Macroscopic Characterization of Mechanical Properties in Electric Current Treated Dry Drawn High Strength Wires
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Macroscopic Characterization of Mechanical Properties in Electric Current Treated Dry Drawn High Strength Wires Osamudiamen Omoigiade1 , Arunansu Haldar2 , and Rongshan Qin3,1 1
Department of Materials, Imperial College London Exhibition Road, London SW7 2AZ, UK 2 TATA Steel Swinden Technology Centre, Moorgate, Rotherham S60 3AR, UK School of Engineering I& Innovation, The Open University, Milton Keynes MK7 6AA, UK
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ABSTRACT The present paper investigates the use of electric current treatment in improving the drawability of plain carbon steel wire for high strength steel applications. The mechanical properties for wires of composition 0.80C 0.65Mn 0.27Si wt.% of diameters 4.09 and 3.00 mm dry drawn from 10.00 mm rods are characterised. The total number of passes for 4.09 and 3.00 mm diameter wires are 7 and 10 respectively resulting in true strains of 1.79 and 2.41. Samples are treated with electric currents in-between the two drawing stages of 4.09 and 3.00 mm, and tested at both stages in tension, torsion and reverse bending along with control samples for comparison. The applied currents are pulsed at a frequency of 100 Hz with each pulse being approximated by a square wave of loading width 80μs and modest current densities of 7.96 Amm−2 . Thus the influence of electric current on the drawability of plain carbon steel wire is assessed between stages of reduction.
INTRODUCTION High strength steel wires are used in a range of applications including pneumatic tires in the automotive industry. These are composites of an elastomeric matrix reinforced with steel. These steels have a fully pearlitic microstructure hence exhibit high work hardening rates which enable them to achieve competitive strengths demanded of their applications while retaining sufficient ductility. The strength of the steel is limited by the starting microstructure in particular the pearlite interlamlellar spacing, where a finer spacing results in greater strength [1]. Although the drawing procedure, which is the driving force for work hardening in the microstructure contributes most significantly to strength increases [2]. The drawing procedure is a step by step reduction in wire diameter by pulling the feedstock through holes in drawing dies inlaid with a hard metal carbide to successively smaller diameters. However, severe drawing distorts the pearlite microstructure which eventually becomes prone to delamination before the target diameter is reached. Delamination is characterised by cracking along the length of a wire such that the wire is no longer in one piece. Types of delamination in steel wire are described in detail by Godecki [3]. In order to circumvent early fracture, the microstructure is restored through a patenting process before conducting wet wire drawing; the third and last wire manufacturing stage precedent to dry drawing and mill-rolling. Whereby the steel is re-austenitized at 900-1000o C and then quenched into a molten salt or lead bath of ∼ 540o C to form a fine pearlite microstructure that can be further drawn. Unfortunately
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