On microstructure formation and mechanical properties in grey cast iron

Sammanfattning: A major user of cast components is the automotive industry, where the functionality of the components is related to environmental demands. Internal combustion engines are constantly being improved to emit less pollution. A vital part in this development is to increase the material properties of engine components during their life cycle. In particular, cylinder heads, cylinder blocks and piston rings for diesel engine are produced in grey cast iron. Cast iron is expected to be in use far into the foreseeable future, due to favourable properties and low production costs. This work has been devoted to study microstructure formation, the tensile properties of cast iron and to some extent defect formation.The microstructure develops during solidification and solid state transformations. An inverse thermal analysis method was developed to study the kinetics of the microstructure formation. The inverse thermal analysis used, the Fourier method, analyses the cooling curves of two thermocouples to study the solidification or transformation. To decrease experimental errors, simulations have been done and the cooling curves were analysed. The best results were obtained when the thermocouples were placed close to each other.With the help of the thermal analysis a time dependent and fading nucleation law of the eutectic cells was found to fit the experimental results best. The experiments were made by multiple thermal analyses, and six different types of inoculants were investigated. The eutectic growth behaviour during solidification was evaluated with inverse thermal analysis, and it was found that commercial inoculants not only affect the eutectic nucleation but they also control the eutectic growth rate.Models of densities and volume changes are an integral part of a microstructure simulation of cast irons. These models are important for the inverse thermal analysis and an understanding of the porosity and expansion penetration in cast iron.The tensile strength of grey cast iron has been discussed by examining the fracture mechanism of the material at failure. The ultimate tensile strength is a result of the intimate collaboration between the graphite flake and the primary phases. Several parameters, including the graphite morphology, carbon content, inoculation and cooling conditions influence the ultimate tensile strength by offseting the equilibrium between the major constituents, the graphite flakes embedded in the primary metallic matrix. A model to predict the ultimate tensile strength is developed based on the interpretation of the stress intensity behaviour in a eutectic cell.The models developed for nucleation, eutectic growth and prediction of tensile strength were introduced into a casting simulation program. Mould filling, solidificauon, microstructure development and tensile strength of a complex. shaped cylinder head were simulated.