Characterisation of aluminium diffusion coatings elaborated on austenitic stainless steels and on ferritic-martensitic steels

Slurry aluminide coatings were elaborated on IN-800HT and HR3C austenitic stainless steels (ASS) and on P92 ferritic-martensitic steels. The thermal treatments conducted in Ar enabled the melting of Al and the high temperature synthesis with the substrate elements to result in an aluminium diffusion coating. Whether for the ferritic-martensitic or the austenitic stainless steels, the coatings were formed by the simultaneous Al inward diffusion into the steel matrix and the outward diffusion of Fe (and Ni for the ASS) on both steel substrates. As a result, the coatings exhibited a B2-(Fe,Ni)Al phase for the ASS and B2-FeAl phase for the P92 substrate. A reduction of the grain size after annealing was noticed for the ASS but their microstructures remained mostly austenitic. However, a significant increase of the grain size occurred in the P92 steel with a transformation from the initial tempered martensitic structure to an austenitic structure. The microhardness of the ASS did not change significantly whereas for P92, a large increase occurred.