2007 年 56 巻 12 号 p. 1126-1132
In order to investigate the hardness dependence on the fatigue strengths for surface fracture and internal fracture, rotating bending fatigue tests were carried out for maraging steels using specimens with different hardness obtained by heat-treating at different under-aging conditions. Fatigue limit for surface fracture was evaluated in electro-polished specimens and the one for internal fracture was in shot-peened ones, where fatigue limits were defined as fatigue strengths at 107 cycles for surface fracture and 108 cycles for internal fracture, respectively. Fatigue limit for internal fracture increased with increase in hardness similar to the one for surface fracture. However, the proportional relationship between the fatigue limit and the hardness existed till higher hardness in the fatigue limit for internal fracture than in the one for surface fracture. This difference in the hardness dependence on fatigue limit was discussed from the viewpoint of the influence of humidity on the fatigue strength.