Fatigue Behavior of High Carbon Chromium Steel in Controlled Humidity
Haeng-Nam KO, Hisatake ITOGA, Keiro TOKAJI and Masaki NAKAJIMA
Abstract:The present paper describes the effect of humidity on the fatigue behavior of a high carbon chromium steel having very high strength. Cantilever-type rotary bending fatigue tests were performed in controlled relative humidity of 5, 45, 55 and 85%. It was found that step-wise S-N curves were seen in relatively lower humidity up to 55%. The transition stress at which the crack initiation site changed from the surface to the subsurface decreased with increasing humidity. In high humidity of 85%, however, cracks were always generated at the specimen surface regardless of stress level, thus step-wise S-N curve was not observed. The decrease of the transition stress and the disappearance of step-wise S-N curve in the high humidity were attributed to humidity-assisted surface crack initiation. At stress levels where surface or subsurface crack initiation occurred, fatigue lives were nearly the same regardless of humidity because of shorter time to fracture in the former and crack initiation from an inclusion inside the specimens in the latter. In subsurface fracture, a fish-eye was always seen with an inclusion at its center from which cracks initiated, and the morphology of fish-eyes was independent of humidity. Fatigue life increased with decreasing stress intensity factor range for inclusion, which was not influenced by humidity. Key Words:Fatigue, Step-wise S-N curve, Fracture mode, Fish-eye, Humidity, High strength steel