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Michael Morgan
Tritium Aging Effects on Forged Stainless Steel
0
1000
2000
3000
4000
5000
0 0.02 0.04 0.06 0.08 0.1
da, in.
J, lb
s / i
n.
As-Received
Hydrogen Charged (5000 psi)
Tritium Charged (5000 psi)253 appm He
Background
Pressure vessels for tritium service 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. 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.
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.
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
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
0
1000
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3000
4000
5000
6000
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
0 0.1 0.2
Displacement, in.
Load
, lbs
1.001.101.201.301.401.501.601.701.801.902.00
0 0.1 0.2
Displacement, in.
Nor
mal
ized
Vol
tage
A
cros
s C
rack
Mou
th
J-R Curves for CF and HERF Steels
0
1000
2000
3000
4000
5000
6000
0 0.02 0.04 0.06 0.08 0.1
da, in.
J, lb
s / i
n.Conventionally Forged (CF)
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
0
1000
2000
3000
4000
5000
6000
0 0.02 0.04 0.06 0.08 0.1
da, in.
J, lb
s / i
n.Conventionally Forged (CF)High-Energy-Rate Forged (HERF)
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
0
1000
2000
3000
4000
5000
0 0.02 0.04 0.06 0.08 0.1
da, in.
J, lb
s / i
n.As-Received
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
0
1000
2000
3000
4000
5000
0 0.02 0.04 0.06 0.08 0.1
da, in.
J, lb
s / i
n.As-Received
Hydrogen Charged (5000 psi)
Tritium Charged (5000 psi)253 appm He
Effect of Decay Helium on Fracture Toughness
0
500
1000
1500
2000
2500
0 200 400 600 800 1000
Helium Content, appm.
JQ V
alue
, lbs
/ 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
0 5 10 15 20 25 30
Time @ 647 C, h
Frac
ture
Tou
ghne
ss,
lbs/
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
HERF Type 21-6-9 stainless steels had lower fracture toughness values than Conventionally Forged Type 21-6-9 steel because of its larger grain size and sensitization that occurred during the forging process. Hydrogen and tritium exposures lowered the JQ values and 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