Effect of Sub-Zero Treatment on Microstructure, Retained Austenite, and Hardness of HSLA Steel for Automotive Applications

This research focused on analyzing the effect of high cooling rates at cryogenic temperatures, referred to as sub-zero treatment, on the retained austenite fraction in the High Strength Low Alloy steel during the hardening heat treatment process. Sub-zero treatment was carried out after the quenching process. The treatment involved cooling the steel to −176 °C, followed by a double tempering process. This study focused on analyzing the percentage of retained austenite in as-quenched and as-subzero-treated samples, as well as in as-quench double-tempered (QTT) and as-quench sub-zero double-tempered (QSTT) samples. Furthermore, the hardness of all treatment variables is also compared. The microstructure characterization was carried out using an optical microscope and a Scanning Electron Microscope (SEM), and the calculation of the percentage of retained austenite phase was performed using ImageJ software. Hardness testing was conducted using Rockwell macro hardness testing. The results of this study show that a decrease in the amount of retained austenite from 6.8% to 3.9% occurs in as-quenched samples without and with sub-zero treatment. After a double temper, the retained austenite decreases from 3.37% to 1.2% in as-QTT and as-QSTT samples. Additionally, there was an increase in hardness from 48 HRC to 49 HRC in the sample with double temper treatment. Thus, sub-zero treatment could reduce the percentage of residual austenite and increase the hardness of the HSLA steel.