Effects of Tempering on Microstructure, Hardness, Wear Resistance, and Fracture Toughness of Cr 3 C 2 /Steel Surface Com
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INTRODUCTION
THOUGH steels have been widely used as structural materials in many industrial fields because of their excellent combination of strength, ductility, and fracture toughness, they have recently been increasingly exposed to severe industrial working environments. The exposure is generally limited to the surface region, and thus efforts to improve surface properties or to strengthen the surface have been conducted. Active research has been conducted on steel-based surface composites in which the excellent resistance to wear, heat, and corrosion of carbides are fully utilized by direct irradiation of highenergy electron beam.[1–4] This high-energy electronbeam irradiation can achieve excellent surface properties while maintaining substrate properties, because the cooling rate is fast and input energy hardly affects the substrate.[5,6] Upon irradiating the metal surface with high-energy electron beam, high kinetic energy of electrons is transformed to thermal energy, which can easily melt ceramics. When the metal substrate on which ceramic
DUKHYUN NAM, Research Assistant, and SUNGHAK LEE, Professor, jointly appointed with the Materials Science and Engineering Department, are with Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784 Korea. Contact e-mail: [email protected] Manuscript submitted November 8, 2007. Article published online August 19, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
powders are evenly deposited is irradiated with electron beam, the ceramic powders and substrate surface region are melted. In this process, ceramic elements are dispersed and infiltrated into the substrate, thereby fabricating ceramic/metal surface composites. In the case of steel-based surface composites, residual stress can be retained inside because the melted surface layer is rapidly cooled due to heat transfer into the substrate, and the composites can be deformed, distorted, or bent.[7–9] In order to remove this residual stress, the subsequent heat treatment, such as tempering, is needed as in fusion welded regions.[10–12] In addition, because the matrix of the surface composite layer is generally composed of bainite or martensite due to rapid cooling, it is difficult for steel-based surface composites to be applied to areas requiring high fracture toughness. Proper combination of hardness and fracture toughness, together with the secondary hardening effect of alloy carbide precipitates, can also be achieved by the tempering. In this study, chromium carbide (Cr3C2) powders were deposited on the surface of a plain carbon steel substrate, and then irradiated with high-energy electron beam to fabricate Cr3C2/steel surface composites. These composites were then tempered and mechanisms for property enhancement were elucidated by correlating microstructure, hardness, wear resistance, and fracture toughness. The microfracture behavior was observed by an in-situ loading stage installed inside a scanning electron microscope (SEM) chamber, while simultaneously measuring appare
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