THERMODYNAMIC ASSESSMENT OF WO3 REDUCTION BY CARBON AND SILICON

Abstract

Technology of arc surfacing with flux-cored wire, in which tungsten oxide WO3 and substances containing reducing agents: carbon and silicon are used as fillers, is of interest for implementation in terms of resource saving. Thermodynamic estimation of probability of 21 reactions proceeding under standard conditions was carried out with the use of tabular thermodynamic data for reagents in temperature range of 1500–3500 K. This interval includes temperatures at the arc periphery and in the upper layers of surfacing bath. Among the reactions are direct reduction of tungsten oxide WO3 by carbon and silicon; indirect reduction of tungsten oxide WO3 by carbon; reaction of tungsten combination with carbon and silicon with formation of tungsten carbides and silicides. W, WC, W2 C, WSi2 , W5 Si3 , CO, CO2 , SiO, SiO2 were regarded as possible reaction products. Oxidation reduction reactions were recorded for 1 mole of O2 , and reactions of tungsten combination with carbon and silicon – for 2/3 moles of W. Probability of reactions proceeding was estimated based on the standard Gibbs energy of reactions. As a standard for reagent substances in the range of 1500 – 3500 K, the following states were selected: W(s), WO3 (s, l) with phase transition at 1745 K, WC(s), W2 C(s), C(s), CO(g), CO2 (g), WSi2 (s, l) with phase transition at 2433 K, W5 Si3 (s, l) with phase transition at 2623 K, Si(s, l) with phase transition at 1690 K , SiO(g), SiO2 (s, l) with phase transition at 1996 K. To estimate the degree of influence of reactions of possible evaporation in WO3 tungsten oxide arc (Tboil = 1943 K) on thermodynamic properties, thermodynamic characteristics of two reactions were calculated in which WO3 (g) was chosen as a standard state in the same temperature interval. Thermodynamic analysis of WO3 reduction shows that temperature of melt along with composition of flux-cored wire can affect composition and service properties of deposited layer. In the system under consideration, formation of tungsten, tungsten silicides and carbides is likely at high temperatures of melt (more than 2500 K). The flow of reactions significantly changes composition of gas phase, but not slag phase in surfacing bath. At temperatures below 1500 K, formation of tungsten and tungsten silicides is most likely due to reduction of WO3 by silicon, with the slag phase becoming more acidic due to SiO2 silicon oxide formation. However, this temperature range is below the melting point of WO3 tungsten oxide (1745 K). In the temperature range of 1500 – 2500 K, a number of competing reduction reactions occur, as a result of which both tungsten and its silicides and carbides are being formed in metallic melt. Reactions of tungsten combination with silicon and carbon with formation of silicides and carbides are less likely than reduction reactions. Evaporation of tungsten oxide WO3 in the arc increases thermodynamic probability of reduction reactions occurrence, but more likely at low temperatures.