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NUREG/CR-6787 ANL-01/25

Mechanism and Estimation of Fatigue Crack Initiation in Austenitic Stainless Steels in LWR Environments

Argonne National Laboratory

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

NUREG/CR-6787 ANL-01/25

Mechanism and Estimation of Fatigue Crack Initiation in Austenitic Stainless Steels in LWR EnvironmentsManuscript Completed: September 2001 Date Published: August 2002

Prepared by O. K. Chopra

Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439

J. Muscara, NRC Project Manager

Prepared for Division of Engineering Technology Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 NRC Job Code Y6388

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Mechanism and Estimation of Fatigue Crack Initiation in Austenitic Stainless Steels in LWR Environmentsby O. K. Chopra

AbstractThe ASME Boiler and Pressure Vessel Code provides rules for the construction of nuclear power plant components. Figures I9.1 through I9.6 of Appendix I to Section III of the Code specify fatigue design curves for structural materials. However, the effects of light water reactor (LWR) coolant environments are not explicitly addressed by the Code design curves. Existing fatigue strainvs.life (eN) data illustrate potentially significant effects of LWR coolant environments on the fatigue resistance of pressure vessel and piping steels. This report provides an overview of fatigue crack initiation in austenitic stainless steels in LWR coolant environments. The existing fatigue eN data have been evaluated to establish the effects of key material, loading, and environmental parameters (such as steel type, strain range, strain rate, temperature, dissolvedoxygen level in water, and flow rate) on the fatigue lives of these steels. Statistical models are presented for estimating the fatigue eN curves for austenitic stainless steels as a function of the material, loading, and environmental parameters. Two methods for incorporating environmental effects into the ASME Code fatigue evaluations are presented. The influence of reactor environments on the mechanism of fatigue crack initiation in these steels is also discussed.

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ContentsAbstract.................................................................................................................................... Executive Summary................................................................................................................. Acknowledgments .................................................................................................................... 1 2 Introduction .................................................................................................................... Overview of Fatigue eN Data ......................................................................................... 2.1 Air Environment.................................................................................................. 2.1.1 2.1.2 2.2 Fatigue Life .......................................................................................... Cyclic Hardening Behavior .................................................................. iii xi xv 1 5 5 5 7 9 9 10 11 11 13 15 16 16 19 19 20 25 29 31 33 35 35

LWR Environments ............................................................................................. 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7 2.2.8 Strain Amplitude ................................................................................. HoldTime Effects ................................................................................ Strain Rate........................................................................................... Dissolved Oxygen................................................................................. Temperature ........................................................................................ Sensitization Annealing....................................................................... Flow Rate ............................................................................................. Cast Stainless Steels ...........................................................................

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Mechanism of Fatigue Crack Initiation .......................................................................... 3.1 3.2 3.3 3.4 3.5 Formation of EngineeringSize Cracks............................................................... Growth of Small Cracks in LWR Environments................................................. Fracture Morphology........................................................................................... Surface Oxide Film.............................................................................................. Exploratory Fatigue Tests...................................................................................

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Operating Experience in the Nuclear Power Industry.................................................... Statistical Model.............................................................................................................. 5.1 LeastSquares Fit................................................................................................

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5.2

The ANL Statistical Model................................................................................... 5.2.1 5.2.2 Air Environment .................................................................................. LWR Environments..............................................................................

35 35 36 38 38 40 43 43 45 47 53 55

5.3 5.4 5.5 6

Japanese MITI Guidelines .................................................................................. Model Developed by the Bettis Laboratory......................................................... Comparison of Various Estimation Schemes.....................................................

Incorporating Environmental Effects into Fatigue Evaluations .................................... 6.1 6.2 Fatigue Design Curves........................................................................................ Fatigue Life Correction Factor ............................................................................

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Conservatism in Fatigue Design Curves ........................................................................ Summary.........................................................................................................................

References ................................................................................................................................

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Figures1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Fatigue eN data for carbon steels and austenitic stainless steels in water.............. Fatigue eN behavior for Types 304, 316, and 316NG austenitic stainless steels in air at various temperatures ......................................................................................... Influence of specimen geometry on fatigue life of Types 304 and 316 stainless steel .............................................................................................................................. Influence of temperature on fatigue life of Types 304 and 316 stainless steel in air Effect of strain range and strain rate on cyclichardening of Type 316NG stainless steel in air at room temperature and 288C ............................................................... Effect of strain range and strain rate on cyclic hardening of Type 304 stainless steel in air at 288C ..................................................................................................... Results of strain rate change tests on Type 316 SS in lowDO water at 325C ........ Fatigue life of Type 304 stainless steel tested in highDO water at 260288C with trapezoidal or triangular waveform but with comparable tensile strain rates... Dependence of fatigue lives of austenitic stainless steels on strain rate in low and highDO water ...................................................................................................... Dependence of fatigue life of Types 304 and 316NG stainless steel on strain rate in high and lowDO water at 288C .......................................................................... Effects of conductivity of water and soaking period on fatigue lives of Type 304 SS in highDO water ......................................................................................................... Change in fatigue lives of austenitic stainless steels in lowDO water with temperature.................................................................................................................. Waveforms for change in temperature during exploratory fatigue tests .................... Fatigue life of Type 316 stainless steel under constant and varying test temperature.................................................................................................................. Effect of sensitization annealing on fatigue life of Types 304 and 316 stainless steel in lowDO water at 325C................................................................................... Effect of sensitization anneal on the fatigue lives of Types 304 and 316NG stainless steel in highDO water ................................................................................. Fatigue strain amplitudevs.life data for CF8M cast SSs in air.............................. Effect of strain rate on cyclichardening behavior of wrought and cast SSs in air at 288C ....................................................................................................................... Fatigue strain amplitudevs.life data for CF8M cast SSs in water ........................ 2 6 6