Effect of Processing Techniques on Microstructure and Mechanical Properties of Carbide-free Bainitic Rail Steels

Rail manufacture processing techniques, such as roller straightening, tempering and air-forced quenching, have shown to make a great influence on the microstructure and mechanical properties of rail steels. In this study, five carbide-free bainitic rail steels with same chemical composition were developed from the above rail processing routes in an industrial production line. In general, the carbide-free bainitic steels consisted of bainitic ferrite (BF) plates with distinct morphologies of retained austenite (RA) and minor occurrence of martensite in some conditions. Electron microscopy confirmed the orientation relationship between BF and RA was in line with the Nishiyama-Wassermann (N-W) orientation relationship, i.e., {111}(gamma)//{110}(alpha), < 112 >(gamma)//< 110 >(alpha). The roller straightening and tempering processes could facilitate the transformation of unstable RA. The small volume fraction of stable RA contributed to an obvious improvement of yield strength, hardness and impact toughness, but at the cost of ductility and strain hardening ability. In addition, the in-line heat treatment resulted in the refinement of bainitic ferrite plates, leading to an increase in the tensile strength of carbide-free bainitic steels. In summary, there was a clear positive relationship between the rail processing techniques and the mechanical properties of the carbide-free bainitic steels. This study can benefit the development of new bainitic steels with a combination of wear and rolling contact fatigue resistance by appropriate selection of processing techniques.