Microstructural study of the thermal stability of a thermomechanically treated T91 steel by TEM in situ annealing

Thermal stability of a conventional 9Cr-1Mo ferritic/martensitic steel submitted to a thermomechanical treatment has been investigated by means of an in-situ annealing experiment up to 700 ?. For comparison, same experiment was carried out in the as received material, before thermomechanical treatment. The results showed an evolution of the microstructure in both cases, more acute for as received material. Before thermomechanical treatment, during annealing small precipitates within grain grew significantly, new ones precipitated and dislocations started to move around 600 ?, finding no effective obstacles to their movement and disappearing at grain boundaries and the free surface. After thermomechanical treatment, the microstructure developed was much more stable. Only a slight increase in the original precipitates size was observed and the dislocation structure remained stable after annealing. To compare the results of in-situ experiments, short term heat treatments up to 700 ? were carried out for both materials on bulk pieces. The principal difference observed was the dislocation density in the case of as received material. After in-situ experiment, the analyzed area was free of dislocations while after heat treatment a certain density of dislocations remained within grains. This fact can be related to the proximity of the free surface in the case of thin foils.