Microstructure and properties of electropulsed high carbon steel wire rod

Abstract

This thesis explores the influence of pulsed electric current (electropulsing) on the microstructure and mechanical properties of high carbon wire rod steels with the aim of improving wire drawability and reducing manufacturing costs. Industrial standard wire tests and microstructural characterisation were performed alongside neutron strain scanning to assess the effects of electropulsing on drawability and microstructure.

High carbon steel wires exploit the exceptional work hardening mechanisms of pearlite to produce versatile components used in high strength applications such as bridge wires and pneumatic tyre reinforcement. However, pearlite’s work hardening is a double-edged sword, as it accumulates severe strain during wire drawing and becomes prone to failure before achieving target wire diameters. To avoid premature failure, wires undergo multiple annealing and patenting steps, including intermediate patenting, which involves passing wires through a 900-1000 °C austentising furnace, forming coarse austenite, before being quenched into a ~540 °C lead bath or forced air cooled to form relatively fine pearlite. However, having coarse austenite grains compromises pearlite’s capacity to absorb energy and ductility, and as a consequence additional annealing steps are required, increasing manufacturing costs and slowing production.

Previously, electropulsing has induced recovery and recrystallisation of fine grains in cold rolled single phase alpha-Ti and Cu strips at room temperature compared with their original grain size. Therefore, pulsed electric currents present an attractive alternative processing tool to alleviate strain accumulation in high carbon steel wire and promote grain refinement.

Here, the capability of electropulsing to relieve residual strain in drawn wires are assessed by testing wires in torsion, tension and reverse bending at stages before and after pulsing at room temperature. The mechanical properties of the pulsed wires showed marginal increases in tensile strength but reduced ductility. Contrary to previous notions, neutron diffraction revealed observed changes in mechanical properties were due to augmented compressive strains in the (110) plane which is thought to be due to the formation of Cottrell atmospheres stabilising dislocations.

To determine whether the refinement process persists in iron and at elevated temperatures, the microstructures of 5.5 mm diameter wire rods are examined in three stages of transformation: prior austenite; partially transformed pearlite nodules and fully transformed pearlite. Grain refinement of austenite was found to persist at elevated temperatures and the austenite grain size was refined by 60 % applying pulsed currents of 20 - 160 A, 100 Hz, 80 μs pulse widths with a potential of 24 V over the austenitising period of 150 seconds. This translated into generally improved ductility to failure, measuring a 10.8 % engineering strain (100 A) in comparison to 6.7 % the non-electropulsed sample.