On the microstructural development of the tempered martensitic Cr-steel P 91 during long-term creep—a comparison of data

Microstructural data for creep of the tempered martensitic 9 wt.%CrMoV steel P 91 at 873 K were analyzed. It was found that the variation of the subgrain size with strain conforms to an exponential change from the initial to the stress dependent steady-state value. Analysis of the size distributions of precipitated particles confirmed the previous result that in P 91 there is a superposition of growth of particles with the dynamic precipitation of new particles mostly of type MX. From the distributions the numerical fractions of two types of particles, a predominating population of M23C6 carbides and a population of fine particles representing mostly carbonitrides of type MX, and their sizes could be deduced. The variation of the creep rate with strain is in qualitative agreement with these microstructural changes.