Abstract: The hardness of airscrew bush parts of helicopter after heat treatment was lower than the required value of design specification.The results of microstructure observation,composition analysis and process checking-up indicated that the essential reason of hardness decreasing the parts was as follows.The surface layer carbureting during the solid solution and aging process resulted in the decrease of martensite and increase of retained austenite in the surface layer microstructure and thus led to the reduction of hardness.In addition finish machining was not preformed to remove the surface carbureting layer so that the hardness was lower than the design specification.