Cutting performance and microstructure of high speed steels: Contributions of matrix strengthening and undissolved carbi
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I.
INTRODUCTION
THE alloy compositions and microstructures of today’s high speed steels are the result of decades of empirical development.[1,2,3] Alloy profiles are customarily described in the dimensions of wear resistance, hot strength, and toughness. This article deals with two of these, hot strength and wear resistance, trying to assess their relative contributions to tool life in uninterrupted cutting, and to establish quantitative relations with microstructural features. Toughness, while important in intermittent cutting, is a secondary concern in uninterrupted chip forming. Therefore, we will merely outline our philosophy. Although the relevant property is actually the resistance to edge chipping,[4–7] the tool materials community prefers to characterize toughness by the more easily measured rupture strength. This property can be understood[4,6,8] in terms of the size distribution of crack initiating defects or inhomogeneities[5,6,8] and of the material’s crack propagation resistance. The latter is fairly constant in the relevant state of heat treatment, varying little with alloy composition and microstructure.[4,5,9] Thus, ‘‘toughness’’ in the sense of rupture strength mainly reflects the defect population, which is a function of cleanliness and processing rather than alloy composition. Wear resistance in high speed steels is customarily as¨ Z, formerly on leave at the Max-Planck-Institut fu¨r S. KARAGO Metallforschung, D-70174 Stuttgart, Germany, is Professor of Metallurgy and Head of the Department of Metallurgy, Kocaeli University, TR-41040 Izmit, Turkey. H.F. FISCHMEISTER, FASM, formerly Professor of Metallurgy, University of Stuttgart, and Director, Max-Planck-Institut fu¨r Metallforschung, is retired. Present address: Department of Materials Engineering and Welding, Technical University, Graz, A-8010 Graz (Austria). Manuscript submitted May 28, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
sociated with a population of carbide particles in the micrometer size range, down to several tenths of a micrometer, which have survived the austenitization treatment without going into solution.[6,10–13] They are embedded in a matrix of tempered martensite, which derives its strength from very fine (nanometer-sized) carbides precipitated during tempering in a process known as secondary hardening.[14–23] We will refer to the two populations as undissolved (or blocky) carbides and secondary precipitates, respectively. The blocky carbides are too coarse to strengthen the material other than by the load transfer mechanism which acts in particle composites; their main role is to protect the material from abrasive and adhesive wear.[23] Hot strength is important in two respects: the matrix must be strong enough at operating temperature to prevent plucking of undissolved carbides from the contact surface and to resist plastic blunting.[24,25] It has been demonstrated[23] that the secondary hardening of the matrix can be accounted for in terms of Orowan strengthening with pertinent precipitate spacings. The composit
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