通过金相和扫描电镜观察、显微硬度检验、化学成分分析对某门式起重机主梁连接螺栓的断裂原因进行了分析。结果表明:裂纹源头位于螺纹根部加工缺口处;由于螺栓内部组织是板块状回火马氏体和针状下贝氏体,脆性较大,当螺栓承受较大外应力时,裂纹沿着内部微裂纹和孔洞扩展,最终发生断裂。 Through the metallographic and scanning electron microscope observation, microhardness test and chemical composition analysis, the cause of the fracture of the connecting bolt of the main girder of a gantry crane was analyzed. The results show that the source of the crack is located at the notch of the root of the thread. Because the internal structure of the bolt is tempered martensite and needle-like lower bainite, the brittleness is large. When the bolt is subjected to large external stress, Cracks and holes expand and eventually break.