抄録
In order to improve the fatigue strength of a nickel base super alloy, Alloy 718, by nitriding, and make clear the crack initiation site in the nitrided alloy, fatigue tests of the plain specimen were carried out under rotating bending. The suitable treatment condition of radical nitriding was investigated from the view points of thickness of the compound layer and strength of the base alloy in combination of nitriding with aging conditions. The selected condition was the combination of treatments of nitriding at 570°C for 20h after aging at 720°C for 8h and then at 620°C for 4h in consideration of practical application. By this treatment, compound layer was formed on the specimen surface with the thickness of about 10μm without softening of the base alloy. Crack initiation was markedly suppressed by the compound layer, causing the increase in fatigue strength. A crack initiated at the base alloy beneath the compound layer. The initiated crack propagated gradually to both directions of the compound layer and the base alloy, though the crack in the compound layer showed a brittle manner.
