AbstractDepletion, continuous wear, and over-tempering of high-speed steel cutting tools are discussed, and the differences between these three processes are clarified. All three mechanisms include diffusion, but continuous wear and over-tempering do not yield any concentration gradients in the tool material. Calculations show that depletion of the high-speed steel is not likely to contribute to tool wear. Continuous wear may occur, but could not be detected experimentally. Overtempering does not result in material loss, but makes the tool more prone to wear by other mechanisms, such as superficial plastic deformation and shear fracture. Thus, diffusion does not seem to contribute directly to tool wear.MST/61