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

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. (C) 2011 Elsevier Ltd. All rights reserved.