Understanding and avoiding intergranular stress corrosion cracking of welded supermartensitic stainless steel

Paul Woollin

Paper presented at Corrosion 2007, Paper 07094, Nashville, Tennessee, 11-15 March 2007.

Abstract

Supermartensitic stainless steels (SMSS), including both lean grades (without molybdenum) and high grades (with 2% molybdenum), have shown sensitivity to intergranular corrosion or stress corrosion cracking (IGSCC) in weld HAZsduring laboratory testing in chloride solutions acidified with CO ₂ at above about 100°C. ^[1-9] Also, lean SMSS grades have cracked in service when exposed to hot acidic brines. ^[4,6] This paper describes a Joint Industry Project designed to test existing understanding of the mechanism of sensitisation to IGSCC of welded SMSS. The project also sought to determine whether IGSCC could be reliably avoided viacontrol of the welding process or by brief PWHT. Existing data were reviewed and two series of welding trials and corrosion tests were undertaken. The review and first series of tests allowed a hypothesis of the mechanism ofsensitisation in SMSS to be developed, which was then tested via the second series of tests. Several SMSS grades were examined. Welds were made with a range of welding thermal cycles, by varying the heat input and interpass temperatureand by use of active water cooling. Some welds were given PWHT at 650°C for 5 minutes. Stressed bend specimens from the welds were tested in 25% NaCl solution acidified with 10bar CO ₂ at 110°C. Some tests included a small addition of HCl, to acidify the solution to pH 3.3 prior to introduction of CO ₂ . From the results of the tests and measurements of the weld thermal cycles, it was concluded that a complete understanding of the mechanism of IGSCC in supermartensitic steels does not yet exist, although themain contributing factors are known. Consequently, where the service environment is sufficiently aggressive, the risk of IGSCC cannot be entirely eliminated by control of the welding process alone, although brief PWHT offers apractical means of avoiding IGSCC.

Introduction

A review of published information indicated that many but not all girth welds in SMSS, in the as-welded condition, are susceptible to stress corrosion cracking in hot acidic brines, by an intergranular mechanism following prioraustenite grain boundaries. ^[1-6] The crack appearance suggests that sensitisation occurs at specific HAZ locations rather than throughout the HAZ. Specimens with a machined root or ones given brief PWHT at 650°C for 5 minutes were not found to besusceptible to cracking. ^[7] No clear links to HAZ hardness, HAZ microstructure or composition were found for the alloys tested and it was not clear how widely applicable the beneficial effect of PWHT was.

The most commonly observed macroscopic cracking mechanism follows prior austenite boundaries, therefore a hypothesis was developed which proposed that carbide precipitation on these boundaries provides the main mechanism forsensitisation. At least two thermal cycles are required, with the first thermal cycle forming fresh martensite and the second and subsequent cycles forming the damaging carbides.

Therefore, a series of welds was made with different heat inputs, and corrosion tests performed, to see if a distinction could be made between welds that were susceptible to IGSCC and those which were not, on the basis of weldingprocedure. Following this, a second series of welds was produced to further explore which aspects of welding procedure were critical with respect to sensitisation to IGSCC.

Phase I experimental programme

A series of GTA welds was produced, with a range of heat input, using Ar shielding and back purge gas, in three steels: a lean grade without Ti addition (steel C: 11Cr1.5NiCu), a high grade without Ti addition (steel D: 12Cr6Ni2Mo)and a high grade with Ti addition (steel A: 12Cr6.5Ni2.5MoTi), Table 1. Superduplex (Zeron 100X) and 22%Cr duplex (ER2209) filler wires were used for the high grade and lean grade steels respectively. For experimental convenience, fairly small pieces of steel ~150 x 200mm with thicknessesof 11-18mm, were used and single, double and multi-pass welds were produced. A maximum interpass temperature (IPT) of 150°C was specified to reflect typical industrial practice. The HAZ thermal cycles were recorded bythermocouples attached to the root surface. Details of the welding procedures used are given in Table 2a.

Table 1 Chemical composition of pipes used.

Table 2a Welding parameters used to prepare the mechanised GTA welds. No PWHT was used.

Table 2b STT GMA/PGMA girth welding parameters. Parts of welds NWA1, 2, 3 and NWB1, 2, 3 were given PWHT at 650°C for 5 minutes, and subsequently identified with a letter 'P'.

Practical implications

The likelihood of intergranular SCC occurring in as-welded supermartensitic steel in service will depend on a combination of the degree of sensitisation of the HAZ and the severity of the environment. Very aggressive environmentswill tend to cause general corrosion rather than intergranular SCC and more benign environments may cause neither.

One consequence of the proposed mechanism of sensitisation of prior austenite boundaries is that although higher heat input appears to have been beneficial, particularly for high grade steel, as it has apparently allowed 'healing',this would not apply if the heat input of any pass was sufficiently high to re-transform the root surface to austenite, and hence form fresh martensite on cooling. In this latter case, the material would then have been returned to acondition in which it may be sensitised by subsequent passes. Significant variation of heat input between passes may therefore contribute to sensitisation. For this reason, manual welding probably cannot be relied upon to give weldsthat are reliably free from sensitisation to IGSCC. In this respect, it is noted that when lean grade pipe failed by IGSCC in service, it typically coincided with the side of the weld on which the final capping pass was applied. ^[4] The lean grade steel used here was taken from such a failed pipe and it was found that heating of the HAZ to >Ac ₁ , would have occurred for several passes. The capping pass probably reheated material into the critical range for sensitisation.

In principle, it should be possible to select welding parameters that produce welds that are not sensitive to IGSCC, based on a knowledge of the critical thermal cycles that must be avoided. This work has hinted at the damagingthermal cycles, and published data for simulated HAZ microstructures give additional values of time and temperature for developing sensitivity to IGSCC. ^[11] However, the wide range of thermal cycles experienced in a multi-pass weld HAZ and the difficulty of accurately measuring or predicting them, imply that reliably avoiding IGSCC by quantitative means will be very difficult, ifnot impossible. An empirical approach, based on trying to avoid rapid cooling of the root during second and subsequent passes, superficially offers the best chance of producing as-welded welds that are not sensitive to IGSCC althoughsensitisation of the ferrite seems to occur for longer thermal cycles. However, even if such welds were produced and qualification corrosion tests showed acceptable performance, the lack of understanding of the range of applicabilityof the result, ie what range of heat input and interpass temperature would be acceptable in production welding, make this approach unreliable. Manual welding would presumably be more variable and have less chance of reliably producingacceptable welds than mechanised welding.

Surface condition and weld geometry probably contribute to susceptibility to intergranular corrosion/stress corrosion but there are no data to confirm this conclusively at present. In practice, efforts to achieve low oxidationlevels and a smooth weld toe are considered prudent, to reduce the likelihood of initiating IGSCC.

It is recommended that PWHT should be applied to welds in supermartensitic stainless steel where there is a risk of intergranular SCC in service, ie in some hot acidic environments, provided that an appropriate qualification processhas been undertaken. It may be noted that the environmental conditions required for IGSCC of lean and high grade steels have not been defined. Due to the limited information available, the use of welded supermartensitic stainless steelin the PWHT condition will require qualification on a case by case basis. There are insufficient data to conclude whether 90-day corrosion testing is required but it is advisable. The qualification process should consider the extremesof the range of PWHT thermal cycles that may be experienced, as the acceptable range has not been established. Qualification testing should address the effects of PWHT on toughness and sour service performance, in addition to IGSCC.Based on current data, a PWHT temperature of 620-660°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 butthe current work extended up to about 670°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 appropriateduration.

Conclusions

• The results obtained do not provide a complete understanding of the mechanism of sensitisation to intergranular stress corrosion cracking in welded SMSS but are consistent with a mechanism based on formation of Cr-carbides and associated Cr-depleted zones.
  
  
• The most commonly observed macroscopic cracking mechanism follows prior austenite boundaries, suggesting that carbide precipitation on these boundaries provides the main mechanism for sensitisation. At least two thermal cycles are required, with the first thermal cycle forming fresh martensite and the second and subsequent cycles forming the damaging carbides. In the context of arc welding of pipes of the size examined here, fairly short reheating thermal cycles are apparently more damaging with respect to IGSCC on prior austenite boundaries than long thermal cycles. Higher interpass temperatures (about 140-150°C) seem to inhibit this form of sensitisation.
  
  
• Several welds developed microscopic IGSCC, some of which was at the weld toe and coincident with the band of retained delta ferrite at that location. This suggests that a second sensitisation mechanism might exist associated with carbide precipitation on delta ferrite/martensite phase boundaries. It is not clear whether this mechanism also requires two or more thermal cycles although it was apparently favoured by longer thermal cycles and greater numbers of welding passes.
  
  
• The significance of shallow cracks developed during tests of up to 90 days duration has not been established. They might be related to localised sensitisation caused by oxidation of the surface, in which case they will not propagate, or they might be associated with a microstructure with a low level of sensitisation or a discontinuous sensitised microstructure, in which case they might continue to propagate over longer periods.
  
  
• The results indicate that control of the welding process to reliably avoid sensitisation to IGSCC is not straightforward. Each steel will have its own range of critical thermal cycles for causing sensitisation. Mechanised welding giving reproducible thermal cycles for each weld pass provides a higher likelihood of avoiding sensitisation to IGSCC than manual welding, where a wider range of thermal cycles will inevitably result.
  
  
• Welds in supermartensitic stainless steel that experience more than one thermal cycle during welding should be given a brief PWHT at about 650°C for five minutes at the root if the environment that they will be exposed to is sufficiently aggressive to cause IGSCC of a sensitised HAZ microstructure.

Acknowledgements

The following organisations are thanked for sponsoring the work, for their technical input and for supply of materials: BP, ConocoPhillips, JFE Steel, Shell, Statoil, Sumitomo Metal Industries and TenarisNKK.

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