Influence of QLT treatment on microstructure and mechanical properties of a high nickel steel

After direct quenching from 870 °C, the samples were subjected to secondary quenching (L) at different intercritical temperatures within the two-phase region and various tempering temperatures (T). Optical microscopy and scanning electron microscopy were employed to analyse the distributions of phases and grain sizes, and transmission electron microscopy was used to reveal the morphology of martensite and retained austenite. Charpy impact specimens of the steel after different heat treatment were tested. Results show that QLT treatment gives the best impact toughness values compared with the rolling condition and conventional quenching and tempering process, and the optimum QLT heat treatment parameters are determined as Q: 870 °C, L: 770 °C, and T: 580 °C.