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.
Spun cast alloy HP50-Nb tubing retired from service in a steam reformer after approximately 8 years of service had stress rupture properties suggesting very short remaining life. Microstructural examination, however, indicated little or no creep damage (cavitation) but did reveal extensive microstructural modification. The microstructure, mechanical properties, and stress rupture properties of the ex service material were determined and compared with virgin as cast material. The microstructural changes that occurred during service included the formation of phases rich in alloying elements such as chromium and niobium that would otherwise be expected to contribute to high creep strength. The creep life of the service exposed material was correspondingly short. The creep ductility of the service exposed material, however, was significantly higher than that of the as cast material. The implications of these findings for remaining life assessments of reformer furnaces using these materials are discussed.MST/3207
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