Niobium-Alloyed high speed steel by powder metallurgy
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I.
INTRODUCTION
THE use of niobium as an alloying element in high speed steels seems first to have been suggested in 1955. [1] In the thirty years since, its possibilities have been further explored by several r e s e a r c h e r s . [2'3/q The exploitation of large pyrochlore deposits in Brazil [5] has stimulated these developments, and a steel containing 2 pct niobium has been commercially introduced there. [6~ During this phase, the development of niobium-alloyed high speed steels followed largely empirical lines, because it was not clear which of the alloying elements could be substituted by niobium, and in which of their specific roles. Any such substitution will change the phase equilibria which control the solidification process, and affect the composition of both carbides and matrix. The relevant multicomponent equilibria are not known. The situation was further complicated by incomplete understanding of the influence of carbide and matrix compositions on tool performance and other properties of the steels. Some of these gaps have been filled during the eighties by research into the physical metallurgy of high speed steels. [7- 107 On the basis of this improved understanding, alloys with moderate niobium contents have been developed for conventional processing. [7'u-~2] During the course of that work, it became clear that the full potential of niobium in high speed steels could be realized only by a powder metallurgy process. Some experiments along these lines are reported in this paper.
II. A S T R A T E G Y FOR THE USE OF N I O B I U M IN H I G H SPEED STEELS
The functional components in the microstructure of high speed steels are a population of blocky ("primary") carbides of types MC and M6C , whose main function is to provide protection against abrasive wear, and a matrix of tempered martensite strengthened by extremely fine carbides precipiS. KARAGOZ, Assistant Professor, Department of Metallurgy, Yildiz University, Istanbul, Turkey, is on leave of absence at Max-Planck Institut fOr Metallforschung, Seestr. 92, D 7000, Stuttgart, Germany. H. E FISCHMEISTER is Director, Max-Planck Institut fiir Metallforschung, Seestr. 92, D 7000, Stuttgart, Germany. Manuscript submitted July 20, 1987. METALLURGICALTRANSACTIONS A
tated during secondary hardening. The function of the matrix is to keep the wear-resisting particles in place despite the high temperatures and shear stresses operating at the interface between the cutting edge and the work material, and to resist plastic blunting of the edge. The blocky carbides are formed by primary or eutectic crystallization from the melt (MC, M 6 C ) , o r by a subsequent solid state reaction, M2C + ~/ - Fe ---> M6C 4- M C , which occurs in the heat of hot working, t13-16]In traditional steels, the composition of MC is dominated by vanadium, while M r C is made up mainly of Mo, W, and Fe. tlTa8] MC is by far the harder phase, and consequently is more valuable for wear resistance, but in conventional steels it is outweighed by the softer and thermodynamically less sta
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