Abstract: Tempering treatments for a direct quenched 800 MPa grade hydropower steel were studied, and the effect of three different tempering temperature on microstructure and mechanical properties of the steel was analyzed. The results show that after controlled rolling and rapid water cooling quenching at 770-820 ℃, with the increase of tempering temperature in the range of 620-680 ℃, the yield strength and tensile strength decrease, while the elongation and impact absorbed energy of the steel increase. The best match of strength and impact properties is achieved when tempered at 650 ℃. In the tempering temperature range of 620-680 ℃, the microstructure of the steel is tempered bainite, and as the tempering temperature rising, the carbides gradually grow and are granular distributed, and bainite appears polygonal features.

Key words: hydropower steel, direct quenching, tempering, mechanical properties, microstructure