Hot Oxidation Resistance of a Novel Cast Iron Modified by Nb and Al Addition for Exhaust Manifold Applications

In this study, high-temperature oxidation behavior of ductile cast irons designed by Nb and Al addition (3.5 wt pct C, 4 wt pct Si, 1 wt pct Nb, 0 to 4 wt pct Al) is studied to develop an alloy that can perform better at elevated temperatures compared to commercial SiMo alloy. In the designed alloy, Mo is replaced by Nb as ferrite stabilizer and carbide former and has advantages compared to Mo since it does not form a network structure in the solidified matrix. Aluminum is added to the composition to inhibit the pearlite formation by causing inverse segregation of silicon that results in stabilizing ferrite and to obtain Al-rich protective oxide forms on the surfaces. Initially microstructural features of SiMo and designed alloys were examined, and modeling studies were carried out by Thermo-Calc software to determine the phases formed at high temperatures that are slightly below and above the A₁ temperatures of the alloys. Oxidation kinetics of SiMo and the designed alloys were determined by thermogravimetric analyses followed by oxidation tests at 750 °C, 800 °C and 850 °C in an air atmosphere furnace. The cross-sections and surfaces of the oxidized alloys were then characterized by microscopical studies and X-ray diffraction. The results revealed that all designed alloys exhibited better oxidation resistance at all studied temperatures than commercial SiMo alloy, and as aluminum addition increases, better performance is obtained because of the formation of Al-rich protective oxide layers. Thus, the designed alloys can be suggested as alternative exhaust manifold materials at elevated temperatures.