Duplex Stainless Steel - Welding-twi

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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 delta­ferrite. They are designed to provide better corrosion resistance, partlcularly chlor1de stress corrosion and chloride pitting corrosion, and higher strength than standard austenltlc stalnless 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 nlckel, up to 9% and 0.05 - 0.5% nitrogen. Both the low nlckel content and the high strength (enabling thinner sections to be used) give significant cost benefits. They are therefore used extenslvely In the offshore oll and gas Industry for pipework systems, manifolds, risers, etc and in the petrochemical industry in the form of pipelines and pressure vessels.

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In addition to the Improved corrosion resistance compared with the 300 series stalnless steels duplex steels also have higher strength. For example, a Type 304 stalnless 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 stalnless steels are hlghly 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 315ac, other codes specify even lower service temperatures, perhaps as low as 250°C 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 slmple 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, whllst the lean grades typlcally have lower nickel and hence lower pr1ce.

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, UN5 531803 has wldely become known as 2205 regardless of Its suppller, 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 UN5 numbers, 531803 and 532205, with S32205 being a more recent and controlled composition.

Typical compositions and proof strengths of common duplex stainless steels

0.2%

Common BSEN Steel ~pical Chemical Composition O/e proof

Name UNSNo

No Type strength

O/DC Cr Ni Mo N Cu N/mm2

(min) 2304 532304 1.4362 duplex 0.015 23.0 4.0 0.055 0.13 400 2205 531803 1.4462 duplex 0.015 22.0 5.5 3.0 0.14 - 450 2205 532205 1.4462 duplex 0.015 22.S 5.5 3.3 0.17 450 255(UR52N) 532520 1.4507 super duplex 0.015 25.0 7.0 3-5 0.28 0.13 550 2507 532750 1,4410 suDer duDlex 0.015 25.0 7.0 4.5 0.28 0.3 550 Zeran 100 532760 1.4501 suDer duDlex 0.015 25.0 7.0 3.5 0.25 0.8 550 Sandvik SAF3207 533207 - hvDer duDle>e 10.03 31 7.5 4.0 0.50 0.75 700 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 no1 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 O.SkJ/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 475°C embrittlement. Sigma and chi phases form at temperatures between 550 and 1000°C with the fastest rate of formation around 850°C. 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 - 550°C 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.SkJ/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 5°C or there is condensation on the surface. In these situations a preheat of around 50 - 75°C should be adequate. Very thick section joints, particularly those welded with the submerged arc process, can also benefit from a low preheat of around 100°C.

Interpass temperature can have a significant effect on the microstructure of the weld and its heat affected zones. For a duplex steel 250°C is regarded as an acceptable maximum and for a superduplex 150°C maximum. Note, however, that many codes do not separate the grades into duplex and superduplex and 150°C 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.

Part 2

This article was written by Gene Mathers.

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Duplex stainless steel - Part 2 Job Knowledge Part 1

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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 undeslrable 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 weld Ing 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 weldlng Is not allowed to take place outside the llmlts of the quallfled 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. Cleanllness of the joint Is therefore stlll Important. Machining or gr1ndlng 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, whllst unusual, Is not unknown and can occur In the ferr1te 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 weldlng gives very clean weld metal with good strength and toughness. Mechanisation has substantlally 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 austenlte former, Is an Important alloylng element, partlcularly 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 wlll, 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 till passes pure argon is generally used although small amounts of nitrogen may be added and pure nitrogen has occaslonally been used.

TlG welding may be perfomied 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 addltlonal 2 to 4% nlckel to ensure that sufficient austenlte Is formed. Any stray arc str1kes wlll 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 Smm 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 weldlng 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 appllcatlons. The flux core Is generally rutlle; the shleldlng gas co2, argon/20%C02 or argon/2%02• 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 avallable that require no slag removal; better suited to mechanised appllcadons than flux-cored wires. Because of differences In flux formulatlon and wire composldon between manufacturers It Is recommended that procedure quallflcadon Is carried out using the specific make of wire used In production even though the wires may fall within the same speclflcadon classlflcatlon.

Submerged arc weldlng (SAW) Is generally confined to weldlng thick wall pipes and pressure vessels. Solld wires, slmllar to those avallable for TIG weldlng, are avallable. Fluxes are generally acld-rutlle or basic, the latter giving the best toughness values In the weld metal. As with any continuous mechanised weldlng process the lnterpass temperature can rapldly Increase and care needs to be taken to control both lnterpass temperature and process heat Input. Because of the need to control heat Input the wire diameter Is normally llmlted to 3.2mm permltdng a maximum weldlng current of SOOA at 32V although larger diameter wires are avallable. However, any productivity gains from the use of a large diameter wire and high weldlng current may not be reallsed due to the need for lnterpass coollng.

There Is often the need to weld duplex/superduplex steel to lower alloyed fen1tlc steel, a 300 series stalnless steel or a dlsslmllar grade of duplex steel. The 300 series stalnless steels are generally welded to duplex steels with a 309Mol (23Cr/13Nl/2.5Mo) flller metal. Low carbon and low alloy steels may be welded to duplex steels using either a 309L (23Cr/13NI) or a 309Mol tlller metal.

These two flller metals, however, have yleld and ultlmate tenslle strengths substantlally 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 weldlng engineer has to select a flller metal that both matches the strength of the weaker steel and Is compatlble with the two parent metals. These considerations narTOw the chola to one of the nlckel-based alloys such as alloy 82 or, for higher strength, a niobium-free high alloyed nlckel flller, 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 falllng out of favour.

Duplex steel welds are seldom post-weld heat treated. Due to sigma phase formadon they cannot be given a heat treatment at the low temperatures of 600-7006 C, the normal range for stress rellef unless a quallflcatlon programme has been undertaken to demonstrate that the loss of toughness Is acceptable. If PWKT Is required then Ideally the whole component must be given a solutlon anneal at 1000-11006 C followed by a water quench; an lmpract:lcal operation with most welded structures.

Lastly, any process that heats the steels above 300°C wlll affect the mechanlcal properties. Heat straightening to control distortion should therefore not be can1ed out. lhe HAZs produced by hot cutdng processes llke plasma or lase1 may contain undeslrable mlcrostructures. Cut edges that wlll 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 com>slon resistance.

If the cut edges are welded after cutting then the HAZs are generally sufflclendy 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.

lhls artlde was written by Gene lfldlten