Simultaneous control of the nitrogen and carbon activities during nitrocarburising of iron

A new method is proposed for composing nitrocarburising gas atmospheres allowing simultaneous control of the chemical potential/activity of both N and C at the surface of the specimen via local equilibrium with the gas phase. The method relies on use of a gas atmosphere composed of NH3, H2, CO, CO2, H2O, CH4 and N2, the latter as inert gas. The difficulty of assigning a unique chemical potential for carbon owing to several possibly operative carburising reactions, is circumvented by choosing the partial pressures of the involved gas-phase components H2, CO, CO2, H2O and CH4 such that these pressures correspond to chemical gas-phase equilibrium. A nitrocarburising facility dedicated to the method proposed and operating with a vertical quartz-tube furnace was constructed. Results of thus performed nitrocarburising experiments on pure α-iron plates using differently composed gas atmospheres at 580 °C were performed and the results were compared with presently available thermodynamic assessments of the Fe–N–C system.