Effects of Microstructure on Eddy Current Residual Stress Characterization of Shot-Peened Inconel 718

Recent studies have shown the possibility of exploiting the eddy current (EC) method for nondestructive characterization of near-surface residual stresses in surface-treated nickel-base superalloy components. The method involves measuring swept frequency EC signals to obtain near-surface electrical conductivity depth profiles, which can be converted into residual stress profiles using the empirical piezoresistivity relationship. However, the relationship between EC responses and residual stress profiles appears to depend on the microstructure. This paper reports on a systematic study of microstructural effects on the EC responses and near-surface conductivity deviation profiles induced by shot peening, using a series of heat-treated Inconel 718 samples with different secondary phase contents and thus a variety of hardness levels. Calibrated EC signals from 100 kHz to 50 MHz were measured from aged and unaged samples, before and after shot peening at various Almen intensities. Strong dependence of the EC responses on the sample hardness and microstructure has been observed. A procedure is introduced to convert the EC signals into shot-peening-induced conductivity deviation profiles where the microstructure dependency is suppressed. The resulting estimates of the conductivity changes are larger than expected, indicating that characterization of shot-peening-induced residual stresses by EC requires further understanding of the material responses beyond the empirical piezoresistivity relationship. Other mechanisms that could contribute to the observed conductivity changes are discussed.