Tailoring the Microstructure to Enhance the Cryogenic Mechanical Properties of a Fe–34.5mn–0.04c Steel

Samples of a Fe–34.5Mn–0.04C (wt. %) austenitic steel with average grain size ranging from 0.45 to 21.0 μm were produced by cold rolling and subsequent annealing. The effects of microstructure on room–temperature (RT) and cryogenic tensile properties and on the related fracture modes were investigated. It is found that for fully recrystallized samples (average grain size in the range from 2.3 to 21 µm), both the yield strength and uniform elongation increase with decreasing grain size at RT and -180 °C, while for smaller grain sizes (in partly recrystallized samples) the strength increases at the expense of uniform elongation. In general, samples with an average grain size of 2.3 µm show a better combination of strength and ductility than those with larger grain sizes. A detailed characterization of the microstructure and fracture surfaces after failure reveals that grain refinement results in elimination of grain boundary martensite, thereby leading to a transition from transgranular and intergranular fracture modes to a ductile fracture mode, characterized by dimpled fracture surfaces. The results are discussed with respect to stacking fault energy as well as the Mn and C content. The study provides important inputs for the design of advanced high–Mn steels for cryogenic applications