Cause of Fracture on Large Spring in a Hydraulic System

A large cylindrical spiral compression spring used in a hydraulic system was fractured after operating 200 000 times. The causes of fracture on spring were analyzed by fracture morphology, microstructure observation, and chemical composition and hardness test. The results show that the fracture of the spring was low cycle fatigue fracture, and the crack source region was located at the large inclusions near the surface of the spring steel wire. The existence of inclusions reduced the internal continuity of the spring steel, and cracks initiated when the spring steel was subjected to tensile stress and torsional force, and the cracks expanded in the subsequent service process, eventually fracture. The depth of hardening layer formed during peening was not enough to effset the adverse effect of decarbonized layer on fatigue life of spring steel wire, resulting in the reduction of fatigue life.