microstructural degradation of two cast heat resistant reformer tubes after long term service exposure

microstructural evolution of a gas reformer tube made of the two dissimilar heat-resistant cast alloys, 25cr–35ni and 28cr–48ni–5w, was studied using light optical microscopy and scanning electron microscopy equipped with energy dispersive spectroscopy point analysis. specimens were cut from an ex-service, failed reformer tube aged 8 years. the phases and carbides at each section of the reformer tube were determined and compared with as-cast conditions. it was found that the specimens located far from the bottom or the roof of the reformer box had mainly meta-stable phases such as g-phase or m₂₃c₆ due to exposure to intermediate temperatures. those inside the reformer box, however, had phases such as m₆c carbides which are more stable than m₂₃c₆, and nbc due to exposure to high temperatures above 1000 °c.