چكيده به لاتين
Suitable combination of mechanical properties together such as strength, toughness, hardenability and high fatigue strength with good wear resistance in Austempered Ductile Irons, ADI, made this type of cast iron to be used in many engineering parts. In some applications, ADI is used instead of forged steel because of its lower costs.
In this study, a two-step austempering process on aluminum ductile iron containing 2.2% aluminum was performed. At first, specimens were austenitized for 90 minutes at 900 °C. Then they were divided into two groups. During two-step austempering, the first group was exposed to a higher temperature in first stage than the second stage, and second group was exposed to a lower temperature in the first stage than the second stage. Temperature in first stage of the first group was determined to be 400 °C with a constant time of 30 minutes, and 250 and 300 °C were selected to be the second stage temperatures with constant times of 5, 20, 40, 60 and 120 minutes. For the second group, first stage temperature of 300 °C with a constant time of 30 minutes was choosed, and the second stage temperatures of 350 and 400°C with constant times of 5, 20, 40, 60 and 120 minutes were selected. All samples were finally quenched in water. The effect of temperature and time of the second step and the type of two-step austempering on alloy behaviors were investigated by microstructural observations, X-ray diffraction patterns and hardness tests. It was found that more retained austenite content was produced in the first group. Maximum retained austenite content of 30 percent for the first group and this of 13 percent for the second group was obtained. austempering reaction progressed more and completed sooner in the second group. It was also found that there is a much wider processing window in the first group than the second group. Sample hardnesses of the second group (with maximum hardness 483 HV30) were higher than those of the first group (with maximum hardness 346 HV30) due to the higher content of lower ausferrite in their structures. The amount of Aluminum in this cast iron, increases austemperability during Austempering in ductile iron, and enhances the diffusion rate of carbon in austenite, thereby increase the reaction rate of austempering. The carbon content of retained austenite in this study reached to 2.63 percent, which implies an increase in the amount of carbon content of retained austenite in the presence of aluminum in the composition of ductile iron. That's because aluminum increases the austenite lattice parameter and this increase in the austenite lattice parameter add to the amount of carbon.
