Comparing microstructure and mechanical properties of AISI D2 steel after bright hardening and oil quenching

D2 is a high carbon-high chromium cold work tool steel. The main problems of hardening treatment of this steel include risk of crack distortion and formation because of thermal shocks induced by oil quenching (conventional hardening treatment for D2 steel). For this purpose, in this study, the possibility of improving microstructure and mechanical properties of AISI D2 steel and decreasing the risk of cracking by bright hardening in comparison with oil quenching was investigated. The quenchant used in bright hardening included 60% of KOH and 40% of NaOH. The bright-hardened samples showed no cracks during their heat treatment, which was in contrast to oil-quenched ones. Moreover, microstructural examinations of the samples revealed that, compared with oil-quenched samples, quenching in hot alkaline salt bath resulted in finer and more uniform distribution of carbides and also lower amount of retained austenite in the matrix. Different microstructures caused the samples to possess different mechanical properties; e.g. bright-hardened samples possessed higher impact toughness (3.7 J) and tensile strength (1764 MPa) as well as higher level of hardness (792 HV) than those of oil-quenched samples, which were 3.2 J, 1608 MPa and 710 HV, respectively.