Influence of graphite morphology on the nitriding and wear beahaviour of stable solidified cast iron compared to metastable solidified surface layer of cast iron
Grey cast iron is a widely used material in industry due to its near-net-shape and cost-effective production. The mechanical properties of the matrix microstructure (ferritic/perlitic) can be significantly improved by volume heat treatment, e.g. quenching and tempering or ADI, but the soft graphite is retained. It is well known that the graphite morphology has a significant influence on the stress behaviour. For improving the tribological load behaviour thermal or thermochemical surface processes and their combinations are suitable.
The paper is focused on comparative investigations of the nitriding and wear behaviour of cast iron materials with perlitic matrix but different graphite morphologies (EN-GJL250: lamellar graphite; EN-GJS600: nodular graphite) and ledeburitic cast iron surfaces generated by electron beam remelting (EBR). For all variants, the gas nitriding (GN) was carried out at 540 °C/16h. The aim was to generate a detailed knowledge about both the depth-dependent layer structure after nitriding depending on the matrix microstructure and/or the graphite morphology and the resulting tribological behaviour. For this purpose, the surface layers were removed stepwise (approx. 3-5 µm) and then at each level examined using XRD, hardness measurements and wear testing.
The phase composition of the compound layer is only influenced from the matrix microstructure (perlite or ledeburite). The graphite morphology has a significant influence on the surface coverage with compound layer and thus on the wear behaviour. Despite large areas are not covered in the case of nodular graphite, they act as a lubricant and wear particle reservoir under moderate stresses. In the case of lamellar graphite, a crack network is formed and abrasive particles are produced subsequently, which resulting in a wear increasing effect. The EBR produces a graphite- free surface, which enables the formation of a closed compound layers (EBR+GN) and thus leads to a significant improvement in abrasive wear behaviour in the scratch test.