Through-wall carburization detection in ethylene pyrolysis tubes

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