Duplex stainless steel. Part 1

Job Knowledge

The name 'duplex' for this family of stainless steels derives from the microstructure of the alloys which comprises approximately 50/50 mixture of austenite and delta-ferrite. They are designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as Type 304 or 316. The main differences in composition, when compared with an austenitic stainless steel is that the duplex steels have a higher chromium content, 20 - 28%; higher molybdenum, up to 5%; lower nickel, up to 9% and 0.05 - 0.5% nitrogen. Both the low nickel content and the high strength (enabling thinner sections to be used) give significant cost benefits. They are therefore used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc and in the petrochemical industry in the form of pipelines and pressure vessels.

In addition to the improved corrosion resistance compared with the 300 series stainless steels duplex steels also have higher strength. For example, a Type 304 stainless steel has a 0.2% proof strength in the region of 280N/mm2, a 22%Cr duplex stainless steel a minimum 0.2% proof strength of some 450N/mm2 and a superduplex grade a minimum of 550N/mm2.

Although duplex stainless steels are highly corrosion and oxidation resistant they cannot be used at elevated temperatures. This is due to the formation of brittle phases in the ferrite at relatively low temperatures, see below, these phases having a catastrophic effect on the toughness of the steels. The ASME pressure vessel codes

therefore restrict the service temperature of all grades to below 315C, other codes specify even lower service temperatures, perhaps as low as 250C for superduplex steels.

Duplex alloys can be divided into three main groups; lean duplex, 22%Cr duplex and 25%Cr superduplex, and even higher alloyed, hyperduplex grades have been developed, this division being based primarily on the alloy's alloying level, eg in terms of 'PREN' (pitting resistance equivalence number), a measure of the alloy's resistance to pitting corrosion. PREN is calculated from a simple formula: PREN = %Cr + 3.3%Mo +16%N and an allowance for W is sometimes made, having a factor of 1.65. A duplex steel has a PREN less than 40; a superduplex a PREN between 40 and 45 and hyperduplex a PREN above 45, whilst the lean grades typically have lower nickel and hence lower price.

The commonest shorthand method of identifying the individual alloys is by the use of the trade name, particularly for the superduplex grades, eg UR52N+, Zeron 100, 2507 or DP3W, whilst the most common 22%Cr grade, UNS S31803 has widely become known as 2205 regardless of its supplier, although this is a trade name.

The UNS numbering system offers an independent alternative. Typical compositions and minimum proof strengths of the more common duplex alloys are given in the Table. Note that the commonly used 2205 applies to two UNS numbers, S31803 and S32205, with S32205 being a more recent and controlled composition.

Typical compositions and proof strengths of common duplex stainless steels

Common

Name

UNS

No

BS EN

No

Steel

Type

Typical Chemical

Composition %

0.2%

proof

%C Cr Ni Mo N Cu strength

N/mm2 (min)

2304 S32304

1.4362

duplex

0.015

23.0

4.0

0.055

0.13

400

2205 S31803

1.4462

duplex

0.015

22.0

5.5

3.0 0.14

- 450

2205 S3220

5

1.446

2

duple

x

0.01

5

22.

5

5.

5

3.3 0.1

7

450

255(UR52N)

S32520

1.4507

super duple

x

0.015

25.0

7.0

3-5 0.28

0.13

550

2507 S3275

0

1.441

0

super

duplex

0.01

5

25.

0

7.

0

4.5 0.2

8

0.3 550

Zeron 100 S3276

0

1.450

1

super

duplex

0.01

5

25.

0

7.

0

3.5 0.2

5

0.8 550

Sandvik

SAF3207

S3320

7

- hyper

duplex

0.03 31 7.

5

4.0 0.5

0

0.7

5

700

The metallurgy of the duplex stainless steel family is complex and requires very close control of composition and heat treatment regimes if mechanical properties and/or corrosion resistance are not to be adversely affected. To

produce the optimum mechanical properties and corrosion resistance the microstructure or phase balance of both the parent and weld metal should be around 50% ferrite and 50% austenite. This precise value is impossible to achieve repeatably but a range of phase balance is acceptable. The phase balance of parent metals generally ranges from 35 - 60% ferrite.

Whilst composition and, perhaps more importantly, heat treatment parameters are relatively easy to control this is not the case during welding. The amount of ferrite is dependant not only on composition but also on the cooling rate; fast cooling rates retain more of the ferrite that forms at elevated temperature. Therefore to minimise the risk of producing very high ferrite levels in the weld metal it is necessary to ensure that there is a minimum heat input and therefore a maximum cooling rate. A rule of thumb is that heat input for duplex and superduplex steels should be not less than 0.5kJ/mm although thick sections will need this lower limit to be increased.

Welding consumables are also generally formulated to contain more nickel than the parent metal, nickel being one of the elements that promotes the formation of austenite. A duplex filler metal may contain up to 7% nickel, a superduplex up to 10% nickel.

Reference to the phase diagrams and CCT curves shows that the duplex stainless steels fall within the area where the production of brittle intermetallic phases is a major risk during welding and heat treatment, markedly reducing both toughness and corrosion resistance.

The main culprits are sigma phase, chi phase and 475C embrittlement. Sigma and chi phases form at temperatures between 550 and 1000C with the fastest rate of formation around 850C. The time to form these phases can be as short as 30 or 40 seconds in a superduplex alloy. 4750C embrittlement, as the name suggests, occurs at lower temperatures of some 350 - 550C with times for the start of formation of perhaps 7 - 10 minutes.

Short times such as these are within the ranges that may be encountered during interpass cooling so, once again, heat input and cooling rates become very important welding parameters except that this time it is the maximum heat input that needs to be controlled. A maximum heat input of 2.5kJ/mm should be acceptable for the duplex steels and 2.0kJ/mm maximum for superduplex. Many codes and contract specifications, however, further restrict heat inputs to less than 1.75 - 2kJ/mm for duplex steels and 1.5 - 1.75kJ/mm for superduplex.

Two other factors that also affect cooling rates are preheating and interpass temperatures. Preheat is not generally regarded as necessary for duplex stainless steels unless the ambient conditions mean that the steel is below 5C or there is condensation on the surface. In these situations a preheat of around 50 - 75C should be adequate. Very thick section joints, particularly those welded with the submerged arc process, can also benefit from a low preheat of around 100C.

Interpass temperature can have a significant effect on the microstructure of the weld and its heat affected zones. For a duplex steel 250C is regarded as an acceptable maximum and for a superduplex 150C maximum. Note, however, that many codes do not separate the grades into duplex and superduplex and 150C is often required as the norm. Such low interpass temperatures can have a serious effect on joint completion times and forced cooling by blowing dry air through the bore of a pipe once the bore purge has been removed has been used. This is generally only beneficial when thick wall vessels or pipes are being welded using a rotated pipe mechanised TIG

process or submerged arc. If this technique is used then it is advisable to force cool the procedure qualification test piece to ensure that cooling rates (and the resultant microstructures) are within the permissible range.

Care therefore needs to be taken to read through code and contract specification requirements and to ensure that the requirements with respect to heat input, interpass temperature etc. are incorporated in welding procedure documentation prior to welding procedure qualification. The next Job Knowledge will provide some guidelines for the welding of the duplex stainless steels.

The previous article highlighted some of the problems encountered when welding duplex and superduplex stainless steels, in particular the need to control closely the heat input if an undesirable phase balance or the formation of brittle intermetallic phases are to be avoided.

This requirement has implications with respect to quality control. Variations in weld preparations which would be compensated for by the welder changing his welding technique, wide root gaps for example, may result in a significant change in heat input. Weld preparations therefore need to be more closely controlled than for a conventional stainless steel.

It is recommended that weld preparations are machined for greatest accuracy but, if hand-ground, close attention must be paid to the weld preparation dimensions. Welding supervisors and inspectors also need to understand the importance of heat input control, ensuring that welding is not allowed to take place outside the limits of the qualified procedures with regular checking of welding parameters and interpass temperature.

Hot cracking is rarely a problem due to the high ferrite content but has been observed, particularly in submerged arc welds. Cleanliness of the joint is therefore still important. Machining or grinding burrs and any paint should be removed and the joint thoroughly degreased and dried prior to welding. Failure to do so can affect corrosion resistance and joint integrity.

Hydrogen cold cracking, whilst unusual, is not unknown and can occur in the ferrite of weld metal and HAZs at quite low hydrogen concentrations. It is recommended that the hydrogen control measures used for low alloy steel consumables should apply for duplex consumables. Submerged arc fluxes and basic coated electrodes should be baked and used in accordance with the manufacturer's recommendations; shield gases must be dry and free of contaminants.

Most commercially available welding consumables will provide weld metal with yield and ultimate tensile strengths exceeding those of the parent metal but there is often difficulty in matching the notch toughness (Charpy V) values of the wrought and solution treated base metal.

TIG welding gives very clean weld metal with good strength and toughness. Mechanisation has substantially increased the efficiency of the process such that it has been used in applications such as cross-country pipelining.

Gas shielding is generally pure argon although argon/helium mixtures have given some improvements by permitting faster travel speeds. Nitrogen, a strong austenite former, is an important alloying element, particularly in the super/hyper duplex steels and around 1 to 2% nitrogen is sometimes added to the shield gas to compensate for any loss of nitrogen from the weld pool. Nitrogen additions will, however, increase the speed of erosion of the tungsten electrode. Purging the back face of a joint is essential when depositing a TIG root pass. For at least the first couple of fill passes pure argon is generally used although small amounts of nitrogen may be added and pure nitrogen has occasionally been used.

TIG welding may be performed without any filler metal being added but is not recommended on duplex steels as the corrosion resistance will be seriously impaired. Filler metals are be selected to match the composition of the parent metal but with an additional 2 to 4% nickel to ensure that sufficient austenite is formed. Any stray arc strikes will be autogenous and must be removed by grinding.

MMA welding is carried out with matching composition electrodes overalloyed with nickel and either rutile or basic flux coatings. Basic electrodes give better notch toughness values. Electrodes of up to 5mm diameter are available with the smaller diameters providing the best control when welding positionally.

MAG welding is generally carried out using wires of 0.8 to 1.2mm diameter, rarely exceeding 1.6mm and of a similar composition to the TIG wires. Shielding gases are based on high purity argon with additions of carbon dioxide or oxygen, helium and perhaps nitrogen. Because of the presence of carbon dioxide or oxygen the weld metal notch toughness (Charpy V values) are less than can be achieved using TIG. Microprocessor-controlled

pulsed welding gives the best combination of mechanical properties. Mechanisation of the process is easy and can give significant productivity improvements although joint completion times may not be as short as anticipated due to the need to control interpass temperatures to below the recommended maximum.

Flux-cored arc welding (FCAW) is used extensively with major productivity gains being possible in both manual and mechanised applications. The flux core is generally rutile; the shielding gas CO2, argon/20%CO2or argon/2%O2. The presence of carbon dioxide or oxygen leads to oxygen, and, in the case of CO2, carbon pickup in the weld metal, thus notch toughness is reduced. Metal cored wires are also available that require no slag removal; better suited to mechanised applications than flux-cored wires. Because of differences in flux formulation and wire composition between manufacturers it is recommended that procedure qualification is carried out using the specific make of wire used in production even though the wires may fall within the same specification classification.

Submerged arc welding (SAW) is generally confined to welding thick wall pipes and pressure vessels. Solid wires, similar to those available for TIG welding, are available. Fluxes are generally acid-rutile or basic, the latter giving the best toughness values in the weld metal. As with any continuous mechanised welding process the interpass temperature can rapidly increase and care needs to be taken to control both interpass temperature and process heat input. Because of the need to control heat input the wire diameter is normally limited to 3.2mm permitting a maximum welding current of 500A at 32V although larger diameter wires are available. However, any productivity gains from the use of a large diameter wire and high welding current may not be realised due to the need for interpass cooling.

There is often the need to weld duplex/superduplex steel to lower alloyed ferritic steel, a 300 series stainless steel or a dissimilar grade of duplex steel. The 300 series stainless steels are generally welded to duplex steels with a

309MoL (23Cr/13Ni/2.5Mo) filler metal. Low carbon and low alloy steels may be welded to duplex steels using either a 309L (23Cr/13Ni) or a 309MoL filler metal.

These two filler metals, however, have yield and ultimate tensile strengths substantially less than most low carbon/low alloy steels and all duplex steels. This means the designer has to take this reduction of strength into account by increasing the component thickness or the welding engineer has to select a filler metal that both matches the strength of the weaker steel and is compatible with the two parent metals. These considerations narrow the choice to one of the nickel-based alloys such as alloy 82 or, for higher strength, a niobium-free high alloyed nickel filler, such as C22. or 59. Alloy 625 has been used but problems with reduced toughness due to the formation of niobium nitride precipitates along the fusion boundary have resulted in the alloy falling out of favour.

Duplex steel welds are seldom post-weld heat treated. Due to sigma phase formation they cannot be given a heat treatment at the low temperatures of 600-700C, the normal range for stress relief unless a qualification programme has been undertaken to demonstrate that the loss of toughness is acceptable. If PWHT is required then ideally the whole component must be given a solution anneal at 1000-1100C followed by a water quench; an impractical operation with most welded structures.

Lastly, any process that heats the steels above 300C will affect the mechanical properties. Heat straightening to control distortion should therefore not be carried out. The HAZs produced by hot cutting processes like plasma or laser may contain undesirable microstructures. Cut edges that will enter service 'as-cut' must be ground or machined back for a minimum of 2mm to remove the HAZ and ensure there is no loss of toughness or corrosion resistance.

If the cut edges are welded after cutting then the HAZs are generally sufficiently narrow that the effects of the cutting operation are lost although it is recommended that, as above, the edges are ground or machined back 2mm.

Welding of austenitic stainless steel

Job Knowledge

There are a number of different types of steels that may be referred to as 'stainless'; previous articles have considered ferritic and precipitation hardening steels for example. It is therefore advisable to be specific and to refer to the group to which the steel belongs in order to avoid confusion. Although commonly referred to as 'stainless steel', the steels covered in this article should be more correctly referred to as austenitic, 18/8 or chromium-nickel stainless steels.

As with the other types of stainless steels, the austenitic stainless steels are corrosion and oxidation resistant due to the presence of chromium that forms a self-healing protective film on the surface of the steel. They also have very good toughness at extremely low temperatures so are used extensively in cryogenic applications. They can be hardened and their strength increased by cold working but not by heat treatment. They are the most easily weldable of the stainless steel family and can be welded by all welding processes, the main problems being avoidance of hot cracking and the preservation of corrosion resistance.

A convenient and commonly used shorthand identifying the individual alloy within the austenitic stainless steel group is the ASTM system. This uses a three digit number '3XX', the '3' identifying the steel as an austenitic stainless, and with additional letters to identify the composition and certain characteristics of the alloy eg type 304H, type 316L etc; this ASTM method will be used in this article.

Typical compositions of some of the alloys are given in Table 1. The type 304 grade may be regarded as the archetypal austenitic stainless steel from which the other grades are derived and changes in composition away from that of type 304 result in a change in the identification number and are highlighted in red.

Table 1 Typical compositions of some austenitic stainless steel alloys

ASTM No. (type)

Composition wt% Microstructure

C

(max)

Si

(max)

Mn

(max)

Cr Ni Mo Others Austenite - A

Ferrite - F

304 0.08 0.75 2.0 18/20

8/11 - - A+2/8%F

304L 0.035 0.75 2.0 18/2

0

8/11 - - A + 2/8%F

304H 0.04 - 0.10

0.75 2.0 18/20

8/11 - - A + 2/8%F

304N 0.08 0.75 2.0 18/20

8/11 - 0.1/0.16N

A + 2/8%F

316 0.08 0.75 2.0 16/1

8

11/1

4

2/

3

- A + 3/10%F

347 0.08 0.75 2.0 17/2

0

9/13 - Nb :

10xC

A + 4/12%F

321 0.08 0.75 2.0 17/19

9/12 - Ti: 5xC A + 4/12%F

310 0.15 0.75 2.0 24/2

6

19/2

2

- - 100% A

309 0.08 1.0 2.0 22/2

4

12/1

5

- - A + 8/15%F

308L (generall

y filler metal

only)

0.03 1.0 2.0 19/21

10/12

A + 4/12%F

The 3XX may followed by a letter that gives more information about the specific alloy as shown in the Table. 'L' is for a low carbon austenitic stainless steel for use in an aggressive corrosive environment ; 'H' for a high carbon

steel with improved high temperature strength for use in creep applications; 'N' for a nitrogen bearing steel where a higher tensile strength than a conventional steel is required. These suffixes are used with most of the alloy designations eg type 316L, type 316LN, type 347H, where the composition has been modified from that of the base alloy.

Austenitic stainless steels are metalurgically simple alloys. They are either 100% austenite or austenite with a small amount of ferrite (see Table 1). This is not the ferrite to be found in carbon steel but a high temperature form known as delta () -ferrite. Unlike carbon and low alloy steels the austenitic stainless steels undergo no phase changes as they cool from high temperatures. They cannot therefore be quench hardened to form martensite and their mechanical properties to a great extent are unaffected by welding. Cold (hydrogen induced) cracking (Job Knowledge No. 45) is therefore not a problem and preheat is not necessary irrespective of component thickness.

Alloying elements in an austenitic stainless steel can be divided into two groups; those that promote the formation of austenite and those that favour the formation of ferrite. The main austenite formers are nickel, carbon, manganese and nitrogen; the important ferrite formers are chromium, silicon, molybdenum and niobium. By varying the amounts of these elements, the steel can be made to be fully austenitic or can be designed to contain a small amount of ferrite; the importance of this will be discussed later.

In 1949 Anton Schaeffler published a constitutional or phase diagram that illustrates the effects of composition on the microstructure. In the diagram Schaeffler assigned a factor to the various elements, the factor reflecting the strength of the effect on the formation of ferrite or austenite; these factors can be seen in the diagram. The elements are then combined into two groups to give chromium and nickel 'equivalents'. These form the x and y axes of the diagram and, knowing the composition of an austenitic stainless steel, enables the proportions of the phases to be determined.

Fig 1. Shaeffler diagram (A-austenite; M - martensite; F - ferrite)

Typical positions of some of the commoner alloys are given in Fig.1. Also superimposed on this diagram are coloured areas identifying some of the fabrication problems that may be encountered with austenitic stainless steels.

Although all the austenitic stainless steels are sensitive to hot cracking (Job Knowledge No.44), the fully austenitic steels falling within the vertically blue area in Fig.1 such as type 310 are particularly sensitive. The main culprits are sulphur and phosphorus. To this end, these tramp elements have been progressively reduced such that steels with less than 0.010% sulphur and phosphorus less than 0.020% are now readily available. Ideally a type 310 or type 317 alloy should have sulphur and phosphorus levels below some 0.003%. Cleanliness is also most important and thorough degreasing must be carried out immediately prior to welding.

The steels such as type 304, type 316, type 347 that fall within, or close to, the small uncoloured triangular region in the centre of the diagram contain a small amount of delta-ferrite and, whilst not being immune to hot cracking, have improved resistance to the formation of sulphur-containing liquid films. The reasons for this are that a) ferrite can dissolve more sulphur and phosphorus than austenite so they are retained in solution rather than being available to form liquid films along the grain boundaries and b) the presence of quite a small amount of ferrite increases the grain boundary area such that any liquid films must spread over a greater area and can no longer form a continuous liquid film. The 100% austenitic steels do not have this advantage.

One problem that has arisen with very low sulphur steels is a phenomenon known as 'cast to cast variation' or 'variable penetration'. The weld pool in a low sulphur steel (

used. The steel and filler metal should be selected with as low a ferrite content as possible, say 1

to 3% for best Charpy-V test results.

Conversely, for best creep resistance an 'H' grade steel should be selected and rutile or acid/rutile

electrodes and acid submerged arc fluxes should be used. These improve the creep strength by

increasing the titanium and niobium content of the weld metal, forming a greater concentration

of grain strengthening carbides.

TIG (GTAW) welding of the root pass must always be carried out with an inert gas back purge to

prevent loss of chromium (and hence of corrosion resistance), argon being the gas generally used

for this purpose. Nitrogen may be used but there is a risk of the weld deposit absorbing nitrogen,

thereby becoming fully austenitic and hot crack sensitive.

Two characteristics of austenitic stainless steels that differentiate them from ferritic steels are the

coefficients of thermal conductivity and expansion. Austenitic stainless steels have a low

coefficient of thermal conductivity, approximately 1/3rd that of ferritic steel at room temperature

and a coefficient of thermal expansion some 30% more than that of a ferritic steel.

Higher expansions in a narrower HAZ result in higher residual stresses and more distortion. This

is a particular problem with thin sheet fabrications where the achievement of the desired

dimensional tolerances can be extremely difficult and costly to achieve. The use of accelerated

cooling techniques such as copper chills or a freezing gas (the liquid CO2 low stress-no distortion

technique typifies this approach) have been used to reduce distortion to acceptable levels.

One of the main reasons for using an austenitic stainless steel is its corrosion resistance. Whilst

this is primarily a function of the chromium content of the steel, carbon also has a major but

adverse effect resulting in a form of corrosion known as intergranular or intercrystalline

corrosion (ICC) or weld decay, a localised effect confined to the HAZ.

Carbides present in the HAZ of an austenitic stainless steel dissolve on heating and reform on

cooling during the welding heat cycle. Unfortunately, these new precipitates form preferentially

as chromium carbides on the grain boundaries, depleting chromium from the region immediately

adjacent to the boundary, resulting in a local loss of chromium and a reduction in corrosion

resistance. If sufficient chromium carbides are formed this results in a network of steel along the

grain boundaries sensitive to corrosion; the steel has been sensitised. This sensitisation occurs in

the HAZ region that has seen temperatures between 600 and 900C and times that may be as

short as 50 seconds.

There are several methods that may be used to overcome this difficulty. A solution heat

treatment (1050C followed by a water quench) will re-dissolve the carbides and these will be

retained in solution on rapid cooling. Whilst this will eliminate the chromium depleted regions it

is rarely practical to solution-treat complex welded structures.

The most obvious alternative technique is to reduce the carbon content. This has two beneficial

effects:

The lower the carbon content, the longer the time required to form the carbides. At 0.08% carbon

this time is around 50 seconds; at 0.03% carbon the time required is about eight hours, most

unlikely to be achieved during welding!

The lower the carbon content then the fewer carbides there are to form a continuous chromium

depleted network. Hence the 'L' grades, type 304L, or 316L, are preferred where best corrosion

resistance is required.

One other method is the addition of alloying elements that will form carbides in preference to

chromium; thus the stabilised type 321 and 347 grades containing titanium and niobium

respectively were developed.

Titanium and niobium are very strong carbide formers that precipitate carbides at higher

temperatures than those at which chromium carbides will form so there is no carbon available to

react with the chromium. However, even these stabilised grades may corrode in a very narrow

band close to the fusion line (the so-called knife-line attack) in the presence of hot acids. This is

due to the higher and more restricted temperature range at which the niobium or titanium

carbides dissolve. The solution, as above, is to limit the carbon to 0.03% maximum.

Welding consumables must also be selected with low carbon content if best corrosion resistance

is required. Most arc welding consumables contain less than 0.03% carbon but there are filler

metals available with carbon contents of up to 0.10%; these should only be used to weld the 'H'

grades of steel where good creep resistance is required.

Although MAG (GMAW) welding is often used it should be remembered that carbon pick-up is

possible when argon/CO2 mixtures are used, particularly if the welding is carried out in the dip

transfer mode. Argon/2% oxygen mixtures are therefore generally preferred where best corrosion

resistance is required but argon/10% CO2/2% oxygen is a good compromise that can be used for

a broad range of applications.

The other major service problem encountered with the austenitic stainless steels is that of stress

corrosion cracking. This may be caused by strong alkali solutions but it is the halides (chlorides,

fluorides and bromides) that are primarily responsible. Cracking takes place in areas of high

stress, as the name suggests, and is not therefore confined solely to welds, but it is at and

adjacent to welds that stresses approaching the yield point of the metal are found and these

present a particular problem.

The cracking is transgranular and propagation rates can be extremely rapid given the ideal

conditions. In hot concentrated chloride solutions, for example, penetration can occur in thin,

sheet components within a few minutes. However, the lower the temperature and/or the acid

concentration then the rate of crack propagation is correspondingly slower. Austenitic stainless

steels are therefore not generally used where halides are present. Even here, stress corrosion

cracking (SCC)may occur due to contamination, either of the product in the pipe or vessel or

externally from sea water, particularly where the liquid is able to concentrate in crevices.

To eliminate any chance of SCC, the only solution is to stress relieve the weld at a temperature

of around 700 to 900C. It should be remembered that:

this may sensitise the steel so only low carbon grades should be used and

the steel may embrittle due to sigma phase formation (see Job Knowledge 103) at the lower heat

treatment temperatures.

Stress corrosion cracking (SCC) in Type 316L stainless steel

Local stress relief should be approached with caution as the temperature gradients may result in stresses developing outside the heated band; wider heated bands and more stringent control of temperature gradients than required by specifications or codes may therefore be necessary. Solution treatment (1050C soak followed by very rapid cooling, ideally a water quench) will eliminate all residual stresses whilst avoiding both sensitisation and embrittlement but is rarely practical on a welded assembly.

The alternative is to select a steel that is more resistant; the molybdenum bearing grade type 316 is better than 304 or 321. The ferritic stainless steels (Job Knowledge 101) are not susceptible to chloride SCC.

Precipitation hardening stainless steels

Job Knowledge

The precipitation hardening (PH) stainless steels are a family of corrosion resistant alloys some of which can be heat treated to provide tensile strengths of 850MPa to 1700MPa and yield strengths of 520MPA to over 1500MPa - some three or four times that of an austenitic stainless steel such as type 304 or type 316. They are used in the oil and gas, nuclear and aerospace industries where a combination of high strength, corrosion resistance and a generally low but acceptable degree of toughness is required. Precipitation hardening is achieved by the addition of copper, molybdenum, aluminium and titanium either singly or in combination.

The family of precipitation hardening stainless steels can be divided into three main types - low carbon martensitic, semi-austenitic and austenitic - typical compositions of some of the steels are given in Table 1.

Table 1 Typical Compositions of some commoner precipitation hardening stainless steels

Specificati

on

Commo

n

Name

Type Typical Chemical Analysis %

C Mn Cr Ni Mo Cu Al Ti Others

A693 Tp630

17/4PH martensitic

0.05

0.75

16.5 4.25

- 4.25

- - Nb 0.3

FV 520 austeniti

c-martensi

tic

0.0

5

0.6 14.5 4.7

5

1.4 1.7 - - Nb

0.3

A693

Tp631

17/7PH austeniti

c-

martensitic

0.0

6

0.7 17.2

5

7.2

5

- - 1.2

5

- -

PH 15/7

Mo

austenitic-

martensitic

0.06

0.7 15.5 7.25

2.6 - 1.3 - -

A 286 austeniti

c

0.0

4

1.4

5

15.2

5

26.

0

1.2

5

- 0.1

5

2.1

5

V

0.25 B

0.007

JBK 75 austenitic

0.01

0.04

14.75

30.5

1.25

- 0.30

2.15

V 0.25

B 0.001

7

17/10P austenitic

0.07

0.75

17.2 10.8

P 0.28

The martensitic PH steels, of which 17/4PH is the most common, transform to martensite at low temperatures, typically around 250C, and are further strengthened by ageing at between 480 and 620C.

The austenitic-martensitic PH steels are essentially fully austenitic after solution treatment and require a second heat cycle to 750C/2 hours before cooling to room temperature to form martensite. Some of these alloys need to be refrigerated (-50/-60C for eight hours) following this heat treatment to ensure full transformation to a stable austenitic/martensitic structure although the two most commonly used alloys, FV520 and 17/7PH, do not require refrigeration to develop optimum properties.

Ageing of these alloys occurs at temperatures between 500 to 600C. The austenitic grades are stable down to

room temperature, improvements in strength being from the precipitates formed by ageing at 650 to 750C. These fully austenitic grades can exhibit good toughness and some may be used at cryogenic temperatures.

For best weldability it is recommended that all three types of alloys are supplied in the annealed, solution treated or overaged condition. Alloys in the form of sheet or strip may be in a cold worked condition and weldability is

seriously compromised. As with many precipitation hardening alloys, achieving mechanical properties in the weld and HAZs to match those of the parent material is a problem. Even with matching welding consumables, a full solution treatment and age hardening the maximum strength of a joint in the semi-austenitic and austenitic alloys is likely to be only some 90% of that of the base metal.

Martensitic PH steels in the solution-treated condition can be welded with most of the conventional arc welding processes although the best toughness will be achieved with the TIG (GTAW) process as this provides the cleanest weld metal. Even better toughness can be achieved using power beam processes (electron beam or laser welding). Matching filler metals are available for most of the steels in this group enabling matching mechanical properties to be achieved by carrying out a post weld ageing heat treatment.

If a joint is very highly restrained then 17/4PH may fail along the fusion line by a form of reheat cracking during the ageing heat treatment. In these circumstances the component should be welded in the overaged condition and then given a solution heat treatment followed by the PWHT described below. Austenitic filler metals such as 308L or, for higher weld metal strength, a duplex filler metal such as 2205, can be used where lower strength joints can be tolerated or cracking due to high restraint is a problem. PWHT is not possible if a duplex filler metal is used or recommended for austenitic weld metal due to embrittlement.

The martensite in these steels is relatively soft due to the low carbon content so preheat is not generally necessary although for thick, (above 25mm) highly restrained joints, a preheat of around 100C has been found to be useful in reducing the risk of cracking. Because of the low temperature at which these steels transform to martensite a maximum interpass temperature of 200C is recommended.

Maintaining a very high interpass temperature results in the entire weld transforming to martensite on cooling to room temperature and the volume change that occurs when this happens can then lead to a form of quench cracking. The stress raising effect of the notch in the root of fillet welds and partial penetration butt welds has been found to cause cracking. Provided the reduction in strength can be tolerated, a Tp308L root pass can be used to

solve this problem. It has also been found that 17/4PH castings may form HAZ hot cracks during welding; for cast items the copper content is therefore limited to 3% maximum.

PWHT generally comprises a 750C soak and cool to room temperature to ensure that the steel is 100% martensitic followed by ageing at 550C. This should give UTS of 900 to 1000MPa, yield strength 800 to 900MPa and ductility of some 15% depending upon the composition of the alloy and the temperature of the ageing heat treatment.

The semi-austenitic alloys are generally supplied in the solution treated condition. This means that the steel is fully austenitic and preheat is not generally required although for welding of thick and highly restrained joints a preheat of around 100C has been found to be helpful. All the common arc welding processes may be used although, as above, TIG (GTAW) will give the best properties.

For alloys containing aluminium, eg 17/7PH, MMA and submerged arc welding should be avoided as a good proportion of the aluminium is lost during welding; inert gas shielded processes are therefore preferred. The weld pool is less fluid than the non-aluminium alloys. Matching composition filler metals for FV520 are readily available

but 17/7PH consumables are difficult and expensive to obtain so parent metal sheared from strip is often used for TIG welding. Alternatively a 17/4PH or FV520 filler may be used; a preheat of 100C is advisable if the 17/4PH filler is used. PWHTs are similar to those used for the martensitic steels but, without a full solution heat treatment and matching filler metal, strengths matching those of the parent metal are unlikely to be achieved.

It is recommended that the fully austenitic PH steels are welded in the solution treated condition; a water or oil quench from around 980C. The ageing process is very sluggish, requiring some 15 hours at 720C to develop full strength and this means that the HAZ is virtually unchanged from the parent metal. Optimum strength can therefore be developed during the post-weld ageing treatment. These steels, like the austenitic stainless steels, are insensitive to cold cracking and do not require to be pre-heated. They are, however, very sensitive to hot cracking due to them being fully austenitic. This makes the welding of thick sections problematic and requires the welding conditions to be very closely controlled with low heat input, small weld beads and interpass temperature controlled to less than 150C.

Aerospace alloys such as AMS 5858, equivalent to A286, have been produced with improved weldability. The 17/10P grade is particularly sensitive and cannot be welded with matching fillers; a type 312 (29Cr/9Ni) filler gives the best chance of success, although hot cracking in the HAZ may still occur.

Due to the presence of aluminium and/or titanium in many alloys only the inert gas shielded arc welding processes should be used. Some matching composition filler metals are available, again in aerospace grades such as AMS 5804 and these can be aged to give strengths close to those of the parent metal. Alternatively either austenitic, duplex or nickel based weld filler metals may be used.

As is apparent, the metallurgy of these steels can be complex and if there is any doubt concerning welding or heat treatment the advice of specialists should be sought.

Welding of ferritic/martensitic stainless steels

Job Knowledge

Stainless steels are 'stainless' i.e. are corrosion resistant, due to the presence of chromium in amounts greater than 12%, where it forms a passive film on the surface of the steel. Note that these stainless steels are not the 'stainless steels' that generally first spring to mind; the 18% Cr/8% Ni austenitic stainless steels of the Type 304 or Type 316 grades; but two separate groups of alloys with different mechanical and corrosion resistant properties.

The ferritic stainless steels contain up to some 27% chromium and are used in applications where good corrosion/oxidation resistance is required but in service loads are not excessive, e.g. flue gas ducting, vehicle exhausts, road and rail vehicles.

The martensitic grades contain up to 18% chromium and have better weldability and higher strengths than the ferritic grades. They are often found in creep service and in the oil and gas industries where they have good erosion and corrosion resistance.

Now for a little metallurgy! Chromium is an alloying element that promotes the formation of ferrite in steel; in the case of the ferritic stainless steels, this ferrite is the high temperature form known as delta-ferrite. Unlike the low

alloy steels, therefore, this type of steel undergoes no phase changes as it cools from melting point down to room temperature; they cannot therefore be hardened by heat treatment and this has implications with respect to the properties of welded joints.

Carbon and nitrogen, however, are two elements that promote the formation of austenite so, as the percentage of carbon and/or nitrogen increases, the ferritic steel can be designed to transform, wholly or partially, to austenite before transforming back to ferrite. This series of phase changes are similar to those in a low alloy steel, enabling the steel to be hardened by producing martensite - the martensitic stainless steels. Compositions and typical properties of some of the alloys are given in Table 1.

Table 1 Typical properties of ferritic and martensitic steels

AISI Number

Steel Type

Chemical Composition (max %) Mechanical Properties

(annealed cond; typical)

C Mn Cr Ni Mo UTS (MPa

)

Y.S. (MPa

)

El.%

409 ferritic 0.0

8

1.0

0

10.5/11.

75

- - 480 240 25

430 ferritic 0.1

2

1.0

0

16.0/18.

0

520 345 25

434 ferritic 0.12

1.00

16.0/18.0

0.75/1.25

530 370 22

446 ferritic 0.2

0

1.5 23.0/27.

0

550 350 20

410 martensit

ic

0.1

5

1.0

0

11.5/13.

00

- - 480 310 25

420 (API 5CT

L-80)

martensitic

0.15

min

1.00

12.0/14.0

- - 650 345 25

422 (12CrMo

V)

martensitic

0.25

1.3 10.0/12.0

0.8 1.2 (V 0.4)

720 550 22

431 martensit 0.2 1.0 15.0/17. 1.25/2. 860 670 20

ic 0 0 0 5

There are a number of welding problems with the ferritic steels. Although they are not regarded as hardenable, small amounts of martensite can form, resulting in a loss of ductility. In addition, if the steel is heated to a sufficiently high temperature, very rapid grain growth can occur, also resulting in a loss of ductility and toughness.

Although the ferritic steels contain only small amounts of carbon, on rapid cooling carbide precipitation at the grain boundaries can 'sensitise' the steel making it susceptible to inter-crystalline corrosion. When this is associated with a weld it is often known as weld decay. Developments in recent years of extra low carbon, titanium or niobium containing grades have, however, improved this situation.

The ferritic stainless steels are generally welded in thin sections. Most are less than 6mm in thickness where any loss of toughness is less significant. Most of the common arc welding processes are used although it is regarded as good practice to limit heat input with these steels to minimise grain growth (1kj/mm heat input and a maximum interpass temperature of 100-120C is recommended) implying that the high deposition rate processes are inadvisable. Preheat is not required although it may be helpful when welding sections over, say, 10mm thick, where grain growth and welding restraint may result in cracking of the joint.

Welding consumables for the ferritic steels are generally of the austenitic type; type 309L (low carbon grade) is the most commonly used. This is to ensure that any dilution that occurs does not result in a low ductility austenitic/ferritic/martensitic weld metal micro-structure. However, provided care is taken to control dilution, types 308 and 316 may be used. Nickel based consumables may also be used and will result in better service performance where the component is thermally cycled. A matching filler metal is available for welding of Grade 409 steel, often used in vehicle exhaust systems.

Post weld heat treatment (PWHT) at around 620C is rarely carried out although a reduction in residual stress will give an improved fatigue performance: nickel based fillers are a better choice in this context than the Cr/Ni austenitic consumables.

The martensitic grades are used in more challenging environments and, as the name suggests, present rather more problems than the ferritic steels. Both the higher carbon (>0.1%) and low carbon (

Conventionally, when welding dissimilar metal joints the filler metal is selected to match the composition of the lower alloyed steel. Experience has shown that this can cause cold cracking problems so filler metals matching the martensitic steel should be used. An alternative is to weld with austenitic stainless steel fillers, type 309 for example, but the weld may then not match the tensile strength of the ferritic steel and this must be recognised in the design of the weld. Nickel based alloys may also be used; alloy 625 for instance, has a 0.2% proof strength of around 450MPa; and will give a better match on coefficient of thermal expansion.

The metallurgy of these types of steels is complex and they are frequently used in challenging and safety related environments. An article such as this can only give a partial picture so if there are any doubts surrounding their fabrication it is recommended that advice is sought from suitable specialists.

Welding of ferritic cryogenic steels

Job Knowledge

Ferritic cryogenic steels are nickel containing low alloy steels designed to operate safely at temperatures substantially below 0C and are characterised by good tensile properties and high impact strength at low temperatures.

The nickel content ranges from around 1.5 to 9%, although there are some fine grained carbon-manganese steels that may be operated at temperatures as low as -50C. These grades of steel are generally found in the oil and gas

and petrochemical industries where they are used for the handling and storage of liquefied petroleum gases (LPG) at temperatures down to approximately -100C and, in the case of the 9% nickel steel, down to -196C. They are also found in the gas processing industry for the production and handling of gases such as carbon dioxide and oxygen as shown in Table 1.

Table 1. Approximate minimum service temperatures and applications of the cryogenic steels

Steel Type Specification

(Plate)

Minimum

service temperature

C

Typical storage/

processing application

Fine grained Al

killed C/Mn steel

EN10028-3

P460NL2

-50 Ammonia, propane

(LPG)

1.5% Ni steel EN10028-4 15NiMn6

-60 Ammonia, propane, carbon disulphide

2.5% Ni steel ASTM A203 GrB -60 Ammonia, propane,

carbon disulphide

3.5% Ni steel ASTM A203Gr E

EN10028-4 12Ni14

-101 Carbon dioxide,

acetylene, ethane

5% Ni steel EN10028-4

X12Ni5

-130 Ethylene (LEG)

9% Ni steel ASTM A353/A553Tp1

EN10028-4 X8Ni9

-196 Methane (LNG), oxygen, argon

Austenitic stainless steel

ASTM 304L EN10088-1

1.4305

-273 Nitrogen, hydrogen, helium

The choice of which steel to use for any particular application depends not only on the temperature but also on such aspects as section thickness required by design and the possibility of stress corrosion.

The applications of these steels require that the mechanical properties, in particular the toughness, of welds and their associated heat affected zones match or are very close to those of the parent metals. The fabrication of the cryogenic steels into pipework and vessels therefore requires careful selection of welding consumables and close control of welding parameters.

Manual metallic arc (SMAW) electrodes matching the composition and Charpy-V impact strength of the fine grained carbon manganese steels at -50C can be obtained, for example, AWS A5.5 E7018-1 electrodes, although the addition of a small amount of nickel, up to 1%, will give added confidence in achieving the required toughness. Matching C/Mn composition MAG (GMAW), flux cored (FCAW) and submerged arc (SA) consumables will not give adequate toughness at -50C and require nickel to provide the required as-welded toughness.

This is generally limited to a maximum of 1%Ni to comply with the NACE International ISO15156-2/MR0175 requirement for use in sour service. For even greater confidence that acceptable Charpy-V values can be achieved and to provide an improved tolerance to procedural variations then 2.5% nickel containing consumables may be used.

The 1.5%Ni and 2.5%Ni steels may be welded with 2.5% Ni consumables and these will provide adequate toughness down to -60C in both the as-welded and post weld heat treated (PWHT) condition. A word of caution, however; the tensile strength of PWHT'd TIG and MAG weld metal may fall below the minimum specified for the parent metal MAG weld metal deposited using a shield gas with a high proportion (>20%) of CO2 appears to be particularly sensitive.

Consumables are available for the MMA and SAW welding processes but not for the TIG, MAG or FCAW processes.

For depositing TIG root passes in the 3.5 Ni alloys, a 2.5% Ni filler metal is normally used. Although the 3.5% Ni consumables are capable of providing adequate toughness at -101C they are very sensitive to variations in

welding parameters, heat input and welding position. This sensitivity results in a wide variability of impact test results so for the more demanding applications alternative, nickel based filler metals such as AWS ENiCrFe-2 orEniCrFe-3 are often used enabling all of the conventional arc welding processes to be used.

The 5% Ni and 9% Ni alloys are conventionally welded using a nickel based filler metal. 6.5% Ni MMA electrodes are available but these are not capable of consistently providing adequate toughness much below -110C. Consumables for welding the 9% Ni alloy have been developed; these typically contain 12% to 14% nickel. However, the cost of production is such that they do not compete with the nickel based alternatives.

A problem with the nickel based consumables that were initially used to weld these steels is that their tensile strength is substantially less than that of the parent metal. Higher strength fillers of the AWS EniCrMo-3 (alloy 625) type are now readily available and these enable all the arc welding processes to be used. They also match parent metals with respect to toughness and ultimate tensile strength although the 0.2% proof strength of TIG, MIG and SAW weld metals may fall below that specified for the 9%Ni steel.

As with any steel where good toughness is required, heat input must be controlled. It is recommended that interpass temperatures are limited to a maximum of 250C and ideally less than 150C for the 9%Ni alloy. Heat input from welding should be limited to approximately 3.5kj/mm for SAW and 2.5kj/mm for MMA.

Preheat may be required for the carbon-manganese and up to 3.5% Ni alloys, depending upon section thickness, joint type and restraint to reduce the risk of hydrogen cold cracking. ASME B31.3, for example, recommends minimum preheat temperatures of 79C for carbon steels greater than 25mm thick, 93C for all thicknesses of the 1.5%, 2.5% and 3.5% nickel steels but only 10C for the 5% and 9% Ni alloys. The reason for this low preheat temperature isthat these high nickel content alloys contain a large amount of austenite that can tolerate large amounts of hydrogen. This austenite therefore substantially reduces the risk of cold cracking; in addition, they are conventionally welded with nickel based alloys that reduce the risk even further.

Post weld heat treatment is not generally required for the 9%Ni steels; indeed, EN 13445-4 recommends that

PWHT should be avoided. The ASME codes, however, specify a PWHT of 552C to 585C for both 9%Ni and 5%Ni alloys when thickness exceeds 51mm (2 inches). There are also differences in PWHT requirements in the EN and the ASME specifications for the other types of low temperature steels discussed in this article as tabulated below.

Steel type EN 13445-4 ASME B31.3

Thickness

(mm)

Temperature

range (C)

Thickness

(mm)

Temperature

range (C)

FG C/Mn >35 550-600 >19 593-649

1.5%Ni >35 530-580 >19 593-635

2.5%Ni >35 530-580 >19 593-635

3.5%Ni >35 530-580 >19 593-635

5%Ni >35 530-580 51 552-585

9%Ni all none 51 552-585

Close control of the PWHT temperature is most important as nickel reduces the lower transformation temperature.

Exceeding the specified temperatures, particularly of the 3.5%Ni and above alloys, may cause the parent metal to transform, resulting in a substantial loss of tensile strength.

One significant problem that is frequently encountered with the nickel steels is that of residual magnetism causing arc blow. This is a particular problem with the 9%Ni steel which can become easily and very strongly magnetised, making it impossible to weld with the arc welding processes. Extreme care needs to be taken during handling, transportation and erection to minimise the effect. Use of alternating current during welding can help overcome some of the difficulties but it may be necessary to degauss the area surrounding the weld.

Welding of ferritic creep-resistant steels

Job Knowledge

Creep is a long term failure mechanism that, in most metals, occurs at elevated temperatures (see Job Knowledge No. 81). Creep strength in the ferritic steels is achieved by alloying with elements that will provide enhanced

strength at high temperatures. Chromium (Cr) and molybdenum (Mo) are the two principal alloying elements but vanadium (V) and niobium (Nb) may also be added.

Table 1 gives the nominal composition of the commoner creep resistant steels. In addition to the use of these steels in creep service they also have resistance to hydrogen attack and corrosion by sulphur bearing hydrocarbons. They are therefore found in power generation and the oil and gas industries.

Table 1 Nominal composition and mechanical properties of the creep resistant steels

Steel grade Composition - nominal

%

Mechanical properties - typical

C

max

Cr Mo V Nb UTS

N/mm2

0.2%

Proof N/mm2

Elongn

%

Charpy-

V J/C

min

C1/2Mo 0.3 0.5 520 320 35 27@20

//CrMoV 0.14 0.5 0.6 0.25 585 330 25

1CrMo 0.13 1.2 0.5 530 350 35 27@20

2Cr1Mo 0.13 2.25 1.0 555 350 35 27@20

5CrMo 0.13 5.0 0.5 690 475 28 27@23

9Cr1Mo 0.13 9.0 1.0 675 475 30

9CrMoVNb (9Crmod or

P91)

0.13 8.75 1.0 0.23 0.08 650 480 30 40@20

The creep resistant steels all contain strong carbide and/or nitride forming elements. These are intended to provide a fine dispersion of precipitates that both increase the tensile strength and impede the formation of the voids illustrated in Fig 1 and Fig 2 of Job Knowledge No. 81. Chromium is also added to reduce the scaling or oxidation of the steel at high temperatures. Each steel grade has a creep limit (a stress and temperature above which it should not be used) and a similar limit on oxidation resistance. The allowable temperature increases with the alloy content, enabling the more highly alloyed steels to be used up 650C.

The //CrMoVsteel is a special case. It was developed for the power generation industry in the UK and is unlikely to be encountered elsewhere but some notes have been included as it may be found in older plant scheduled for repair.

As the alloy content increases then so does the hardenability (the ability to form martensite) of the steel. CMo, CMV and 1CrMo steels form ferritic/bainitic structures, the other more highly alloyed steels forming martensite, even at relatively slow cooling rates. This should give some hint as to one of the problems encountered when welding this family of steels; that of hydrogen induced cold cracking (see Job Knowledge No. 45), since martensite is generally hard, brittle and sensitive to the presence of hydrogen. Low hydrogen welding processes are therefore essential. This includes ensuring that any shield gases are of high purity and are dry; ideally with a dew point less than 50C.

Preheat is essential for most of the alloys (the IIW carbon equivalent method is not valid for these grades of steel) and few welding specifications give much guidance regarding recommended preheat temperatures. However, ASME B31.3 and EN 1011 Part 2 both contain recommendations. Table 2 is adapted from the EN specification for processes with hydrogen limited to between five and 10mls of hydrogen in 100gms of weld metal (Scale C). It may be permissible to use lower preheats if the hydrogen content is reduced to less than 5mls/100gm; for instance when depositing a TIG root pass. This could be confirmed during welding procedure development.

Table 2 Recommended preheat and interpass temperatures

Steel Grade Thickness

(mm)

Min. Preheat

(C)

Max. Interpass

(C)

C1/2Mo 15 >1530 >30

20

75 100

250

//CrMoV All 150 300

1CrMo 15 >15

100 150

300

2Cr1Mo 15 >15

150

200

350

5CrMo All 200 350

9Cr1Mo All 200 350

9Cr1MoVNb All 200 350

11/4Cr1/2Mo power station boiler header

An additional problem that may be encountered with the creep resistant steels is that of reheat cracking (see Job Knowledge No. 48). This is a cracking mechanism that takes place, as the name suggests, during reheating of the welded joint, either when the weld is post weld heat treated (PWHT) or is put into high temperature service without PWHT.

The most sensitive grades are those containing vanadium; the //CrMoV steel being one of the most sensitive. It is so sensitive that it may be necessary to maintain the preheat and hot grind and blend the weld toes of a thick, highly restrained weld to reduce stress concentrations before immediately performing the PWHT operation.

Solutions to this problem are control of residual elements to low levels, low heat input to minimise grain growth in the HAZ and devising a welding procedure that results in the maximum amount of grain refinement in the HAZ. Rapid heating through the temperature range 350 - 600C at which the steel is most sensitive can also help. This approach must be treated with some caution as too rapid a temperature rise can cause unacceptable stresses and distortion and may violate code requirements.

Most of the creep resistant steels require PWHT; mandatory in all of the application codes. This is to ensure that

the hard microstructures formed during welding are softened and toughness improved. It is also necessary to heat treat the weld and HAZs to ensure that the precipitates, required to give best creep performance, are of the correct size and distribution. PWHT temperatures and soak times must therefore be closely controlled to develop the required mechanical properties. Typical temperatures and times are given in Table 3. These figures are typical only and it is important that the item is heat treated precisely in accordance with the relevant application code; ASME VIII, BS PD5500, EN 13445 etc.

Table 3 Typical PWHT temperatures and times

Steel Grade Temperature

Range (C)

Soak Time

(hours)

C1/2Mo 630 - 670 1 per 25mm

//CrMoV 650 - 680 1 per 25mm

1CrMo 650 -700 1 per 25mm

2Cr1Mo 680 - 720 2 min

5CrMo 710 - 750 2 min

9Cr1Mo 730 - 760 2 min

9Cr1MoVNb 730 - 760 2 min

The PWHT temperature of the 1CrMo and the 2Cr1Mo steels are sometimes changed from the ranges given in Table 3 in order to develop specific properties; see for example Table 4.4.1 in BS PD5500.

The 9CrMoVNb steel is particularly sensitive to PWHT times and temperatures and great care must be exercised when PWHT'ing this particular grade of steel.

Any alloy containing more than 2% chromium will need to be bore purged with an inert gas such as argon when depositing a TIG root pass. Exposure of the molten weld pool to the atmosphere results in some of the chromium boiling off giving rise to a porous or 'coked' bead on the reverse side of the weld. This adversely affects both mechanical properties and corrosion resistance.

Welding consumables matching the parent metal composition are readily available for all of these steels for most of the welding processes. An exception to this is the //CrMoV steel which is conventionally welded with a 2Cr1Mo filler. Dissimilar metal joints made between components from within this group of ferritic steels or with the carbon manganese steels are usually welded using a filler metal that matches the less highly alloyed steel. PWHT temperature for the dissimilar metal joints can be a problem and tends to be a compromise between overtempering the lower alloyed steel and undertempering the more highly alloyed metal.

Welding of HSLA Steels

Job Knowledge

The development and use of high strength low alloy (HSLA) steels has been driven by the need to reduce costs, the higher strength compared with a conventional carbon-manganese steel enabling thinner and lighter structures to be erected. The majority of these steels are to be found in structural applications; offshore structures, yellow

goods, buildings, shipbuilding etc. Tensile strengths of up to 690MPa are achievable whilst still maintaining good weldability and high notch toughness, often better than 50J at -60C.

There are two methods by which both high tensile strength and toughness is achieved - by micro-alloying, adding small amounts of strong carbide and nitride formers and by very careful control of the rolling temperature - controlled rolling or thermo-mechanically controlled processing (TMCP steels).

The highest strengths are achieved by a combination of the two methods. The aim of both methods is to produce as small a grain size as possible, fine grain giving the best notch toughness and each halving of the grain diameter producing a 50% increase in tensile strength.

Improved weldability is an additional objective and this is achieved by reducing the hardenability of the steel, the carbon content of some steels being lower than 0.05%C, and reducing undesirable elements such as sulphur and phosphorous to as low a level as possible.

To compensate for the loss of carbon and to increase tensile strength small additions of alloying elements such as niobium (

without undue loss of strength. For a definitive statement on heat input control the advice of the steel manufacturer should be sought.

These steels must under no circumstances be normalised or tempered although post weld heat treatment (PWHT) is often a requirement when the component thickness is greater than some 35 to 40mm. Care needs to be taken if PWHT is applied that the soak temperature does not exceed 600C; a temperature range of 550C to 600C is often specified. The reason for this is that many of the TMCP steels are accelerated cooled to a temperature of around 620C; heat treating at or close to this temperature will result in a substantial reduction in tensile strength due to over-tempering. The same restriction applies to any hot working activity - plate must not be hot rolled and the temperature of local heating for correction of distortion must not be allowed to exceed 600C.

Further advice on the welding of these steels can be found in the trade literature and in the specification EN 1011 Part 2 Welding - Recommendations for welding of metallic materials: Arc welding of ferritic steels.