Utilisation of Carbon Dioxide for Electro-Carburisation of Mild Steel in Molten Carbonate Salts

The CO 2 gas was utilised as a source of carbon for electro-carburisation of mild steel in carbonate containing molten salts at 800°C. In the process, the mild steel to be carburised was made the cathode. An inert anode of SnO 2 was used to ensure oxygen gas as the by-product. Two molten salt baths, i.e. Na 2 CO 3 -NaCl (molar ratio = 4:1) and Li 2 CO 3 -K 2 CO 3 (molar ratio = 1:1), were investigated as the electrolyte and also the medium for CO 2 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 (Fe 3 C) 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 (Fe 3 C, 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.