Abstract: Martensite structure differences between P91 steel not in service and in service over 80 000 hours were compared by means of SEM, EBSD and TEM at different scales. The statistical and quantitative comparison of martensitic structure was made, the degradation behavior of martensitic structure in long-term service was analyzed, and the influence mechanism of martensitic structure degradation on mechanical properties was discussed. The results show that the structure of martensite block is coarsened, the average size of block increases by 61.6%, and the vertex value of size normal distribution increases by 70%. The martensite packet structure is also coarsened, the average size increases by 21%, and the normal distribution vertex value increases by 25.4%. The structure coarsening of packet is realized by the coarsening of block, and the degree of coarsening of block is greater than that of packet. The dislocation density decreases, resulting in a decrease of 8.6% in the proportion of structure with misorientation less than 15°. The coarsening of block structure and the decrease of dislocation density in martensitic structure are the important factors leading to the decrease of strength of the P91 steel after long-term service.

Key words: P91 steel, long-term service, martensite structure, degeneration behavior