An experimental analysis and modeling of the work-softening transient due to dynamic recrystallization
Résumé
The flow softening brought about by dynamic recrystallization (DRX) during the plastic deformation of metals and alloys under hot-working conditions is of a great practical significance for the computation of the loads, torques and power consumption required in industrial hot forming operations. The present communication reports the main results of an investigation that was carried out in order to analyze in detail the work-softening transient present on the flow curves of a C-Mn steel deformed in a wide range of temperatures and strain rates. The analysis of the experimental stress-strain curves allowed the description of the temperature and strain rate dependence of four important stress parameters: yield, critical, saturation and steady-state, by means of the Sellars-Tegart-Garofalo model (STG). Also, it has been possible to derive an expression for the time required to achieve 50% recrystallization as a function of the deformation conditions, as well as the computation of the Avrami exponent of the material. All this information has been subsequently employed in the description of the flow stress of the material as a function of deformation conditions. For this purpose, an original constitutive description in differential form, which combines a work-hardening and dynamic recovery term, described by the phenomenological equation earlier advanced by Sah et al. and an additional softening term, which involves the Avrami relationship, is proposed. The evolution equation that has been advanced is independent of strain. Therefore, it is shown that, in principle, it is possible to describe satisfactorily the evolution of the flow stress during transient loading conditions as a consequence of changes in strain rate or deformation temperature, regardless if the material undergoes DRX during such a transient. Contrary to many different models reported in the literature, the approach here proposed is independent of the peak parameters exhibited on the flow curves.