Stress Analysis of the Steam-Side Oxide of Boiler Tubes: Contributions from Thermal Strain, Interface Roughness, Creep, and Oxide Growth

Abstract

Stresses in the steam-side oxide of boiler tubes are evaluated based on analytical derivations and numerical simulations. With only thermal strain considered, analytical solutions of stress distribution are obtained for a cylindrical geometry representative of boiler tubes and a flat-plate geometry—a typical simplification assumed in the literature to represent boiler tubes with extremely thin oxide scale. In these analytical derivations, the substrate metal has finite dimensions or is assumed to be very thick. The solutions under the approximations of flat-plate geometry and very thick substrate are employed to examine the accuracy of various approximations adopted in the literature to analyze the stress distribution in boiler tubes. For more complicated situations where the contributions from thermal strain, interface roughness, creep of the oxide and metal, and oxide growth are considered, numerical simulations are performed for a cylindrical geometry to evaluate the stress distribution in boiler tubes. The simulation results reveal that: (1) the local radial stress at curved oxide–metal interfaces is enhanced by interface roughness with implications about interfacial crack growth; (2) contrary to the general belief that creep relieves the oxide stresses, creep may actually increase the stresses in the oxide due to different creep rates of the oxide and substrate metal; and (3) the geometrically induced oxide growth strain substantially increases the magnitudes of the hoop and axial stresses in the oxide. Based on the assumption that the failure of the oxide scales is caused by crack growth which is dominated by the stress intensity factor, damage maps are plotted directly using the hoop, axial, and radial stresses as the critical variables. Our work provides a quantitative understanding of the interactions between different thermomechanochemical processes and oxide scale failure in boiler tubes.