White layers in hard turned surfaces are identified, characterized and measured as a function of tool flank wear and cutting speed. White layer depth generally increases with flank wear. It also increases with speed, but approaches an asymptote. A thermal model based on Jaeger’s moving heat source problems (Jaeger, 1942) is applied to simulate the temperature field in machined surfaces and to estimate white layer depth in terms of the penetration depth for a given critical temperature. The analysis shows good agreement with the trend in experimental results. White layer formation seems to be dominantly a thermal process, possibly plastic strain activated; flank wear land rubbing may be a primary heat source for white layer formation. A strong material dependence of surface alteration is also observed.