Estimation of Fracture Toughness of Cast Stainless Steels During Thermal Aging in LWR Systems (NUREG/CR-4513, ANL-15/08,Revision 2)

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Publication Information

Manuscript Completed: June 2015
Date Published: May 2016

Prepared by:
Omesh K. Chopra
Argonne National Laboratory
Argonne, Illinois 60439

Appajosula S. Rao, NRC
Technical Monitor and Project Manager

NRC Job Code Number V6455

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC FIN A-2243

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Abstract

Cast austenitic stainless steel (CASS) materials are used extensively in reactor coolant pressure boundary systems as well as core support structure and reactor internals. However, these materials have a duplex structure consisting of austenite and ferrite phases and are susceptible to thermal aging embrittlement during reactor service. In addition, the prolonged exposure of these materials to neutron irradiation changes their microstructure and microchemistry, which can degrade their fracture properties even further. This report is a revision of NUREG/CR-4513, Rev. 1, ANL-93/22 (August 1994); it revises the procedure and correlations used for predicting the change in fracture toughness and tensile properties of CASS components due to thermal aging during service in light water reactors at 280–330°C (535–625°F). The updated correlations are based on the current fracture toughness database for CASS materials aged up to 100,000 h at 290–350°C (554–633°F). The methodology for estimating fracture properties has been extended to cover CASS materials with a ferrite content of up to 40%. The correlations for estimating the change in tensile stress, including the Ramberg/Osgood parameters for strain hardening, are also described. The fracture toughness J-R curve, tensile stress, and Charpy-impact energy of aged CASS materials are estimated from material composition. The mechanical properties of a specific CASS material are estimated from the extent and kinetics of thermal embrittlement. Embrittlement is characterized in terms of room temperature Charpy-impact energy. The extent or degree of thermal embrittlement at "saturation" (i.e., the minimum impact energy that can be achieved for a material after long-term aging) is determined from the chemical composition of the material. Charpy-impact energy as a function of the time and temperature of reactor service is estimated from the kinetics of thermal embrittlement, which are also determined from the chemical composition. Data on the initial impact energy and tensile flow stress of the unaged material are required for these estimations. The fracture toughness J-R curve for the material is then obtained by correlating room temperature Charpy-impact energy with fracture toughness parameters. The values of J_Ic are determined from the estimated J-R curve and flow stress. A common "predicted lower-bound" J-R curve for CASS materials of unknown chemical composition is also defined for a given grade of steel, range of ferrite content, and temperature. In addition, guidance is provided for evaluating the combined effects of thermal and neutron embrittlement of CASS materials used in the reactor core internal components.