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High temperature oxidation of iron-based alloys reinforced by hafnium carbides. Part 2: Metallography results

Patrice Berthod, Elodie Conrath

The behaviour in oxidation at 1200°C of three HfC-containing iron-based alloys – Fe - 25Cr - 0.25 or 0.50C - 3.72 or 5.58Hf (wt.%) – was studied at 1200°C in dry synthetic air. After a first part dealing with the mass variations recorded by thermogravimetry during the oxidation tests, the present second part aims to characterize the surface, subsurface and bulk deteriorations or modifications by observing (optical and electron microscopy) and analysing (EDS, XRD) the oxidized samples before and after cross-sectioning. The results show that no catastrophic oxidation occurred neither for the Hf-containing alloys nor for the corresponding Hffree ones added for comparison. However iron oxideswere present together with chromia in the external scale – with additionally some hafnium oxides – and internal oxidation seemingly occurred deeply. Despite that the chromiumcontents in extreme surface were still high oxidation acceleration starts were observed in some isolated areas. The oxidation behaviour of these iron-based Hf-containing alloys was globally much better than for their cobalt-based homologues and slightly worse than for their nickelbased ones.