Effects of microstructure and corrosive environment on fatigue crack growth behaviour in a duplex stainless steel

Fatigue tests have been conducted on a duplex stainless steel, SUS329J4L, in order to clarify the influence of volume fraction of ferrite and austenite phases on crack growth behaviour in room air and in 3% NaCl solution. Three materials with different volume fractions were prepared with solution treatment at 1050°C, 1150°C and 1250°C, resulting in the volume fractions of austenite phase of 50%, 28% and 12%, respectively. In room air, when the data were characterized in terms of ΔK, the crack growth rates decreased considerably with increasing solution treatment temperature, Ts. After allowing for crack closure, i.e. evaluated by ΔKeff, all materials showed almost the same growth behaviour, indicating that the observed difference in crack growth resistance was attributed to crack closure derived from residual stress and crack path deflections. In 3% NaCl solution, the crack growth rates for all materials were enhanced in high ΔK or ΔKeff region. Cleavage fracture in ferrite phase was responsible for the enhanced crack growth, which increased with increasing Ts and ΔK.