TEM study on the surface white layer in two turned hardened steels

The structure of surface white layers was examined using transmission electron microscopy. Surface specimens were machined from a BS 817M40 steel (0.4C 1.2Cr 1.4Ni 0.2Mo) of 52 HRC and a low alloy tool steel (0.8C 1.7Cr 0.4Mo) of 58 HRC, with unworn and worn alumina/TiC composite cutting tools. Thin foil specimens were prepared such that the direction of observation was normal to the machined surface. The as-tempered microstructure of both steels was lath martensite. The structure of the machined surfaces of both steels was characterised by very fine, mis-orientated cells, less than 100 nm in size. The accompanying selected area electron diffraction patterns indicated the presence of retained austenite, the volume fraction of which increased with cutting tool wear. A refinement in the size of cementite particles was also evident. In the surface of the BS 817M40 steel machined with a worn cutting tool, there was evidence to suggest a degree of recrystallization. This may be accounted for by a transition from dynamic recovery to dynamic recrystallization during surface generation; a phenomenon which is favoured by the decrease in the work materials stacking fault energy as a result of the reverse martensite transformation.