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


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


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


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

6 © 2014 Electric Power Research Institute, Inc. All rights reserved. Welding and Repair Technology Center 6

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


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


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


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


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


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


52M Narrow Groove Test Results Ductility-dip Crack Locations

3.0 mm

Weld Start Region Weld End Region

3.0 mm


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


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)


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°


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)


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.


Questions or Comments?

Welding and Repair Technology Center


Together…Shaping the Future of Electricity

EPRI Contacts: Jon Tatman – [email protected] 704-595-2762

Steve McCracken – [email protected] 704-595-2627

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06/04/2014 update on 52 weldability. - nrc: home pageupdate on 52 weldability comparison of...

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