High temperature oxidation of inoculated high Si/SiMo ductile cast irons in air and combustion atmospheres

The present work aims to investigate the effect of heating temperature (400, 600 and 800 °C) and inoculating elements (Ca-Ca, Ba-Ca, RE) on oxidation behavior of ductile irons containing 5.25% Si and 4.8% Si-2.3% Mo in dry air and combustion gas containing water vapour (natural gas burning). The oxidation is influenced by the gas atmosphere type, the iron alloying system, and the inoculating elements depending on the heating temperature. The weight gain increases from 0.001%–0.1% (400 °C) to 0.05%–0.70% (600 °C) and up to 0.10%–2.15% (800 °C). No particular effects of the considered influencing factors are found when heating at 400 °C, while at 600 °C, mainly the oxidation gas atmosphere type shows a visible influence. At the highest heating temperature of 800 °C, a limited increase of the weight gain is found for dry air atmosphere (up to 0.25%), but it drastically increases for combustion atmospheres (0.65%–2.15%). The water vapour presence in the combustion atmosphere is an important oxidising factor at 600–800 °C. The alloying system appears to influence the oxidation behavior mainly at a heating temperature of 800 °C in the combustion atmosphere, as evidenced by the lower weight gain in 5.25% silicon cast iron. Positive effects of inoculating elements increase with the heating temperature, with Ca and Ba-FeSi inoculation generally showing better performance. Irons inoculated with CaRE-FeSi exhibit a higher degree of oxidation. These results are in good relationship with the previous reported data: Ca-Ba-inoculation system appears to be better than simple Ca for improving the graphite parameters, while RE-bearing inoculant negatively affects the compactness degree of graphite particles in high-Si ductile irons. As the lower compactness degree is typical for graphite nodules in high-Si ductile irons, which negatively affects the oxidation resistance, it is necessary to employ specific metallurgical treatments to improve nodule quality. Inoculation, in particular, is a potential method to achieve this improvement.