Effect of austempering time on multiphase microstructure evolution and properties of carburized Cr-Mn-Si alloyed steel subjected to bainitization quenching & partitioning heat treatment

Abstract

An innovative multistage heat treatment, that combines Bainitization and Quenching & Partitioning processes (BQ&P) can be applied to obtain multiphase microstructure in the carburized surface layer comprising nanobainite, ultra-fine martensite and retained austenite. In this study, a carburized medium-carbon Cr-Si-Mn alloyed steel was treated by BQ&P treatments with 20%, 40% and 100% of advancement of bainitic transformation in the top surface to investigate the effect of isothermal holding time on microstructure evolution and properties.Extending the bainitization time leads to an increase in the bainitic ferrite content in the microstructure and more significant fragmentation of the residual austenite blocks. It was shown that multiphase microstructure consisting of nanobainite, ultra-fine martensite and retained austenite results in the higher hardness and wear resistance of the case as compared to the fully bainitic microstructure. The enhanced properties results from the strengthening effect of additional martensite fraction and higher amount of stabilized austenite prone to transformation under friction (transformation induced plasticity, TRIP effect). Furthermore, adjusted parameters of the process lead to favorable mechanical properties in a medium-carbon core (YS ∼ 1430 MPa, UTS ∼ 1820 MPa, KV ∼ 20 Jcm−2). The ability to tailor mechanical and service properties of steel by controlling the fraction of particular phases along with a significant reduction of heat treatment time might designate the BQ&P process to industrial application, e.g. carburized gear wheels.