EPRI 2000 Session: Material Properties Measurement
Microstructural Investigations and Monitoring of Degradation of LCF Damage of Austenitic Steel Type X6CrNiTi(Nb) 18-9(10)
D. Kalkhof, M. Grosse, M. Niffenegger, Paul Scherrer Institute; D. Stegemann, University of Hannover; U. Gampe, Siempelkamp Prüf- und Gutachter-Gesellschaft MBH
The development of advanced diagnostic systems that are able to identify, locate and monitor material degradation in the microstructures of steels is a new challenge for NDT techniques. Especially in nuclear power plants, early detection of material degradation can contribute to improve safety and reliability of the primary circuit boundary as well as plant life time management of certain components. The following contribution deals with the characterisation of microstructural changes in the pre-crack stage of low-cycle fatigue damage (LCF) in austenitic piping steels and with monitoring of the LCF degradation. Thermo-mechanical loading which is typical for thermal stratification in piping under start-up, shut-down and transient load conditions was simulated. The LCF damage evolution in the meta-stable austenitic steel causes a deformation-induced phase transformation from austen ite to martensite. Different LCF damage levels (usage factors) were generated in hour-glass specimens in total strain-controlled testing. Neutron diffraction measurements at different axial positions with a beam height of 2mm enable the correlation between the local martensite content and the appropriate load amplitude. It has been demonstrated that thresholds exist for the formation of martensite as a function of both the load amplitude and the number of LCF cycles. Magnetic stray field and eddy current measurements were chosen to transfer the results of material characterisation to an on-line monitoring NDT method. The density and distribution of martensite obtained from metallographic and neutron diffraction methods are used for calibration of the magnetic methods. Both the magnetic stray field measurements as well as the eddy current techniques were able to detect the very low amount of martensite in the different aged specimens (0.5 - 3.1 vol. % martensite, usage factors from 0 up to 1.0).
Publication Source: 2nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, May 24-26, 2000, New Orleans, Louisiana USA. Publisher: EPRI - [Homepage]
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