浸炭材の表層組織とショットピーニングで発生する最大圧縮残留応力に関する研究

抄録

Shot peening treatment has been applied to strengthen automotive parts, such as carburized gears for transmission and heat treated coil springs for suspension. The treatment effectively increases compressive residual stress on and beneath the surface of parts, leading to high fatigue strength parts. Therefore, researches on optimizing shot peening condition have been conducted intensively. In this research we changed surface microstructure in carburized steels and measured residual stress distribution in the carburized case after shot peening. Considered plastic deformation of shot media during the peening, two types of media were prepared; media 1 has lower hardness and media 2 has higher hardness comparing to the hardness of the case.The results are as follows.(1) For eutectoid carburized specimen, maximum compressive residual stress decreased with the hardness of the case when the shot media 1 was applied. On the contrary, when the media 2 was shot peened, maximum value increased with the hardness of the case. (2) Retained austenite, which changed into martensite during shot peening, decreased the increment of the maximum compressive residual stress. It was explained by the change in hardness in the case due to work hardening.(3) In case of carbide dispersed case, maximum compressive residual stress became lower than that of eutectoid carburized case. Because of the higher hardness of the carbide, media deformed plastically at the moment of peening, and loosened its energy to induce compressive residual stress in the case.