Effect of austempering temperature and time on microstructure and mechanical properties of austempered ductile iron with dual matrix structure

Austempered ductile cast iron with dual matrix structure (DMS-ADI) is a relatively new group of austempered ductile cast iron (ADI) with a good combination of strength and ductility. DMS-ADI is produced by intercritical austenitizing temperature + austempering heat treatments and they contain ferrite + ausferritic structure in their microstructure. In this study, the effect of austempering temperatures and times on the microstructure and mechanical properties of DMS-ADI was investigated. For this purpose, tensile specimens were prepared by machining from Y block ductile graphite cast iron. Tensile specimens were austenitized at intercritical austenitizing temperature of 810°C for partial austenitizing followed by austempered at different austempering temperatures (300°C, 325°C, 350°C, and 375°C) and different austempering times (45 min, 60 min, 90 min, 120 min, and 180 min). Microstructural characterization was performed by optical microscope, scanning electron microscope (SEM) and X-ray diffraction (X-RD). Tensile and hardness tests were carried out to determine the mechanical properties. Austempered ductile iron with a dual-matrix structure consisting of ferrite + ausferrite was produced by intercritical temperature austenitization + austempering heat treatment. It has been determined that the austempering kinetics of austempered ductile graphite irons with the dual-matrix structure are similar to those of conventional ADIs. It was determined that as the austempering time increased, the ausferritic structure became significantly more pronounced, and the ausferrite volume fraction remained almost constant (45-47%), especially after 90 minutes of austempering time. With increasing austempering time, yield and tensile strength of the samples decreased, total elongation and fracture energy increased. However, after 120 minutes of austempering time, both the total elongation and impact toughness of the samples decreased. It was determined that austempering times have no significant effect on the ausferrite morphology. The austempering temperature and time that provides the best mechanical properties in austempered ductile iron with dual matrix structure (DMS-ADI) were determined as 350°C and 70-130 min.