- Email: [email protected]
Helium embrittlement of stainless steels at ambient temperature
Scripta
METALLURGICA
Vol. 16, pp. 9 6 9 - 9 7 2 , 1982 Printed in t h e U . S . A .
HELIUM EMBRITTLEMENT
OF STAINLESS
Pergamon
Press
Ltd.
STEELS AT AMBIENT TEMPERATURE
George R. Caskey, Jr., David E. Rawl, Jr., and David A. Mezzanotte, Jr. E. I. du Pont de Nemours and Co., Savannah River Laboratory, Aiken, SC 29808 (Received May (Revised June
17, 15,
1982) 1982)
Introduction The effects of helium on the mechanical properties of metals are a potential limitation to the useful lifetime of fusion reactor materials (I). Helium may be introduced by (n,u) reactions, ion implantation, or radioactive decay of tritium which has permeated into the metal. Only radioactive decay of tritium is free of concurrent radiation damage to the metal. The effects of the decay reaction should be negligible since the ~ particle produced has an average energy of 5.7 keV, too low for tritium recoil to be significant. Helium generated by decay of tritium has been shown to embrittle several alloys at high temperature or following a high temperature anneal (2-7). Recently, low temperature helium embrittlement was convincingly demonstrated in an austenitic stainless steel that had been exposed to tritium (4). In retrospect, several earlier investigations had shown helium damage at ambient temperature but the mechanical property changes had been small and the results had been inconclusive (5). The present investigation was undertaken to verify our earlier work (4) and widen the study by including other stainless steels. Initial results from this study are presented here. Additional specimens are still in storage and will be tested after additional helium has been generated. The data obtained so far demonstrate that low temperature helium embrittlement occurs in a variety of stainless steels that have been charged with tritium. Severity of helium damage depends on both stainless steel composition and prior thermomechanical treatment. Experimental
Procedure
& Results
Notched C-shaped specimens 3.8 mm thick and 25 mm outer radius (8) were machined from five stainless steels: Type 304L, Type 316, Nitronic-40, A286, and a modified A286 (Table I). The modified A286 was in the annealed condition, all others were high-energy-rate forged (HERF). Specimens were not fatigue precracked. The specimens were exposed to tritium at 61 MPa pressure at 422 K for six months. TABLE
1
Nominal Alloy Compositions (weight percent) Alloy
C*
Mn*
Si*
Cr
Ni
Mo
304L
0.03
2.00
1.00
19
I0
-
316
0.08
2.00
1.00
17
12
25 -
Nitronic-40
0.08
9.00
1.00
21
6
A286
0.08
1.35
0.70
15
26
1.25
A286 (modified)
0.03
0.20
0.i0
15
30
1.25
*
Max imum
**
2.15 Ti, 0.30 V, 0.20 AI, and 0.003 B.
*** 2.15 Ti, 0.25 V, 0.25 AI, and 0.002 B.
969 0036-9748/82/080969-04503.00/0
N
0.3
Other
970
HELIUM
EMBRITTLEMENT
Vol.
16,
No.
8
Calculated tritium concentrations were 270 mol T2(STP)/m3 at the surface and 60 mol T2(STP)/m3 at the center (9). Actual concentrations may be slightly lower except for Nitronic-40 where measured hydrogen solubility at 470 K was 50% higher than in the other steels (10). Tritium and helium contents averaged over the cross section were 167 mol T2(STP)/m3 and I0 mol He(STP)/m 3. Following the 15-month aging, average tritium and helium contents were 160 mol T2(STP)/m3 and 24 mol He(STP)/m 3. Tensile tests were run immediately after exposure and following 15 months' storage at 273 K. Control specimens were exposed to air at 422 K for six months and stored at low temperature. Tensile tests were made in air at room temperature and the load-deflection curves were analyzed by the J-integral technique (8). J-integral at maximum load (Jm) and deflection at maximum load are given in Table 2 for the several test conditions. TABLE 2 Tritium and Helium Effects on Fracture Toughness of Stainless Steel
Alloy
* C C T T
+ +
History*
at mm
Fracture Toughness Jm, kJ/m 2
304L HERF
C T C + A T + A
3.71 1.55 3.33 1.32
316 HERF
C T C + A T + A
1.95"* 1.80 1.70 1.14
Nitronic-40 HERF
C T C + A T + A
2.67 1.50 2.62 1.02
1200 480 960 75
A286 HERF
C T C + A T + A
3.51"* 2.39 2.97 2.03
1500"* 890 q50 630
A286 (modified) Annealed
C T C + A T + A
2.86 1.50 2 . 7 9 ~'* 0.84**
840 350 680** 40**
Control A - Control + Aged Tritium Charged A - Tritium Charged
** Single
Deflection Max. Load,
Specimen.
All
1280 360 1120 190 610"* 530 350 130
+ Aged
others
in duplicate,
The d a t a d e m o n s t r a t e that helium has degraded the mechanical properties of all five steels. Immediately aft,~r the six-month exposure, the difference i n am b e t w e e n t h e c o n t r o l and charged snecimens was due mostly to the tritium with only a small helium effect. After 15 m o n t h s ' storage a t 2 7 3 K, t h e d i f f e r e n c e between char~ed and control specimens w a s d u e t,) t h e a d d i t i o n a l helium generated duri.ng storage. HERF A286 a n d HERF T y p e 3 0 4 L s t a i n l e s s steels were affected the least by the helium, a n d HERF N i t r o n i c - 4 0 and annealed A286 (modified) were affected the most. Th,~ o c c u r r e n c e o f .~ l a r R e h e l i u m e f f e c t in Nitronic-40 was not s,rprlsing, as a comnarable result was observed earlier (4). The difference in response t,) t h e r ~ r e s e n c e o f h e l i u m s h o w n b y RERF A286 a n d a n n e a l e d A286 ( m o d i f i e d ) ar~pears to be associated with microstructural differences arisln~, from ~r'ior ther~nal or thermo-mechanical treatments. Composition differences between the two varit:ties o f A 2 8 6 a r e s m a l l b u t raav h a v e c o n t r i b u t e d to the relative helium effect also.
Vol.
16,
No.
8
HELIUM
EMBRITTLEMENT
971
Control specimens of all five alloys had lower Jm values after the 15-month storage than immediately after the six-month anneal at 422 K. This change in fracture toughness was not expected and the reason for the change has not been identified as yet. An explanation is probably related to solute redistribution and dislocation rearrangement during the six-month anneal at 422 K and subsequent aging effects at low temperature. The fracture mode tended to change from void coalescence to intergranular in all alloys except HERF A286 (Figure 1). Fracture of annealed A286 (modified) was partly intergranular after tritium charging and entirely intergranular after subsequent aging. Intergranular fracture was evident in isolated areas of the Nitronic-40 after tritium charging and became dominant after aging. Both Type 304L and Type 316 stainless steels failed by mixed fracture (void coalescence and intergranular) after charging and aging, whereas only void coalescence was observed in the tritium charged specimens of these alloys. Discussion In this investigation, helium and hydrogen (tritium) damage occurred simultaneously, and no attempt was made to factor the measured damage between the two causes. The magnitudes of the two effects will change with time, as the tritium decays to helium. Some samples are currently being heated to off-gas the tritium (while leaving the less mobile helium intact) prior to testing. The data accumulated so far suggest however, that helium damage is more potent than hydrogen (tritium) damage for equal atomic concentrations of the two elements. There were decreases in Jm of 50 to 90% as a consequence of the 15-month aging during which time the average helium content increased by a factor of 2 and the average tritium content decreased by 4%. Distribution of the helium within the specimens has not been established as yet. In the Nitronic-40, helium is presumably distributed in fine bubbles throughout the matrix as in the earlier specimens (4). There is no reason to believe that this same helium distribution should apply to the other alloys, particularly when their widely differing microstructures are taken into account. For example, fine carbo-nltride particles occur more corm~only in Nitronic-40 than in Type 304L or Type 316 stainless steels, and both varieties of A286 may contain gamma prime (Ni3 [AI , Ti]), eta (Ni3Ti) and TiC phases in varying quantities and distributions (II). All of these phases could act as traps for tritium and thus create localized high helium concentrations or bubbles which would be distributed within the alloy in the same manner as the phases. We can envision two possibilities for tritium positioning the helium in the lattice. Decay of tritium in an interstitial site produces a helium atom which diffuses until trapped by a vacancy or other lattice defect. Decay of previously trapped (or otherwise segregated) tritium would lead to segregation of helium at the trap site. Severity of helium damage would be related then to trap characteristics such as, quantity of trapped tritium at the interface, interface coherency and whether the trap were reversible or irreversible (12). However, the relation between helium damage and the trap characteristics need not be the same as between hydrogen damage and trap characteristics because the strain fields of helium and hydrogen differ and hydrogen is chemically reactive. Acknowledgement The information contained in this article was developed during the course of work under Contract No. DE-ACO9-76SRO0001 with the U.S. Department of Energy. References (i) J. R. Cost, R. G. Hickman, J. B. Holt, and R. J. Borg. "Helium Release from Type 304 Stainless Steel." Proceedings of International Conference on Radiation Effect and Tritium Technology for Fusion Reactors II. CONF-751026-13, p 235, (1976). (2) A. W. Thompson. "Mechanical Behavior of Face-Centered Cubic Metals Containing Helium." Mat. Sci. and Eng. 21, 41 (1975). (3) M. R. Louthan, Jr., G. R. Caskey, Jr., D. E. Rawl, Jr., and C. W. Krapp. "Tritium Effects in Austenitic Steels," Proceedings of the Conference on Radiation Effects and Tritium Technology for Fusion Reactors IV. Conference-751026-13 (1976). (4) D. E. Rawl, Jr., G. R. Caskey, Jr., and J. A. Donovan. "Low Temperature Helium Embrittlemerit of Tritium Charged Stainless Steel," iO9th Annual AIME Meeting, Las Vegas, Nevada, February 24-28, 1980. (5) A. J. West and D. E. Rawl, Jr. "Hydrogen in Stainless Steel: Isotopic Effects on Mechanical Properties," Proceedings of the Conference on Tritium Technology in Fission, Fusion and Isotopic Applications. Conference-800427 (1980).
972
HELIUM EMBRITTLEMENT
Vol.
16,
No.
8
(6) J. A. Donovan. "Effects of Helium on Mechanical Properties of Armco Iron," Fall TMS-AIME Meeting, Niagara Falls, New York, September 20-23, 1976. (7) J. A. Donovan, R. J. Burger, and R. J. Arsenault. "The Effect of Helium on the Strength of Niobium," 109th Annual AIME Meeting, Las Vegas, Nevada, February 24-28, 1980. (8) M. R. Dietrich, G. R. Caskey, Jr., and J. A. Donovan. J-Controlled Crack Growth as an Indicator of Hydrogen-Stainless Steel Compatibility, Hydrogen Effects in Metals, ed. I. M. Bernstein and A. W. Thompson, Metallurgical Society of AIME, Warrendale, PA, 1981. p 637. (9) M. R. Loutban and R. G. Derrick, "Hydrogen Transport in Austenitic Stainless Steel" Corros. Sci. 15 565 (1975). (I0) G. R. Caskey, Jr., and R. D. Sisson, Jr., "Hydrogen Solubility in Austenitic stainless Steel." Scripta Met 15, 1187 (1981). (II) A. W. Thompson and J. A. Brooks, Hydrogen Performance of Precipitation-Strengthened Stainless Steel Based on A-286, Met. Trans 6A, 1431 (1975). (12) G. M. Pressouyre, Trap Theory of Hydrogen Embrittlement, A~ta Met 28, 895 (1980).
FIGURE 1 - Fracture Mode vs. Helium Content
24 mol He/m 3 Metal
I() mol He/m 3 Metal
Fracture surfaces typical of HERF 304L, HERF 316, and HERF 21-6-9
t
Fracture surface of HERF A-286
0.001 inch
!
METALLURGICA
Vol. 16, pp. 9 6 9 - 9 7 2 , 1982 Printed in t h e U . S . A .
HELIUM EMBRITTLEMENT
OF STAINLESS
Pergamon
Press
Ltd.
STEELS AT AMBIENT TEMPERATURE
George R. Caskey, Jr., David E. Rawl, Jr., and David A. Mezzanotte, Jr. E. I. du Pont de Nemours and Co., Savannah River Laboratory, Aiken, SC 29808 (Received May (Revised June
17, 15,
1982) 1982)
Introduction The effects of helium on the mechanical properties of metals are a potential limitation to the useful lifetime of fusion reactor materials (I). Helium may be introduced by (n,u) reactions, ion implantation, or radioactive decay of tritium which has permeated into the metal. Only radioactive decay of tritium is free of concurrent radiation damage to the metal. The effects of the decay reaction should be negligible since the ~ particle produced has an average energy of 5.7 keV, too low for tritium recoil to be significant. Helium generated by decay of tritium has been shown to embrittle several alloys at high temperature or following a high temperature anneal (2-7). Recently, low temperature helium embrittlement was convincingly demonstrated in an austenitic stainless steel that had been exposed to tritium (4). In retrospect, several earlier investigations had shown helium damage at ambient temperature but the mechanical property changes had been small and the results had been inconclusive (5). The present investigation was undertaken to verify our earlier work (4) and widen the study by including other stainless steels. Initial results from this study are presented here. Additional specimens are still in storage and will be tested after additional helium has been generated. The data obtained so far demonstrate that low temperature helium embrittlement occurs in a variety of stainless steels that have been charged with tritium. Severity of helium damage depends on both stainless steel composition and prior thermomechanical treatment. Experimental
Procedure
& Results
Notched C-shaped specimens 3.8 mm thick and 25 mm outer radius (8) were machined from five stainless steels: Type 304L, Type 316, Nitronic-40, A286, and a modified A286 (Table I). The modified A286 was in the annealed condition, all others were high-energy-rate forged (HERF). Specimens were not fatigue precracked. The specimens were exposed to tritium at 61 MPa pressure at 422 K for six months. TABLE
1
Nominal Alloy Compositions (weight percent) Alloy
C*
Mn*
Si*
Cr
Ni
Mo
304L
0.03
2.00
1.00
19
I0
-
316
0.08
2.00
1.00
17
12
25 -
Nitronic-40
0.08
9.00
1.00
21
6
A286
0.08
1.35
0.70
15
26
1.25
A286 (modified)
0.03
0.20
0.i0
15
30
1.25
*
Max imum
**
2.15 Ti, 0.30 V, 0.20 AI, and 0.003 B.
*** 2.15 Ti, 0.25 V, 0.25 AI, and 0.002 B.
969 0036-9748/82/080969-04503.00/0
N
0.3
Other
970
HELIUM
EMBRITTLEMENT
Vol.
16,
No.
8
Calculated tritium concentrations were 270 mol T2(STP)/m3 at the surface and 60 mol T2(STP)/m3 at the center (9). Actual concentrations may be slightly lower except for Nitronic-40 where measured hydrogen solubility at 470 K was 50% higher than in the other steels (10). Tritium and helium contents averaged over the cross section were 167 mol T2(STP)/m3 and I0 mol He(STP)/m 3. Following the 15-month aging, average tritium and helium contents were 160 mol T2(STP)/m3 and 24 mol He(STP)/m 3. Tensile tests were run immediately after exposure and following 15 months' storage at 273 K. Control specimens were exposed to air at 422 K for six months and stored at low temperature. Tensile tests were made in air at room temperature and the load-deflection curves were analyzed by the J-integral technique (8). J-integral at maximum load (Jm) and deflection at maximum load are given in Table 2 for the several test conditions. TABLE 2 Tritium and Helium Effects on Fracture Toughness of Stainless Steel
Alloy
* C C T T
+ +
History*
at mm
Fracture Toughness Jm, kJ/m 2
304L HERF
C T C + A T + A
3.71 1.55 3.33 1.32
316 HERF
C T C + A T + A
1.95"* 1.80 1.70 1.14
Nitronic-40 HERF
C T C + A T + A
2.67 1.50 2.62 1.02
1200 480 960 75
A286 HERF
C T C + A T + A
3.51"* 2.39 2.97 2.03
1500"* 890 q50 630
A286 (modified) Annealed
C T C + A T + A
2.86 1.50 2 . 7 9 ~'* 0.84**
840 350 680** 40**
Control A - Control + Aged Tritium Charged A - Tritium Charged
** Single
Deflection Max. Load,
Specimen.
All
1280 360 1120 190 610"* 530 350 130
+ Aged
others
in duplicate,
The d a t a d e m o n s t r a t e that helium has degraded the mechanical properties of all five steels. Immediately aft,~r the six-month exposure, the difference i n am b e t w e e n t h e c o n t r o l and charged snecimens was due mostly to the tritium with only a small helium effect. After 15 m o n t h s ' storage a t 2 7 3 K, t h e d i f f e r e n c e between char~ed and control specimens w a s d u e t,) t h e a d d i t i o n a l helium generated duri.ng storage. HERF A286 a n d HERF T y p e 3 0 4 L s t a i n l e s s steels were affected the least by the helium, a n d HERF N i t r o n i c - 4 0 and annealed A286 (modified) were affected the most. Th,~ o c c u r r e n c e o f .~ l a r R e h e l i u m e f f e c t in Nitronic-40 was not s,rprlsing, as a comnarable result was observed earlier (4). The difference in response t,) t h e r ~ r e s e n c e o f h e l i u m s h o w n b y RERF A286 a n d a n n e a l e d A286 ( m o d i f i e d ) ar~pears to be associated with microstructural differences arisln~, from ~r'ior ther~nal or thermo-mechanical treatments. Composition differences between the two varit:ties o f A 2 8 6 a r e s m a l l b u t raav h a v e c o n t r i b u t e d to the relative helium effect also.
Vol.
16,
No.
8
HELIUM
EMBRITTLEMENT
971
Control specimens of all five alloys had lower Jm values after the 15-month storage than immediately after the six-month anneal at 422 K. This change in fracture toughness was not expected and the reason for the change has not been identified as yet. An explanation is probably related to solute redistribution and dislocation rearrangement during the six-month anneal at 422 K and subsequent aging effects at low temperature. The fracture mode tended to change from void coalescence to intergranular in all alloys except HERF A286 (Figure 1). Fracture of annealed A286 (modified) was partly intergranular after tritium charging and entirely intergranular after subsequent aging. Intergranular fracture was evident in isolated areas of the Nitronic-40 after tritium charging and became dominant after aging. Both Type 304L and Type 316 stainless steels failed by mixed fracture (void coalescence and intergranular) after charging and aging, whereas only void coalescence was observed in the tritium charged specimens of these alloys. Discussion In this investigation, helium and hydrogen (tritium) damage occurred simultaneously, and no attempt was made to factor the measured damage between the two causes. The magnitudes of the two effects will change with time, as the tritium decays to helium. Some samples are currently being heated to off-gas the tritium (while leaving the less mobile helium intact) prior to testing. The data accumulated so far suggest however, that helium damage is more potent than hydrogen (tritium) damage for equal atomic concentrations of the two elements. There were decreases in Jm of 50 to 90% as a consequence of the 15-month aging during which time the average helium content increased by a factor of 2 and the average tritium content decreased by 4%. Distribution of the helium within the specimens has not been established as yet. In the Nitronic-40, helium is presumably distributed in fine bubbles throughout the matrix as in the earlier specimens (4). There is no reason to believe that this same helium distribution should apply to the other alloys, particularly when their widely differing microstructures are taken into account. For example, fine carbo-nltride particles occur more corm~only in Nitronic-40 than in Type 304L or Type 316 stainless steels, and both varieties of A286 may contain gamma prime (Ni3 [AI , Ti]), eta (Ni3Ti) and TiC phases in varying quantities and distributions (II). All of these phases could act as traps for tritium and thus create localized high helium concentrations or bubbles which would be distributed within the alloy in the same manner as the phases. We can envision two possibilities for tritium positioning the helium in the lattice. Decay of tritium in an interstitial site produces a helium atom which diffuses until trapped by a vacancy or other lattice defect. Decay of previously trapped (or otherwise segregated) tritium would lead to segregation of helium at the trap site. Severity of helium damage would be related then to trap characteristics such as, quantity of trapped tritium at the interface, interface coherency and whether the trap were reversible or irreversible (12). However, the relation between helium damage and the trap characteristics need not be the same as between hydrogen damage and trap characteristics because the strain fields of helium and hydrogen differ and hydrogen is chemically reactive. Acknowledgement The information contained in this article was developed during the course of work under Contract No. DE-ACO9-76SRO0001 with the U.S. Department of Energy. References (i) J. R. Cost, R. G. Hickman, J. B. Holt, and R. J. Borg. "Helium Release from Type 304 Stainless Steel." Proceedings of International Conference on Radiation Effect and Tritium Technology for Fusion Reactors II. CONF-751026-13, p 235, (1976). (2) A. W. Thompson. "Mechanical Behavior of Face-Centered Cubic Metals Containing Helium." Mat. Sci. and Eng. 21, 41 (1975). (3) M. R. Louthan, Jr., G. R. Caskey, Jr., D. E. Rawl, Jr., and C. W. Krapp. "Tritium Effects in Austenitic Steels," Proceedings of the Conference on Radiation Effects and Tritium Technology for Fusion Reactors IV. Conference-751026-13 (1976). (4) D. E. Rawl, Jr., G. R. Caskey, Jr., and J. A. Donovan. "Low Temperature Helium Embrittlemerit of Tritium Charged Stainless Steel," iO9th Annual AIME Meeting, Las Vegas, Nevada, February 24-28, 1980. (5) A. J. West and D. E. Rawl, Jr. "Hydrogen in Stainless Steel: Isotopic Effects on Mechanical Properties," Proceedings of the Conference on Tritium Technology in Fission, Fusion and Isotopic Applications. Conference-800427 (1980).
972
HELIUM EMBRITTLEMENT
Vol.
16,
No.
8
(6) J. A. Donovan. "Effects of Helium on Mechanical Properties of Armco Iron," Fall TMS-AIME Meeting, Niagara Falls, New York, September 20-23, 1976. (7) J. A. Donovan, R. J. Burger, and R. J. Arsenault. "The Effect of Helium on the Strength of Niobium," 109th Annual AIME Meeting, Las Vegas, Nevada, February 24-28, 1980. (8) M. R. Dietrich, G. R. Caskey, Jr., and J. A. Donovan. J-Controlled Crack Growth as an Indicator of Hydrogen-Stainless Steel Compatibility, Hydrogen Effects in Metals, ed. I. M. Bernstein and A. W. Thompson, Metallurgical Society of AIME, Warrendale, PA, 1981. p 637. (9) M. R. Loutban and R. G. Derrick, "Hydrogen Transport in Austenitic Stainless Steel" Corros. Sci. 15 565 (1975). (I0) G. R. Caskey, Jr., and R. D. Sisson, Jr., "Hydrogen Solubility in Austenitic stainless Steel." Scripta Met 15, 1187 (1981). (II) A. W. Thompson and J. A. Brooks, Hydrogen Performance of Precipitation-Strengthened Stainless Steel Based on A-286, Met. Trans 6A, 1431 (1975). (12) G. M. Pressouyre, Trap Theory of Hydrogen Embrittlement, A~ta Met 28, 895 (1980).
FIGURE 1 - Fracture Mode vs. Helium Content
24 mol He/m 3 Metal
I() mol He/m 3 Metal
Fracture surfaces typical of HERF 304L, HERF 316, and HERF 21-6-9
t
Fracture surface of HERF A-286
0.001 inch
!