This paper is a study on the structure of the surface layer of a grinded plane workpiece. It establishes the time variation of the surface layer temperature field, the workpiece layer where the melting temperature is attained. It is possible, in this way, to know the maximum width of the layer where the fast austenite/martensite transformations are changing the material properties. The study also establishes the variation of the surface layer width as a function of time, feed velocity and grinding tool thickness.