Effects of working temperature and carbon diffusion on the microstructure of high pressure heat-resistant stainless steel tubes used in pyrolysis furnaces during service condition

Highlights: → Microstructure of HP tubes used in pyrolysis furnaces during service condition was studied. → Two major phases, namely chromium and niobium carbides, existed in HP tubes in as-cast conditions. → Temperature and carbon diffusion influence the composition and volume fraction of secondary precipitates. → Chromium and niobium carbides were transformed to M23C6 and G-phase respectively during service. -- Abstract: In the present study, high pressure heat-resistant cast stainless steels (HP steels) modified with niobium and titanium were investigated in as-cast conditions and after being used in pyrolysis furnaces. Life span of the studied specimens obtained from pyrolysis furnace was 5 years. Microstructural changes were studied via scanning electron microscopy (SEM) equipped with energy dispersive spectrum (EDS), optical microscopy (OM), and X-ray diffraction (XRD). The effect of temperature and carbon diffusion on the microstructure, chromium-rich carbides, the NbC transformation to G-phase and other precipitates formed during service condition were discussed. The results showed that two major phases, namely chromium and niobium carbides, existed in the as-cast specimens. Temperature and carbon diffusion influenced the composition and volume fraction of secondary precipitates. Chromium and niobium carbides were transformed to M23C6 and G-phase respectively during service. Higher working temperatures do not always cause coarsening of precipitates. However, factors such as decarburization and carbon diffusion have important roles, too.