Effect of austempering time and temperature on microstructure and mechanical properties of nodular cast irons

Austempered ductile cast iron (ADI) prior to casting molten metal with a small amount of Mg and the addition of elements such as Ce, graphite (carbon) is a type of cast iron thus formed spherical. This type of cast iron, also known as ductile cast iron, has higher strength, toughness, ductility and workability than other cast irons. Due to these superior properties, it has wide application area in many different sectors such as automotive industry, machinery manufacturing industry, defense industry, construction machinery, railways and agricultural machinery. By heat treatment, the superior mechanical properties possessed by the ADI austempering name is further improved. Austempered ductile cast irons are new generation materials that can replace forged steels due to their excellent mechanical properties such as high strength, high toughness, high abrasion and creep resistance and ductility, as well as low density and high strength / weight ratios. The unique physical and mechanical properties of austempered ductile cast iron are due to the microstructure of oesferrite (bainitic needle ferrite) and high-carbon austenite resulting from heat treatment. In this study, as a result of the austempering heat treatment applied to commercially produced GGG-50 grade ductile cast irons, the effect of the material on microstructure and mechanical properties was investigated. The cast material used in the thesis was produced without alloy. The production cost was lower as no alloying elements were added. Experiments were performed on commercially produced material. In this way, the commercially used casting material was used instead of using tensile sample due to the fact that the cooling rate was faster in the tensile sample and the error rate was higher due to reasons such as austempering process easier. As austempering process to austempered ductile cast iron, austenitizing heat treatment was performed for 120 minutes at 930˚C and the structure was completely austenite. The second treatment was heat treatment in a salt bath for 90 and 120 minutes at 320 and 350°C. As a result of the austempering heat treatment, the microstructure of the material was observed to change depending on the austempering temperature and the salt bath temperature. Carbon content in austenite was determined by XRD process and hardness change was compared. The hardness values of the specimen in casting and the hardness values were observed as a result of austempering heat treatment. Keywords: Cast Iron, ADI, Austempering Heat Treatment, Mechanical Properties