α+γ域加熱でえられる微視組織を有する11%Ni鋼の低温延性

抄録

Tensile testing has been carried out in the temperature range from -196°C to room temperature. Highly tempered martensite (plus fine dispersed retained austenite) microstructure, observed in specimens heated in the vicinity of A_s temperature, shows the best low temperature ductility. Heating in the vicinity of A_f temperature, lath-type fresh martensite with some small recrystallized equiaxed grains is observed and shows relatively a good ductility. On the other hand, a highly recovered ferrite with fresh martensite structure, obtained by heating in the intermediate range of α-γ duplex phase region, yields a poor ductility. All these structures, especially the former two ones, exhibit larger uniform elongation at lower temperature down to -196°C. No trace of martensitic transformation of retained austenite is detected by cooling down to -269°C, but a little plastic deformation at -78°C makes most of retained austenite transform to martensite. Such a transformation induced by deformation is also observed at room temperature. Considering the results of some other Fe-Ni alloys, the interrelationship between low temperature ductility and microstructures of 11%Ni steels is discussed.