Abstract
Nodular cast iron is widely used in combustion engines, for example as the row material of crankshafts, connecting rods, camshafts. The introduction of this material - instead of steels - have brought significant cost reduction but at the same time there are significant difficulties in surface finishing process and surface properties. The reasons of these difficulties are occurring from the inhomogeneous microstructure of cast iron, as in some cases only 1..3 μm metallic layer can be found above the graphite particles and this thin layer may crack or bend during the machining process. It is obvious, that the probability of the occurrence of particles on the surface or directly under the surface depends on the size and distribution of graphite. The surface defects like cracks or bended metallic layers can be originated from the manufacturing process, especially from the finishing operations. The paper introduces a quantitative stereological model which is suitable for describing the size distribution and location of graphite particles in nodular cast iron. The probability of occurrence of graphite particles under the surface at a given distance can be calculated using the model. If a requirement is given that the allowable number of particles on a unit area do not exceed a limit value, the optimal grain size distribution can be calculated. This size distribution can be reached by an appropriate casting process. The other possibility to avoid dangerous cracks and bended layers on the surface is the optimisation of finishing process. Pre-finishing and finishing operations influence the final properties of the surface. The quality management of finishing operations needs an appropriate non-destructive method for checking the final surface. As the crankshafts and camshafts can not be observed directly by optical microscope, a replica technique has been developed for that purpose. Experiments have proved that both the quantitative stereological model and the quality assurance of surface finish fit the requirements.
Keywords: nodular cast iron, surface finish, quantitative stereological model, quality assurance of surface