Precipitate stability in creep resistant 9-12% chromium steels

Doctoral thesis, 2006

9-12% chromium steels are used extensively in modern steam power plants at service temperatures up to 600°C. These materials are subjected to creep deformation during exposure which is the limiting factor in increasing the steam temperature and thus the thermal efficiency of the plants. A detailed characterization of the microstructure and its development during aging has been carried out to understand the changes in microstructure of 9-12% chromium steels during creep. The focus has been put on studying the coarsening and growth VN and M23C6 precipitates at service temperatures (600°C and 650°C). It was shown that boron segregates to austenite grain boundaries during quenching and is incorporated in M23C6 precipitates during tempering. It was also shown that this element reduces the coarsening rate of M23C6 precipitates. Atom probe measurements showed that slow transport of boron is due to both a low solubility in the matrix and a low diffusion constant resulting from the formation of Mo-B clusters. VN precipitates were highly stable even at 650°C. In many steels Z-phase precipitates after long time (10,000 to several 100,000 hours) and forms as coarse precipitates at the expense of fine VN precipitates and is detrimental for creep strength. The nucleation stage of Z-phase was studied using energy-filtered TEM. It was shown by electron diffraction that VN precipitates in the vicinity of NbC precipitates act as nucleation sites for Z-phase due to a small misfit in the (100) planes of VN and Z-phase. A new high boron trial steel with similar composition and treatment as the well-known TAF steel was studied. It was shown that tempering at 690°C for 2 hours is sufficient for the full precipitation of M23C6 but not VN. Coarse, partly agglomerated borides rich in molybdenum, chromium, vanadium and niobium were found but no BN was observed. Very fine VN precipitates were studied using TEM and atom probe tomography. They contained only vanadium, nitrogen and some chromium but no boron or carbon. A large fraction of the nitrogen content in the steel is present in the matrix as V-N clusters, giving a potential for dynamic precipitation during service.