Optimization of heat treatment parameters in austempered spheroidal graphite cast iron

In this research, influences of austempering parameters (time and temperature) on the mechanical properties and microstructure of ductile iron have been studied. The investigation was applied on a set of tensile samples and Charpy samples which were provided from Dikkan Döküm A.Ş. company. The material of samples was GGG 40 which a grade of ductile iron according to German standard. The samples were divided into 10 groups upon austempering temperatures and time so that austempering process was applied on 9 groups and one group was stayed as-cast for comparing the results. All samples of 9 groups were heated to 900 oC for 2.5 hours. This stage was followed by cooling process to 300, 375 and 450 oC using salt bath contained a mixture of sodium nitrate and potassium nitrate with percentages of 55 and 45 respectively, so that every 3 groups were cooled to a temperature. Every group in temperature was held in salt bath for 1, 2 and 3 hours. Then, they were cooled to room temperature by air. Mechanical properties of samples were evaluated by using tensile, impact and hardness tests. The microstructures of samples were obtained by using optical microscope. This test gave ability to predict to reasons of mechanical behavior which are affected by the phases, graphite content and shape. It was observed that austempering process enhanced the mechanical properties of samples considerably but harmed the elongation. Tensile and yield strength decrease with the increase in austempering temperature. The highest elongation is obtained at 375oC. Impact energy at this temperature is also higher than those of the other temperatures. When the holding time of every austempering temperature is exceeded 2 hours, the nodule size and graphite content increase. Keywords: Austempering, Ductile Iron, GGG 40, Spheroidal Graphite Cast Iron