The 9%Cr high resistant steels, especially the modified 9Cr-1Mo steel (T91), have proved good performances in thermal power plants, petrochemical industries and nuclear reactors since the eighties. These performances can be traduced by the high creep strength, the resistance to oxidation cracking and the adequate cost.

Thermal exposure changes of the microstructure of the modified 9Cr-1Mo steel have been investigated, through long term experience at 550°C in furnace, up to 7000 hours.

Detailed analysis of the microstructural evolution and changes in secondary carbides (M23C6) were carried out using SEM, TEM, XRD and EDX analysis. Electrochemical extractions were done because of the small volume of carbides. A progressive restoration of the tempered martensite matrix was observed. Moreover, a continuous increase of M23C6 size is revealed until stabilization after about 5000 hours of exposure. The nucleation of Laves phases is here found; two inverse contributions may be concluded. When nucleating far from secondary precipitates, these phases grow by consuming matrix elements, which can trigger creep damages. Nevertheless, by surrounding the M23C6 carbides like a shell, Laves phases can slow down their growth and so contribute in solid solution hardening. 

X-ray diffraction analysis lead to determine the temperature-time dependence of the matrix and M23C6 lattice parameter.