Corrosion of Austenitic Stainless Steel Welds in Chloride ...
Understand the Phase Transformation of Thermally Aged and ... · stainless steel welds and cast...
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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.
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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)
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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
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Stre
ss (M
Pa)
Strain
CF-8, 23% δRT, 1x10-3 s-1
Unaged
Aged
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2 3 45 6
a
bc
de
f
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a b c
d
e
f
WAXS Results
0.8 1 1.2 1.4 1.6 1.8 2 2.2
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0.8 1 1.2 1.4 1.6 1.8 2 2.2
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d spacing, Angstrom
Inte
nsity
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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
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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.