A series of neutron diffraction measurements have been carried out to determine the elastic residual strains deep within a large, 40-cm-diameter, forged and water-quenched IN718 aeroengine compressor disc. Neutron path lengths of up to 6 cm were necessary to probe the thickest parts of the forging, and three-dimensional strain and stress components have been derived for the first time in such a large superalloy specimen. Measurements have been compared with the results from a coupled thermal-mechanical finite-element model of the quenching process, based upon appropriate temperature-dependent material properties, with some success. The general residual stress state in the disc is one of near-surface compression, balanced by tension within the disc interior. The steepest stress and strain gradients occur in the transition region from compression to tension, about 1 cm below the surface all around the disc. The largest stress component is in the disc tangential direction and reaches a magnitude of 400 to 500 MPa near the disc surface and at its core. This exceeds the effective yield stress because of the presence of significant hydrostatic stress.
The residual stresses present in a quenched IN718 aeroengine compressor disc forging have been characterized using neutron diffraction and the results compared to those obtained from a finite element (FE) model for the quenching process. The ~40 kg forging had a diameter of 400 mm and a maximum thickness of ~45mm. Neutron path lengths of up to 60 mm were required to record strain components at the deepest points within the material. The residual hoop and radial stresses measured are generally compressive at the surface, up to 600 MPa and tensile at depth, up to 400 MPa, whilst the radial stresses are generally small. The deviatoric stresses are generally <150 MPa. FE model predictions of the residual stress are reasonable agreement with the measured values.
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