Low Stress Creep Deformation in High Chromium Ferritic Heat-resistant Steel Evaluated by Helicoid Spring Creep Test Method

We measured a creep deformation in Grade T91 steel under low stress condition using a helicoid spring creep test method. By applying this method, we measured a very low strain rate creep deformation in a short time as there is no need to consider the contribution of the microstructural change to the creep deformation behavior. We distinguished magnitude of several types of time-dependent deformation, including viscoelastic deformation, which occurs during the creep test in order to evaluate a time-dependent plastic deformation in strain-time curves. This consideration derives a conclusion that the time-dependent plastic deformation occurs even in a short time and low stress condition. Moreover, we confirmed a strain rate of whole creep deformation is controlled by the strain rate of the time-dependent plastic deformation at the time of 270 ks in the creep test. We confirmed that though there are almost no changes in the microstructure of Grade T91 steel during creep tests, the susceptibility on strain rate to applied stress has changed depending on the applied stress region. In addition, activation energy of the creep deformation was also different in each stress region. From these results, we concluded that the dominant creep deformation mechanism is different in the low stress region and high stress region.