Microstructural Characterization and Image Analysis in Ex-Service HP Alloy Stainless Steel Tubes for Ethylene Pyrolysis

Carburization is the major degradation mechanism of the HP alloy stainless steel tubes used for ethylene pyrolysis. Due to the changes in the microstructure and magnetic properties of the heat-resistant HP grade (Fe-Cr-Ni) alloy tubes over their service life, the level of carburization can be detected non-destructively using eddy current methods. In the current project, the microstructure of a carburized ex-service Nb-modified HP alloy tube has been characterized via image analysis. The phase fractions and phase distributions have been measured in order to determine the microstructural features that contribute to the eddy current non-destructive testing response, to assist in improving the accuracy of remaining life estimates. Issues in differentiating the large number of phases were encountered when using electron backscattered images for image analysis, and thus, energy-dispersive x-ray spectroscopy maps have been used, in combination with the software programs ilastik, for automated image segmentation, and FIJI, for image processing. The chromium depletion of the austenite matrix of the tube, and thus the magnetic response, was observed to be predominantly influenced by the volume fraction of M₇C₃ and M₂₃C₆ chromium carbides, with potential contribution from the chromium-containing η-carbide intermetallic. Carbonitrides of the type Cr₂(C,N) were seen to have negligible impact. A comparison between microstructures and magnetic response has determined that eddy current non-destructive testing can detect and measure carburization.