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Review Article

Methodology for Estimating Thermal and Neutron Embrittlement of Cast Austenitic Stainless Steels During Service in Light Water Reactors

[+] Author and Article Information
O. K. Chopra

Environmental Science Division,
Argonne National Laboratory,
Argonne, IL 60439

A. S. Rao

Division of Engineering,
U.S. Nuclear Regulatory Commission,
Washington, DC 20555

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received June 22, 2015; final manuscript received October 19, 2015; published online April 28, 2016. Assoc. Editor: Marina Ruggles-Wrenn.The United States Government retains, and by accepting the article for publication, the publisher acknowledges that the United States Government retains, a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States government purposes.

J. Pressure Vessel Technol 138(4), 040801 (Apr 28, 2016) (24 pages) Paper No: PVT-15-1130; doi: 10.1115/1.4031909 History: Received June 22, 2015; Revised October 19, 2015

Cast austenitic stainless steel (CASS) materials, which have a duplex structure consisting of austenite and ferrite phases, are susceptible to thermal embrittlement during reactor service. In addition, the prolonged exposure of these materials, which are used in reactor core internals, to neutron irradiation changes their microstructure and microchemistry, and these changes degrade their fracture properties even further. This paper presents a revision of the procedure and correlations presented in NUREG/CR-4513, Rev. 1 (Aug. 1994) for predicting the change in fracture toughness and tensile properties of CASS components due to thermal aging during service in light water reactors (LWRs) at 280–330 °C (535–625 °F). The methodology is applicable to CF-3, CF-3M, CF-8, and CF-8M materials with a ferrite content of up to 40%. The fracture toughness, tensile strength, and Charpy-impact energy of aged CASS materials are estimated from known material information. Embrittlement is characterized in terms of room-temperature (RT) 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. The fracture toughness J-R curve for the aged material is then obtained by correlating RT Charpy-impact energy with fracture toughness parameters. A common “predicted lower-bound” J-R curve for CASS materials of unknown chemical composition is also defined for a given grade of material, 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. The correlations for estimating the change in tensile strength, including the Ramberg/Osgood parameters for strain hardening, are also described.

Copyright © 2016 by ASME
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References

Figures

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Fig. 1

Time–temperature curve for the formation of various phases in CASS materials [3]

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Fig. 2

Deformation twins in a Charpy-impact specimen of CF-8 material aged for 30,000 hrs at 350 °C and tested at 290 °C [22]

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Fig. 3

Pseudobinary diagram for Fe–Ni–19%Cr alloy [13]

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Fig. 4

Ferrite content and morphology along the circumferential section of centrifugally cast CF-8 (heat P1) pipe from regions near the inside and outside diameter

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Fig. 5

Plots of measured ferrite content and values calculated from Hull's equivalent factor for various CASS materials

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Fig. 6

Decrease in Charpy-impact energy for various heats of CASSs aged at 400 °C (752 °F)

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Fig. 7

Correlation between RT Charpy-impact energy at saturation and the material parameter ϕ for CF-3, CF-3 M, CF-8, and CF-8 M materials

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Fig. 8

(a) Correlation between RT Charpy-impact energy and coefficient C at RT for CF-3, CF-8, CF-3 M, and CF-8 M CASS materials and (b) correlation between RT Charpy-impact energy and coefficient C at 290–320 °C for CF-3, CF-8, CF-3 M, and CF-8 M CASS materials

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Fig. 9

The lower-bound correlations between RT Charpy-impact energy and exponent n of the power-law J-R curve at 290–325 °C and RT for CASS materials

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Fig. 10

Fracture toughness J-R curve data for thermally aged heat 68 of CF-8M plate at 54 °C. The solid line represents the predicted curve and the chain-dashed line represents the lower-bound curve at RT for static-cast CF-8 material with 15–25% ferrite [74].

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Fig. 11

Fracture toughness J-R curves for sensitized Type 304 SS in simulated BWR coolant at 288 °C and three different displacement rates [77]

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Fig. 12

Flow diagram for estimating mechanical properties of thermally aged CASS materials

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Fig. 13

Fracture toughness J-R curves for (a) unaged CASS materials at 290–320 °C and (b) wrought austenitic SSs at various temperatures [16,73,8082]

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Fig. 14

Correlation between estimated saturation RT Charpy-impact energy and ferrite content for CF-3, CF-8, and CF-8M CASS materials

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Fig. 15

Estimated lower-bound J-R curves at RT and 290–320 °C for static-cast CASS materials with 15–20% ferrite

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Fig. 16

Change in saturation J at 2.5-mm crack extension at RT and 290–320 °C as a function of ferrite content for static-cast CF-3, CF-8, and CF-8M CASS materials

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Fig. 17

Change in saturation J at 2.5-mm crack extension at RT and 290–320 °C as a function of ferrite content for centrifugally cast CF-3, CF-8, and CF-8M CASS materials

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Fig. 18

Change in fracture toughness JIc as a function of neutron exposure for LWR-irradiated austenitic SSs. Dashed lines represent the scatter band for the fast reactor data on SSs irradiated at 350–450 °C [3845,4749,83,8690].

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Fig. 19

Fracture toughness JIc values as a function of neutron dose for CASS materials. The data points plotted at 0.007 dpa are for nonirradiated materials.

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Fig. 20

Coefficient C of the J-R curve as a function of neutron dose for CASS materials. The data points plotted at 0.007 dpa are for nonirradiated materials.

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Fig. 21

Fracture toughness J2.5 values as a function of neutron dose for CASS materials. The data points plotted at 0.007 dpa are for nonirradiated materials.

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Fig. 22

Flow stress ratio Rf of aged CF-3, CF-8, and CF-8M materials at RT and 290 °C as a function of the normalized aging parameter

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