The intergranular stress corrosion cracking (IGSCC) in heat affected zone (HAZ) of weldable super martensitic stainless steels has been reported. The Cr-carbide precipitation surrounded by Crdepletion zone was discussed as the cause of IGSCC for Ti-free martensitic stainless steels. The Crdepletion along grain boundary in the vicinity of the surface oxide layer on HAZ formed by welding was also reported and the initiation mechanism was discussed based on the Cr-depletion of the surface area. The improvement of SCC resistance by Post Weld Heat Treatment (PWHT) has been recognized. PWHT can be effective to reduce the IGSCC susceptibility because of the healing for Cr-depletion (sensitized zone and surface area). It has been also proposed that the effect of PWHT on the propagation of IGSCC can be explained by the enrichment of molybdenum at the grain boundary, which has the positive effect for enhancing the corrosion resistance, and/or by the elimination of the harmful effect of phosphorus segregation to be drained from the grain boundary to the molybdenum enriched phase. In this study, the molybdenum precipitation behavior along grain boundary by PWHT of super martensitic stainless steels is determined in detail. The quantitative analysis by the extracted residue is carried out with the varied combination of temperatures and durations of PWHT. The relation between the IGSCC susceptibility and the molybdenum precipitation along grain boundary is discussed.
The intergranular stress corrosion cracking (IGSCC) in heat affected zone (HAZ) of weldable super martensitic stainless steels has been reported1). The post weld heat treatment (PWHT) is considered to be beneficial to avoid the IGSCC in HAZ2, 3). The Cr-depletion along the grain boundary in the vicinity of the surface oxide layer on HAZ formed by welding was also reported and the initiation mechanism was discussed based on the Cr-depletion of the surface area4). The propagation mechanism of IGSCC in Ti-free weldable martensitic stainless steels has been discussed in association with the sensitization due to the formation of chromium carbides (M23C6) in the coarse-grained HAZ5). The TTS (Time - Temperature - Sensitization) behaviors by the simulated thermal cycles of the lean grade (Moless type) and the high grade (Mo-alloyed type, without titanium) martensitic stainless steels were also reported using the U-bend method in the lower pH environment6). Titanium addition to steels is considered to contribute to the stabilization by avoiding Cr-carbides formation, however, it was reported that the addition of titanium to martensitic stainless steel apparently did not prevent IGSCC in HAZ, even if the crack morphology was different between Ti-free and Ti-alloyed martensitic stainless steels7). The propagation mechanism for IGSCC with Ti-alloyed high grade steel, where no Cr-carbides may precipitate, has not yet been thoroughly understood. Therefore, it is important to consider a reasonable propagation mechanism to explain the IGSCC in HAZ near fusion boundary other than the sensitization by Cr-carbide formation. In the previous paper8), the influence of the intergranular phosphorus segregation in HAZ of as welded condition was discussed as an important factor for IGSCC propagation. It was also reported that PWHT was effective to improve the IGSCC resistance because of the enrichment of molybdenum at grain boundary which has the positive effect of enhancing the corrosion resistance, and/or because of the elimination of the harmful effect of phosphorus segregation to be drained from the grain boundary to the molybdenum enriched phase.
