Low Cyclic Fatigue Behavior of 32%Mn Nonmagnetic Steel and the Effects of C and N in Liquid Nitrogen and Liquid Helium

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Abstract
The effects of testing temperature, C, and N on the low cyclic deformation behavior of 32% Mn nonmagnetic steels have been investigated in ambient air, liquid nitrogen, and liquid helium. It was observed that several problems exsisted in fatigue tests in liquid helium due to special phenomena occurred at very low temperatures. The steel containing 0.3 % N, which showed large fatigue softening at room temperature, increased the trend toward the softening at low temperatures. The steel containing 0.14% C and 0.13% N also increased the tendency of softening with the temperature decrease, while it was not so large at room temperature. Dislocation configuration in steels showing the softening tended to be mainly planar at very low temperatureas same as at room temperature. The steel with a very low content of C and N, the 0.3%C steel, and the 0.12% N steel did not show the softening at low temperatures, but showed only fatigue hardening. The hardening of the former two steels increased remarkably as the temperature decreased. This phenomenon was attributable to ε martensite induced by the cyclic deformation. The fatigue softening behavior observed at low temperatures could qualitatively be explained with the hypothesis that the softening occurred through the breakdown of solid solution strengthening due to IS complexes during the cyclic deformation.