Carbon – Cobalt – Iron

Abstract

Interest in the C-Co-Fe system is related mainly to steel production. An early study of the phase equilibria in the system was provided by [1932Vog]. Using thermal analysis and microscopical techniques, they determined 6 vertical sections at Fe:Co ratios of 49:1, 29:1, 9:1, 7:3, 1:1 and 3:7 (by mass) in the Fe rich corner of the system. Subsequently, work has focused on the effects of Co on C levels in Fe solutions. The solubility of C in liquid Fe with Co was studied at 1550 and 1350°C by [1956Tur, 1969Sch], 1450°C by [1960Fuw], 1600°C by [1963Sch] and at 1375°C by [1964Dai]. [1983Jan1, 1983Jan2] determined the solubility at 1600, 1550 and 1500°C. The experimental technique employed was virtually the same in each case. Mixtures of Fe and Co were melted in a graphite crucible under argon gas, and after equilibrium was reached, the melt was quenched. The C content was then determined by combustion analysis. The solubility of C in (Fe was studied by [1969Fra]. Rod samples of induction melted master Co-Fe alloys were packed in graphite crucibles with graphite powder and pure Fe foil. The crucible was heated to 1000°C for 72 h under flowing dried hydrogen before quenching by moving to the water cooled end of the furnace. The C levels in the samples were then determined. Much of the research into the system, however, was focused on the C activity in solid alloys. A number of researchers have studied the effects of Co on the C activity in solid [1960Pet, 1963Bro, 1965Smi, 1972Wil] and liquid [1959Fuw] ternary alloys. Known CO/CO2 mixtures were passed over a charge of the three components, held in a high-frequency induction furnace at different temperatures. After equilibration (~ 5 h) the alloys were solidified for chemical analysis leading to C activities. Because of concerns in the experimental technique relating to gas mixture inhomogeneity owing to thermal diffusion and carbon deposition in cold parts of the apparatus, [1972Foo] employed a closed chamber where the gas employed acts purely as a transport medium. The affect of Co content on the activity of C was also determined by measuring the C solubility [1959Fuw]. The melt was continually stirred with a carbon rod until equilibrium was achieved. The melt was sampled for analysis, and further Co additions made and the stirring and sampling process repeated. These data were reviewed by [1963Ell]. A similar gas equilibration technique was used by [1963Bro]. The effect of Co on the carbon activity in austenite alloys was studied by [1972Wil]. Spiral specimens of alloy, pure Fe and a source of C in the form of cast Fe particles were heated at temperatures of 850, 925 and 1050°C. After equilibration, the carbon activity was determined from the amount of C in the pure Fe specimen. They found that Co increased the carbon activity of austenite. [1968Chi] reviewed the experimental data for the activity of carbon in alloyed austenite at 1000°C. The Phase equilibrium data were critically reviewed by [1984Riv, 1988Ray, 1994Rag, 2002Rag]. The enthalpies of mixing of the ternary liquid were determined calorimetrically at 1900°C by [1993Wit, 1994Wit]. Partial and integral enthalpies were presented and also, they suggested that repulsive forces take place between the like and unlike atoms. In addition to the experimental studies of phase equilibria and thermodynamics, many researchers took a theoretical approach. [1965Bur] extrapolated the thermodynamic properties of the binary systems in order to calculate isotherms in the ternary system. [1966Aar] took three different statistical thermodynamic approaches to the thermodynamics of interstitial solid solutions. They found that that based on the Lacher-Fowler-Guggenheim model best fit the experimental activity data available in the literature. [1968Zup] employed an extension of the Langmuir lattice gas model to calculate the carbon activity, and [1971Ale] used a quasi-chemical approach to calculate the thermodynamics of the ternary liquid. More recently however, Calphad modelling techniques have been employed. [1977Uhr] used experimental carbon activity data from the literature [1965Smi, 1968Bod] to determine a temperature independent interaction parameter. [1988Hoc] calculated metal-carbon interactions for solid ternary alloys, and in the same year, a full Calphad assessment of all available experimental phase equilibrium and thermodynamic information was carried out by [1988Gui]. [1985Eno] used a central atom model to calculate ortho and