Fatigue properties of high nitrogen steels

The paper reports the different effects that nitrogen produces on the fatigue properties of austenitic and duplex stainless steels. High nitrogen steels exhibit higher stress levels to strain cycling than conventional ones and a strong tendency to softening. In general, the fatigue resistance is increased with nitrogen alloying but there exists a limit upon which the effect becomes negligible. In the single phased alloys, these properties result from strong interactions between nitrogen atoms and dislocations, occurrence of planar slip and of short range order and in some cases decrease of stacking fault energy. In dual phased alloys, the role of nitrogen appears in addition with an indirect way. Being a γ stabilizer, addition of nitrogen in duplex stainless steels tends to produce more austenite than ferrite which appears beneficial for controlling the softening and for the distribution of fatigue damage. Finally, it is shown that a too large amount of nitrogen restricts the safe fields of application.