Correlation of Microstructure, Chip-Forming Properties, and Dynamic Torsional Properties in Free-Machining Steels
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INTRODUCTION
FREE-machining steels are plain carbon steels containing a number of metallic or non-metallic inclusions to improve machinability and are mainly used for automotive or precision machinery components. Typical examples of inclusions are MnS and soft metal additives such as Pb, Bi, Sn, and Sb.[1–5] These inclusions work as stress concentrators during machining processes, which promote the initiation and propagation of voids or microcracks at interfaces between inclusions and the steel matrix, and improve the machinability by reducing the force required for machining. They are often softened or melted during machining to act as lubricants at interfaces between chips and machining tools, thereby working for preventing the wear of tools.[1–10] The machining process of a free-machining steel is schematically drawn in Figure 1, which shows shear deformation zones and crack formation processes by YONGJIN KIM, formally Research Assistant with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea, is now Senior Researcher with the Plate Research Group, Technical Research Laboratories, POSCO, Pohang 790-300, Korea. HYUNMIN KIM and MINJU KANG, Research Assistants, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology. KIHO RHEE, Senior Researcher, is with the Product Solution Research Group, Songdo Product Application Center, POSCO, Incheon 406840, Korea. SANG YONG SHIN, Postdoctoral Researcher, is with Department of Microstructure Physics and Alloy Design, Max-PlanckInstitut fu¨r Eisenforschung, GmbH, 40237, Du¨sseldorf, Germany. SUNGHAK LEE, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Jointly appointed with Materials Science and Engineering, Pohang University of Science and Technology, Pohang 90-784, Korea. Contact e-mail: [email protected] Manuscript submitted April 18, 2012. Article published online June 11, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
various inclusions. Here, the lubricating effect is generally achieved in the primary shear deformation zone, secondary shear flow zone, and machined surface layer.[1] Yaguchi[11,12] reported that in low-carbon PbS-based free-machining steels, MnS inclusions indirectly influenced the formation of chips. Kishi et al.[1] investigated the roles of MnTe, MnS, MnSe, and Pb in freemachining steels during machining. MnS inclusions did not have lubricating effects by themselves, but had them by compounding with MnTe, MnSe, and Pb inclusions. Akasawa et al.[13] also investigated the effects of freemachining additives such as S, Ca, Cu, and Bi on machinability of austenitic stainless steels. Many studies of free-machining steels have been mainly focused on machining mechanisms under various machining conditions and on the roles of MnS or soft metal additives in machinability. Experimental methods are mostly based on observations of chips after machining, measurements of cutting force during machining,[10
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