Effect of Radical Nitriding on Fatigue Strength of Alloy 718 under Rotating Bending

Rotating bending fatigue tests were carried out using a radical nitrided nickel base superalloy, Alloy 718, to investigate the effect of the nitriding on initiation and propagation behavior of a fatigue crack. The nitriding was done for 12h at 500°C and for 20h at 570°C in consideration of practical application. Compound layers were formed on the specimen surface with the thickness of about 4mm and 10mm respectively under these conditions. Hardness of the base alloy was hardly affected by nitriding at 500°C, but it was decreased at 570°C. Fatigue strength increased by nitriding. The increase in fatigue strength was mainly caused by the suppression of a crack initiation through the compound layer. A surface crack in the aged alloy initiated in ductile manner and one in the nitride alloy initiated in brittle manner from the specimen surface. Also, crack growth rate in the alloy was slightly decreased by nitriding at 500°C, but it was accelerated by nitriding at 570°C.