New insights for preformed martensite of carbide-free bainitic steels austempered below Ms temperature: Inconsistent effects of microstructure, stability of retained austenite, and mechanical properties

Previous studies have shown that the effect of preformed martensite (PM) on bainitic steel is complex. In this study, the austempering process for medium-carbon carbide-free bainitic steel was conducted in a salt bath environment at various temperatures to investigate the influence of PM on microstructure of bainitic steel, especially the development rule of bainitic structure, and the mechanical stability of retained austenite (RA), and tensile properties. Results indicate that with the volume fraction of PM increases, the carbon diffusing from the PM also increases correspondingly. This leads to more carbon enriched at the preformed martensite/undercooled austenite (PM/UA) interface, which enhances the inhibition of bainite nucleation. Moreover, the number of PM/UA interfaces increases slowly. These factors lead to a bilinear distribution of the thickness of bainitic ferrite (BF) lath austempered below and above the Ms temperature. For the sample with PM, the increase rate of BF lath thickness is 65.1% lower than that without PM. The mechanical stability of RA is improved with the introduction of PM because it refines RA and shares the stress on RA. This leads to the RA transforms slowly under stress. The tensile results show that strength of the tested steel increases after the introduction of PM. However, given the stress concentration around PM promoting crack initiation, reduce the plasticity of the tested steel.