Effect of long-term thermal aging in heat exchange equipment of fast neutron switchgears on the structure and properties of austenitic chromium-nickel steel

The influence of long-term operation at 515 °C on structure and properties of 09Cr18Ni9 steel was investigated. Structure and phase composition were obtained using optical and scanning electron microscopy. The phase composition of the steel in equilibrium state was determined by thermodynamic modeling in the software package Fact-Sage. As a result of the study, it was found that during the operation at 515 °C with a duration of 195,000 h, the structure changes occurred in the 09Cr18Ni9 steel with the formation of secondary phases, initiated by the release of elements with limited solubility from the supersaturated solid solution. The following secondary precipitates in structure of the solid solution of austenite presented: Cr23C6 chromium carbide, ferrite (a), G-phase. Based on comparison of the thermodynamic modeling results and on experimental determination of the phase composition, it was established that the steel structure is in a state close to equilibrium. The mechanism of structural transformations course and sequence of the secondary phases’ formation were revealed and described. At the initial stage, chromium carbide is formed, then a-ferrite is formed near the carbides, and then G-phase is formed. Results of the tests for impact strength and static elongation have shown that the change in phase composition in process of thermal aging leads to embrittlement of the steel - a reduction in ductility and impact energy. Fractografic studies of fracture surfaces of the samples have shown that the decrease in plasticity during long-term high-temperature operation is associated with softening of the grain body and strengthening of the boundaries due to secondary precipitations of the carbide phase. As a result of this process, plastic deformation is localized in the weakened volume of the body of grain surrounded by strong boundaries. The structure evolution during prolonged heat aging has the greatest effect on impact strength. At the same time, the change in ultimate and yield stress is insignificant. The main contribution to the change in mechanical characteristics of steel is made by the secondary precipitates of the carbide phase.

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