Improvement of Mechanical Properties of Spheroidized 1045 Steel by Induction Heat Treatment
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SPHEROIDIZATION is an important heat treatment process to increase the ductility of steels that undergo considerable deformation at room temperature.[1] During spheroidization, a steel specimen is typically held at a temperature below the Ae1 temperature (e.g., 973 K (700 °C)) for a long time (12 to 20 hours), which causes the formation of highly stable spheroidized cementite particles dispersed in a ferrite matrix. Pearlite with a lamellar structure composed of plate-type cementite particles and a ferrite matrix is transformed just below the Ae1 temperature during the austenite decomposition. The cementite lamellae divide into small spherical cementite particles. The driving force for the spherical particle formation is the reduction in the surface energy caused by a decrease in the ratio of the interfacial surface area to the unit volume between ferrite and cementite. The total interfacial surface area can be minimized by growing or merging the small cementite particles during spheroidization, which is referred to as coarsening or Ostwald ripening.[2] The spheroidization of steels is a prolonged heat treatment, typically lasting several tens of hours. Many researchers have investigated the potential reduction in
MINWOOK KIM, Graduate Student, and SEOK-JAE LEE, Assistant Professor, are with the Division of Advanced Materials Engineering, Research Center for Advanced Materials Development, Chonbuk National University, Jeonju 561-756, Republic of Korea. Contact e-mail: [email protected] JUNG-HO SHIN, Principal Researcher, is with the Product Research Center, R&D Center, SeAH Besteel, Soryong-dong, Gunsan 573-711, Republic of Korea. YOUNG CHOI, Senior Researcher, is with the Convergence Components and Agricultural Machinery Application Group, Jeonbuk Regional Division, KITECH, 222 Palokro, Jeonju 561-202, Republic of Korea. Manuscript submitted June 21, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
the spheroidization process time over the last few decades. The use of cycling heat treatment is effective for accelerating the spheroidization process. Chou et al.[3] obtained a spheroidized structure of hypoeutectoid steel with 0.45 wt pct C brought about after quenching a sample to a lower temperature and immediately heating it to the spheroidization temperature below Ac1. During this thermal cycle, supercooled austenite was directly decomposed into spheroidized cementite that was homogeneously distributed in a ferrite matrix. Saha et al.[4,5] proposed the cyclic annealing of steel with 0.6 wt pct C to accelerate spheroidization. Their sample was repeatedly heated to a temperature higher than the Ac3 temperature and immediately quenched to room temperature. This process produced almost fully spheroidized cementite after 8 cycles. Lv et al.[6] investigated the cementite spheroidization behavior in Fe-0.8 pctC steel during repeated cyclic heat treatment. They found that as the number of cyclic treatments was increased, the fraction of spheroidized cementite increased and that spheroidization was completed
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