A metallographic study of aging in a cast heat-resisting alloy

Microstructural evolution of a heat-resisting cast alloy was studied by lig ht and electron optical methods. Samples from a cast pipe, used in a reform er furnace, were cut and aged for up to 6 weeks at 750 degreesC. The materi al was prepared for metallographic examination in the standard way, and ele ctrolytically etched with an oxalic acid solution. The samples were examine d with a light optical and a scanning electron microscope. Examination with the electron microscope was carried out with secondary and backscattered e lectron detectors; X-ray analysis was conducted at selected areas of the sa mples. It was found that the microstructure in the as-cast samples consiste d of an austenitic matrix and a network of two types of primary carbides (c hromium and niobium), whereas that of the aged materials also exhibit a fin e dispersion of small chromium carbide precipitates; decomposition of prima ry niobium carbides was found to occur as aging progressed. (C) 2000 Elsevi er Science inc. All rights reserved.