Simulation of the long-term cyclic creep behaviour of a low alloyed ferritic chromium steel

Low alloyed ferritic chromium steels of the 21/4 %Cr 1%Mo-type (German designation: 10 CrMo 9 10) have been used in electric power plants for many years. During service startand stop-operations cause cyclic creep. Long-term deformation tests showed that cyclic creep is often accelerated compared to monotonie creep (at constant load and constant temperature). The aim of the present work is to simulate the long-term cyclic creep behaviour on a microstructural basis. The simulation was carried out with the composite model of plastic deformation, which has been applied in the past to simulate the monotonie and cyclic creep behaviour of martensitic 9–12% chromium steels. The model was adapted to the present case by specifying the microstructural development of the 21/4 %Cr 1%Mo-heat through transmission electron microscopic investigations of the initial state prior to creep. The simulation is able to predict the creep behaviour for constant as well as cyclic loading and thus to quantify the degree of cyclic creep acceleration.