Effect of intercritical austenitizing and air cooling heat treatments on microstructure and mechanical properties of ductile cast iron

In this study, the effect of the intercritical austenitizing temperature and air cooling on the microstructure and mechanical properties of ductile iron was investigated. For this purpose, samples were prepared by machining from Y block ductile iron castings. The samples were air-cooled from intercritical austenitizing temperatures at 770°C, 780°C, 790°C, 800°C, 810°C, and 820°C for 1-hour. Optical microscope was used for microstructural characterization and scanning electron microscope (SEM) was used for analyzing fracture surface of samples. Tensile and hardness tests were conducted in order to determine the mechanical properties. Experimental results showed that pearlite volume fraction increased with increasing intercritical austenitizing temperatures. Moreover, the hardness and tensile strength increased with increasing pearlite volume fraction. However, total elongation, reduction in area and energy at break values decreased with increasing intercritical austenitizing temperatures. Fracture surface analysis showed that the number of dimples regions decreased with increasing intercritical austenitizing temperatures. This can be explained as ductility decreased as well. Ductile cast irons which have different pearlite volume fraction can be produced by controlling intercritically austenitizing in the same chemical composition. When mechanical properties of intercritically austenized and air cooled nodular cast irons compared with relevant ASTM standarts, it can be seen that tensile and total elongation values take place between austempered ductile irons and as cast ferritic/pearlitic ductile irons.