Abstract: A mixed-grain phenomenon was observed in the P91 thick-walled steel pipes， which would reduce the high-temperature properties of the pipes. Heat treatment process， the category of ingot and the microstructural evolution during hot rolling were analyzed. The results indicated that the difference of the categories for the P91 forging ingot and continuous casting ingot did not lead to microstructure difference. The pipes appeared obvious mixed-grain microstructure after hot rolling and the optimal microstructure could not be achieved only through the conventional normalizing and tempering treatment. The pipes’ microstructure uniformity can be optimized effectively by transforming from non-equilibrium microstructure to equilibrium one and rational heat treating process. The mixed-grain microstructure was not discovered in the steel pipes under the hot rolling process with the same processing parameters. However， abnormal grains growth was observed in the pipes after Pilger rolling.Therefore， the Pilger rolling process was the stage of mixed-grain initiation and improvement of microstructure uniformity.

Key words: P91 thick-walled steel pipes, mixed-grain, heat treatment, equilibrium microstructure, Pilger rolling