某20Cr2Ni4钢活塞杆在装机使用一天后表面出现一条环状裂纹。采用低倍检验、化学成分分析、断口分析、金相检验以及力学性能测试等一系列试验手段对活塞杆环状裂纹属性及成因进行了分析。结果表明:该活塞杆环状裂纹属于疲劳裂纹;磨削工艺不当导致在活塞杆表层局部区域产生异常淬火马氏体组织，最终致使活塞杆在交变工作应力条件下发生疲劳开裂。

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.