This work deals with the effects of high temperature exposition of low alloyed creep resistant steels on their structural stability and structural stability of their weld joint. The theoretical part of this work gives an overview of creep resistant steels considering their chemical and phase composition and their connection to high temperature mechanical properties. The CALPHAD method is presented as generally accepted approach for equilibrated calculations and Thermo-Calc and DICTRA software packages are introduced as CALPHAD applications. Thermo-Calc software is used for calculation of phase composition of the examined materials and for calculation of the phase diagrams. Software DICTRA is used for simulations of diffusion controlled phase reactions in the analysed weld. Experimentally one weld joint of the steels 15 128 and 15 313 has been studied after four different annealing procedures. The experimental results include chemical profiles across the welds, microhardness profiles and metallographic analysis of significant regions of the welds. These experimental results are compared with the simulation results. The comparison shows a reasonable agreement between experimental and simulation data.

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This doctoral thesis summarizes the theoretical and experimental knowledge in the field of dissimilar weld joint of progressive austenitic and ferritic creep-resistant steels. The following materials were selected for the presented study: 316Ti stabilized austenitic stainless steel, martensitic 9-12 %Cr steel P92 and ferritic ODS steel MA 956. The main attention was focused on the long-term microstructural stability during high temperature exposure of heterogeneous joints of the austenite / ferrite type. The literature analysis critically evaluates the current state of knowledge in the field of microstructural stability of advanced creep-resistant steels weld joints. The practical experimental part was carried out in two directions. On the basis of the chemical composition phase equilibrium calculations were performed for each steel using the ThermoCalc software, giving the basic concepts about the dependence of the phase composition and the chemical composition of phases on temperature. In parallel with these calculations the laboratory joints 316Ti/P92 made by resistance welding and the MA 956/316Ti electron beam weld joints were prepared, analyzed in as-weld state and further annealed at different temperature conditions. Exposed joints were subjected to microstructure and phase analysis. The stability of the weld interface was mainly observed. Attention was also focused on the agreement between the calculation and experimental data in comparison with data published in the literature. Based on the calculations, experimental results and published data the suitability of the combination of materials is discussed in the thesis and reasoning about the behavior of studied weld joints during long-term high temperature exposure was formulated. Based on the results the expected degree of microstructural stability of 316Ti/P92 joint was confirmed, while the joints MA 956/316Ti were found to be unstable.

No description available.