High Temperature Oxidation of HfC-Containing Chromium-Rich Iron-Based Alloys

Three HfC-containing iron-based alloys were successfully prepared in the foundry. The alloys, all containing 25 wt% Cr, 0.25 or 0.50 wt% C and 3.7 to 5.6 wt% Hf, are characterized by a dendritic matrix and the presence of interdendritic script-like HfC carbides. These alloys were subjected to thermogravimetry test in air at 1,200 °C for 46 h. Such exposure at this high temperature did not significantly modify the characteristics of the bulk HfC carbides. In contrast the presence of hafnium led to non-parabolic oxidation kinetics and fast mass gain rates, by comparison with similar Hf-free ternary alloys. The scales formed all over the samples’ surfaces were never composed exclusively of chromia and iron oxides or (iron, chromium) oxides were present in significant quantities. Some particles of hafnium oxides were also present in the external scales. A subsurface zone in which the HfC carbides were in situ replaced by hafnium oxides developed from the oxidation front. Catastrophic oxidation started in several locations and threatened to expand all around the samples beyond the stage duration. The high temperature oxidation behavior of these HfC-containing iron-based alloys appeared better than their cobalt-based counterparts, but worse than their nickel-based counterparts, for the same conditions of oxidation test.