Formation of chromium carbide coatings on HT250 steel by thermal diffusion processes in fluoride molten salt bath

LiF-NaF-KF (46.5-11.5-42 mol %, FLiNaK) molten salt was used in the thermal diffusion (TD) process to form chromium carbide coatings on gray cast iron HT250 at 900 °C for 8 h. The structure and chemical composition of the coating were observed by scanning electron microscopy coupled with energy dispersive X-ray microanalysis (SEM-EDX) and X-ray diffraction (XRD). Both the nanoindentation hardness and Vickers hardness were measured. A 3-5 μm thick chromium carbides layer and approximately 10 μm thick transition layer were formed on the surface of gray cast iron HT250. The chromium carbides layer was composed of Cr23C6 and Cr7C3. The hardness of gray cast iron HT250 was increased 6 times after treatment. The tribological behavior was also improved. These results demonstrate that fluoride salts can be used as base salts to form excellent chromium carbides coatings via the TD process.