There were 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, sub-grain size and dislocation density in a P91 steel that had been creep tested for 113,431 h at 600C. 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 sub-grain size after creep exposure. MX carbonitrides were very stable during thermal and creep exposure of P91 steel at 600C up to 113,431 h. Electron backscatter diffraction investigations also revealed a significant change in the substructure of the steel after creep exposure.

Evolution of Dislocation Density, Size of Subgrains and MX-Type Precipitates in a P91 Steel during Creep and during Thermal Ageing at 600C for More Than 100,000h. C.G.Panait, A.Zielińska-Lipiec, T.Koziel, A.Czyrska-Filemonowicz, A.F.Gourgues-Lorenzon, W.Bendick: Materials Science and Engineering A, 2010, 527[16-17], 4062-9