Microstructure and Properties of a Low Carbon Bainitic Steel Produced by Conventional and Inverted Two-Step Austempering Processes

The microstructure evolution and strain hardening behaviour of a low-carbon carbide-free bainitic steel prepared by either single-step austempering (420 °C or 365 °C), conventional two-step austempering (420 °C then 365 °C) or inverted two-step austempering (365 °C then 420 °C) treatments were investigated. The results show that when the total isothermal holding time was the same, the inverted two-step austempering treatment (first completing bainitic transformation at low-temperature and then annealing at high-temperature austempering) led to the highest toughness (30.7 GPa%) due to the finer bainitic microstructure and higher fraction of film-like retained austenite. Grain refinement and transformation-induced plasticity allowed the material to achieve high ductility without sacrificing strength. Comparing single-step austempering at 365 °C with the inverted two-step austempering process indicates that annealing at a higher temperature after completion of the bainitic transformation resulted in better tensile properties because of a lower dislocation density and more stabler retained austenite. In addition, the samples heat-treated by the conventional two-step austempering process exhibited slower bainite transformation kinetics and the worse tensile properties than the sample which was heat-treated using a single-step austempering treatment at 365 °C or the one which was heat-treated using an inverted two-step heat treatment. Through the analysis of the orientation relationships, it is observed that the original austenite and the bainitic plates mainly followed the K-S orientation relationships regardless of whether the bainite plates were formed in the first or the second heat-treatment step.