Understand the Phase Transformation of Thermally Aged and Neutron Irradiated Duplex Stainless Steels Used in LWRs Using Advanced High Energy X-ray

Technologies

Yong Yang (PI), University of Florida Jeff Terry (Co-PI), Illinois Institute of Technology Yiren Chen (Co-PI), Argonne National Laboratory

Light Water Reactor Sustainability

  Most of U.S. Nuclear Power Plant (NPP) is scheduled to retire between 2029 and 2056

  Extending NPP lifetimes to 80 years –  To be cost effective –  To reduce greenhouse gas emission –  To meet electricity demand –  To ensure the national energy security

  Significant R&D is needed to provide technical basis for licensing –  Materials Aging and Degradation

–  Understanding, Prediction and Mitigation of Primary System Aging Degradation –  Advanced Instrumentation, Control, and Information Systems and Technologies –  Risk-informed Safety Margin Characterizations –  Advanced Light Water Reactor (LWR) Nuclear Fuels

Cast Austenitic Stainless Steels (CASS) and Stainless Steel (SS) welds

  From practical perspective   CASS and SS welds are Fe-Cr-Ni alloys used widely in

light water reactors (LWRs).   Many CASS and weld components in primary pressure

boundaries, and in reactor core internals   Good compatibility with wrought stainless steels   Casting is a near-net-shape manufacturing technology

necessary for producing complex shapes.   High impact on license renewal and inspection decision,

and aging management strategy

  From scientific perspective   Hardening and embrittlement of a dual-phase

microstructure à deformation behaviors of two phases   Inherent instability of delta ferrite à vulnerability to

thermal aging   Interaction between thermal aging and irradiation damage à interaction of different degradation mechanisms

Lower CRGT Flange Weld

http://www.ndt.net/article/cofrend2014/papers/ME1B4_J_PONTON.pdf

Control Rod Guide Card

§  Dual-phase microstructure of delta ferrite (δ) and austenite (γ)

–  L à L + δ à L + δ + γ à δ + γ –  Ferrite content is controlled by alloy

composition. –  Ferrite morphology is affected by local

chemical composition, cooling rate, fluid flow of solidification pool, etc.

§  Beneficial effects of delta ferrite –  Help prevent “hot cracking” –  Provide a strengthening mechanism for

solidification microstructure –  Improve sensitization and SCC resistance

Duplex-Phases Microstructure

S.A. David, et al, JOM, June, 2003. Acicular

Thermal Aging Effects on Duplex SSs

The main thermal aging effects at <500°C are associated with the phase changes in ferrite: –  Spinodal decomposition into Fe-rich α phase and Cr-rich α’ phase –  Precipitations of G-phase (Mn/Mo, Ni, and Si rich), ε phase (Cu rich),

carbide, and π phase –  Growth of existing carbides at the ferrite/austenite phase boundaries

Embrittled by Thermal Aging and Irradiation

  The extent of irradiation-induced embrittlement is greater in unaged than aged specimens.

  When both degradations are present, neutron irradiation seems to play an dominant role.

0

100

200

300

400

500

600

700

800

0.0 0.5 1.0 1.5 2.0 2.5

J (k

J/m

2 )

Crack Extension (mm)

CF-8, 23% δ, in low-DO water, ~315oC

Unaged, unirradiated,estimated

Unaged,0.08 dpa

Aged, unirradiated

Aged, 0.08 dpa

(b)

0

200

400

600

800

0.0 0.5 1.0 1.5 2.0 2.5

J (k

J/m

2 )

Crack Extension (mm)

CF-3, 24% δ, in PWR or low-DO water, ~315oC

Unaged, unirradiated

Unaged,0.08 dpa

Aged, unirradiated

Aged, 0.08 dpa

(a)

Thermal Aging on Fracture Toughness

[Chopra]>30% ferrite 15% ferrite

Irradiation Effects on Duplex SS

  R&D needs identified in 2013 EPRI’s Materials Degradation Matrix (MDM, rev. 3).

  “Insufficient ongoing R&D resolve technical uncertainties in a reasonable, near time frame”. Multiple degradation modes are possible when neutron irradiation is taken in account.

  Very limited studies and contradictory results.

Westinghouse components as cast stainless steel and/or structural welds

Region 1: <1x1020 n/cm2

Region 2: 1x1020 - 7x1020n/cm2

Region 3: 7x1020 - 7x1021n/cm2

Region 4: 1x1021 - 7x1022n/cm2

Region 5: 1x1022 - 5x1022n/cm2

Region 6: >5x1022n/cm2

Objectives

  Further probe the elemental segregations, phase precipitations using high energy X-ray diffraction, Extended X-ray Absorption Fine Structure Spectroscope (EXAFS)

  Understand the lattice straining status in different phases and the load partition between phases using Wide Angle X-ray Scattering (WAXS) under a tensile load

  To clarify the combined effects from irradiation and thermal aging

  To contribute to construction of a physics based model for predicting duplex structure stainless steel long term mechanical behaviors for NEAMS

Materials and Conditions

Neutron irradiated specimens:

Cast and welds thermally aged at 400°C for various hours ( up to 20,000), provided by PNNL.

Proposed High Energy X-ray Experiments

Characterization on BOR-60 irradiated CF3

Vaporized

LEAP 4000

Characterization on BOR-60 irradiated CF3

Wavelength vs Neutron Irradiation Dose

20.3

Dose (dpa) 0.08 5 10 20 40 λ ( nm ) 9.02 15.08 17.6 19.6 20.07

Exponential

Coarsening of G Phase Precipitates

Relative spatial distribution of G phase and Spinodal Decomposition

Distributions of Cr and Ni in the slice of 10nm in thickness from the middle of the APT tips

G phase precipitates perfectly locate in the Cr depleted regions, as an obvious proof that a competition of precipitation coarsening and SD growth.

5 dpa 10 dpa 20 dpa 40 dpa

Understand the distribution of near-neighbor atoms using EXAFS

  Synchrotron EXAFS measurements on 9Cr-1Mo model alloy irradiated to 1, 4, and 10 dpa.

Atomic environment around Fe and Mo before and after irradiation [M.Li, JNM, 2013]

Anticipated challenge: deconvolution between two phases.

Proposed In-situ WXAS Tensile Test

As cast Thermally aged

Irradiated Thermally aged and irradiated

  Understand the deformation behavior of each phase

  Quantify the load partition and phase boundary strain mis-match

§  To analyze load partitioning among phases à The role of ferrite in the hardening and embrittlement behavior of CASS/welds.

Wide-Angle X-ray Scattering (WAXS) with in-situ straining (conducted by Co-PI at ANL)

In-situ tensile tests at RT

0

200

400

600

800

0 10 20 30 40 50 60 70

Stre

ss (M

Pa)

Strain

CF-8, 23% δRT, 1x10-3 s-1

Unaged

Aged

1

2 3 45 6

a

bc

de

f

1

2

3

4 5 6

a b c

d

e

f

WAXS Results

0.8 1 1.2 1.4 1.6 1.8 2 2.2

0

2000

4000

6000

8000

0.8 1 1.2 1.4 1.6 1.8 2 2.2

0

2000

4000

6000

d spacing, Angstrom

Inte

nsity

In

tens

ity

CF-8, unaged

CF-8, aged

(110) (200) (310) (400) (211) (222)

(111) (200) (220) (311) (400) (331) (222) (422)

(333) fcc

bcc 0 0.03 0.09 0.19 0.32 0.47 Strain

0 0.05 0.08 0.21 0.32 0.45 Strain

CF-8 with 23% ferrite, Unaged vs. Aged

0.0000

0.0050

0.0100

0.0150

0 20 40 60 80 100

unaged, γ (111)unaged, δ (110)aged, γ (111) aged, δ (110)

Latti

ce s

train

Engineering strain (%)

CF-8

Unaged

Aged

Phase boundary Strength Measurement

Correlation between mechanical property and ferrite degradation

Relationship between the SD amplitude and hardening

[Khatkhatay 2013]

Fracture toughness vs. aging time [Chopra]

Phase Boundary Strength

Preliminary Results on Modeling

  Sub-size CT test specimen constructed in COMSOL (in mm)   COMSOL doesn’t support built-in crack propagation interface, but carries a 2-D

stationary stress strain analysis   J-integral was calculated as a function of Young’s modulus of matrix, addition of

secondary phase, and the morphology of second phase.

  Future plan is to use the MOOSE framework for simulation.

Summary

1.  This is a multi-PIs and multi-institutions research program. The anticipated outcomes will fill the large gap in knowledge with regard to how the stainless steel welds and cast stainless steels respond to multi-decade thermal aging with or without neutron irradiation.

2.  The program impacts are tri-fold: advance the scientific understanding of the degradation of duplex stainless steels in LWRs, support the development of more practical model for the LWRs extension license renewal and regulation, and train the next generation workforce.

3.  Preliminary results are very promising though several challenges have to be overcome.