Unusual Chemical Composition of Carbides in Rapidly Solidified High Speed Steels
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Unusual Chemical Composition of Carbides in Rapidly Solidified High Speed Steels A. Tauqir, H. Nowotny, and P. R. Strutt Metallurgy Department University of Connecticut Storrs, Connecticut 06268 ABSTRACT Studies of the elemental composition of carbide phases formed by rapid solidification of a Mo-base high speed steel (M7) have yielded an intriguing finding. Micro-analysis of the extracted carbides (M2 C, M23 C6 and MC)showea that these were unusually rich in refractory elements with values of Mo in the range 46-53 wt.%, corresponding values for Fe and Cr were low (6-12 wt.% Cr and 1.5-13 wt% Fe). This was in marked contrast with analyses of MC, M6 C, M7 C3 and M23 C6 in the 'unglazed' material where Fe and Cr were major constituents. A distinctive feature was M23 C6 in the rapidly solidified material which contained large amounts of Mo and Wand very little Cr. It is proposed that this phase, initially formed as M6 C, underwent solid-state transformation. INTRODUCTION Conventional processing of Mo-W based steels, regarded as Fe-X-C (ternary equivalent) where X stands for Mo+W, results in the formation of MCand M6 C carbides [1]. The metallic sites in the carbide can be replaced by other elements, depending on the composition of the alloy; for example in M6 C, an otherwise Mo-W rich carbide, Fe can replace the metallic atoms up to 30%, while in MC, vanadium can replace W and Mo [2]. The Cr content of the material does not have much effect on the composition of M6 C [2]; however, it forms Cr-Fe rich M2 3 C6 which has crystallographic similarity to M6 C [3,4]. As for abundance, in M2 tool steel, 90% of the primary carbides are M6 C; but M2 C replaces M6 C if cooling rates are increased, specially in the regions of high carbon conenctration [2]. Workers [ 5-7 ] have observed similar carbide phases in the rapidly solidified steels, but the chemical composition of carbide phases in rapid solidification has not been the subject of studies reported to date. In the present work thin foils and carbide extraction replicas from rapidly solidified surface areas are prepared and studied under AEM Philips EM420 to investigate the crystal structure and also the chemical composition of the submicron size carbides present. EXPERIMENTAL TECHNIQUE
An oscillated beam of focused electrons was used to produce rapidly solidified surfaces (0.6 cm wide) on M7 high speed steel. Translation velocity of the ipecimen and the beam power were varied in the range of 0. 2 to 5 cm.s- and 200 to 1000 kWrespectively. Detailed and specific micro-structural features were observed and analyzed using (i) single stage extraction replicas and (ii) thin films. Specimens prepared by these techniques were examined under Philips EM420 analytical electron microscope to determine carbide morphology, size, distribution, crystal structure and chemical analysis. Carbide extraction replicas provided particularly useful data since the (i) crystal structure and (ii) chemistry of fine carbide particles could be studied without interference from the iron-base matrix mat
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