Abstract: Metallographic, chemical composition, crack morphology and other physical and chemical tests were carried out to analyze the causes of cracks in the failed joint bottom. The results show that the joint bottom cracks are quenched cracks; The main reason is the small angle radius of the transition arc R between the center cylinder of the joint bottom and the bottom, which leads to the stress concentration during the quenching process leading to cracking; The central part of the raw material is loose and cooled quickly, which further promotes crack initiation;By increase the R angle, adjust the heat treatment process and control the raw material quality, can reduce the quenching stress to avoid the joint bottom quenching cracking.