Effect of Tempering Temperature on Microstructure and Mechanical Properties of Intercritical Quenched 40CrNiMo Steel

40CrNiMo steel was treated by oil intercritical quenching at 740 ℃, and then was tempered at different temperatures (570, 600, 630 ℃). The effects of tempering temperature on the microstructure and mechanical properties of the test steel were investigated and compared with those of the steel treated by fully quenching at 850 ℃ and tempering. The results show that after intercritical quenching and tempering, the microstructure of the test steel consisted of tempered sorbite, ferrite and cementite. With the increase of tempering temperature, the granular cementite precipitated rapidly and aggregated to grow up, the recrystallization degree of α ferrite increased, and the hardness of the test steel decreased from 203 HV to 194 HV and was lower than that of the fully quenched and tempered test steel. Under intercritical quenching and tempering, the change of tempering temperature had no significant effect on the low-temperature impact toughness of the test steel, and the variation range of the impact absorption energy at −20 ℃ was 4–5 J. The impact absorption energy at −20 ℃ of the intercritical quenched and tempered test steel was higher than that of the fully quenched and tempered test steel. The impact fracture at different tempering temperatures showed obvious ductile fracture characteristics, and the size and number of dimples did not change much. Within the range of test parameters, The optimal tempering temperature for the intercritical quenching and tempering treatment of the test steel was 630 ℃; at this time, the −20 ℃ impact absorption energy of the test steel was 33 J, which met the engineering application requirements, and the steel also had a relatively high hardness (194 HV).