The Effect of Hardness on Eddy Current Residual Stress Profiling in Shot-Peened Nickel Alloys

Abstract Recent research results indicate that eddy current conductivity measurements can be exploited for nondestruc-tive evaluation of subsurface residual stresses in surface-treated nickel-base superalloy components. According to this approach, first the depth-dependent electric conductivity profile is calculated from the measured frequency-dependent apparent eddy current conductivity spectrum. Then, the residual stress depth profile is calculated from the conduc-tivity profile based on the piezoresistivity coefficient of the material, which is determined separately from calibration measurements using known external applied stresses. This paper presents new results that indicate that in some popular nickel-base superalloys the relationship between the electric conductivity profile and the sought residual stress profile is more tenuous than previously thought. It is shown that in delta-processed IN718 the relationship is very sensitive to the state of precipitation hardening and, if left uncorrected, could render the eddy current technique unsuitable for resid-ual stress profiling in components of 36 HRC or harder, i.e., B.A. Abu-Naba