Reasons Analysis on Circular Crack of a 20Cr2Ni4 Steel Piston Rod

A piston rod made of 20Cr2Ni4 steel appeared a ring-shaped crack on the surface after assembling and one day operating. A series of tests such as macro structure examination, chemical composition analysis, fracture analysis, metallographic examination and mechanical property test were performed on the piston rod for nature and causes of the crack. The results show that the ring-shaped crack was fatigue crack in nature. Due to the improper grinding process, the abnormal quenched martensite formed at the local area of the rod surface layer, which resulted in the fatigue cracking of the piston rod under alternating stress service condition.