Electromagnetic nondestructive evaluation of tempering process in AISI D2 tool steel

• D2 steel parts were tempered at 200-650 °C to produce various microstructures.
• Precipitation of ɛ and Fe3C carbides and spheroidization of carbides were detected.
• Retained austenite decomposition and secondary hardening effect were determined.
• Variations of electrical resistivity (ρ) and magnetic saturation (Bs) were studied.
• Combined effects of ρ and Bs on the EC outputs were evaluated.

The present paper investigates the potential of using eddy current technique as a reliable nondestructive tool to detect microstructural changes during the different stages of tempering treatment in AISI D2 tool steel. Five stages occur in tempering of the steel: precipitation of ɛ carbides, formation of cementite, retained austenite decomposition, secondary hardening effect and spheroidization of carbides. These stages were characterized by destructive methods, including dilatometry, differential scanning calorimetry, X-ray diffraction, scanning electron microscopic observations, and hardness measurements. The microstructural changes alter the electrical resistivity/magnetic saturation, which, in turn, influence the eddy current signals. Two EC parameters, induced voltage sensed by pickup coil and impedance point detected by excitation coil, were evaluated as a function of tempering temperature to characterize the microstructural features, nondestructively. The study revealed that a good correlation exists between the EC parameters and the microstructural changes.