The intergranular stress corrosion cracking (IGSCC) in heat affected zone (HAZ) of weldable super martensitic stainless steels was reported. The Cr carbide precipitation surrounded by Cr depletion zone was discussed as the cause of IGSCC. 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 area. The improvement of SCC resistance by Post Weld Heat Treatment (PWHT) has been recognized. PWHT can be effective to reduce SCC 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 is 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 such as Laves.
In this study, the enrichment of Mo and the precipitation behavior on the grain boundary of super martensitic stainless steels by PWHT is determined in detail. The quantitative analysis by the residual extraction is carried out with the varied combination of temperatures and durations of PWHT. The relation between the IGSCC susceptibility and the precipitation along the grain boundary will be discussed.