Abstract: During the commissioning period of a nuclear power plant unit, the containment isolation valve did not close as expected. Upon disassembling the valve and inspecting it, it was found that there were cracks on the surface of gear shaft gear. The causes of valve gear cracking were analyzed by means of macroscopic inspection, chemical composition analysis, metallographic examination and hardness testing. The results show that the cracks are quenching cracks, which occur at the transition point between the tooth root and the outer edge of the gear shaft, and there are obvious rough machining tool marks at the transition point of the tooth root fillet. Due to the overall high hardness of the gear area and the hardening of all teeth, it does not meet the requirements of JB/T 6395-2010. The analysis suggests that the occurrence of cracks is mainly related to the incorrect selection of heat treatment methods and improper control of heat treatment processes. The obvious rough machining tool marks at the transition of the tooth root fillet are the contributing factors to the occurrence of cracks.

Key words: containment isolation valve, gear, cracking, heat treatment