某电厂中压调节阀在运行过程中因活塞杆断裂而发生故障,采用化学成分分析、宏观和微观检验、扫描电镜及硬度测试等方法对断裂件进行了分析。结果表明,活塞杆在调质处理中存在淬火冷却不足或保温时间不足等原因,使其显微组织自表面至心部为回火索氏体回火索氏体+少量屈氏体贝氏体+屈氏体+沿晶界呈网状或半网状析出的铁素体,从而导致其硬度和屈服强度均低于设计要求,加上在活塞杆的螺杆螺纹处存在应力集中倾向,最终在运行应力的作用下发生疲劳断裂。建议在调质处理时要控制好各种参数,同时在加工螺纹时应避免尖角过大问题。

The intermediate pressure control valve of certain power plant conked out during operation. The piston rod had ruptured. Its material is 40Cr steel. Low cooling speed of after quenching caused insufficient cooling structures after quenching and high-temperature tempering. The microstructure of piston rod from surface to center was tempered sorbite→tempered sorbite+little troostite→bainite+ troostite+netlike or half netlike ferrite. The hardness and yield strength of piston rod materials do not meet technical requirements of designs,and the fatigue strength declined. The fracture was related with lower strength of piston rod,and was fatigue fracture caused by stress concentrating effect. Its suggested that sharp corner shall not be too large in processing screws for piston rod,quenching cooling speed shall be controlled in quenching and high temperature tempering.