The influence of long-term operation as a part of the fast-breeder reactor heat exchange equipment on the structure and properties of austenitic chromium-nickel steel

The influence of long-term operation at 515°C on structure and properties of steel grade 09Cr18Ni9 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 FactSage.
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 steel grade 09Cr18Ni9 with the formation of secondary phases, initiated by the release of elements with limited solubility from the supersaturated solid solution. The presence of the following secondary precipitates in structure of a solid solution of austenite: Cr23C6 chromium carbide, ferrite (α), G-phase.
Based on a comparison of the thermodynamic modeling results and the experimental determination of the phase composition, it was established that the structure of the steel is in a state close to equilibrium.
The mechanism of the course of structural transformations, the sequence of formation of secondary phases was revealed and described. At the initial stage, chromium carbide is formed, then α-ferrite is formed near the carbides, and then G-phase is formed.
The results of the tests for impact strength and static elongation showed 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 samples showed 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 greatest influence of the evolution of the structure with prolonged thermal aging 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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