Utilisation of carbon dioxide for electro-carburisation of mild steel in molten carbonate salts

The CO₂ gas was utilised as a source of carbon for electro-carburisation of mild steel in carbonate containing molten salts at 800C. In the process, the mild steel to be carburised was made the cathode. An inert anode of SnO₂ was used to ensure oxygen gas as the by-product. Two molten salt baths, i.e. Na₂CO₃-NaCl (molar ratio 4:1) and Li₂CO₃-K₂CO₃ (molar ratio 1:1), were investigated as the electrolyte and also the medium for CO₂ absorption. Microstructural changes in the electro-carburised samples, as revealed by either optical or scanning electron microscopy, were featured by the increase of the carbon rich cementite phase (Fe₃C) at the expense of the original ferrite phase near the surface of the samples. Micro-hardness profiles measured from the surface to the centre of the electro-carburised sample presented clear evidence of carbon penetration as a function of the electrolysis voltage, and the activity of carbonate ions in the molten salts. The carbon-hardened case was up to 0.60 mm in thickness with the carbon content in the near surface region reaching saturation (Fe₃C, 6.69 wt.). The current efficiency of electro-carburisation depended on the cell voltage, and possible causes are discussed with the aid of a simple model correlating the hardness and carbon content.