Deformation mechanism in a single phase austenitic and ferrito-austenitic stainless steel

Abstract

The low cycle fatigue behaviour of a duplex stainless steel aged at 475oC has been studied by mechanical testing, scanning and transmission electron microscopy. It has been observed that the fatigue life is higher in the aged condition at lower strain amplitude (^e/2=0.4) and with gradual increase in ^E/2 the fatigue life becomes similar (^E/2=0.8). The cyclic hardening and softening behaviour also changes with change in ^E/2. The plastic deformation is mostly confined to the austenitic phase in the aged condition and this phase cyclically softens immediately after reaching stabilisation when ^E/2 is low. This information is confirmed by fatigue crack initiation studies at stress levels below the yield strength of the material. However, in the virgin condition the stabilizat-ion takes place over a period of time followed by gradual softening. At higher ^E/2 the stress level increases for the aged material indicating the involvement of the ferritic phase in the plastic deformation. However, at higher ^E/2, there is no remarkable change observed in the cyclic hardening and softening behaviour between virgin and aged conditions reveals in an orientation contrast for the needle shaped precipitates. The fractographs of the aged material reveal secondary cracks. At higher ^E/2 the ferritic phase in the virgin condition shows formation of dislocation cells and ladder structure in contrast to the ferritic phase in the aged condition that shows coale-scence of the precipitates, which are the probable sites for the secondary crack formation.