Weld Analysis in Combustion Chambers Subjected to Thermo-Mechanical Fatigue Load Conditions

Sammanfattning: At cyclic operation of gas turbines, components are subjected to thermo-mechanical fatigue (TMF) due to the high temperature gradients that arises. In this thesis life assessment of welds in combustion chambers are at focus. Siemens Industrial Turbomachinery AB uses a method they call the HQ-method to estimate the TMF life of components, but how well this estimation work with welds have not been investigated before and is therefore unclear. Because of this unclearness, an additional reduction factor is used take this into account. The goal of this thesis is to conduct a study of welds in the combustion chambers to get an overview of welds that are present, and to identify eventual problems with those. Furthermore, an analysis of a selected weld is performed and the results are evaluated by using the HQ-method. For this analysis, a TIG weld on a test specimen of Hastelloy X is selected as both the weld method and the material are commonly used in combustion chambers. The specimen is chosen due to the possibility to verify the results using a test rig designed for TMF-tests. The results show that the HQ-method, with the assumptions made today, yields a life of welds considered very low compared with the base metal. Also the location of the most severely loaded point is questionable. Further investigations also show that residual stresses in the weld relax quickly and the creep rate of the weld does not make any particular difference to expected life. To verify the results and clarify some questions it is therefore suggested that the commenced investigation continues with real tests on the weld. As the results show that the creep rate is unimportant to the fatigue life, the parameter should be ruled out from the investigation and the objective should instead be to find the accurate yield strength of the weld, and study if failure occurs in or outside the weld.