Abstract: The crosshead of a large low-speed marine diesel is an important moving component,which is prone to damage due to its poor lubrication conditions.Cracks were found during magnetic particle testing of the 5S50ME-B crosshead finished product.The reasons for the occurrence of cracks in the crosshead were analyzed by methods such as hardness testing,macroscopic analysis of cracks,metallographic examination,and chemical composition analysis.The results indicate that the crack is quenching crack,which is caused by local tensile stress concentration during the medium frequency induction quenching process.By improving the heat treatment process and changing the heating method to rotary heating,cracks were avoided.