Evaluation of Residual Stress Relaxation in Surface-Treated Engine Alloys

摘要

Recent research results indicate that eddy current conductivity measurements can be exploited for nondestructive 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 conductivity profile based on the piezoresistivity coefficient of the material, which is determine separately from calibration measurements using known external applied stresses. This report presents results that indicate that in some popular nickel-base superalloys the relationship between the electric conductivity profile and the sought residual stress depth profile is more tenuous than previously thought. In particular, it is shown that in IN718 the relationship is very sensitive to the state of precipitation hardening and could render this technique unsuitable for eddy current residual stress profiling in components of 36 HRC or harder, i.e., in most critical engine applications.