Abstract: In the actual production process, cracks are found in the grinding surface of the parking ratchet after carburizing and quenching, and it would lead to the scrap of the parts. In order to find out the cause of surface cracking after heat treatment and grinding of the parking ratchet and subsequent control measures, the crack parts were tested and analyzed by using microhardness tester, scanning electron microscope, energy spectrometer, X-ray stress analyzer and other characterization means, and the reproducibility test was conducted with normal parts and relevant indicators were compared. The results show that the carburizing and quenching microstructure of the cracked parts of the parking ratchet is normal, the surface residual stress is normal, there is no oxidation decarburization and carburization at the crack, there is no burn in grinding, and there are metal oxide inclusions on the surface of the parts. The root cause of surface cracks is the defects of oxide inclusions in the raw materials. This study provides a feasible direction for failure analysis. Some control measures to prevent cracks are put forward, such as monitoring the inclusions on the surface of raw materials, shortening the interval time of quenching and tempering, controlling the microstructure of the surface of heat treatment and optimizing the grinding parameters.

Key words: 20MnCrS5 HL steel, parking ratchet, carburizing and quenching, cracks, grinding, compressive stress, tensile stress, oxide inclusion