Einfluß der Zusammensetzung auf die Strahlenversprödung von Eisenlegierungen

The radiation embrittlement of the reactor pressure vessel is highly safety-relevant for VVER-type pressure vessels. The sensitivity against radiation embrittlement depends on the chemical composition of the pressure vessel steel. Using an irradiation experiment at surveillance positions in two Russian VVER 440-type reactors the effects of copper, phosphorus and nickel on the radiation embrittlement should be investigated. For that, eight mock-up alloys were selected. Their chemical composition varied between 0.015 and 0.42 % Cu, 0.002 and 0.039 % P, 0.01 and 1.98 % Ni, 0.09 and 0.37 % Si, and 0.35 and 0.49 % Mn. Charpy-V impact tests and tensile tests were performed with specimens machined from these alloys. The specimens were tested in the as-received state, in the irradiated state (fluence: 1x10¹⁹ and 8x10¹⁹ /cm² [E>0.5 MeV]) an in the post-irradiation annealed state. In the as-received state, the alloys have a ferritic microstructure. Apart from Cu, the alloyed elements are solved in the matrix.
Irradiation produces strong hardening and embrittlement. The effect increases with the Cu and P content. Ni causes an additional embrittlement. It is independent on the Ni concentration within the range of 1.1 to 2 % Ni and results in a shift of the ductile-brittle transition temperature of about 120 °C after a fluence of 1x10¹⁹ /cm² by a flux of 4x10¹¹ /cm²s. The shift does not depend on the Cu or P content. Furthermore the upper shelf energy is especially reduced by the Ni-rich alloys. For very low content of Cu and P these relations are not valid. The irradiation effect can be eliminated by annealing at 475 °C /100 h. For high content of Cu or P the recovery is incomplete, it remains a residue of 20 to 25 % of the irradiation effect. Ni has no influence on the recovery.
Comparing the results of this study with the ones of the surveillance programmes of the VVER 440-type reactors, the alloys with low Ni content show the same irradiation behaviour as the weld metal. For the Ni rich alloys such well-walidated references are missing.
The experiment is part of an extended research programme. It supposed to continue in order to gain information about the synergistic effects of these elements.