Microstructural Effects on Stress Corrosion Crack Growth

in Cold-Worked Alloy 690 Tubing and Plate Materials

Steve Bruemmer, Matt Olszta, Nicole Overman and Mychailo Toloczko

Pacific Northwest National Laboratory

Research Supported by U.S. Nuclear Regulatory Commission

and Rolls Royce

16th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors

August 2013 Asheville, North Carolina, USA

Disclaimer: The work reported in this paper was supported by the Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission. The views expressed in this paper are not necessary those of the U.S. Nuclear Regulatory Commission.

Measured SCC Growth Rates for Alloy 690 Materials without Cold Working

Measured SCC growth rates are low or very low on alloy 690 materials in the non cold-worked condition, however the total number of CRDM tubing and plate heats evaluated is limited.

1.E

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10 15 20 25 30 35 40 45

CG

R (

mm

/s)

K (MPa√m)

Alloy 690 CGR vs K

TT CRDM - 3 heats (PNNL)

SA CRDM (PNNL)

Ciemat A52/690 HAZ (PNNL)

HAZ NG KAPL (GE)

TT CRDM PNNL (ANL)

HAZ CRDM (ANL)

HAZ (Ciemat)

HAZ (Studsvik)

MRP-55, 75% (alloy 600)

MRP-55 curve

Some rates are uncorrected

Non-CW Material

PNNL 11/2012

2

MA B25K Plate (PNNL) MA ANL Plate (PNNL) HAZ Ciemat (PNNL) HAZ ANL Plate (PNNL) HAZ KAPL (PNNL)

PNNL 05/2013

Measured SCC Growth Rates for Alloy 690 CRDM Tubing Materials

The number of SCC growth tests on cold-worked CRDM alloy 690 heats has increased significantly over the last 3 years. High levels of CW (>~20%) can promote moderate-to-high propagation rates.

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CG

R (

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K (MPa√m)

Alloy 690 CGR vs K RE243 TT CRDM (PNNL) RE243 SA CRDM (PNNL) WP140 TT CRDM (PNNL) WP142 TT CRDM (PNNL) RE243 TT+17% CR SL CRDM (PNNL) RE243 SA+17% CR SL CRDM (PNNL) RE243 TT+30% CR TL CRDM (PNNL) RE243 SA+30% CR TL CRDM (PNNL) RE243 TT+31% CR SL CRDM (PNNL) RE243 SA+31% CR SL CRDM (PNNL) WN415 20% CF TL/LT CRDM (GE) WN415 41% CF TL/LT CRDM (GE) WQ199 31% CF SL CRDM (GE) WQ199 21% CF SL CRDM (GE) RE243 TT+31% CR SL CRDM (GE) RE243 SA+31% CR SL CRDM (GE) WP142 TT CRDM (ANL) WP142 HAZ CRDM (ANL) RF103 20% CR TL CRDM (Serco) RF103 20% CR SL CRDM (Serco) WP787 20% CR TL CRDM (Ciemat) WP787 20% Tens TL CRDM (Ciemat) MRP-55 (alloy 600)

MRP-55 curve

PWR Primary Water 320-360°C

CRDM Heats

Some rates may be uncorrected

PNNL 07/2013

S670743 30% CF CRDM (GE) WQ199 30% CF CRDM (GE) WQ199 20% CF CRDM (GE) SB167 31% CF CRDM (PNNL) WP787 20% Tens TL CRDM (PNNL)

Summary of Alloy 690 Measurements of SCC Growth Rates – All Data

Full spectrum of measured SCC growth rates illustrating significant effect of 1D cold work, however initial Bettis results remain at upper end of data with extremely high growth rates at lower K values.

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CG

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K (MPa√m)

Alloy 690: Effect of Cold Work CRDM (PNNL) SA CRDM (PNNL)

17% CR SL CRDM (PNNL) SA+17% CR SL CRDM (PNNL)

30% CR TL CRDM (PNNL) SA+30% CR TL CRDM (PNNL)

31% CR SL CRDM (PNNL) SA+31% CR SL CRDM (PNNL)

31% CR SL, RT CRDM (PNNL) 26% CR SL ANL (PNNL)

20% CR SL GE (PNNL) 26% CR SL ANL (GE)

20% CR SL GE many (GE) 20% CF TL/LT (GE)

21% CF SL/ST ENSA (GE) 20% CF TL/LT CRDM (GE)

41% CF TL/LT CRDM (GE) TT+31% CR SL CRDM (GE)

SA+31% CR SL CRDM (GE) TT CRDM PNNL (ANL)

26% CR SL ANL (ANL) 26% CR ST ANL (ANL)

20% CR SL GE (Serco) 20% CR TL (INSS)

50% CR TL (INSS) 12% CR ST (Bettis)

24% CR ST (Bettis) 24% CR SL (Bettis)

30% CR SL (Bettis) 31% CF SL CRDM (GE)

21% CF SL CRDM (GE) 20% CR TL CRDM (Serco)

20% CR SL CRDM (Serco) 20% CR TL CRDM (Ciemat)

20% Tens TL CRDM (Ciemat) 20% CR SL p-band (Serco)

20% CR SL o-band (Serco) MRP-55, 75% (alloy 600)

MRP-55 curve

Some rates may be uncorrected

PNNL 7/2013 22%CR SL ENSA (PNNL) 32%CF SL/ST ENSA (PNNL) MA GE Plate (PNNL) MA ANL Plate (PNNL) *

* *

* ^

^

^

^

30%CF SL/ST ANL (PNNL) HTA + 30%CF ANL (PNNL) > <HTA + 20%CR GE (PNNL)

> >

<<

+

+

+

31% CF SL/ST CRDM (PNNL) *

*

20% CR TK-VDM Plate (PNNL) +

+

Constant K or Load Data

! Alloy 690 CRDM Tubing (15 tests) ! Heat-to-heat response with tests on three as-received TT heats:

Valinox RE243, WP140 and WP142

! As-received TT (high density of GB Cr carbides + Cr depletion) versus HTA/SA (no GB Cr carbides or Cr depletion); cold rolling effects: 0%, 17% (S-L), 30% (T-L), 31% (S-L); post-cold rolling recovery anneal: 31%CR alloy 690TT + 700°C (S-L)

! As-received TT + 21%CF & 30%CF Valinox RE243, 30%CF Sumitomo SB167 and 20%TS Valinox (from CIEMAT)

! Alloy 690 Plate Materials (10 tests) ! As-received MA heats: ANL (NX3297HK12) and GEG (B25K)

! MA versus HTA (most carbides removed) for cold-worked 30%CF ANL (S-L/S-T) and 20%CR (S-L) GEG heats

! 26%CR ANL (S-L), 22%CR (S-L) ENSA, 30%CF (S-L/S-T) ENSA and 20%CF TK-VDM

PNNL Alloy 690 Testing Summary August 2013

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! Seven recirculating autoclave systems

! DCPD in-situ crack length measurement with <2 µm peak-to-peak noise

! Simulated PWR primary water (1000 ppm B, 2 ppm LiOH)

! Most testing at 360°C with H2 at Ni-NiO line

! Focus on establishing constant K response after various SCC transitioning steps

! Growth rates adjusted for post-test crack length observations

PNNL Crack Growth Rate Testing

Example of testing approach for two as-received CRDM heats: constant K typically evaluated several times in different microstructural regions and often at different K levels during long-term tests.

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! Microstructural Characterization ! Essential for material assessment and comparisons including heat-

to-heat, processing and heat treatment effects.

! Important to assess general microstructure (grain size/shape, banding), precipitate microstructures (size/distribution IG and TG), local microchemistry (grain boundary depletion/segregation), matrix hardness, strain distributions and local CW damage characteristics.

! Characterization Methods ! Optical, SEM and EBSD for general microstructure

! SEM and TEM for precipitate and CW damage microstructure ! EBSD for strain distributions, hardness

! TEM for phase identification and grain boundary composition

! APT for grain boundary composition

! Optical, SEM, TEM and APT of SCC cracks and crack tips

PNNL Alloy 690 Characterization Activities

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! Alloy 690 CRDM Tubing (15 tests) ! Heat-to-heat response with tests on three as-received TT heats:

Valinox RE243, WP140 and WP142

! As-received TT (high density of GB Cr carbides + Cr depletion) versus HTA/SA (no GB Cr carbides or Cr depletion); cold rolling effects: 0%, 17% (S-L), 30% (T-L), 31% (S-L); post-cold rolling recovery anneal: 31%CR alloy 690TT + 700°C (S-L)

! As-received TT + 21%CF Valinox RE243, 30%CF Valinox RE243, 30%CF Sumitomo SB167 and 20%TS Valinox (from CIEMAT)

! Alloy 690 Plate Materials (10 tests) ! As-received MA heats: ANL (NX3297HK12) and GEG (B25K)

! MA versus HTA (most carbides removed) for cold-worked 30%CF ANL (S-L/S-T) and 20%CR (S-L) GEG heats

! 26%CR ANL (S-L), 22%CR (S-L) ENSA, 30%CF (S-L/S-T) ENSA and 20%CF S-L/S-T) TK-VDM

PNNL Alloy 690 Testing Summary August 2013

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CRDM Alloy 690TT vs Alloy 690SA

Solution anneal at 1100°C and water quench removed nearly all grain boundary carbides, isolated TiN particles remain. This material was then cold rolled to 17, 30 and 31% as for the alloy 690TT.

Semi-continuous

grain boundary carbides

Isolated TiN particles

Alloy 690TT

Isolated TiN particles

Alloy 690SA

Alloy 690TT CRDM

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%CR S-L

crac

k g

row

th r

ate

(mm

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~10x

~500x

350-360°C

Alloy 690SA CRDM

~15X

No grain boundary carbides

9

ANL HTA + 30%CF plate has local regions of larger grains with lower twin density and lower average misorientation density than MA + 30%CF.

Most areas show similar grain size, twinning and misorientation density plus overall average Vickers hardness is essentially identical.

Microstructure and Strain Distributions in ANL MA versus HTA 30%CF Alloy 690 Plate

ANL  HTA  +  30%CF:    Average  Vickers  Hardness  =  317  kg/mm2

ANL  MA  +  30%CF:    Average  Vickers  Hardness  =  316  kg/mm2

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SEM (and TEM) examinations reveal that the ANL MA alloy 690 plate has a high density of µm-size Cr carbides at nearly all high-energy grain

boundaries. HTA dissolves most of these carbides resulting in a much lower density of fine (5-50 nm) carbides at isolated sections of grain boundaries.

Precipitation Microstructures in ANL MA versus HTA Alloy 690 Plate

Semi-­‐continuous  

0.2-­‐1  µm    IG  M23C6

ANL  MA

Isolated    <50  nm  IG  M23C6

ANL  HTA

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High Temperature Anneal on SCC Crack Growth for Cold-Worked Alloy 690 Plate

Initial high temperature anneal results in much lower (~15X) SCC growth rates in highly cold-worked alloy 690 plate.

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CT073&74 - 0.5T CT ANL alloy 690 MA+30%CF & HTA+30%CF (S-L/S-T) 360°C, 1000 ppm B, 2.0 ppm Li, 25 cc/kg H2

HTA+30%CF 30 MPa√m

constant load

Comparisons to early in the test at 30 MPa√m at constant K MA: 5.1e-08 mm/s CM: 3.0e-09 mm/s

2.6e-09 mm/s

MA+30%CF 35 MPa√m

3.2e-08 mm/s

8.3e-08 mm/s

test end

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Cracking Morphology on 30%CF ANL Alloy 690: MA versus HTA Materials

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Extensive IGSCC growth is observed during the final constant K evaluation in ANL MA + 30%CF alloy 690 specimen, much more limited IG cracking is present for the ANL HTA + 30%CF alloy 690 specimen.

ANL MA + 30%CF ANL HTA + 30%CF

---- ----

Air Fatigue

Air Fatigue

IG SCC

Mixed IG/TG

GEG HTA + 20%CR plate has a larger grain size, lower twin density, lower average misorientation density and

lower average hardness than GEG MA + 20%CR.

Microstructure and Strain Distributions in GEG MA versus HTA 20%CR Alloy 690 Plate

GEG  HTA  +  20%CR:    Average  Vickers  Hardness  =  291  kg/mm2

GEG  MA  +  20%CR:    Average  Vickers  Hardness  =  321  kg/mm2

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GEG MA plate has a significant microstructural variability, finer grain size, matrix carbides and a low density of carbides on most grain boundaries.

HTA alters microstructure and dissolves carbides, results in a low density of fine (5-50 nm) carbides at isolated sections of grain boundaries.

Precipitation Microstructures in GEG MA versus HTA Alloy 690 Plate

TG  M23C6

GEG  MA+  20%CR

IG  M23C6

Primarily    TG  

M23C6

TG  M23C6

IG  M23C6

GEG  HTA  +  20%CR Few  IG  

M23C6

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High Temperature Anneal on SCC Crack Growth for Cold-Worked Alloy 690 Plate

Initial high temperature anneal results in much lower (~20X) SCC growth rates in 20%CR alloy 690 plate.

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CT070 - 0.5T CT GE Alloy 690 B25K HTA+20%CR S-L 360°C, 30 MPa√m, 1000 ppm B, 2.0 ppm Li, 25 cc/kg H2

HTA+20%CR S-L

Third Constant K Evaluation

2.4e-09 mm/s

Previous Results on MA+20%CR S-L

4.5x10-8 mm

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Influence of Initial High Temperature Anneal on Alloy 690 Microstructure and SCC

Alloy 690 Material Microstructure Test Plane

Hardness PNNL CGRs

TT+31%CR

Nearly continuous GB carbides, isolated GB and matrix TiN, elongated grains, very high dislocation density, moderate density of IG

voids, some cracked carbides and TiN

315 kg/mm2 1.2x10-7 mm/s

(S-L, 360°C)

SA+31%CR Isolated GB and matrix TiN, elongated grains, very high dislocation density, few cracked TiN 316 kg/mm2

9.7x10-9 mm/s (S-L, 360°C)

ANL Bar MA+30%CF

Semi-continuous GB carbides and TiN, matrix TiN, slightly elongated grains, high dislocation density, moderate-to-high density of IG cracked

carbides/TiN precipitates and voids

316 kg/mm2 8.2x10-8 mm/s

(S-L, 360°C)

ANL Bar HTA+30%CF

Fine GB carbides and TiN, matrix TiN, slightly elongated grains, high dislocation density, low

density of IG voids, cracked TiN 317 kg/mm2

3.0x10-9 mm/s (S-L, 360°C)

GEG Billet B25K MA+20%CR

Occasional GB carbides and TiN with more in banded regions, many matrix carbides and TiN,

slightly elongated grains, high dislocation density, isolated IG voids, cracked carbides

321 kg/mm2 4.5x10-8 mm/s

(S-L, 360°C)

GEG Billet B25K HTA+20%CR

Fine GB carbides and TiN, slightly elongated and larger grains, high dislocation density,

isolated IG voids, cracked carbides 291 kg/mm2

2.4x10-9 mm/s (S-L, 360°C)

Preliminary Data

Initial HTA treatment produces a consistent decrease in SCC growth response for highly cold-worked alloy 690 materials. 16

SCC Crack Growth Rates for Cold-Worked Alloy 690 CRDM and Plate Materials

SCC growth rate does not scale with %CW, different behavior indicated at high levels of cold work for CRDM and plate materials with important effect of initial material condition.

Simulated PWR Primary Water 360oC, 25 cc/kg H2

10-10

10-9

10-8

10-7

10-6

0 5 10 15 20 25 30 35

RE243TT CR

RE243SA CR

Cra

ck G

row

th R

ate

, m

m/s

Reduction due to Cold Rolling, %

CRDM Alloy 690TT

CRDM Alloy 690SA

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ANL MA

ANL HTA

GEG HTA

GEG MA TT

SA

TT

SA

EBSD Measurements of Average Misorientation in CW Alloy 690 Materials

Reasonably good correlation for most materials using average EBSD misorientation densities except for cold-worked GEG and ANL MA plate materials. Cold-worked GEG and ANL plates fit general correlation if given initial HTA treatment. Clearly initial material condition and microstructure can influence subsequent strain distribution from cold working.

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ANL MA

ANL HTA GEG MA

GEG HTA

Correlations Between Average EBSD Misorientation Density and SCC Growth Rates

Good correlation with SCC growth rates for most cold-worked alloy 690 heats including 20%CR GEG, 26%CR ANL, 30%CF ANL and 31%CR CRDM + recovery. However, HTA + CW materials show lower SCC growth rates at equivalent EBSD strains.

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CRDM TT ANL

MA

GEG MA

CRDM SA

ANL HTA GEG HTA

SCC correlation with hardness also helps explain outlier points for 31%CR + Recovery and 20%CR GEG specimens. HTA + CW materials again show lower SCC growth rates at equivalent hardness values.

Correlations Between Hardness and SCC Growth Rates

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150

200

250

300

350

0 5 10 15 20 25 30 35

CRDM TT

GE MA

CRDM 31%CRTT+Rec

CRDM SA

ANL MA 26%CR

GE MA 20%CR

ENSA 32%CF

ENSA 22%CR

AMEC 3PU

AMEC 34HU

ANL HTA 30%CF

AML MA 30%CF

ANL MA

GE HTA 20%CR

Har

dnes

s, k

g/m

m2

Reduction due to Cold Work, %

GE MA

CRDM 31%CR + Recovery

10-10

10-9

10-8

10-7

10-6

150 200 250 300 350

CRDM TTGE MACRDM 31%CR+RecovCRDM SAANL MA 26%CRGE MA 20%CRENSA TT 32%CFENSA TT 22%CRGE HTA 20%CRAMEC 3PUAMEC 34HUANL HTA 30%CFANL MA 30%CFANL MA

Cra

ck G

row

th R

ate,

mm

/s

Hardness, kg/mm2

Vickers: 100 g Load

GE HTA

ANL HTA

CRDM SA

! Results demonstrate that alloy 690 tubing and plate materials with different starting microstructures become susceptible to IGSCC in the highly deformed (>~20%CR) condition. Significant heat-to-heat and material condition differences in SCC susceptibility are observed for these highly CW materials.

! High levels of CW produces slightly elongated grains, high dislocation densities particularly at grain boundaries and some degree of IG void formation and cracked carbides/nitrides depending on the precipitate distribution. Pre-existing grain boundary voids and cracks do not directly promote IGSCC.

! Best correlation to SCC growth rates is for EBSD-measured strains and hardness in the alloy 690 materials. Current data indicates that matrix strength and deformation structures near grain boundaries control IGSCC susceptibility.

! Initial high temperature anneal and quench improves SCC resistance in highly CW alloy 690 heats. It appears that this results from the removal of pre-existing matrix damage and/or modification of the grain boundary carbide distribution.

Microstructure Effects on IGSCC in Cold-Worked Alloy 690 Materials

21