Microstructural Evolution of Hypoeutectic, Near-Eutectic, and Hypereutectic High-Carbon Cr-Based Hard-Facing Alloys
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NTRODUCTION
HARD-FACING is a commonly employed method to improve surface properties of agricultural tools, components for mining operation, soil-preparation equipment, and others.[1,2] An alloy is homogeneously deposited onto the surface of a soft material (usually low- or medium-carbon steels) by welding, with a view to increasing hardness and wear resistance without significant loss in ductility and toughness of the substrate. In recent years, many investigations have been conducted on the high-carbon ferrochromium for hardfacing cladding to improve the hardness and wear resistance of industrial parts.[3–5] The high-carbon Cr-based hard-facing alloy has revealed the formation of microstructures comprising of a-ferrite and complex carbides, such as M23C6 and M7C3, depending on the chemical composition of the alloy. As the high-carbon content Cr-Fe-C hard-facing alloys are in hypereutectic structure, i.e., primary M23C6 is surrounded by the Fe-Cr and M23C6 eutectic structures, they can reduce the occurrence of the crack because the lamellar eutectic structure will resist crack spreading along the grain boundary.[4] As the high-carbon content Cr-Fe-C hardfacing alloys are in hypereutectic structure, i.e., primary M7C3 is surrounded by the Fe-Cr and M7C3 eutectic structures, they have the properties of high hardness and superior wear resistance. The M7C3 is well known for its CHI-MING LIN, CHIA-MING CHANG, JIE-HAO CHEN, and CHIH-CHUN HSIEH, PhD Students, and WEITE WU, Professor, are with the Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan R.O.C. Contact e-mail: [email protected] Manuscript submitted October 22, 2008. Article published online February 21, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
excellent combination of high hardness and excellent wear resistance, as well as good corrosion and oxidation resistance, so it has been widely used as the reinforcing phase in the composite coatings.[6–12] Advances in surface modification of materials can be achieved also with gas tungsten arc (GTA) synthesis.[13] Gas tungsten arc surface modification by means of alloying is a process in which an alloy powder of a desirable composition and a thin surface layer of the substrate material are melted and then rapidly solidified to form a dense coating metallurgically bonded to the base material. A coated surface obtained by GTA technique has the potential to produce a fine microstructure with high hardness and wear resistance for synthesis onto various substrate materials.[14–17] In this article, the gas tungsten arc welding (GTAW) process is used as a high-energy density beam to form a high-carbon Cr-based hard-facing alloy cladding above the S45C steel with chromium and chromium carbide (Cr:C = 4:1) alloy fillers. These claddings were designed to observe hypoeutectic, near-eutectic, and hypereutectic structures with various (Cr,Fe)23C6 and (Cr,Fe)7C3 carbides at room temperature. The microstructural evolution on the top of cladding layers were systematically characte
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