Annealing study of thin chromium layers on cemented steel substrates

• New Cr carbide coating process starting from hard Cr plating of a carburized steel
• Study of the annealing treatment to transform chromium coating into chromium carbide
• Study of the kinetics of the transformation as a function of the annealing temperature
• Comparison of hardness and adhesion in function of the annealing temperature

The present study has been conducted in the objective of obtaining chromium carbide layers using a conversion treatment involving three main steps: 1) carburizing treatment in order to increase the amount of carbon atoms at the steel surface, 2) deposition of a pure chromium thin layer using electrolytic method and finally, 3) annealing treatment used for carbon diffusion and carbide formation until complete transformation of chromium layer. Depending on the treatment temperature, partial or complete conversion is obtained as a result of the diffusion process. The role of the annealing temperature on transformation rates of chromium into chromium carbide films was investigated. It is shown that for 1 h of treatment the layer is totally transformed at 1100 °C. Hardness and adhesion of layers produced were found to increase whatever the annealing treatment even for partial transformation at the lower annealing temperatures.