Transition temperature of unalloyed nodular cast iron

Abstract

The relation between ductile-to-brittle transition temperature and the austempering temperature for an unalloyed nodular cast iron was studied. Un-notched impact samples were used in accordance to the ASTM 327 standard. These were austenitized at 920°C for 2 hours, and then austempered at 300°C during 90 minutes and at 350°C and 400°C for 50 minutes. These combinations of time and temperature were chosen because they correspond to the highest energy absorber for each austempering temperature. In order to determine the transition temperature, Charpy-type tryouts were done with the 30 kg hammer, modifying the temperature between -70°C and 150°C.
The results showed a decrease in the ductile-to-brittle transition temperature when the austempering temperature decreases, which could be a consequence of martensite formation at sub-zero temperatures. On the other hand, the chemical stability also determines the transition temperatures for different austempering temperatures, showing a higher carbon content in the structures obtained at lower austempering temperatures (Ta = 300°C).
With the purpose of supporting the hypothesis of martensite transformation, a measurement was done on the samples impacted in try out temperature interval (-70°C to 150°C); the Vickers hardness curves against the try out temperature showed that at sub-zero temperature the hardness increases a 15% regarding the hardness obtained at environment temperature. To make the microstructure changes evident, the samples in which the maximum and minimum impact energy were analyzed in an optical and electronic scanning microscope.