06/04/2014 Update on 52 Weldability. - NRC: Home PageUpdate on 52 Weldability Comparison of...
Transcript of 06/04/2014 Update on 52 Weldability. - NRC: Home PageUpdate on 52 Weldability Comparison of...
Update on 52 Weldability
Comparison of Ductility-Dip Cracking in a Narrow Groove Weld to Strain Accumulation Determined by Computer Simulation
Industry/U.S. NRC Materials Program Technical
Information Exchange Meeting
Rockville, MD Wednesday June 4, 2014
Jon Tatman and Steve McCracken EPRI Welding & Repair Technology Center
2 © 2014 Electric Power Research Institute, Inc. All rights reserved. Welding and Repair Technology Center
Ductility-Dip and Solidification (Hot) Cracking
Ductility-dip Temperature Range (DTR) ductility-dip ~ 10% to 15% strain
~ 950°C to 1150°C temperature range Brittle Temperature Range (BTR)
~ liquidus to terminal solidus
Strain applied Due to weld shrinkage
& joint restraint
Liquation cracking mechanism that occurs during solidification in the brittle temperature range (mushy zone)
Solidification Cracking Solid state cracking
mechanism that occurs in the ductility-dip temperature range (reheated weld metal)
Ductility-dip Cracking
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DDC Susceptibility is Difficult Assess
• Stain-to-fracture (STF) testing is often used for ranking DDC susceptibility – STF testing may not simulate actual welding conditions
• Various other mockups and tests have also been applied – Chabenat DMW circular patch test with crack count – KAPL & Bettis narrow groove test with crack count – Weld pad with side bends and examination
• These methods do not discern between type of cracking • DDC continues to occur in field even with current testing
approaches • A better understanding of DDC is needed to formulate a
more representative test
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DDC Theories
• DDC occurs in reheated HAZ between 950-1150°C temperature range
Theory 1 Mechanism
• “Grain boundary sliding” effect occurs in HAZ, resulting in localized high stress/strain that cause grain boundaries to pull apart1
Theory 2 Mechanism
• Cracking occurs due to high stresses generated at grain boundaries from precipitation of M23C6 carbides2
Theory 3 Mechanism (EPRI Theory)
• HAZ “Strain ratcheting” (i.e., multi-strain cycling) effect. Cracking occurs due to combination of high localized plastic strain build-up and shear stress in HAZ – further supports Theory 1
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Accumulative Strain Theory
Accumulative Strain (τXYAcc) = │0.05 │ + │- 0.05 │ = 0.10
σXX = 100 / Principal σYY = 10 / Principal τXY = 45 / Max. Shear
Strain τXY = 0.05
σXX
σXX
σYYσYY
γXY
γXY
γYX
γYX
σYYσYY
σXX
σXX
γ=0.05 γ=0.10
On-Cooling On-Heating
Compressive Stress
Tensile Stress
σXX = 100 / Principal σYY = 10 / Principal
τXY = -45 / Max. Shear
Strain τXY = - 0.05
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DDC & Cumulative Plastic Strain Multiple Strain Cycles in DTR Region
Plot at right shows cycles of strain vs.
temperature at Node A caused by
adjacent weld passes 28, 32, 33,
37, 38, 42, 43
Node A is heated four times by adjacent passes 32, 33, 37, 38 heat into the ductility-dip temperature range (DTR) where DDC occurs
0.000
0.010
0.020
0.030
0.040
0.050
0.060
0.070
0.080
0.090
0.100
0.110
0.120
0.130
0.140
0.150
0.160
0.170
0.180
0100200300400500600700800900100011001200130014001500
Com
ulat
ive
Stra
in (M
ax, S
hare
/ τX
Y)
Temperature (℃)
g/ g
(28)
(32)
(33)
(37)
(38)
(42)
(43)DTRBTRTheory IITheory 3 Mechanism
(28)
Node A
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New DDC Testing Approach Weld Pad Build-up on Low Alloy Steel Plate
• Weld buildup is made on low alloy steel plate with NiCrFe filler metal to be tested
• Narrow groove is machined into the NiCrFe buildup • NiCrFe narrow groove weld is made with controlled
interlayer wash passes (re-melt cycles) – High restraint condition to induce DDC – Wash passes simulate multiple reheat cycles – Test approach minimizes hot cracking potential
Low Alloy Steel Plate
NiCrFe Weld Pad
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Narrow Groove Welding Sequence – Method 1 High Restraint Narrow Groove Weld
• Narrow weld bead sequence – 2 weld passes per layer – 12 layers to fill the groove
• Wash passes simulate multiple reheat cycles – 1 wash cycle (2 wash passes) on layer 4 – 2 wash cycles (4 wash passes) on layer 7 – 3 wash cycles (6 wash passes) on layer 10
1 wash cycle
3 wash cycles
2 wash cycles
Layer 4
Layer 7
Layer 10 Wash Cycle means
to weld without filler metal with
purpose to cause a reheat cycle in the
deposited weld metal below
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Proof-of-Concept Sysweld™ Simulation
• Proof-of-concept using Sysweld™ model simulation – Thermal history – Strain build-up – Evolution of stresses
• Model was compared to cracking observed on mockup
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Simulation - Accumulative Strain Results
• Accumulative Plastic Strain After 1 Wash Cycle After 2 Wash Cycles After 3 Wash Cycles
Wash Cycle means to weld without filler metal with purpose to cause a reheat cycle in the weld metal below
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High Restraint Narrow Groove DDC Test Setup 52M Weld Pad and Machined Groove
• Low alloy steel base material • 52M filler material (weld pad) • Narrow groove machined
transverse to weld pad direction
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52M Narrow Groove Test Results Ductility-dip Crack Locations
3.0 mm
Weld Start Region Weld End Region
3.0 mm
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Accumulative Plastic Strain vs. Crack Locations Cross Section at Weld End Region
• Actual weld compared to proof-of-concept Sysweld simulation
• Crack locations correspond with areas of high accumulative plastic strain
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Cracking Results of DDC Test Mockup Number of Cracks, Maximum Length,Total Length
• Cracks counted in two cross-sections • Cracks outside of narrow groove weld region not counted • No hot cracking found in cross-sections examined
Crack Evaluation 1 Wash HAZ Susceptible
Region
2 Wash HAZ Susceptible
Region
3 Wash HAZ Susceptible
Region
Number of Cracks 0 7 10
Maximum Crack Length 0 0.33mm (0.013in) 0.51mm (0.020in)
Total Crack Length 0 1.52mm (0.060in) 2.74mm (0.108in)
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52M Pad and Alloy 690 with 52 DDC Mockups Comparison of Simulation & Mockup DDC
• SysWeld™ simulation shows higher accumulative strain in HAZ with increasing wash (remelt) cycles
• HAZ cracks observed in DDC 52M test: – Number of cracks increased with wash cycles – Cracking occurred in areas predicted – Cracking occurred in preferred orientation – Cracking is characteristic of DDC
• Value of new WRTC DDC test – Simplified DDC screening test – Can potentially provide samples with known
DDC for characterization and CGR testing (samples have known thermal history, high DDC density, preferred orientation)
– Potential to show that small ductility-dip cracks in 52 type weld metals are NOT detrimental and are acceptable for service
Dynamic Recrystallization
Crack orientation = ~45°
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Future Work
• Benchmark Sysweld™ model to more accurately estimate accumulative strain values – Use calibrated heat source
parameters to replicate actual weld heating & cooling cycles
– Use laser profilometer to obtain bead profiles
• Test other 52 compositions using new test method to evaluate susceptibility to DDC – 52, 52MSS, 52i, 690Nb, etc.
• Determine critical cumulative strain values (strain ratchet cycles)
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References & Acknowledgments
• Takanori Kanehiro, IHI Corporation • Brian Shula, ESI Group
- SysweldTM technical support References
1 N.E. Nissley, J. C. Lippold, Ductility-Dip Cracking Susceptibility of Nickel-Based Weld Metals: Part 2 – Microstructural Characterization, The Welding Journal, June 2009, pp 131s-141s.
2 G. A. Young, et al, The Mechanism of Ductility Dip Cracking in Nickel-Chromium Alloys, The Welding Journal, February 2008, pp 31s-43s 3 A. Chabenat, A. Thomas, D. Waskey, Hot Cracking Susceptibility of 30% Cr Alloy Filler Metal in GTAW Deposits, Welding & Repair Technology for Power Plants 6th International EPRI Conference, June 2004, Sandestin, Florida 4 C.M. Brown, T.G. Hicks, J. K.Tatman, Fatigue, Fracture Toughness, and Weldability Characteristics of Three Nickel-Chromium-Iron Alloy Welding Products, EPRI WRT Conference, June 2010.
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Questions or Comments?
Welding and Repair Technology Center
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Together…Shaping the Future of Electricity
EPRI Contacts: Jon Tatman – [email protected] 704-595-2762
Steve McCracken – [email protected] 704-595-2627