Microstructure and mechanical properties of surface and subsurface layers in broached and shot-peened Inconel-718 gas turbine disc fir-trees

Metallurgical and mechanical characterization of surface and subsurface regions in broached and shot-peened fir-trees in an industrial gas turbine disc made of Inconel-718 were carried out. High resolution scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectrometry (EDS), electron backscatter diffraction (EBSD), X-ray diffraction, optical microscopy, and microhardness instruments were employed for qualitative and quantitative assessment of alterations at surface and subsurface levels. Five specific locations along the broached and shot-peened path were selected and thoroughly examined. Original metallography methods were developed to clearly and reliably reveal microstructure constituents. Special emphasis was placed on the generated defects in view of the manufacturer's quality indices, formation mechanisms of defects, and their potential impact on the service capability of the disc. Also, advanced analysis of the EBSD data allowed assessment of the deformed layer thickness as well as the misorientation angle and grain size variations from the broached and shot-peened surface towards the bulk parent material (PM). Furthermore, through successive material removal by electropolishing, measurement of residual stresses as a function of depth from the surface was performed by the sin2Ψ method. The obtained results are analyzed in terms of impact of the processing conditions on the evolution of microstructure, microhardness, and residual stresses. The findings are also related to the geometrical location in the disc.