Microstructures and Mechanical Properties of as-Drawn and Laboratory Annealed Pearlitic Steel Wires
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INTRODUCTION
PATENTED pearlitic wires can offer excellent strength (tensile strengths of 5 to 6 GPa) and high torsional ductility.[1–7] Such wires are used for several important applications.[8,9] The manufacturing involves a combination of ‘‘patenting’’ (rapid cooling in a lead-bath to form fine pearlite from the austenite phase), wire drawing and stress relief. The patenting provides a very fine lamellar pearlitic structure that is reported to have limited alignment with the wire axis.[1,8,9] Wire drawing reduces the cementite spacing and provides an almost complete alignment of the lamellae with the wire axis.[4–7,10–17] The final product, considered as nano-composite of alternate layers of
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, Jamshedpur 831 001, 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 April 1, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
ferrite and cementite, derives its extraordinary strength from the proximity of cementite lamellae. A drop in tensile ductility through wire drawing is expected from the strain hardening.[5,7,17,18] Although the final failure has been attributed to the formation of localized shear bands,[5–7,12,15–19] ductile cup and cone fractures are reported.[20] Wire drawing has also been found, somewhat surprisingly: given the high strengths produced by drawing, to significantly enhance torsional ductility.[5] This increase may be correlated with any of the microstructural changes associated with wire drawing: finer lamellar spacing, strong lamellar alignment with the wire axis, strain hardening, texture change, and residual stresses. But which is the most significant influence on torsional ductility does not yet appear to be established. Many scientific and technological questions remain unresolved on the wire drawing enforced microstructural changes and their resulting effects on mechanical property. The microstructure consists of bcc ferrite and orthorhombic cementite. Embury and Fisher[2] proposed strengthening from both increased dislocation density and diminishing distance between substructural barriers (the cementite/ferrite interfaces). Their model assumed homogenous and axially symmetric deformation with no fragmentation of cementite. Langford[3] modified this simple picture with transmission electron microscopy evidence of increased dislocation density but with some limited indications
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