The fatigue properties and microstructure of 08Ch18N10T steel used for production of the interior of a pressure water reactor VVER 440 were investigated. The influence of the service conditions was simulated using thermo mechanical strain controlled low cycle fatigue tests in the range from 150°C to 270°C. The cyclic stress behaviour was investigated at different total strain amplitudes. The cyclic test to rupture and fractographic analysis were carried out. Selected tests were interrupted at different stages of fatigue life and changes in microstructure were evaluated. Dislocation density was determined using transmission electron microscopy and X-ray diffraction. Dislocation rich and dislocation poor regions appeared as a result of cyclic loading. Vein structure and successive formation of persistent slip bands and dislocation cells were observed similar to those in FCC metal monocrystals fatigued at room temperature. Dislocation density increased considerably at the beginning of cyclic loading; maximum was reached at 0.05 of life time. The following gradual decrease was related to rearrangement of dislocations into the walls. Simultaneously the nature of the dislocations changed. A majority of the screw dislocations occurred in as received conditions and at the beginning of cyclic loading. The fraction of edge dislocations increased with the increasing number of cycles. Distance between persistent slip bands and size of dislocation cells were related to the distance between striations on the fracture surface.Keywords: Low cycle fatigue, austenite, dislocation substructure, X-ray diffraction, transmission electron microscopy
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