力矩扳手连接杆在使用中发生断裂，通过宏观观察、化学成分分析、力学性能试验、显微组织观察和断口分析等方法，对该连接杆断裂的原因进行分析。结果表明：连接杆硬度合格，抗拉强度、屈服强度和延伸率不合格，显微组织为回火索氏体+网状铁素铁+魏氏组织，不符合技术要求；断口属于疲劳断裂，裂纹起源于连接杆外表面，裂纹源处无夹杂物；断裂的主要原因是连接杆热处理工艺不当导致力学性能偏低；通过降低淬火温度、缩短保温时间和淬火转移时间以及提高淬火冷却速率等措施后，连接杆服役了18个月，未发生断裂。

The connecting rod of torque spanner was fractured during use. The reasons for the fracture of connecting rod were analyzed by methods such as macro observation, chemical composition analysis, mechanical performance test, microstructure observation and fracture analysis. The results showed that the hardness of connecting rod were qualified, the tensile strength,elongation and yield strength were unqualified. The microstructure was tempered sorbite+reticulated ferrite+widmanstatten structure, which did not meet technical requirements. The fracture belonged to fatigue fracture, and crack originated from the outer surface of connecting rod. The main reason for the fracture was the improper heat treatment process of connecting rod, which led to low mechanical properties. After reducing quenching temperature, shortening holding time and quenching transfer time and increasing quenching cooling rate and other measures, the connecting rod has been in service for 18 months without fracture.