AbstractThe rate at which microstructural features in H P50 (niobium modified) change with time and temperature has been studied in an attempt to allow evaluation of the thermal history of reformer tubes. The as cast microstructure consists of an austenite matrix and an interdendritic distribution of primary niobium and chromium carbides. After short exposures to service temperatures, a fine distribution of secondary carbides is generated. Samples of HP50 (modified) were aged at a range of temperatures both with and without the application of stress, and the population density of niobium carbide particles in the material was determined. The rate of change of the population of niobium carbide particles in the microstructure was found to vary according to first order rate kinetics, with an activation energy of 473 kJ mol−1. This suggests that a structured programme of microstructural examination of tubes at maintenance shutdowns would yield valuable information on the temperature profile in a reformer furnace.MST/1607