某45钢法兰零件调质处理后内孔边缘发生开裂。通过宏观检验、硬度测试、化学成分分析、金相检验的方法，对该45钢法兰的开裂原因进行了分析。结果表明：法兰开裂的主要原因是法兰内孔边缘未倒角，淬火过程产生极大的表面拉应力，造成拉向应力开裂；另外基体存在枝晶状组织，降低了材料的强度和韧性；热处理过程中法兰表面产生较严重的脱碳层，进一步降低了材料的表面强度，最终导致内孔边缘淬火开裂。

A 45 steel flange cracked on the edge of the inner hole after quenching and tempering treatment. The cracking reasons of the 45 steel flange were analyzed by macroscopic examination, hardness testing, chemical composition analysis and metallographic examination. The results show that:the main reason for the cracking of the flange was that the hole edge of the flange wasn't chamfered, which produced large tensile stress on the surface during quenching process and caused the tensile stress cracking; in addition, dendrite structure existed in the matrix and reduced the strength and toughness of the material; in the process of heat treatment, the severe decarburization layer appeared on the surface of the flange, which further reduced the surface strength of the material, and eventually led to the cracking on the edge of the inner hole.