Microstructure–Property Relationship in Cold-Drawn Pearlitic Steel Wires

Abstract

The difficulty in predicting flow stress of a cold-drawn pearlitic steel wire arises from the complex microstructural transformations induced by cold drawing. For assessing properly the different contributions to strengthening, four quantities must first be expressed as a function of strain $$ \varepsilon $$: (i) residual volume fraction of cementite; (ii) interlamellar spacing $$ \lambda $$, taking initial orientation of cementite lamellae into account; (iii) carbon concentration in ferrite deduced from strain-induced carbon partitioning between ferrite and cementite; and (iv) density of dislocations. Combination of these four expressions shows that cementite is able to significantly deviate from stoichiometry before dissolving, and that dislocations alone in ferrite are not able to control the decomposition of cementite unless $$ \varepsilon $$ reaches 4, implying involvement of solid solution. At $$ \varepsilon > 1.5 $$, when all lamellae are parallel to the wire axis and Voigt model can be applied, it is found that strain hardening is negligible, ferrite hardening by carbon in solid solution is canceled out by cementite softening and flow stress is better described by the Orowan mechanism ($$ 1/\lambda $$ dependence) than by the Hall–Petch law ($$ 1/\sqrt \lambda $$ dependence).