The maintenance and repair of jet or gas turbine components has a considerably high share in the overall turbine operating costs. The authors deal with the regeneration process of complex capital goods considering jet engines as an example, with turbine blades being the most important components to be regenerated. In order to decide on a reasonable and economical regeneration path, maintenance approaches typically require detailed knowledge of the shape and wear condition of the components. In order to select suitable repair strategies for each component, the best possible knowledge about geometry, damages and surface topologies is necessary. In order to meet these requirements, a novel combination of non-destructive testing and measuring methods will be presented. Each process can be adapted for inline operation. The presented methods also enable quality control of the regenerated components that have completed their individual regeneration path. Due to the high variety of possible defects on turbine blades, the individually presented methods can be combined to form an inspection sequence. Detailed status monitoring before and after maintenance becomes possible for each component. This provides the basis for further decisions in the regeneration process.
Components made of nickel-based alloys are typically used for high-temperature applications because of their high corrosion resistance and very good creep and fatigue strength, even at temperatures around 1000 °C. Corrosive damage can significantly reduce the mechanical properties and the expected remaining service life of components. In the present study, a new method was introduced to continuously determine the change in microstructure occurring as a result of exposure to high temperature and cyclic mechanical loading. For this purpose, the conventional low-cycle fatigue test procedure was modified and a non-destructive, electromagnetic testing technique was integrated into a servohydraulic test rig to monitor the microstructural changes. The measured values correlate with the magnetic material properties of the specimen, allowing the microstructural changes in the specimen’s subsurface zone to be analyzed upon high-temperature fatigue. Specifically, it was possible to show how different loading parameters affect the maximum chromium depletion as well as the depth of chromium depletion, which influences the magnetic properties of the nickel-based material. It was also observed that specimen failure is preceded by a certain degree of microstructural change in the subsurface zone. Thus, the integration of the testing technology into a test rig opens up new possibilities for improved prediction of fatigue failure via the continuous recording of the microstructural changes.
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