Abstract: The 42CrMoA quenched and tempered steel pull rod on an aluminum alloy die casting machine broke after 2 years of service. The machining quality, chemical composition, mechanical properties, microstructure and fracture morphology were examined, the fracture reason of the pull rod was analyzed. The results show that the leakage of release agent caused corrosion on the thread surface of pull rod, which led to premature corrosion fatigue fracture.