Correlation of microstructure with hardness and wear resistance in (CrB, MoB)/steel surface composites fabricated by hig
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I. INTRODUCTION
STEELS have been widely used as basic structural materials in all industries because of their excellent combination of strength and ductility, but they have been increasingly exposed to severe industrial environments. Since these exposures are mostly limited to the surface region, efforts to improve their surface properties by coating or surface treatment have been made. Recently, research has been conducted on surface composites in which the excellent resistance to heat, corrosion, and wear of ceramics are fully utilized by direct irradiation of high-energy electron beam.[1–7] This highenergy electron beam irradiation method can achieve required surface properties, while maintaining substrate properties, since the cooling rate is fast and input energy hardly affects the substrate during the irradiation.[8–11] When the highenergy (several MeV energy range) electron beam irradiates the material surface, incident electrons having high kinetic energy collide with electrons inside the material, and transfer their energy to excitation energy and kinetic energy of secondary electrons. Since the excited electrons subsequently thermalize and transfer their excitation energy to the lattice by electron-phonon collisions, ceramics having high melting point can be easily melted by this thermal energy.[1,12,13] Upon the electron beam irradiation of the metal substrate surface, where ceramic powders are evenly deposited, the substrate surface and ceramic powders completely melt with the ceramic dispersed into the substrate. In the melted layer, KYUHONG LEE and DUK-HYUN NAM, Research Assistants, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is also with the Department of Materials Science and Engineering, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted January 31, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
carbides, borides, and nitrides are precipitated during solidification, thereby fabricating a ceramic/metal surface composite. This fabrication can be continuously performed in the air, and a very large area can be treated at one time, which makes it advantageous for the fabrication of large structures or parts. In this study, (CrB,MoB)/steel surface composites with improved surface properties were fabricated by depositing CrB or MoB powders on the surface of a plain carbon steel substrate and irradiating with the high-energy electron beam. Transition metals usually form borides such as CrB and MoB in steels, and these borides can be used as reinforcing or coating materials because they have very high hardness and excellent wear resistance.[14,15] For the fabrication of the surface composites through surface melting, a flux has to be used to protect melted boride powders and substrate from the air and to promote homogeneous melting.[16,17] Microstructure, hardness, and
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