EFFECT OF ALLOYING ELEMENT ON INTERGRANULAR STRESS CORROSION CRACKING SUSCEPTIBILITY OF SUPER MARTENSITIC STAINLESS STEEL WELDED JOINTS IN SWEET ENVIRONMENT

摘要

The effect of alloying elements such as titanium or phosphorus, and the effect of PWHT on SCC susceptibility of super martensitic stainless steel welded joints were examined. On the basis of the sensitization mechanism due to the formation of Cr-carbides, the effect of titanium on the SCC susceptibility was determined. It is revealed that the super martensitic stainless steel (0.01C-12Cr-6.5Ni- 2.5Mo) without titanium suffered SCC in the specific region of the TTS diagram (Time - Temperature - Sensitization) during the 2nd thermal cycle of the simulation HAZ. On the other hand, the super martensitic stainless steel of the Ti-alloyed type (0.01C-12Cr-6.5Ni-2.5Mo-0.1Ti) was immune to SCC in the same thermal cycle conditions. From these results, titanium is considered to prevent the formation of Cr-carbide, which can promote the Cr-depletion along the prior austenite grain boundary, by TiC formation. However, even if the titanium could suppress the Cr-carbide formation, it was reported that the Ti-alloyed high grade steel girth welds in as-welded condition showed IGSCC along the high temperature HAZ. Therefore, in this study, the effect of phosphorus was also examined. It was revealed that the SCC susceptibility of the welded joints in as-welded condition decreased with the lowering of the phosphorus content in steels. The TEM observation revealed that Cr-depletion along the grain boundary in the vicinity of the surface oxide layer on HAZ disappeared after PWHT, and molybdenum enrichment along the grain boundary was also observed after PWHT. The Laves (λ) phase which contained phosphorus was observed on the grain boundary after long duration PWHT. The good SCC resistance of Mo-alloyed type martensitic stainless steel welded joint after PWHT is considered to be achieved by this enrichment of molybdenum along the grain boundary which may eliminate the harmful effect of phosphorus on SCC propagation, as well as by the disappearance of the Cr-depletion for initiation of SCC in the vicinity of the surface oxide layer.