Delamination of Pearlitic Steel Wires: The Defining Role of Prior-Drawing Microstructure
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PATENTED, wire-drawn, and then stress-relieved near-eutectoid pearlitic wires are a unique grade of steel: a material with up to 7 GPa tensile strength[1] and excellent tensile/torsional ductility.[2–4] Lead patenting of the pearlitic steel wires involves the heating of cold-drawn wires to austenite region, followed by sudden quenching in a lead bath.[3,4] Patenting achieves fine interlamellar spacing, while wire drawing refines it further and also enforces pearlite alignment with wire axis.[5,6] However, very high wire drawing strains (> 3.5) have been reported to promote carbide dissolution.[4,7–9] The later may cause ‘delamination’: reduced ductility A. DURGAPRASAD and I. SAMAJDAR are with the Department of Metallurgical Engineering and Materials Science, IIT Bombay, Mumbai, 400076, India. Contact e-mail: [email protected] S. GIRI is with the Department of Metallurgical Engineering and Materials Science, IIT Bombay and also with the Research and Development Division, Tata Steel, Jamshedpur, 831 001, India. S. LENKA, S. KUNDU, and S. CHANDRA are with the Research and Development Division, Tata Steel. SUDIP KUMAR SARKAR and ANIRUDDHA BISWAS are with the Glass and Advanced Materials Division, Bhabha Atomic Research Centre, Mumbai, 400085, India. S. MISHRA is with the Department of Mechanical Engineering, IIT Bombay, Mumbai, 400076, India. R.D. DOHERTY is with the Department of Metallurgical Engineering and Materials Science, IIT Bombay and also with the Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104. Manuscript submitted October 25, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and longitudinal fracture during stranding steel wire for cables.[10–13] It is not surprising that wire drawing-induced delamination had attracted considerable scientific and industrial interests.[9–23] Bae et al.[23] proposed that originally misaligned coarse pearlites might generate larger voids during wire drawing, which may lead to delamination. Shimizu and Kawabe[13] and Lee et al.[19] also attributed delamination to the increased tendency of microvoid formation. Languillaume et al.[7] reported delamination through carbide dissolution after a drawing strain of 3.5 which gave an interlamellar spacing of only 20 nm. Maruyama et al.,[11] with an atom-probe study, showed that increased carbon concentration in lamellar ferrite correlated with the loss of ductility. Another classical study of carbide dissolution is from Hono et al.[8] They showed cementite decomposition at a drawing strain of 5.1, creating nanoscale carbon-supersaturated ferrite. It appears from the literature[4,10,11,20] that delamination is mostly attributed to carbide dissolution and the corresponding increase in carbon content of the ferrite. These may lead to higher tendency of microvoid formation[13,19,22,23] and/or shear nature of the delamination fracture associated with local severe plastic deformation.[20] However, there are arguments[14] that shear stress distributions within wires are affected by both the applied
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