在加工某型45钢螺帽时,数控车过程中发现有2个螺帽发生断裂。采用宏观观察、化学成分分析、扫描电镜及能谱分析、金相检验、硬度测试和热处理工艺分析等方法,研究了螺帽断裂的原因。结果表明:螺帽断裂处存在淬火裂纹;螺帽毛坯孔壁的厚度处于淬火裂纹敏感厚度的区间内,且底孔与内壁的过渡圆弧较小,产生了应力集中,并形成了淬火裂纹,裂纹沿晶扩展,最终在数控车过程中螺帽发生断裂。

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.