Causes of carburized pits and black spots on the surface of ball valve

The surface of 316L steel ball valve used in a thermal power plant produced pits and black spots after carburizing heat treatment, which seriously affected the sealing effect of the ball valve during use. The causes of the pits and black spots were analyzed by means of macroscopic observation, scanning electron microscopy and energy spectrum analysis. The results show that the improper control of temperature and heat treatment time in the carburizing process of ball valve results in the formation of local segregation of surface carbon elements and the formation of local black areas. The Cr element formed a short-range diffusion in the surface area and coverd the surface layer. After carburization, it precipitated in the form of M₂₃C₆ and Fe-Cr-C compound phase. M₂₃C₆ segregated in spherical form and fell off with the operation process, and formed pits. The elements segregated at the edge of the pits and precipitated harmful phases. The precipitation and falling off alternate, and eventually led to the accelerated formation of pits.