Shot-peening is an effective solution to mitigate scale exfoliation from steamside oxidation. Little data are readily available cataloging the long-term performance of shot-peened steels. Quarter-tube specimens from two commercially shot-peened type 304H tubes were exposed to 1 bar steam at 550o, 600o, 625o, and 650oC for up to 15,000 hours with intermittent removal of specimens at predefined times. Commercial shot-peening proved effective over 15,000 hours for temperatures up to 625oC. At 650oC, the shot-peened surface exhibited nodule formation, a sign of a loss of protection. Thick Fe-rich oxides were observed on the cut and polished faces on both the coupons and quarter-tube specimens. Unexpectedly, machining the outer diameter produced enough cold-work in the material to allow Cr to diffuse to the surface and provide oxidation protection. Shot-peening demonstrated increased hardness at 15,000 hours and 650oC. Research was sponsored by U.S. Department of Energy, Office of Fossil Energy, Crosscutting Technology Program.