Evolution of dislocation density, size of subgrains and MX-type precipitates in a P91 steel during creep and during thermal ageing at 600 deg. C for more than 100,000 h

There are rather few quantitative data on the microstructure of the 9-12%Cr heat resistant steels after long-term creep. This paper presents results of the quantitative measurement of the size of MX precipitates, subgrain size and dislocation density in a P91 steel that had been creep tested for 113,431 h at 600 deg. C. The same measurements were conducted in the same P91 steel in the as received conditions. Transmission electron microscopy investigations were conducted using thin foils and revealed a decrease in dislocation density and an increase in subgrain size after creep exposure. MX carbonitrides are very stable during thermal and creep exposure of P91 steel at 600 deg. C up to 113,431 h. Electron backscatter diffraction (EBSD) investigations also revealed a significant change in the substructure of the steel after creep exposure.