Abstract
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 As temperature, shows the best low temperature ductility. Heating in the vicinity of Af 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.
