Effect of prior austenitic grain size and tempering temperature on the energy absorption characteristics of low alloy quenched and tempered steels

The aim of the present work was to investigate the combined effect of prior austenitic grain size and the tempering temperature on the energy absorption characteristics of low alloy quenched and tempered steel. High frequency (HF) electric resistance welded tubes made of Boron added low carbon steels were used for the study. Induction hardening at two different temperatures (above upper critical temperature) led to two different prior austenitic grain sizes 20 µm and 100 µm. Both the set of samples were subjected to conventional tempering at different temperatures and it resulted in variation in the distribution and morphology of the carbides. Microstructural evolution at each tempering temperature, carbide morphology and distribution of carbides was investigated using SEM. Low temperature tempering leads to precipitation of rod like carbides and with increase in tempering temperature the carbide morphology turns spherical followed by carbide coarsening at higher temperatures. Three point bend test of the tempered samples was carried out using Schimadzu Universal testing machine to determine the energy absorption characteristics. Test results indicate that optimum combination of high energy absorption and better mechanical properties was delivered by tempered martensite with fine spherical carbides and fine prior austenitic grain size. Thus the work established the correlation between energy absorption; prior austenitic grain size and tempering temperature in low alloy quenched and tempered steel.