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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.