Causes of fracture of 2Cr12NiMo1W1V steel door stem of medium pressure governing valve in a power plant

The fracture of 2Cr12NiMo1W1V steel door bar of medium pressure speed regulating valve in a power plant occurred during operation. The fracture reasons of the door stem were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the fracture of the door stem originated from the wall of the air leakage balance hole, which was not protected by anti-seepage nitrogen during the nitriding process. After nitriding, the brittleness of the material was enhanced, which further aggravated the stress concentration. Under the action of alternating load, the material formed microcracks and gradually expanded to the matrix, eventually leading to the fracture of the door stem. The elongation after fracture and impact absorption energy of the door bar were low, the strength was high, and the toughness was poor. To a certain extent, it accelerated the crack propagation and shortened the service life of the door bar.