STRESS CORROSION CRACKING SENSITIVITY OF SUPER MARTENSITIC STAINLESS STEELS IN HIGH CHLORIDE CONCENTRATION ENVIRONMENT

摘要

It has been reported that the super martensitic stainless steel welded joints may be susceptible to stress corrosion cracking (SCC) in CO_2 environment at elevated temperatures. In this investigation, the effect of the welding conditions on SCC sensitivity of super martensitic stainless steel was investigated with the welded joints by Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW). The GMAW as welded sample suffered SCC in the heat affected zone (HAZ), however, no SCC was observed in the GMAW with the post weld heat treatment (PWHT) sample, GTAW as welded sample and GTAW with PWHT sample. The difference of the crack sensitivity in HAZ depends on welding conditions was recognized, therefore, the factors relating to crack initiation were investigated based on the TEM analysis. TEM analysis was carried out to confirm the existing of Cr-carbide at the grain boundary in HAZ, and TEM/EDS analysis with FIB was also carried out to examine the Cr distribution under the oxide layer in the surface area of HAZ. No Cr carbide was observed in HAZ of both GMAW and GTAW with or without PWHT. The Cr depleted zone near the grain boundary in the vicinity of the surface oxide layer was revealed directly by applying nano-level analysis with FIB-TEM in this study. The Cr depleted zone was observed only near the grain boundary in the vicinity of the surface oxide layer of the GMAW as welded sample, nevertheless no Cr depleted zone was observed in GMAW with PWHT sample and GTAW with or without PWHT samples. The susceptibility to SCC correlates closely with the existence of the Cr depleted zone near grain boundary in the vicinity of the surface oxide layer, therefore, it is suggested that the Cr depleted zone near the grain boundary formed by the GMAW due to the oxidation during welding would be the initiation of the SCC in the GMAW welded joint.