Metal dusting of ferritic Fe-Al-M-C (M = Ti, V, Nb, Ta) alloys in CO-H2-H2O gas mixtures at 650 °C

Iron aluminides are known for their resistance to high temperature oxidation and sulphidation. Only little information is available about carburisation and metal dusting of Fe-Al alloys. Metal dusting experiments with Fe-15Al and Fe-15Al-2M-1C alloys (in at.%) with M = Ti, V, Nb. or Ta were conducted at 650 C in CO-H-2-H2O gas mixtures with the carbon activity a(C) = 28. The kinetics of the carbon transfer was measured using thermogravimetric analysis (TGA). It is shown that the mass gain kinetics decreases by adding the alloying elements Nb, Ta, V, or Ti with C. Alloying with titanium and carbon leads to the most significant decreasing effect. The metallographic cross section observation showed a general metal wastage for Fe-15Al, but local pitting for the Fe-15Al-2Nb-1C and Fe-15Al-2Ta-1C alloys. For the Fe-15Al-2V-1C and Fe-15Al-2Ti-1C alloys no significant attack was observed. Needle- or plate-like Fe3AlCx precipitates were detected in the carburised samples. The existence of this ternary carbide with perovskite structure was predicted by thermodynamic calculations using the software Thermo-Calc. The morphology of graphite on the surface was analysed by scanning electron microscopy (SEM). Mainly fine filaments with iron containing particles were detected. Cementite was detected in the coke layer by X-ray diffraction analysis (XRD).