Asymptotic creep deformation behavior of modified HP steel after long-term service

Creep behavior of modified HP steel was investigated through the tensile creep tests under the stress conditions of 25–55 MPa at 950 °C with four reformer tubes of different service periods: Unused (virgin), 8.50, 9.67, and 16.17 year aged. The minimum creep strain rates, creep rupture life, as well as elongation and reduction of area of the ruptured specimen were measured. It was observed that all the measured properties did not vary with different service periods. The creep deformation behavior was modeled based on the experimentally measured creep data utilizing the secondary creep (SC) model, the tertiary creep (TC) model, and the combined model of two latter ones (SC-TC model). The results indicated the asymptotic creep deformation behavior of the secondary creep strain rate. The tertiary creep also demonstrated asymptotic behavior, however, its strain rate slightly increased, as the service period increased. The creep curves predicted by the SC-TC model were more accurate than those obtained using the SC or TC model. Microstructural observations supported the observed asymptotic behavior.