某5 t自由锻电液锤锤杆在锻打过程中突然发生断裂，采用宏观检验、化学成分分析、金相检验和力学性能测试等方法对锤杆断裂原因进行了分析。结果表明：由于该锤杆活塞和杆的圆弧过渡处未按设计要求做到光滑过渡，机加工刀痕明显，造成该处应力集中严重，促使裂纹源在此处萌生；加之锤杆的力学性能也略低于技术要求，导致锤杆在拉-压载荷的作用下发生了低应力高周早期疲劳断裂。

The hammer rod of a 5 t free forging electro-hydraulic hammer fractured suddenly in the course of forging. By means of macroscopic examination, chemical composition analysis, metallographic examination, mechanical properties test and so on, the fracture causes of the hammer rod were analyzed. The results showed that there were obvious machining marks at the arc transition position of hammer piston and rod because it didnt machine smoothly according to the design requirement. The machining marks induced serious stress concentration and initiation of the cracking origin at the arc transition, and at the same time the mechanical properties of the hammer rod was lower than the requirement, which resulted the hammer rod fractured with mode of low strain and high cycle early fatigue under the condition of tension-compression load.