Thermodynamic Assessment of the Reduction of WO3 by Carbon and Silicon

An interesting process in terms of resource conservation is the arc surfacing of worn components by means of powder wire in which the filler contains tungsten oxide WO₃ and a reducing agent (carbon and silicon). Thermodynamic assessment of the probability of 21 reactions in standard conditions is based on tabular data for the reagents in the range 1500–3500 K. This range includes the temperatures at the periphery of the arc and in the upper layers of the surfacing bath. The reactions assessed include direct reduction of WO₃ by carbon and silicon, indirect reduction of WO₃ by carbon, and reaction of tungsten compounds with carbon and silicon to form tungsten carbides and silicides. The possible reaction products considered are W, WC, W₂C, WSi₂, W₅Si₃, CO, CO₂, SiO, and SiO₂. The reduction of the oxide is written for 1 mole of O₂, while the reactions of tungsten compounds with carbon and silicon compounds are written for 2/3 mole of tungsten W. The probability of the reactions is estimated in terms of the standard Gibbs energy. In the range 1500–3500 K, the standard states of the reagents are assumed to be as follows: W(so); WO₃(so, li), with phase transition at 1745 K; WC(so); W₂C(so); C(so); CO(g); CO₂(g); WSi₂(so, li), with phase transition at 2433 K; W₅Si₃(so, li), with phase transition at 2623 K; Si(so,li), with phase transition at 1690 K; SiO(g) and SiO₂(so, li), with phase transition at 1996 K. To assess the influence of the possible evaporation of tungsten oxide WO₃ in the arc (T_b = 1943 K) on the thermodynamic properties, the thermodynamic characteristics of two reactions are considered; the standard state in this temperature range is assumed to be WO₃(g). Thermodynamic analysis of the reduction of tungsten oxide WO₃ shows that the temperature of the melt and the composition of the powder wire may affect the composition and properties of the layer applied. At high melt temperatures (>2500 K), the formation of tungsten and also tungsten carbides and silicides is likely. These reactions significantly change the composition of the gas phase, but not that of the slag phase in the surfacing bath. Below 1500 K, the most likely processes are the formation of tungsten silicides and tungsten on account of the reduction of WO₃ by silicon. In that case, the slag phase becomes more acidic on account of the silicon dioxide SiO₂ formed. However, this temperature range is below the melting point of WO₃ (1745 K). In the range 1500–2500, numerous competing reduction processes result in the formation of tungsten and also tungsten carbides and silicides in the melt. The reaction of tungsten compounds with carbon and silicon to form carbides and silicides is less likely than reduction processes. Evaporation of tungsten oxide WO₃ in the arc increases the thermodynamic probability of reduction; this effect is greatest at low temperatures.