Abstract: The hardness abnormality of striking covers made of 20Cr2Ni4 steel led to serious decrease of machining tool life. The abnormal and normal workpieces were compared and analyzed by macroscopic analysis, chemical composition analysis, hardness analysis and metallographic examination. The results show that granular bainite with high hardness was produced in 20Cr2Ni4 steel after hot working. The subsequent normalizing and high tempering heat treatment can decompose granular bainite and reduce the hardness of products. The abnormal workpieces were products without heat treatment, whose coarse austenite grain and high hardness resulted in serious tool damage. It is suggested classifying the products and strengthening the on-site management and monitoring.