Nitriding of aluminium-containing steels
This thesis describes a study of the nitriding behaviour of some aluminium-containing low alloy steels with and without chromium. The main aim of the study has been to establish the relationship between prior heat treatment and nitriding process variables on the nitriding response of the aluminium containing steels. Emphasis was placed on establishing the metallography of the steels nitrided under controlled conditions, using transmission electron microscopy of thin foils and carbon replicas, optical microscopy and microhardness measurements. A potentiostatic etching method was used for the extraction of carbides and nitrides, and those phases which were present were determined by X-ray diffraction and X-ray microanalysis. In addition to the known nitride phases, i. e. f. c. c CrN and hexagonal A1N, another phase with a-Fe type d-spacing was observed, and subsequently identified as f. c. c. A1N of NaCl structure with a ti 4.05 A. This phase transforms to hexagonal A1N if the nitrided specimen is annealed at 700°C. The latter phase was found to be rich in silicon. Unlike CrN, the precipitation of A1N is greatly influenced by the nitriding temperature and the NH3 content of the gas mixture. The formation of the carburized layer depends on the internal compressive stress of nitrided layer. The size of the carbides is an impörtant factor in the formation of this layer. Larger: carbides are more stable and less likely to dissolve during nitriding. In bulk specimens, nitrided in high NH3 content so as to form a white layer on the surface, carbide concentration takes place under the white layer. The effect of process variables on the fatigue resistance of nitrided steel has been investigated. Specimens of "Nitralloy" En41A have been gas nitrided under controlled conditions to produce steep and shallow case-hardness profiles with and without white layer. The resultant fatigue properties have been related to the microstructure, hardness profile, residual stress and case depth. The fatigue resistance of the nitrided En41A steel depend on the residual stress level in the case, and to a certain extent on the microstructure of the nitrided layer. Nitriding at 570°C produces low residual stress and also causes embrittlement of the nitrided layer.