Microstructural Evolution and Wear Resistance of Friction Stir-Processed AISI 52100 Steel
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INTRODUCTION
IN industrial point of view, components working at load-bearing and severe wear conditions are required to be hard on the surface and tough in the bulk. Among the wear resistant alloys, AISI 52100 steel (100Cr6) is a high-carbon, chromium-containing low-alloy steel which is widely used as the bearing material in heavy rotating machineries. However, when the carbon content of the steel exceeds the eutectoid composition, the strength increases at the expense of toughness and ductility, due to the formation of cementite network on the primary austenite grain boundaries.[1] Moreover, it is well accepted that the wear resistance is directly proportional to the hardness. Therefore, the production of wear resistant materials is not probable without using the surface-hardening treatments. Generally, surface-hardening methods are divided into two categories: (1) methods based on changing the chemical composition (e.g., carburizing and nitriding) and (2) surface hardening by thermal, mechanical, or thermomechanical processing.[2] The heat treatment methods are usually very time-consuming and final properties of component are significantly affected by the process parameters, i.e., temperature, time, and cooling rate. On the other hand, the liquid-state surface processes, such as laser surface hardening, result in the formation of solidification cracking, hydrogen embrittlement, and brittle intermetallic compound in sensitive materials.[3] Also, due to R.A. SERAJ, Ph.D. Candidate, A. ABDOLLAH-ZADEH, Professor of Metallurgy, Dean of Faculty, F. KARGAR, Laboratory Technician, and R. SOLTANALIZADEH, M.Sc. Student, are with the Department of Materials Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran. Contact e-mail: zadeh@ modares.ac.ir M. HAJIAN, Assistant Professor, is with the Department of Materials Science and Engineering, Shahrood University of Technology, Shahrood, Iran. Manuscript submitted August 19, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
the development of cellular/dendritic microstructures,[4] the post-heat treatment is necessary for these methods. In addition, a deep-hardened depth cannot be obtained by mechanical processes such as shot pinning, surface mechanical attrition treatments (SMAT), etc.[5] Thus, the attention has been attracted to the solid-state thermomechanical treatments. Friction stir processing (FSP) is a solid-state process which is derived from friction stir welding (FSW). It can be used for localized or near surface treatment as well as microstructural modification of metals and alloys.[6] Due to the low and the localized heat input in FSP, distortion is minimized and the size and shape of the processed component do not change.[6,7] In this process, a rotating, non-consumable tool is plunged into the surface of a monolithic piece and traversed along the surface with specified velocity.[7,8] The process can be performed as single or multi-pass for homogenizing and/ or modifying the microstructures, and for producing surface composites on small or relatively
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