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Temperedmartensiticsteelsarecommonlyusedforpowerplantcomponentsatelevatedtemperatures upto903K. Inadditiontothecreepdeformationinducedbymechanicalloadsatthehightemperatures, the components are subjected to cyclic loads because of the frequent start and stop operations of power plants. Due to their mechanical and thermal properties, tempered martensitic steels are ideal candidates to withstand these conditions. Nevertheless, it is well known that tempered martensitic steels suffer from softening effects under constant and cyclic loads. The thesis at hand presents a framework for modeling the mechanical behavior of this type of steels at high temperatures. Here, the applicability of the proposed methods is demonstrated using the alloy X20CrMoV12-1, which is a typical representative of tempered martensitic steels. A phase mixture model is used to simulate the mechanical behavior of these alloys at elevated temperatures. The phase mixture model describes the alloy under consideration by means of an iso-strain approach including a hard and a soft phase. The hard phase is related to the subgrain boundaries and areas withahighdislocationdensity,whilethesoftphaserepresentstheinteriorofthesubgrainsandregions with a low dislocation density. Softening effects are taken into account based on the assumption that the volume fraction of the hard phase decreases during deformation. In order to make the calibration of the model based on macroscopic material tests possible, a backstress of ARMSTRONGFREDERICK-type and a dimensionless softening variable are introduced. This procedure results in a coupled system of three evolution equations with respect to the inelastic strain, the backstress, and the softening variable. Based on these evolution equations, the phase mixture model provides a unified description of the rate-dependent inelastic deformation including hardening and softening effects. As a basis for the calibration of the model, the results of numerous high temperature tensile and creep tests are presented. During the high temperature tensile tests, a constant temperature and strain rate are prescribed in the intervals 673K≤T ≤923K and 5.0×10−5 s−1≤ ˙ ε≤1.0×10−3 s−1,respectively. In addition, several creep tests are conducted at a constant temperature of 873K under different load levels. The test results serve as basis for the calibration of the one-dimensional phase mixture model. In the first step of the complex calibration procedure, the elastic parameters are determined, which is succeeded by the calibration of the inelastic behavior, the hardening regime, and the softening range. The subsequent verification of the calibrated model using the results of additional creep tests taken from literature reveals that the model provides accurate approximations of the experimental data for wide ranges of both temperature and stress, i.e. 673K≤T ≤923K and 100MPa≤σ≤700MPa, respectively. The calibrated phase mixture model requires only 14 temperature-independent parameters for simulations with respect to the indicated validity ranges. Furthermore, the phase mixture model is extended to multiaxial stress and deformation states, which resultsagaininacoupledsystemofthreeevolutionequationswithrespecttotheinelasticstraintensor, the backstress tensor, and the scalar softening variable. In addition, the thermodynamic consistency of the model is demonstrated based on the CLAUSIUS-PLANCK inequality. The three-dimensional phasemixturemodelisimplementedintothefiniteelementcodeABAQUS,whilethebackward EULER method is used for the implicit time integration of the evolution equations. The implementation of the model into the finite element method is verified by various examples, covering both uniaxial and multiaxial stress and deformation states. As a final step of the proposed framework, the mechanical behavior of an idealized steam turbine rotor is simulated. Therefore, a decoupled thermo-mechanical finite element analysis is employed to simulate a cold start and a subsequent hot start of a power plant. Within the preceding heat transfer analysis, the instationary steam temperature and the heat transfer coefficients are prescribed, and the resulting temperature distribution in the rotor is computed. Based on the obtained temperature fields, the stress and strain tensors are determined in a subsequent structural analysis. For future applications, these results could lay the foundation for the estimation of creep and fatigue damage, thus allowing for a precise prediction of the lifetime of power plant components in use. |
