Quality monitoring of thermally degraded stationary gas turbine blade material by surface wave technique

For the quality monitoring and reliable application of stationary gas turbine blade (vane), near surface damages in the isothermally degraded vein material (i.e., cobalt based superalloy) were characterized by ultrasonic surface wave technique. Surface wave velocity and attenuation were measured for the artificially degraded specimens at 1100 degrees C, together with microstructural analysis and micro-hardness measurement. Surface wave velocity increased with thermal degradation time, which was attributed to the increasing depletion of solute chromium near the surface. Strong frequency dependence of surface wave velocity was observed in the specimens with surface depletion layer. Attenuation coefficient of surface wave increased with increasing degradation time. The potential of ultrasonic surface wave technique to assess near surface damages in vein material was discussed with an emphasis on the relationship between the microstructural damage and the governing principles of ultrasonic response.