Investigations to the mechanisms of the layer formation using gas nitriding on remelted ledeburitic surfaces of unalloyed cast irons

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Within the framework of the project (3+2 years), the layer formation mechanism during nitriding for graphitic and remelted (ledeburitic) high C and Si cast iron alloys was researched and clarified for the first time. The chosen approach of carrying out the investigations on model alloys as well as on technical alloys allowed a separation of cause and effect and thus simplified the definition of objective functions. As a result, a model was developed for the time-dependent phase and precipitation formation in the different areas of the nitriding layer (compound/diffusion layer) depending on the microstructural constituents (eutectic and eutectoid cementite, ferrite, silicocarbide) of the white solidified Fe-xC-ySi alloys. Nitriding experiments at 540 °C on Fe-Si and Fe-C-Si alloys gave new conclusions on the Fe-Si-C-N phase diagram (metastable considering the amorphous Si nitride phase X). It was shown that the Mn and Cu contents additionally present in the cast iron alloys have an influence on the phase formation in the compound layer. While Mn obviously promotes the formation of ε-nitride, Cu supports the formation of γ‘-nitride. In addition, in the presence of Mn and Cu, there is a coupled effect that accelerates the precipitation of the Si-containing nitride. Mn apparently prevents the diffusion of carbon and thus the transformation of the eutectic cementite into α-Fe.The wear and corrosion behaviour of the nitrided cast irons is mainly determined by the inhomogeneous, not fully formed compound layers due to the graphite. In contrast, the stress behaviour of the combined treated edge layers is significantly better and is mainly determined by the phase composition of the compound layers.The aim of the continuation period applied for here is to investigate and analyse the essential findings on the nitriding mechanism of combined treated cast iron edge layers with regard to their effects on the depth-dependent (in the removal process) tribological (ball-plate test) and corrosive (current density potential curve) stress behaviour (damage mechanisms). A target-oriented delimitation of the treatment variants for an in-depth analysis of the stress behaviour on the basis of ablation tests in 3 levels was therefore carried out on the basis of different nitride layer structures and includes the limit states (in addition to Si nitride): I. ε-rich; II. γ'-rich; III. ε/ γ' (≈ 50:50 %). With the extensive database thus determined, the developed model conception of the nitriding mechanism is to be extended by the phase- and depth-dependent stress behaviour on the one hand. On the other hand, practical specifications for the nitriding process and the development of a nitriding-suitable alloy design for cast iron materials are to be derived and particularly suitable layer structures identified.