Microstructure and Carburization Detection in HP Alloy Pyrolysis Tubes

A highly carburized HP40-Mod alloy ethylene pyrolysis tube was characterized by means of scanning electron microscopy with backscatter electron imaging, electron back-scattered diffraction, energy dispersive spectroscopy, and etching using the NACE International Standard Test Method for evaluation of carburization of ethylene pyrolysis tubes. The response of the tube to eddy current non-destructive testing was measured using the carburization crawler under development at Quest Integrity NZL Ltd. The matrix was significantly depleted of chromium, as low as 4 wt% Cr at the inner wall. M23C6 carbides transformed to M7C3 at the inner wall region and NbC carbides partially transformed to the chromium-rich η-carbide at the outer wall region, both of which likely contributed to the chromium depletion of the matrix. The present results indicate that the NACE etchant attacks the austenitic matrix where chromium content is below approximately 12wt%. A comparison to other ex-service tubes indicates that matrix chromium content and the location of the M23C6/M7C3 transformation front are useful microstructural characteristics for interpreting eddy current NDT response.