An Aluminide Surface Layer Containing Lower-Al on Ferritic-Martensitic Steel Formed by Lower-Temperature Aluminization

An aluminide (AlFe and α-(FeAl)) surface layer containing lower-Al was formed on ferritic-martensitic steel P92 by means of surface mechanical attrition treatment (SMAT) combined with a duplex aluminization process at lower temperatures, i.e. a packed aluminization followed by a diffusion annealing treatment below its tempering temperature. Indentation tests indicated that the lower-Al surface layer formed on the SMAT sample is more resistant to cracking and has better adhesion to the substrate in comparison with the Al5Fe2 layer formed on the as-received sample after the duplex aluminization process. Isothermal steam oxidation measurements showed that the oxidation resistance is increased significantly by the lower-Al surface layer due to the formation of a protective (Fe,Cr)Al2O4 layer. The rate constant of oxidation was estimated to decrease from ~0.849 mg2 cm−4 h−1 of the as-received material to ~0.011 mg2 cm−4 h−1 of the AlFe layer at 700 °C.