A New Procedure for Determining Volume Fraction of Primary Carbides in High-Speed and Related Tool Steels
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INTRODUCTION
P R I M A R Y carbides constitute one of the essential microstructural features of high-speed tool steels. They contribute to hardness and wear resistance. The dissolution of primary carbides during austenitizing enriches the matrix with carbon and alloying elements, and results in an exceptional as-quenched hardness and the ability to undergo secondary hardening by precipitation of secondary carbides during subsequent tempering. Therefore, in the research of tool steels it is important to be able to measure the quantity of primary carbides. Conventional metallography, consisting of polishing, etching, and point counting, has most often been used to estimate volume fraction of carbides in tool steels. A classical example is a paper by Kayser and Cohen, ~ in which quantities and species of primary carbides were analyzed for several high-speed steels in both annealed and hardened conditions. In the course of tool steel studies at Climax Molybdenum Company, however, the somewhat subjective nature of the point counting method was painfully realized, especially when the method was tried on annealed or under-hardened tool steels containing too numerous fine carbide partich.. To avoid the subjectivity and difficulty associated with the point counting method, a new analytical procedure was developed in this study. Carbides are extracted by electrolysis. The species and unit cell sizes of carbides are determined by X-ray powder diffraction, and their individual chemical composition by energy dispersive X-ray microanalysis (EDX). The weight fraction of the individual carbide species is determined by equating the bulk carbide composition to the sum of individual carbide species. Finally, volume fractions are calculated from the weight fractions and carbide densities computed from the results of X-ray diffraction arid composition analysis.
C. KIM and V. BISS are both Senior Research Associates with Climax Molybdenum Company of Michigan. a subsidiary of AMAX, Inc., Ann Arbor, MI 48105. W. F. HOSFORD is Professor, Department of Materials and Metallurgical Engineering. The University of Michigan, Ann Arbor, MI 48109. Manuscript submitted December 15, 1980. METALLURGICAL TRANSACTIONS A
The new procedure of determining volume fractions of carbides was) applied to microstructures of two tool steels in the present study, AISI M2 high-speed steel (6 pct W - 5 pct Mo - 4 pct Cr - 2 pct V - 0.8 pct C - balance Fe) and a tool steel with a composition of 2 pct W - 2.75 pct Mo - 4.5 pct Cr - 1 pct V - 0.5 pct C - balance Fe. The latter steel is commercially known as VASCO-MA, and for convenience will be called MA. The MA steel derives its composition from the matrix of AISI M2 austenitized according to industrial practice. Such steel is called a matrix steel because of the origin of its composition. Volume fractions of carbides in annealed AISI M2 highspeed steel determined by the new procedure were considerably different from values previously estimated using the point counting method. Therefore, further analyses were
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