Thermodynamic calculations for Nb-containing high-speed steels and white-cast-iron alloys
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Thermodynamic Calculations for Nb-Containing High-Speed Steels and White-Cast-Iron Alloys G.C. COELHO, J.A. GOLCZEWSKI, and H.F. FISCHMEISTER Thermodynamical equilibria have been calculated for a wide variety of high-speed steel compositions belonging to the multicomponent system Fe-C-W-Mo-V-Cr-Nb as well as for two series of whitecast-iron alloys containing niobium. Some temperature-concentration diagrams for both classes of alloys are presented and calculated quantities (melting and transformation temperatures, amounts and compositions of phases) are compared with experimental data. Good agreement between calculated and experimental information has been obtained, with the exception of the MC phase compositions and transformation temperatures for white-cast-iron alloys with high carbon and chromium contents. These differences can, however, be satisfactorily explained by plausible kinetic effects.
I. INTRODUCTION
HIGH-SPEED steels (HSS), as the name suggests, are steels mainly used as tools for cutting metals and other materials at high speeds. High cutting forces, intense friction, and high temperatures occur during tool operation, demanding high yield strength, high fracture toughness, and high wear resistance of the cutting material, all at both low and high temperatures. The microstructure of the high-speed steels after tempering consists of the following. 1. It consists of coarse, blocky carbides of the MC and M6C type (size on the order of micrometers), which crystallize directly from the melt, either as blocky particles or together with fcc-Fe in a eutectic structure, and are therefore called “primary” carbides. Among the microstructure components, these carbides have the highest hardness. They play an important role in protecting the tool surface against wear caused by sliding chips. With special solidification conditions or contents of some particular alloying elements such as Mo, V and C, M2C, carbides may also form during primary crystallization.[1–5] 2. The microstructure consists of a tempered “martensitic matrix,” which surrounds and supports the coarse carbides. Before tempering, it is oversaturated with carbideforming elements. 3. It consists of a fine dispersion of MC and M2C type carbides (size on the order of few nanometers), which precipitate during tempering.[6–11] They are responsible for the “secondary hardening” of the HSS and are, therefore, called “secondary carbides,” although in the sequence of phase formation they belong to a later generation.
G.C. COELHO, Associate Professor, on leave at the Max-Planck Institut für Metallforschung, D-70569, Stuttgart, Germany, is with the Faculty of Chemical Engineering of Lorena, Department of Materials Engineering, C.P. 116, CEP 12600-000, Lorena-SP, Brazil. Contact e-mail: coelho@demar. faenquil.br J.A. GOLCZEWSKI, Senior Associate Researcher, is with the Max-Planck-Institut für Metallforschung. H.F. FISCHMEISTER, FASM, formerly Director, Max-Planck-Institut für Metallforschung, and Professor, Department of Metallurgy,
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