Small-scale specimen testing for fatigue life assessment of service-exposed industrial gas turbine blades

• Residual fatigue life assessment of service-exposed gas turbine blades.
• Successful application of small-scale specimen testing.
• Service-exposure causes microstructural degradation in the airfoil section.
• Microstructural degradation leads to reduced tensile and LCF properties at 850 °C.
• Investigation of fatigue crack growth by optical microscopy.

Service-exposed industrial gas turbine blades made from conventionally cast nickel-base superalloy IN738 were investigated in order to study the effect of service-induced microstructural changes on the tensile and cyclic material properties. Optical microscopy and scanning electron microscopy were used for the microstructural characterization of the gas turbine blades. The microstructural examinations showed a degraded microstructure in the airfoil section, such as γ′ coarsening, MC carbide decomposition as well as M23C6 carbide formation. Uniaxial tensile and isothermal low cycle fatigue tests were carried out at 850 °C using small-scale specimens manufactured from the blade root and airfoil section. The service-induced microstructural changes in the airfoil section yielded to a reduction of the tensile properties. Furthermore, a lower fatigue life of airfoil specimens compared to the blade root specimens was observed. Based on the determined fatigue data the residual fatigue life of the gas turbine blades was estimated. Finally, the fatigue crack paths were studied by optical microscopy.