Residual stress and cold work assessment in shot-peened IN718 using a dual-mode electromagnetic technique

Abstract

This paper investigates the feasibility of a novel dual-mode electromagnetic NDE technique based on broadband Hall impedance and eddy current conductivity measurements for characterization of IN718 coupons shot-peened at Almen 4A−12A intensities. The measured data were inverted into residual stress and cold work depth profiles using an iterative inversion procedure and elastic and plastic gauge factors obtained by independent calibration tests conducted under uniaxial stress. Comparison between the inverted dual-mode NDE results and residual stress and cold work depth profiles obtained by destructive XRD testing showed good qualitative agreement. However, the quantitative agreement was found to leave much to be desired as the residual stress levels were significantly underestimated while the cold work levels were slightly overestimated. To mitigate this problem caused by simple but inaccurate approximations used in the inversion, an empirical correction was applied that reduced the elastic gauge factors by 32% and increased the plastic gauge factors by 32%. This single-parameter empirical correction significantly improved the accuracy of the inverted NDE residual stress and cold work depth profiles. To check if this method worked in thermally relaxed states, preliminary dual-mode NDE data were obtained from some of the shot-peened IN718 coupons after thermal exposure to 600 °C for 100 h. The measured data showed trends that were different from those expected based on linearized gauge factors and indicated that the highly accelerated thermal relaxation used in these tests led to thermally activated microstructural changes in the highly cold worked near-surface regions of the shot-peened IN718 coupons. Therefore, future work should be focused on thermally relaxed IN718 coupons exposed to less accelerated thermal relaxation that leads to less severe thermally activated microstructural changes.