Thermogravimetry and post-mortem study of the oxide scale spallation behavior during post-isothermal oxidation cooling of Cr-rich {Co, Ni or Fe}-based cast alloys with various Hf contents

摘要

Co, Ni and Fe-based alloys containing 25wt.%Cr, and in the microstructures of which chromium carbides or HfC were present, were obtained by foundry way. Samples prepared from the obtained ingots were oxidized at 1000, 1100 and 1200°C. The spallation at cooling of the isothermally formed oxide scales was studied by exploiting the cooling part of the thermogravimetry curves. It appeared first that, for such {M-25wt.%Cr}-based cast alloys, the base element was of prior importance. Indeed, besides the possible mismatch of thermal expansion behaviour with the external oxide, it significantly influenced the nature of the oxide scale. Another identified important parameter was the quantity of oxide formed before cooling, which depends on the temperature stage but also on the more or less complex nature of the oxides formed. The presence of hafnium, here introduced in great quantities for obtaining many HfC carbides, played its well-known beneficial role for the spallation resistance.