Bauschinger effects and work-hardening in spheroidized steels

Abstract

The effects of cementite particles and subgrain boundaries on the work-hardening behaviour of spheroidized carbon steels were investigated by making direct measurements of residual internal stresses. These internal stresses developed due to plastic incompatibilities between elastic particles and the surrounding elastic-plastic matrix. A continuum analysis of these internal stress fields, based upon a multiple slip model, was found to be in good accord with the experiments regarding its predictions of the Bauschinger effect. The internal stresses appear to saturate in the plastic strain range of ~ 3–5% where a transition in strain-hardening behaviour was observed (double-n behaviour), and to contribute approximately 20% to the total work-hardening. The cementite-particle-pinned subgrain boundaries, formed during a post-quench annealing treatment, were found to lower the internal stress levels, thus indicating that they assisted the relaxation processes of entrapped Orowan loops by acting as sources of dislocations. The flow stress curves of these spheroidized carbon steels were found to be approximately described by a modified mean-squareroot addition of several identifiable contributions to work-hardening. 'Normal interfacial stresses acting on the particles were estimated from the data. The maximum values of these stresses, which ranged upwards to about one half of the flow stress, occurred in the tensile direction at the poles of the particles. It is suggested that these stresses contribute to particle–matrix separation. © 1978 The Metals Society.