Effect of Austempering Route on Microstructural Characterization of Nanobainitic Steel

Abstract

A low-temperature nanobainitic steel was obtained through one-step and two-step austempering. The effect of austempering temperature, time, and route on bainitic lath width, volume fraction of retained austenite, carbon concentration in retained austenite, and nanohardness of bainitic lath and retained austenite was studied. Results showed that the transformation kinetics was slowed down and the bainitic lath was refined as the austempering temperature decreased from 300 to 250 °C. Both coarser and finer bainitic laths were obtained with the two-step austempering, which was consistent with the lath size at 300 and 250 °C austempering, respectively. X-ray diffraction analysis showed that both volume fraction of retained austenite and its carbon concentration decreased with the decrease of austempering temperature for the one-step austempering, and especially the carbon concentration is obviously increased when the two-step austempering is adopted. The nanohardness of the bainitic lath in the sample after two-step austempering treated lies between that of the samples after 300 and 250 °C austempering treated. The product of tensile strength and total elongation of the two-step austempered sample is the highest, which increases monotonously with the product of retained austenite fraction and its carbon concentration. Higher strength–ductility balance may be resulted by the fine bainitic lath, high volume fraction, and high stability of retained austenite in the sample after two-step austempered.