Solidification of high-speed tool steels
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INTRODUCTION
P R O M I N E N T features in the microstructure of the highspeed steels are blocky carbides of types MC and M 6 C . Since these carbides (or their precursor phase M2C) are formed during solidification, they are often called "primary" carbides. With a given alloy chemistry, their phase composition, size, shape, and spatial distribution are determined by the course of solidification, in conjunction with the process of hot working the ingot to a mill shape. The cutting performance of the high-speed steels is determined by the blocky carbide population and by the degree of secondary hardening conferred upon the matrix during tempering. Based on an analysis of the secondary hardening reaction 11'21 and on quantitative analyses of the populations of blocky carbides in a variety of commercial and experimental high-speed steels, t3-71 a semiempirical model has been suggested to describe the contributions of both factors to the cutting performance, tS,gj Analysis of customary and experimental high-speed steels in terms of this model indicates that the contributions of matrix hardening and the blocky carbides can be roughly equal, t81 The distribution of the blocky carbides also has a decisive influence on the toughness of the tools, because large carbide clusters nucleate the microcracks which eventually cause rupture.fl~131 Thus, many important properties of high-speed steels can be rationalized fairly well in terms of their microstructure, but the evolution of the microstructure during solidification has been studied only for a few alloys, I14-221 and it remains the main gap in the understanding of high-speed steel properties, The transformation of the microstructure during subsequent hot working, on the other hand, is somewhat easier to rationalize. 111.23.z4] H.F. FISCHMEISTER, Professor of Metallurgy and Director, is with the University of Stuttgart and Max-Planck-Institut fiir Metallforschung. R. RIEDL, Department Head, is with the PVD Department, ETA SA Fabriques d'Ebauches, Schild-Ruststr. 17, CH2540, Grenchen, Switzerland. S. KARAGOZ, Associate Professor, Department of Metallurgy, Yildiz University, Istanbul, Turkey, is on leave of absence at Max-Planck-Institut for Metallforschung, Seestr. 92, D-7000, Stuttgart, Federal Republic of Germany. Manuscript submitted August 24, 1988. METALLURGICAL TRANSACTIONS A
The present paper reviews unidirectional solidification experiments undertaken to fill this gap. The alloys studied span the whole range of customary high-speed steel compositions. Also included were some experimental alloys from a project devoted to the development of niobium-alloyed high-speed steels, I91 where control of the solidification structure is especially important. Attention is focused on the influence of the solidification parameters upon the crystallization sequence, on the quantitative aspects of dendrite and eutectic (ledeburite) morphology, and on the chemical composition of the carbides. The results are discussed jointly with published data on the solidification of high-speed st
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