Abstract Microstructural evolution of a heat-resisting cast alloy was studied by light and electron optical methods. Samples from a cast pipe, used in a reformer furnace, were cut and aged for up to 6 weeks at 750°C. The material was prepared for metallographic examination in the standard way, and electrolytically etched with an oxalic acid solution. The samples were examined with a light optical and a scanning electron microscope. Examination with the electron microscope was carried out with secondary and backscattered electron detectors; X-ray analysis was conducted at selected areas of the samples. It was found that the microstructure in the as-cast samples consisted of an austenitic matrix and a network of two types of primary carbides (chromium and niobium), whereas that of the aged materials also exhibit a fine dispersion of small chromium carbide precipitates; decomposition of primary niobium carbides was found to occur as aging progressed.