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

Abstract

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.

Steam oxidation of aluminide coatings under high pressure and for long exposures

“…The inner part of the coating (the interdiffusion zone) was similar in terms of thickness (67 µm thick) but was less homogenous compared to the first one. Needle-like precipitates were observed and were attributed to Al nitrides, due to the presence of N in the substrate, as previously reported in [20,29,30]. Moreover, the diffusion coefficients of Al and of the compounds of the substrate were different, a Kirkendall effect was induced involving some porosities in the coating [20].…”

“…The slurry developed in our research group is "environmentally friendly" because water is used as a solvent [16]. The mechanisms of formation of these coatings on austenitic and ferriticmartensitic steels have been largely described in [17][18][19][20]. The good behaviour of these coatings in air, argon or steam at intermediate temperatures (650 < T < 700°C) has been demonstrated by previous works conducted in our group [12,19,21].…”

“…a.). As previously seen in [20,27], the coating elaborated on the P92 ferritic-martensitic steel can be divided in two parts. The first part corresponded to the diffusion zone (or additive layer) and was about 60 µm thick.…”

“…The aluminium micro-particles employed to elaborate the slurry were the same used in our previous works [12,19,20,21,23] (4-5 µm average size, Hermillon, France). The P92 samples were coated by spraying a slurry containing 43wt% Al microparticles dispersed in 57wt% of binder (1/10 polyvynil alcohol/deionized water).…”

“…The P92 samples were coated by spraying a slurry containing 43wt% Al microparticles dispersed in 57wt% of binder (1/10 polyvynil alcohol/deionized water). After drying in ambient air for at least 1 hour [16], the coated samples were annealed in argon (400°C/3h + 650°C/3h + 1050°C/1h) [20]. The O 2 partial pressure in the furnace where the thermal treatment takes place is lower than…”