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2HYDROGEN BEHAVIOR IN STAINLESS STEEL
UNDER RADIATION
I. P. Chernov, Yu. P. Cherdantsev, A. M. Lider, N. N. Nikitenkov, Yu. I. Tyurin, <t A. V. Panin, G. V. Garanin, O. V. Boyarinov, Yu. V. Martynenko1, M. I. Guseva1, § S. N. Korshunov1, M. Kroening2, A. S. Surkov2
CD
u Tomsk Polytechnic University
I Lenin Ave. 30, Tomsk, 634050 Russia
^ 1 RRC Kurchatov Institute
-M Kurchatov sq. 1, Moscow, 123182, Russia 8 2 Fraunhofer Institut Zerstorungsfreie Prufverfahren
g Saarbrucken, Germany
The dynamics of accumulation of hydrogen and helium in austenite stainless steel (12Crl8NilOTi, 03Cr17Ni12Mo2), in ferrite steel (lCrl3MoTi) and in ferrite-martensite steel (5Crl2Ni2Mo) have investigated. Hydrogen migration under effect of heavy ions, helium, electrons and X-rays quantums is researched. It was determined, that: ionizing radiation (electron and ion beams, X-rays) of metals results in intensive hydrogen migration caused by metal-hydrogen system electron excitation, which has lifetime long enough for hydrogen non-equilibrium migration; hydrogen solved in metal decreases helium accumulation at implantation; steel strength and plasticity decrease at hydrogen accumulation.
Introduction
Nuclear power plants materials, as well as materials of containers for storage and transport of radioactive materials should correspond to extreme condition to ensure radiation safety. One of the most important reasons of mechanical and technological properties deterioration is hydrogen and helium accumulation, because it results in embrittle-ment and decrease of constructing materials plasticity, especially in the region of welding joints [1].
The main object of the work is to clarify hydrogen and helium embittlement mechanism on the base of the new knowledge about metal-hydrogen-helium system under ionizing radiation (X-rays, electrons, ions and y-radiation) received by authors [2]. Hydrogen, occupying the regular positions in the metal, forms self-subsystem. The energy introduced by radiation is accumulated by hydrogen subsystem. As a result hydrogen atoms receive an additional energy and begin to migrate and release from the metal at low temperature. Hydrogen movement stimulates defects and additive atoms (helium included) diffusion. This results in material defect structure reordering.
We discuss here the role of the mentioned effect, as well as steel mechanical properties change under radiation.
Investigation methods
Hydrogen volume content in steel was determined by thermo stimulate gas release. The same method was used for investigation of hydrogen behavior under electron and x-ray radiation. Hy-
drogen isotopes and helium distribution in material was studied by elastic recoils analysis. Nitrogen ions with energy 12-16 MeV and 2.4 MeV helium ions were used for the analysis. The method gives the information about hydrogen diffusion during analyzing beam irradiation and can be used as a simulation of fission fragment radiation. Defects structure and concentration were studied by electron-positron annihilation. The average positron lifetime is 109 ps and corresponds to known date, whereas the average lifetime in the samples with hydrogen is 120 ps. It shows the sensitivity of the method to the defects created at hydrogen saturation.
The damage and stress in steel arised at hydrogen saturation were determined also by sound velocity. Sound velocity measurement was performed by pulse auto circulation with Rayleigh waves use. The accuracy of sound velocity measurement was better than 0,01 %. Ultrasonic wave penetration depth is 1.5-2 mm.
Hydrogen and radiation affect on steel mechanical properties was investigated by test machines ComTen 95T and Schenck Sinus-100.
A number samples sets were prepared for investigation with the sizes form 30x3x2 mm3 up to 120x10x3 mm3. The samples were mechanically and electrochemically polished and annealed in vacuum (10-5 T) at temperature from 550 up to 1000 °C for 60 min with following cooling in oven.
Some of the samples were tensile deformed with the velocity ~0,005 mm/s. Electrolytic hydrogen saturation was performed in 1 M alkali solution LiOH+H2O or in acid solution H2SO4+H2O. A
Доклад на Первом Всемирном конгрессе «Альтернативная энергетика и экология» WCAEE-2006, 21—25 августа 2006 г., Волга, Россия.
Paper at the First International Congress "Alternative energy and ecology" WCAEE-2006, August 21—25, Volga, Russia.
part of samples were irradiated by electrons, ions and X-rays beams.
Hydrogen and defects accumulation dynamic in stainless steel
Hydrogen and defects accumulation dynamic was studied in stainless steel AISI316 (03Cr17Ni12Mo2). The hydrogen volume distribution was measured by thermo stimulated desorption. Defects were studied by positron annihilation and by acoustic methods.
Several samples sets were studied after exposure at room temperature for more than a day to remove the rapid relaxation processes such as hydrogen diffusion and residual stress release. The Fig. 1 shows hydrogen accumulation (a), sound velocity (b) and positron lifetime (c) as functions of titanium saturation time. All the shown characteristics grow with saturation time. Positron lifetime grows due to large defects number increase at hydrogen saturation. Sound velocity increase vs is caused by elastic modulus increase at hydrogen saturation. Sound velocity measured after a long exposition after hydrogen saturation differs from that measured within a short (10 min) stop of saturation. Thus we confirm the influence of relaxation processes on the metal-hydrogen system properties.
Hydrogen affect on steel mechanical properties
Hydrogen affects mechanical properties were investigated for steel 03Cr17Ni12Mo2. Tensile deformation diagrams for various saturation times in 1M solution of LiOH+H2O (J = 0,5 A/cm2) are shown in the Fig. 2. Strength and plasticity decrease occurs at hydrogen saturation (Fig. 3 a, b). Authors of [3] received the same results for the hydrogen saturate
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Fig. 2. Strain diagrams for steel 03Cr17Ni12Mo2 samples with different hydrogen content
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Fig. 1. Dependencies of Hydrogen content (a), sound velocity (b), positron lifetime (c) on hydrogen saturation time
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Fig. 3. The dependence of hydrogen on mechanical properties: a) breaking point; b) unit elongation
samples both at active tension and at long static load. For iron mono crystal the hydrogen affect on stress-deformation curve is low. It means that steel properties change at hydrogen saturation is determined by grain boundary mechanisms.
Hydrogen and plastic deformation affect on stainless steel defects was investigated by positron annihilation. At tensile deformation positron mean lifetime increases. It shows the defect number increase (Fig. 4). Kind of the defects depends on the
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sequence of deformation-hydrogen saturation processes. At tensile stress of hydrogen saturated sample hydrogen being localized in porous stimulates pores grows. Hydrogen introduced after deformation does not influence on the defects growth. In this case hydrogen is trapped by already existing defects.
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Fig. 4. Positron mean lifetime dependence on samples relative elongation
Steel mechanical properties under hydrogen ion irradiation
Samples deformation at a constant tensile stress was measured as a function of hydrogen ion radiation time. Creep of steel loaded up to yield point initiated by hydrogen ion irradiation was observed. Steel 18-10 samples with sizes 25x5x0,3 mm were studied. Samples were annealed at 10-3 Pa, 1170 K for one hour with the following cooling in oven. Samples thickness has more than 30 grains. Standard mechanical and electrolytic samples polishing were used. The samples were irradiated by mono energetic ions 15-keVH2+ up to dose 1022 m-2 at current density 0,6 A/m2 in accelerator ILU [4].
One axis tension during the ion irradiation was used for creep investigation. For this purpose a special device was designed and placed within the accelerator ILU. Current through the samples performed the heating. The temperature was constant during the irradiation and varies in the range 570-770 K for the aim of investigation. The temperature difference along the sample was below 10 K. The tension value was 300 MPa.
Experiment sequence was the following. The sample was heated up to the required temperature, was sustained for an hour, than it was loaded up to total elastic-plastic deformation g = 0.5%. The tension for steel samples 300 MPa independent of temperature. Then for an hour elongation was measured. After that H2+ ion irradiation starts.
Isothermal creep curves for steel 18-10 for different temperatures are shown in the figure 5. After a latent period the deformation growth under hydrogen ion irradiation is observed. The latent period decreases with the temperature increase.
Fig. 5. Creep curves for steel 18-10 during H2+ ion irradiation at various temperatures
Affect of hydrogen ion irradiation is the most evident at the temperatures when there is not thermal creep of steel samples (curves 2-4). At the temperature of thermal creep, T = 770 K, hydrogen ion irradiation results in a fast creep rate. Below T = 570 K the effect of radiation induced creep was not observed. Deformation measurements under hydrogen ion irradiation show that the creep curves ware wave like with strengthening followed after s weakening. Some curves show saturation. It means that the hydrogen ion irradiation affects on steel 18-10 residual deformation only.
Model of metal creep initiated by hydrogen ion irradiation
Explanation of metal creep observed during hydrogen ion irradiation is based on the following assumptions:
1. The observed effect is a direct result of hydrogen affect and don't relate to radiation defects introduced during ion irradiation. In our experiment the ratio of damaged layer depth to the sample thickness is 10-3, and the maximal depth of possible radiation defects influence is limited within one-grain size that is one order of magnitude lower than sample thickness.
2. At the temperatures of the investigation the creep is determined by inter grain mechanism. It follows from the hydrogen mobility estimation made below.
3. Hydrogen accumulation on the grain boundaries results in weakening of the boundaries and removing of the sliding obstacles.
We assume that hydrogen is localized both within grain and on the grain boundary, hydrogen being capable transit from grain boundary to the grains in a time t and back from the grain to the grain boundary in the time (D • r'2 • C)"1, where r is the grain size, C is hydrogen concentration within the grain. Effective diffusion coefficient through the sample is
Deff = Db D• r2• t/(1 + D• r2• t) « DDbT/ r2.
Hence Deff increases and the latent time decreases with grain size decrease. The other requirement for the creep is that total hydrogen con-
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centration in the sample should be higher than a critical concentration evaluated on the base of requirement of hydrogen clusters formation on the grain boundary
N> Ncr = r/(D-Db-x2).
Creep latent time tL is determined by hydrogen diffusion time through the sample (tL ~ L2, L is the sample thickness) or by time when hydrogen concentration reaches the critical concentration Ncr ~ r. Thus, creep latent time increase with samples thickness and grain sizes growth and decreases with temperature increase (it is e squence of temperature dependences of D, Db and t.
The proposed model explains the hydrogen ion induced creep by hydrogen accumulation on the grain boundaries. The model explains the temperature dependence of the effect and predicts effect increase with grain size decrease.
Hydrogen affect on helium diffusion
Hydrogen affect on helium diffusion in steel was investigated by elastic recoils analysis. The samples were prepared from cold rolled stainless steel with the thickness 1-2 mm and the sizes 10x30 mm. The samples were mechanically and electrochemically polished. Than a part of the samples were annealed in vacuum or irradiated by Ar ions. Hydrogen concentration in surface layer of non annealed samples was 1019-1020 at/cM3. Then the samples were irradiated by 30 keV He+ ions with current density j = 10...50 mA/cm2. Investigation shows that helium accumulation at temperature below 100 °C in non annealed samples is much lower than in annealed samples (Fig. 6). Helium ion radiation stimulates hydrogen migration and hydrogen migration stimulates helium diffusion.
on irradiation intensity, hydrogen release from the total sample at room temperature at narrow electron beam irradiation, when electron beam diameter is much lower than the samples size, atom hydrogen release from metals, whereas at heating only molecular hydrogen releases.
To receive reproducible results the samples were annealed in vacuum before electrolytic deuterium saturation. The samples surface was cleaned in UHF H+H2 discharge. Deuterium release was stimulated by 10-100 keV electron beam. The Fig. 7. shows deuterium release intensity from stainless steel under 20 keV electron beam radiation. The temperature of face sample side was ~40 °C and that on backside were not higher than 60 °C. At such temperature without electron beam deuterium release is negligible. Deuterium average release rate grows super linear with beam intensity increase (Fig. 8).
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Time, s
Fig. 7. Deuterium release intensity from stainless steel under 20 keV electron beam radiation
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Thickness, ^m
Fig. 6. Hydrogen (-) and helium ( - - - ) distributions
in surface layer of steel 12Cr18Ni10Ti, implanted by 30 keV He+ with dose 51016cm-2 at temperature: 1, 6 — 293 K; 2, 5 — 673 K; 3, 4 — 723 K
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Fig. 8. Deuterium release intensity from stainless steel under 20 keV electron beam radiation as a function of electron beam current density.
Hydrogen non-equilibrium migration stimulated by ionizing irradiation
There are a number experimental evidences if hydrogen subsystem excitation by ionizing irradiation: hydrogen release from metals at room temperature and below, super linear dependencies hydrogen release rate on hydrogen concentration and
Besides mass spectral measurements of deuterium release from metals were performed at linear temperature increase and simultaneous electron beam radiation. The fig. 9 shows comparison of deuterium release from stainless steel, Ti and Pd. Gas release has maximum for stainless steel at 180 °C without electron beam and at 80 °C with electron beam; for Ti — 475 and 350 °C correspondingly and for
Pd — 150 and 85 °C (Fig. 9). Electron beam stimulation is the most evident for stainless steel.
For investigation of x-ray affect on deuterium release from 12Cr18Ni10Ti stainless steel the samples were prepared the same way as for investigation of electron radiation affect. Electrolytic deuterium saturation was performed from D2O solution at current density j = 130 |A/cm2 for 2 hours. X-rays radiation with power 105R/h was performed for one our. X-ray radiation decreases thermo gas release peak intensity 2-3 times (Fig. 10). Relative peak decrease grows with deuterium concentration in steel increase.
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Fig. 9. Deuterium thermo release from stainless steel at linear temperature increase 0.4 K/s: 1 — without electron beam; 2 — with electron beam
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The Fig. 11 shows typical dependences of hydrogen concentration in austenite steel (12Cr18Ni10Ti) on nitrogen ion dose. Hydrogen surface concentration in He implanted samples with dose samples decreases with dose increase. This decrease is the most evident in sample non irradiated by helium. Helium concentration does not change under nitrogen ion irradiation. Hydrogen behavior in ferrite and in ferrite-mar-tensite steel is the same.
Hence intensive hydrogen migration under X-rays, electron and ion radiation was shown.
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Fig. 11. Helium implantation affect on hydrogen concentration stainless steel
Ionizing radiation affect on defects in steel
Four sets of steel 316 samples were prepared for investigation of ionizing radiation affect on defects in steel. The first set was plastic deformed, the second set after plastic deformation was x-ray irradiated for 40 s with the dose 70 R/h, the third set after plastic deformation was saturated by hydrogen for 4 hours with current density 0.1 A/cm2 in LiOH electrolyte. The fourth set after the treatment like third set was X-ray radiated. Positron annihilation was use to study defects. The Fig. 12 shows average positron lifetime for stainless steel dependence on deformation at deferent treatments.
irradiation; 2 — with x-ray radiation.
Hydrogen migration under heavy ion irradiation
Hydrogen migration in austenite steel (12Cr18Ni10Ti), in ferrite steel (1Cr13MoTi) and in ferrite-martensite steel (5Cr12Ni2Mo) under 12 MeV 14N ion bombardment was investigated by ERDA with the same ions used for analysis. Nitrogen ion radiation simulates neutron and fission fragments radiation damage. Initial hydrogen concentration in surface layer of the samples was about 51019 at/cm3. Ion current density was j = 0,03... 0,08 mA, radiation time. A part of the samples was implanted by He ions to study helium introduced defects affect on hydrogen migration.
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Fig. 10. D2 thermo release for stainless steel: 1 — without
Plastic deformation results in positron lifetime increase caused by vacancy defects number and their size growth (Fig. 12, curve 1). Radiation does not affects on defect structure at low deformation but at at deformation >2 % results in positron lifetime increase (Fig. 12, curve 2). Steel saturation by hydrogen leads to positron lifetime decrease. It is caused by hydrogen localization at the defects and makes them hidden (but not removes the defects). Radiation of hydrogen-saturated samples after plastic deformation decreases considerably positron lifetime (Fig. 12, curve 4). It is a result defect annihilation initiated by hydrogen migration.
Phenomenology models of radiation affect on metal-hydrogen system behavior
Conventional mechanism of radiation stimulated hydrogen migration is based on diffusion acceleration as a result of additional defects accumulation.
Low energy electrons and X-rays do not create new defects. However experiments show acceleration of hydrogen atom migration. These kinds of radiation excite electron subsystem. Energy of electron subsystem should be transferred the hydrogen atoms. However one electron excitation lifetime in metals is only 10-15 s and energy transfer is not possible.
A possible model is the model of collective electron excitation within the track of a swift particle moving through a crystal [5]. One can assume energy transfer from one electron to the neighboring ones and a region of collective excitation with relaxation time about 10-11 s. In this case energy transfer to additive hydrogen atoms is possible.
We have the other opinion. In a perfect metal energy of radiation cannot be stored in electron degree of freedom because of very low relaxation time. However, hydrogen presence in metal was shown to lead considerable change of electron spectra. Below Fermi level and 13-15 eV above Fermi level areas of higher electron states density appear. It results in peak of energy adsorption appearance at 13 eV. Hydrogen atoms being excited can move far away from regular sites before electron system relaxation. It is possible due to very low diffusion activation energy and its quantum character. One of the de-excitation channels is energy transfer to hydrogen atom. The received energy can be transferred to the other hydrogen atoms due to oscillation exchange. Such way the energy of radiation can be transferred to the hydrogen subsystem without energy transfer to the metal atoms, in contrast to thermal heating. This decreases hydrogen-metal bounds and promotes hydrogen movement to sinks and release from metal. More detailed description of hydrogen property to absorb radiation energy is in the work [6].
Conclusion
1. Ionizing radiation (electron and ion beams, X-rays) of metals results in intensive hydrogen migration caused by metal-hydrogen system electron excitation, which has lifetime long enough for hydrogen non-equilibrium migration.
2. Hydrogen migration and release from metals under electron (below defects formation threshold) and X-rays radiation is accompanied by defect structure reordering. Hydrogen induced defects disappear due to interstitial atoms annihilation with vacancies appeared after hydrogen release.
3. Hydrogen solved in metal decreases helium accumulation at implantation. It is a result of small complexes HV and HV2 formation and as a sequence decrease of large vacancy clusters creation, which are helium traps. In hydrogen saturated metal helium is more uniform distributed within the samples and more easy releases from it as mobile complex ^V2.
4. Investigation of hydrogen accumulation dynamic shows an important role of relaxation processes in metal-hydrogen subsystem.
5. Steel strength and plasticity decrease at hydrogen accumulation.
6. Creep of steel loaded up to yield point initiated by hydrogen ion irradiation was observed. Creep begins after a latent time after hydrogen ion radiation start.
Aknowlegments
Russian authors are supported by ISTC Project No. 2864.
References
1. MorozL. S., Chechulin B. B. Hydrogen em-brittlement of metals. Moscow, Metalurgy, 1967.
2. Kroening M., Baumbach H., Tyurin Yu. I., Chernov I. P., Cherdantsev Yu. P. Metal-hydrogen non-equilibrium systems. Tomsk State University Publishing House, Tomsk, 2002.
3. Швед M. M. Изменение эксплуатационных свойств железа и стали под влиянием водорода. Киев: Наук. думка, 1985.
4. Gusev V. M., Busharov N. P., Naftulin S. M. et al. Ion accelerator ILU, 100 KeV, mass separation. / / Pribory I tekhnika eksperimenta, 1969. №4. P. 19-25; Гусев M. И., Бушаров Н. П., Нафтулин С. M. Ионный ускоритель ИЛУ на 100 кэВ с сепарацией ионов по массе // Приборы и техника эксперимента, 1969. № 4. С. 19-25.
5. Shalaev A. M. Radiation stimulated diffusion in metals. Atomizdat. 1972.
6. Chernov I. , Mamontov A., Tj yrin Y., Cherdancev Y. Hydrogen migration in stainless steel and titanum alloys, stimulation by ionizing radiation. // J. of Nucl. Mat. 1996. Vol. 233237. Р. 1118-1122.
