The evaluation of quenching temperature effect on microstructural and mechanical properties of advanced high strength low carbon steel after quenching partitioning treatment

The influence of quenching temperature on microstructural and mechanical properties of low alloy steel of the following chemical composition: 0.26 C, 1.70 Mn, 1.42 Si, 1.10 Cr, 1.10 Ni, 0.94 Cu, 0.24 Mo, 0.1 V, Bal. Fe (Wt.%) was investigated after applying a quenching-partitioning (Q-P) treatment. The steel samples were isothermally quenched at 260, 280, and 300 °C, from the austenitizing temperature and then Q-P treated at 340 °C. After the Q-P treatment, the steel showed a multiphase microstructure containing bainite, martensite, and retained austenite. It was determined that the tensile strength and Charpy impact energy increased with a decrease in quenching temperature to 1415 MPa and 43 J, respectively. This effect was attributed to an increase in the volume fraction of austenite/martensite micro blocks that introduces a hard phase mixture strengthening factor and the presence of tempered martensite, which is strengthened by fine particle dispersion and moreover, a decrease in thickness of the bainitic-ferrite subunits that refine the microstructure. The fractographic examination of the Charpy tested specimens showed that the sample quenched at 260 °C contained finer and deeper dimples, which indicates that more energy was spent on the nucleation and growth of ductile fracture microvoids, thus increasing the toughness.