Michael Morgan

Hydrogen and Helium Isotopes in MaterialsFebruary 6-7, 2008

Sandia National LaboratoryAlbuquerque, NM

Effect of Forging Process on Tritium Compatibility of Stainless Steel

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Hydrogen Charged (5000 psi)

Tritium Charged (5000 psi)253 appm He

Background

Tritium reservoirs are constructed from stainless steel forgings. Tritium and its decay product, helium, change the structural properties of stainless steels and make them more susceptible to cracking. Tritium-Exposed Steels have decay helium bubble microstructures and are hardened and less able to deform plastically and become more susceptible to embrittlement by hydrogen isotopes.Material and forging specifications have been developed for optimal material compatibility with tritium. They include: Composition, tensile properties, and select microstructural characteristics like grain size, flow line orientation, inclusion content, and ferrite distribution and content. For years, the forming process of choice was high-energy-rate forging (HERF)Today, some reservoir forgings are being made that use a conventional, more common process known as press forging (PF or CF). Conventional hydraulic or mechanical forging presses deform metal at 4-8 ft/s, about ten-fold slower than the HERF process. The material specifications continue to provide successful stockpile performance by ensuring that the two forging processes produce similar reservoir microstructures. A comparison on the fracture toughness properties of steels having similar composition, and mechanical properties, but produced by different forming processes, has not been conducted until now.

Purpose

The purpose of this study was to measure and compare the fracture toughness properties of Type 21-6-9 stainless steel for:High-energy-rate and conventional forgings; in theUnexposed, hydrogen-exposed and tritium-exposed-and-aged conditions.

Summary

Fracture toughness data, particularly for stainless steel weldments and base metals with high decay-helium content, are needed for assessing tritium reservoir structural integrity. The effects of tritium and decay helium on the fracture toughness properties of Type 21-6-9 stainless steel were measured. Included in the study is data from J-Integral fracture toughness tests on unexposed, hydrogen-exposed and tritium-exposed-and-aged base metals (800 atomic parts per million decay helium content). The results indicate that as-received conventionally forged steels have higher fracture-toughness properties than high-energy-rate-forged steels. This difference has been attributed to the fact that the particular high-energy-rate-forged heat used in the study had a larger grain size than the conventionally forged heats and, more importantly, had been sensitized during the forging process. Fracture occurred by a microvoid- nucleation-and-growth process in both unexposed and tritium-exposed steels and welds; the size distribution and spacing of microvoids on the fracture surfaces depended on the exposure conditions with the lowest-toughness samples having an extremely fine void size.

Unexposed

Effect of Tritium Exposure on Stainless Steels

Tritium-Exposed & Aged

Helium Hardened Microstructure

Materials and Compositions

Table I. Compositions of Stainless Steel Forgings, Plates and Weld Filler Wires (Weight %)

Material Forging Sample ID Cr Ni Mn P Si Co Mo C S N O Al Cu

HERF* 21-6-9 A4582 F97-X 19.4 6.4 8.5 0.021 0.33 0.04 <.001 0.28 0.0022 <.001 CF** 21-6-9 B7073 H94-X 19.1 6.7 9.9 0.01 0.41 0.03 0.004 0.28 0.001 0.005 CF** 21-6-9 B6275 F9-X 19.3 6.7 9.9 0.01 0.38 0.03 0.001 0.28 0.002 0.004 Filler Wire 308L

308L Weldment 98-X 20.5 10.3 1.56 0.006 0.5 0.068 <0.01 0.028 0.012 0.055 - - 0.015

*High-Energy-Rate Forged **Conventionally Forged Manufacturers’ supplied compositions

Forging and Sample Orientation

Mechanical Properties and Grain Size

-33.3988006030098 (21-6-9/308L Weld)

5/337.6139400104800F97 (HERF)

10/7;7 < 5%

44.313930099400H94 (CF)

10/7; 7 < 5%

48.313140087100F9 (CF)

Grain Size% ELUltimate Strengthpsi

Yield Strength

psi

Sample ID

Sample Cartridge and Aging Calculation

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7000

0 24 48 72 96

Time (months)

Con

cent

ratio

n (a

ppm

)

Helium (304L)Tritium (304L)Helium (21-6-9)Tritium (21-7-9)

1st Age

2nd Age

Hydrogen and Tritium exposures conducted at 350 C for three weeksTritium samples aged at -40C to build-in decay heliumAll samples were tested at ambient temperature in air

Fracture Toughness Testing

050

100150200250300350400450500

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Displacement, in.

Load

, lbs

1.001.101.201.301.401.501.601.701.801.902.00

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Displacement, in.

Nor

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ized

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tage

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rack

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J-R Curves for CF and HERF Steels

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High-Energy-Rate Forged (HERF)

JQ

Fracture Appearance Unexposed Heats

20 µm20 µm20 µm

Conventionally Forged High-Energy-Rate Forged

20 µm20 µm20 µm

Conventionally Forged Microstructures

_____40 µ m_____40 µ m

_____20 µ m

90 ksi Y.S. Heat 100 ksi Y.S. Heat

_____20 µ m

_____40 µ m

•_____•40 •µ•m•_____

•µ•m

_____20 µ m

HERF Microstructures

_____40 µ m_____40 µ m

_____20 µ m_____20 µ m

J-R Curves for As-Received and Heat-Treated Steels

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CF Heated 650 C 5 MinHERF Heated 650 C 5 Min

Fracture Appearance Heat Treated Steels

20 µm20 µm20 µm

Conventionally Forged High-Energy-Rate Forged

20 µm20 µm20 µm

Effect of Hydrogen Exposure on J-R Behavior

Conventionally Forged Type 21-6-9 Stainless Steel

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Hydrogen Charged (5000 psi)

Hydrogen Charged (10000 psi)

Effect of Hydrogen on Fracture Appearance

100 µm100 µm100 µm

Conventionally Forged Type 21-6-9 Stainless Steel

20 µm20 µm20 µm

Effect of Hydrogen, Tritium, and Decay Helium on J-R Behavior

Conventionally Forged Type 21-6-9 Stainless Steel

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Tritium Charged (5000 psi)253 appm He

Effect of Decay Helium on Fracture Toughness

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Helium Content, appm.

JQ V

alue

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/ in

.CF 21-6-9 100 ksiyield strength heat

CF 21-6-9 90 ksiyield strength heat

HERF 21-6-9 100ksi yield strenthheat

Tritium Aging Effects on Fracture Appearance

•20 µm•20 µm•20 µm

253 appm Helium 627 appm Helium

•20 µm•20 µm•20 µm

253 appm Helium 627 appm Helium

Tritium Aging Effects on Fracture Appearance

4 µm4 µm4 µm4 µm4 µm4 µm

Fracture Appearance of Tritium Exposed HERF Type 21-6-9 Stainless Steel

100 µm100 µm100 µm

627 appm Helium

20 µm20 µm20 µm

Fracture Toughness - Heat Treated

Fracture Toughness of Heat Treated 21-6-9 SS

0250500750

10001250150017502000

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Time @ 647 C, h

Frac

ture

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ghne

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in.

Comparison of Fracture Appearance –Heat Treated vs. Tritium-Exposed-and-Aged

5 µm5 µm5 µm

Heat Treated 650 C 24 h Tritium-Exposed-Aged for Seven Years (627 appm helium)

10 µm10 µm10 µm

Conclusions

High-energy-rate forged Type 21-6-9 stainless steels had lower fracture toughness values than conventionally forged stainless steels. The fracture toughness difference was attributed to a larger grain size and sensitization that occurred during the high-energy-rate forging process. Weldments of Type 21-6-9 stainless steel had higher fracture toughness than base metals. The toughness improvement was attributed to the presence of the ductile ferrite phase in the microstructure.Hydrogen and tritium exposures lowered the fracture toughness properties of both base metals and weldments. This was characterized by lower JQ values and lower J-da curves. The degree of sensitization did not seem to affect the fracture toughness at high helium levels. Fracture modes of the forged steels were dominated by the dimpled rupture process in unexposed, hydrogen-exposed and tritium-exposed steels and welds. Heavily sensitized steels had a similar fracture appearance as tritium-exposed-and-aged steels