Broaching has long been used for machining of fir-trees in gas turbine discs. The fir-tree arrangement is employed for mechanical attachment of blades to disc. Surface and subsurface microstructure changes induced by broaching affect the fatigue resistance of the disc. Therefore, a reliable and quantitative microstructure mapping of the broached fir-trees is essential for the basic optimization of the broaching process. In this study, the microstructure of the surface and subsurface layers of fir-trees in an industrial gas turbine Inconel-718 disc has been analyzed using optical microscopy and scanning electron microscopy. The focus has been on the characterization of defects generated by broaching at the surface and subsurface layers of fir-trees. Also, characterization of the grain size, γ″, γ′, and δ particles from the broached surface to the parent material has been carried out. Characteristics of these microstructural features are key inputs for the development of material based FEM models predicting the fatigue life of the disc. From microstructure studies, the presence of defects such as plucking and distorted layer was observed. The characteristics of these defects (size and morphology) were compared with those of the design limits determined by gas turbine engine manufacturer. Furthermore, significant variation in the volume fraction of δ particles was observed in the area affected by broaching compared with the parent material. These observations are related to the thermomechanical history of the material during the broaching process. Finally, the impact of microstructure evolution on microhardness variation from the broached surface to the parent material is discussed.
Determination. of Distorted Layer Occurred in Particle Surface by D. T. A.