Evolution of fracture behaviour in progressively drawn pearlitic steel

Abstract

In this paper the fracture behaviour of progressively drawn pearlitic steels is studied. To this end, samples from different stages of an industrial manufacturing process were analyzed to elucidate the consequences of steelmaking by cold drawing on the fracture toughness and the microscopic fracture modes. The real manufacture chain was stopped in the course of the process, and samples of five intermediate stages were extracted, apart from the original material or base product (hot rolled bar: not cold drawn at all) and the final commercial product (prestressing steel wire: heavily cold drawn). Results demonstrated that progressive cold drawing affects clearly the fracture performance of the materials, so that the most heavily drawn steels exhibit anisotropic fracture behaviour with crack deflection, i.e., a change in crack propagation direction which deviates from the original mode I propagation and approaches the wire axis or cold drawing direction, thereby producing a mixed mode stress state. At the microscopical level, clear changes are observed in the micrographs with appearances from cleavage-like in the slightly drawn steels to predominant micro-void coalescence in the heavily drawn steels. From the macroscopic fracture mechanics viewpoint, the manufacture process by cold drawing is beneficial, since the fracture toughness is progressively increased by steel-making. These important results demonstrate that both the traditional mechanical properties (e.g. the yield strength) and the fracture mechanics properties (e.g. the fracture toughness) are improved by cold drawing.