Effect of Chills on the Microstructure and Mechanical Properties of Carbidic Austempered Ductile Iron

Carbidic Austempered Ductile Iron (CADI) is a recent addition to the Austempered Ductile Iron (ADI) family. The effect of chills on the microstructure and mechanical properties of CADI was investigated after Austempering. Three samples of chromium alloyed CADI, the first sample without chill, the second sample with bottom chill and the third sample with bottom and side chills were produced in order to evaluate the effect of chills on its mechanical properties. The samples were austenised for 2 hours at 925° C and then austempered at 325° C for 2 hours in a salt bath furnace. The microstructural features of the as-cast and the austempered CADI samples were analysed using Optical Microscope and Scanning Electron Microscope (SEM). The mechanical properties of the CADI samples (as-cast and austempered) were evaluated for hardness, impact and wear. By austempering at 325° C for 2 hours a typical microstructure of bainite was produced in all the three samples. Hardness and wear resistance of austempered samples produced using bottom and side chills were considerably higher than the corresponding values in samples produced without using any chill and also by using only bottom chill. This enhanced mechanical property in the bottom and side chill sample is attributed to the presence of bainite, carbides and more of uniform fine graphite nodules.