Solidification microstructure and M 2 C carbide decomposition in a spray-formed high-speed steel
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INTRODUCTION
AMONG several of today’s emerging techniques for producing advanced materials with superior qualities relative to conventional ingot metallurgy, the spray-forming process has successfully demonstrated its capability to produce highly alloyed materials.[1,2] Spray-formed alloys have superior properties compared to ingot metallurgy alloys and are comparable to those produced by powder metallurgy. In addition, spray forming has also achieved low production costs at an industrial production level. Recently, application of the spray-forming process has been focused on advanced materials with special qualities or materials with superior properties relative to those of conventional ingot casting.[3,4] The major advantage of the spray-forming process is that a near-net-shaped product consisting of fine equiaxed grains can be fabricated in a single operation through the gas atomization of a liquid melt and the subsequent consolidation of these droplets.[4] By using the spray-forming process, alloys containing large amounts of alloying elements, such as high-speed steels or superalloys, can be fabricated without significant problems in segregation.[1,2] High-speed steels are characterized by a long freezing range and complex eutectic reactions. This results in segregation of alloying elements and formation of several different types of carbides during solidification.[5–8] The major carbides of high-speed steels are MC, M2C, and M6C, depending on the cooling rate and the alloy composition.[7,8] For conventionally processed high-speed steels, it is inevitable that a coarse carbide network will be formed during solidification. Coarse primary carbides tend to result in uneven carbide bands even after a substantial amount of hot forging.[9] Many attempts have been made to develop a proEON-SIK LEE, WOO-JIN PARK, J.Y. JUNG, and S. AHN, Research Scientists, are with the Structural Metals Research Team, Advanced Materials Division, Research Institute of Industrial Science and Technology, Pohang 790-600, South Korea. Manuscript submitted August 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
cess to minimize carbide segregation utilizing the rapid solidification process.[10–15] The powder metallurgy and spray-forming routes have been shown to produce highspeed steels with considerable microstructural refinement.[11,14] The carbide type, size, and its distribution affect the mechanical properties of high-speed steel. In conventional ingot casting, using a high-molybdenum series of high-speed steels lead to the formation of a plate-shaped M2C carbide by eutectic reaction, which deteriorates hot workability and toughness.[17] The solidified microstructure of high-speed steels formed during spray forming can be varied by affecting the thermal enthalpy inflow to the deposit surface of the growing billet.[15] It has been reported that M2C carbide was formed during spray forming of ASP30*-grade *ASP30 is a trademark of Erasteel Kloster AB, Langshyttam, Sweden.
high-speed steel.[16] The M2C carbide is a metastable pha
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