Coarsening resistance of M 2 C carbides in secondary hardening steels: Part III. Comparison of theory and experiment
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I.
INTRODUCTION
O N L Y carbide structures capable of high coherency with body-centered cubic (bcc) iron will precipitate on a sufficiently fine scale, less than 5 nm in diameter, to provide the high strength levels. Such carbides generally consist of the face-centered cubic (fcc) group (MC, M = Nb, Ta, Ti, V) and hexagonal close-packed (hcp) group (M2C, M = Fe, Cr, Mo, W). Coherency of the fast group is permitted by near-coincidence in the cube planes of both the carbide and bcc iron, favoring {100}~ platelets, while coherency of the second is allowed by nearcoincidence of the carbide close-packed direction and the bcc iron cube direction, promoting (100)~ rods. In contrast, less coherent (but often more stable) carbides, such as M 6 C , M7C3, and M23C 6, precipitate in coarser form with less strengthening and even embrittlement v/a interfacial precipitation. When the strong carbide-forming elements Mo, Cr, and W are added to high Co-Ni steels, secondary hardening is accomplished by the precipitation of fine-scale M2C alloy carbides. In this case, the resistance of the M2C dispersion with respect to Ostwald ripening is an important matter. A model was introduced for the coarsening resistance of multicomponent carbides, o] While the model treats the coarsening of shape-preserving particles, it is applicable to nonspherical, in particular, rodlike particles. The alloy AF1410 I21 and four experimental aUoys [31 will be tested experimentally in terms of the secondary hardening reaction and coarsening kinetics of the M2C carbides. Peak hardness is obtained when M2C particles are still coherent at 783 K in AF1410. t4j However, at this stage, particles are too small to be observed easily, and the effect of coherency strain energy on the coarsening
HYUCK MO LEE, Assistant Professor, is with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea. SAMUEL M. ALLEN, Associate Professor, is with the Center for Materials Science and Engineering~ Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted January 30, 1991. METALLURGICAL TRANSACTIONS A
behavior may not be negligible because the coherent equilibrium state is different from the incoherent equilibrium state, t5,61 Therefore, this study covers the tempering time range of 8 to 400 hours, where incoherent (but effective in hardening) M2C carbides are supposed to be in equilibrium with the matrix. The mechanism governing the coarsening kinetics of M2C carbides is also of interest. The experimental results are compared in detail with predictions of Thermo-CalcI3'7]and the modified coarsening theories, m
II.
EXPERIMENTAL PROCEDURES
The chemical compositions of the materials used in this work, commercial AF1410 steel and experimental SRG1, SRG2, SRG3, and SRG4 steels, are listed in Table I (in weight percent). The chemical compositions of the experimental alloys are not exactly the same as those of calculations, t3J This slight change in composition can have an effect on the
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