Investigation of cooling rate dependent dendrite morphology in hypoeutectic lamellar cast iron

Shrinkage porosity and metal expansion penetration are two fundamental defects appearing at production ofcomplex shaped lamellar cast iron components. In previous work it has been shown that both shrinkageporosity and metal expansion penetration are related to the primary austenite dendrite network and itsformation mechanisms. The purpose of the present work is to study the morphology of primary austenite intest casting with a high tendency to form shrinkage porosity and metal expansion penetration. Simplified testmodels simulating the thermal and geometrical conditions similar to the conditions existing in complex shapedcasting have been successfully used to provoke shrinkage porosity and metal expansion penetration.Stereological investigation of the primary dendrite morphology indicates a maximum interdendritic space inconnection to the casting surface where the porosity and the penetration defect appear. Away from the defectformation area the interdendritic space decreases. Furthermore the local solidification times of the investigatedsamples were calculated in a 3D simulation software. Comparison of the simulated local solidification timesand measured interdendritic space indicates a strong relation of the same shape as it is known from theliterature when dynamic coarsening mechanism is characterized. The main outcome of the present paper is theobserved gradient of increasing interdendritic space from sections with high local solidification to sectionswith low solidification time. The mechanism of increasing the interdendritic phase can be explained by thedynamic ripening process. The unfortunate thermal conditions with the slowest local solidification timesituated in the border between the casting surface and its surrounding are considered the reason to form anaustenite morphology which can promote the mass flow between dendrite provoking shrinkage porosity ormetal expansion penetration.QC 20131210