A number of research works have been conducted for clarification of the mechanism of intergranular stress corrosion cracking (IGSCC) in girth welds of high grade supermartensitic stainless steel (SMSS). Although some evidence indicates that the IGSCC is caused by chromium depletion due to carbide precipitation, more confident evidence is required for clear understanding of the mechanism. In this context, behavior of the carbide precipitation was investigated using strain aging phenomena. Higher aging index was observed in solution treated steel with higher carbon content. However, no increase in aging index was observed in the sensitization treated steel with increase in carbon content. These results suggest that carbide precipitation occurs during the sensitization treatment. That is, it supports that the mechanism of IGSCC in high grade SMSS would be chromium depletion accompanied by chromium carbide precipitation during girth welding.
Low carbon martensitic stainless steels, which are called supermartensitic stainless steels (SMSS), have been developed for linepipes under sweet environments since the late 1990's 1-3. They have contributed to oil and gas industry as alternative materials for duplex stainless steels or carbon steel with inhibitor 4-8. However, a new type of cracking of the steel has been reported on the basis of laboratory tests 9, 10. The cracking occurs in the heat affected zone (HAZ) of girth welds and its morphology is intergranular stress corrosion cracking (IGSCC). In-service failure caused by this type of cracking has also been reported in lean grade SMSS, which contains no molybdenum 11. No experience of in-service failure has been reported in high grade SMSS, which contains 2-3% of molybdenum. A lot of research has been focused upon IGSCC of SMSS after its recognition 12-14. The authors reported that chromium depletion was detected on the grain boundary of the sensitized lean grade SMSS 12. They also reported in the subsequent paper 13 that lowering carbon content and addition of titanium in high grade SMSS was effective in preventing the cracking and it was suggested that the mechanism of IGSCC of high grade SMSS was chromium depletion on the grain boundary accompanied by precipitation of chromium carbide. However, more confident evidence has been requested to understand the mechanism of IGSCC. Hardening is generally observed when steels are cold-worked followed by aging, and well known as strain aging 15. One of the causes of this phenomenon is fixation of dislocations by Cottrell atmosphere formed by dissolved carbon and nitrogen. The other is precipitation of some second phase such as carbides. Strain aging has close relation with dissolved carbon and carbide precipitation. In these contexts, this phenomenon has been assessed for understanding the mechanism of IGSCC from a new point of view. This paper describes the results of the study.
Materials High grade SMSS with 30 to 100 ppm of carbon were prepared for the study in the quenched and tempered condition. Both titanium free and bearing steels were used. The compositions of the steels used are shown in Table 1.
