Application of bending creep for examining effect of service conditions on creep response of steel

We demonstrate that creep testing of cantilevers in bending is suitable for evaluating steady-state creep parameters, including the stress exponent. In particular, a low alloy bainitic boiler tube steel, namely T11, in as-fabricated condition and after 27 years of service in a coal-fired power-plant were tested in bending at 600 °C at effective steady-state stresses of 100–200 MPa. Steady-state strain rates obtained from the bending tests matched reasonably well with those obtained from uniaxial compression tests performed at equivalent stresses, thereby unambiguously validating the bending test methodology for the entire range of stress. All types of samples showed a stress exponent of 6; however, the steady-state strain rates of the in-service samples were ~7 times larger than those of as-fabricated samples. Although this suggests a substantial loss in the creep resistance of the tubes at the end of the design-life, the “to-be-discarded” tube may still have considerable residual life. Microstructural evaluation of the samples revealed coarsening of carbides in bainite and metal enrichment of carbides, with excess of M6C, with exposure to the service conditions. A structure-property correlation is established to explain the experimental observations, and results are discussed in light of existing literature.