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The microstructure of the Sanicro 25 steel after steam oxidation studied by advanced electron microscopy and spectroscopy methods

Rutkowski B., Gil A., Ratuszek W., Woźniak B., Czyrska-Filemonowicz A.

Inżynieria Materiałowa

2016

Vol. 37, nr 5

223--227

Increase of the coal fired power plants efficiency is inseparable with an increase of the steam conditions. Currently used 9÷12% Cr steels are not able to withstand pressure of 30 MPa at 700°C for a long time due to their microstructure instability leading to fast damage. Development of new Fe-based materials able to work under advanced ultra-supercritical (A-USC) conditions for a long time is the key of importance. Present paper deals with a microstructure of the prospective, 22% Cr austenitic steel, Sanicro 25, heat treated or oxidized in water vapour at 700°C. Detailed characterization of the steel was performed using X-ray diffractometry as well as scanning and transmission electron microscopy techniques. Investigation led to establish the effect of temperature and water vapour environment on the microstructure stability of this modern austenitic steel. The results showed that the microstructure of the aged steel consists of M23C6 and Laves phase precipitated on the grain boundaries as well as ε-Cu, NbN, M23C6 and Z-phase precipitated within the grains. After oxidation at 700°C up to 5000 h in water vapour, Sanicro 25 developed a thin protective oxide scale at the surface, consisting mainly of Cr2O3 plates, characteristic for steels oxidation in vapour.

W pracy przedstawiono wyniki badań stabilności mikrostruktury stali Sanicro 25 (22Cr25NiWCoCu) po starzeniu w temperaturze 700°C w czasie krótszym niż 1000 h oraz po utlenianiu w atmosferze pary wodnej w tej samej temperaturze do 5000 h. Szczególną uwagę poświęcono mikrostrukturze zgorzeliny wykształconej na powierzchni stali podczas utleniania w temperaturze 700°C oraz zmianom mikrostruktury materiału rodzimego.