Abstract: During the processing of a certain type of 45 steel nut, two nuts were found to be broken during the computerized numerical control lathe process. The fracture reasons of the nuts were studied by means of macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination, hardness test and heat treatment process analysis. The results show that there were quenching cracks at the nut fracture. The thickness of the hole wall of the nut blank was within the range of the sensitive thickness of the quenching cracks, and the transition arc between the bottom hole and the inner wall was small, resulting in stress concentration and the formation of the quenching cracks. The cracks propagated along the crystal, and finally the nut fracture occurred during the computerized numerical control lathe process.