Carbide Transformations during Aging of Wear-Resistant Cobalt Alloys
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I.
INTRODUCTION
COBALT alloys are used as wear-resistant materials for hardfacing. Their good resistance to abrasion is due to the presence of hard carbide precipitates. The presence of cobalt improves high temperature properties and resistance to various corrosive agents. In previous work ~ it was shown how the tungsten and silicon content may influence the nature and composition of carbides which appear during solidification. The purpose of the present paper is to examine the behavior of such alloys when aged for a long time in the temperature range 650 ~ to 850 ~ The composition of the alloys selected for the investigation is close to the commercial composition Stellugine 778. So, with regard to phase equilibria, they can be studied by referring to the phase diagrams published for the following systems: Co-Cr-C, z'3 Co-W-C,4 Co-Cr-C-W, or Mo. 1,6The phases expected are an austenitic y matrix, M7C3, M23C6, and M6C type carbides, and, depending on temperature, some hexagonal-closepacked cobalt (E). Besides the phases appearing during the solidification process, secondary carbide precipitation occurs. These precipitates coarsen and their crystal structure is altered by aging. So one of the purposes of our study is to determine to what extent these modifications occur and subsequently influence the mechanical properties of the alloy. Thermal aging was performed at two temperatures, 650 and 850 ~ (corresponding to the normal temperature range of application): 850 ~ is the temperature of ready secondary precipitation, and aging at 650 ~ allows cobalt transformation (y--->e) to occur. Annealing was carded out for 1000 hours. After this time, homogenization of the matrix is almost totally achieved and nothing else occurs, other than coarsening of the precipitates.
II.
EXPERIMENTAL PROCEDURE
The alloys were prepared by "Alliages Fritt6s S.A.". Four compositions were studied. The nominal compositions S. HAMAR-THIBAULT and M. DURAND-CHARRE are with Laboratoire de Thermodynamique et Physico-Chirnie M6tallurgiquesENSEEG-Domaine Universitaire BP44, 38401 St. Martin d'H~res, France. B. ANDRIES is with Alliages Fritt6s S.A. 54, Avenue RhinDanube 38100, Grenoble, France. Manuscript submitted August 11, 1981. METALLURGICALTRANSACTIONS A
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of the alloys investigated are indicated in Table I. They include two commercial compositions I and III. Number II has the composition of a three-phase eutectic as determined in a previous study.l Numbers III and IV have the same composition except for nickel content, in order to establish the effect of cobalt substitution. The specimens were sealed in evacuated tubes of silica and annealed for 30, 300, and 1000 hours at 850 ~ and for 30 hours at 650 ~ Such annealings were performed on two kinds of microstructures, as-cast or undirectionally solidifled. These latter exhibited a coarser structure, facilitating microprobe analysis. The microstructure was examined by optical microscopy on samples etched in Murakami's reagent. Scanning electron microscopy was performed on samples with a sligh
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