Synthesis and oxidation performance of low-temperature pack aluminide coatings on ferritic-martensitic steels

Evaluations of low-temperature pack aluminized coatings were investigated with respect to their oxidation and creep performance. A pack cementation process was developed to coat commercial 9% Cr ferritic-martensitic steel T91 at temperatures below its tempering temperature to avoid any potential detrimental effect on the mechanical properties of the coated alloy. To prevent formation of the Fe2Al5 phase in the coatings, the Al activity in the pack cementation process was reduced by substituting pure Al masteralloy with binary Cr-Al masteralloys containing 15 and 25 wt.% Al. When the Cr-25Al masteralloy was used, a duplex coating was formed at 700°C, consisting of a thin Fe2Al5 outer layer and an inner layer of FeAl. With the Cr-15Al masteralloy, a FeAl coating of ∼12 mum thick was achieved at 700°C.;Additional low temperature coatings were investigated utilizing a surface mechanical treatment method in order to determine what effect a higher surface stress in the alloy would have on the coating process. A process that combined the alloy's heat treatment and/or post weld heat treatment process was also explored. This process used a typical annealing and tempering cycle as the pack cementation processing temperature profile in order to achieve a one-step coating process.;Evaluation of the coatings involved long-term oxidation testing and creep testing. The oxidation testing was conducted in air + 10 vol.% H2O at 650 and 700°C. The creep testing was conducted at 650¢ªC in air with 100MPa load and showed that the Cr-15Al masteralloy coating reduced the rupture life by 11.6% when compared against the as-received alloy.