Microstructure of HK40 alloy after high-temperature service in oxidizing/carburizing environment: II. Carburization and carbide transformations

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, M23C6 eutectoid carbides of the as-cast condition were observed to have coarsened and transformed into M7C3 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 M7C3. The second material that presumably had been exposed to less severe service conditions displayed only coarse M23C6 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.