Examination of rate-controlling mechanisms for plastic deformation of pearlitic steel at low homologous temperatures

Abstract

The rate-controlling mechanisms of plastic deformation of pearlitic steel before and after cold drawing at low homologous temperatures (T/Tm) were explored because cold-drawn pearlitic steel is an important structural material that requires long-term sustainability at low T/Tm. Both the undrawn and the cold-drawn pearlitic steels were deformed plastically under constant loads at low T/Tm of 0.25–0.30. The strain rate-stress data were analyzed in terms of a power law model based on low temperature dislocation climb, a lattice friction controlled plasticity model, and an obstacle controlled plasticity model. The first two models were inadequate to explain low T/Tm plastic deformation of the present steels: (a) much higher activation energy than that for core diffusion resulted from the low temperature dislocation climb model, and (b) athermal stress comparable to that for pure α-Fe was estimated from the lattice friction controlled plasticity model, so the role of cementite on athermal stress in pearlite was unexplainable. By contrast, the obstacle controlled plasticity model estimated activation energy on the same order as that for overcoming forest dislocations which were regarded as the main obstacles in the present steels, as evidenced by microstructural observation. In addition, this model not only gave an estimate of the athermal stress for the present pearlitic steel that was higher than that of pure α-Fe (reflecting the effect of cementite on athermal stress), but also provided an estimate of the athermal stress of the cold-drawn steel that was higher than that of the undrawn steel (reflecting the effect of pre-existing dislocations caused by cold-drawing in the former). Therefore, it is probable that low T/Tm plastic deformation of the present pearlitic steels is rate-controlled by dislocation glide overcoming forest dislocations generated by low T/Tm deformation and/or by cold drawing.