Method of making surface-hardened metal shot

摘要

When a shot (26) having a hardness equal to or higher than a ferrous or nonferrous metal shot material (29) is blasted against the surface of the metal shot material (29) at the blasting speed of 80 m/s or above, the collision causes heat to be generated only in portions of the metal shot material against which the shot has collided. The temperature is raised in the vicinity of the surface of the metal shot material (29). Alternatively, the metal shot material may be blasted against a metal body having a hardness at least equal to that of the temperature of the metal shot material. In either case, the temperature in the vicinity of the surface of the metal shot material (29) is increased to or above an A3 transformation temperature when the material (29) is ferrous and is increased to or above a recrystallization temperature when the material (29) is nonferrous. Subsequently, the metal shot material (29) is quickly cooled. As a result, the metallurgical structure of the surface layer 20 mu deep from the surface of the metal shot material (29) is refined such that a highly hardened and tough structure is obtained. Part of the metal shot material (29) and the shot (26) is recovered and the recovered metal shot material (29) and the shot (26) are reblasted against unrecovered metal shot material (29) and the shot (26) repeatedly.