Performance of Wrought Superalloys in Extreme Environments

As power generation systems move towards higher efficiency operation above 700 ℃, wrought superalloys are the leading structural alloy candidates, including precipitation strengthened (PS) alloys 740 and 282 for the highest temperatures. To evaluate the performance of these alloys for these applications, a range of 500–5000 h evaluations have been conducted in environments including steam, supercritical CO₂ (sCO₂) and simulated combustion exhaust with H₂O and/or SO₂ at 700–800 ℃ and compared to baseline exposures in laboratory air and 1 bar CO₂. These alloys primarily rely on the formation of an external Cr-rich oxide layer or scale for environmental protection and the reaction rates in all of these conditions are similar and relatively low. However, compared to a conventional solid solution strengthened alloy, like 625, the mass gains are higher for PS alloys due to the internal oxidation of the γ′ forming additions, Al and Ti. Post-exposure characterization has quantified the reaction products and the depth of internal oxidation is not a concern and does not appear to increase above the baseline behavior in laboratory air. Likewise, there is no indication of internal carburization in the sCO₂ environment at 750 ℃/300 bar. The addition of 0.1% SO₂ in CO₂-10% H₂O at 800 ℃ actually suppressed the internal oxidation at 1 bar but SO₂ may be a concern when the total pressure is higher.