Study of the fatigue behaviour and damage of a aged duplex stainless steel

Cast duplex stainless steels are commonly used in components of pressurized water reactors primary circuit. When submitted to in-service temperatures embrittlement occurs because of the nucleation and growth of a harder phase in the ferrite by spinodal composition. Macrostructure of this steel (ferritic primary grain size is about 4-5 mm) and embrittlement of ferrite due to aging lead to a very high scattering of mechanical properties for monotonous loadings. We showed that, in spite of this macrostructure, the cyclic behaviour of aged duplex stainless steels fits usual Manson-Coffin law while initial hardening is followed by softening, in part because of the demodulation of the composition. The fatigue crack propagation rate of material follows a Paris law. While crack initiation mainly appears next to the millimetric cast defects, fatigue crack propagation remains a continuous mechanism. Ferritic and austenitic elements break successively (ferrite first breaks by cleavage, then austenite breaks by ductile fatigue). In spite of the fact that the aged ferrite is embrittled, cleavage microcracks, for load levels examined, seldom appear in ferrite at the crack tip and on both sides of the main crack. Effects of cast defects and crystallographic ferrite orientation were also studied. Propagation fatigue crack behaviour was modeled assuming that the crack tip material behaves as if it was submitted to low cycle fatigue loadings. If we consider a homogeneous material, results are in good agreement with experiments. (authors)