Effect of the section size, holding temperature and time on the kinetics of the ausferritic transformation and mechanical properties of as-cast ausferritic ductile iron

The present research work studies the effect of section size, holding temperature and time on the microstructure and mechanical properties of as-cast ausferritic ductile iron. To that aim, Y block standard samples of 12.5 and 25.0 mm thickness were produced through a controlled cooling process. The holding temperatures of 300 °C and 400 °C were studied. The transformation time was changed in the range of 10–180 min. Furthermore, interrupted cooling experiments on the temperature range of 550 °C to 300 °C were performed to reveal the ausferritic nose. The experimental results indicated that for a holding temperature of 300 °C, an effect of the section size is noticed due to the cooling rate differences before the temperature arrest, while for a holding temperature of 400 °C no significant change on the microstructure evolution was observed. Besides, at the studied temperatures of 300 °C and 400 °C, and holding times of 80 and 65 min, respectively, after the beginning of the ausferritic reaction, the microstructure and the mechanical properties remained constant. The low cooling rate (air cooling) from the early shake out, to the temperature arrest, allowed the formation of some upper acicular ferrite during the cooling process from the ausferrite formation start temperature (defined for the considered alloy around 550 °C) down to the holding temperature, even for low temperatures (300 °C).