Sulphide Stress Cracking of Joints in Martensitic Steels

Sulphide Stress Cracking of Welded Joints in Weldable Martensitic Stainless Steels

Paul Woollin

In Advancing Corrosion Technology, Proc. 9th Middle East Corrosion Conference, Bahrain, 12-14 Feb 2001, pp 427-435.

Abstract

Weldable martensitic stainless steels are an attractive alternative to duplex stainless steel for CO ₂-resistant piping. However, the resistance of the martensitic grades to sour media is limited and is likely to be controlled by the behaviour of any welded joints, due to local hardening. Sulphide stress cracking tests were performed on cross-weld specimens from five different weldable martensitic stainless steel pipes. Observations were made on the nature of cracking and limits, in terms of H ₂S partial pressure and pH, are proposed for variants with high and low levels of Mo and Ni, when exposed to fairly high chloride brine representative of a formation water.

1. Introduction

In recent years, low carbon martensitic stainless steels with chromium content in the range 10-13% and additions of nickel and molybdenum have been developed as CO ₂-resistant readily weldable pipe products for the oil and gas industry ^[1,2] . These steels have become known as 'weldable 12/13%Cr', 'weldable martensitic stainless' or 'supermartensitic stainless' steels and are currently being employed in a number of oil and gas industry developments. This paper describes sulphide stress cracking test results for welded joints in a range of such grades and examines the effect of brief postweld heat treatment cycles. The aim of the work was to illustrate the range of behaviour obtained and to explore the limitations of high and low alloy variants.

All of these steels are predominantly martensitic, with composition adjusted to give little or no delta ferrite, and are supplied in the tempered condition. Such tempering operations precipitate carbides, to lower hardness, and generally encourage formation of some retained austenite. Austenite contents up to 11% have been reported ^[3] . Such parent steel microstructures have useful resistance to sulphide stress cracking (SSC) in environments containing H ₂S, especially when Mo and Ni are included ^[4-6] but, in general, hardness levels are such that these steels cannot be considered immune to SSC, regardless of heat treatment condition.

When welded joints are made in these low carbon martensitic stainless steels, sound joints with good HAZ toughness can be made without need for pre-heat or PWHT, hence the description of these steels as 'weldable' ^[7,8] . However, in the weld HAZ, Fig.1, a region is inevitably re-transformed to austenite on heating and, subsequently, virgin martensite is formed, with associated local increase in hardness. In addition, adjacent to the fusion boundary, transformation to delta ferrite will occur, such that a small amount may be retained on cooling and substantial grain growth may occur ^[3] . Consequently, a weld HAZ will have different microstructure, hardness and toughness than the tempered parent material and will be expected to be more susceptible to SSC. An extended duration PWHT would undoubtedly improve resistance ^[9] but is generally impractical for laying of pipelines, especially offshore, hence the interest in the effect of brief PWHT cycles, which might be economically incorporated into a production welding sequence. 

2. Experimental programme

Five steels were examined, all in pipe form, with nominal strength levels ranging from 70-90ksi. These steels have the following approximate compositions: (i) 12%Cr6.5%Ni2.5%Mo, (ii) 12%Cr6%Ni2%Mo, (iii) 11%Cr4%Ni1%Mo, (iv) 12%Cr3%Ni and (v) 11%Cr1.5%Ni. Steels (iii), (iv) and (v) also had small additions of copper, Table 1. Girth welds were produced in each steel using TIG or MMA processes for the root pass and either GTA, SMA or FCA for fill passes, with a range of consumables, including 12%Cr 4%Ni (AWS ER410NiMo), 22%Cr duplex (AWS E2209), 25%Cr superduplex (Zeron 100X) and alloy 625 (AWS E/ERNiCrMo-3). All welding was with the pipe fixed and horizontal, with welding vertically up (5G position) to represent likely industrial practice. No pre-heat was used and heat input was in the range 0.5-1.5kJ/mm, depending on the particular pipe and consumable in question.

Table 1 Steel compositions

Sulphide stress cracking tests were performed in autoclaves containing deoxygenated brines (5 and 8%NaCl, to represent formation waters), pressurised by continuous bubbling with CO ₂/H ₂S/N ₂ gas mixtures at 25°C and with pH adjusted by addition of NaHCO ₃. Calculation of pH employed CORMED ^TM software ^[10] . All tests involved cross-weld bend specimens, 100x15x3mm, taken from the root of the weld with the weld root profile intact. Applied strain in the HAZ was measured using small strain gauges attached to the machined, compression face of the specimens, so as not to modify the specimen test surface. Dummy specimens were used to correlate the HAZ strains on the compression and tension faces during deflection. Test specimens were deflected to give HAZ stresses of 90% or 100% of the actual measured proof stress but otherwise procedures followed the guidance given in EFC 17 ^[11] . Comparative tests at 90% and 100% of the proof stress showed no significant difference, in test outcome, for the condition selected (0.03 bar H ₂S, pH=4.5). Test duration was 30 days. Some specimens were given a brief PWHT at 650°C for 5 minutes in a furnace prior to testing. Each specimen was thermocoupled, so that time at the required temperature, which was held within ±5°C, could be monitored. Heating normally took around 5 minutes and specimens were air cooled. After test, specimens were examined visually and sectioned for metallographic examination.

3. Results and discussion

The results highlight the increased sensitivity of the weld region to SSC in a wide range of weldable martensitic stainless steel pipe materials ( Table 2, Fig.2 to 4). Comparison with published data for parent steel indicates a marked reduction, after welding, in the acceptable H ₂S level for steel with around 6%Ni and 2%Mo ^[4] . Highest resistance to SSC for welded joints was found in the steels with highest Ni and Mo content, even though these also tended to give higher HAZ hardness than the lower alloy grades.

Table 2 SSC test results

Most cracking occurred in the high temperature supercritically reheated HAZ region but some cracks were found also in parent steel, in weld metal and at the fusion boundary. Most cracking was transgranular in nature but some areas of intergranular cracking were observed in 11Cr1.5Ni steel specimens. The intergranular path was presumably with respect to the prior austenite grain structure. Cracking in parent material was only found in 11Cr4Ni1Mo steel after PWHT at 650°C for 5 minutes, whilst weld metal cracking was only found in a 12%Cr4%Ni (ER 410NiMo) deposit, presumably reflecting its low Mo content (about 0.5%). Fusionline cracking may be of greater significance, as it may indicate particular sensitivity to cracking in this area, although no particular trend with pipe or consumable type was found. The fusionline structure is a high alloy martensite when compositionally overmatched consumables are used and locally high microhardness was measured in this area both before and after PWHT. Maximum microhardness was around 400HV0.05 as-welded and 370HV0.05 after PWHT for the 12Cr6Ni2Mo steel, and around 420HV0.05 as-welded and 330HV0.05 after PWHT for 11Cr1.5Ni steel. Also, the fusionline and HAZ material immediately adjacent to it may be enriched in nitrogen due to diffusion across the interface when duplex or superduplex consumables are used. As the parent steel is deliberately produced with low carbon and nitrogen, local enrichment of nitrogen may be significant and could enhance hardness locally, possibly with an associated loss of SSC resistance. From the data presented, no particular preference of welding consumable type can be identified with respect to SSC resistance. However, provided weld metal resistance to SSC is adequate, the current trend towards development of matching consumables seems to have merit in view of (i) the potential concerns relating to the fusion boundary area when dissimilar consumables are used and (ii) the potential for precipitation of deleterious phases in high alloy weld metal if PWHT is required for the HAZ.

Efforts were made to derive limiting conditions for SSC in simulated formation water (5%NaCl) in two of the steels, i.e. 12Cr6Ni2Mo and 11Cr1.5Ni. This was complicated, in particular for the former steel, by the observations of very short 'microcracks', typically 50-150µm deep, in many of the specimens. The origin of these cracks was not clear. Bending of dummy specimens from the same steel, which were not exposed to a test environment, indicated that small HAZ microcracks (<50µm in depth) may form simply on bending to 100% of the actual proof stress, as measured by a strain gauge covering an area of a few square millimetres. The limiting conditions for SSC proposed here are based on the assumption that any crack >50µm deep was a result of SSC. However, it may be noted that for steel with 2-2.5%Mo and 6-6.5%Ni, cracking to a depth of >0.5mm was only observed in this test series after exposure at a pH of 3.2, with 0.016 bar H ₂S. For 12Cr6Ni2Mo pipe welded with superduplex or alloy 625 consumables, the data suggest H ₂S limits in a formation water of around 0.035 bar at a pH of 5 and less than 0.01 bar at a pH of 4 for the as-welded condition. Limits for the 11Cr1.5Ni steel in the as-welded condition were not established, as all specimens cracked in all the tests performed. After brief PWHT (5 minutes at 650°C), the 11Cr1.5Ni steel cross-weld specimens gave very similar results to the as-welded specimens from the 12Cr6Ni2Mo steel. The effect of brief PWHT on the SSC resistance of the 12Cr6Ni2Mo steel was minor in the tests performed. Crack-depths for the 12Cr6Ni2Mo steel were typically less after PWHT but H ₂S limits based on the criterion described previously were not shifted significantly. This may reflect on the criterion used and the test conditions selected here, as previously published work on an approximately 12Cr6.5Ni2.5Mo steel has shown a beneficial effect of a similar PWHT for a particular practical application with 0.04 bar H ₂S ^[2] . Nevertheless, PWHT seems to have a more significant effect on SSC resistance for the 11Cr1.5Ni grade, which will have fairly high Ac1 temperature and hence will not tend to austenitise during PWHT at around 650°C, in contrast to the 12Cr6Ni2Mo grade. This is reflected in the hardness change. PWHT at 650°C reduced the maximum root HAZ hardness in each case, by up to 90HV for the low alloy steel and by up to 40HV for the higher alloy.

4. Summary and conclusions

Sulphide stress cracking tests were performed in simulated formation waters on specimens taken from girth welds in five grades of weldable martensitic stainless steel, made with a range of consumable types. The effect of brief PWHT at 650°C for 5 minutes was examined.

1. Welding reduced the SSC resistance of weldable martensitic stainless steel pipe materials, with cracking in tests occurring in HAZ, fusionline and occasionally weld metal, depending on pipe and weld metal composition.
2. Steels with the highest Mo and Ni contents showed the highest SSC resistance, at welded joints, especially in the as-welded condition.
3. In 5%NaCl solution, approximate limiting H ₂S partial pressures of 0.035bar at a pH of 5 and 0.01bar at a pH of 4 are suggested for 12Cr6Ni2Mo steel in the as-welded condition, at 25°C.
4. Similar H ₂S limits were found for 11Cr1.5Ni steel after PWHT at 650°C for 5 minutes but, in the as-welded condition, the limits were lower, with cracking noted at the lowest H ₂S levels examined, i.e. 0.03 bar/pH=5 and 0.01bar/pH=4.
5. Brief PWHT at 650°C for 5 minutes gave a marked improvement of SSC performance for 11Cr1.5Ni steel but had much less effect for 12Cr6Ni2Mo steel in the tests performed.
6. Establishing limits of SSC resistance from bend tests on samples with the weld root intact was difficult due to the formation of microcracks around 50-150µm deep at the welds toe in several instances, especially in the12Cr6Ni2Mo steel. Similar cracks up to 50µm in length were found in specimens that were bent but not exposed to the test environment, hence the origin of the small cracks in the SSC tests was not always clear.

5. Acknowledgement

This paper is dedicated to the memory of Dr Trevor Gooch, without whose knowledge, enthusiasm and tuition this work would not have been possible. The following organisations are thanked sincerely for funding the work and for their permission to publish these results:
Agip SpA, ARCO, British Gas, BP, Fabrique de Fer, Nickel Development Institute (NiDI), NKK, Shell Research, Sumitomo Metal Industries and The Pipeline Research Committee International (PRCI).

6. References

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