Morphology,Composition,and Residual Stresses of Compound Layers of Nitrocarburized Iron and Steels

Specimens of pure iron, an iron-carbon (1 wt-%C) alloy, and steels Ck 45 and En 40B (=24CrMo13) were salt bath nitrocarburized. M etallographic examination was performed after Murakami etching. This etchant provides a sensitive discrimination between pure nitrides and carbon containing nitrides (carbonitrides). Thus the two-phase (ε and γ’) constitution, as well as the carbon concentration profile within the compound layer, was revealed. Nitrogen, carbon, oxygen, and sulphur concentration profiles were quantitatively determined by electron microprobe analysis. Residual stress measurement and phase analysis were carried out by X-ray diffraction methods. Both macros trains (by analysis of line shift using the sin² Ψ method) and microstrains (by analysis of line broadening using a Voigt single-line method) were determined. In view of a possible interaction between residual stresses in the compound layer and the diffusion zone, residual stresses were also determined in the diffusion zone beneath the compound layer. The results obtained provide a basis for a model to describe the constitution of the compound layer.