Influence of Tempering on the Microstructure & Properties of Martensite and Bainite developed in a Low-C High-Si Steel

Martensitic and bainitic steels are two types of widely used steels with excellent mechanical behaviors in automatic industry. It’s universally acknowledged that as- quenched martensite possesses poor ductility and impact toughness, which should be tempered before putting into application. During tempering, as-quenched martensite changes from a hard and brittle microstructure to more ductile and softer microstructure, leading to improved mechanical properties for manufacturing. Bainitic microstructures is less sensitive to tempering in the improvement of mechanical properties, while the explicit tempering effects are still unknown.The primary goal of the current research is to study the effect of tempering on the microstructure and properties of martensitic and bainitic microstructures developed in the same low carbon steel with high content of silicon and manganese (0.2wt.%C- 2wt.%Si-3wt.%Mn). Results show that for tempered martensite, the hardness, yield strength and tensile strength have a decreasing tendency with higher tempering temperatures. Through tempering, martensite became more ductile at the expense of strength. Based on the literatures, transitional carbide precipitation is expected to take place instead of cementite at early stage of tempering due to the retarding effect of high silicon content. Investigations also show that bainite is less sensitive to tempering compared to martensite. The yield strength and elongation firstly increase due to the TRIP effect of retained austenite and then decrease with higher tempering temperature. Hardness and tensile strength have a similar decreasing tendency as martensite during tempering. The fracture modes of tempered martensite for all temperatures show a typical ductile fracture, while for bainite, a morphology of quasi-cleavage fracture is observed