Austenite decomposition to carbide-rich products in Fe-0.30C-6.3W
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I. INTRODUCTION
THE kinetics and morphology of steels alloyed with strong carbide-forming elements (e.g., V, Cr, and Mo) differ markedly from those which contain exclusively weak/noncarbide-forming elements (e.g., Mn, Co, and Ni). Inasmuch as the use of strong carbide-forming alloying elements has enormous implications for the hardenability of steels,[1] their systematics will be briefly summarized. Goldschmidt[2,3,4] has reviewed the occurrence of carbide formation in binary M-C and ternary (and higher) Fe-M-C systems (where M refers to transition metal(s)). These surveys showed that the carbide-forming tendency (as measured by the carbide melting point, enthalpy of formation, etc.) diminishes as the atomic number increases across any given transition-metal period on the periodic table. An example is in the M-C systems, where M is in the first transition-metal group, Ti and V are the strongest carbide formers, Cr is less strong, and Mn and Fe are fairly weak, and carbide formation is very weak or nonexistent for Co, Ni, and Cu. For ternary Fe-M-C systems, a distinction can be made between elements whose sufficient concentration will lead to the equilibrium formation of alloy carbides (i.e., carbides other than cementite) and those which merely substitute for Fe within the cementite structure. For all practical purposes, the alloy carbide formers all lie in Ti, V, and Cr columns, while the remainder fall into the Mn, Fe, Co, Ni, and Cu columns. The alloy carbide formers themselves fall into two major classes, defined for the present study as class I and class II. The elements in class I (Ti, Zr, Hf, V, Nb, and Ta) primarily form fcc binary carbides (formula MC, NaCl prototype). These elements typically have low solubility in austenite, R.E. HACKENBERG, formerly Graduate Student, Univ. of Virginia, is now Technical Staff Member, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. D.G. GRANADA, formerly Undergraduate Student, Univ. of Virginia, is now Engineer, is with the National de Ingenieros Electromecanica, Tegucigalpa, Honduras. G.J. SHIFLET, WG Reynolds Professor, is with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904. Contact e-mail: [email protected] Manuscript submitted May 31, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
usually less than 1 at. pct, due in part to the high thermodynamic stability of the carbides, whose heats of formation are more negative than ⫺100 kJ/gram-atom.[4] Those in class II (Cr, Mo, and W) form complex double carbides (e.g., fcc M6C, fcc M23C6, and rhombohedral M7C3) in addition to binary hexagonal carbides (MC, WC prototype and M2C, AsNi prototype). The class II elements have larger solubilities in austenite than those in class I, which reflect their more moderate carbide heats of formation, which range from ⫺100 to ⫺10 kJ/gram-atom.[4] The high-temperature austenite decomposition behavior of alloys containing class I elements have been investigated for Ti,[5,6] V,[7,8,9] Nb,
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