Constitutive description of plain carbon steels deformed at high temperatures: Effect of carbon content

The mechanical threshold stress (MTS) model has been employed to conduct a rational analysis of the flow stress data, previously reported in the literature, of a number of plain carbon steels of different carbon content, deformed in the temperature range of 850–1200°C, in a wide spectrum of strain rates. The main objective of the work has been the interpretation of the weakening effect of the addition of carbon to the steel when it is deformed under such conditions. The MTS, as a structural parameter, has been assumed to be composed of two distinct components: one that arises as a result of the interaction of dislocations with solute carbon atoms and another that results as a consequence of the interaction of dislocation among themselves and the re-arrangement of the dislocation substructure by means of dynamic recovery. The results of the present investigation indicate that the weakening influence of the carbon addition is a result of the decrease of both components with the increase in carbon content. The dislocation/solute interaction component has been observed to be reduced by approximately 8 MPa/wt-% C, independently of deformation temperature and strain rate, whereas the dislocation/dislocation interaction component of the MTS, has been found to be reduced between approximately 5–30 MPa/wt-% C, depending on the deformation temperature and strain rate. The latter effect has been related to a possible decrease in the work hardening rate because of an increase in the rate of dynamic recovery as a consequence of a rise in the self-diffusivity of iron in austenite, with the addition of carbon. In general, the constitutive model developed on these bases allows a close prediction of the flow stress and work hardening rate of the different steels analysed, at any deformation temperature, strain and strain rate, for any carbon content. The possible effect of the austenitic grain growth in the reduction of the MTS has been analysed and a modification of the constitutive description in order to incorporate grain size effects into the MTS has also been proposed.