Microstructural changes during creep and aging of a heat resistant MARBN steel and their effect on the electrochemical behaviour

The high creep strength of advanced boron alloyed 9 wt% Cr steels results from a precipitate stabilised martensitic microstructure. Grain boundaries and sub-boundaries are effectively pinned by boron stabilised precipitates. However, during creep exposure several degradation mechanisms with regards to precipitates are active such as, particle coarsening, phase transformations, dissolution of precipitates and precipitation of new phases. This diffusion driven phenomena affect the type, size and distribution of precipitates and lead to a deterioration of creep strength with increasing time of exposure. Within this work, an experimental melt of a 9Cr3Co3WVNbBN steel grade was investigated in terms of microstructural evolution during creep and aging and electrochemical behaviour. Creep specimens, tested up to 24,606 h were analysed to quantify the microstructural and electrochemical behaviour of the grip and the necked portion of the creep exposed specimens by z-phase indication of the main precipitates and Electrochemical potentiodynamic reactivation (EPR)-test. The change in diameter, phase fraction, number density and current density was quantified. The results of microstructural changes during creep exposure are compared with the electrochemical response of the specimens in the EPR-test. By understanding the correlation between microstructural changes and the results of the EPR-test, a possibility for a nearly non-desctructive on-site evaluation of the creep state of power plant components might be possible in future.