LNG温度域におけるオーステナイト系ステンレス鋼の引張特性と疲労特性

抄録

Tensile strength, fatigue strength and fatigue crack growth rate of metastable and stable austenitic stainless steels at the temperatures between 20°C and ·162°C have been studied. Microstructural changes of specimens during tensile and fatigue deformation were also observed. The results obtained are as follows.
1) The proof stress of metastable steels slightly drops at low temperatures below about -100°C. This is due to the inducement of ε-martensite by tensile stress. Addition of nitrogen makes austenite matrix stable and rises proof stress.
2) The fatigue strength increases with the increase in tensile strength which is raised by the inducement of α′-martensite in metastable steels and by the solid solution strengthening of matrix due to the addition of nitrogen in stable ones.
3) Microstructural changes of tensile and fatigue deformation are characterized by α′-phase formation in metastable steel and by planar arrays of dislocation pile-ups in nitrogen added stable steel at-162°C.
4) At-162°C, fatigue crack growth rate in high ΔK region is larger in metastable steels than in stable ones. This will be attributed to the formation of α′-phase at the tip of crack. In low ΔK region, stable steels exhibit large crack growth rate and this is due to a rapid propagation of fatigue crack along the slip line.