Abstract: Types and contents of precipitated phases in the high temperature range of a new low cobalt secondary hardening steel were calculated by Thermo-Calc thermodynamic software. Combined with the experimental methods of optical microscope (OM), scanning electron microscope (SEM) and mechanical testing, the effect of quenching temperature on the microstructure and properties of the tested steel was studied. The results show that there are a large number of spherical W/Mo-rich M₆C precipitates on the lath martensite matrix when quenching at lower temperature. With the increase of quenching temperature, M₆C precipitates dissolves rapidly and completely dissolves at 1060 ℃. The dissolution of M₆C precipitates enhances the secondary hardening effect and improves the impact properties. At the same time, it causes the prior austenite grains to be significantly coarsened, which has an adverse impact on the strength and toughness. Finally, the tested steel obtains the optimal match of comprehensive mechanical properties after quenching at 1060 ℃.

Key words: secondary hardening steel, quenching temperature, mechanical properties, precipitated phases