Hydrogen Isotope Degradation of Fracture Toughness on Wrought and Welded Austenitic Stainless Steel

Joe Ronevich, Doug Medlin, Josh Sugar, Chris San Marchi, Eun Ju Song, Ryan Sil ls, Dorian Balch

October 24th, 2018

40th Trit ium Focus GroupSandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy?s National Nuclear Security Administration under contract DE-NA0003525.

SAND2018-11917 C

Exposure to high pressure hydrogen-isotopes results in continuous degradation over life of component

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◦ Structural metal integrity is essential for safe storage of high-pressure hydrogen gas isotopes over the life of a component. Tritium diffuses into steel, decays to helium bubbles resulting in evolving degradation as component ages

How microstructural variations (forging, welding, residual stress) are affected by tritium and helium are not understood

Fracture toughness decreases with increasing time (e.g. increasing helium content)

Lack microstructure-property relationships necessary to inform predictive models

Tritium pre-charged

*Courtesy of Michael Morgan (SRNL)

Evaluating effects of aging on mechanical properties necessitates combined efforts of Sandia and SRNL

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Examining how hydrogen embrittles material provides a baseline comparison for understanding how tritium + helium further degrades

Aging problem has two components:1. Hydrogen isotope embrittlement2. Helium bubble hardened microstructure

Doug Medlin Josh SugarJulian SabischDale HitchcockTim Krentz

Michael MorganTim Krentz

Joe RonevichChris San MarchiEun Ju Song

Approach: Fracture tests on H2-precharged stainless steel welds/HAZ at ambient and low temperature

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• Extracted 3-point bend bars/Arc specimens from forged gas tungsten arc welded (GTAW) or electron beam welds (EBW)• 304L, 316L, XM-11 (21Cr-6Ni-9Mn)

• GTAW with 308L filler metal

• Machined notch in area of interest• Weld, HAZ, base metal

• Precracked & Thermal precharge with H2• 300oC for 16+ days at 138 MPa

• Saturates material with ~ 1 at% H2

• Elastic-Plastic Fracture (J-R curves) ASTM E1820 tests at 223 K and 293 K

• Liquid Nitrogen chilled environmental chamber

Arc3-pt bend

Modified weld joint to measure properties of HAZ5

• Precrack grows parallel to FZ boundary and provides greater potential for terminating precrack in HAZ

3 Pt-bend

Single J

Consistently observed crack remaining in HAZ Ensures we are measuring HAZ fracture toughness

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3-point bend (3PB)

Results show similar fracture toughness (JIH) among different specimen geometries examined Provides confidence that future experiments in tritium will be comparable to results in H2 with all test geometries.

• Historical data has been generated using multiple specimen geometries Each geometries have advantages/disadvantages Need to ensure similitude in results

ArcCompactTension(CT)

Non-standard test geometries require comprehensive validation of test methods

J IH

(kJ/

m^2

)Arc C(T)3-pt bend

304L Forged 140 wppm H2

(4) (2)(5)# of measurements

*Error bars indicate standard deviation

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Fracture Thresholds (JIH) exhibited degradation in hydrogen-precharged condition

𝐾𝐾 =𝐽𝐽𝐽𝐽

1 − 𝜗𝜗2

• Without hydrogen, no crack propagation was observed• With hydrogen, welds were degraded more than base metal and HAZ

304L (JIH)GTA weld < E-beam weld < Base metal

21-6-9 (JIH)GTA weld < HAZ

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Low temperature can exacerbate ductility loss in GTA weldsTensile Ductility Fracture Toughness

Jackson et al ( 2012,2016)Ronevich et al (2017)

304L Forging 304L Weld

• Tensile ductility is reduced further at low temperature• Fracture toughness:

• Forgings reduced in low temperature• Welds appear insensitive

As-received

H2-precharged293K 293K223K 223K

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Testing rate appears to affect welds more than base metals

Decreasing testing rate yields lower fracture toughness in welds, likely due to faster hydrogen diffusion in weld delta-ferrite.

Testing rate

Weld microstructure

Weld Fracture surface

δ-ferrite (skeletal)

Stainless steel performance in H2 relative to other alloy systems10

304L Stainless steel Forgings

Stainless welds

304L

E-b

eam

304L

/308

L G

TAW

21-6

-9/2

1-6-

9 G

TAW

Pipeline steels(in 21 MPa H2)

Pressure vessel Steels (in 100 MPa H2)

X52 X60 X80

3T3AAX Gr. J

Despite degradation of fracture toughness, stainless steels remain quite tough relative to other alloys

Ref:San Marchi et al. (2010)San Marchi et al. (2011)Jackson et al (2013)Somerday et al.(2009)

21-6

-9/3

08L

GTA

W

*Error bars represent std. dev including testing rate effects

Arc

3-pt

ben

d

C(T)

Low Temperature Welds

Microstructural study (role of strength)

Stress triaxiality (notched tests)

21-6-9 & 304L tensile curves (interrupted strains)

FY18 FY20 FY22 FY24 FY26

Single / Oligocrystal Stainless steel

E-beam welds

Low Temperature Base Metals

Aluminum

Multi-year testing & characterization plan in progress to deconstruct the complex physics of aging in structural metals

Elucidating embrittlement mechanisms by examining a diverse set of microstructures in different environments will improve & reduce uncertainty in predictive models

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Over 400+ experiments planned

12Summary

Fracture toughness of hydrogen-precharged stainless steel welds were measured using rising displacement test Welds were observed to be more severely embrittled than base metals and HAZ Welds appeared to be more testing rate sensitive than base metals Likely due to ferrite

Despite hydrogen degradation, toughness of stainless steel welds/HAZ/base metals remains high compared to other alloys

Strategic experiments focused on understanding fundamentals of embrittlement behavior will enable enhanced predictive capabilities for lifetime assessment of tritium containing vessels Weld

Hydrogen Isotope Degradation of Fracture Toughness on Wrought and Welded Austenitic Stainless SteelExposure to high pressure hydrogen-isotopes results in continuous degradation over life of componentEvaluating effects of aging on mechanical properties necessitates combined efforts of Sandia and SRNLApproach: Fracture tests on H2-precharged stainless steel welds/HAZ at ambient and low temperatureModified weld joint to measure properties of HAZSlide Number 6Slide Number 7Slide Number 8Slide Number 9Stainless steel performance in H2 relative to other alloy systemsMulti-year testing & characterization plan in progress to deconstruct the complex physics of aging in structural metalsSummary