Postweld heat treatment to avoid intergranular stress corrosion cracking of supermartensitic stainless steels

P Woollin

Paper presented at Stainless Steel World 2005 Conference, Maastricht, The Netherlands, 8-10 November 2005.

Abstract

Supermartensitic stainless steels (SMSS) are attractive materials for flowlines transporting produced fluids with high levels of CO ₂ and low levels of H ₂ S. However, recent cracking of lean grade material in service and both lean and high-alloy grades during qualification testing have revealed sensitivity to intergranular stress corrosion cracking (IGSCC) at somegirth welds ^[1-11] , although all flowlines in high alloy SMSS have apparently had no such problems in service. One potential solution is to use a brief postweld heat treatment, typically at around 630-650°C for five minutes, which has beenshown to overcome susceptibility to IGSCC in laboratory tests. ^[4-11]

The paper considers existing information on the effects of brief PWHT on welded SMSS, presents additional data for a range of pipes and weld types and discusses the likely mechanism by which PWHT may be effective in preventingIGSCC. It is concluded that a microstructural effect is probably dominant. Based on this preliminary conclusion and a consideration of the potential detrimental effects of an inappropriate PWHT cycle, the necessary control of the PWHTprocess is addressed and recommendations are made with respect to application of PWHT, highlighting best practice based on current knowledge.

1. Introduction

Intergranular SCC of SMSS pipe girth welds represents an obstacle to the exploitation of these materials in flowlines for some applications, although they are still being used extensively by some operators. It is recognised that areliable way to prevent susceptibility to IGSCC of as-welded SMSS via control of welding parameters and without PWHT may still take some time to develop, assuming that it is possible. However, experimental evidence to date suggeststhat the use of brief PWHT at around 650°C will eliminate sensitivity to IGSCC. ^[4-11] No examples of IGSCC have been reported after PWHT at around 650°C for 5 minutes. Such PWHT is therefore an attractive interim solution to the problem, albeit one that will add to the cost of producing welded fabrications.Nevertheless some flowlines are operating successfully without PWHT, although the ability to do this will probably depend on the operating environment. It is noted that welded SMSS is also susceptible to cracking in sour environmentsbut this is by a different mechanism to the IGSCC discussed here and PWHT does not prevent cracking in sour environments, although it may improve resistance.

Several authors have previously examined the effects of PWHT on the properties of SMSS welds, ^[12,13] although only more recently has its effect on sensitivity to IGSCC been explored. ^[4-11] The range of PWHT treatments studied on actual welds is from 600-700°C for 3-5 minutes, ^[6-11] although simulated HAZ studies have suggested that a wider range of thermal cycles, from 550-700°C for 1 to 17 minutes may also be effective at eliminating sensitivity to IGSCC. ^[5] However, it should be noted that the heat treatment required to eliminate sensitivity to IGSCC will presumably depend its severity and the composition of the steel, notably C and perhaps N content, and the levels of othercarbide/nitride forming elements such as Ti, Nb and Mo.

When specifying a PWHT cycle in practice, it is essential that it should not only provide acceptable IGSCC properties but other mechanical and corrosion properties must also be acceptable after PWHT. Therefore this study alsoexamines effects of PWHT on microstructure, hardness and toughness.

2. Experimental work

2.1 Materials

Six low carbon martensitic stainless steel pipes with 10.9-13.5%Cr were selected, all of which could broadly be considered as 'supermartensitic' but not all representing currently commercially available grades, Table 1. Two of the steels were variants of the same grade, with very similar composition (C1 and C2). Nickel content varied from 1.55-6.4% and Mo ranged from 0-2.5% for the steels examined.

Table 1 Chemical analyses of the materials used

2.2 Welding

Three types of girth weld were examined, (i) three automatic pulsed MIG welds made with superduplex solid filler wire throughout (W1-W3), (ii) two automatic pulsed MIG welds made with approximately matching composition metal coredfiller wires (W4 and W5) and (iii) two manual welds made using the TIG process for the root and second pass and the MMA process for the fill and cap passes, using superduplex consumables (W6 and W7).

Table 2 lists the welding consumable analyses, which were either direct analyses of the solid wires or were from all-weld metal pads deposited using the coated electrodes and the metal cored wires. Analyses were obtained byOES and inert gas fusion for O and N.

Table 2 Welding consumable analyses

Table 3 summarises the welding matrix. All welding was in the 5G position (pipe horizontal, fixed). For the automatic pulsed MIG welding, a copper backing shoe and Ar backing gas were used and interpass temperature wasrestricted to <150°C, whilst pre-heat was just sufficient to remove moisture. Travel speed was in the range 350-500mm/min and heat input approximately 0.5kJ/mm. An Ar/He/CO ₂ /N ₂ shielding gas mixture was used for welding with the superduplex wires and Ar+0.5%CO ₂ was used for the matching composition wires. A narrow gap J preparation was used. For the manual welding, interpass temperature was again <150°C, the heat input for the root and second pass was1.1-1.5kJ/mm and for the fill and cap passes it was 0.5-1.4kJ/mm. Argon shielding gas was used for TIG welding and Ar back purge gas was used throughout. A 30° bevel was used with no root face and a 4mm root gap.

Table 3 Girth welding matrix

2.3 PWHT

Examples of each weld type were subjected to brief PWHT. For the pulsed MIG welds W1-W5, the PWHT was applied on the whole weld by induction heating, whilst pieces from welds W6 and W7 were heat treated in a furnace. The specifiedheat treatment cycle was rapid heating to 650°C, followed by holding for 5 minutes and air cooling. A volume of metal around 40mm wide, including the weld metal and approximately 15-20mm of pipe either side of the root and 10-15mmeither side of the weld cap was heated by the induction coil. Temperature was controlled via thermocouples on the weld metal cap and measurements were made also on the root. Heating was fairly rapid to 600°C and then temperaturerose to 650°C over about two minutes. During the five minute hold period, the cap temperature remained between 640 and 657°C. The root temperature was typically 15-35°C less than the cap temperature, i.e. 620-640°C,depending on the pipe wall thickness. For furnace heat treatment, heating was fairly slow taking about 10 minutes to reach 650°C. Temperature was again monitored by thermocouples, this time on the root weld metal. The welds weregiven a 'P' designation after PWHT.

2.4 Weld characterisation

Sections were taken through the welds for microstructural examination and Vickers hardness measurement (HV10) in the weld metal and HAZ both before and after PWHT.

2.5 Toughness testing

Charpy V-notch impact tests were performed on through-thickness notched specimens from weld W7 (12Cr6Ni2.5MoTi pipe welded with superduplex consumables) before and after PWHT, with the notch on the weld metal centreline or at thefusionline mid-thickness position. Tests were performed over the temperature range -80 to +40°C.

In addition, fracture toughness tests were performed to BS7448 part 1 at -20°C on Bx2B (11.5x23mm) specimens from weld W7 both before and after PWHT, both given 1% local compression to reduce the effects of residual stress.Specimens were through-thickness notched on the weld metal centreline or on the fusion line mid-thickness position. A loading rate of 0.4mm/min was employed.

2.6 Corrosion testing

Four point bend tests were performed in two environments (i.e. 25%NaCl solutions acidified to calculated pH = 3.3 and 4.5 respectively, Table 4) on 100x15x3mm specimens from each of the girth weld types W1-W6, in both as-welded and PWHT conditions. Two specimen types were examined (i) with the root machined flush and ground to a 600 grit finish and (ii) withthe root in the as-welded condition. Both specimen types had strain gauges applied (i) on the test face for flush ground specimens and (ii) on the non-test face for specimens with the profile intact. Specimens were deflected to give astrain equivalent to 100% of the parent material 0.2% proof stress in the HAZ. After deflection the specimens were placed in a nitrogen-blanketed autoclave filled with deoxygenated test solution. The vessel was then heated to testtemperature and finally pressurised with the test gas. Test exposure was for 30 days. After test, specimens were examined visually under a binocular microscope, photographed and, if cracking was not observed, they were sectionedtransverse to the weld at the mid-width position, to look for small cracks.

Table 4 Corrosion test environments

3. Results

3.1 Weld characterisation

The HAZs were generally visibly tempered by PWHT, i.e. showed slightly greater etching response, particularly for the lean grades D and E, but no other microstructural changes were observed optically. In some areas, precipitation onHAZ prior austenite boundaries in steel C1 was visible at high magnification after PWHT. Under a light microscope, this leads to a clear definition of the HAZ prior austenite grain boundaries in the high temperature HAZ within about150µm of the fusionline, Fig.1. The superduplex weld metals showed evidence of precipitation of very fine secondary austenite after PWHT but no intermetallic phases were observed, Fig.2.

4.3 Avoiding potential detrimental effects of PWHT

In order for a PWHT cycle to be successfully applied to a SMSS weld, in addition to eliminating sensitivity to IGSCC, it must also be such that it does not have any significantly detrimental effects on other weld properties.

One undesirable effect of PWHT on the HAZ would be associated with heating to a temperature such that an excessive amount of austenite re-forms, leading to formation of un-tempered martensite on subsequent cooling. Un-temperedmartensite has high hardness and low toughness in conventional martensitic stainless steels, which have carbon contents in excess of 0.03%, although for the low carbon SMSS grades, these effects are not pronounced and may not besignificant. Examination of the effect of PWHT in simulated HAZs showed that 650°C was typically the temperature giving most hardness reduction of the steels studied but also showed substantial variation in response between SMSSgrades, with some giving more hardness reduction at 625°C. ^[14] This indicates the importance of choosing PWHT for the specific steel in question, although broadly similar behaviour is expected for all SMSS grades based on the data obtained here. The reformed, stable austenite content wasgenerally found to increase on tempering at 600-650°C, indicating that Ac ₁ was exceeded over this range, hence some virgin martensite formation is possible if the upper temperature during PWHT is above this range. With induction heating, a temperature gradient develops through the pipewall thickness, with the outside being hotter than the inside. For wall thicknesses of 11-18mm, induction PWHT trials indicated that the root was typically 15-35°C cooler than the cap. The greatest risk of un-tempered martensiteformation and associated hardening is therefore in the weld cap, whilst it is essential for eliminating sensitivity to IGSCC in the internal environment that the temperature at the root is controlled. This requires that both root andcap temperatures are held within an acceptable range during PWHT. The limiting upper temperature will vary from grade to grade but based on the current data, which only extends to a cap temperature of up to 660°C, it isrecommended that temperatures in excess of 660°C should be avoided. Further work is required to explore the suitability of PWHT temperatures exceeding 660°C.

Another potential detrimental effect of PWHT is that it will tend to increase oxidation of the weld area. Oxidation during welding has been demonstrated to have a detrimental influence on the pitting resistance of SMSS HAZs inmildly sour media ^[16] and hence any further oxidation from PWHT might also be detrimental. However, published work has indicated that PWHT at 650°C may be beneficial for service under mildly sour conditions, ^[17] presumably by lowering hardness, but it does not give immunity to cracking in sour media. Further work is required to explore this issue, although use of an inert gas shield during PWHT would eliminate the concern.

Detrimental microstructural effects at the edge of the PWHT zone, where intermediate temperatures will be experienced, are not anticipated, provided that the whole of the weld HAZ is heat treated, i.e. that the intermediatetemperatures are experienced by parent steel. This assumes that the parent steel will have been tempered such that the carbon content in solution is very low. Detrimental microstructural effects in the HAZ and weld metal are of greaterconcern. These may include precipitation of (i) further carbides, e.g. on prior austenite boundaries or within or on the interface of any delta ferrite retained in the HAZ and (ii) intermetallic phase, secondary austenite or alphaprime phase in the delta ferrite in weld metal deposited with a duplex or superduplex consumable. These precipitation reactions may act to lower corrosion resistance and toughness in the weld metal or HAZ very close to the fusion line,although the present study showed that the toughness effects are not significant for a high grade SMSS HAZ and superduplex weld metal subject to PWHT at 650°C for 5 minutes. To avoid loss of toughness, it is recommended that thePWHT duration should not be substantially longer than 5 minutes whilst recognising that longer PWHT may still give acceptable results for many applications. Substantially shorter PWHT periods are not recommended due to the absence ofdata. Sensitisation is not expected provided that the whole of the HAZ sees the intended PWHT temperature. No loss of corrosion resistance associated with precipitation on delta ferrite in SMSS HAZs has been noted to date, although onereference cites it as an issue for conventional 13%Cr 4%Ni steels, but does not indicate the precise temperature range of concern, although it does state that tempering at around 600°C gives good corrosion resistance, ^[15] and hence problems are only likely to occur below this. Based on the results of the present work, a suitable lower temperature limit of 620°C is suggested for the HAZ.

Although some precipitation occurred in superduplex weld metal during PWHT, this was apparently restricted to the formation of secondary austenite. Secondary austenite tends to reduce corrosion resistance but this should not be aproblem when welding SMSS. This implies that, although PWHT of superduplex weld metal is not normally considered advisable, in this case 5 minutes PWHT at 650°C does not seem to be detrimental. If longer PWHT times or highertemperatures were used, some loss of toughness in superduplex weld metal might occur, although this was not studied here.

5. Conclusions

1. The sensitisation of lean grade SMSS HAZs has been linked to the formation of Cr-carbides on prior-austenite grain boundaries and adjacent Cr-depleted zones but this link has not been established for the high alloy grades. Formation of Cr-depleted zones on prior-austenite boundaries immediately underneath the welding oxide has been observed in high alloy grades. Hence some uncertainty remains over the mechanism of IGSCC of high grade supermartensitic stainless steel and the effect of PWHT. Nevertheless, there is a substantial body of information supporting a consistent beneficial effect of brief PWHT for a broad range of supermartensitic grades.
   
   
2. It is recommended that PWHT should be applied to welds in supermartensitic stainless steel where there is a risk of intergranular SCC in service, i.e. in hot acidic environments. A PWHT temperature of 620-650°C at the root is recommended and the heat treated zone should encompass the whole of the weld metal and HAZ. The maximum allowable cap temperature has not been established but the current work extended up to 660°C. Heating and cooling should be fairly rapid. The most appropriate PWHT duration has not been established but there is fairly common agreement that 5 minutes is an appropriate duration.
   
   
3. Whilst the beneficial effect of PWHT with respect to IGSCC has been demonstrated for 30 day exposure tests, longer term data are required to confirm the applicability of the effect to long term service.
   
   
4. Due to the limited information available, the use of welded supermartensitic stainless steel in the PWHT condition will require qualification on a case by case basis. The qualification programme should consider the effects of PWHT on toughness and sour service performance, in addition to IGSCC. The qualification process should consider the extremes of the range of PWHT thermal cycles that may be experienced, as the acceptable range has not been established.
   
   
5. No substantial change in toughness of superduplex weld metal was observed for PWHT at 650°C for 5 minutes, although secondary austenite was formed. A small reduction of root HAZ hardness was generally associated with PWHT.
   
   
6. Postweld heat treatment may also have detrimental effects if not adequately controlled, e.g. (i) thickening of weld area oxides and associated loss of general/pitting corrosion resistance, (ii) formation of virgin martensite in the HAZ and increased hardness leading to reduced toughness and resistance to sour environments, (iii) loss of toughness in superduplex stainless steel weld metal, (iv) tempering of HAZ at temperatures that could induce sensitisation to intergranular SCC if the heat treated area is not wide enough.
   
   
7. The precise response to PWHT is specific to each individual grade of supermartensitic steel, although the data indicate that all steels examined here were fairly similar and the beneficial effect of 5 minutes at 620-650°C, with respect to IGSCC, is applicable to 'lean' grades, with <1%Mo and 'high' grades with >2%Mo both with and without Ti addition.

Acknowledgements

The following companies, who sponsored the work, are thanked for their permission to publish the results: Advantica, BP, Chevron, ENI, ESAB, Industeel, JFE Steel, J Ray McDermott, Lincoln Electric, Nickel Institute, Serimer Dasa,Shell, Sumitomo Metal Industries, Tenaris and Total.

7. References

1. Rogne T, Lange H I, Svenning M, Aldstedt S, Solberg J K, Ladanova E, Olsen S, Howard R and Leturno R E: 'Intergranular corrosion/cracking of weldable 13%Cr steel at elevated temperature', Proc conf Corrosion 2002, USA, NACE International, paper 02428.
   
   
2. van Gestel W: 'Girth weld failures in 13Cr sweet wet gas flow lines', Proc conf Corrosion 2004, USA, NACE International, paper 04141.
   
   
3. Ladanova E and Solberg J K: 'Transmission electron microscopy investigation of precipitation reactions in coarse-grained heat affected zone in two 13%Cr supermartensitic stainless steels', Proc Conf, Supermartensitic Stainless Steels 2002, Belgium, KCI publishing, pp205-214.
   
   
4. Coudreuse L, Ligier V, Lojewski Ch, Toussaint P, 'Environmental induced cracking (SSC and SCC) in supermartensitic steels (SMSS), Proc Conf, Supermartensitic Stainless Steels 2002, Belgium, KCI Publishing, pp163-172.
   
   
5. Amaya H, Kondo K, Taniyama A, Sagara M, Ogawa K, Murase T, Hirata H, Takabe H and Ueda, M: 'Stress corrosion cracking sensitivity of supermartensitic stainless steels in high chloride concentration environment', Proc conf Corrosion 2004, USA, NACE International, paper 04124.
   
   
6. Ladanova E, Solberg J K and Rogne T: 'Investigation of the precipitation mechanism in HAZ of supermartensitic stainless steels. Two-pass weld simulation'. Proc conf Eurocorr '04, Nice, France.
   
   
7. Hashizume S and Ono T: 'Laboratory results of cracking behaviour of weldable 13%Cr steels girth weld joints in hot acid environments', Proc conf Eurocorr '04 , Nice, France.
   
   
8. Hoerner B, Bayle B, Ligier V and Delafosse D: 'Stress corrosion cracking of welded joints of supermartensitic stainless steel in H2S free environment', Proc conf Eurocorr '04, Nice, France.
   
   
9. Miyata Y, Kimura M, Nakamichi H, Sato K, Itakura N and Masamura K: 'Mechanism of intergranular stress corrosion cracking in HAZ for supermartensitic stainless steel', Proc conf Eurocorr '04, Nice, France.
   
   
10. Enerhaug J, Olsen S, Rorvik G, Mikkelsen A, Rogne T Rostadsand P A, Haabrekke T: 'Robustness of supermartensitic stainless steel girth welds - experiences from the Kristin Field Development Project', Proc conf Corrosion 2005, USA, NACE International, paper 05097.
    
    
11. Hashizume S and Ono T: 'Effect of PWHT temperature on SCC behaviour of low C-13%Cr steels weld joints in hot acidic production environments', Proc conf Corrosion 2005, USA, NACE International, paper 05094.
    
    
12. Karlsson L, Rigdal S, Dhooge A, Deleu E, Goldschmitz M and Broek K van den: 'Mechanical properties and ageing response of supermartensitic (stainless steel) weld metals', Proc conf Stainless Steel World 2001, The Netherlands, KCI publishing, pp47-59.
    
    
13. Dhooge A, Deleu E and Holy A: 'Weld simulation testing of different supermartensitic stainless steel grades', Proc Conf Supermartensitic Stainless Steels 2002, Belgium, KCI Publishing, pp225-232.
    
    
14. Woollin P and Carrouge D: 'Heat affected zone microstructure in supermartensitic stainless steels', Proc Conf Supermartensitic Stainless Steels 2002, Belgium, KCI Publishing, pp199-204.
    
    
15. Folkhard E, 'Welding metallurgy of stainless steels', Springer Verlag, 1988.
    
    
16. Olsen S and Enerhaug J: 'Common pitfalls during SSC (sulphide stress cracking) and pitting testing of supermartensitic stainless steels for use in pipelines', Proc conf Corrosion 2002, USA, NACE International, paper 02038.
    
    
17. Enerhaug J, Kvaale P E, Bjordal M, Drugli J M, Rogne T: 'Qualification of welded super 13%Cr martensitic stainless steels for the Asgaard Field, Proc conf Corrosion 99, USA, NACE International, Paper 587.
    
    
18. Gregori A, Woollin P, van Gestel W: 'Analysis of in-service failures in girth-welded supermartensitic pipes', Proc conf Stainless Steel World 2003, Netherlands, KCI Publishing, p195.
    
    
19. Smithells Metals Reference Book, 6th Edition, Brandes, 1983.