ABSTRACT
Low carbon martensitic stainless steels, called supermartensitic stainless steels (SMSS), have widely been applied for flowlines transporting corrosive oil and gas. However, intergranular stress corrosion cracking (IGSCC) in girth welds of the steel has become a major concern recently. A number of research works have been conducted to clarify the mechanism and to establish the measures. The most likely mechanism of IGSCC is chromium depletion on grain boundary because it is observed in lean grade SMSS, which contains no molybdenum. However, neither depletion nor carbide of chromium has been detected so far in 2-3% Mo-added high grade SMSS, although the steel suffers IGSCC under some conditions. In order to clarify the mechanism in high grade steel, effects of chemical components on IGSCC resistance were investigated. Simulated HAZ specimens with various conditions of thermal cycles were tested in hot acid environments by U-bend SCC test. The test results showed that reduction of carbon and addition of titanium are effective to improve the resistance to IGSCC. Both would inhibit to form chromium carbide that causes to chromium depletion. Therefore, it suggests that the mechanism of IGSCC in high grade SMSS is chromium depletion, as in lean grade.
INTRODUCTION
Low carbon martensitic stainless steels, which are called supermattensitic stainless steels (SMSS), have been developed for linepipes under sweet environments since the late 1990's [I]-[3]. They have contributed to oil and gas industry as alternative materials for duplex stainless steels or carbon steel with inhibitor [4]-[8].
However, 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 same type of cracking has also been reported in lean grade SMSS, which contains no molybdenum [I I]. No experience of in-service failure has been reported in high grade SMSS, which contains 2-3% of molybdenum. The most likely mechanism of the IGSCC is chromium depletion on grain boundary accompanied by re-precipitation of chromium carbide during girth welding. The fact that chromium depletion was observed in lean grade SMSS [I21 supported the idea. However, no clear evidence has been obtained so far in high grade SMSS. In order to clarify the mechanism in high grade SMSS, effects of chemical components such as C and Ti on resistance to IGSCC were studied. This paper describes the results of the study.
EXPERIMENTAL PROCEDURES
Materials
High grade SMSS with various content of carbon and titanium were prepared for the study. The compositions of the steel used are shown in Table 1. Each ingot in 50 kg weight was hot-rolled into 15 mm thickness sheet followed by quenching and tempering.
HAZ simulation
The steels were subjected to thermal cycles for HAZ simulation by Gleeble tester. The specimens for the HAZ simulation were smoothly machined and their dimension was 4 mm thick and 15 mm wide, as shown in Figure 1 (a).
