Microstructural Evolution and Microstructure–Mechanical Property Correlation in Nano/ultrafine-Grained Fe-17Cr-6Ni Austenitic Steel

Abstract

The effect of cold-rolled microstructures and subsequent reversion annealing on the microstructural characterization, reversion behavior, and microstructure–mechanical property correlation of Fe-17Cr-6Ni steel was evaluated with a heavy cold reduction of approximately 75 pct. Reversion annealing was conducted in a temperature range of 600 °C to 750 °C. The microstructures were evaluated using X-ray diffraction, optical microscope, scanning electron microscope, and transmission electron microscope. The mechanical properties were determined using tensile tests and microhardness tests. The results indicate that the reversion of lath martensite occurred in a diffusional manner over the entire temperature range, whereas that of dislocation martensite occurred in a diffusional manner at 600 °C, a combination of diffusional manner and shear manner at 650 °C, and a shear manner at 700 °C to 750 °C. The difference in the reversion conditions of lath martensite and dislocation martensite resulted in the heterogeneous microstructure made up of micrograins and nano/ultrafine grains. The tensile test results revealed that the mechanical behavior is closely related to the reversed microstructure, and can be classified into three different types, according to the engineering stress–strain curves. Moreover, a good combination of tensile strength and ductility was obtained in the samples that contained a suitable amount of retained martensite (~ 40 pct), which was due to long-range Lüders deformation.