Oxidation behavior of chromium‐rich Fe‐based alloys containing HfC carbides at 1100 °C

Abstract

Three HfC‐containing iron‐based alloys were successfully elaborated by foundry. The alloys, all containing 27 wt% Cr, 0.25 or 0.50 wt% C and 4 to 6 wt% Hf, are composed of a dendritic matrix and of HfC carbides of two types: pre‐eutectic blocky carbides and interdendritic script‐like HfC carbides. These alloys were subjected to thermogravimetry tests in air at 1100 °C for 46 h. The presence of hafnium led to oxidation kinetics globally parabolic and low 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. Iron oxides or (iron, chromium) oxides were also present. Some particles of hafnium oxides were noticed in the subsurface and in the external scales. These particles obviously pegged the external oxide scale on the alloy surface. The long exposure at this high temperature did not significantly modify the characteristics of the bulk HfC carbides. For the alloys in which they were present in the as‐cast state, the eutectic chromium carbides played an important role. They dissolved and helped to supply chromium by enriching the interdendritic spaces which act as fast diffusion paths. In the alloy containing 0.50 wt% Cr and 4 wt% Hf, a parabolic constant of only 2.5 ×10−10 g2 × cm−4 × s−1 was thus obtained, which is a very low value for 1100 °C.