Assessment of thermal aging embrittlement of cast stainless steel using pulsed eddy current testing signals

Thermal aging of cast stainless steel (CSS) degrades its mechanical properties such like toughness and poses risks to structural integrity. In this study, pulsed eddy current testing (PECT) signals were applied to evaluate thermal aging in specimens of CF8M steel, a typical CSS material. First, thermally aged CSS specimens were prepared by exposing CF8M material to elevated temperatures for various durations. Second, PECT signals at different pulse repetition frequencies are measured for specimens of different thermal aging state, i.e., different aging duration. Typical PECT signal features in time-domain and frequency-domain were then extracted, and their dependence on the aging time was investigated. It was found that the amplitude-based signal features show clear monotonic correlation with aging time, and the full envelope area of PECT signal at 250 Hz was identified as the optimal signal feature to evaluate the thermal aging degradation of mechanical property such as the embrittlement. Finally, electromagnetic property measurements and finite element simulations were conducted to investigate the evaluation mechanism of the thermal aging of the CF8M material. Both experimental and simulation results indicate that the reduction in magnetic permeability due to thermal aging damage is the main contributor to the perturbation of PECT signals. Based on the experimental and numerical results, it is concluded that the PECT method is an efficient technique for non-destructive evaluation of material degradation due to thermal aging in structures of CSS material.