The influence of coupling effect between stress and high temperature on the degradation of microstructure and hardness in P91 steel

Interrupted aging and creep tests were performed in the condition of 650 degrees C/70 MPa for P91 steel. The mechanisms of hardness and microstructure evolution during aging and creep were studied through hardness test, SEM/EBSD and TEM/EDS. By comparing the difference between aging and creep, the coupling effect of temperature and stress in P91 steel was elucidated. Results show that creep stress could accelerate the entire degradation process in P91 steel. Firstly, the dislocation density increased rapidly at the beginning of creep, and then decreased also at a high rate after reaching its peak value. On the other hand, the dislocation density decreased slowly in aging condition. Additionally, with the migration of grain boundaries under creep stress, some intergranular M23C6 entered inside grains and afterwards dissolved into matrix. On the other hand, other M23C6 which remained intergranular continuously ripened and congregated with time, and finally turned into large ones at triple junctions of grain boundaries. Furthermore, stress had the greatest impact on dislocation density, whereas temperature affected volume fraction of M23C6 the most. In creep condition, the degradation of mechanical properties of P91 steel was mainly caused by the reduction of dislocation strengthening which was followed by grain boundary strengthening.