Fracture Reason and Heat Treatment Process Improvement of Connecting Rod of Torque Spanner

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.