Flail knife is one consumable part of aluminum scrap impact crusher made in Germany. In order to lower production cost, localized production of flail knife has been researched. Flail knife in steel 42CrMo has been developed instead of that of Germany. In course of using, neck of the self-developed flail knife has been broke up and fractured. For seeking the reason of failure, microstructure and mechanical properties of new flail knife has been analyzed, and force condition has been calculated by finite element analysis, heat treatment process and installation technology has been investigated. The reason of failure in flail knife has been summarized, and the heat treatment process optimized has been designed and took good effect.