Cracking of Cast Austenitic Stainless Steels after Thermal Aging and Neutron Irradiation

Y. Chen,1 W.-Y Chen, 1 Z. Li, 2 Y. Yang, 2 A. S. Rao,3

B. Alexandreanu,1 and K. Natesan 1

1Argonne National Laboratory, 9700 S. Cass Ave., Lemont, IL 60439 2University of Florida, 549 Gale Lemerand Dr, Gainesville, FL 32611 3US Nuclear Regulatory Commission, 11545 Rockville Pike, Rockville, MD 20852

For presentation at the International Light Water Reactor Materials Reliability Conference and Exhibition 2016, Chicago, August 1-4, 2016

Outline • Background

– Cast Austenitic Stainless Steels (CASS) and Dual-phase microstructure – Effects of Neutron Irradiation and Compositional Details of CASS – Irradiation and Specimens – Crack Growth Rate (CGR) and J-R Curve tests

• Cyclic CGR results • Stress Corrosion Cracking (SCC) CGR Results • Fracture Toughness (J0.2 mm) • Fracture Surfaces • Microstructural Characterizations

– Transmission Electron Microscopy (TEM) Characterization – Atom Probe Tomography (APT) on ferrite – G-phase precipitation and phase separation in ferrite

• Summary

CF grades of cast austenitic stainless steels (CASSs) are cast version of 300-series austenitic SSs, and are used for components with complex shapes, e.g., pump casings, valve bodies, elbows, control rod guide tube spacers, etc.

CASS alloys consist of a duplex microstructure of delta ferrite and austenite, which also can be seen in SS welds.

CF-8 SS weld metal, Metals Handbook, 9th ed. CF-3

Background

Dual-phase microstructure of delta ferrite (δ) and austenite (γ) – L L + δ L + δ + γ δ + γ – Ferrite morphology is affected by local chemical composition, cooling rate, fluid flow of

solidification pool, etc.

S.A. David, S.S. Babu, and J.M. Vitek, JOM, June, 2003.

Beneficial effects of delta ferrite – Help prevent “hot cracking” – Provide a strengthening mechanism for

solidification microstructure – Improve the resistance to sensitization and SCC

Detrimental effect of delta ferrite – Vulnerable to embrittlement after thermal aging

• Carbide, G-phase, spinodal decomposition …

Cast Stainless Steels (CASS) Dual-phase microstructure

Neutron irradiation induces dislocation loops and defect clusters Irradiation hardening and embrittlement.

Irradiation also produces lots of point defects that enhance lattice diffusion, and thus

could affect phase stability and precipitation, radiation induced hardening and embrittlement.

What is the combined effect of thermal aging and neutron irradiation on CASS?

Will the two degradation processes interact with each other?

Effects of Neutron Irradiation

6 6

Static casts with different ferrite contents Thermal aging conducted at 400°C for 10,000 hr, or

at 320°C for 55,000 hr.

1 Measured with ferroscope. 2 Calculated with Hull’s equations.

Cast Grade Composition (wt. %) Ferrite Content Mn Si P S Mo Cr Ni N C Measured 1 Calculated 2

CF-3 0.63 1.13 0.015 0.005 0.34 20.18 8.59 0.028 0.023 24% 21%

CF-3 0.57 0.92 0.012 0.005 0.35 19.49 9.4 0.052 0.009 14% 10%

CF-8 0.64 1.07 0.021 0.014 0.31 20.46 8.08 0.062 0.063 23% 14%

CF-8 0.65 1.01 0.007 0.007 0.32 20.65 8.86 0.080 0.054 13% 10%

CF-8M 0.53 0.67 0.022 0.012 2.58 20.86 9.12 0.052 0.065 28% 25%

Compositional Details of CASS

Irradiation and Specimens

Irradiated in helium-filled capsule at Halden – Irradiation temperature: 318-327°C – Dose rate: ~5x10-9 dpa/s, – Dose: 5.0X1019 n/cm2 (E>1MeV), 0.08 dpa.

1/4T-Compact Tension specimens Radiation exposure rate: 0.3-2 R/hr @ 30 cm.

Crack Growth Rate and J-R Curve Tests

CGR test: – Pre-crack in test environments to obtain environmentally enhanced cracking. – SCC CGR tests with and without periodical partial unloading.

J-R curve test: – Use a SCC starter crack. – Use very slow displacement rate, ~0.43 µ m/s. – Use a blunting line of J/4σf in the analyses to be consistent with previous results.

SEM fractographic examination

Test environments: – PWR water: B ~1000 ppm, Li ~2 ppm, DH ~2

ppm, DO<10 ppb, Conductivity ~20 µ S/cm – Low-DO high-purity water: DO<10 ppb, Conductivity 0.07 µ S/cm. – ~1800 psig, ~320°C, Flow rate: 20-30 ml/min

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

CGR en

v (m/s

)CGRair (m/s)

(b) CF-8, low-DO high-purity water, 320oC

CF curve for 0.2 ppm DO by Shack & Kassner

Red: Best fit for E-1 data, unaged, irr. CF-8.

Blue: Best fit for F-1 data, aged, irr. CF-8.

Black: Best fit for E-N1 data, unaged, unirr. CF-8.

Purple: Best fit for F-N1 data, unaged, unirr. CF-8.

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

CG

Ren

v (m

/s)

CGRair (m/s)

(a) CF-3, PWR or low-DO high-purity water320oC

CF curve for 0.2 ppm DO by Shack & Kassner

Red: Best fit for A-1 data, unaged, irr. CF-3 in PWR water.

Blue: Best fit for A-2 data, unaged, irr. CF-3 in Low-DO water.

Black: Best fit for B-1 data, aged, irr. CF-3 in PWR water.

Purple: Best fit for B-N1 data, aged, unirr. CF-3 in PWR water.

Brick: Best fit for A-N1 data, unaged, unirr. CF-3 in Low-DO water.

Cyclic CGR data are fitted to a superposition model CGRCF = A*CGRair0.5

Neither thermal aging nor irradiation (0.08 dpa) resulted in a statistically significant effect on cyclic CGR behavior.

Cyclic CGR results

10-12

10-11

10-10

10-9

10-8

10 15 20 25

High ferrite, Unaged, PPU (1, 2 hr)

High ferrite, Unaged, Constant-load

High ferrite, Aged, PPU (1, 2 hr)

High ferrite, Aged, Constant-load

Low ferrite, Unaged, PPU 2hr

Low ferrite, Unaged, Constant-load

Low ferrite, Aged, PPU 2 hr

Low ferrite, Aged, Constant-load

CG

R (m

/s)

K (MPa m1/2)

NUREG-0313Curve

Unirradiated and 0.08-dpa CF-3, CF-8, and CF-8M, tested in low-DO high-purity or PWR water, ~320 oC.

SCC CGR Results SCC susceptibilities of aged

and unaged, or low- and high-ferrite CASS specimens are similar.

Neutron irradiation (0.08 dpa) does not elevate the cracking susceptibility in low- corrosion-potential environments.

The favorable cracking response is mainly due to the low-corrosion-potential environments.

Fracture Toughness (J0.2 mm)

Both thermal aging and neutron irradiation can reduce the fracture toughness.

The combination of thermal aging and irradiation reduces the fracture resistance further.

The extent of irradiation-induced embrittlement is greater in the unaged specimens compared to the aged specimens.

Fracture Surface of Unirradiated Aged CF-3

Transgranular (TG) cracking during CGR test Ductile dimple fracture during J-R curve test.

Fracture Surface of Irradiated Aged CF-3

Fracture modes are similar to that of the unirradiated specimen (previous slide).

Fracture morphology of CF-3 CASS at different stages

Fracture morphologies in irradiated CF-3 CASS

γ

δ

δ

γ

(a) Unaged + Irradiated, CGR test region (b) Unaged + Irradiated, J-R test region

(c) Aged + Irradiated, CGR test region (d) Aged + Irradiated, J-R test region

Microstructural Characterizations

Transmission electron microscopy – Per-chloric acid/methanol solution, -30°C to -40°C, current ~100 mA – Weak-beam dark-field imaging condition for irradiation defects – Centered dark-field imaging condition for precipitates

Atom probe tomography – APT tips prepared from ferrite using focused ion beam – UV laser-pulse APT was used for collecting data, and 3-D reconstruction and

analyses were conducted with IVAS 3.6.8

APT sample tip obtained from ferrite phase

CF-8, unaged – Very low density

of dislocations in ferrite and austenite

– M23C6 carbides at phase boundary

δ

γ

M23C6

δ

G-phase

CF-8, aged at 400°C for 10k hr. – Carbides at phase boundary unchanged – No aging-induced change in austenite – High density of G-phase precipitates in

ferrite

Dark-field image with G-phase diffraction spot

Aging γ γ

TEM Characterization

CF-8, aged – Ni-Si solute clusters and G-phase

precipitates

CF-8, unaged – Ni-Si solute clusters -- embryos

of G-phase precipitates

7NiSi isosurface

7NiSi isosurface

Aging

Possible phase separation after thermal aging

50 nm

Cr Map, CF-3

Atom Probe Tomography

19

Ferrite, unaged CF-3, DF w/t G-phase

Ferrite, unaged CF-8, DF w/t G-phase Austenite, unaged CF-3, DF

Austenite, unaged CF-3, BF Ferrite, aged CF-3, DF w/t G-phase

Ferrite, aged CF-8, DF w/t G-phase

Microstructure of irradiated - unaged and aged CF-3 CASS

In austenite, very few visible defects, possible faulted dislocation loops or precipitates

In ferrite, very few dislocation loops, but a high density of G-phase precipitates.

G-phase precipitation enhanced by irradiation in CASS

Neutron irradiation increases the size of G-phase precipitates.

Cr Map, CF-3

Irradiation Aging

Irradiation

Atom Probe Tomography

Neutron irradiation and thermal aging produce similar Cr distribution in ferrite, suggesting similar phase separation behavior.

With prior thermal aging, neutron irradiation increases the magnitude and spacing of Cr-concentration, implying a mixed effect on hardening.

In low-dose irradiated samples: • Very few dislocation loops in both austenite and ferrite • High density and coarsened G-phase precipitates in

ferrite • Advanced phase separation in ferrite

Cr Map, CF-3

Irradiation Aging

Irradiation

Chen et al., JNM, 466 (2015) 560

Microstructural origins of neutron embrittlement

The microstructures of thermally aged and neutron irradiated CASS are very similar. Therefore the loss fracture toughness in both cases may be due to these defects.

(a) Unaged, unirradiated weak beam dark field, (g,5g)

(c) Unaged, 0.08 dpa centered dark-field with G-phase reflection

(d) Aged, 0.08 dpa centered dark-field with G-phase reflection

(b) Aged, unirradiated centered dark-field with G-phase reflection

Irradiation

Aging

Irradiation

(a) Unaged, unirradiated weak beam dark field, (g,5g)

(b) Aged, unirradiated - centered dark-field with G-phase reflection

(c) Unaged, 0.08 dpa - centered dark-field with G-phase reflection

(d) Aged, 0.08 dpa - centered dark-field with G-phase reflection

Thermal aging and irradiation are two degradation processes related at microstructural level.

Thermal aging and neutron irradiation

Secondary phases (α’ or G-phase) within ferrite or at austenite/ferrite boundaries are the microstructural origin of thermal aging embrittlement.

Neutron irradiation at ~320°C can also induce the same types of microstructure observed in 400°C thermal aging.

Neutron irradiation enables or facilitates the precipitation or phase separation that would otherwise be absent or would occur too slowly at the irradiation temperature.

Summary – CASS alloys with various ferrite contents showed good resistances to both

corrosion fatigue and SCC in the low-corrosion-potential environments. – Neutron irradiation (0.08 dpa) did not elevate its cracking susceptibility.

– Neutron irradiation had a significant effect on the fracture toughness.

– The combined effect of thermal aging and neutron irradiation decreased the fracture resistance further.

– The precipitate microstructures that were observed both in aged and irradiated specimens are similar.

– The evolution of micro-structural and micro-chemical changes in CASS subjected to thermal aging or neutron irradiation appears to be similar.

– Thermal aging and neutron irradiation are related degradation mechanisms and may interact with each other at a microstructural level.

Acknowledgements The TEM work was performed at the IVEM - Tandem Facility funded

by the US Department of Energy Office of Nuclear Energy. The APT study was conducted at the CAES facility through the ATR-

National Scientific User Facility Rapid Turnaround Program. This work is sponsored by the U.S. Nuclear Regulatory Commission,

under Job Code V6380, and by the U.S. Department of Energy, under contract # DE-AC02-06CH11357.

Special thanks go to O. K. Chopra, W. J. Shack, Xuan Zhang, Chi Xu,

T. M. Karlsen for their contribution to the project.