オーステナイト系ステンレス鋼溶接金属の鋭敏化に関する理論的解析 : オーステナイト系ステンレス鋼溶接金属の低温鋭敏化とレーザ表面溶融処理によるその改善(第5報)

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The formation of chromium depleted zone due to the precipitation of chromium carbides along grain boundaries in weld metal of Type 308 stainless steels with δ-γ duplex structure was theoretically analyzed in this study. The kinetics theory of grain boundary precipitation, which was developed by modifying the nucleation and growth theory proposed by Johnson, was applied to the theoretical analysis to predict precipitation of chromium carbides at grain boundaries. Profiles of chromium concentration adjacent to grain boundaries were analyzed by using a two-dimensional finite difference method which included the impinging effect of the chromium profile along grain boundaries. Calculation of nucleation time showed that precipitation of chromium carbides in δ/γ boundary occurred faster than that in γ/γ boundary. Numerical analysis for the chromium distribution around chromium carbides revealed that chromium depleted zone in δ grain side of δ/γ boundary increased much faster than in γ grain side. It was also indicated that the continuous network of chromium depleted zone along δ/γ boundaries formed in shorter time than in γ/γ boundary. Thus provided that the δ/γ boundary was linked continuously in weld metal, serious intergranular corrosion and intergranular stress corrosion crack may take place in weld metal in much shorter time.
1999-11-05