Excess nitrogen and discontinuous precipitation in nitrided iron-chromium alloys

• Published in: Journal of materials science 1985-02, Vol.20 (2), p.718-729
• Main Authors: Hekker, P M, Rozendaal, H C F, Mittemeijer, E J
• Format: Article
• Language: English
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/BF01026547

FeCr (1.88 and 3.61 wt.% Cr) specimens were gas nitrided (10 vol.% NH sub 3 /90 vol.% H gas mixture at 833K). The structure after nitriding was studied employing metallographic methods (optical microscopy, hardness measurements), scanning electron microscopy and X-ray diffractometry (residual macrostress determination and phase identification). Further the weight and thickness increases of the specimens during nitriding were determined. Excess nitrogen uptake, discontinuous precipitation and void formation were observed. The amount of excess nitrogen observed in an initial stage of precipitation corresponds well with the predicted amount applying thermodynamics of stressed solids. The occurrence of a discontinuous precipitation reaction at later stages of precipitation was accompanied by the loss of excess nitrogen from the supersaturated ferritic matrix. In this way void formation and changes in the residual macrostress distribution within the specimen during nitriding can be explained. As a result a model is presented for the description of the nitriding behaviour of Fe--Cr alloys. 27 ref.--AA