Effect of tempering on stability of retained austenite and tensile properties of nanostructured bainitic steel

In this investigation, the evolution of the mechanical stability of retained austenite (RA) and the strength-plasticity of nanostructured bainitic steel tempered at a temperature ranged from 250 °C to 510 °C were studied. Results show that the mechanical stability of RA decreased after tempering mainly due to the weakened shielding effect of the bainite ferrite (BF) on RA. The RA with high mechanical stability generated a little mechanically induced martensite when the tempering temperature was below 380 °C, leading to less contribution to elongation. In addition, the poor deformation ability of the bainitic matrix resulted in nanostructured bainitic steel with low elongation. With increasing tempering temperature to 380 °C and higher, the mechanical stability of RA decreased significantly and mechanically induced martensite transformation occurred easily. However, the contribution of RA to the elongation remained low due to the formation of a substantial quantity of mechanically induced martensite after 380 °C specimen yield. The elongation was improved remarkably with increasing tempering temperature to 450 °C and above. The main reason was the improvement of the coordinated deformation ability of bainite matrix with a large number of carbide particles precipitated in BF and RA. Finally, the specimen tempered at 450 °C presented the optimum tensile mechanical properties. The yield and ultimate tensile strength of nanostructured bainitic steel increased at first and then decreased with increasing tempering temperature, and both of them reached the maximum in the 250 °C specimen.