Contribution to X-ray analysis of carbo-nitrided steel layers

The non-destructive X-ray diffraction method is used to analyse carbo-nitri ded steel layers after wear testing. These measurements are carried out on the two major phases of the material, i.e. the martensite and the retained austenite. Such measurements are particularly difficult for three reasons. First, strong gradients exist across the wear track. Second, the diffractio n peaks obtained for the martensite are broadened, as a result of the overl ap of different reflections of the tetragonal structure. Third, the studied material is multiphase. Its major phases are martensite and austenite, but it also contains carbide and nitride clusters, which lead to incoherent sc attering of X-rays. A new quantitative phase analysis method is thus propos ed to define the volume fractions of these different constituents of the ma terial. This method accounts for the evolution of the background level duri ng wear. A micro-mechanical model is then developed to process the diffract ion peak positions obtained for the martensite and the retained austenite. This model defines the 'true' stress and carbon content of both phases. It also allows separation of the reflections of the martensite. The true width s of the diffraction peaks, which characterize the plastic deformation, can thus be quantified. Results for wear-test specimens show a strong plastic deformation of the retained austenite during contact fatigue. This leads to a partial transformation of this phase into martensite. In the martensite, on the contrary, the plastic deformation remains low but the carbon conten t decreases. This is caused by a stress-induced precipitation of carbides.