Microstructure and creep rupture strength of 2 kinds of P91 steels with abnormally low hardness

Abstract

P91 samples were prepared by means of 2 kinds of heat treatments which involved low normalizing cooling rate &tempering temperature exceeding Ac1 followed by low cooling rate respectively. The correlation between low hardness, microstructure and creep rupture strength was discussed. Results showed that there was a clear microstructure disparity between samples enduring different heat treatments, even though low hardness emerged in both samples. The sample undergoing low normalizing cooling rate featured ferrite structure with coarse grains and continuously distributed precipitates at grain boundaries after normalizing, and intermittently distributed precipitates at grain boundaries after tempering. The sample witnessing tempering temperature exceeding Ac1 followed by low cooling rate represented precipitates inside grains and the morphology of lath martensite in the majority of areas, whereas other areas were characterized by fine blocks of ferrite. The creep rupture strength of the sample experiencing low normalizing cooling rate was higher than the one experiencing tempering temperature exceeding Ac1 followed by low cooling rate, although hardness of the former was lower than that of the latter. For P91 steel, hardness and creep rupture strength did not show a positive correlation, because the grain size and distribution of precipitate also mattered.