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

在该项目(3+2年)的框架内，首次研究和阐明了石墨化和重熔(莱氏体型)高碳硅铸铁合金渗氮过程中的渗层形成机理。所选择的对模型合金和技术合金进行调查的方法使因果分开，从而简化了目标函数的定义。因此，建立了渗氮层(化合物/扩散层)不同区域的随时间变化的相和析出物形成模型，该模型取决于白色凝固Fe-xC-YSi合金的组织成分(共晶和共析渗碳体、铁素体、碳化硅)。对Fe-Si和Fe-C-Si合金进行了540℃的渗氮实验，得出了Fe-Si-C-N相图(考虑非晶氮化硅X相的亚稳)的新结论。结果表明，铸铁合金中添加的锰和铜含量对化合物层中相的形成有影响。当锰明显促进ε-氮化物的形成时，铜支持γ‘-氮化物的形成。此外，在锰和铜的存在下，存在加速含硅氮化物析出的耦合效应。锰明显地阻止了碳的扩散，从而阻止了共晶渗碳体向α-Fe的转变。渗氮铸铁的磨损和腐蚀行为主要是由石墨引起的不均匀的、未完全形成的化合物层决定的。相反，复合处理的边缘层的应力行为明显更好，主要由化合物层的相组成决定。本文应用连续周期的目的是调查和分析复合处理铸铁边缘层渗氮机理的基本结果，即它们对深度相关(去除过程中)摩擦学(球板试验)和腐蚀(电流密度电位曲线)应力行为(损伤机制)的影响。因此，根据不同的氮化物层结构和包括极限状态(除氮化硅之外)：I.富含ε；ii.富含γ‘；i.ε/γ’(≈50：50%)，在3个水平的烧蚀试验的基础上进行了用于深入分析应力行为的处理变体的目标定界。在确定了广泛的数据库后，一方面，所开发的渗氮机制的模型概念将被与相和深度相关的应力行为所扩展。另一方面，将制定实用的渗氮工艺规范和开发适合铸铁材料渗氮的合金设计，并确定特别适合的层结构。

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.