Verformungsverhalten dickwandiger Rohre aus X10 NiCrAlTi 3220 unter zyklischen thermomechanischen Beanspruchungen (Ratcheting)

In heat exchanger components which are exposed to temperatures of more than 500°C, creep deformations must be taken into account. In the context of start up and shut down procedures or hot flow streaks secondary stresses are induced in these components. If additionally load controlled stresses (primary stresses) are present, e. g. in a tube under internal pressure, enhanced strain rates may occur (so called creep ratcheting). But also without any load controlled stresses, a progressive reduction in the length of the tube can occur. The strong temperature dependence of creep is responsible for this effect. Creep ratcheting was evaluated in axially unloaded and loaded thickwalled tubes of INCOLOY 800 under cyclic inductive heating conditions. The test of tubes under cyclic heating conditions showed a progressive axial contraction. In the case of cyclic thermal stresses with a superimposed primary stress, the creep rate increased up to a factor between 2 and 9 compared to the creep rate for the same primary stress under constant temperature conditions. Under constant temperature conditions the effect of negative creep due to microstructural instabilities could be observed. For the calculation of the temperature distributions in thickwalled tubes under cyclic heating conditions a computer program has been used based on finite differences method. By using this calculated temperature fields the creep strain behaviour was investigated with the FE-code ABAQUS. Here without any primary stress the FE-calculations showed a progressive axial contraction in good consistency with the experiments. In the case of cyclic secondary stresses with additional primary stresses the relationship between the calculated ratcheting strain rates were slightly underestimated. Nevertheless the theoretical analysis fairly confirmed the experimental results