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The Mechanism and Estimation of Fatigue Crack Initiation ... · PDF fileNUREG/CR-6787 ANL-01/25 Mechanism and Estimation of Fatigue Crack Initiation in Austenitic Stainless Steels

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  • NUREG/CR-6787 ANL-01/25

    Mechanism and Estimation ofFatigue Crack Initiation inAustenitic Stainless Steels inLWR Environments

    Argonne National Laboratory

    U.S. Nuclear Regulatory CommissionOffice of Nuclear Regulatory ResearchWashington, DC 20555-0001

  • NUREG/CR-6787 ANL-01/25

    Mechanism and Estimation ofFatigue Crack Initiation inAustenitic Stainless Steels inLWR EnvironmentsManuscript Completed: September 2001Date Published: August 2002

    Prepared byO. K. Chopra

    Argonne National Laboratory9700 South Cass AvenueArgonne, IL 60439

    J. Muscara, NRC Project Manager

    Prepared forDivision of Engineering TechnologyOffice of Nuclear Regulatory ResearchU.S. Nuclear Regulatory CommissionWashington, DC 20555-0001NRC Job Code Y6388

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

    by

    O. K. Chopra

    Abstract

    The ASME Boiler and Pressure Vessel Code provides rules for the construction of nuclearpower plant components. Figures I9.1 through I9.6 of Appendix I to Section III of the Codespecify fatigue design curves for structural materials. However, the effects of light waterreactor (LWR) coolant environments are not explicitly addressed by the Code design curves.Existing fatigue strainvs.life (eN) data illustrate potentially significant effects of LWR coolantenvironments on the fatigue resistance of pressure vessel and piping steels. This reportprovides an overview of fatigue crack initiation in austenitic stainless steels in LWR coolantenvironments. The existing fatigue eN data have been evaluated to establish the effects of keymaterial, 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 stainlesssteels as a function of the material, loading, and environmental parameters. Two methods forincorporating environmental effects into the ASME Code fatigue evaluations are presented. Theinfluence of reactor environments on the mechanism of fatigue crack initiation in these steelsis also discussed.

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    Contents

    Abstract.................................................................................................................................... iii

    Executive Summary................................................................................................................. xi

    Acknowledgments .................................................................................................................... xv

    1 Introduction .................................................................................................................... 1

    2 Overview of Fatigue eN Data ......................................................................................... 5

    2.1 Air Environment.................................................................................................. 5

    2.1.1 Fatigue Life .......................................................................................... 5

    2.1.2 Cyclic Hardening Behavior.................................................................. 7

    2.2 LWR Environments............................................................................................. 9

    2.2.1 Strain Amplitude ................................................................................. 9

    2.2.2 HoldTime Effects ................................................................................ 10

    2.2.3 Strain Rate........................................................................................... 11

    2.2.4 Dissolved Oxygen................................................................................. 11

    2.2.5 Temperature ........................................................................................ 13

    2.2.6 Sensitization Annealing....................................................................... 15

    2.2.7 Flow Rate ............................................................................................. 16

    2.2.8 Cast Stainless Steels ........................................................................... 16

    3 Mechanism of Fatigue Crack Initiation.......................................................................... 19

    3.1 Formation of EngineeringSize Cracks............................................................... 19

    3.2 Growth of Small Cracks in LWR Environments................................................. 20

    3.3 Fracture Morphology........................................................................................... 25

    3.4 Surface Oxide Film.............................................................................................. 29

    3.5 Exploratory Fatigue Tests................................................................................... 31

    4 Operating Experience in the Nuclear Power Industry.................................................... 33

    5 Statistical Model.............................................................................................................. 35

    5.1 LeastSquares Fit................................................................................................ 35

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    5.2 The ANL Statistical Model................................................................................... 35

    5.2.1 Air Environment .................................................................................. 35

    5.2.2 LWR Environments.............................................................................. 36

    5.3 Japanese MITI Guidelines .................................................................................. 38

    5.4 Model Developed by the Bettis Laboratory......................................................... 38

    5.5 Comparison of Various Estimation Schemes..................................................... 40

    6 Incorporating Environmental Effects into Fatigue Evaluations .................................... 43

    6.1 Fatigue Design Curves........................................................................................ 43

    6.2 Fatigue Life Correction Factor ............................................................................ 45

    7 Conservatism in Fatigue Design Curves ........................................................................ 47

    8 Summary......................................................................................................................... 53

    References ................................................................................................................................ 55

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    Figures

    1. Fatigue eN data for carbon steels and austenitic stainless steels in water.............. 2

    2. Fatigue eN behavior for Types 304, 316, and 316NG austenitic stainless steels inair at various temperatures......................................................................................... 6

    3. Influence of specimen geometry on fatigue life of Types 304 and 316 stainlesssteel .............................................................................................................................. 6

    4. Influence of temperature on fatigue life of Types 304 and 316 stainless steel in air 7

    5. Effect of strain range and strain rate on cyclichardening of Type 316NG stainlesssteel in air at room temperature and 288C ............................................................... 8

    6. Effect of strain range and strain rate on cyclic hardening of Type 304 stainlesssteel in air at 288C..................................................................................................... 8

    7. Results of strain rate change tests on Type 316 SS in lowDO water at 325C ........ 10

    8. Fatigue life of Type 304 stainless steel tested in highDO water at 260288Cwith trapezoidal or triangular waveform but with comparable tensile strain rates... 11

    9. Dependence of fatigue lives of austenitic stainless steels on strain rate in lowand highDO water ...................................................................................................... 11

    10. Dependence of fatigue life of Types 304 and 316NG stainless steel on strain ratein high and lowDO water at 288C .......................................................................... 12

    11. Effects of conductivity of water and soaking period on fatigue lives of Type 304 SSin highDO water ......................................................................................................... 13

    12. Change in fatigue lives of austenitic stainless steels in lowDO water withtemperature.................................................................................................................. 14

    13. Waveforms for change in temperature during exploratory fatigue tests .................... 14

    14. Fatigue life of Type 316 stainless steel under constant and varying testtemperature.................................................................................................................. 15

    15. Effect of sensitization annealing on fatigue life of Types 304 and 316 stainlesssteel in lowDO water at 325C................................................................................... 15

    16. Effect of sensitization anneal on the fatigue lives of Types 304 and 316NGstainless steel in highDO water ..................