Effect of ageing at 475°C on fatigue crack growth behavior of (α + γ) two-phase stainless steel

The effect of ageing at 475°C on fatigue crack growth behavior was studied using (α + γ) two-phase stainless steel with two different grain sizes, which is expected to be used widely in industrial fields. The main results obtained are as follows; (1) In the low ΔK range, the fatigue crack growth rate (da/dN) decreased with ageing at 475°C for the coarse grained specimens. This is caused by the enhancement of the roughness induced crack closure due to the change of the deformation mode in α phase with ageing. However, da/dN was not affected very much by ageing for the fine grained specimens. (2) In the low ΔK range, da/dN became to be fluctuated very much with ΔK by the ageing. This is closely related to the fact that the fatigue crack growth mechanism changes from slip plane fracture to that of cleavage in every α grain with ageing. (3) In the high ΔK range, da/dN increased with ageing particularly for the coarse grained specimens, since the cleavage fatigue crack growth mechanism becomes to be more dominant as ΔK increases.