Abstract: Microstructure, carbide precipitation behavior and mechanical properties of C61 gear steel specimens after quenching at 1000 ℃ and tempering were studied by using SEM, TEM, XRD, microhardness tester, tensile testing machine and impact testing machine, etc. The results show that under the quenched and cryogenic conditions, the primary carbides in the tested steel are basically dissolved and the matrix is lath martensite. In this condition, the solid solution strengthening can provide a better basis for strengthening and toughening. When the tempering temperature is 420 ℃, the precipitated M₃C cementite provides higher strength, but the presence of which has greater damage to the impact property. The M₃C cementite will dissolve when tempered at 482 ℃, and the rod-shaped M₂C carbides with a size of 10-20 nm are dispersed and precipitated in the lath martensite, which not only provides higher strength but also improves the impact property. With the increase of tempering temperature, a large amount of reverse-transformed austenite is formed which not only effectively improves the impact property but also makes the strength decrease more obviously, meanuhile, the M₂C carbides become coarser and the strengthening effect of the second phase decreases. It can be concluded that the tested steel can achieve better matching of strength and toughness under the tempering condition of 482 ℃, at which the tensile strength is 1781 MPa, yield strength is 1546 MPa, impact absorbed energy is 97 J, and peak value hardness is 52 HRC.

Key words: tempering temperature, gear steel, microstructure, mechanical properties, carbide