The influence of graphite morphology on the elastic behaviour of cast irons

Abstract: The morphology of the graphite phase largely controls the resulting properties of different grades of cast iron. In flake graphite cast irons, the mechanical properties are low while the thermal conductivity is high. In contrast, with spheroidal graphite cast irons, the mechanical properties are high while the thermal conductivity is low. These differences must be accounted for in the design work of diesel engines when choosing the material grade of different components. In this work the influence of graphite morphology on the deformation behaviour of cast irons has been studied with an emphasis on the elastic region.The studied material grades originated from castings for marine diesel engine piston rings with different chemical analyses. This gave rise to six material grades with different nodularities and three material grades with different carbon equivalents. For these different material grades the mechanical properties were determined and the deformation behaviour was examined by means of mechanical testing and acoustic emission measurements. These data were correlated to microstructural parameters.When studying the modulus of elasticity it was found that the modulus of elasticity of the inherent graphite phase depends on the roundness of the graphite particles and is due to the different elasticity in the two main crystallographic directions of the graphite lattice. The graphite particles are structured in different ways in flake graphite and spheroidal graphite, which gives rise to different stiffness at loading. The modelling and experiments support the observation that the average modulus of elasticity of the graphite phase depends on the morphology. This improves the understanding of the differences in the modulus of elasticity of the different studied material grades. The correlation between the modulus of elasticity of the graphite phase and the bulk nodularity of the cast iron specimen was found to be linear. This linear function was used when modelling the effective modulus of elasticity of the different cast iron grades with very high accuracy between experimental and theoretical values.Another factor affecting the elastic response when subjecting a cast iron component to tensile load was found to be the plastic deformation that actually occurs at very low strains for all of the studied cast iron grades. lt was found that the plastic deformation in the low strain, elastic region governs the modulus of elasticity of cast irons. The plastic deformation in the seemingly elastic region was quantified by using acoustic emission measurements. These measurements showed that the amount of plastic deformation in the elastic region was largely controlled by the graphite morphology. It was concluded that, as the roundness of the graphite particles increases, the plastic activity in the elastic region decreases and the energy absorption ability increases.