Abstract: An improved technique is described for effecting decarbonization protection for the surfaces of ferrous metals during heat treatment in an oxidation atmosphere. The surface to be protected is coated, prior to the heat treating step, with a paste-like substance preferably formed from a mixture of boron carbide, potassium boron fluoride, borax and an aqueous binder. The resulting protective coating, which may be sprayed on the relevant surface, has a maximum thickness of .1 mm. Such relatively thin coating having the described composition, which in larger thicknesses is useful for boron-hardening of metallic surfaces, is effective to provide the required decarbonization protection without the accompanying disadvantages of excessive boron atom diffusion.