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Study of hydrogen influence on 1020 steel by low deformation method
Author’s Accepted Manuscript Study of hydrogen influence on 1020 steel by low deformation method B.G. Mytsyk, Ya.L. Ivanytskyi, A.I. Balitskii, Ya.P. Kost’, O.M. Sakharuk www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)31344-1 http://dx.doi.org/10.1016/j.matlet.2016.08.065 MLBLUE21349
To appear in: Materials Letters Received date: 10 February 2016 Revised date: 1 August 2016 Accepted date: 13 August 2016 Cite this article as: B.G. Mytsyk, Ya.L. Ivanytskyi, A.I. Balitskii, Ya.P. Kost’ and O.M. Sakharuk, Study of hydrogen influence on 1020 steel by low deformation method, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.08.065 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Study of hydrogen influence on 1020 steel by low deformation method B.G. Mytsyk, Ya.L. Ivanytskyi, A.I. Balitskii, Ya.P. Kost’, O.M. Sakharuk Karpenko Physico-Mechanical Institute of NAS of Ukraine; 5, Naukova str., Lviv 79601, Ukraine *
Corresponding author. Tel.: +380 322 633388; fax: +380 322 649427. [email protected]
Abstract Low mechanical deformation method was utilized to reveal effects of hydrogen plasticification of steel 1020, which do not depends on value of mechanical stress as in known mechanisms of hydrogen embrittlement, but only depends on hydrogen state in metal: diffused hydrogen plasticizes metal, and residual hydrogen, concentrated in defects, causes considerable increase of its embrittlement. “Giant” increase of elastic hysteresis amplitude during long-continued (4 months) ageing of hydrogenated steel 1020, which is indication of appropriate decrease of its cyclic durability, was detected. New conception of hydrogen plasticification and embrittlement increase in metals were formulated. It was shown that metal after hydrogen yield does not renovate its mechanical characteristics. Graphical abstract
Keywords: low mechanical stress, steel 1020, hydrogen influence, plasticity, embrittlement.
1. Introduction Hydrogen influence on metals is a subject of intensive study during recent years (see for instance, reviews in [13]). It is caused by important applicative importance of such researches.
This paper presents study of gas hydrogen influence on steel 1020 under high temperature (420 °С) and 8 MPa pressure. Its chemical composition (in %) is the following: C = 0.17-0.24; Si = 0.17-0.37; Mn = 0.35-0.65; Ni 0.25; S 0.04; P 0.04; Cr 0.25; Cu 0.25; As 0.08; Fe 98.5. This steel is a good constructional material for service under temperatures below 450 °С [4]. Hydrogen influence on this material was studied by the method with low mechanical stresses σ 0.3σy (σy – yield strength) [5]. Appropriate characteristics (elastic hysteresis amplitude ωh and residual deformations βres) were proved to be very sensitive to the material state. That is why it was predictable that mentioned ωh and βres parameters are also sensitive to the action of hydrogen, absorbed in steel 1020. Uncovered in this paper steel plasticity change, during hydrogen yield, is impossible to interpret in conventional way on the basis dislocations creation and displacement under action of high σ ≥ σy and their accumulation near barriers [1,68]. Then dislocations transport hydrogen to these barriers (defects, grain boundaries etc.). Hydrogen is being accumulated near barriers and retards dislocation displacement under action of high σ. Metal plastic deformation decreases and its embrittlement increases, correspondingly. Such general mechanism of hydrogen influence on metal is the main while interpreting increase of their hydrogen embrittlement [1,3,7,8]. Below, we will demonstrate that revealed in the given paper effects of hydrogen influence on steel under the action of low σ should be interpreted only on the basis of diffuse (migrating) hydrogen, as well as hydrogen, absorbed by “natural” defects (collectors), such as micro gaps, microcracks, non metallic inclusions, ultra microscopic cells defects (vacancies) and grain boundaries. Dislocation displacements under the action of σ ≥ σy we will exclude, as only low σ σy were used during studies.
2. Specimens and experiment methodology Specimens for the study were produced in the form of a cylinder with the inner diameter 60 mm and thin bottom ~0.9 mm (see Fig.1a). Such specimens correspond to the known model of a thin rigid round plate, pinched over the contour [5]. Experimental dependencies between mechanical stress and deformation (plate bending) were built by applying pneumatic pressure Р to the specimen. Maximum stress σ was calculated on the basis of the equation [5]
3 PR 2 PR 2 2 σ = σ σ 1 μ 0,775 2 , 4 h2 h 2 t
2 r
(1)
where σt and σr – tangential and radial mechanical stress on the plate contour correspondingly, h and R – plate thickness and radius, μ = 0.26 – Poisson's ratio of steel 1020. Dependence Рω and correspondingly, σω (here ω – displacement of plate center, which is called bending) were built with a high accuracy (~0.01%) due to use of pressure and displacement meters with relative error of Р and ω values measurement no greater than 0.01%. High accuracy of ω measurement was provided by the use of strain meter of remote action (there is no contact between strain meter and specimen). High accuracy of σω dependencies building allows detecting elastic hysteresis and residual deformations at the beginning of Hooke region (for σ 0.3σy).
2
It should be noticed that elastic hysteresis amplitudes ωk were determined under the condition of residual deformations (bending) absence. But small residual deformations ωres in steel 1020 are presented even at the beginning of Hooke region (Fig. 1b, loop 1), when σ = (0.040.3)σy. And after several (57) loading cycles N, the next cycle does not increase ωres value (then the beginning and the end of hysteresis loop coincide and ωres gets N maximum value ωres ; see Fig. 1b, loop 2) and the specimen acts as an elastic body. Exactly in this case one determines ωh value (Fig.1b, loop 2). Dependencies of residual deformations versus loading cycle’s quantity N are given in relative βres deformations which can be presented as βres = (ωres /ω)·100%, where βres – relative residual deflections, ωres – absolute residual maximum deflection under action of maximum σ value.
(2) deflections, ω –
Fig.1. Schemes of the specimen (а) and inelastic (1) and elastic (2) hysteresis (b). Specimen hydrogenation by gaseous hydrogen under pressure РH was fulfilled under the conditions: РH = 8 MPa, Т = 420 °С, τ = 4 hours (hydrogenation time). Evaluation of concentration СH of hydrogen, absorbed by steel 1020, was carried out on the basis of known Sieverts law:
CH K S PH ,
(3)
where KS – coefficient of hydrogen dissolubility in metal (Sieverts coefficient). By introducing KS = 2.63 mole/(m3 MPa ), which is specific for steel 1020 and Т = 420 С, hydrogen pressure РН = 8 MPa, into Eq. (3), we will get bulk concentration of hydrogen absorbed by specimens: СH 0.95 mppm = 0.95104 % (in mass fractions).
3. Results and their discussion 3.1. Residual deformations Figure 2 presents dependencies of specimen residual deflections βres versus quantity loading cycles N. Comparing curves А and В, one can see that βres values of hydrogenated specimen (measurements were fulfilled immediately after hydrogenation) prevail considerably over appropriate values of initial specimen (area 4 – about 35%). Thus specimen plasticity increased just after hydrogenation. 3
Considerably different metal state, namely its embrittlement increase, is demonstrated by the curve В′ (Fig. 2), which were built, when diffusive hydrogen leaves the specimen (after 2 days and Troom). Value of βres decreases 5.5 times concerning curve B (area 4) and 4 times concerning curve A (initial specimen). Thus, specimen embrittlement increased considerably. These results give two conclusions: 1) increasing of steel 1020 plasticity (thus, βres increase), immediately after hydrogenation, is caused by the diffusive hydrogen and its decohesive action [1,3,911], which leads to decrease of bound energy between crystal lattice nodes and energy of natural defects bound with crystal lattice and that is why σ action causes bigger residual deformations; 2) considerable decrease of βres value, when diffusive hydrogen has left the specimen (curve В′ on Fig. 2), in other words increase of metal embrittlement – is the result of influence of residual hydrogen concentrated in and around metal defects.
Fig.2. Examples of residual deformations (deflections) versus loading cycles quantity dependencies (at Troom): A – initial (without hydrogenation) specimen, B – hydrogenated specimen, B – after diffusive hydrogen left the specimen (time 2 days); areas 1, 2, 3 and 4 received under action of cyclic loading σ with value 12.6 (0.04σy), 37.8, 63.0 and 88.2 MPa, correspondingly.
Thus, effects of hydrogen plastification and embrittlement increase exist, without any action, of high σ and, correspondingly, without creation and displacement of dislocations, which transport hydrogen to collectors. This fact allows us to form the next conception of metal hydrogen plasticification and embrittlement increase: only hydrogen action is the reason of considerable metal plasticification at the first ageing stage (at the presence of high concentration of diffusive hydrogen in the crystal lattice) and reason of considerable metal embrittlement increase at the second ageing stage, when there is high concentration of hydrogen in both molecular and ion states (residual hydrogen), which retard defects displacements. 3.2. Elastic hysteresis
4
Figure 3 presents dependencies of elastic hysteresis amplitudes ωh versus mechanical stress value σ. For hydrogenated specimen ωh value increases on ~ 25% (curve 2) concerning ωh of the initial (without hydrogenation) specimen (curve 1). This effect is caused, as described above βres increase, by decohesive action of diffusive hydrogen. Correspondingly, defects, whose reverse displacement during σ increase and decrease, are the reason of hysteresis, at the presence of diffusive hydrogen move on greater amplitude, thus increasing ωh. But after ~2 days of ageing (diffusive hydrogen leaves) hysteresis amplitude decreases to the value ωh = 105 nm (Fig. 3, insertion), which is considerably lower than ωh of initial specimen. It is the result of specimen embrittlement increase, after diffusive hydrogen has left it (due to retard of defect displacements by residual hydrogen). It should be noted that approximately the same time of hydrogen evacuation is typical for other lowcarbon steels (for example, [12]), but for medium-carbon steels, which have lesser hydrogen diffusion coefficient, time t is higher (~ 300 hours) [13, 14].
Fig.3. Dependencies of ωh(σ) of initial (1) and hydrogenated (2) specimen (at Troom). At the insertion – temporal change of ωh for σ = 88.2 MPa (0.28 σy) after hydrogenation. Thus, residual deformation and elastic hysteresis, under the action of low σ, are sensitive indicators of hydrogen influence on steel 1020. Presence of diffusive hydrogen increases metal micro-elasticity (βres and ωh increase, correspondingly), and residual hydrogen, which is concentrated in natural defects and around them, increases its embrittlement (βres and ωh decrease, see curve В' in Fig. 2 and the insertion of Fig. 3). It should be noticed that these results were obtained by the method that utilizes low mechanical stresses σ 0.3σ y , which do not take part in the processes of hydrogen plasticification and embrittlement change, as in known methods for study of conventional mechanical characteristics of metals (for instance, ultimate strength σ u [15,16] or contraction ratio ψ [17]), when high σ σ y are acting. Low σ in this method are only the instrument for metal plasticification and embrittlement study and appropriate mechanical characteristics (βres and ωh) are sensitive to both, diffusive and residual hydrogen. 3.3. Prolonged ageing of hydrogenated metal
5
During long observation of the specimen, after diffusive hydrogen leaving, we have detected “giant” increase (4 times) of elastic hysteresis amplitude ωh (Fig.4, to the right from point А). Increase of ωh is a sign of metal plasticification. Let’s consider revealed effect on the basis of known fact of residual hydrogen removal from collectors [13]. Due to hydrogen accumulation in collectors, during metal hydrogenation and atomic hydrogen transformation into the molecular one, hydrogen pressure in collectors increases. This pressure starts collector destruction by creating micro-cracks. During long time hydrogen is leaving collectors, and micro-cracks, created in collectors, are the source of high hysteresis. It is clear that metal, dehydrogenated due to ageing, will have worse characteristics of strength and cyclic durability owing to micro-cracks presence. This fact can be the reason of known uncontrolled fracture of constructional elements, which operate in hydrogen environment.
Fig.4. Dependence of ωh (for σ = 88.2 MPa) versus ageing time t at Тroom: point А corresponds to the end of hydrogen diffusion from the specimen for time 2 days (see Fig. 3, insertion), and further increase of ωh corresponds to hydrogen leaving collectors; dark points ωh(t) dependence for initial specimen.
4. Conclusions New conception of hydrogen influence on metal plasticity was formulated on the basis of residual deformations βres and elastic hysteresis amplitudes ωh, determined by low mechanical stresses (σ 0.3σy) method, and their changes during diffusive hydrogen leaving steel 1020. This conception is the following: diffusive hydrogen, due to its’ decohesive action, increases metal plasticity (βres and ωh increase) and, residual hydrogen, concentrated in collectors and around them, increases its’ embrittlement (βres and ωh decrease). This conception does not contradict the old one (for σ > σy), but supplement it with new content: if σ is low, then hydrogen influence is the only reason of metal plasticity and embrittlement change.
6
“Giant” increase of ωh value during prolonged (4 months) ageing was detected. This effect is caused, obviously, by micro-cracks in collectors, which are being created during specimen hydrogenation. Micro-cracks in collectors worsen mechanical properties of metals, which are probably the reason of metal uncontrolled destruction in hydrogen environment.
References [1] V.V. Panasyuk, Decohesive concept of the interaction of hydrogen with metals, Materials Science 50 (2014) 161169. [2] J.W. Hanneken, Hydrogen in metals and other materials: a comprehensive reference to books, bibliographies, workshops and conferences, Int. J. Hydrogen Energy 24 (1999) 10051026. [3] Gaseous hydrogen embrittlement of materials in energy technologies. V. 2. Mechanisms, Modelling and Future Developments, R.P. Gangloff and B.P. Somerday (Eds.). Woodhead Publishing Limited, Cambridge, 2012. [4] Y.Y. Zhiguts, Technology of creation and official properties of thermit steel 20, Metallurgy 2 (2013) 4852. (in Ukrainian) [5] B.G. Mytsyk, Y.P. Kost’, N.M. Demyanyshyn, Effect of heat treatment of the D16 alloy on its mechanical properties at low stresses, Phys. Solid State 56 (2014) 2227–2232. [6] O.P. Ostash, V.I. Vytvyts’kyi, Duality of the action of hydrogen on the mechanical behavior of steels and structural optimization of their hydrogen resistance, Materials Science 47 (2012) 421437. [7] P. Novak, R. Yuan, B.P. Somerday, P. Sofronis, R.O. Ritchie, A statistical, physicalbased, micro-mechanical model of hydrogen-induced intergranular fracture in steel, J. Mechanics and Physics Solids. 58 (2010) 206–226. [8] V.V. Panasyuk, A.E. Andreikiv, V.S. Kharin, Origin and growth of microcracks produced by blocked accumulation of dislocations, Materials Science 21 (1985) 105115. [9] I.M. Robertson, The effect of hydrogen on dislocation dynamics, Eng. Fract. Mech. 68 (2001) 671692. [10] Y.A. Du, L. Ismer, J. Rogal, T. Hickel, J. Neugebauer, R.Drautz, First-principles study on the interaction of H interstitials with grain boundaries in α- and γ-Fe, Phys. Rev. B 84 (2011) 144121 (9). [11] Motomichi Koyama, Cemal Cem Tasan, Eiji Akiyama, Kaneaki Tsuzaki, Dierk Raabe, Hydrogen-assisted decohesion and localized plasticity in dual-phase steel, Acta Materialia 70 (2014) 174–187. [12] Pokhodnia I.K., Shvachko V.I., Upyr V.N., Shyjan A.V., Smiyan O.D. Hydrogen influence on the structural steels and their welding joining embitterment, Automatic Welding 5 (1989) 14. (in Russian) [13] Yan Liu, Maoqiu Wang, Guoquan Liu, Effect of hydrogen on ductility of high strength 3Ni–Cr–Mo–V steels, Mater. Scie. Engineering A 594 (2014) 40–47.
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[14] C.J. Carneiro Filho, M.B. Mansur, P.J. Modenesi, B.M. Gonzalez, The effect of hydrogen release at room temperature on the ductility of steel wire rods for pre-stressed concrete, Mater. Scie. Engineering A 527 (2010) 4947–4952. [15] I.M. Dmytrakh, R.L. Leshchak, A.M.Syrotyuk, Effect of hydrogen concentration on strain behaviour of pipeline steel, Int. J. Hydrogen Energy 40 (2015) 40114018. [16] Yihong Nie, Yuuji Kimura, Tadanobu Inoue, Fuxing Yin, Eiji Akiyama, Kaneaki Tsuzaki, Hydrogen embrittlement of a 1500-MPa tensile strength level steel with an ultrafine elongated grain structure, Metalurgical and Materials Transactions A 43 (2012) 16701687. [17] A. Balitskii, V. Vytvytskyii, L. Ivaskevich, J. Eliasz. The high- and low-cycle fatigue behavior of Ni-contain steels and Ni-alloys in high pressure hydrogen, Int. J. Fatigue 39/6 (2012) 32–37.
Highlights
Gas hydrogen influence on steel 20 was studied by low mechanical stress method
Hysteresis amplitude and residual deformations are sensitive to hydrogen action
New conception of hydrogen embrittlement increase in metals was formulated
Cyclic durability of hydrogenated metals decreases during prolonged ageing
8
PII: DOI: Reference:
S0167-577X(16)31344-1 http://dx.doi.org/10.1016/j.matlet.2016.08.065 MLBLUE21349
To appear in: Materials Letters Received date: 10 February 2016 Revised date: 1 August 2016 Accepted date: 13 August 2016 Cite this article as: B.G. Mytsyk, Ya.L. Ivanytskyi, A.I. Balitskii, Ya.P. Kost’ and O.M. Sakharuk, Study of hydrogen influence on 1020 steel by low deformation method, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.08.065 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Study of hydrogen influence on 1020 steel by low deformation method B.G. Mytsyk, Ya.L. Ivanytskyi, A.I. Balitskii, Ya.P. Kost’, O.M. Sakharuk Karpenko Physico-Mechanical Institute of NAS of Ukraine; 5, Naukova str., Lviv 79601, Ukraine *
Corresponding author. Tel.: +380 322 633388; fax: +380 322 649427. [email protected]
Abstract Low mechanical deformation method was utilized to reveal effects of hydrogen plasticification of steel 1020, which do not depends on value of mechanical stress as in known mechanisms of hydrogen embrittlement, but only depends on hydrogen state in metal: diffused hydrogen plasticizes metal, and residual hydrogen, concentrated in defects, causes considerable increase of its embrittlement. “Giant” increase of elastic hysteresis amplitude during long-continued (4 months) ageing of hydrogenated steel 1020, which is indication of appropriate decrease of its cyclic durability, was detected. New conception of hydrogen plasticification and embrittlement increase in metals were formulated. It was shown that metal after hydrogen yield does not renovate its mechanical characteristics. Graphical abstract
Keywords: low mechanical stress, steel 1020, hydrogen influence, plasticity, embrittlement.
1. Introduction Hydrogen influence on metals is a subject of intensive study during recent years (see for instance, reviews in [13]). It is caused by important applicative importance of such researches.
This paper presents study of gas hydrogen influence on steel 1020 under high temperature (420 °С) and 8 MPa pressure. Its chemical composition (in %) is the following: C = 0.17-0.24; Si = 0.17-0.37; Mn = 0.35-0.65; Ni 0.25; S 0.04; P 0.04; Cr 0.25; Cu 0.25; As 0.08; Fe 98.5. This steel is a good constructional material for service under temperatures below 450 °С [4]. Hydrogen influence on this material was studied by the method with low mechanical stresses σ 0.3σy (σy – yield strength) [5]. Appropriate characteristics (elastic hysteresis amplitude ωh and residual deformations βres) were proved to be very sensitive to the material state. That is why it was predictable that mentioned ωh and βres parameters are also sensitive to the action of hydrogen, absorbed in steel 1020. Uncovered in this paper steel plasticity change, during hydrogen yield, is impossible to interpret in conventional way on the basis dislocations creation and displacement under action of high σ ≥ σy and their accumulation near barriers [1,68]. Then dislocations transport hydrogen to these barriers (defects, grain boundaries etc.). Hydrogen is being accumulated near barriers and retards dislocation displacement under action of high σ. Metal plastic deformation decreases and its embrittlement increases, correspondingly. Such general mechanism of hydrogen influence on metal is the main while interpreting increase of their hydrogen embrittlement [1,3,7,8]. Below, we will demonstrate that revealed in the given paper effects of hydrogen influence on steel under the action of low σ should be interpreted only on the basis of diffuse (migrating) hydrogen, as well as hydrogen, absorbed by “natural” defects (collectors), such as micro gaps, microcracks, non metallic inclusions, ultra microscopic cells defects (vacancies) and grain boundaries. Dislocation displacements under the action of σ ≥ σy we will exclude, as only low σ σy were used during studies.
2. Specimens and experiment methodology Specimens for the study were produced in the form of a cylinder with the inner diameter 60 mm and thin bottom ~0.9 mm (see Fig.1a). Such specimens correspond to the known model of a thin rigid round plate, pinched over the contour [5]. Experimental dependencies between mechanical stress and deformation (plate bending) were built by applying pneumatic pressure Р to the specimen. Maximum stress σ was calculated on the basis of the equation [5]
3 PR 2 PR 2 2 σ = σ σ 1 μ 0,775 2 , 4 h2 h 2 t
2 r
(1)
where σt and σr – tangential and radial mechanical stress on the plate contour correspondingly, h and R – plate thickness and radius, μ = 0.26 – Poisson's ratio of steel 1020. Dependence Рω and correspondingly, σω (here ω – displacement of plate center, which is called bending) were built with a high accuracy (~0.01%) due to use of pressure and displacement meters with relative error of Р and ω values measurement no greater than 0.01%. High accuracy of ω measurement was provided by the use of strain meter of remote action (there is no contact between strain meter and specimen). High accuracy of σω dependencies building allows detecting elastic hysteresis and residual deformations at the beginning of Hooke region (for σ 0.3σy).
2
It should be noticed that elastic hysteresis amplitudes ωk were determined under the condition of residual deformations (bending) absence. But small residual deformations ωres in steel 1020 are presented even at the beginning of Hooke region (Fig. 1b, loop 1), when σ = (0.040.3)σy. And after several (57) loading cycles N, the next cycle does not increase ωres value (then the beginning and the end of hysteresis loop coincide and ωres gets N maximum value ωres ; see Fig. 1b, loop 2) and the specimen acts as an elastic body. Exactly in this case one determines ωh value (Fig.1b, loop 2). Dependencies of residual deformations versus loading cycle’s quantity N are given in relative βres deformations which can be presented as βres = (ωres /ω)·100%, where βres – relative residual deflections, ωres – absolute residual maximum deflection under action of maximum σ value.
(2) deflections, ω –
Fig.1. Schemes of the specimen (а) and inelastic (1) and elastic (2) hysteresis (b). Specimen hydrogenation by gaseous hydrogen under pressure РH was fulfilled under the conditions: РH = 8 MPa, Т = 420 °С, τ = 4 hours (hydrogenation time). Evaluation of concentration СH of hydrogen, absorbed by steel 1020, was carried out on the basis of known Sieverts law:
CH K S PH ,
(3)
where KS – coefficient of hydrogen dissolubility in metal (Sieverts coefficient). By introducing KS = 2.63 mole/(m3 MPa ), which is specific for steel 1020 and Т = 420 С, hydrogen pressure РН = 8 MPa, into Eq. (3), we will get bulk concentration of hydrogen absorbed by specimens: СH 0.95 mppm = 0.95104 % (in mass fractions).
3. Results and their discussion 3.1. Residual deformations Figure 2 presents dependencies of specimen residual deflections βres versus quantity loading cycles N. Comparing curves А and В, one can see that βres values of hydrogenated specimen (measurements were fulfilled immediately after hydrogenation) prevail considerably over appropriate values of initial specimen (area 4 – about 35%). Thus specimen plasticity increased just after hydrogenation. 3
Considerably different metal state, namely its embrittlement increase, is demonstrated by the curve В′ (Fig. 2), which were built, when diffusive hydrogen leaves the specimen (after 2 days and Troom). Value of βres decreases 5.5 times concerning curve B (area 4) and 4 times concerning curve A (initial specimen). Thus, specimen embrittlement increased considerably. These results give two conclusions: 1) increasing of steel 1020 plasticity (thus, βres increase), immediately after hydrogenation, is caused by the diffusive hydrogen and its decohesive action [1,3,911], which leads to decrease of bound energy between crystal lattice nodes and energy of natural defects bound with crystal lattice and that is why σ action causes bigger residual deformations; 2) considerable decrease of βres value, when diffusive hydrogen has left the specimen (curve В′ on Fig. 2), in other words increase of metal embrittlement – is the result of influence of residual hydrogen concentrated in and around metal defects.
Fig.2. Examples of residual deformations (deflections) versus loading cycles quantity dependencies (at Troom): A – initial (without hydrogenation) specimen, B – hydrogenated specimen, B – after diffusive hydrogen left the specimen (time 2 days); areas 1, 2, 3 and 4 received under action of cyclic loading σ with value 12.6 (0.04σy), 37.8, 63.0 and 88.2 MPa, correspondingly.
Thus, effects of hydrogen plastification and embrittlement increase exist, without any action, of high σ and, correspondingly, without creation and displacement of dislocations, which transport hydrogen to collectors. This fact allows us to form the next conception of metal hydrogen plasticification and embrittlement increase: only hydrogen action is the reason of considerable metal plasticification at the first ageing stage (at the presence of high concentration of diffusive hydrogen in the crystal lattice) and reason of considerable metal embrittlement increase at the second ageing stage, when there is high concentration of hydrogen in both molecular and ion states (residual hydrogen), which retard defects displacements. 3.2. Elastic hysteresis
4
Figure 3 presents dependencies of elastic hysteresis amplitudes ωh versus mechanical stress value σ. For hydrogenated specimen ωh value increases on ~ 25% (curve 2) concerning ωh of the initial (without hydrogenation) specimen (curve 1). This effect is caused, as described above βres increase, by decohesive action of diffusive hydrogen. Correspondingly, defects, whose reverse displacement during σ increase and decrease, are the reason of hysteresis, at the presence of diffusive hydrogen move on greater amplitude, thus increasing ωh. But after ~2 days of ageing (diffusive hydrogen leaves) hysteresis amplitude decreases to the value ωh = 105 nm (Fig. 3, insertion), which is considerably lower than ωh of initial specimen. It is the result of specimen embrittlement increase, after diffusive hydrogen has left it (due to retard of defect displacements by residual hydrogen). It should be noted that approximately the same time of hydrogen evacuation is typical for other lowcarbon steels (for example, [12]), but for medium-carbon steels, which have lesser hydrogen diffusion coefficient, time t is higher (~ 300 hours) [13, 14].
Fig.3. Dependencies of ωh(σ) of initial (1) and hydrogenated (2) specimen (at Troom). At the insertion – temporal change of ωh for σ = 88.2 MPa (0.28 σy) after hydrogenation. Thus, residual deformation and elastic hysteresis, under the action of low σ, are sensitive indicators of hydrogen influence on steel 1020. Presence of diffusive hydrogen increases metal micro-elasticity (βres and ωh increase, correspondingly), and residual hydrogen, which is concentrated in natural defects and around them, increases its embrittlement (βres and ωh decrease, see curve В' in Fig. 2 and the insertion of Fig. 3). It should be noticed that these results were obtained by the method that utilizes low mechanical stresses σ 0.3σ y , which do not take part in the processes of hydrogen plasticification and embrittlement change, as in known methods for study of conventional mechanical characteristics of metals (for instance, ultimate strength σ u [15,16] or contraction ratio ψ [17]), when high σ σ y are acting. Low σ in this method are only the instrument for metal plasticification and embrittlement study and appropriate mechanical characteristics (βres and ωh) are sensitive to both, diffusive and residual hydrogen. 3.3. Prolonged ageing of hydrogenated metal
5
During long observation of the specimen, after diffusive hydrogen leaving, we have detected “giant” increase (4 times) of elastic hysteresis amplitude ωh (Fig.4, to the right from point А). Increase of ωh is a sign of metal plasticification. Let’s consider revealed effect on the basis of known fact of residual hydrogen removal from collectors [13]. Due to hydrogen accumulation in collectors, during metal hydrogenation and atomic hydrogen transformation into the molecular one, hydrogen pressure in collectors increases. This pressure starts collector destruction by creating micro-cracks. During long time hydrogen is leaving collectors, and micro-cracks, created in collectors, are the source of high hysteresis. It is clear that metal, dehydrogenated due to ageing, will have worse characteristics of strength and cyclic durability owing to micro-cracks presence. This fact can be the reason of known uncontrolled fracture of constructional elements, which operate in hydrogen environment.
Fig.4. Dependence of ωh (for σ = 88.2 MPa) versus ageing time t at Тroom: point А corresponds to the end of hydrogen diffusion from the specimen for time 2 days (see Fig. 3, insertion), and further increase of ωh corresponds to hydrogen leaving collectors; dark points ωh(t) dependence for initial specimen.
4. Conclusions New conception of hydrogen influence on metal plasticity was formulated on the basis of residual deformations βres and elastic hysteresis amplitudes ωh, determined by low mechanical stresses (σ 0.3σy) method, and their changes during diffusive hydrogen leaving steel 1020. This conception is the following: diffusive hydrogen, due to its’ decohesive action, increases metal plasticity (βres and ωh increase) and, residual hydrogen, concentrated in collectors and around them, increases its’ embrittlement (βres and ωh decrease). This conception does not contradict the old one (for σ > σy), but supplement it with new content: if σ is low, then hydrogen influence is the only reason of metal plasticity and embrittlement change.
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“Giant” increase of ωh value during prolonged (4 months) ageing was detected. This effect is caused, obviously, by micro-cracks in collectors, which are being created during specimen hydrogenation. Micro-cracks in collectors worsen mechanical properties of metals, which are probably the reason of metal uncontrolled destruction in hydrogen environment.
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Highlights
Gas hydrogen influence on steel 20 was studied by low mechanical stress method
Hysteresis amplitude and residual deformations are sensitive to hydrogen action
New conception of hydrogen embrittlement increase in metals was formulated
Cyclic durability of hydrogenated metals decreases during prolonged ageing
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