Effect of Quenching Temperature on Quenching Cracking of 45 Steel

45 steel rods in forged and normalized state with a diameter of 8 mm for sealing plug were quenched at 750-880℃ for 15 min and high temperature tempered at 550℃ for 30 min. The microstructure, fracture morphology and hardness of the specimens at different quenching temperatures were studied. The effect of quenching temperature on quenching cracking was analyzed and the heat treatment process was optimized. The results show that the specimens did not crack after quenching at 750℃ and 780℃, and cracked after quenching at 800-880℃. With increasing quenching temperature, the ferrite content in microstructure decreased, the grain size increased, and the hardness first increased and then decreased. When the quenching temperature ranged from 800℃ to 830℃, the cause of quenching cracking was that the cooling rate of undercooled austenite in the martensite transformation phase region was too large, and the structural stress was concentrated in the outer layer of the specimen, which led to the propagation of crack in a mixed mode of intergranular and transgranular. When the quenching temperature ranged from 850℃ to 880℃, the grain boundary was weakened due to the higher quenching temperature, and cracks propagated along grain boundaries under the combined action of the structural stress and the thermal stress. The best tempered sorbite structure and relatively high hardness of 45 steel could be obtained and the quenching cracking could be avoided by setting a slow cooling process of 3-5 s at room temperature before quenching at 830℃ and then tempering.