Intergranular stress corrosion cracking (IGSCC) in heat affected zone (HAZ) for supermartensitic stainless steel was studied. Two grades of the steel, lean and high grades, were heat- treated for simulating welding thermal cycles. Cracks were observed in some simulated HAZ specimens by all four methods of SCC test, U-bend, four point bent (4PB), slow strain rate technique (SSRT) and single edge notch bend (SENB) methods. It suggests that even smoothly machined specimen can detect IGSCC as long as the specimen is sensitized sufficiently and immersed in severe corrosion environment. Thermal cycle conditions inducing the cracking were clarified by U-bend SCC test for the lean and high grade steels. The results revealed that the high grade steel has higher resistance to IGSCC than the lean grade steel, and that post welding heat treatment (PWHT) is effective to prevent IGSCC. Chromium depleted zones were confirmed on prior austenite grain boundary adjacent to carbides that precipitated on the grain boundary for the lean grade steel. In these results, it was concluded that IGSCC in HAZ for supermartensitic stainless steel is caused by chromium depletion on prior austenite grain boundary accompanied by re-precipitation of chromium carbide during girth welding.
Low carbon martensitic stainless steels, which are called supermartensitic stainless steels, 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, 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). Laboratory works show that post welding heat treatment (PWHT) is effective to prevent IGSCC and that cracks are observed only in root intact specimen on four point bent (4PB) SCC test, while no crack was observed in root machined specimen [9]. The most likely mechanism of the cracking was supposed to be sensitization, in other words, chromium depletion from the results of TEM analysis [10]. However, more precise analysis is required for a clear understanding of the mechanism. In-service failure caused by same type of cracking has also been reported [11 ]. Although the analysis suggested that welding conditions affect the cracking, they have not yet been proven quantitatively. Amaya et al. reported that the crack initiated the chromium depletion caused by surface oxidation during welding [12]. However, this cannot explain the propagation of the crack.
In this context, we studied effects of welding thermal cycle conditions on sensitization behavior by HAZ simulation. We also tried more precise analysis to investigate chromium distribution around grain boundary.
