深冷処理したＳＵＳ３０１鋼単結晶の応力腐食割れ

Abstract

The stress corrosion cracking (SCC) of subzero treated 301 steel single crystal in boiling 42% MgCl2 solution at 416K was studied crystallographically with the aid of electron microscopy.The subzero martensite which has the same crystal structure as strain-induced martensite (ε, α') was formed preferentially along the primary slip plane in austenitic matrix. The SCC threshold stress obtained by the constant loading method increased markedly as a result of the increase of mechanical strength by martensitic transformation. However, the ratio of threshold stress to yield strength (0.2% off-set) in oil at 416K reduced to half in comparison with that before subzero treatment. The SCC specimen surface presented an appearance of general corrosion, which was caused by the active dissolution of subzero martensite. In this case, the crack initiated at the area where slip steps were apt to occur, and then propagated preferentially along the primary slip plane. Consequently, the fracture appearance was characterized by the plate-like pattern, which was closely related to the morphology of subzero martensite.The above results suggest that ε-martensite formed along {111}γ plane contributes to the susceptible path for hydrogen cracking.