stainless steel welding, a flux-cored wire instead of a solid wire can

be used in the root run. This protects the underbead from oxidation without the need for gas backing.

Inserts

A pre-placed insert can be used to improve the uniformity of the root penetration. Its main use is to prevent suck-back in an autogenous weld, especially in the overhead position. The use of an insert does not make welding any easier and skill is still required to avoid problems of incomplete root fusion and uneven root penetration.

Protective equipment

A slightly darker glass should be used in the head or hand shield than that used for MMA welding.

Recommended shade number of filter for TIG welding:

Shade number Welding current A9 Less than 20

10 20 to 40 11 40 to 100

12 100 to 175 13 175 to 250

14 250 to 400

Equipment for Plasma Welding Plasma welding derives its unique operating characteristics from the torch design. As in TIG welding, the arc is formed between the end of a small diameter tungsten electrode and the workpiece. However, in the plasma torch, the electrode is positioned behind a fine bore copper nozzle. By forcing the arc to pass through the nozzle, the characteristic columnar jet, or plasma, is formed.

As described in Job Knowledge for Welders, No 7, three different operating modes can be produced by the choice of the nozzle bore diameter, current level and plasma gas flow rate:

• Microplasma (0.1 to 15A) is equivalent to microTIG but the columnar arc allows the welder to operate with a much longer arc length. The arc is stable at low welding current levels producing a 'pencil-like' beam, which is suitable for welding very thin section material.

• Medium current plasma (15 to 100A) similar to conventional TIG is also used for precision welding operations and when a high level of weld quality is demanded.

• Keyhole plasma (over 100A) produced by increasing the current level and the plasma gas flow. It generates very powerful arc plasma, similar to a laser beam. During welding, the plasma arc slices through the metal producing a keyhole, with the molten weld pool flowing around the keyhole to form the weld. Deep penetration and high welding speeds can be achieved with this operating mode.

As the plasma arc is generated by the special torch arrangement and system controller, the equipment can be obtained as an add-on unit to conventional TIG equipment to provide additional pilot arc and separate plasma and shielding gases. Alternatively, purpose-built plasma equipment is available. Despite similarities in plasma and TIG equipment, there are several important differences in the following components:

• Power source • Torch • Backing system • Protective equipment

Power source

The power source for plasma welding is almost exclusively DC and, as in TIG, the drooping, or constant current, output characteristic will deliver essentially constant current for a given power source setting. The power source is ideal for mechanised welding as it maintains the current setting even when arc length varies and, in manual welding, it can accommodate the natural variations of the welder.

The plasma process is normally operated with electrode negative polarity to minimise heat produced in the electrode (approximately 1/3rd of the heat generated by the arc is produced at the cathode with 2/3rds at the anode). Special torches are available, however, for operating with electrode positive polarity which rely on efficient cooling to prevent melting of the electrode. The positive electrode torch is used for welding aluminium, which requires the cathode to be on the material to remove the oxide film.

AC is not normally used in the plasma process because it is difficult to stabilise the AC arc. Problems in reigniting the arc are associated with constriction by the nozzle, the long electrode to workpiece distance and balling of the electrode caused by the

alternate periods of electrode positive polarity. The square wave AC (inverter, switched DC) power source, with an efficiently cooled torch, makes the use of the AC plasma process easier; rapid current switching promotes arc reignition and, by operating with very short periods of electrode positive polarity, electrode heating is reduced so a pointed electrode can be maintained.

The plasma system has a unique arc starting system in which HF is only used to ignite a pilot arc held within the body of the torch. The pilot arc formed between the electrode and copper nozzle is automatically transferred to the workpiece when it is required for welding. This starting system is very reliable and eliminates the risk of electrical interference through HF.

Torch

The torch for the plasma process is considerably more complex than the TIG torch and attention must be paid, not only to initial set up, but also to inspection and maintenance during production.

Nozzle

In the conventional torch arrangement, the electrode is positioned behind the water-cooled copper nozzle. As the power of the plasma arc is determined by the degree of nozzle constriction, consideration must be given to the choice of bore diameter in relation to the current level and plasma gas flow rate. For�soft� plasma, normally used for micro and medium current operating modes, a relatively large diameter bore is recommended to minimise nozzle erosion.

In high current keyhole plasma mode, the nozzle bore diameter, plasma gas flow rate and current level are selected to produce a highly constricted arc, which has sufficient power to cut through the material. The plasma gas flow rate is crucial in generating the deeply penetrating plasma arc and in preventing nozzle erosion; too low a gas flow rate for the bore diameter and current level will result in double arcing in the torch and the nozzle melting.

The suggested starting point for setting the plasma gas flow rate and the current level for a range of the bore diameters and the various operating modes is given.

Electrode

The electrode is tungsten with an addition of between 2 and 5% thoria to aid arc initiation. Normally, the electrode tip is ground to an angle of 15 degrees for microplasma welding. The tip angle increases with current level and for high current, keyhole plasma welding, an angle of 60 degrees to 90 degrees is recommended. For high current levels, the tip is also blunted to approximately 1mm diameter. The tip angle is not usually critical for manual welding. However, for mechanised applications, the condition of the tip and the nozzle will determine the shape of the arc and penetration profile of the weld pool penetration, so particular attention must be paid to grinding the tip. It is also necessary to check periodically the condition of the tip and nozzle and, for critical components, it is recommended the torch condition is checked between welds.

Electrode set-back

To ensure consistency, it is important to maintain a constant electrode position behind the nozzle; the torch manufacturer provides guidance on electrode setback and a special tool. The maximum current rating of each nozzle has been established for the maximum electrode setback position and the maximum plasma gas flow rate. Lower plasma gas flow rates can be used to soften the plasma arc with the maximum current rating of the nozzle providing electrode setback distance is reduced.

Plasma and shielding gas

The usual gas combination is argon for the plasma gas and argon-2 to 8% H2 for the shielding gas. Irrespective of the material being welded, using argon for the plasma gas produces the lowest rate of electrode and nozzle erosion. Argon - H2 gas mixture for shielding produces a slightly reducing atmosphere and cleaner welds. Helium gives a hotter arc; however, its use for the plasma gas reduces the current carrying capacity of the nozzle and makes formation of the keyhole more difficult. Helium - argon mixtures, e.g. 75% helium - 25% argon, are used as the shielding gas for materials such as copper.

Plasma gas flow rate must be set accurately as it controls the penetration of the weld pool but the shielding gas flow rate is not critical.

Backing system

The normal TIG range of backing bar designs or shielding gas techniques can be employed when using micro and medium current techniques. When applying the keyhole mode a grooved backing bar must be used, with or without gas shielding or total shielding of the underside of the joint. Because the efflux plasma normally extends about 10mm below the back face of the joint, the groove must be deep enough to avoid disturbance of the arc jet; if the efflux plasma hits the backing bar, arc instability will disturb the weld pool, causing porosity.

Protective equipment

Protective equipment for plasma welding is as described for TIG in Job Knowledge for Welders No 17. Regarding protection from arc light, a similar Shade number to TIG at the same welding current level should be used in head or hand shield. The

glass will be slightly darker than that used for MMA welding at the same current level.

Recommended shade number of filter for plasma welding:

Welding Current, A Shade Number Micro Plasma Plasma

5 0.5 to 1

6 1 to 2.5

7 2.5 to 5

8 5 to 10

9 10 to 15

10 15 to 30

11 30 to 60 Less than 15012 60 to 125 150 to 250

13 125 to 225 Above 250 14 225 to 450

See BS 639:1989 for further information on shade numbers.

The Manual Metal Arc process

Manual metal arc welding was first invented in Russia in 1888. It involved a bare metal rod with no flux coating to give a protective gas shield. The development of coated electrodes did not occur until the early 1900s when the Kjellberg process was invented in Sweden and the Quasi-arc method was introduced in the UK. It is worth noting that coated electrodes were slow to be adopted because of their high cost. However, it was inevitable that as the demand for sound welds grew, manual metal arc became synonymous with coated electrodes. When an arc is struck between the metal rod (electrode) and the workpiece, both the rod and workpiece surface melt to form a weld pool. Simultaneous melting of the flux coating on the rod will form gas and slag which protects the weld pool from the surrounding atmosphere. The slag will solidify and cool and must be chipped off the weld bead once the weld run is complete (or before the next weld pass is deposited).

The process allows only short lengths of weld to be produced before a new electrode needs to be inserted in the holder. Weld penetration is low and the quality of the weld deposit is highly dependent on the skill of the welder.

Types of flux/electrodes

Arc stability, depth of penetration, metal deposition rate and positional capability are greatly influenced by the chemical composition of the flux coating on the electrode. Electrodes can be divided into three main groups:

• Cellulose • Rutile • Basic

Cellulose electrodes contain a high proportion of cellulose in the coating and are characterised by a deeply penetrating arc and a rapid burn-off rate giving high welding speeds. Weld deposit can be coarse and with fluid slag, deslagging can be difficult. These electrodes are easy to use in any position and are noted for their use in the 'stovepipe' welding technique.

Features:

• Deep penetration in all positions • Suitability for vertical down welding • Reasonably good mechanical properties • High level of hydrogen generated - risk of cracking in the heat affected zone

(HAZ)

Rutile electrodes contain a high proportion of titanium oxide (rutile) in the coating. Titanium oxide promotes easy arc ignition, smooth arc operation and low spatter.

These electrodes are general-purpose electrodes with good welding properties. They can be used with AC and DC power sources and in all positions. The electrodes are especially suitable for welding fillet joints in the horizontal/vertical (H/V) position.

Features:

• Moderate weld metal mechanical properties • Good bead profile produced through the viscous slag • Positional welding possible with a fluid slag (containing fluoride) • Easily removable slag

Basic electrodes contain a high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid than rutile coatings - this is also fast-freezing which assists welding in the vertical and overhead position. These electrodes are used for welding medium and heavy section fabrications where higher weld quality, good mechanical properties and resistance to cracking (due to high restraint) are required.

Features:

• Low weld metal produces hydrogen • Requires high welding currents/speeds • Poor bead profile (convex and coarse surface profile) • Slag removal difficult

Metal powder electrodes contain an addition of metal powder to the flux coating to increase the maximum permissible welding current level. Thus, for a given electrode size, the metal deposition rate and efficiency (percentage of the metal deposited) are increased compared with an electrode containing no iron powder in the coating. The slag is normally easily removed. Iron powder electrodes are mainly used in the flat and H/V positions to take advantage of the higher deposition rates. Efficiencies as high as 130 to 140% can be achieved for rutile and basic electrodes without marked deterioration of the arcing characteristics but the arc tends to be less forceful which reduces bead penetration.

Power source

Electrodes can be operated with AC and DC power supplies. Not all DC electrodes can be operated on AC power sources, however AC electrodes are normally used on DC.

Welding current

Welding current level is determined by the size of electrode - manufacturers recommend the normal operating range and current. Typical operating ranges for a selection of electrode sizes are illustrated in the table. As a rule of thumb when

selecting a suitable current level, an electrode will require about 40A per millimeter (diameter). Therefore, the preferred current level for a 4mm diameter electrode would be 160A, but the acceptable operating range is 140 to 180A.

What's new

Transistor (inverter) technology is now enabling very small and comparatively low weight power sources to be produced. These power sources are finding increasing use for site welding where they can be readily transported from job to job. As they are electronically controlled, add-on units are available for TIG and MIG welding which increase the flexibility. Electrodes are now available in hermetically sealed containers. These vacuum packs obviate the need for baking the electrodes immediately prior to use. However, if a container has been opened or damaged, it is essential that the electrodes be redried according to the manufacturer's instructions.

The oxyacetylene process Process features Oxyacetylene welding, commonly referred to as gas welding, is a process, which relies on combustion of oxygen and acetylene. When mixed together in correct proportions within a hand-held torch or blowpipe, a relatively hot flame is produced with a temperature of about 3,200°C. The chemical action of the oxyacetylene flame can be adjusted by changing the ratio of the volume of oxygen to acetylene.

Three distinct flame settings are used, neutral, oxidising and carburising.

Neutral flame

Oxidising flame

Carburising flame

Welding is generally carried out using the neutral flame setting, which has equal quantities of oxygen and acetylene. The oxidising flame is obtained by increasing just the oxygen flow rate while the carburising flame is achieved by increasing acetylene flow in relation to oxygen flow. Because steel melts at a temperature above 1,500°C, the mixture of oxygen and acetylene is used, as it is the only gas combination with enough heat to weld steel. However, other gases such as propane, hydrogen and coal gas can be used for joining lower melting point non-ferrous metals, and for brazing and silver soldering.

Equipment Oxyacetylene equipment is portable and easy to use. It comprises oxygen and acetylene gases stored under pressure in steel cylinders. The cylinders are fitted with regulators and flexible hoses which lead to the blowpipe. Specially designed safety devices such as flame traps are fitted between the hoses and the cylinder regulators. The flame trap prevents flames generated by a 'flashback' from reaching the cylinders; principal causes of flashbacks are the failure to purge the hoses and overheating of the blowpipe nozzle.

When welding, the operator must wear protective clothing and tinted coloured goggles. As the flame is less intense than an arc and very little UV is emitted, general-

purpose tinted goggles provide sufficient protection.

Operating characteristics The action of the oxyacetylene flame on the surface of the material to be welded can be adjusted to produce a soft, harsh or violent reaction by varying the gas flows. There are of course practical limits as to the type of flame, which can

be used for welding. A harsh forceful flame will cause the molten weld pool to be blown away, while too soft a flame will not be stable near the point of application. The blowpipe is therefore designed to accommodate different sizes of 'swan neck copper nozzle which allows the correct intensity of flame to be used. The relationship between material thickness, blowpipe nozzle size and welding speed, is shown in the chart. When carrying out fusion welding the addition of filler metal in the form of a rod can be made when required. The principal techniques employed in oxyacetylene

welding are leftward, rightward and all positional rightward. The former is used almost exclusively and is ideally suited for welding butt, fillet and lap joints in sheet thicknesses up to approximately 5mm. The rightward technique finds application on plate thicknesses above 5mm for welding in the flat and horizontal-vertical position. The all-positional rightward method is a modification of the rightward technique and is ideally suited for welding steel plate and in particular pipework where positional welding, (vertical and overhead) has to be carried out. The rightward and all- positional rightward techniques enable the welder to obtain a uniform penetration bead with added control over the molten weld pool and weld metal. Moreover, the welder has a clear view of the weld pool and can work in complete freedom of

movement. These techniques are very highly skilled and are less frequently used than the conventional leftward technique.

Solid wire MIG welding Metal inert gas (MIG) welding was first

patented in the USA in 1949 for welding aluminium. The arc and weld pool formed using a bare wire electrode was protected by helium gas, readily available at that time. From about 1952 the process became popular in the UK for welding aluminium-using argon as the shielding gas, and for carbon steels using CO2. CO2 and argon-CO2 mixtures are known as metal active gas (MAG) processes. MIG is an attractive alternative to MMA, offering high deposition rates and high productivity.

Process characteristics

MIG is similar to MMA in that heat for welding is produced by forming an arc between a metal electrode and the workpiece; the electrode melts to form the weld bead. The main difference is that the metal electrode is a small diameter wire fed from a spool. As the wire is continuously fed, the process is often referred to as semi-automatic welding.

Metal transfer mode

The manner, or mode, in which the metal transfers from the electrode to the weld pool largely, determines the operating features of the process. There are three principal metal transfer modes:

• Short circuiting • Droplet / spray • Pulsed

Short-circuiting and pulsed metal transfer are used for low current operation while spray metal transfer is only used with high welding currents. In short-circuiting or 'dip' transfer, the wire dipping into the weld pool transfers the molten metal forming

on the tip of the wire. This is achieved by setting a low voltage; for a 1.2mm diameter wire, arc voltage varies from about 17V (100A) to 22V (200A). Care in setting the voltage and the inductance in relation to the wire feed speed is essential to minimise spatter. Inductance is used to control the surge in current, which occurs when the wire

dips into the weld pool.

For droplet or spray transfer, a much higher voltage is necessary to ensure that the wire does not make contact i.e. short-circuit, with the weld pool; for a 1.2mm diameter wire, the arc voltage varies from approximately 27V (250A) to 35V (400A). The molten metal at the tip of the wire transfers to the weld pool in the form of a spray of small droplets (about the diameter of the wire and smaller). However, there is a minimum current level, threshold, below which droplets are not

forcibly projected across the arc. If an open arc technique is attempted much below the threshold current level, the low arc forces would be insufficient to prevent large droplets forming at the tip of the wire. These droplets would transfer erratically across the arc under normal gravitational forces. The pulsed mode was developed as a means of stabilising the open arc at low current levels i.e. below the threshold level, to avoid short-circuiting and spatter. Spray type metal transfer is achieved by applying pulses of current, each pulse having sufficient force to detach a droplet. Synergic-pulsed MIG refers to a special type of controller, which enables the power source to be tuned (pulse parameters) for the wire composition and diameter, and the pulse frequency to be set according to the wire feed speed.

Shielding gas

In addition to general shielding of the arc and the weld pool, the shielding gas performs a number of important functions:

• Forms the arc plasma • Stabilises the arc roots on the material surface • Ensures smooth transfer of molten droplets from the wire to the weld pool

Thus, the shielding gas will have a substantial effect on the stability of the arc and metal transfer and the behaviour of the weld pool, in particular, its penetration. General-purpose shielding gases for MIG welding are mixtures of argon, oxygen and C02, and special gas mixtures may contain helium. The gases, which are normally used for the various materials, are:

• Steels o CO2 o Argon +2 to 5% oxygen o Argon +5 to 25% CO2

• Non-ferrous o Argon

o Argon / helium Argon based gases, compared with CO2, is generally more tolerant to parameter settings and generates lower spatter levels with the dip transfer mode. However, there is a greater risk of lack of fusion defects because these gases are colder. As CO2 cannot be used in the open arc (pulsed or spray transfer) modes due to high back-plasma forces, argon based gases containing oxygen or CO2 are normally employed.

Applications

MIG is widely used in most industry sectors and accounts for almost 50% of all weld metal deposited. Compared to MMA, MIG has the advantage in terms of flexibility, deposition rates and suitability for mechanisation. However, it should be noted that while MIG is ideal for 'squirting' metal, a high degree of manipulative skill is demanded of the welder.

Submerged-arc Welding

The first patent on the submerged-arc welding (SAW) process was taken out in 1935 and covered an electric arc beneath a bed of granulated flux. Developed by the E O Paton Electric Welding Institute, Russia, during the Second World War, Saw�s most famous application was on the T34 tank.

Process features

Similar to MIG welding, SAW involves formation of an arc between a continuously fed bare wire electrode and the workpiece. The process uses a flux to generate protective gases and slag, and to add alloying elements to the weld pool. A shielding gas is not required. Prior to welding, a thin layer of flux powder is placed on the

workpiece surface. The arc moves along the joint line and as it does so, excess flux is recycled via a hopper. Remaining fused slag layers can be easily removed after welding. As the arc is completely covered by the flux layer, heat loss is extremely low. This produces a thermal efficiency as high as 60% (compared with 25% for manual metal arc). There is no visible arc light, welding is spatter-free and there is no need for fume extraction.

Operating characteristics

SAW is usually operated as a fully mechanised or automatic process, but it can be semi-automatic. Welding parameters: current, arc voltage and travel speed all affect bead shape, depth of penetration and chemical composition of the deposited weld metal. Because the operator cannot see the weld pool, greater reliance must be placed on parameter settings.

Process variants

According to material thickness, joint type and size of component, varying the following can increase deposition rate and improve bead shape.

Wire

SAW is normally operated with a single wire on either AC or DC current. Common variants are:

• Twin wire • Triple wire • Single wire with hot wire addition • Metal powdered flux addition

All contribute to improved productivity through a marked increase in weld metal deposition rates and/or travel speeds.

Flux

Fluxes used in SAW are granular fusible minerals containing oxides of manganese, silicon, titanium, aluminium, calcium, zirconium, magnesium and other compounds such as calcium fluoride. The flux is specially formulated to be compatible with a given electrode wire type so that the combination of flux and wire yields desired

mechanical properties. All fluxes react with the weld pool to produce the weld metal chemical composition and mechanical properties. It is common practice to refer to fluxes as 'active' if they add manganese and silicon to the weld, the arc voltage and the welding current level influence the amount of manganese and silicon added. The main types of flux for SAW are:

• Bonded fluxes - produced by drying the ingredients, then bonding them with a low melting point compound such as a sodium silicate. Most bonded fluxes contain metallic deoxidisers, which help to prevent weld porosity. These fluxes are effective over rust and mill scale.

• Fused fluxes - produced by mixing the ingredients, then melting them in an electric furnace to form a chemical homogeneous product, cooled and ground to the required particle size. Smooth stable arcs, with welding currents up to 2000A and consistent weld metal properties, are the main attraction of these fluxes.

Applications

SAW is ideally suited for longitudinal and circumferential butt and fillet welds. However, because of high fluidity of the weld pool, molten slag and loose flux layer, welding is generally carried out on butt joints in the flat position and fillet joints in both the flat and horizontal-vertical positions. For circumferential joints, the workpiece is rotated under a fixed welding head with welding taking place in the flat position. Depending on material thickness, either single-pass, two-pass or multipass weld

procedures can be carried out. There is virtually no restriction on the material thickness, provided a suitable joint preparation is adopted. Most commonly welded materials are carbon-manganese steels, low alloy steels and stainless steels, although the process is capable of welding some non-ferrous materials with judicious choice of electrode filler wire and flux combinations.

TIG Welding Tungsten inert gas (TIG) welding became an overnight success in the 1940s for joining magnesium and aluminium. Using an inert gas shield instead of a slag to protect the weld pool, the process was a highly attractive replacement for gas and manual metal are welding. TIG has played a major role in the acceptance of aluminium for high quality welding and structural applications.

Process characteristics

In the TIG process the arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding. Because the electrode is not consumed during welding, the welder does not have to balance the heat input from the arc as the metal is deposited from the melting electrode. When filler metal is required, it must be added separately to the weld pool.

Power source

TIG must be operated with a drooping, constant current power source - either DC or AC. A constant current power source is essential to avoid excessively high currents being drawn when the electrode is short-circuited on to the workpiece surface. This could happen either deliberately during arc starting or inadvertently during welding. If, as in MIG welding, a flat characteristic power source is used, any contact with the workpiece surface would damage the electrode tip or fuse the electrode to the workpiece surface. In DC, because arc heat is distributed approximately one-third at the cathode (negative) and two-thirds at the anode (positive), the electrode is always negative polarity to prevent overheating and melting. However, the alternative power source connection of DC electrode positive polarity has the advantage in that when the cathode is on the workpiece, the surface is cleaned of oxide contamination. For this reason, AC is used when welding materials with a tenacious surface oxide film, such as aluminium.

Arc starting

The welding arc can be started by scratching the surface, forming a short-circuit. It is only when the short-circuit is broken that the main welding current will flow. However, there is a risk that the electrode may stick to the surface and cause a tungsten inclusion in the weld. This risk can be minimised using the 'lift arc' technique where the short-circuit is formed at a very low current level. The most common way of starting the TIG arc is to use HF (High Frequency). HF consists of high voltage sparks of several thousand volts, which last for a few microseconds. The HF sparks will cause the electrode - workpiece gap to break down or ionise. Once an electron/ion cloud is formed, current can flow from the power source.

Note: As HF generates abnormally high electromagnetic emission (EM), welders should be aware that its use can cause interference especially in electronic equipment. As EM emission can be airborne, like radio waves, or transmitted along power cables, care must be taken to avoid interference with control systems and instruments in the vicinity of welding.

HF is also important in stabilising the AC arc; in AC, electrode polarity is reversed at a frequency of about 50 times per second, causing the arc to be extinguished at each polarity change. To ensure that the arc is reignited at each reversal of polarity, HF sparks are generated across the electrode/workpiece gap to coincide with the beginning of each half-cycle.

Electrodes

Electrodes for DC welding are normally pure tungsten with 1 to 4% thoria to improve arc ignition. Alternative additives are lanthanum oxide and cerium oxide, which are claimed to give superior performance (arc starting and lower electrode consumption). It is important to select the correct electrode diameter and tip angle for the level of welding current. As a rule, the lower the current the smaller the electrode diameter and tip angle. In AC welding, as the electrode will be operating at a much higher temperature, tungsten with a zirconia addition is used to reduce electrode erosion. It should be noted that because of the large amount of heat generated at the electrode, it is difficult to maintain a pointed tip and the end of the electrode assumes a spherical or 'ball' profile.

Shielding gas

Shielding gas is selected according to the material being welded. The following guidelines may help:

• Argon - the most commonly used shielding gas, which can be used for welding a wide range of materials including steels, stainless steel, aluminium and titanium.

• Argon + 2 to 5% H2 - the addition of hydrogen to argon will make the gas slightly reducing, assisting the production of cleaner-looking welds without surface oxidation. As the arc is hotter and more constricted, it permits higher welding speeds. Disadvantages include risk of hydrogen cracking in carbon steels and weld metal porosity in aluminium alloys.

• Helium and helium/argon mixtures - adding helium to argon will raise the temperature of the arc. This promotes higher welding speeds and deeper weld penetration. Disadvantages of using helium or a helium/argon mixture are the high cost of gas and difficulty in starting the arc.

Applications

TIG is applied in all industrial sectors but is especially suitable for high quality welding. In manual welding, the relatively small arc is ideal for thin sheet material or controlled penetration (in the root run of pipe welds). Because deposition rate can be quite low (using a separate filler rod) MMA or MIG may be preferable for thicker material and for fill passes in thick-wall pipe welds.

TIG is also widely applied in mechanised systems either autogenously or with filler wire. However, several 'off the shelf' systems are available for orbital welding of pipes, used in the manufacture of chemical plant or boilers. The systems require no manipulative skill, but the operator must be well trained. Because the welder has less control over arc and weld pool behaviour, careful attention must be paid to edge preparation (machined rather than hand-prepared), joint fit-up and control of welding parameters.

Plasma Welding

Process characteristics

Plasma welding is very similar to TIG as the arc is formed between a pointed tungsten electrode and the workpiece. However, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. Plasma is

then forced through a fine-bore copper nozzle, which constricts the arc. Varying bore diameter and plasma gas flow rate can produce three operating modes:

• Micro plasma: 0.1 to 15A. The micro plasma arc can be operated at very low welding currents. The columnar arc is stable even when arc length is varied up to 20mm.

• Medium current: 15 to 200A. At higher currents, from 15 to 200A, the process characteristics of the plasma arc are similar to the TIG arc, but because the plasma is constricted, the arc is stiffer. Although the plasma gas flow rate can be increased to improve weld pool penetration, there is a risk of air and shielding gas entrainment through excessive turbulence in the gas shield.

• Keyhole plasma: over 100A. By increasing welding current and plasma gas flow, a very powerful plasma beam is created which can achieve full penetration in a material, as in laser or electron beam welding. During welding, the hole progressively cuts through the metal with the molten weld pool flowing behind to form the weld bead under surface tension forces. This process can be used to weld thicker material (up to 10mm of stainless steel) in a single pass.

Power source

The plasma arc is normally operated with a DC, drooping characteristic power source. Because its unique operating features are derived from the special torch arrangement and separate plasma and shielding gas flows, a plasma control console can be added on to a conventional TIG power source. Purpose-built plasma systems are also available. The plasma arc is not readily stabilised with sine wave AC. Arc reignition is difficult when there is a long electrode to workpiece distance and the plasma is constricted, Moreover, excessive heating of the electrode during the positive half-cycle causes balling of the tip which can disturb arc stability.

Special-purpose switched DC power sources are available. By imbalancing the waveform to reduce the duration of electrode positive polarity, the electrode is kept sufficiently cool to maintain a pointed tip and achieve arc stability.

Arc starting

Although the arc is initiated using HF, it is first formed between the electrode and plasma nozzle. This 'pilot' arc is held within the body of the torch until required for welding then it is transferred to the workpiece. The pilot arc system ensures reliable

arc starting and, as the pilot arc is maintained between welds, it obviates the need for HF, which may cause electrical interference.

Electrode

The electrode used for the plasma process is tungsten-2%thoria and the plasma nozzle is copper. The electrode tip diameter is not as critical as for TIG and should be maintained at around 30-60 degrees. The plasma nozzle bore diameter is critical and too small a bore diameter for the current level and plasma gas flow rate will lead to excessive nozzle erosion or even melting. It is prudent to use the largest bore diameter for the operating current level. Note: too large a bore diameter, may give problems with arc stability and maintaining a keyhole.

Plasma and shielding gases

The normal combination of gases is argon for the plasma gas, with argon plus 2 to 5% hydrogen for the shielding gas. Helium can be used for plasma gas but because it is hotter this reduces the current rating of the nozzle. Helium's lower mass can also make the keyhole mode more difficult.

Applications

Micro plasma welding

Micro plasma was traditionally used for welding thin sheets (down to 0.1 mm thickness), and wire and mesh sections. The needle-like stiff arc minimises arc wander and distortion. Although the equivalent TIG arc is more diffuse, the newer transistorised (TIG) power sources can produce a very stable arc at low current levels.

Medium current welding

When used in the melt mode this is an alternative to conventional TIG. The advantages are deeper penetration (from higher plasma gas flow), and greater tolerance to surface contamination including coatings (the electrode is within the body of the torch). The major disadvantage lies in the bulkiness of the torch, making manual welding more difficult. In mechanised welding, greater attention must be paid to maintenance of the torch to ensure consistent performance.

Keyhole welding

This has several advantages, which can be exploited: deep penetration and high welding speeds. Compared with the TIG arc, it can penetrate plate thicknesses up to l0mm, but when welding using a single pass technique, it is more usual to limit the thickness to 6mm. The normal methods are to use the keyhole mode with filler to ensure smooth weld bead profile (with no undercut). For thicknesses up to 15mm, a vee joint preparation is used with a 6mm root face. A two-pass technique is employed and here, the first pass is autogenous with the second pass being made in melt mode with filler wire addition.

As the welding parameters, plasma gas flow rate and filler wire addition (into the keyhole) must be carefully balanced to maintain the keyhole and weld pool stability, this technique is only suitable for mechanised welding. Although it can be used for positional welding, usually with current pulsing, it is normally applied in high speed welding of thicker sheet material (over 3 mm) in the flat position. When pipe welding, the slope-out of current and plasma gas flow must be carefully controlled to close the keyhole without leaving a hole

Ceramics - materials, joining and applications Ceramics are an incredibly diverse family of materials whose members span traditional ceramics (such as pottery and refractories) to the modern day engineering ceramics (such as alumina and silicon nitride) found in electronic devices, aerospace components and cutting tools.

Whilst the most extravagant claims of the 1980s in favour of advanced ceramic materials (such as the all ceramic engine) have largely proved inaccurate, it is true to say that ceramics have established themselves as key engineering materials.

When used in conjunction with other materials, usually metals, they provide added functionality to components thereby improving application performance, once the appropriate joint design and technology have been identified.

Ceramic materials Ceramics exhibit very strong ionic and/or covalent bonding (stronger than the metallic bond) and this confers the properties commonly associated with ceramics: high hardness, high compressive strength, low thermal and electrical conductivity and chemical inertness.

This strong bonding also accounts for the less attractive properties of ceramics, such as low ductility and low tensile strength. The wider range of properties, however, is not widely appreciated. For example, whilst ceramics are perceived as electrical and thermal insulators, ceramic oxides (initially based on Y-Ba-Cu-O) are the basis for high temperature superconductivity. Diamond, beryllia and silicon carbide have a higher thermal conductivity than aluminium or copper.

Control of the microstructure can overcome inherent stiffness to allow the production of ceramic springs, and ceramic composites have been produced with a fracture toughness about half that of steel.

The main compositional classes of engineering ceramics are the oxides, nitrides and carbides. The Table gives the general properties of the most used ceramics.

Table 1 Properties of ceramics Ceramic Melting

point (°C)

Density (g/cm3)

Strength (MPa)

Coefficient of thermal expansion (x 10-6/°C)

Thermal conductivity

(W/m.K)

Elastic Modulus

(GPa)

BeO 2530 3.1 246 7.4 210 400 Al2O3 2050 4.0 455 8.0 40 380 ZrO2 2700 5.6 175 10.5 19 140 AlN 1900 3.3 441 4.4 180 320 Si3N4 1900 3.2 210 3.0 17 175 B4C 2350 2.5 350 4.3 25 450 SiC 2700 3.2 140 4.3 50 210 WC 2377 15.8 600 5.2 - 700

Diamond 3000 3.5 1500 0.5 2000 500

Oxides

Aluminium oxide (Al2O3) and zirconia (ZrO2) are the most commonly used engineering grade oxide ceramics, with alumina being the most used ceramic by far in terms of both tonnage and value.

Nitrides

Silicon nitride (Si3N4), and aluminium nitride (AlN) are the main advanced engineering ceramics in this category. There is a wide range of grades and types of these materials, particularly of silicon nitride with each grade having specific properties

Carbides

Silicon carbide (SiC) is widely used for its high thermal conductivity, corrosion resistance and hardness, although as an engineering ceramic its toughness is lower than that of some silicon nitride grades. Boron carbide (B4C) is the third hardest industrial material (after diamond and cubic boron nitride) and is used for components needing very high wear performance.

Ceramic-based composites

Ceramics are used as the reinforcement of composite systems such as GRP (glass reinforced plastics) and metal matrix composites such as alumina-reinforced aluminium (Al/Al2O3). Advanced ceramic materials are also used as the matrix materials in composites. Currently the most widely available materials are based on SiC and carbon.

Joining

There are many possible techniques for joining ceramics to themselves and to dissimilar materials. These technologies range from mechanical fixturing to direct bonding. Fig.1 gives an overview of these methods.

Fig.1. An overview of processes for joining ceramics

The selection of one of these techniques to manufacture a particular component will depend on a number of factors including:

• Desired component function e.g. strength, electrical insulation or wear resistance

• Materials to be joined • Operational temperature • Applied stress • Required level of joint hermeticity • Component design • Cost

Whilst all these considerations must be taken into account, generally the two important factors are the similarity of the materials to be joined and the required temperature capability. Fig. 2 gives the temperature capability of a number of joining media.

Fig.2. Temperature capability of a number of joining media

When joining ceramics to metals it is necessary to create an interface between the materials. In general the interface must accommodate the following:

• The difference in coefficient of thermal expansion (CTE) • Bond type i.e. ionic/covalent for ceramics ranging to the metallic bond • Crystallographic lattice mismatch between the ceramic and metal

Applications Compared to metals and plastics, ceramics are hard, non-combustible and inert. Thus they can be used in high temperature, corrosive and tribological applications. These applications rely on combinations of properties that are unique to industrial ceramics and which include:

• Retention of properties at high temperature • Low coefficient of friction (particularly at high loads and low levels of

lubrication) • Low coefficient of expansion • Corrosion resistance • Thermal insulation • Electrical insulation • Low density

Engineering ceramics are used to fabricate components for applications in many industrial sectors, including ceramic substrates for electronic devices (Fig. 3), turbocharger rotors (Fig. 4), and tappet heads for use in automotive engines. Other examples of where advanced ceramics are used include oil-free bearings in food processing equipment, aerospace turbine blades, nuclear fuel rods, lightweight armour, cutting tools, abrasives, thermal barriers and furnace/kiln furniture.

Fig.3. Ceramic substrates for electronic devices

Fig.4. Ceramic turbocharger rotor assembly made from silicon nitride Courtesy of NGK/NTK Spark Plug Co

Summary When selecting a material for use in a specific component the applicability and suitability of the candidate materials need to be considered in detail. When a ceramic material is being selected the fitness-for-purpose criteria that should be applied include:

• Operational environment - atmosphere, temperature, applied stress, fatigue, exposure time

• Predictable excursions beyond the usual, including mechanical impact or rapid heating/cooling

• Design - ceramic materials are relatively intolerant of abrupt changes in cross-section such as notches, holes and corners

• Joining - the role of the joint, its operational conditions and performance requirements and the joining techniques suitable for manufacture

• Cost - as with all materials selection and component design questions, the cost and availability of the raw materials and all necessary fabrication techniques must be considered in the light of their suitability to provide a component with the required performance profile at a viable cost

Future development is likely to come from improved processing and fabrication techniques that will lower component costs or improve behaviour, an increasing demand for higher performance materials necessitating the use of more ceramics. Whilst it is difficult to predict new materials, improvements in existing ones can be readily foreseen. The most significant area of development is likely to be in the ceramic matrix composites.

Whilst existing composites based on SiC will improve as porosity levels are reduced by improved processing techniques, the development of high temperature oxide-based composites is likely to provide a competitor material system with wider applicability

in the near future. In the future we can expect to see a still greater contribution to industrial growth and technological development from these materials.

Welding techniques for thermoplastics The purpose of this article is to give an overview of the variety of techniques available to industry for the thermal joining of thermoplastics.

The techniques used can be divided into three distinct groups based on the method used to introduce heat to the weld. These are:

• By mechanical movement, • By an external heat source • From electromagnetism

Welding techniques where heat is generated by mechanical movement

Linear vibration

In linear vibration welding the parts to be joined are brought into contact under pressure before being rubbed together in a linear reciprocating motion. The resulting friction melts the material at the interface after which the vibration stops; the parts are then aligned and held together until the weld solidifies.

Most thermoplastic materials can be welded using this technique, which is used extensively in the automotive industry for joining components such as two-part bumpers, fuel tanks, air ducts and inner door panels.

Spin

Fig. 1. Spin welding machine

In spin welding the joint areas are always circular and the motion is rotational. The technique has been exploited for applications as diverse as the manufacture of polyethylene floats, aerosol bottles, transmission shafts and PVC pipes and fittings.

Ultrasonic

Ultrasonic welding involves the use of high frequency mechanical energy to soften or melt the thermoplastic at the joint line. Parts to be joined are held together under pressure and then subjected to ultrasonic vibrations, usually at a frequency of 20 or 40kHz. Ultrasonic welding is a fast process, with weld times typically less than a second, and can be easily automated. It is a popular choice for assembling components in the automotive, medical, electronic and packaging markets.

Welding techniques using an external heat source

Hot plate

Hot plate welding is possibly the simplest plastic joining technique, used for various applications ranging from small automotive fluid reservoir vessels to pipelines in excess of 1000mm in diameter.

The technique involves heating the ends of the parts to be joined against an electrically heated platen until they are sufficiently molten. The heater plate is then removed and the parts pressed together. A cooling cycle follows, allowing the weld to develop strength.

Hot bar and impulse

This technique is mainly used for joining thermoplastic films with a thickness of less than 0.5mm. It works on the principle that if two films are pressed against a heated metal bar, they will soften and allow a joint to be made between them. Weld times are rapid, around two seconds for 100mm film.

The principle of impulse welding is the same. Here the heat comes from a brief burst of electrical energy through a nickel chromium wire triggered as the films are pressed together. This method is used in packaging for the rapid sealing of polyethylene bags.

Hot gas

In hot gas welding of thermoplastics, the parts to be joined, typically sheet sections up to 30mm in thickness, are prepared in a V-butt or T-butt configuration before a stream of hot gas is directed towards the joint area. This causes melting of the joint area and also of a consumable filler rod of the same polymer type as the parts being joined. The weld is formed from the fusing together of the joint with the filler material.

The main advantage of hot gas welding is that the equipment is easily portable. However, the process is slow and weld quality depends greatly on the skill of the

operator. Training and Certification of operators is recommended to achieve high standards.

Extrusion

Fig. 2. Extrusion welding

Extrusion welding is similar to hot gas welding, sharing some of its characteristic advantages and disadvantages. Molten thermoplastic filler material is fed into the joint preparation from the barrel of a mini hand-held extruder based on an electric drill. The molten material emerges from a PTFE shoe shaped to match the profile being welded. At the leading edge of the shoe a stream of hot gas is used to pre heat the substrate prior to the molten material being deposited, ensuring sufficient heat is available to form a weld.

The process is used typically for assembly of large fabrications such as chemical storage vessels, with wall thicknesses up to 50mm.

Welding techniques, which directly use electromagnetism. Fig. 3. Overview of welding processes for thermoplastics, grouped by heating mechanism

Resistive implant

This involves trapping an electrically conducting implant between the two parts to be joined before applying a high electric current to cause resistive heating. As the implant heats, the surrounding thermoplastic material softens and melts. Application of pressure ensures the molten surfaces fuse together to form a weld.

A widely used application of resistive implant welding is the electrofusion technique for joining thermoplastic pipes using specially designed socket couplers containing an integral electrical heating coil.

Induction

Induction is similar to resistive implant welding, as an implant is generally needed at the joint line. However, in this process a work coil connected to a high frequency power supply is placed close to the joint. As high frequency electric current passes through the work coil, a dynamic magnetic field is generated whose flux interacts with the implant. Eddy currents are induced in the implant, heating it and the surrounding joint area.

High frequency (dielectric)

High frequency (dielectric or radio frequency) welding relies on the ability of the plastic being joined to generate heat in a rapidly alternating electric field. Hence the technique is generally restricted to PVC, EVA and polyurethane�s.

During the process, the parts to be joined are subjected to a high frequency electric field applied between two metal bars. The dynamic electric field causes molecular vibration in the plastic. Some of the resulting oscillatory motion is converted into thermal energy, causing the material to heat.

Products manufactured by high frequency welding include stationery wallets, inflatables, tarpaulins and blood bags.

Infrared

During infrared welding the parts to be joined are brought into very close proximity with an electrically heated platen. The technique is similar to hot plate welding although no actual physical contact is made with the heat source. After sufficient time has elapsed the parts become molten and can be forced together to form a weld.

Infrared welding is generally faster than hot plate welding with typical welding times being reduced by around 50%. The fact that heating is achieved without physical contact eliminates the possibility of contamination entering the weld from the surface of the hot plate. The technique is used for joining thermoplastic pipes.

Laser

The laser welding technique uses a focused beam of intense radiation, usually in the infrared area of the electromagnetic spectrum, to melt the plastic in the joint region. The type of laser used and the absorption characteristics of the plastic determine the extent of welding possible.

ClearWeldTM transmission welding, recently patented by TWI, uses a colourless infrared absorbing medium at the joint interface of two transmissive plastics. Thus two optically clear plastics may be laser welded with an almost invisible joint.

Laser welding has the advantage of being a quick, clean, non-contact process, which generates minimum flash and distortion