Effect of Deep Cryogenic Treatment on the Microstructure and Wear Resistance of a Novel Nanobainite Steel

Abstract

A novel nanobainitic (NB) steel is treated by three different heat treatment routes: quenching‐tempering (QT), quenching‐austempering‐tempering (AT), and quenching‐austempering‐deep cryogenic treatment‐tempering (ACT). To investigate the effects of retained austenite (RA) with different morphologies, stabilities, and volume fractions on the wear resistance of NB steel, the microstructure is observed by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The volume fraction and stability of RA are determined by quantitative X‐ray diffraction (XRD) analysis. It is found that deep cryogenic treatment (DCT) after low‐temperature austempering can effectively eliminate the blocky RA, increase the stability of filmy RA and have no effect on the NB microstructure. The AT and ACT treatments have higher surface residual compressive stress and better wear resistance than conventional QT treatments. For the ACT treatment, a multiphase microstructure composed of NB, martensite, and filmy RA is obtained near the surface, and the wear resistance of the steel is optimized, with increases of 23%, 52%, and 93% for austempering times of 8, 12, and 24 h, respectively. The results show that DCT can be combined with low‐temperature austempering treatment, thereby effectively eliminating unstable blocky RA, avoiding the transformation of brittle martensite, and obtaining improved wear resistance.