Effects of Initial Austenite Grain Size on Microstructure and Mechanical Properties of 5% Nickel Cryogenic Steel

Abstract

In the present study, the relationship between initial austenite grain size and mechanical properties of a 5% nickel cryogenic steel was investigated. The microstructures of initial austenite grains and tempered martensites were analyzed by an optical microscope. In addition, a transmission electron microscope was employed to examine the morphologies of reversed austenite and refined tempered martensite laths. Further, tensile and cryogenic impact tests were conducted to evaluate the mechanical properties of the experimental steel. It was found that the impact toughness of the sample steel increased as the size of initial austenite grain decreased and the thicknesses of martensite laths decreased with the decrease in initial austenite grain size. The average grain size of initial austenite in the specimen austenitized at 830 °C for 60 min followed by tempering at 600 °C for 60 min was found to be 19.2 μm, and the maximum value of impact toughness reached 292.5 J. Therefore, the improvement in impact toughness can be attributed to the existence of refined tempered martensite laths and more amount of reversed austenites in the specimen.