Effects of above- or below-A austenitization on bainite transformation behavior, microstructure and mechanical properties of carbide-free bainitic steel

The isothermal bainite transformation kinetics, microstructure, crystallography and mechanical properties of a carbide-free bainitic steel, subjected to pre-quenching treatment, followed by austenitization at various temperatures above or below AC3 and then by above-MS austempering, have been investigated via dilatometric measurements, microstructural characterization and mechanical tests. The results show that, as the austenitization temperature decreases from 980 °C to 860 °C (above AC3), bainite transformation first decelerates and then accelerates; this is primarily a result of the competition between the availability of parent austenite grain (PAG) boundaries as nucleation sites for bainite transformation and the shear resistance of bainite growth, both of which increase with decreasing the PAG size but play a reverse role in bainite transformation rate. Further decreasing the austenitization temperature down to 820 °C and 800 °C (below AC3) even more notably accelerates bainite transformation and shortens the transformation completion time, since the intercritical ferrite (or tempered martensite), which is formed in the intercritical region, favor the nucleation of initial bainitic laths. The resulting microstructure after austempering is composed of bainite, retained austenite and intercritical ferrite and/or fresh martensite, the feature of which depends on the austenitizing temperature below or above AC3. Below-AC3 austenitization largely refines the microstructure and decreases the amount of large, brittle fresh martensite/austenite blocks, mainly attributed to the synergetic role of the refinement of austenite grains and pre-existence of intercritical ferrite prior to austempering. In addition, the intercritical ferrite and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block. As compared with above-AC3 austenitization, below-AC3 austenitization shows large increases in elongation, especially for specimens subjected to shorter time austempering, despite some decreases in yield and tensile strengths.