10-Year Experience With T23(2.25Cr-1.6W) and T122(12Cr-0.4Mo-2W) in a Power Boiler

Tungsten strengthened ferritic steels, 2.25Cr-1.6W-V-Nb and 12Cr-0.4Mo-2W-V-Nb-Cu have been developed and approved by the ASME Boiler and Pressure Vessels Code Committee for use in Section I construction, designated as T23 and T122, respectively. A field exposure test installing both steel tubes in service along with comparative materials in the tertiary superheater and secondary reheater of a $156MW$ utility power boiler has been conducted since April 1993. The tubes were removed to confirm their material properties and corrosion/steam oxidation behaviors after $1-year$⁠, $3-year$⁠, $6-year$⁠, and $10-year$ periods of service. The tensile and creep rupture strengths of both steels showed no remarkable change during service. Examination of steam oxidation scale on the inner surface of the tubes indicated that the scale growth rate of T122 was extremely small following $1-year$ service. The growth rate and morphology of steam oxidation scale is discussed, as compared with conventional materials.