Behaviour of carbides-rich {Co, Ni or Fe (bal.) - 30Cr - 2.5 to 5wt.C} ternary alloys in oxidation at high temperature

摘要

Eighteen Fe-based, Ni-based and Co-based alloys, all containing about 30wt.% of chromium and different carbon amounts (2.5, 3.0, 3.5, 4.0, 4.5 and 5.0wt.%) were elaborated by high frequency induction melting. These alloys were exposed to hot air (1000, 1100 and 1200°C) for 50 hours then air quenched. Their surface states were then characterized by Scanning Electron Microscopy (external oxides, sub-surface deterioration, carbide-free zones) and by Wavelength Dispersion Spectrometry mea surements (minimal chromium contents still existing on the extreme surface of the alloys). The as-cast alloys of the study show very high fractions of chromium car bides (Cr_(23)C_6, Cr_7C_3 and/or Cr_3C_2, this depending on the alloy's base element and on the carbon content), either eutectic or both eutectic and pro-eutectic (coarse and aci cular) carbides. Additionally, some of the C-richest alloys also contain lamellar gra phite. Generally, the alloys correctly resisted to oxidation at high temperature, thanks to their high chromium contents. However, some alloys appeared being affected by significant changes concerning their initial carbides, notably in the neighborhood of the oxidation front. Catastrophic oxidation sometimes appeared to be imminent, by considering some very low chromium contents on surface. Oxidation had become really catastrophic for the carbon-richest iron alloys exposed at 1200°C, as revealed by their very thick external scales and the loss of all their carbides elsewhere in the bulk.