Cyclic plasticity of 17-7 precipitation-hardenable semi-austenitic stainless steel

grantor: University of TorontoSince their development in the 1940s, precipitation-hardenable stainless steels have been used predominantly in sheet form. In more recent times there has been renewed interest in these alloys from the aerospace and automotive industries as candidates for spring materials as current economies dictate a much higher strength to weight ratio. This requires a more thorough understanding of their cyclic deformation and sag resistance or stress relaxation behaviour. The present research is carried out to investigate the cyclic plasticity of 17-7 precipitation-hardenable semi-austenitic stainless steel in terms of the Bauschinger effect, cyclic hardening and/or softening behaviour and cyclic creep or ratchetting behaviour. Asymmetrical loading of the material revealed that miniscule forward and reverse ratchetting could occur under certain loading conditions during the initial fatigue life, while compressive creep dominated the latter stage. Prior strain hardening in the form of tensile pre-strain did not cause cyclic softening which was observed in most materials. Instead cyclic hardening was found to be prevalent and the underlying mechanism for this behaviour is suggested to be the partial plastic deformation of the martensitic phases. The cyclic stress ratio, R, was found to have no significant effect on the cyclic hardening or softening behaviour but was found to have a prominent effect on the cyclic creep behaviour. From transmission electron microscopy study, it was found that elongated-cell dislocation structures existed in the material regardless of the R ratio and peak stress amplitudes. These dislocation structures were inherited from the pre-strain conditions, and cyclic loading only changed the wall width of these cells but the general layout appeared to be largely unaffected. Two major types of dislocation cell morphologies are observed, and a mechanism is provided for explaining the cyclic hardening and creep behaviour in the material. The accommodation and release of energy by the material during cycling were examined and results have proven that the Bauschinger effect alone could not account for the cyclic creep behaviour of the material.Ph.D