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14PHYSICAL AND MECHANICAL PROPERTIES OF STRUCTURAL MATERIALS IN GASEOUS MEDIA CONTAINING HYDROGEN ISOTOPES
A. V. Bazunov, I. E. Boitsov, S. K. Grishechkin, V. Z. Ismagilov, I. L. Malkov, Yu. A. Khabarov, A. A. Yukhimchuk
' Russian Federal Nuclear Center — All-Russia Research Institute of Experimental Physics Mira av., 37, Sarov, Nizhegorodskiy Region, 607188, Russia
Experimental capabilities are described which are intended for physical and mechanical studies of structural materials (SM) when they are exposed to hydrogen isotopes environment at pressures up to 100 MPa and temperatures up to 900 K. The paper presents research on the hydrogen effects on mechanical properties of the alloy CrNi40MoCuTiAl and the temperature dependence of this alloy's permeability PH to diffusion-purity hydrogen isotopes, with reference to its use as SM in a radiationsafe container for D/T filling of laser targets.
Introduction
Hydrogen is known to produce unfavorable effects upon SM involving their plasticity degradation, or the phenomenon called «hydrogen em-brittlement» [1]. Hydrogen embrittlement poses the risk of early and unexpected damage for structures exposed to hydrogen during their operation. Investigating the hydrogen interaction with SM is a challenge because of no theory of hydrogen em-brittlement existing to serve the guidance for operating structures in hydrogenous environments. Therefore, recommendations on preferred SM, stress limits for critical structural components, operation time in a hydrogenous medium etc. can be mostly provided through experimental research into the hydrogen exposure of SM under near-operational conditions. Given the need to design gas supply facilities for muonic catalysis physics experiments and develop systems to fill laser targets with D/T mixture, ext., it is of special interest to study the SM mechanical properties in a high-pressure hydrogen isotope environment over a wide temperature range.
Test Procedure
This paper describes experimental capabilities existing at RFNC-VNIIEF, that can be used to investigate physical and mechanical properties of SM exposed to high-pressure hydrogen isotopes. Fig. 1 shows a schematic setup for mechanical testing of SM by subjecting standard cylindrical samples to tension in hydrogenous media at high pressure values.
The setup includes the frame (1), the high-pressure electrically heated chamber (2), the upper (3) and lower (4) pull rods to fix the sample and transfer axial load thereto, the axial load gage (5) and the hydraulic support unit (6). A high-pres-
sure generator (HPG) is used to make the in-chamber medium pressurized to the required value. During the test, the sample is loaded by gas pressure in the chamber with the hydraulic support plunger (7) moving downwards due to the hydraulic compressor changing the liquid level inside the cylinder. Pressure in the high-pressure
Fig. 1. Schematic of the setup for tensile testing of samples in hydrogen environment: 1 — frame; 2 — electrically heated pressure chamber; 3 — upper rod; 4 — low rod; 5 — axial load gage; 6 — hydraulic support unit; 7 — hydraulic support plunger.
The high-pressure hydrogen tensile test setup offers the following basic performance:
— loading maximum
— operational size (height/diameter)
— working pull rod stroke
— loading rate
— hydrogen pressure
— test temperature
20 kN
120/12 mm 15 mm
3.310-2 mm/s 50-150 MPa 300-1100 K
Статья поступила в редакцию 05.06.2006 г.
The article has entered in publishing office 05.06.2006.
chamber is measured using a super high-pressure meter (measurement range is 0-600 MPa). The sample loading rate in tensile testing is constant and equal to 3.310-2 mm/s. Load measurement on the sample is provided by the axial load gage which is designed to avoid any effects of gas pressure or sealing friction on the loading measurement. An electrical signal from the gage is transmitted to the register (oscilloscope). The resulting oscilloscope patterns are processed as required by the State Standard 1497-84, «Metals and Tensile Test Procedures». This involves standard characterization of mechanical properties, such as ultimate tensile strength ctb, yield strength ct0 2, elongation ratio 8 and contraction ratio y. The sample temperature is measured indirectly with a chromel-alumel thermoelectrical thermometer used as primary pickup.
For the purpose of hydrogen immunity of SM used under combined stress conditions as typical of high-pressure vessels, the experiments are used to test tubular samples against hydrogen pressure loading concurrently with tensile tests in a hydrogenous medium. Schematic of the test setup for tubular samples shown in fig. 2.
The test sample is placed into a containment vessel (fig. 2) intended for two functions: (a) heater safety when the tubular sample gets damaged, (b) tight sample enclosure. The containment vessel together with the copper heat exchanger to allow uniform sample heating is positioned into the electric heater. During the test, temperature (1100 K is the highest) is measured with a thermocouple put into the hole 5 as deep as mid-length of the sample, and an automated potentiometer is used to control and monitor the specified temperature. Using high-pressure pipes, the sample is coupled to the HPG which can allow as high hydrogen pressure as 500 MPa.
The tubular sample fracture stress calculated by formula [2]:
CTfrac = Pfrac ' ln ' (dout/ din ) =
where Pfrac — the tubular sample fracture pressure, d t, din — outside and inside diameter correspondingly.
In testing of standard cylindrical and tubular-shaped samples, the severity of hydrogen exposure for SM is evaluated by comparing between mechanical properties found out by hydrogenous testing (H2) and those observed by experiments in inert medium (XHe), and thus the hydrogen
effect factor is calculated as: Д =
XHe - Xh.
X
2 -100%.
He
Characterization of hydrogen isotopes permeation through SM is done using an experimental setup as shown schematically in fig. 3 [3]. The technique employed here is determination of the kinetics
of hydrogen permeation (HP) through SM based on the analysis of the heat conduction change of the carrier gas of the exit side of test sample, which results from the carrier gas dissolving the hydrogen isotope when it has diffused through the sample.
The HP tests of SM are performed on tubular-shape samples at up to 300 MPa pressure of diffusion-or engineering-purity hydrogen isotopes and temperatures up to 1100 K.
The setup can be operated both in integral mode (time-based accumulation and subsequent quantification of the diffusion gas amount) which is used at early time of the experiment and at low operating temperature, and differential mode (online data reading) used at higher temperatures and, accordingly, for short-term experiments. The measurement technique has sensitivity threshold for hydrogen of 210-9g and the accuracy within ±7 %. With the knowledge hydrogen permeation behavior through SM at different temperatures and pressure, one can calculate the hydrogen concentration in metal and thus establish a relationship between the SM mechanical properties and the hydrogen content therein.
Together with hydrogen embrittlement, there is another effect observed during long-term operation of SM in tritium-containing environment, which is the so-called «helium embrittlement» [4], a phenomenon resulting from helium atoms produced in the SM matrix by the radioactive decay of material-dissolved tritium. What makes helium embrittlement a challenge is that it is virtually impossible to reduce the He3 content in SM because of its low diffusisity. From available publication [5, 6], critical helium content for most materials is ~30 apmm and higher, however there are same steels for which the helium content even as low as ~2-3 apmm may cause considerable degradation in plasticity. Helium embrittlement studies of SM have been comprehensive enough for the case with He atoms implantation or production in the material matrix by a or neutron irradiation. However, the information provided by these studies is insufficient to predict properties for the SM in which helium production is due to tritium decay. First of all, this is because irradiation results in defects in the metal lattice, while these defects do not occur when metals are saturated with tri-
Fig. 2. Schematic of the test setup for tubular samples: 1 — tubular-shape sample; 2 — containment vessel; 3 — copper heat exchanger; 4 — electrical furnace; 5 — thermocouple hole
Fig. 3. Schematic of the test setup for hydrogen permeation through SM: HC — hydraulic compressor, GH — gas hydraulic apparatus; MT — multiplicator, DF — diffusion filter; R — reductor; BV — buffer vessel; K — electromagnetic valve; NT — nitrogen trap; DV — dose valve; V — high pressure valve; RV — restrictor valve; Kt — differential thermal conductibility cell; P — probes volume; 2NVR-5DM — forevacuum pump; D — pressure sensor; OM-6 — optical manometer; EH — electric resistance heater, DC — diffusion cell
tium. Also, mechanical properties are significantly affected by helium distribution pattern. During irradiation, helium is generated in the surface layer several tenths of millimeter thick, while radiogenic helium distribution depends on the distribution of tritium which having high diffusivity factor can permeate much deeper into the metal and come to concentrate in traps. Helium effects on SM mechanical properties are difficult to study because helium is featured by its slow buildup in material, which is due to tritium having a relatively low rate of decay (about 12-years' half-life). As long as helium concentration in SM is directly proportion to that of tritium, then if may be a possible approach to accelerated buildup of radiogenic helium in SM to make the SM matrix have increased tritium concentration. Again, the higher tritium concentration in SM can be achieved through holding a sample of test SM in tritium gas environment at high pressure and temperatures. To this end, a specialized set of equipment has been designed and built at RFNC-VNIIEF using for basic facility a system for making hydrogen isotopes gas a cylindrical vessel as basic component, in which standard tensile test samples are to be held in tritium environment at pressures up to 100 MPa and temperatures up to 900 K as long as needed to reach the required helium concentration. Following this, the samples undergo decontamination to remove tritium and them used to continue experiments under conditions free of specific safety requirements. To investigate the impact on the SM mechanical properties by helium occurring in matrix, samples are tested using the setup for tensile testing high-pressure hydrogen gas environment, as described above. The same setup can be used in experiments for combined helium and hydrogen effects on SM mechanical properties.
Results and discussion
As an illustration of how the above-described experimental capabilities are used to investigate physical and mechanical properties of SM exposed to high-pressure hydrogen isotope environment, test data are presented below in this paper on the alloy CrNi40MoCuTiAl as applied to its use as SM for radiation-safe container with D/T mixture.
The high-nickel CrNi40MoCuTiAl refers to austenitic precipitation-hardened alloys whose strengthening is achieved by precipitating the reinforcing y'-phase from solid solution in aging. Fig. 4-6 shows experimental data on the hydrogen effects at 100 MPa pressure and temperatures between 273-873 K for 200 hours of exposure on this alloy's mechanical properties.
The experimental data have been analyzed to show that the alloy CrNi40MoCuTiAl under significant plastic strain condition features apparent hydrogen sensitivity, with the hydrogen contribution to ctb, ct0 2, 8 and y properties ranging from 19 to 88 %. Moreover, the plasticity quantities 8, measured at sample failure, i. e. plastic strain is the highest, are much more decreased by hydrogen exposure, than the strength quantity ctb measured at peak loading on the sample when plastic strain accounts for about several percent. However, is specific of metal resistance to low plastic strains, is not very sensitive to hydrogen, so that its decrease is ~4 % at most. We can conclude from the research data obtained, that in terms of effective stress levels the yield strength value is what must limit the use of the alloy CrNi40MoCuTiAl under long-term exposure to high-pressure hydrogen isotope-mixed gas at higher temperatures.
With helium concentrated in CrNi40MoCuTiAl alloy at ~1.5apmm (calculated) after it has been
d„ а0,2, MPa 1500
1400
1300
1200
1100
1000
900
800
Ов
/
г-
00,2 \ Не
\ \
Д^—и к. . / \ н2
200
400
600
800
1000
Test temperature, К
S,
50 40 30
Не * л
V
\ \ \ W «
Н2 s
200
400 600
'l'est temperature, К
800
1000
Fig. 4. Mechanical properties vs test temperature
1400
1300
1200
1100
айао, MPa
1000
Не -—_ /
\ Jk
200
400
600
Test t
800 , К
1000
t temperature.
Fig. 5. Tubular sample fracture stress vs test temperature
400 600 800 1000 Test temperature, K
Fig. 6. Hydrogen effect factor vs test temperature
IgPn
D, Нг
lgDH,
110
110
-4
1-Ю
d2 / Н2 /
/
0.9
1.1
1.3 1000/T, к
1.5
1.7
Fig. 8. Coefficient diffusion of hydrogen isotopes vs temperature
1.3
1000/T, к
Fig. 9. Hydrogen isotopes solubility vs temperature
Fig. 7. Permeation PH vs temperature T for hydrogen isotopes through alloy
Fig. 10. Permeation of PD vs pressure deuterium through alloy at 873 K
held in tritium for 100 hours at 20 MPa pressure and 500 K temperature, its mechanical properties are not affected. Investigations into helium effects upon the CrNi40MoCuTiAl mechanical properties will be continued.
Results of study hydrogen isotopes permeation though CrNi40MoCuTiAl alloy shows in fig. 7-10.
For example fig. 7 presents permeation PH vs temperature curves for diffusion-purity protium and deuterium through CrNi40MoCuTiAl as the data have shown, there is isotopic composition de-
Chemical composition of alloy CrNi40MoCuTiAl
Composition (wt. %)
Si Mn Cr Ni Ti Al Mo Cu S P
<0.5 <0.80 14.0-17.0 39.0-42.0 2.5-3.2 0.7-1.2 4.5-6.0 2.7-3.3 <0.020 <0.035
Ej Heat treatment: aging, for 8-10 hours at 890 ± 15 K; air cooling.
cu
1 pendence of gas permeation for CrNi40MoCuTiAl.
15 The experimental permeation ratio through
| CrNi40MoCuTiAl for different hydrogen isotopes
h" in the temperature range of interest is ~1.4-1.7,
| which agrees well with the theory (|p ~1.4 ,
g where |d and |p are the molecule masses of deute™ rium and protium, respectively).
From the results obtained, the permeability of CrNi40MoCuTiAl alloy to tritium can be evaluated, so that the appropriate container design would be identified for this material, which provides for adequate radiation safety in operation.
Thus, the research data on its physical and mechanical properties indicate, that the alloy CrNi40MoCuTiAl can be recommended for use as SM for radiation-safe container operated under long-term exposure (up to 200 hours) to high-pressure hydrogen isotope-mixed gas (up to 98 MPa) at high temperatures (up to 873 K).
The study of physical and mechanical properties of alloy CrNi40MoCuTiAl in a high-pressure hydrogen isotope environment has been supported by ISTC (project #025-95).
References
1. Kolachev B. A. Hydrogen embrittlement of metalls. Moscow: Metallurgica, 1985. P. 217.
2. Rumjanzev O. V. Оборудование цехов синтеза высокого давления в азотной промышленности. Moscow: Chemistry, 1970. P. 376.
3. Yukhimchuk A. A., Gaevoy V. K. // J. of Nuclear Materials. 1996. Vol. 233-237. P. 11931197.
4. Embrittlement of engineering alloys/Ed. by C. L. Briant, S. K. Banerji. N.Y., 1983.
5. Caskey G. R. // Fusion Technology. 1985. Vol.8, No. 2. P. 2293-2298.
6. Robinson S. L., Moody N. R. // J. of Nuclear Materials. 1986. Vol. 140. P. 245-251.
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