Abstract: 42CrMo large ring forgings were widely used in fields such as nuclear power, aerospace, and aviation due to their excellent mechanical properties. Based on the theory of deformation heat treatment, 42 Cr Mo steel was studied. A new process of waste heat controlled temperature quenching between ordinary quenching and waste heat quenching was adopted to study the influence of cooling and certain time of insulation on the size and hardness of steel grains. The results show that with the increasing insulation temperature after cooling, the steel undergoes a transformation from bainite to pearlite and ferrite in the heat treatment process. The size of the lath martensite in the final quenching state first increases and then stabilizes. After heat treatment, the size of the lath martensite decreases to（1.91 ±0.017） μm when insulated at 530 ℃, with the highest hardness reaching（710±10） HV. With prolonged insulation time, the size of the lath martensite in the final quenching state gradually refines and reaches（1.73 ±0.016） μm after 300 s of insulation, with the highest hardness of（736 ±8） HV. The analysis indicates that the new waste heat controlled-temperature quenching process retains the advantages of conventional quenching in grain refinement and energy savings from waste heat quenching, further refining the grain size and increasing hardness. The new process can provide references for optimizing and improving the production and manufacturing process of 42CrMo steel large ring forgings.