Fatigue behaviour of duplex stainless steels in the very high cycle regime

Very high cycle fatigue tests for two austenitic and ferritic duplex stainless steel grades have been performed and multi-stage duplex S–N curves obtained. The plateau stresses that are comparable to conventional fatigue endurance depend on the strength of the steel grades. The mechanisms for the transition of fatigue crack initiation from surface to subsurface were also investigated. It was found that surface fatigue crack initiation started mainly at the ferrite phase, due to extrusion and intrusion of slip bands, where the applied stresses were higher than the yield strength. The subsurface fatigue crack initiation also started in the ferrite phase, due to the presence of subsurface stress-raiser like inclusions that cause the formation of localised fatigue damage or strain localisation. A micro-fracture mismatch between austenite and ferrite was observed in ODA, and the stress intensity factor range at the front of ODA is comparable with the effective threshold value for fatigue crack propagation.