INITIAL OXIDATION STAGE OF SiMo AND SiNb-xAl DUCTILE CAST IRONS IN AIR AND CO2-CONTAINING ATMOSPHERES

In this study, novel SiNb-xAl (x:0-4 wt.%) ductile cast irons were developed to be used as exhaust manifold material. Initial oxidation stages of these alloys were studied at 800 degrees C in air and in a gas atmosphere (9 % CO2, 4 % O-2 and 87 % N-2) and compared with a commercial SiMo ductile iron. It was found that the newly developed ductile cast irons had higher oxidation resistance compared to the SiMo ductile iron under the studied test conditions, and this resistance increased further, as aluminum content of the ductile cast iron increased. Both surfaces and cross sections of the oxidized cast irons were characterized by 3D profilometer, scanning electron microscope equipped with energy-dispersive spectrometer and X-ray diffraction technique. Dense Fe-rich nodules and an extremely thick bilayer scale were detected on the surface of the SiMo ductile iron in both test environments. However, the number of nodules on the surface and their distribution frequency were decreased in SiNb cast iron. Aluminum addition to SiNb cast iron caused a further reduction in the number of nodules and also a finer scale formation due to the presence of an Al-rich protective layer on the surface. All these findings are encouraging that new cast irons can be used as alternative materials to SiMo ductile iron.