Carburization phenomenon in two tubular materials made from Fe-Cr-Ni-based HK40 alloy has been investigated after a service life of approximately 25,000 h in an ethylene pyrolysis furnace and compared with the original as-cast structure. One of the materials involved unique microstructural features associated with a macrocrack in the tube wall. In this material, M 2 3 C 6 eutectoid carbides of the as-cast condition were observed to have coarsened and transformed into M 7 C 3 carbides with a heavily faulted structure. This carbide transition was observed to have occurred via an in-situ mechanism and also resulted in γ precipitation in M 7 C 3 . The second material that presumably had been exposed to less severe service conditions displayed only coarse M 2 3 C 6 type carbides. Chemical compositions of the microstructural constituents have been obtained via microanalysis and the distribution of the alloying elements among the present phases by using X-ray mapping.