Influence of Mechanical Surface Strengthening on Nickel-base Superalloy DA718

Author:  Ehrl, Planetzki, Hesser, Wagner
Source:  Conf Proc 2011: ICSP-11 South Bend, IN USA (pgs. 213-218)
Doc ID:  2011035
Year of Publication:  2011
Abstract:  
Abstract To ensure safe turbine engine service life despite decreasing wall thicknesses of rotating parts, the component surfaces are subjected to specific mechanical strengthening pro-cesses. Widely-used materials for these rotating components are those of the group of nickel-base superalloys. One representative example is DA718 (Direct Aged) which is chemically identical with IN718 but differs in the production process. For DA718, the solution heat treatment in the production of the forging is omitted. Only a two-stage ageing process is used following quenching from the forging temperature. DA718 possesses a tensile strength superior to that of IN718 and a comparable high creep resistance. Mechanical surface strengthening is achieved by shot peening or deep rolling. To investigate the influence of both strengthening methods on the fatigue behavior of the material, parame-ter studies were conducted. Shot peening was performed using peening media of various shot sizes and different Almen intensities. For deep rolling, the ball diameter, pressure, roll-ing speed and overlap of adjacent rolling traces were varied. Residual stress-depth profiles were measured by means of X-ray diffraction on flat speci-mens that were also used for surface topography characterization. Specimens were sub-jected to load-controlled axial tests and rotating bending tests at room temperature to deter-mine fatigue lifes and endurance limits as well. Keywords DA718, shot peening, deep rolling, fatigue life, residual stress, rotating bending loading, axial loading