The carburization of ethylene pyrolysis tubes is characterized using hardness, x-ray diffraction (XRD), scanning electron microscopy (SEM) and optical microscopy. The through-wall changes in magnetic properties due to carburization are quantified using vibrating sample magnetometer (VSM) measurements. The thickness and strength of the carburized layers are correlated with non-destructive measurements of the impedance of a C-core electromagnet, placed on the outside of the tubes, as a function of driving frequency. A two-dimensional finite-element mathematical (FEM) model is used to interpret the experimental impedance measurements. Good agreement was attained between the fitted model, the optical micrographs, hardness and VSM measurements.

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