对SCM435钢制备的10.9级螺栓进行不同的加载，使其发生过载、疲劳和氢脆断裂，结合断裂的宏观和微观形貌，研究螺栓不同断裂类型断口的形成机理。结果表明：各断面均出现在螺纹部位，且微观特征明显，过载断口均表现为韧窝特征，其中盲孔拧断断口的韧窝为剪切韧窝；疲劳断口疲劳辉纹鲜明，高、低周疲劳断口的疲劳区面积占整个断面的2/3和1/2；氢脆断口晶粒清晰，沿晶断裂特征显著。不同类型的断口中，拧紧+拉伸断口缩颈和螺纹伸长最为明显，高、低周疲劳断口基本没有缩颈和伸长，其余类型的螺纹断口略有伸长。

The 10.9 grade bolts made of SCM435 steel were subjected to overload, fatigue and hydrogen embrittlement fracture. Combined with the macro and micro morphology, the formation mechanism of different fracture types of bolt surfaces were studied. The results show that each fracture surface occurred in the thread part, and the micro characteristics were obvious. The overload fracture surface shows the characteristics of dimple, and the dimple of the screw fracture surface of the blind hole was shear dimple. The fatigue fracture surface had bright fatigue lines, the fatigue area of high and low cycle fatigue fracture surface accounted for 2/3 and 1/2 of the whole section. The grain of hydrogen embrittlement fracture surface was clear and the characteristics of intergranular fracture were obvious. Among different types of fracture surfaces, tightening+stretching fracture surfaces necking and thread elongation were the most obvious, the high and low cycle fatigue fracture surfaces basically had no necking and elongation, and the other types of thread fracture surfaces had a slight elongation.