Effects of first-step controlling on ultra-fine bainitic steel produced by two-step austempering process

Two-step austempering process has been demonstrated to be an effective method to accelerate the bainitic transformation at low temperature, but the rule of first-step controlling is still unclear. This paper comprehensively studies the effects of the first-step transformation on bainitic transformation, microstructural evolution and the related mechanical properties of ultra-fine bainitic steel obtained by two-step processes. Results show that the two-step austempering process notably accelerates the bainitic transformation, with the maximum reduction in total transformation time of 93.1% when the first-step bainitic fraction is controlled to be 22%. The thickness and length of bainitic ferrite plates and the size of blocky retained austenite of the two-step austempering samples are smaller than those of the conventional first-step austempering sample. Although the hardness and strength of the two-step austempering samples decrease slightly as compared with traditional one-step sample, they also meet the requirements of the product, such as bearing, especially the sample with 22% and 50% first-step fractions, which achieve higher toughness and yield strength, respectively. This study provides a theoretical support on process design of ultra-fine bainitic steel, based on short-period heat treatment process, optimized microstructure and mechanical properties.