Welding and Fabrication

Influence on Stress Corrosion

Cracking (SCC)

ATI-CSC-11
Lake Louise, Alberta, Canada
Dana Couch, EPRI

September 29, 2011

 Outline

•!Discussion of SCC and it
s Relevance

•!Discussion of SCC in the Nuclear Power Industry
•!New Nuclear Plant Builds
•!Welding and Fabrication Influence on SCC
•!EPRI Developed Tool for Managing Welding and
Fabrication Influence on SCC
•!Implementation Examples
•!Weld Repairs
•!Conclusions

© 2011 Electric Power Research Institute, Inc. All rights reserved. 2

 Acknowledgements

•!EPRI-WRTC: Steve McCracken, Eric Willis

•!EPRI-ANT: Jeff Hamel, Letitia Midmore
•!Alliance Engineering: Wayne Lunceford, T. DeWees, and
David Beal
•!XGEN Engineering: Dave Sandusky
•!Hermann and Associates: Bob Hermann and Tony
Giannuzzi

© 2011 Electric Power Research Institute, Inc. All rights reserved. 3

 Stress Corrosion Cracking

•!Requires simultaneous
presence of tensile stress,
a corrosive environment,
and a susceptible material
•!Most alloys have an
environment that will cause
SCC
•!Changes in one or more of
the three interactions can
prevent or mitigate SCC

© 2011 Electric Power Research Institute, Inc. All rights reserved. 4

 Factors Contributing to the Three Interactions

•! Susceptible Material
–! Sensitization
–! Surface Condition
–! Heat Treatment
•! Environment
–! pH
–! Temperature
–! ECP
•! Stress
–! Weld Residual
–! Fit-up Stress
–! Restraint
–! Service

© 2011 Electric Power Research Institute, Inc. All rights reserved. 5

 Stress Corrosion Cracking (SCC)

•!Environmentally-assisted cracking phenomena occurring

under tensile stress (primary or secondary stresses)
•! Composition
•! Service Stress
•! Microstructure
(sensitization) •! Fit-up Stress

•! Surface condition •! Weld residual

(cold work) Material stress

•! Heat treatment •! Restraint

(e.g. solution anneal)
A number of contributing
•! pH conditions are affected
by welding & fabrication
•! temperature Environment Stress
•! anions / cations
•! Oxygenation/ECP
•! Crevices

© 2011 Electric Power Research Institute, Inc. All rights reserved. 6

 SCC in the Nuclear Industry

•!SCC is one of the most serious metallurgical problems

facing the industry
•!In the 70
s & 80
s – BWR Primary Piping
–!Intergranular Stress Corrosion Cracking (IGSCC) -
Stainless steel
•!In the 80
s – PWR Components
–!Primary Water Stress Corrosion Cracking (PWSCC) –
Nickel-base Alloys
•!Steam Generator Tubes
•!Vessel Penetrations
•!Nozzles

© 2011 Electric Power Research Institute, Inc. All rights reserved. 7

 SCC Experience (Stainless Steels)

Source: BWRVIP-186

•! BWR Examples:
–! 304SS recirculation piping
(sensitization induced IGSCC)
–! 304L/316L core shrouds
(cold work induced)
–! Core spray (304SS & 304L)
(oxidizing conditions)
•! PWR occurrences limited in
comparison with BWRs
–! Occurrences associated with
oxygen ingress / off-normal
chemistry; Ex., CRDM upper
housings, dead legs, PZR heaters
–! Weld Repairs performed – further
discussion later
© 2011 Electric Power Research Institute, Inc. All rights reserved. 8
 BWR Remedies

•!Material remedies, especially replacement of affected

piping using IGSCC resistant nuclear grades of stainless
steel or solution annealed piping with corrosion resistant
ID surface cladding at field welds.
•!Stress remedies, such as Mechanical Stress Improvement
(MSIP), to reduce the stresses causing the IGSCC.
•!An ECP remedy, involving use of hydrogen water
chemistry (HWC), to reduce the electrochemical driving
force for the IGSCC.

© 2011 Electric Power Research Institute, Inc. All rights reserved. 9

 SCC Experience (Nickel-base Alloys)

•! PWRs
–! PWSCC of Alloys 600/82/182
–! RPV upper head penetrations
–! RPV lower head penetrations
–! Nozzle DM butt welds
–! PZR heater sleeves
•! BWR Examples:
–! Creviced Alloy 600
(shroud head bolts, access
hole covers, nozzle safe ends
with thermal sleeves)
–! Alloy 182 (CRD housing
penetrations, shroud support)
–! Nozzle DM butt welds
(Ex. Recirculation Inlet &
Outlet, Core Spray Nozzles)
© 2011 Electric Power Research Institute, Inc. All rights reserved. 10
 PWR Remedies

•!Application of full structural weld overlays

•!Innovative repairs for vessel penetrations
•!Use and development of Alloy 690 base and weld
materials

•!Point being from all these examples:

–! plenty of opportunity to learn, improve, and implement
methods to reduce the risk of materials degradation
mechanisms such as SCC

© 2011 Electric Power Research Institute, Inc. All rights reserved. 11

 Evolution of Reactor Designs

© 2011 Electric Power Research Institute, Inc. All rights reserved. 12

 Generation II Reactors

Class of commercial reactors built up to

the end of the 1990
s

Design Examples: PWR, CANDU, BWR,

AGR, and VVeR

Most commercial nuclear reactors

currently in operation are Generation II
reactors.

The term Generation II distinguishes

Diablo Canyon – Westinghouse 4-Loop these reactors from early prototype
PWR Generation I reactors.
Start of commercial operation:
Unit 1 – May 7, 1985
Unit 2 – March 13, 1986
© 2011 Electric Power Research Institute, Inc. All rights reserved. 13
 Generation III Reactors

Incorporate improvements to Generation

II reactors including: superior thermal
efficiency, passive safety systems,
standardized designs.

Design Examples: ABWR, APWR,

Enhanced CANDU 6

Longer operational life (60 years vs. 40)

compared to Gen. II Extendable to 120
years

Hamaoka Nuclear Power Plant

Unit 5 is an ABWR design Core Damage Frequencies are lower
than Generation II
Unit 5 first criticality: January 18, 2005

© 2011 Electric Power Research Institute, Inc. All rights reserved. 14

 Generation III+ Reactors

Design Improvements
•! Passive Safety Systems
•! Simplified Designs using fewer
valves, less piping
•! Standardized Design to expedite
licensing, construction
•! Better load follow capability

Design Examples: AP1000, ESBWR,

EPR, APR-1400

Cutaway drawing of GE
s Economic

Simplified Boiling Water Reactor Generation III+ Reactor designs
(ESBWR) represent most of the new units
expected to come online in the next 20
years

© 2011 Electric Power Research Institute, Inc. All rights reserved. 15

 NRC Received Applications for Combined License
(COL) – 28 Applications for New Reactors

© 2011 Electric Power Research Institute, Inc. All rights reserved. 16

 Moving Forward

•!To ensure the reliability and longevity of future nuclear

plants, the Electric Power Research Institute (EPRI) has
worked with utilities and equipment manufacturers to
develop welding and fabrication best practice guidelines
for new nuclear plant construction.
•!Such guidelines will equip the welding community and
utility engineers with practical tools for identifying and
implementing the most efficient, timely, and cost-effective
methods to reduce the risk of degradation mechanisms
such as stress corrosion cracking.

© 2011 Electric Power Research Institute, Inc. All rights reserved. 17

 Material Degradation Industry Guidance

Environmental
ASME
Code X Degradation
IGSCC, PWSCC
etc.

Weld and HAZ

Degradation

Welding & Fabrication GAPS

-No Industry Guidelines
-No Guidance on Mitigation

© 2011 Electric Power Research Institute, Inc. All rights reserved. 18

 EPRI-Issued Document

•!Welding and Fabrication Critical Factors for New Nuclear

Power Plants, 1019209
•!Issued December 2009
•!Advanced Nuclear Technology (ANT) sponsored within
EPRI
•!Expert panel comprised of welding and fabrication experts
to survey welding and fabrication practices in nuclear
plants and in other industries and rank critical factors
•!Considered a tool to assist ANT members in developing,
reviewing, and implementing requirements for welding
and fabrication

© 2011 Electric Power Research Institute, Inc. All rights reserved. 19

 Introduction

•!The following slides provide an overview of the EPRI

Product - Welding and Fabrication Critical Factors for New
Nuclear Power Plants
•!While intended primarily for members involved in new
construction of nuclear power plants, the concepts used
could be applied to other applications

© 2011 Electric Power Research Institute, Inc. All rights reserved. 20

 Degradation Mechanisms:
Welding & Fabrication Influence Screening

Degradation Mode Degradation Mechanism Welding & Fabrication

Influence

Intergranular /Transgranular Significant

SCC
Irradiation-Assisted SCC (IASCC) Significant

Wastage No significant influence

Degradation Initiator /
Corrosion Crevice Corrosion Accelerator

Flow Accelerated Corrosion (FAC) No significant influence

Wear Wear No significant influence

Fatigue Low-Cycle/High-Cycle Significant

Thermal Aging (Note 3) No significant influence

Reduction in Fracture
Resistance
Environmental Reduction in Fracture Resistance (RiFR) No significant influence

Irradiation Embrittlement (Reduced Toughness) Significant

Irradiation Effects Void Swelling No significant influence

Stress Relaxation No significant influence

© 2011 Electric Power Research Institute, Inc. All rights reserved. 21

 Degradation Mechanism

Welding
Degradation Mechanism

SCC
-TGSCC
-IGSCC
Fatigue IASCC
-Low cycle
-PWSCC
-High cycle

MMM Degradation Matrix

Three Degradation Mechanisms are considered to be affected by

welding, fabrication and mitigation factors

© 2011 Electric Power Research Institute, Inc. All rights reserved. 22

 Degradation Contributor

Deleterious material condition that is promoted or caused

by a welding or fabrication process and which increases
the susceptibility to a known degradation mechanism.

© 2011 Electric Power Research Institute, Inc. All rights reserved. 23

 Terms: Degradation Contributor

SE RS
Degradation Contributors
CW RT

Sensitization Cold Work Residual Reduced

(SE) (CW) Stress Toughness
(RS) (RT)

Promotes increased susceptibility to a Degradation Mechanism

SCC
-TGSCC
-IGSCC
Fatigue IASCC
-Low cycle
-PWSCC
-High cycle

© 2011 Electric Power Research Institute, Inc. All rights reserved. 24

 Degradation Contributor: Sensitization

•! A welding or other heat treatment

process that produces a condition SE RS
in an austenitic stainless steel or
nickel-based alloy whereby the
heat-affected region of the CW RT
component becomes susceptible
to IGSCC
•! Characterized by formation of
chromium carbides at the grain
boundary, combined with a
corresponding reduction of
chromium in the vicinity of the
grain boundary

© 2011 Electric Power Research Institute, Inc. All rights reserved. 25

 Degradation Contributor: Cold Work

•! Results from fabrication processes

(e.g., machining, grinding, SE RS
forming)
•! Found to reduce the SCC CW RT
resistance of stainless steel and
nickel alloys
•! Produces a surface and near
surface layer with substantially
elevated yield strength and a
corresponding reduction in
toughness

© 2011 Electric Power Research Institute, Inc. All rights reserved. 26

 Degradation Contributor: Weld Residual Stress

•! Weld residual stress develops

through the thickness of the SE RS
welded joint as a result of the
alternate melting and solidification
and shrinkage associated with the CW RT
weld deposition process
•! Residual stress is a function of the
distortion and radial shrinkage
during welding and is determined
on the ID surface by the final layer
or two on the OD surface

© 2011 Electric Power Research Institute, Inc. All rights reserved. 27

 Degradation Contributor: Weld Residual Stress
Example Residual Stress Profile

Source: MRP-106

© 2011 Electric Power Research Institute, Inc. All rights reserved. 28

 Degradation Contributor: Reduced Toughness

•! Can result from the SE RS

use of some flux
shielded welding
processes (e.g. CW RT
SMAW, SAW, FCAW)
•! Can contribute to
material degradation
in the form of reduced
resistance to crack
propagation under
loading

SMAW
© 2011 Electric Power Research Institute, Inc. All rights reserved. 29
 Degradation Initiator/Accelerator

Degradation Initiator/
Accelerator

Crevice
Microfissures Weld Defects
Corrosion
(MF) (WD)
(CC)

Degradation Initiators aggravate, accelerate, or initiate a degradation

mechanism, but are not essential for a Degradation Mechanism to occur

SCC
-TGSCC
-IGSCC
Fatigue IASCC
-Low cycle
-PWSCC
-High cycle

© 2011 Electric Power Research Institute, Inc. All rights reserved. 30

 Degradation Initiator/Accelerator:
Crevice Corrosion

•! Electrochemical reaction caused

by an oxygenated media within a CC MF WD
fluid-containing system.
•! Regions containing crevices
(narrow gaps) can result in oxygen
depletion and a relatively high
concentration of chloride or other
negatively charged ions.
•! Result is an increase susceptibility
of materials to stress corrosion
cracking in the occluded region.

© 2011 Electric Power Research Institute, Inc. All rights reserved. 31

 Degradation Initiator/Accelerator:
Microfissures
•! Defined as small cracks (often less than
100 !m) associated with the welding CC MF WD
process. These may occur in the weld
metal or in the adjacent base metal.
•! Often too small to be detected by
conventional NDE methods

© 2011 Electric Power Research Institute, Inc. All rights reserved. 32

 Degradation Initiator/Accelerator:
Weld Induced Fabrication Flaws
•! Welding flaws are associated
with the welding process and CC MF WD
operator.
•! These flaws include:
–! Lack of fusion
–! Porosity
–! Lack of penetration
–! Slag
–! Other flaws resulting from
poor process control or
application.
•! By definition, flaws meet ASME
Code acceptance criteria and
do not require removal and
repair Radiograph of slag inclusions

© 2011 Electric Power Research Institute, Inc. All rights reserved. 33

 Four Important Areas Addressed

•! Welding Process
•! Fabrication Process
•! Restraint Conditions and Configurations
•! Mitigation

© 2011 Electric Power Research Institute, Inc. All rights reserved. 34

 Document Structure:

•! 3.1 – Low C Stainless Steels

Influence Results
•! 3.2 – XM-19
•!Chap 3 – Advanced BWRs
•! 3.3 – CASS
•!Chap 4 – Advanced PWRs •! 3.4 – SS Weld Metal (Low C)
•! 3.5 – N-580 (modified A600)
Welding & Fabrication Factors •! 3.6 – Alloy 82

•!Ch. 5 - Welding & Weld Repairs •! 3.7 – Low Alloy Steel

•! 3.8 – Carbon Steel
•!Ch. 6 - Fabrication
•! 4.1 – Low C Stainless Steel
•!Ch. 7 - Restraint & Configuration
•! 4.2 – Stainless Steel
•!Ch. 8 – Mitigation
•! 4.3 – CASS
•! 4.4 – SS Weld Metal (Low C)
Suggested Enhancements •! 4.5 – SS Weld Metal
•!9.1 – Advanced BWRs •! 4.6 – Alloy 690
•!9.2 – Advanced PWRs •! 4.7 – Alloy 52, 52M, & 152
•! 4.8 – Low Alloy Steel

© 2011 Electric Power Research Institute, Inc. All rights reserved. 35

 Welding Factors

•!Welding process factors

Influences sensitization and weld residual stress
–!High Heat Input (> 20 kJ/cm)
–!Moderate Heat Input (8 -20 kJ/cm)
–!Low Heat Input (< 8 kJ/cm)

© 2011 Electric Power Research Institute, Inc. All rights reserved. 36

 Fabrication Factors

Machining Single point tool

(6.1)
Form Cutting Tools

Grinding Variables
Grinding
(6.2) Grinding Process
Controls
Cold Stamping, Punching & Shearing
Forming &
Cold Rolling
Cutting
Fabrication (6.3) Cold Bending and Forming

Factors Hot
Forming High heat input processes (arc
(6.4) gouging, plasma, oxyacetylene)

Thermal Laser Cutting

Cutting
(6.5) EDM
Solution Anneal
~1900-2000°F
Heat (1040-1149°C)
Treatment
PWHT
(6.6)
1100-1300°F
(593-704°C)

© 2011 Electric Power Research Institute, Inc. All rights reserved. 37

 Restraint / Configuration Factors

–! Pipe Joint ! 25.4 cm dia.

–! Weld Nominal Thickness > 2.54 cm

–! Weld Nominal Thickness ! 2.54 cm

–! Vessel Partial Penetration Welds

–! Pipe Socket Weld

–! Fillet and Lap joints

–! Single Bevel Standard Groove

–! Single Bevel Narrow Groove Weld

–! Double Welded Groove Weld

(final pass on inner diameter surface)

© 2011 Electric Power Research Institute, Inc. All rights reserved. 38

 Mitigation Factors

Surface Peening Shot Peening

Processes Water Jet Peening
(8.1) Laser Peening

Mechanical Polishing
Polishing Processes
Wet Surface Comp. Polishing
(8.2)
Electropolishing

Compressive Stress Heat Sink Welding

Welding Processes Last Pass Heat Sink Welding
(8.3) Dry I.D. Compression Welding

Post Weld Stress

Mitigation Improvement
Induction Heat Stress Improvement

Processes
Factors Mechanical Stress Improvement
Process
(8.4)
Corrosion Resistant Cladding
Surface Cladding &
Noble Metal Cladding & Coating
Coating Processes
Protective Insulating Coatings
(8.5)
Titanium Oxide Coating
Water Chemistry Hydrogen Water Chemistry
Modifications Noble Metal Injection
(8.6) Zinc Injection

© 2011 Electric Power Research Institute, Inc. All rights reserved. 39

 Terms: Influence

•! Influence refers to the impact of a welding or fabrication factor on the

applicable degradation mechanisms
Relates to the combined effect of:
–! Welding and fabrication factors on degradation contributors
(independent of the coolant environment)
–! Degradation contributors on degradation mechanisms
(dependent on the coolant environment)

•! Influence is shown for each degradation contributor and initiator as

applicable to a specific material and environment (BWR vs. PWR)

© 2011 Electric Power Research Institute, Inc. All rights reserved. 40

 Influence Assessment

© 2011 Electric Power Research Institute, Inc. All rights reserved. 41

 Document Structure:

•! 3.1 – Low C Stainless Steels

Influence Results
•! 3.2 – XM-19
•!Chap 3 – Advanced BWRs
•! 3.3 – CASS
•!Chap 4 – Advanced PWRs •! 3.4 – SS Weld Metal (Low C)
•! 3.5 – N-580 (modified A600)
Welding & Fabrication Factors •! 3.6 – Alloy 82

•!Ch. 5 - Welding & Weld Repairs •! 3.7 – Low Alloy Steel

•! 3.8 – Carbon Steel
•!Ch. 6 - Fabrication
•! 4.1 – Low C Stainless Steel
•!Ch. 7 - Restraint & Configuration
•! 4.2 – Stainless Steel
•!Ch. 8 – Mitigation
•! 4.3 – CASS
•! 4.4 – SS Weld Metal (Low C)
Suggested Enhancements •! 4.5 – SS Weld Metal
•!9.1 – Advanced BWRs •! 4.6 – Alloy 690
•!9.2 – Advanced PWRs •! 4.7 – Alloy 52, 52M, & 152
•! 4.8 – Low Alloy Steel

© 2011 Electric Power Research Institute, Inc. All rights reserved. 42

 Use in Welding & Fabrication Critical Factors
Document: Influence Tables (BWR Example)

•! Illustrates how welding &

fabrication techniques can
influence degradation.
1) Contributor influence
•!SE: Sensitization
•!RS: Residual Stress
•!CW: Cold Work
•!RT: Reduced Toughness
2) Relative Overall Influence
•!How much can this factor
influence degradation?
•!How can detrimental
influence be reduced?
(e.g. process controls)

© 2011 Electric Power Research Institute, Inc. All rights reserved. 43

 Example

•!The four influence tables (Welding, fabrication, restraint,

and mitigation) for low carbon stainless steel in a BWR
environment

© 2011 Electric Power Research Institute, Inc. All rights reserved. 44

 Welding Influence

© 2011 Electric Power Research Institute, Inc. All rights reserved. 45

 Fabrication Influence

Text underlined in blue

are hyperlinked to
applicable section within
the document

© 2011 Electric Power Research Institute, Inc. All rights reserved. 46

 Restraint Condition Influence

© 2011 Electric Power Research Institute, Inc. All rights reserved. 47

 Mitigation Influence

© 2011 Electric Power Research Institute, Inc. All rights reserved. 48

 Weld Repairs

•! Service related stress corrosion cracking in nuclear plant primary pressure

retaining welds are know to occur in areas where repairs were performed
during construction

•! Examples include:
•! VC Summer 2000 (RPV to hot leg)
•! Tsuruga-2 (Pressurizer Safety/Relief nozzle)
•! Tsuruga-2 (SG nozzle)
•! TMI-1
•! Tomari-1
•! Takahama-3
•! Ringhals-3
•! Ringhals-4 -2000 (RPV nozzle to hot leg)
•! Davis Besse -2006 (Cold leg drain nozzle)
•! Pilgrim -2007 (Safe end to nozzle)

© 2011 Electric Power Research Institute, Inc. All rights reserved. 49

 Weld Repairs

•!EPRI has worked with the ASME Section III Standards

Committee to develop an alternate approach for
acceptance of full penetration Class 1 and Class 2
pressure boundary butt welds
•!Fitness-for-Service approach allows the use of UT as an
alternative to RT based on demonstration and feasibility of
UT in detecting fabrication weld flaws
•!Flaw acceptance criteria based on fracture mechanics
evaluation

© 2011 Electric Power Research Institute, Inc. All rights reserved. 50

 Conclusions

•!The next generation of nuclear power plants will likely

have a design life of 60 years or more.
•!Improved welding and fabrication practices will be
essential in achieving this increased life expectancy and
minimizing the potential for unexpected and costly repairs
and maintenance.
•!EPRI has issued a document which provides a tool to
assist ANT members in developing, reviewing, and
implementing requirements for fabrication and installation
of new nuclear plant components from a materials
degradation perspective.

© 2011 Electric Power Research Institute, Inc. All rights reserved. 51

 Conclusions

•!This tool can be used to identify areas where additional

resource allocation during the fabrication or installation
phases can substantially reduce the risk of long-term
environmental degradation.
•!Optimizing welding, fabrication, or surface-conditioning
practices in the susceptible areas of critical components
can significantly improve the life and reliability of new
nuclear power plants, preventing forced shutdowns and
reducing outage maintenance costs.

© 2011 Electric Power Research Institute, Inc. All rights reserved. 52

 Conclusions

•!Working together, utilities, equipment manufacturers,

vendors, and the welding community can seize the
opportunity to apply improved welding and fabrication
practices to ensure that new nuclear plants will operate
reliably over their designed 60-yearlifetimes.

© 2011 Electric Power Research Institute, Inc. All rights reserved. 53

 Together!Shaping the Future of Electricity

© 2011 Electric Power Research Institute, Inc. All rights reserved. 54