Fracture Causes of 38CrMoAl Nitriding Steel Valve Stem in a Thermal Power Plant

The electric gate valve stem made of 38CrMoAl nitriding steel for the main feedwater pipeline in a thermal power plant was broken during the start-up operation. The cause of the failure was analyzed through macroscopic examination, chemical composition analysis, microstructure observation, mechanical property testing, and fractography. The results show that the fracture of the valve stem was attributed to deficiencies in both the metallurgical quality of the material and the heat treatment process. A significant amount of AlN inclusions was observed inside the valve stem, leading to inferior metallurgical quality. Inappropriate control of the heat treatment process resulted in microstructural inhomogeneity. After quenching and tempering, banded martensite was formed in the microstructure, while the hardness and local thickness of the surface nitrided layer failed to meet the specified requirements. These factors collectively caused the yield strength of the valve stem to fall below the standard, severely insufficient impact toughness, and overall poor mechanical properties. During pipeline vibration, stress concentration occurred at the root of the relief groove due to the absence of a transition radius. Cracks initiated from the nitrided layer surface at the relief groove and propagated rapidly inward, ultimately leading to brittle fracture. It ass recommended that manufacturers control the nitrogen content in molten steel to reduce inclusions, optimize the quenching-tempering and nitriding processes, improve the relief groove design to mitigate stress concentration, and strengthen the incoming inspection of critical components, thereby ensuring that the comprehensive mechanical properties of the valve stem meet the required standards.