The migration of substances from the place of higher to the place of lower concentration with the aim of balancing the concentration, this process is called "diffusion". Diffusion occurs mainly in hot wear-resistant carbide tools. Only they can withstand temperatures at which diffusion can occur.

Other tooling materials (such as high speed steel - 660°C) usually soften before diffusion can even occur. High-speed steel cannot scale because it already loses its hot hardness below the temperature required for scale formation and softens. It thus fails as a tool (fails) and an increase in temperature is not possible.