The effect of shot peening on steam oxidation of stainless steel at 600°-650°C

Shot peening is the boiler industry solution to scale exfoliation problems from steamside oxides formed on austenitic stainless steel superheater and reheater tubing in coal-fired power plants. However, relatively little data are publicly available regarding the performance of shot peened specimens in steam environments. This study exposed sections from two commercially shot peened S30409 tubes in 1 bar steam for up to 15,000 h at 600°, 625° and 650°C and specimens were removed at increments to characterize the oxide thickness and microstructure. The shot-peened inner diameter generally formed thin oxides with occasional Fe-rich oxide nodules at 600° and 625°C. However, the benefit of shot peening began to degrade after 5,000h at 650°C. Cut and polished faces of these specimens formed thick Fe-rich oxides similar to polished S30409 coupons. Surprisingly, the machined outer diameter of the tube specimens also showed similar behavior as the shot peened inner diameter, suggesting it had sufficient cold work to gain a similar benefit as shot peening in these exposures. The Cr content in the metal beneath the oxide was measured after 10,000 h but did not show the same amount of depletion as was observed in field exposed shot peened tubes.