The ternary system Fe-W-C is a basis for the interpretation of the microstructure of many commercial steels as high speed steels and tool steels. Nevertheless the phase equilibria in the ironrich corner remain point of controversy. Some discrepancies are found in the extension of phase field of the primary tungsten carbide. But the main critical point concerns the ternary eutectic : seven different proposals have been made for this eutectic, all combinations involving austenite g with two other phases, namely Fe3C, M6C,WC or graphite.
This scattered amount of data reveals the need for supplementing investigations of this system. Our purpose is to reinvestigate the liquid/solid equilibria in the iron-rich side of the ternary system and to acurately determine the invariant transformations. The study was undertaken using thermal analysis techniques and the electromagnetic phase decantation in the temperature range 1300K - 1500K. This latter technique enables the determination of liquid/solid equilibria by maintaining the isothermal treatment during several hours. For our investigation two experimental methods were carried out : sintering from elemental powders or melting and casting from pure metals. The phase equilibria have been investigated by SEM, X-ray diffraction and EPMA.
Three different eutectic have been identified in this study depending on the solidification conditions : g=M6C=Fe3C, g=WC=Fe3C and g=M6C=graphite. The g=M6C=Fe3C constituent is found to occur systematically in the as-solidified alloys. The corresponding eutectic temperature is perfectly reproductible (1085_C). Even the structure obtained after a slow cooling rate may be considered out of equilibrium. The microstructures actually observed have been compared to the calculated diagram : disagreements concern the range of existence of WC and the calculated ternary eutectic g=WC=graphite. The experimental method used for the alloy preparation plays an important role owing to the difficult nucleation of the WC tungsten carbide and its high density which induded its decantation. The facetted morphology of the WC particles is a characteristic of a high supercooling. Thus a marked chemical supersaturation is necessary for WC to form from the liquid, otherwise metastable phases will tend to replace it. The solidifi-cation paths can then be interpreted in the absence of WC, as in the case of the settling out of these carbides to the bottom of the crucible. If WC is suspended from the calculations, the phase diagram obtained then explains the microstructures observed.
A modification of the phase diagram is thus proposed, compatible with the experimental observations and the remaining microstructures. An isothermal section at 1473K and the evolution with temperature of the ternary phase fields are given.
References
[1] J.Y. Shen, G. Giron, P. Jay and M. Durand-Charre, 18e JEEP, Paris XI Chatenay-Malabry, 1992.
[2] M. Durand-Charre, La microstructure des aciers et des fontes, Genèse et interprétation, Ed. SIRPE,
2003.