Aluminum cast iron has excellent oxidation resistance, resistance to sulfide and corrosion. Compared with Ti and Ni alloys, it is expected to be a substitute material for structural materials and stainless steels because it is relatively inexpensive to use Fe, which is a non-strategic element. In case of iron, excellent non-strength has the same level of strength when manufacturing parts. This results in a weight reduction effect of about 30% as compared with the case of using stainless steel. However, their practical use has been limited because their room-temperature ductility is insufficient and strength is rapidly reduced at temperatures of 600 °C or higher. For heat-resistant cast iron, high-temperature materials containing Cr and Ni account for 30∼50% or more. However, such high-temperature materials are costly and aluminum heat-resistant cast iron is considered a heat-resistant cast iron material to replace such expensive heat-resistant materials.
To this end, in this study, aluminum cast iron specimens with 1∼4 wt.% aluminum added to flake graphite cast iron are fabricated using high-frequency induction melting furnace. The microstructure and phase analyses, mechanical properties, and oxidation resistance according to the heat treatment conditions are investigated to establish the optimal austempering heat-treatment conditions. The results are quantitatively evaluated by varying the aluminum content and the austempering heat-treatment conditions.
The microstructure analysis in terms of aluminum content and austempering heat treatment conditions revealed two phases: austenite and ferrite. The cooling curve experiment and solidification simulation show that the austenitizing temperature is insufficient for forming austenite phase in the 4 wt.% Al cast-iron to establish the optimum os tempering heat treatment condition. When the austenitizing temperature is 1,000 °C, the ausferrite fraction is higher than that obtained at 900 °C. Based on the mechanical properties, phase map analysis, and distribution fraction of 2 wt.% Al cast-iron, the most optimized heat treatment condition was identified as the os tempering temperature of 320 °C.
Oxidation by aging temperature and holding time the weight measurement and oxide layer thickness measurement showed that the oxidation weight and oxide layer thickness increased more for 0 wt.% Al cast-iron than for 2 and 4 wt.% Al cast-iron. Observation of the cross section of the oxide layer revealed that the 0 wt.% Al cast-iron contained a silicon oxide-containing SiO2 or Fe2SiO4 oxide film. For cast-iron containing aluminum, as the aging temperature and the holding time increase, the internal oxide layer due to aluminum increases in thickness and the amount of the iron-oxide layer generated on the surface decreases.