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Reproduction and distribution are prohibited. Thank youPWSCC mitigation: engineering justification of CPAntoine MARION – FROG Corrosion - February 3rd - 5th 2015 - p.1

Engineering & Projects Organization


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Nicolas HUIN, A. MARION, F. STELTZLEN, L. FOURNIER, G. POLING, D. WOOD,

M. GALLANT, O. CALONNE

17th International Conference on Environmental Degradation of Materials in NuclearPower Systems – Water Reactors

August 9-13, 2015, Ottawa, Ontario, Canada

Effects of Cavitation Peening on surfaceconditions and PWSCC behavior of Nickelbase alloys 600 & 182


Reproduction and distribution are prohibited. Thank youEffects of CP on surface conditions and PWSCC behavior of Alloys600 and 182 – 17th Env. Deg – Ottawa, August 9-13, 2015 - p.3

IntroductionMost of the components in Alloy 600/182/82 have been replaced byAlloy 690/152/52 due to PWSCC issues, nevertheless, componentsin Alloy 600/182/82 are still in operation

To prevent these components fro m PWSCC, Cavitation Peeningcan be an effective technique

Locations in the Reactor Vessel

Reactor pressure vessel top headpenetration nozzles

Reactor vessel bottom mountednozzles (BMNs)

Dissimilar metal welds (DMWs) ofprimary system piping

Lower radial support welds


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1. Cavitation Peening (CP)process

The CP process uses water, no heat is involved, no material isbeing removed or transferred

Effects of CP on surface conditions and PWSCC behavior of Alloys600 and 182 – 17th Env. Deg – Ottawa, August 9-13, 2015 - p.4



2. Effects on the surfaceEffects of CP on the surface ofmaterials

« as machined » surface

PEENED UNPEENED

No difference in the general aspectinduced by CP

Topographic measurements on industrialsurfaces

on Alloy 182 ground coupons

After grinding After grinding and CP

Maintopographicparameters

Ra = 6 µmRt = 40 µm

Ra = 6 µmRt = 38 µm

No significant modification of the roughness



Micro-hardness in depth

2. Effects on the surface

200

250

300

350

400

450

0 100 200 300 400 500

Depth (µm)

Ha

rdn

es

sH

V0

.02

5

Grinding

Grinding + CP

Depth of the cold worked layer unchanged !

Slight hardening in the first 100 µm



3. Effects on residual surface stresses

Studied on Alloy 182 ground U-bends and flat coupons

Surface stress on Alloy 182 specimens

Large tensile stress level after the initial grinding step

Large compressive stress level after CP

-600

-400

-200

0

200

400

600

800

Lo

ng

itu

din

al

str

es

s(M

Pa

)

σL after grinding

σL after grinding and CP

U-bend(-3% / +9%)

Significant effect of CP on the surface stress of the material



4. Effects on PWSCC initiation

Effect of CP on PWSCC initiation

Studied on ground U-bend specimens in Alloy 182

PWSCC test performed in simulated PWR primarywater (360°C, 1200 ppm B, 2 ppm Li, [H2] = 100cc/kg, 2000 h)

Clear preventing effect of CP on PWSCC initiation

MitigationPresence of crack

after exposure

U-bend

No treatment

Large crack

Thin multi-cracking

Large crack

CP

-

-

-

2000 µm

U-bend(-3% / +9%)



Evaluation of mechanical loading

Surface stress measurements before and after corrosion test (2000h / 360°C)

Remaining of compressive stresses after exposure with CP

After grinding+ CP

After grinding+ CP+ exposure

U-bends

Flat coupons

4. Effects on PWSCC initiation



5. Maintenance of compressivestresses for intended service life

Effect of thermal cycles on stress evolution at the surface

Surface stress on ground Alloy 600 coupons after CP and several thermal cycles at 350°C

3 * 3 hours + 3 * 30 hours + 3 * 300 hours = 999 hours

No significant evolution after the first thermal cycle (Δσ ≈ 100-150 MPa)

-500

-400

-300

-200

-100

0

1 10 100 1000

Time (hours)

Re

sid

ua

ls

urf

ac

es

tre

ss

(MP

a)

Longitudinal residual stress

Transversal residual stress



5. Maintenance of compressivestresses for intended service life

Effect of thermal cycles on stress evolution in depth

Residual stress on ground Alloy 600 specimens after CP and up to 2 cycles (3 hours at350°C)

No significant evolution in depth after the first cycle

-500

-400

-300

-200

-100

0

1000 200 400 600 800 1000 1200

Resid

ualstr

ess

(MP

a)

Depth (µm)

After CP

After CP + 1 cycle (3h)

After CP + 2 cycles (2x3h)



Example of stress measurements in depth on full size BMI nozzlemockups (B&W design, peripheral nozzle) after CP

Axial stress in the weld

6. Mockups (1/2)

Compressive stresses up to at least 1 mm



Example of stress measurements in depth on full size BMI nozzlemockups (B&W design, peripheral nozzle) after CP

Axial stress in the base metal

6. Mockups (2/2)

Compressive stresses and significanteffect on stress up to 1.5 mm



SummaryCP does not significantly modify the surface of the treated material interms of roughness, cold work and hardness

CP induces high compressive stresses on the surface of the mitigatedmaterial as shown by results on coupons and SCC specimens. Residualsurface stresses as low as -800 MPa and -600 MPa for, respectively, Alloy182 and Alloy 600 have been measured on mockups

Residual stresses increase with depth in the material but remaincompressive up to approximately 500 µm on Alloy 600 coupons

2000 hour exposure to simulated PWR primary water at 360°C after CPleads to only a slight loss of compressive stresses

No deleterious effect of thermal cycling on the evolution of compressivestresses at the surface and in depth

Initiation tests conducted at 360°C for 2000 hours demonstrate thatsusceptibility to PWSCC initiation is suppressed after CP thanks tocompressive stresses

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PWSCC mitigation: engineering justification of CP The information …envdeg2015.org/.../ENVDEG/presentations/ENVDEG_PPT161.pdf · 2015-10-19 · RESTRICTEDAREVA The information in