Rotating bending fatigue tests were carried out for an 18%Ni maraging steel in order to investigate the effect of humidity on a crack initiation and its propagation of the steel. Moreover, the effect of reverted austenite on the sensitivity of fatigue strength for humidity was also investigated. Many cracks were initiated at the specimen surface and intergranular cracks were observed on the fracture surface when humidity was high. Both of the crack initiation and the early stage of the crack propagation were accelerated by humidity, especially the effect on the crack initiation was large. However, the effect of humidity on the crack initiation and its propagation was decreased and the intergranular cracking was suppressed by the formation of reverted austenite.
