Carbide/austenite equilibria in the Fe-Cr-W-C system have been studied at 900–1100°C by means of a method involving well-controlled heat-treatments and microprobe measurements. The results obtained are analysed in terms of a thermodynamic model. The parameters evaluated for the description of M23C6 and M6C were used to calculate the carbide/austenite equilibria in the iron-rich part of the phase diagram. Good agreement between calculated phase equilibria and the experimental data was obtained. The free energy of formation of FeC6/23 was evaluated. The value obtained at 1000°C is ∆°Gf = −620 J mol−1 Fe.
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