M 2 C precipitates in isothermal tempering of high Co-Ni secondary hardening steel
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I.
INTRODUCTION
THERE are two types of carbide structures that are capable of high coherency with bcc iron and precipitate on a sufficiently fine scale (less than 5 nm in diameter) to provide high strength levels. These are carbides of the fcc group (MC, M 5 Nb, Ta, Ti, and V) and hcp group (M2C, M 5 Fe, Cr, Mo, and W). The coherency of the first group is permitted by a near-coincidence in the cube planes of both the carbide and bcc iron, favoring {100}a platelets, while coherency of the second is allowed by near-coincidence of the carbide close-packed direction and the bcc iron cube direction, promoting ^100&a rods. In contrast, less coherent (but often more stable) carbides such as M6C, M7C3, and M23C6 precipitate in coarser form with less strengthening, and even embrittlement via interfacial precipitation. Secondary hardening is accomplished by the precipitation of fine-scale M2C alloy carbides when the strong carbideforming elements Mo and Cr are added to high Co-Ni steels. However, this reaction is preceded by the formation of metastable cementite precipitates. The cementites are quite coarse and therefore limit the fracture toughness of many ultrahigh-strength martensitic steels. A prolonged tempering treatment is required to dissolve the cementite. The stability and resistance of the M2C dispersion with respect to Ostwald ripening are an important matter, since coarsened particles CHOONG HWA YOO, Researcher, is with Stainless Department Cold Rolling Team, POSCO, Pohang, Korea 790-360. HYUCK MO LEE, Associate Professor, is with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon, Korea 305-701. JIN W. CHAN, Associate Specialist, Materials Science and Mineral Engineering, University of California, Berkeley, is also Participating Guest with the Center for Advanced Materials, Lawrence Berkeley National Laboratory. JOHN W. MORRIS, Jr., Professor of Metallurgy, Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720, is also Program Leader, Structural Materials, Center for Advanced Materials, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. Manuscript submitted November 27, 1995. 3466—VOLUME 27A, NOVEMBER 1996
cause a drop in strength and even embrittlement through the grain boundary precipitation. A model was introduced for the coarsening resistance of multicomponent carbides.[1] While the model treated the coarsening of shape-preserving particles, it was applicable to nonspherical, in particular, rodlike particles in the AF1410 steel.[2] Recently, Carpenter Technology Corporation[3] developed and patented an ultrahigh-strength martensitic steel, named AerMet 100, which is stronger but has lower fracture toughness than AF1410. As the material itself is a recent development, the published studies on it are quite limited and they primarily investigated the mechanical properties after isochronal aging for only about 5 hours.[4,5] In this study, this alloy was tested experimentally and analyzed in terms of the sec
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