Work Hardening in Ferritic Steel

Abstract

Work hardening is one of powerful strengthening methods in metals. In this paper, the work hardening behavior is reviewed on ferritic steel from the initial stage to severe deformation stage. The work hardening behavior is roughly separated to three stages: 1) Dislocation strengthening without the change of ferrite grain size (true strain < 2.66). 2) Dislocation strengthening accompanying dynamic grain refinement (true strain > 2.66). 3) Grain refinement strengthening exceeds the strength obtained by dislocation strengthening (Grain size < 0.3 μm). With decreasing grain size and increasing carbon content, the introduction of dislocations is promoted thus yield stress (or flow stress) is enhanced at an identical percentage of deformation due to the increased dislocation density. In addition, the behavior of dislocation strengthening depends on not only dislocation density but also the character of dislocations and their arrangement. When deformation strain exceeds 2.66 in true strain, dynamic grain refinement starts and the decrease of grain size affects to make the dislocation density increase. The limit of dislocation strengthening is around 1.2 GPa, therefore the mechanism of work hardening changes finally to grain refinement strengthening when the grain size has been refined below 0.3 μm through the dynamic grain refinement. As a result, it is concluded that the mechanism of work hardening is dislocation strengthening on the early stage of deformation but grain refinement strengthening on the latter stage of work hardening.

Effect of ferrite grain size d on the flow stress σt at ε=0.5. The broken line shows the yield stress obtained by Eq.23.