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

Abstract

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 M 23C 6 due to exposure to intermediate temperatures. Those inside the reformer box, however, had phases such as M 6C carbides which are more stable than M 23C 6, and NbC due to exposure to high temperatures above 1000 °C.