Microstructural and fatigue assessment of thermal and mechanically aged austenitic AISI 347 power plant steelWednesday (26.09.2018) 11:30 - 11:45 S1/01 - A02 Part of:
Under service conditions, stainless steel piping and boiling water reactors components undergo several thermal loads in a high pressure state. The occurrence of stresses at variable high temperatures leads to cyclic deformations and material damage. Such a process goes under the nomenclature of ageing. To ensure the safety of the components and the plant operation, it is important to consider ageing-induced fatigue and corrosion damages.
The objective of the current study is to provide the experimental basis for the establishment of an innovative, time- and resource-optimized determination of fatigue data, focusing on microstructural and cyclic investigations of niobium-stabilized austenitic AISI 347 stainless steels (X6CrNiNb18-10, 1.4550). Aged and non-aged specimens were investigated in air and distilled water environments. The ageing process consisted of cyclic loading with a total strain amplitude of 0.3% and a strain rate of 0.4 %/s imposed up to 50% of the specimens’ lifetime at an elevated temperature of 240 °C. For the characterization of the cyclic deformation behavior in total strain-controlled strain increase (SIT) and constant amplitude tests (CAT), non-destructive testing (NDT) such as magnetic, resistometric and electrochemical methods were simultaneously applied until failure of the samples.
No life-time influencing effects under distilled water environment were observed. Within the SIT, the aged specimens show a fatigue life reduction of 30% in comparison to the non-aged specimens. The material reactions for both conditions (ageing states) of the specimens display a qualitatively similar trend concerning the stress amplitude development over life, the change in the tangential magnetic field and the open circuit potential, although with different quantitative values. The change in the electrochemical potential can be assumed as a consequence of crack initiation and growth, due to the constant formation of new active material surface.