Effect of Austenitizing Temperature and Holding Time on Microstructure, Hardness, and Retained Austenite Amount of HSLA Steel as Automotive Components Material

Abstract

Retained Austenite is a metastable phase that can cause isothermal transformation at room temperature, resulting in cracks occurring long after producing components made from HSLA steel. This research focuses on the heat treatment process's austenitizing temperature and holding time. For the study on the effect of temperature, the austenitizing stage was carried out at 850 o C, 870 o C, 900 o C, and 926 o C for 28 minutes. In contrast, for the study of holding time, austenitizing was carried out for 28 minutes, 43 minutes, 58 minutes, and 73 minutes on 926 o C. The characterization conducted was metallography using optical microscopy (OM) and scanning electron microscopy (SEM), micro and macro hardness testing (using Vickers and Rockwell C method), and retained austenite phase quantification using Image-J image analyzer. The microstructure produced was tempered martensite, lower bainite and a small amount of retained Austenite were also found. All the variables produce the same microstructure but have different hardness values and retained austenite percentages. It was found from this research that the optimum variable with the highest hardness value (500 HV) and the lowest amount of retained Austenite (1.50%) is at the austenitizing temperature of 900°C and the holding time of 58 minutes. With a low amount of retained Austenite, the possibility of isothermal transformation at room temperature of the retained Austenite to other phases becomes less, reducing the likelihood of cracking some time after the production process of components.