Microstructure and Mechanical Properties of Service-Exposed HP-MA Heat-Resistant Steel Tube, Used in Cracking Furnaces

Cracking tubes as the main part of an olefin unit, are exposed to very harsh working conditions at both outer and inner surfaces, associated with major microstructural changes during service. Carburization, oxidation, alloying elements depletion, carbide coarsening, and secondary carbide formations are common phenomena, expected to take place in cracking tubes. This research tries to experimentally investigate the implications of cracking service exposure on microstructure and mechanical properties of the service-exposed HP-MA cracking tubes. In this regard, two tube samples after 20000 and 45000 h service at approximately 900 °C were selected. Microstructure degradation at the inner and outer surfaces of the tubes and their mid-thickness were investigated with optical and electron microscopes. By microstructural characteristics of both tube samples, it was concluded that in the 45,000-hour sample, which was more exposed to carbon, the number of secondary carbides formed on the outer surface was higher. Hardness variation across the thickness was also measured for both tubes and according to the results, in total thickness of the 45,000-hour sample, the hardness was more than the 20,000-hour specimen.