The assessment of changes in the microstructure of a gas turbine blade with a non - destructive thermographic method

Diagnostic methods applied nowadays to examine condition of a gas turbine blade have been discussed in the Introduction section. What follows in the body of the paper is presentation of findings on the assessment of changes in the microstructure of a gas turbine blade with a non-destructive thermographic method. A laboratory experiment performed on new turbine rotor blades made of the EI 867WD alloy has been described. The experiment consisted in holding the blades at different temperatures in the exhaust-gas environment produced by an aircraft engine supplied with jet fuel. The pulsed thermography method has been used to determine and analyse the temperature distribution upon the examined area while cooling-down after the earlier uniform warming it up with a heat pulse. Diffusion and effusion of heat in the alloy have been examined. Both phenomena are determined with thermal properties and other physical properties such as size, shape, weight, structure, chemical composition, thickness of the protective coating of the blade. Examination of the microstructure of the blade material has shown the increased thickness of the coating, grain size, and in particular disadvantageous modification in the strengthening gamma' phase against temperature. Finding the relationship between parameters of thermal responses of blade materials to heat pulse against the average size of precipitates of the gamma' phase and thickness of the aluminium protective coating enables inferences on changes in the blade material's condition. With this relationship as the basis and knowing permissible changes to the microstructure one can determine the 'fit-for-use' and 'unfit-for-use' (i.e. serviceable and unserviceable) conditions of the blades. Assuming the material criterion, i.e. some disadvantageous change in the morphology of precipitates of the gamma' phase in the EI 867WD alloy under examination, the 'fit-for-use' (serviceability) thresholds for blades affected with different temperatures have been defined.