Chill formation on the surface of ductile iron and the effects of austempering heat treatments on microstructures and mechanical properties

In this study, the effects of chill formation on the surface of ductile iron and austempering heat treatment on microstructure and mechanical properties were investigated. For this aim, the casting of material having four different chemical compositions were carried out to moulds having different samples diameter (Ø10-50 mm), mould material type (Sand, Stainless Steel, Steel and Copper) and mould wall thickness than all sample were austempered.Experimental study results showed that in the as-cast samples, the chill depth decreased with increasing sample diameter when mould wall thickness was constant and sample diameter is variable. In these samples the graphite volume fraction and the nodularity increased from surface to center and perlitic or ferritic structure was formed depending on the cooling rate. Again, in the as-cast samples, the chill depth was increased with increasing the mould wall thickness on the condition which the sample diameter was constant and the mould wall thickness was variable. This trend became more obvious by using the mould material having high heat transfer coefficient. In the large diameter samples (Ø30-50 mm), the highest chill depth was obtained in the sample having 30 mm diameter and 50 mm wall thickness.The wear resistance increased with increasing carbide volume fraction as impact strength, tensile strength and total elongation reduced in the as-cast samples.It was observed that the chill zone completely dissolved in the alloys of I, II and III at austempered conditions, but the chill zone in the IV. alloy containing low Si and high Cr content preserve its persistence with decreasing to some extend.Generally austempering heat treatments increase the impact strength, tensile strength and total elongation while it decreases wear resistance.It is defined that the value obtained from mathematical model performed with artificial neural network (ANN) is quite similar with experimental results.Keywords : Ductile iron, Chill, Austempering, Cooling rate.