Induction thermography as a tool for reliable detection of surface defects in forged components

Dynamic thermography with inductive excitation is analysed as an alternative to magnetic particle inspection or to eddy current testing. Given by the relation of the electromagnetic skin depth, the thermal penetration depth and the crack dimensions to be detected, different regimes for defect detection are identified. The effect of the crack parameters length, depth and inclination angle are discussed. In ferritic steel, at induction frequencies of 100-200 kHz, perpendicular open cracks with a length of 7.5 mm were detectable when their depth was minimum 0.15 mm. For inclined cracks, the sensitivity is even higher. Experiments were performed on cold and warm forged steel components. The signal-to noise ratio obtained from defects was usually high, the critical limitation on technical surfaces is the background due to surface roughness and due to surface contamination. A series investigation on forged components showed a good probability of detection and a low false alarm rate compared to magnetic particle testing. The short testing times of a few 100 ms per object view will allow short cycle times for mass products.