ESAB TRAINING & EDUCATION

MMA welding

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The History of weldingContent

The history of welding ........................................... 3Covered electrodes ................................................. 5Organic electrodes ................................................... 6Acid electrodes ......................................................... 6Rutile electrodes ....................................................... 7Basic electrodes ....................................................... 7Welding with covered electrodes .............................. 8Some terms associated with weldingwith covered electrodes .......................................... 9

The term “welding” was first used at the beginning of the 20th century, even though the process had been in use for many years. Nowadays, welding comprises a number of methods that are divided into pressure and fusion welding methods, where the history of the pressure method stretches back thousands of years. Fusion welding first became possible in the 19th century, when heat sources which were able to generate sufficiently high temperatures became available.

Pressure welding takes place when the joint sur-faces are pressed together at a temperature under melting point. The heat can be produced by a coal fire, a gas flame or electrically. As the name suggests, fusion welding takes place at higher temperatures which are produced by gas or electrically. In the book of Genesis, Tubal-cain was de-scribed as “an instructer of every artificer (smith) in brass and iron”. Today, very few smiths re-main and we admire their professional skill but are not impressed by either their productivity or quality. The size of the things they can make is naturally limited and the products are restricted to fittings and tools. As industrialisation developed and progressed, the need to produce large, sustainable structures

made of steel grew and, in the mid-19th century, new joining methods began to be developed. The difficulty when it came to joining large work-pieces was the shortage of heat sources that were able to generate sufficiently high temperatures to produce melting temperatures locally in a con-tinuous process. Coal was completely impossi-ble to handle and did not produce sufficient heat, while gases that generated sufficient energy had still not been found. Electricity was tested by Thomson, an Eng-lishman working in the USA. By pressing two plates between copper electrodes and allowing a short-circuit current to flow between the elec-trodes, he produced a welding lens. This was an effective method, but it consumed a great deal of energy and could only be used on thin material.

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coating. The composition of the coating varies sharply depending on the metal to be welded. Many different types of chemicals and minerals are used in electrode production. The covering of a single electrode contains some thirty products. Here are some examples of the contents and their tasks in an electrode.

In 1882, the Russian Bernados patented a meth-od in which an arc, which has been established in an air gap between a carbon electrode and a workpiece, produced the continuous melting of both the joint edges and the consumable, which was supplied from the side. This method failed to attract any interest, as the totally unprotected molten pool became porous and was oxidised by the gases in the air that poured into it. Ber-nados used the method in his workshop where he caulked steam boilers by hammering the mol-ten pool. The term “iron soldering” began to be used. Ten years later, Bernados’ method was devel-oped by his fellow-countryman Slavianoff, who replaced the carbon electrode with a steel rod which functioned as both the electrode and the consumable. The resulting weld displayed the same hopeless characteristics as that produced by the previous method. At the same time, the Frenchmen Piccard and Fouché presented a gas-welding method. The gas was produced by melting carbon and calci-um together in an electric arc furnace to produce calcium carbide. The calcium carbide was then allowed to react with water, thereby producing etyn gas (C2H2=acetylene). When it was com-busted in oxygen, this gas produced a flame with a heat of 3,000°C, which was enough to produce local, continuous heat in large objects. The weld metal displayed excellent characteristics and the joint that was produced offered the desired re-sult. The weld was clean because the acetylene gas that was combusted in oxygen generated the combustion gases CO and CO2. They created a barrier that prevented the gases in the air reach-ing the molten pool. To begin with, the method was a great success, but producing gas locally in water resulted in a fair number of explosions, as the combustion speed was twice the escape velocity. Backfiring in the gas burner was common, especially as flash-back arrestors had not yet been invented, and the whole of the gas plant exploded, causing injuries and damage to people and property. Transporting the gas was impossible and if it was compressed, it disintegrated with increased heat and explod-

ed. Not long afterwards, it was discovered that acetone could absorb 25 times its own volume of acetylene with increasing atmospheric pressure. This meant that, at a pressure of 10 atmospheres, a ten-litre container could hold 2,500 litres of acetylene. Unfortunately, the containers were in-credibly weak and sensitive to explosion should the acetone evaporate from the container. In an air pocket of this kind, the gas poured out, under pressure, disintegrated as heat developed and ex-ploded. This problem was solved by Gustaf Dalén, who eliminated the air pockets by filling the gas containers with porous concrete made of

cement, wood flour and kieselguhr, among other things. The ace-tone is absorbed into the porous mass in the cylinder and the acetylene could be safely pressurised in the acetone. He called his invention the gas accumulator and, in 1902, he founded the

company known as Aktiebolaget Gas Ackumula-tor (AGA). At this time, Oscar Kjellberg from Värmland in Sweden had been repairing boilers for some time in the harbour in Göteborg, where he was using Slavianoff’s iron soldering method. To im-prove the welding characteristics and the quality of the weld metal, he covered his steel electrodes with a mixture of sodium silicate, carbon pow-der, calcium oxide and cellulose. The mixture was burnt in the arc and produced the combus-tion gases of CO and CO2. This produced the an-ticipated result; the shielding gas that was gener-ated produced an excellent weld and after having the method approved by Lloyd’s, he patented his invention which he called the “crater-forming covered electrode”. In 1904, Oscar Kjellberg founded “Elektriska Svetsningsaktiebolaget”, ESAB. Two years later the term “welding” start-ed to appear in the literature.

Oscar Kjellberg

Core wire is both an electrode and a consumable. The core wire alloy is adapted to match the metal for which the electrode is going to be used. This does not mean that the core wire alone complies with the requirements relating to mechanical pro-perties. The final composition of the weld metal is largely determined by the components in the

Covered electrodes

Form weld metal

Iron powder

Ferro-silicon

Ferro-manganese

Ferro-alloys

Slag formation

Ilmenite

Zirconium dioxide

Zirconium sand

Quartz

Felspar

Kaolin

Fluorspar

Flusspat

Gas shield

Limestone

Magnesite

Dolomite

Cellulose

Lubricants and binders

Adhesive

CMC (Carboxyl Methyl Cellulose)

Alginates

Potash- natron- lithium sodium silicate

Water

Lye

Task of the coating:

• To ionise and stabilise the arc• To develop shielding gas• To deposit slag• To add de-oxidants• To add alloying agents

Task of slag:

• To form weld metal• To create a protective cover• To neutralise harmful

substances

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Electrodes are divided up according to the com-position of the covering into organic, acid, basic and rutile types. In organic electrodes, the cover-ing consists primarily of organic substances such as cellulose, wood flour and so on. The terms acid and basic come from the reac-tions the oxides produce and they should not be in-terpreted as meaning that there are acids or hydrox-ides in the electrodes. Rutile electrodes have been given this name because most of the slag-forming components are made of the mineral rutile. Most rutile electrodes are rutile-acid electrodes, but there are also rutile-basic versions.

Core wire: The metal rod of the stick electrode

Electrode coating:

Forms slag, facilitates ionisation, produces shielding gas,adds de-oxidants, adds alloying elements

Droplet forma-tion:

The metal droplets are protected by the slag covering during transport through the arc

Gas shield: Protects the molten pool from the oxygen in the atmosphere

Arc: Electrical conduction through a gas in an applied electric field

Weld metal dilution in base material metal:

70% consumable, 30% parent metal

Slag:Protects the solidifying weld metal from oxidation and forms the weld bead

Molten cove-ring:

Some components evaporate, some form weld metal and others form slag

Core wire

Coating

Melted covering

Droplet formation

Gas shield

Arc

Slag Weld metal

Covering components

Cellulose

Wood flour

Silicates

Water

Organic electrodes

Acid electrodes

Basic electrodes

Coating

Calcite fluorspar

Ferro manganese

Iron silicate

Iron powder

These days’ basic elec-trodes are of the LMA type, Low Moisture Absorbing. The transfer of weld metal is globu-lar in shape. Compared to rutile electrodes they

are harder to strike and even more difficult to re-strike and their slag removal is more demanding too. The bead shape in both fillet and butt welds is convex and penetration into the parent metal is deeper. The primary benefits of basic electrodes are purer weld metal, a lesser degree of unwanted substances in the weld metal and much higher me-chanical properties and impact values.

As the covering has a high content of iron powder, this electrode type has high produc-tivity and excellent welding characteristics horizontally. The slag is porous and easy to re-move. This electrode is

welded with high heat input and has a powerful ability to penetrate. The appearance of the bead is smooth to concave and the mechanical proper-ties are good. As the electrode dilutes with the parent metal, the weld metal is sensitive to con-taminants which could produce a porous weld or hot cracking. The principal application for this electrode type is unalloyed steel with a low car-bon content.

This type of electrode has low productivity but good welding charac-teristics in all positions. It produces a small amount of slag which is easy to remove. The

appearance of the bead is slightly convex and the mechanical properties are satisfactory. The prin-cipal application is pipeline welding, vertically down.

Rutile electrodes

Coating

Rutile (TiO2)

Silicate

Limestone

Mica

Ferromanganese

Possibly cellulose

Water

This electrode type is easy to weld and has good restriking char-acteristics. It produces an attractive bead with good wetability to the joint edges. The slag is relatively thick and easy to remove. These electrodes are very re-

sistant to pore formation and their resistance to cracking is almost as good as that of basic elec-trodes. Hydrogen content restricts their use in C-Mn steel. The applications are standard struc-tural steel and shipbuilding steel of A grade and standard strength classes.

There are other types of coatings. Electrodes de-signed for cast iron with core wire of nickel or nickel/iron have a graphite covering. Electrodes designed for aluminium with a core wire of aluminium have a covering with a fluo-ride/chloride composition. This covering should be regarded as a flux whose task is to dissolve the aluminium oxides that are difficult to melt and otherwise form an insulating layer between the weld metal and the parent material, which results in incomplete penetration. Iron powder content in the coating increases the deposition rate of an electrode. The more iron powder, the higher the productivity. Electrodes with a high iron powder content are called high efficiency electrodes. The efficiency is stated as a percentage of the weight of the core wire in proportion to the weight of the weld metal. For example, the weld metal of an electrode whose core wire weighs 100 g and which has no alloy-ing elements and iron powder in its covering (or-ganic electrode) will weigh around 80 g when it is deposited as a low bead on a flat plate. The missing 20 g have disappeared in metal vapour and spatter. The efficiency level is 80% – a low level. With the addition of iron powder, a normal-efficiency electrode produces 105-140%. With the addition of iron powder, the efficiency can be as high as 250%. These electrodes are known as high-efficiency electrodes. Adding iron powder instead of simply increasing the diameter of the core wire makes the welding process smoother, reduces energy consumption and improves cur-rent durability. High-efficiency electrodes pro-duce a larger molten pool and this is only suit-able for horizontal welding. The choice of electrode is determined by the parent metal. Steel is divided into unalloyed, low-alloy and high-alloy steel. This then gives us a selection of electrodes for steel that are unal-loyed, low-alloy, high-alloy or over-alloyed with an organic, acid, basic or rutile covering and low, normal or high efficiency. The term “overalloyed electrodes” refers to products with a raised alloy content, designed to join dissimilar steels and metals.

Covering components

Quartz

Limestone

Ferromanganese

Felspar

Iron oxide

Water

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Welding with covered electrodes

In the ESAB Welding Handbook, there is a prod-uct description for every electrode. It specifies the materials for which the electrode is designed and its characteristics.

EfficiencyExpressed in per cent.

Type of current Indicates whether the electrode is designed for direct current plus or minus pole or alternating current. If it is suitable for alternating current, it functions perfectly on both plus or minus of direct current. Alternating current requires good ionisation, as the arc is extinguished when al-ternating current changes polarity, which takes place 50 times a second. The plus pole produces the greatest penetra-tion and the minus polarity the least. Intermediate penetration is obtained with alternating current.

Open-circuit voltage (OCV) Is the voltage the welding power source has when it is not subjected to any load. Basic electrodes require at least 65 volts. Hobby transformers nor-mally generate just 50 volts. This is not enough to strike anything other than rutile electrodes. The OCV falls to around 25 volts arc voltage when the arc is struck. Hobby machines normally work on 230 volts single phase mains voltage. With a 10 amp slow blow fuse the maximum power available is 2300 Watts. If welding at 150A 25 volts the power consumption is 3750 Watts. The main fuse will blow due to this (maximum welding current should not be more than 100 Amps). Professional machines are three phase, at least 16 amps x 400 V = 6400 W, which gives 250 amps of welding current on a 16-amp fuse. The ESAB Welding Handbook shows ex- amples of amperage and voltage values of differ-ent electrodes.

Metal arc welding takes place in the following way. A power source is connected to the mains. In the power source, a transformation takes place to a suitable current and voltage, followed nor-mally by rectification. The electrode is connected to the plus pole, making it the anode, while the workpiece is connected to the minus pole, mak-ing it the cathode. The current circuit is closed when the electrode touches the workpiece. At this point, a short-circuit current rushes through the tip of the electrode. The resistance heat melts any unevenness on the tip of the electrode and an air gap is created in which the current continues to rush into an arc. Current is electrons that are moving. They are emitted by cathode spots on the workpiece, but they are also released from atoms in the air. Electrons are negatively charged and they are at-tracted at high speed towards the positive anode where the kinetic energy generated by the colli-sion is converted into heat. To begin with, the at-oms are electrically neutral as the positive charge in the nucleus of the atom is the same size as the negative charge in the electrons. When one or more electrons leave the atom, it becomes positive or an ion. The positive ion is attracted by the negative cathode, where the kinetic energy is converted into heat. Additional electrons are then emitted and they then release electrons from atoms in the air which are then

ionised. The level of ionisation has a decisive effect on arc stability and, for this reason, sub-stances that are easy to ionise make up part of the covering components. The arc is so hot that the material forms of sol-id, liquid and gaseous ensue and the mixture of molecules, positive ions and negative free elec-trons turns into plasma. The temperature can be as high as 30,000°C. Electromagnetism holds the arc together and electromagnetic radiation in the UV range is emitted. The transport of material from the melting electrode is protected by slag, which covers the droplets on their way to the molten pool. In the molten pool, the slag dissolves contaminants. When the slag rises towards the surface of the molten pool because of its lower density, it so-lidifies before the metal and thereby creates the bead. This protects the red-hot weld metal which has high affinity to oxygen. In the heat generated by the arc, some covering components, first and foremost calcium oxides, are burned and produce the combustion gases of CO2 and CO. These combustion gases push away the ambient air and protect the N, O and H in the air reaching the molten pool. Otherwise the gases in the air would cause nitrides, oxides and hydrogen inclusions, as well as porous welds, when the metal solidifies around the gas that is pouring out of the hot molten pool.

Some terms relating to welding with stick electrodes

Electrode class Electrodes are manufactured according to stand-ards which describe the properties and mechani-cal values of the product. AWS A5.1 (American Welding Society electrode standard), EN 499 (European standard for covered electrodes) and ISO 2560 (International Standardisation Organi-sation’s electrode standard) are normally speci-fied.

Approval ESAB complies with the above-mentioned stand-ards and this is something customers can rely on. To protect third parties from accidents and injuries, there are a number of inspection bodies accredited by government authorities. It is their task to safeguard the interests of third parties. These inspection bodies are known as classifica-tion societies. Approval for ESAB’s products is granted by a number of classification societies which moni-tor the welding, sampling and mechanical test-ing of electrodes that are randomly selected from stock. If the test results comply with the standards and the classification societies’ requirements, the product is approved for use in the pressure- and load-bearing structures these societies monitor. These tests take place annually.

Chemical composition Typical values for all-weld metal. All-weld metal is obtained from the special standardised joints from which samples are taken. In principle, all-weld metal is pure electrode weld metal with-out the inclusion of parent metal. Weld metal in structural joints always contains approximately 30% or more parent metal. I-joints in particular may have a far higher par-ent metal content. For this reason, the mechani-cal values specified in the catalogue must not be interpreted as a guarantee that the structural joint will display the same characteristics.

A miniature steelworks

N = the weight of the weld metal

The weight of the core wire

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Mechanical values The mechanical values depend on chemical com-position, but they are also influenced by the parent metal and welding procedure. The tensile yield stress specifies the force that is required for the material to deform plastically. The ultimate tensile strength is the force which causes the material to fail. The length of the de-formation between the tensile yield limit and the ultimate tensile strength is measured in per cent over a length of five times the diameter of the specimens.

Impact strength The impact strength is a gauge of the amount of energy the material can absorb in ductile condition at a given temperature. The colder the metal, the more brittle the fracture. The temperature range in which the material changes from ductile to brittle is called the transition temperature of the steel. The impact strength falls dramatically at the transition temperature and can fall from a ductile fracture with an impact strength of 100 J to one of just a few J. The inappropriate choice of steel and electrode in a snow plough or ice-breaker could have unexpected consequences, Aluminium and nickel have no transition temperature, which ex-plains why these materials are used in tanks for transporting liquid gases at –196°C.

Electrode diameter Specifies the diameter of the core wire. Electrodes with a small diameter should be used on thin ma-terial or in root runs. The heavier the material, the larger the electrode diameter that can be used. The current should be increased to match an increase in electrode diameter. The current range of the elec-trode is calculated between the lowest current at which it can be welded and 10% under the highest temperature it is able to withstand without being overloaded. This range covers a wide area and the current is determined by the metal thickness and the welding position. The electrode length is the length of the core wire. The length varies between electrode types. Some core wires have a high re-sistance to electric current. If the core wire is too long it will overheat. Therefore stainless steel and nickelbased electrodes are normally kept between 300 and 350 mm long.

Arc voltage Varies between different types of electrodes and depends on arc length. The length of the visible arc is more or less the same for all covered elec-trodes, but, depending on the depth of crater for-mation in the covering on the tip of the electrode, it varies from one electrode type to another. High-efficiency electrodes with a thick covering have the highest arc voltage. In normal circumstances, the arc voltage on an MMA power source cannot be regulated. Some machines designed for organic pipeline electrodes are equipped with voltage reg-ulation (arc force).

N, deposition efficiencyDeposition efficiency is the relationship of the weight of the weld metal deposited to the weight of the electrode consumed in making a weld. It can be accurately determined only by making a timed test weld, and carefully weighing the weld-ment and the electrode or wire, before and after welding. The efficiency can then be calculated by the formula: Deposition efficiency = Weight of weld metal ./. weight of electrode used (or) deposition rate kg/hr ./. burn-off rate (kg/hr). The deposition efficiency tells us how many kg of welt metal that can be expected from a given weight of the electrode purchased, As an example, 50 kg of a coated electrode with an efficiency of 65% will produce approximately 32.5 kg of weld metal, less the weight of the stubs discarded.

B, the number of electrodes per kilogram of weld metal If you know the arc time of the electrode, you can calculate how long it takes to produce one kilo-gram of weld metal.

H, deposition rateThe deposition rate is the rate that weld metal can be deposited by a given electrode, expressed in kg per hour. It is based on continuous operation, not allowing time for stops and starts caused by inserting a new electrode, cleaning slag, termi-nation of the weld or other reasons. The deposi-tion rate will increase as the welding current is increased.

T, arc time in seconds per electrodeIs measured at an I-max – 10%. The welding time increases as the current is reduced.

Notes

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ESAB AB

Box 8004, SE-402 77 Göteborg, Sweden

Phone: +46 31 50 90 00. Fax: +46 31 31 50 93 90

info@esab.se www.esab.com

Content

• The history of welding • Covered electrodes• Organic electrodes • Acid electrodes • Basic electrodes• Rutile electrodes• Welding with covered electrodes• Some terms associated with welding with covered electrodes

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