Influence of Nitriding Condition on Fatigue Strength of Maraging Steel

Rotating bending fatigue tests were carried out for maraging steels nitrided under three conditions in order to investigate the influence of nitriding on fatigue strength. The fatigue strength was increased due to nitriding. The increase in fatigue strength and the fracture mechanism were markedly influenced by nitriding condition. That is, fracturing of the steel nitrided at under-aging condition, 480°C-1h and 5h, occurred from the surface of the specimen in the short life region, and from the interior in the long life region, although all of the specimen aged at under and over-aging conditions and nitrided at over-aging condition, 570°C-1h, fractured from the surface. Consequently, S-N curves in the former steel showed a two-step shape. The fatigue strength decreased with the increase in nitriding time in the short life region where the surface fracture occurred, though there is no or little influence of nitriding time on the fatigue strength in the long life region where the internal fracture occurred. The decrease in the fatigue strength was related to the surface brittleness caused by longer nitriding.