Kinetic Behavior and Microstructure of Pearlite Isothermal Transformation Under High Undercooling
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PEARLITE is an important microstructure in structural steels, which are widely used in various engineering applications. The mechanical properties of such steels are closely related to their pearlitic microstructure. For example, the yield strength increases with decreasing lamellar spacing.[1] Refinement of pearlite colonies is conducive to enhancing the steels’ ductility,[2,3] including their reduction in area (RA) and elongation (EL). In addition, the refinement of pearlite colonies also increases fatigue crack growth resistance[4,5] and fracture toughness.[6,7] Thus, a fine pearlitic structure tends to result in steels with desirable mechanical properties. However, in the aforementioned research, pearlite was refined through changes in the original austenite grain size or the pearlite isothermal transformation temperature.[1–5,8] In addition, pearlite refinement is also achieved through enhancement of the phase transformation driving force, such as by the addition of alloying elements[9] or the use of strong magnetic fields.[10] YU LIANG, PINGWEI XU, SONG XIANG, and YILONG LIANG are with the College of Materials and Metallurgy, Guizhou University, Xibei Road, Huaxi District, Guiyang, 550025, P.R. China, with the Guizhou Key Laboratory for Mechanical Behavior and Microstructure of Materials, Guizhou University, Guiyang, P.R. China, and also with the National & Local Joint Engineering Laboratory for High-Performance Metal Structure Material and Advanced Manufacturing Technology, Guiyang, P.R. China. Contact e-mails: [email protected]; [email protected] HU XIONG and JING LI are with the College of Materials and Metallurgy, Guizhou University. Manuscript submitted November 13, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A
However, other methods of increasing driving force, e. g., through modification of the cooling conditions, are not well understood. In all steel products, cold-drawn pearlite wires are overwhelmingly strong.[11–13] For example, a near theoretical strength of ∼ 7 GPa is obtained in cold-drawn high-carbon steel.[14] In the wire-drawing industry, a fully pearlitic microstructure and fine lamellar structure are essential to the production of high-quality steel wire. A fine pearlitic structure can improve the plasticity and homogenization of drawing deformation.[8] Generally, high-carbon hypereutectoid steels with a fully pearlite structure are used in the drawing industry; however, the increased strength due to the increased carbon content makes the subsequent cold drawing process difficult. The achievement of a fully pearlitic structure in hypoeutectoid steels could solve this problem, as experimentally demonstrated through acceleration of the cooling rate in hypoeutectoid Fe-C-Mn steel,[15] which can produce a fine pearlite structure and avoid the precipitation of proeutectoid ferrite.[16] In this case, the pearlite transformation has a stronger driving force than the martensite or proeutectoid ferrite phase transformation, producing cementite with sizes as small as ∼ 20 nm in 0.17C and 0.33C stee
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