Effect of a Novel Bainitization Quenching & Partitioning Heat Treatment on Multiphase Microstructure Evolution and Properties of Carburized Cr-Mn-Si Alloyed Steel

An innovative multistage heat treatment, that combines Bainitization and Quenching & Partitioning processes (BQ&P) can be applied to obtain multiphase microstructure comprising carbide-free nanobainite, ultra-fine martensite and retained austenite. In this study, a carburized medium-carbon Cr-Si-Mn alloyed steel was treated by Bainitic Austempering and BQ&P treatments with 20% and 40% of advancement of bainitic transformation. Carburization leads to an increase of carbon content in steel, which allows obtaining very fine complex microstructure in the case, changing gradually within a carbon gradient on a cross-section. It was shown that BQ&P process allowed obtaining higher hardness and wear resistance of the case as compared to the simple bainitic austempering treatment. 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