An eddy current testing method for detection of subsurface defects in reformer tubes

Subsurface defect detection in stainless-steel tubes is challenging due to low electrical conductivity, closed cross-sectional geometry, surface curvature and in-service inspection constraints. A probe-assisted signal processing method is proposed for high-sensitivity defect detection in reformer tubes under non-saturated conditions using eddy current testing (ECT). Slow spatial variations of the material properties along the tube are regarded as a localised baseline drift and separated from defect-induced perturbations using an information criterion-based adaptive polynomial fitting approach combined with Gaussian filtering. An enhanced ECT probe incorporating a tunnelling magnetoresistance (TMR) sensor is developed to selectively capture perturbation fields. Experimental results demonstrate that the proposed method enables reliable detection of defects located up to 90% of the tube wall thickness exceeding 10 mm and provides a robust and practical solution for non-destructive testing (NDT) of tubular structures.