Effect of intermediate cooling rate normalizing on microstructure and creep rupture strength of P91 Steel

In this work, the P91 sample bearing intermediate normalizing cooling rate(200 degrees C/h) was prepared. Its mechanical and creep properties were compared with the air-cooling (2000 degrees C/h) normalized sample, and the differences of microstructure evolution were also compared by employing OM, SEM/EBSD and TEM/EDS. Results showed that after normalizing, twin martensite appeared in P91 steel with intermediate cooling rate, and its microstructure was coarser than the air-cooled one. Strength and hardness of the slow-cooled P91 steel were somewhat lower than the air-cooled one, but it had much lower toughness because of the formation of twin martensite. After tempering, P91 steel with intermediate cooling rate recovered more completely with twin martensite disappearing. Its strength and hardness dropped evidently, and were still lower than the air-cooled one, but it witnessed a predominant improvement in toughness. Creep rupture strength of the slow-cooled P91 steel was markedly lower than the air-cooled one. The main cause of that was the coarsening and different distributions of M23C6 carbides, which weakened the pinning effect and ultimately resulted in degradation acceleration.