Effect of adding Cr, Ni and Mo on quench and temper microstructure and mechanical properties of a Si-Mn spring steel

In this study, 0.12Cr, 0.12Ni and 0.06Mo were added to the ordinary Si-Mn steel to investigate the effect of chemistry on the microstructure and mechanical properties of the Si-Mn steel. Various combinations were done but these produced optimal results. The steels were hardened and soaked at different temperatures with reports coming from only heat treatment of austenitising at 870 °C for 1800 s and tempering at 560 °C for 7200 s. The samples were quenched in oil that was maintained at 30 °C. The light optical microscope (Olympus BX51M) with the aid of the Olympus Stream Essentials software and the scanning electron microscope (Jeol JSM-IT300LV SEM) were used for the microstructural analysis. The Vickers hardness tester and Instron tensile testing machine were used to evaluate the mechanical properties. The room temperature tensile tests were carried out at a strain rate of 10−3/sec. The microstructure obtained after quenching and tempering was largely tempered martensite. The results also showed that the hardness slightly increased after adding the above alloying elements i.e. 373 Hv and 360 Hv for the modified and unmodified steel respectively. The yield strength of the modified steel was found to be 1020 MPa while that of the unmodified was found to be 960 MPa which corresponded with the hardness values. On the contrary, the Charpy impact energy of the steels remained unchanged with unmodified and modified steels recording average energies of 28 and 27 J respectively. In other words, the modified steel showed improved mechanical properties as compared to the unmodified steel when subjected to the same quench and temper conditions. © 2021