Influence of long-term thermal ageing on creep-fatigue behaviour of ferritic steels

Abstract

For the design and to assess the final life span and the potential for life extension of components, such as, e.g. turbine shafts, piping and valve casing and nuclear pressure vessels, which are subjected to temperature and alternating load, data are required on the long time history of the corresponding material. Especially, it is necessary to know about the material alterations and the consequences of the changes in material properties. For turbine application, the creep resistant ferritic 28 CrMoNiV 49 steels (forged) and 13 CrMo 44 (rolled) as well as the creep resistant cast steel GS 17 CrMoV 5 were investigated. Ageing times between t = 1000 h and t = 81000 h were applied at temperatures of 530 °C.and 560 °C. For the nuclear grade RPV steel (forged) 20 MnMoNi 55 (similar to A 508 Cl 3) the ageing time was 30 000 h at 320 °C. The following tests were conducted: mechanical and technological tests (tensile tests, high temperature tests, notch impact bending tests), creep tests and LCF tests. The interpretation of results were supported by extensive metallographical and microstructural investigations. The creep fatigue behaviour of creep resistant steels can be described and explained with the help of these investigations. It can be decided whether it is possible to simulate structural changes in material caused by service load properties, by means of long-term thermal ageing. Furthermore, the microstructural influence was assessed.