Abstract: Effects of different heat treatment processes on the microstructure and mechanical properties of DH350 steel were studied by means of optical microscope, scanning electron microscope, impact testing machine and hardness tester. The heat treatment processes included: oil quenching at 1030 ℃+tempering at 590 ℃+tempering at 600 ℃ (process I), air quenching at 1030 ℃+tempering at 590 ℃+tempering at 600 ℃ (process II), and oil quenching at 1030 ℃+oil quenching at 980 ℃+tempering at 590 ℃+tempering at 600 ℃ (process III). The results show that the microstructure of the tested steel is mainly tempered martensite+tempered sorbite+carbide under three heat treatment processes. The difference lies in the pronounced lath structure of the microstructure under process I, with fine carbides dispersed throughout. At this time, the hardness and impact property are high, and the fluctuation of impact absorbed energy is small, and the comprehensive property is good. Under process II, there is a pronounced coarsening of the lath structure in the microstructure, and carbides gather along the grain boundaries, resulting in reduced hardness and deteriorated impact property. Under process III, there is a significant refinement of the grain in the microstructure. However, there is a higher amount of undissolved carbides present, and mixed crystal phenomena occur. As a result, the impact absorbed energy is relatively high but exhibits greater fluctuation. Additionally, there is a noticeable decrease in hardness. Therefore, the optimal heat treatment process for the DH350 die steel is oil quenching at 1030 ℃, followed by tempering at 590 ℃, and tempering at 600 ℃. Under these conditions, the hardness and impact absorbed energy are measured to be 45.1 HRC and 22.4 J, respectively.

Key words: hot work die steel, air cooling quenching, double oil quenching, impact property, hardness