Microstructural analysis of vanadium carbide/steel surface-alloyed materials fabricated by high-energy electron-beam irr
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I. INTRODUCTION
NEW surface treatment technology using direct irradiation of high-energy beams such as laser, ion, and electron beams has recently been developed to enhance surface properties of materials. Much research has been put forth on the development of surface composites or surface-alloyed materials taking advantage of excellent resistance to heat, corrosion, and abrasion of ceramics.[1–7] Presently available surface treatment methods such as chemical vapor deposition, laser cladding, and sputtering may cause fracture or separation in the metal/ceramic interface under the conditions of high temperature, impact, and corrosive environment. High-energy beam irradiation methods, however, rarely cause such problems and have an advantage in producing metastable phases during solidification because of the fast cooling rate.[8–11] A number of studies have been published reporting high-energy electron beam irradiation techniques with various substrates and ceramics in order to improve surface properties.[1,12,13] Upon irradiation on the metal surface with high-energy electron beam, high kinetic energy of electrons, being struck into material lattices and forming phonons, is transformed to high thermal energy, which can easily melt ceramics with high melting points. When high-energy beam is irradiated on a metal substrate, ceramic powders are simultaneously melted with the surface region of the substrate, and the melted ceramic elements are dispersed into the melted region, forming new phases during solidification. This way, the processed material can acquire more enhanced properties than the original metal substrate. In the present study, vanadium carbide (VC) powders were KWANGJUN EUH, Research Associate, and SUNGHAK LEE, Professor, are with the the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea. SEONGHUN CHOO, formerly Research Assistant, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is Senior Researcher, Product Design Team, Samsung SDI Co., Ltd., Suwon, 442743 Korea. Manuscript submitted March 12, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
deposited on the surface of a plain carbon steel substrate, and then irradiated with high-energy electron beam in order to fabricate VC/steel surface-alloyed materials. It is required to use flux to help powders homogeneously melt and to protect melted vanadium carbide powders from the air. A mixture of MgO and CaO powders was used as flux. It has been reported that MgO plays a role in controling molten fluidity and CaO reduces impurities during welding.[14,15] Three kinds of specimens were fabricated by varying the mixing ratios of vanadium carbide powders and flux, and their microstructures and hardness variations were investigated to define mechanisms of the surface property improvement. II. EXPERIMENTAL Vanadium carbide powders used in this study have high hardness (2000 to 3000 kg/mm2), excellent resistance to heat, abrasion, and corrosion, and high melting point
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