Influence of Microstructure on the Mechanical Properties of a Pearlitic Steel
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INTRODUCTION
PRESTRESSED concrete steel wire rods, or simply PC wire rods, are widely used in the construction sector. The typical application is fabrication of PC strands. These strands are used in the construction of major bridges, buildings, dams, precast segments, etc. The key composition features a medium to high carbon chemistry, with an appropriate amount of manganese, silicon, etc. The most important feature of this class of material is its high strength combined with adequate ductility. Over several years, researchers have been trying to increase the strength of this material further. The wires nowadays have strength values as high as 2000 to 4000 MPa, for various applications, starting from bridge wires to tyre cords.[1] High carbon wire rod with fully pearlitic microstructure is an established product. The mechanical properties of these steels have a close relationship with the microstructural features such as pearlite colony size, interlamellar spacing, and grain size. Therefore, understanding the processes that control the microstructural parameters, from the point of view of final properties, is extremely important. Processes, such as austenitization, pearlitic transformation, and deformation, are the principal factors that control the final microstructure and properties. It is well known that a diffusion transformation of austenite leads to formation of pearlitic structure, and this can occur in a wide range B. BHATTACHARYA, T. BHATTACHARYYA, and A. HALDAR are with the R&D Division, Tata Steel Ltd., Jamshedpur 831001, India. Contact e-mail: [email protected] Manuscript submitted November 11, 2019. Article published online May 14, 2020 3614—VOLUME 51A, JULY 2020
of temperatures, depending on the cooling rate.[2] Due to different transformation kinetics, the interlamellar spacing of pearlite can also vary widely, which can significantly influence the strength and deformation behavior of the material.[3–6] Microstructure also has an influence on the strength and ductility of pearlitic steels.[7] However, it is also proposed that the toughness is dictated mainly by initial austenite grain size.[8] The austenite grain size also determines the ductility.[9] The austenite grain size depends on various factors such as austenitizing temperature, time, and steel chemistry. A single austenite grain may give rise to the formation of multiple pearlite colonies. The size of these colonies is determined both by austenite grain size and austenite to pearlite transformation temperature. Marder and Bramfitt[10] observed the influence of cooling rate on average nodule diameter and minimum pearlite lamella spacing. They quantitatively derived that both nodule diameter and minimum pearlite lamella spacing decreased almost linearly with the logarithm of the cooling rate. The magnitudes of both these parameters decrease with transformation temperature, as transformation temperature also decreases with increasing cooling rate. In another work,[11] they observed how the morphology of pearlite influenced the mechanical properties
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