Probing microstructural evolution and surface hardening of AISI D ₃ steel after multistage heat treatment: An experimental and numerical analysis

Abstract In this study, different heat treatment cycles such as martempering, austempering, quenching, and quench tempering were applied to American Iron and Steel Institute (AISI) D 3 tool steel. Optical microscopy, scanning electron microscopy, and molecular dynamics (MD) approaches were utilized to evaluate the heat-treated microstructures. Moreover, the hardness and microhardness of the specimens were studied. The austempered specimen showed lower hardness than the partially and fully martempered specimens due to the formation of a bainitic matrix. On the other hand, the hardness loss of the fully martempered specimen was insignificant due to its low-carbon martensite matrix and alloy carbide hardness. Tempering of quenched specimens changed the carbide alloy from M 3 C to M 3 C 7 , increasing the microhardness from 1,150 to 1,756 HV, whereas martensite microhardness decreased from 817 to 485 HV. The observed hardness reduction of the quench-tempered specimen suggests that the matrix part of an alloy significantly contributes to its hardness. The MD simulation results reveal that grain boundaries act as favorable sites for thermal twin formation in the microstructure during the tempering of the quenched specimen. A large concentration of such thermal twins would be another reason for the hardness loss of the tempered specimen.