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White layers (WLs) produced in hard steels by machining have been characterized using nanoindentation, optical microscopy, transmission electron microscopy (TEM), and X-ray diffraction. The WL is found to have a hardness of 12.85±0.80 GPa, which is significantly greater than that of untempered martensite produced by various heat-treatment processes. The grain size in the WL is shown to be in the submicrometer range with values ranging, typically, between 30 and 500 nm. These two characteristics of the WL distinguish it from various structures formed in steels by heat treatment. The formation of WLs is promoted by conditions of moderate to high cutting speed in conjunction with tool flank wear. Based on a consideration of the strain, stress, and temperature states associated with the formation of WLs in machining, it is hypothesized that deformation of material to very large strains is the principal factor …