Effect of subzero treatment on the microstructure and mechanical properties of austempered ductile cast iron

We have investigated the effects of subzero treatment on microstructures and mechanical properties of austempered ductile cast irons. The retained austenite microstructure was transformed to martensite microstructure by subzero treatment and strain. The ratio of the transformation to martensite increased with decreasing the subzero treatment temperature. By decreasing the subzero treatment temperature and increasing the strain, the retained austenite of the specimens with more Cu contents was transformed more to martensite. While the values of strength and hardness increased by decreasing the subzero treatment temperature, the values of elongation and impact decreased. Moreover, the strength and hardness of the specimens with more Cu contents increases, whereas the values of elongation and impact decreased. As a result, it was found that an addition of Cu in subzero treatment of the specimens had a little effect on the strength and hardness, but had a significant effect on the impact values.