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Non-destructive microstructure characterization of materials states after thermal ageing, fatigue loads and neutron irradiation - a state of the art report from the German nuclear safety research program

: Dobmann, G.

Mayr, P. ; TU Graz; Österreichische Gesellschaft für Schweißtechnik -ÖGS-; International Institute of Welding -IIW-:
IIW International Conference Safety and Reliability of Welded Components in Energy and Processing Industry 2008. Proceedings : 10 - 11 July 2008, Graz, Austria
Graz: Verlag der TU Graz, 2008 (Welding in the world 52.2008, Special issue)
ISBN: 978-3-85125-019-0
International Conference, Safety and Reliability of Welded Components in Energy and Processing Industry <2008, Graz>
Fraunhofer IZFP ()
3MA; nuclear power plant

Welded steel components in the primary circuit in nuclear power plants (NPP) like pressure vessels and piping in
service are exposed to long-term thermal loads, fatigue cycles and, as far as the pressure vessel is concerned, also to neutron irradiation. All of the influences change the microstructure states of the steels involved resulting in a possible material degradation or ageing. The paper reports to research results obtained in the nuclear safety program of the German Ministry of Economy and Technology in the last 5 years period. In the case of the steel WB 36 which is in service in some NPP components by Cu precipitation there is a risk of thermal ageing and degradation in toughness and a shift in the ductile-to-brittle transition temperature (dbt). In the case of austenitic stainless steel components produced of Nb or Ti stabilized material as X6 Cr Ni Ti 1810 under fatigue loads a phase transformation from the fcc austenitic phase to the bcc alpha'-martensitic phase is observed which can - depending on the mechanical load amplitude and service temperature - influence the material with secondary hardening before the softening phase to the fatigue break. Neutron irradiation influences the pressure vessel material in core-near mainly in the welds by generation vacancy clusters and precipitation of (Cu, Cr, Ni)-rich precipitates in 1-3 nm size which are coherent in the bcc lattice producing high residual stresses of the 3rd kind which can enhance the dbt tremendously and reduce the Charpy energy. For non-destructive characterization of the degradation the 3MA-technology is applied (Micromagnetic - Multiparameter - Microstructureand Stress Analysis [1]) which allows the prediction of the microstructure changes in terms of hardness increase, shift in dbt or upper shelf value of the Charpy energy.