Phase transformations in surface layers of machined steels investigated by X-ray diffraction and Mِssbauer spectrometry

Machined surfaces of two steels (0.38 and 0.8 wt.% C) were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Mِssbauer spectrometry. Because of the shear and tool wear during the chip formation, a white etching layer appears on these surfaces. X-ray diffraction and Mِssbauer spectrometry data show the presence of retained austenite and suggest the presence of martensite in the white etching layer. The formation of these phases is discussed and from crystallographic data, the temperature reached during turning is estimated.