HYDROGEN-INDUCED COLD CRACKS IN WELDED JOINTS OF … · hydrogen-induced cold cracks in welded joints of high-strength low-alloyed steels (review) i.k. pokhodnya, a.v. ignatenko,

HYDROGEN-INDUCED COLD CRACKS IN WELDEDJOINTS OF HIGH-STRENGTH LOW-ALLOYED STEELS

(Review)

I.K. POKHODNYA, A.V. IGNATENKO, A.P. PALTSEVICH and V.S. SINYUKE.O. Paton Electric Welding Institute, NASU

11 Bozhenko Str., 03680, Kiev, Ukraine. E-mail: [email protected]

Possibility of development of hydrogen-induced cold cracks in welded joint depends on series of intercon-nected and complex physical phenomena. Work represents a short review of investigations carried out inthe E.O. Paton Electric Welding Institute on study of processes of hydrogen absorption by metal, itsdiffusion in the welded joint considering kinetics of temperature gradient, hydrogen traps and residualstresses. Peculiarities of hydrogen diffusion in strain-free and plastically deformed metal were studied byexperiment-calculated methods. Results of experiment-calculation investigations and mathematical model-ling of mechanisms of generation and growth of hydrogen-induced cracks in welded joints on micro- andmacrolevel are stated. It is shown with high reliability that interaction of hydrogen with dislocations makesthe basis of mechanism of hydrogen embrittlement. Hydrogen influences nucleation and growth of mi-crocracks in metal making coalescence of dislocations easier that result in localizing of plastic strain undereffect of hydrogen. As it is showed by computer modelling of development of microdefects in metal, thereduction of grain size, at other factors being equal, increases metal sensitivity to negative influence ofhydrogen. Mechanism of crack growth in metal containing hydrogen is proposed considering effect ofhydrogen-enhanced localized plasticity. 64 Ref., 12 Figures.

K e y w o r d s : brittle fracture, hydrogen embrittle-ment model, hydrogen-enhanced localized plasticity, re-sidual stresses, BCC metals, grain size, hydrogen diffu-sion

Steel is one the most effective structural materi-als. More than 20 bln t of steel are used in dif-ferent parts and structures in present time.1550 mln t of steel were manufactured in theworld in 2012, and in further 40 years volumeof production would hypothetically increase by50—100 % [1]. At that welding remains one ofthe most widespread methods for obtaining ofpermanent metal joints. As far as the require-ments to strength of source materials and qualityof obtained joints constantly rise in a course oftime , it could be supposed that problems existingin development of more safe and long-termwelded joints would be relevant and economicimportance of scientific investigations directedon solution of given problems would permanentlyincrease.

High-strength low-alloyed (HSLA) steels dueto increasing requirements to service charac-teristics of welded structures are used in machinebuilding, construction engineering, shipbuildingand pipeline construction. There is a risk of ap-pearance of cold cracks in zone of welded jointin welding of HSLA steels. It was determinedthat one of the main factors promoting formation

of cold cracks is hydrogen absorbed by liquidmetal from arc plasma. The results of carried outexperimental and theoretical investigations showthat hydrogen-induced cold cracks (HICC) arethe consequence of more general physical effect,i.e. hydrogen embrittlement (HE) under specificconditions of thermal-deformation welding cycle[2].

Possibility of development of HICC in thewelded joint depends on series of interconnectedand complex physical phenomena. For theirstudy the processes of hydrogen absorption bymetal, diffusion of hydrogen in welded joint al-lowing for kinetics of temperature gradient, hy-drogen traps and residual stresses were investi-gated as well as mathematical modeling of mecha-nisms of formation and growth of HICC on micro-and macrolevel was performed, and experiment-calculated investigations of mechanisms of HICCformation in welded joints were carried out atthe E.O. Paton Electric Welding Institute.

Forms of hydrogen in iron and steels. Char-acteristics and state of hydrogen in metal volumeare to be known for deep understanding of HEmechanism. Experimentally stated that processof hydrogen dissolution in iron follows theSiverts’ law, i.e. under thermodynamic equilib-rium a concentration of dissolved hydrogen isdirectly proportional to square root of value of

© I.K. POKHODNYA, A.V. IGNATENKO, A.P. PALTSEVICH and V.S. SINYUK, 2013

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its pressure in gas phase. This means that hydro-gen like other biatomic gases is absorbed in formof atoms.

In spite of the fact that atomic hydrogen isthe simplest substance soluble in iron, corre-sponding methods of calculation of its state basedon different theories have not yet been developed.It is assumed that dissolved hydrogen can be inseveral forms, i.e. protonic, anionic and atomic.Occurrence of metallic bonding characterizingby sharing of valence electrons of iron and hy-drogen electron takes place between atoms inmetal during formation of protonic form. How-ever, experimental observations of directedmovement of hydrogen under effect of electricfield do not provide precise answer to questionabout charge state of hydrogen in metal [3, 4].The effect of transfer will be determined by par-ticle charge [3] only in absence of interactionbetween the particles of migrating componentand charge carrier in metal.

Quantum mechanical calculations were usedfor analyzing of possibility of existence of differ-ent forms of charge state of hydrogen dependingon parameters of electron interaction in «hydro-gen—metal» system [5]. It was concluded basedon obtained results that protons H+, neutral at-oms H0 and negative ions H— can be present inmetal simultaneously with different possibilities.In authors’ opinion, the main question lies in thefact in what condition the hydrogen will havethe maximum effect on physico-mechanical prop-erties. In V.I. Shvacko opinion [6] this conclu-sion causes doubts on alternative problem formu-lation about charge state of hydrogen in metal,but does not clear the situation itself. Conclusionabout necessity to concentrate on determinationof the most active form of hydrogen virtuallymeans return to initial statement of problem sinceproblem about charge state of hydrogen has ap-peared exactly from the necessity of determina-tion of mechanism of its abnormal effect on metalproperties.

Calculating electron structure of iron—hydro-gen FCC system, the authors [7] came to theconclusion that density of valence electrons in-creases near hydrogen atom, i.e. negative chargeis concentrated around hydrogen atoms. Calcu-lations also showed from point of view of electrontheory of metals that density of free electronsaround the atoms of hydrogen being present indislocations increases as well. This results in in-crease of dislocation mobility and reduction ofdistance between them in dislocation cluster [8].

It was shown using method of secondary ionicemission that hydrogen diffusing from metal

depth has negatively charged state on the surface[9, 10].

Model, according to which atoms of hydrogenintroduced in metal lattice are localized in voidof that or another type and make oscillatory mo-tions near equilibrium position in accordance toatomic structure of solid solution, obtained widedistribution in authors’ opinion [11]. Potentialholes for atoms of hydrogen located in quasi equi-librium conditions are sufficiently deep (againsttheir average kinetics energy). Such a model,apparently, describes the most significant pecu-liarities of the solution, i.e. in such form it pro-vides the possibility accurately, by order of mag-nitude, calculate diffusion coefficients and ex-plain the reasons of their exponential dependenceon temperature, that is approved by direct ex-periments.

Hydrogen absorption. Processes of absorp-tion of gases by electrode metal and weld pooldevelop significantly in welding of steels underconditions of high temperatures of arc dischargeand high rates of heating and cooling of metal.Increase of hydrogen concentration in metal risesa risk of initiation of cold cracks in welded jointand, as a result, failure of whole welded structure[2, 12]. Thus, one of the main solutions of prob-lem of HICC prevention is a fundamental inves-tigation of hydrogen behavior in welding, search-ing of ways of reduction of its content in theweld metal and development of new welding con-sumables based on obtained results.

Content of H2 and H2O in arc atmosphere,metal temperature, presence of layer of slag andits properties, kinetics of electrode melting andtransfer of electrode metal in the weld pool aremainly used for determination of hydrogen ab-sorption by molten metal. High partial pressureof molecular and atomic hydrogen in arc gap aswell as temperature of liquid metal at the end ofelectrode and weld pool provide for intensivehydrogen absorption. Experimentally showedthat high rate of cooling of drops of electrodemetal allows registering high contents of hydro-gen [13].

Performance of experimental investigationson interaction of hydrogen with metal at the endof electrode and weld pool under conditions ofarc welding is significantly complicated due tohigh temperatures, dissociation and ionized stateof gases and, as a result, high reaction speeds[14—17]. Therefore, physical and mathematicalmodels of processes, based on ideas about exist-ence of local thermodynamic equilibrium in arccolumn, were developed and applied for investi-gation of processes of hydrogen absorption by

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metal in arc welding. Mathematical model ofprocess of hydrogen absorption by metal pro-posed in [14—16] is based on system of equationsof gas dynamics and equation, which describesmolecular interaction in thin Knudsen layer ad-jacent to metal surface, as well as equation ofhydrogen mass transfer in metal. Evaporation ofmetal from drop surface was considered in cal-culation of absorption of hydrogen by drop ofelectrode metal. It is shown that reduction of arctemperature increases efficiency of hydrogenbonding by fluorine and oxygen. It was deter-mined that effect of abnormal absorption of hy-drogen by metal interacting with plasma of arcdischarge is defined by degree of molecular dis-sociation in plasma volume, which depends onenergy of molecular dissociation and temperatureof plasma, and not by absorption of acceleratedcharged particles.

Maximum solubility in contact of iron withhydrogen under conditions of thermodynamicequilibrium is observed at T = 2600 K. Furtherincrease of temperature results in reduction ofhydrogen solubility caused by intensive ironevaporation. Calculation, proved by experiment,showed that absorption of hydrogen from plasmaof arc discharge multiply exceeds (more than 10times) absorption under equilibrium conditionsat T = 2000 K, and is determined by degree ofhydrogen dissociation depending on arc tempera-ture. Also, monotonous reduction of content ofhydrogen in iron is observed with increase of itstemperature due to iron evaporation in contactwith Ar + H2 plasma.

Entering of fluorine compounds in composi-tion of welding consumables is one of the mostefficient methods of reduction of hydrogen ab-sorption by liquid metal. Thermodynamic analy-sis of behavior of HF in arc zone was carried outin work [18]. HF and OH are completely disso-ciated in accordance with radial distribution oftemperature in central high-temperature regionat T = 6200 K. HF is not dissociated at columnperiphery (2500 K) that shows the possibility ofhydrogen bonding by fluorine in arc zone.

Thermodynamic approach was also used foranalysis of processes of bonding of hydrogen be-ing in gas phase in a form of water vapors (atP = 1⋅105 Pa pressure) by slags of TiO2—CaO—CaF2, Al2O3—CaO—CaF2, SiO2—CaO—CaF2 sys-tem. It is shown that minimum content of hy-drogen in metal is typical for TiO2—CaO—CaF2and Al2O3—CaO—CaF2 slag systems dependingon CaO content in slag melt [19, 20]. In Al2O3—CaO—CaF2 system weight fraction of CaO hasno influence on hydrogen content in metal. Ad-

dition of SiF4 in gas phase has effective influenceon decrease of hydrogen content in metal due toreaction of HF formation and reduction of partialpressure of hydrogen.

Presence of mainly fluorosilicate compoundsand anhydrous hydrogen fluoride was detectedby mass-spectrometric investigations [21] of fluo-rides included in gases emitted from arc zone inwelding by flux-cored wires which contain CaF2,SiO2, MgO, CaO, ZrO2. Composition of forminggases [22] was investigated in process of arc heat-ing of CaF2 specimen at T = 147 K. HF, whichwas formed at interaction of CaF2 and residualvapors of water in mass spectrometer, was foundin mass spectrum of gases in both experiments.

Accurate data on concentration of hydrogenin weld metal is necessary for development ofefficient measures of reduction of hydrogen ab-sorption by liquid metal, control of quality ofwelding consumables, prevention of formation ofcold cracks and pores. Mercury method based onISO 3690:2000(E) is a widely used method ap-plying eudiometers. Developed method of analy-sis of diffusion hydrogen with chromatographicending provides the possibility of measurementof volumes of hydrogen emitted from specimenwith high accuracy and sensitivity, as well asallow accelerating measurement of quantity ofhydrogen due to specimen heating 30—50 times.Chromatographic method is introduced in GOST23338—91.

Investigations of effect technological factorsof welding on hydrogen quantity in the weldmetal [23, 24] were carried out using developedmethod. It is determined that changes of weldingspeed and value of welding current for coatedelectrodes do not virtually change concentrationof hydrogen in the weld metal, but at the sametime, content of hydrogen related to depositedmetal significantly rises with increase of weldingspeed. Thus, a conclusion can be made that acontent of hydrogen in molten weld metal is nec-essary to be determined for accurate estimationof diffusion hydrogen. Average concentration ofhydrogen in multilayer deposited metal does notexceed content of hydrogen in single-run weldmetal.

One of the main methods for reduction ofhydrogen content in weld metal is preliminaryheat treatment of welding consumables, at whichpart of hydrogen in a form of H2O is removed.Allowable temperatures of baking for coated elec-trodes make 400—450 °C, and for flux-cored wiresare 250—270 °C and they do not allow removingall moisture present in components of welding

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consumables. Temperature dependence of re-moval of H2O and H2 (up to 1000 °C) fromgas-slag-forming and alloying components wasinvestigated using thermal desorption analysis[25] which allowed determining methods of theirtreatment for reduction of level of potential hy-drogen in welding consumables. Application ofheat-treated components in composition of thecoated electrodes provided obtaining of ex-tremely low concentrations of diffusion hydrogen(1.0—1.5 ml/100 g) in the weld metal [26].

Hydrogen diffusion. Redistribution of hydro-gen in metal of the welded joint takes place afterits absorption by weld pool. Field of hydrogenconcentration in the joint is necessary to beknown for determining the zones of metal ofwelded joint susceptible to the highest risk ofHICC initiation. Thus, nonstationary problemof hydrogen diffusion considering thermal-defor-mation cycle of welding, structural transforma-tions and hydrogen traps [2] is solved in a generalcase.

Moving force of diffusion is a gradient ofchemical potential, the value of which dependson hydrogen solubility in metal, diffusion coef-ficient, gradients of concentration and tempera-ture, stressed state and plastic deformations [27].At that, thermal-deformation cycle of weldingresults in significant inhomogeniety of hydrogenconcentration field.

Diffusion of hydrogen in the weld metal andredistribution of hydrogen in welded joint afterwelding were studied experimentally and usingmathematic modelling. Experiments on kineticsof removal of hydrogen, being absorbed in processof welding, from cylinder specimens of the weldmetal were carried out, and dependence of speedof degassing V on quantity of hydrogen Q inspecimen V(Q) were obtained for determinationof value of effective coefficient of hydrogen dif-fusion DH. DH was determined based on obtainedexperimental dependence V(Q) using solutionof reverse coefficient isothermal problem. DH == 1⋅10—7 cm2/s for metal welds performed usingrutile welding consumables at T = 20 °C, and DH= 1⋅10—6 cm2/s [28] was obtained for low-alloykilled welds.

Removal and redistribution of hydrogen inmetal of welded joint take place after welding.Using results of experimental investigation [29]a mathematical model of redistribution of hydro-gen between the weld metal and base metal [30]was developed, which allows determining cur-rent concentration of hydrogen in zones of welded

joint considering hydrogen absorption by defectsof metal crystalline structure. Applied stressessignificantly influence on DH value and hydrogenpermeability of steel at plastic strain [31].

Plastic strain in fracture zone precedes as arule metal failure. Plastic strain of weld metaland HAZ is possible as a result of thermal-defor-mation cycle of welding or due to external load-ing. In this case, interaction of hydrogen withformed dislocation structure takes place. The dis-locations are reversible traps, which at metal tem-perature lower than 100 °C start to provide sig-nificant effect on hydrogen diffusion. Besides, asfollows below, basis of HE lies in interaction ofhydrogen with mobile edge dislocations. There-fore, study of hydrogen diffusion in plasticallydeformed metal is of particular interest in scopeof HE investigation.

Results of investigation of kinetics of hydro-gen removal at room temperature are shown inFigure 1 [32]. Character of hydrogen diffusionin undeformed and plastically deformed metalhas noticeable difference. As computer calcula-tions based on experimental data showed, DHremains constant in plastically deformed metalvirtually during the whole degassing processes.Therefore, completely all hydrogen is bondedwith dislocation structure to the moment of de-gassing beginning, and in order that hydrogenatom can move out from the metal it firstly needsto overcome energy barrier and detach from dis-location holding it. Respectively, DH value indeformed metal is determined by average energyof bonding of hydrogen atoms with dislocationsand does not change in the degassing processes.

According to calculations, DH changes by sev-eral orders (Figure 2) in undeformed specimenin a process of dissociation. Only part of hydro-gen is bonded with dislocation structure in the

Figure 1. Results of experiments on kinetics of hydrogenremoval from deformed (1) and underformed (2) specimen

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beginning of degassing in undeformed metal dueto relatively small concentration of dislocations.Initially, when desorption of hydrogen notbonded with dislocations takes place, the dislo-cations have no significant effect on diffusionprocess and, respectively, on DH value. But, asfar as desorption takes place the portion of hy-drogen, which was initially bonded with dislo-cation structure, increases in general flow of de-gassed gas. Thus, increase of time of degassingprovides reduction of concentration of remainedhydrogen, and rise of influence of dislocationson character of hydrogen diffusion in metal isobserved. This leads to gradual reduction of DHvalue and speed of specimen degassing. Whenhydrogen concentration is low, DH value in un-deformed metal is comparable with its value indeformed metal that confirms the conclusionsabout role of dislocation structure made before.

Work [33] proposes a mathematical model ofhydrogen mass transfer in metal considering

traps, which describes redistribution of hydrogenbetween residual and diffusion one. Dislocationsformed as a result of structural transformationsin metal during cooling were considered as traps.Calculation of mass transfer of hydrogen inwelded joint was carried out by finite elementmethod from moment of beginning of weld metalsolidification (Figure 3).

Local concentration of hydrogen CH in weldcenter in moment of its solidification as well asduring structural transformations rapidly in-creases due to solubility jump (see Figure 3).Concentration of residual hydrogen (bondedwith dislocations) increases with cooling of metalup to 100 °C and depends on bond energy of trapswith hydrogen Eb. Thus, at presence in metal oftraps with Eb = 20 kJ/mol the quantity of re-sidual hydrogen makes 0.5 cm3/100 g, and be-havior of diffusion hydrogen does not signifi-cantly change.

In case of traps with Eb = 30 kJ/mol thequantity of residual hydrogen increases up to2 cm3/100 g, and content of diffusion hydrogenrapidly reduces. After traps are saturated, sig-nificant delay of hydrogen diffusion takes placedue to reduction of gradient of concentration ofdiffusion hydrogen as a result of its transfer inresidual one. Thus, resulting reduction of localconcentration of diffusion hydrogen in the weldcenter (in 10 h after welding) appears to besmaller than increase of residual one.

Hydrogen transfer by edge dislocations. Pe-culiarities of reversible hydrogen embrittlement(for example, temperature-speed dependence ofmetal sensitivity to hydrogen embrittlement) ac-cording to current representations are explainedby interaction of hydrogen dissolved in metalwith mobile edge dislocations [6, 34]. Mathe-matical model was proposed in [35] for descrip-tion of process of hydrogen transfer by edge dis-locations. Atom of hydrogen moving inside themetal as a result of interaction with lattice willhave different potential energy in different mo-ments of time. Possibility of transfer of intersti-tial atoms in specific adjacent void depends onmetal temperature, potential energy of atom ininitial and finite void. Based on concepts of mi-croscopic theory of diffusion and considering thatatom of hydrogen can jump in adjacent void onlyif it is not occupied by other atoms, the followingsystem of equations describing diffusion of hy-drogen in the field of mobile edge dislocation[36] can be obtained:

Figure 3. Dependence on time of temperature T (1) andhydrogen concentration CH: 2 – diffusion (without traps);3, 4 – in traps with Eb = 20 and 30 kJ/mol, respectively;5, 6 – diffusion with Eb = 20 and 30 kJ/mol, respectively(concentration of diffusion hydrogen in 10 h after weldingis shown in the right upper corner)

Figure 2. Calculation dependence of DH on hydrogen con-tent in metal

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⎧

⎨

⎩

⎪⎪⎪⎪⎪

⎪⎪⎪⎪⎪

jx = —D ⎡⎢⎣

⎢⎢∂uD

∂x p(1 — p) +

∂p

∂x

⎤⎥⎦

⎥⎥ + V0p,

jy = —D ⎡⎢⎣

⎢⎢∂uD

∂y p(1 — p) +

∂p

∂y

⎤⎥⎦

⎥⎥ + V0p,

∂(jx)

∂x +

∂(jy)

∂y = 0,

where Jx and Jy are the flow of hydrogen alongx and y axes, respectively; D is the coefficientof hydrogen diffusion in defect-free metal; uD isthe potential of interaction of hydrogen with edgedislocation; p = C/Cv is the hydrogen concen-tration related to number of voids; Cv is the num-ber of voids in volume unit; V0 is the speed ofmovement of edge dislocation with conditions atinfinity: p = p0 = C/Cv; Jx = V0C0/Cv = V0p0;Jy = 0 at (x2 + y2) → ∞.

Effect of metal temperature, speed of move-ment of edge dislocations and concentration ofdiffusion hydrogen on quantity of hydrogentransferred by dislocations was investigated. Thecalculations showed that dependence of quantityof hydrogen transported by edge dislocation Non temperature has maximum in field of roomtemperature (Figure 4). It is determined thatincrease of speed of edge dislocation movementor reduction of diffusion hydrogen concentrationdecreases quantity of transported hydrogen andmaximum of N(T) curve is shifted in area ofhigher temperatures [36]. Since movement of dis-location is an elementary act of plastic deforma-tion then increase of speed of plastic deformationrises speed of dislocation movement. The resultsof calculation are well matched with experimen-tal data, i.e. increase of speed of plastic defor-mation reduces metal sensitivity to HE, and mini-mum of brittle strength of specimens containinghydrogen is shifted in area of higher temperatures[12].

Mechanical investigations. Works [37—39]proposed new physically based criterion charac-terizing the degree of reduction of brittle strengthof metal under effect of hydrogen, and procedureof its determination was developed on data ofmechanical tests. In contrast to comparison cri-teria used earlier, new criterion has clear physicalcontent determined by metal structure. Applica-tion of this procedure allows estimating the de-gree of metal HE by means of performance of thesimplest uniaxial tensile tests of standard speci-mens.

Works [40—43] show that measure of brittlestrength of metal is a value of microcleavage re-sistance Rmce, i.e. minimum stress of brittle frac-

ture at uniaxial tension deformed per certain de-gree e. Since Rmce value is structurally deter-mined in relation to temperature, then change ofRmce

H /Rmce relation depending on temperature re-flects influence of hydrogen on this value. Pecu-liarities of Rmce value marked in [40—43] allowexpressing degree of reduction of brittle strengthof metal under effect of hydrogen δH throughdecrease of value of critical fracture stress σ1C inspecimen waist:

δH = Rmсе

H

Rmсе ≈

σ1CH

σ1C,

where RmсеH and σ1C

Н are the characteristics ofmetal containing hydrogen.

σ1C for concerned temperature interval shouldbe calculated based on data of mechanical testsfor determination of δH parameter. Estimation ofδH value is obtained after dividing at fixed e andσ1CН /σ1C.

Results of experimental investigations of hy-drogen effect on mechanism of metal fracture aregiven in work [44]. Thermodesorption analysisdetermined that residual hydrogen which isbonded with formed dislocations and microcracks(Figure 5) is generated in metal containing dif-fusion hydrogen as a result of plastic deforma-tion. Thus, density of dislocations increases andmicrodefects are formed as a result of plastic de-formation of metal that leads to hydrogen redis-tribution.

Specimens from VSt3sp (killed) steel contain-ing 7 cm3/100 g of hydrogen were stretched todifferent degrees of plastic deformation for study-ing of effect of hydrogen on mechanisms of nu-cleation and growth of microdefects in metal.After preliminary deformation the hydrogen was

Figure 4. Dependence of hydrogen quantity N transportedby section of mobile edge dislocation on temperature ofspecimen and speed of its movement V0 = 1⋅10—3 (1), 1⋅10—2

(2) and 1⋅10—1 (3) m/s

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removed and specimens were stretched up to fail-ure. The specimens containing no hydrogen weresubjected to identical cycle of testing. Hydrogenprovides no significant effect on mechanical prop-erties of VSt3sp steel (Figure 6, a) up to 10 %deformation of specimen, and increase of defor-mation from 15 up to 17 % results in significanteffect of hydrogen on metal failure (Figure 6, b,c). Brittle microcrack, the growth of which takesplace on tough mechanism after hydrogen re-moval (Figure 6, d), was found on the fracturesurface of specimens with 7 cm3/100 g contentof hydrogen and preliminary deformation.

Deformation of metal containing hydrogen re-sults in nucleation and growth of microdefectsin it that significantly influence mechanical prop-erties. Effect of hydrogen rapidly increases inachieving of certain level of plastic deformation.Presence of hydrogen on dislocations facilitiestheir coalescence that results in microcrack nu-cleation at lower external stress.

Mechanism of hydrogen embrittlement ofiron and steel. Formation of HICC in weldedjoints from HSLA steels are determined by pe-culiarities of structural transformations in weldmetal and HAZ, value of residual tensile stressesand concentration of hydrogen in metal [9, 45].Mechanism of more general physical phenomenonof degradation of mechanical properties of metalunder effect of dissolved hydrogen, i.e. hydrogenembrittlement [9], is necessary to be consideredfor detection of mechanism of HICC formationunder conditions of thermal-deformation cycle ofwelding.

Hydrogen at plastic strain is transported toplace of crack nucleation by mobile dislocations.Dislocation theory proposes a number of modelsof dislocation reorganizations which can resultin formation of extremely sharp nucleation sub-microcrack [46]. One of them is Zener—Stroh

Figure 5. Spectrum of thermodesorption of residual hydro-gen for VSt3sp steel specimen, containing 8.5 cm3/100 gof diffusion hydrogen, after fracture

Figure 6. Tensile diagrams for VSt3sp steel specimens with preliminary deformation epr of 10 (a), 15 (b) and 17 (c) %(1, 2 – specimens containing no hydrogen and containing it respectively), and microstructure of microcrack (d)

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model, according to which cluster of dislocationsis formed in a place of stop of slip band andoccurrence of tensile stress takes place in its top,which under certain force conditions is developedin microcrack (Figure 7). Presence of hydrogenaround the dislocations results in nucleation ofsubmicrocrack at lower stress due to facilitationof dislocation coalescence.

If stability of microcrack is lost in a processof nucleation, then brittle fracture of metal willtake place [47, 48]. If crack does not loose sta-bility, then its further growth will depend onpeculiarities of development of local plasticstrain in zone around microcrack tip and hydro-gen concentration [49, 50] (Figure 8).

Presence of hydrogen results in change of mor-phology of plastic area due to localized plasticstrain [51—53]. New microdefect [54] (Figure 9)is nucleated in shear band near microcrack tipunder hydrogen effect.

Change of character of microplastic strainaround micropores or nonmetallic inclusions un-der effect of hydrogen promotes transfer from

tough to brittle fracture due to hydrogen-en-hanced localized plastic strain [55, 56] (Fi-gure 10).

Thus, critical factors on the stage of mi-crocrack development are the main tensilestresses and quantity of hydrogen transferred bydislocations to the place of defect formation (de-termined by concentration of diffusion hydrogen,temperature, speed of deformation and disloca-tion density in metal). Mechanism of microcrackgrowth (tough or brittle), if it does not loosestability in the moment of its nucleation, is de-termined by stress intensity factor and hydrogenconcentration in the crack tip.

Plastic strain of metal results in increase ofnumber of mobile dislocations that, in turn, leadsto redistribution of diffusion hydrogen betweenthe lattice and reversible traps-dislocations. Hy-drogen, transferred by dislocations to place ofmicrocrack nucleation, will be molized inside thelatter.

Figure 7. Scheme of microcrack formation: σ – externaltensile stresses; τeff – tangential stresses acting in slideplane of dislocations; L – length of submicrocracks; N –total quantity of edge dislocations in plain cluster; n –quantity of dislocations merged in submicrocrack; b –modulus of Burgers vector

Figure 9. Formation of micropore in specimen from IN903steel containing hydrogen [54]

Figure 8. Mechanism of crack growth: a – tough; b –quasi-brittle; c, d – intergranular

Figure 10. Scheme of nucleation, growth and merging ofmicropores in tough metal fracture

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Model of nucleation and growth of submi-crocrack in metal grain on microcleavage mecha-nism was proposed considering the model of hy-drogen transfer by dislocations. The main mecha-nism of metal embrittlement by hydrogen is effectof hydrogen-enhanced localized plasticity [47]which is considered through change of elasticenergy of edge dislocations and submicrocrackinduced by accumulation of hydrogen atomsaround them. The effect indicated above signifi-cantly reduces value of stress necessary for grainfracture. Proposed mathematical model [57, 58]considers metal temperature, grain size in whichsubmicrocracks appear, complex stressed state,physical characteristics of metal, mobility andconcentration of diffusion hydrogen, speed ofmovement of edge dislocations and influence hy-drogen-enhanced localized plasticity. Multi-fac-tor model allows describing such peculiarities ofreversible hydrogen embrittlement as tempera-

ture-speed dependence of value of fracture stressof metal containing hydrogen.

Computer modeling of influence of metalgrain orientation in relation to external stresseson value of fracture stress was carried out [59].The optimum angle of inclination between slipplane of edge dislocations and main tensile stressαopt equals 45°. It is determine that number ofdislocations in cluster reduces with deviation ofinclination of slip plane of edge dislocations fromoptimum angle or increase of complex stressedstate of metal. Appearance of cluster of edge dis-locations in slip plane is completely impossibleunder certain conditions. Such a dependence be-tween αopt corresponds with conclusions of dis-location theory. It is determined that relativeeffect of hydrogen on metal brittle strength in-creases with rise of complex stressed state of met-al, however, absolute value of fracture stress ofhydrogen-containing metal grain increases.

Hydrogen-enhanced localized plasticity ofmetal significantly reduces value of stress whichis necessary to be applied for grain fracture [57,60]. In some cases decrease of metal strength canachieve 40—50 % (Figure 11). It was determinedusing calculations that, at other factors beingequal, decrease of metal grain results in increaseof degree of metal HE, however, absolute valueof fracture stress of hydrogen-containing metalrises with grain decrease (see Figure 11, curve6). Thus, increase of steel strength due to decreaseof metal grain is reasonable only to certain extent,which depends on number of hydrogen as well assensitivity of steel to HE under given conditions.Calculation results correspond with presented ex-perimental data which were obtained for Armco-iron and low-carbon steel [12, 61].

One of the most possible mechanisms ofmacrocrack development in metal is formationmicrodefect in front of crack tip and its furthercoalescence with crack (Figure 12) [58, 62]. Areaof plastic strains is formed in metal in front oftip of growing crack under effect of stress. In aprocess of crack growth this results in formationof plastically deformed metal under its surface,the thickness of which depends on applied stress,i.e. the higher stress which is necessary to beapplied for formation of microdefect in front ofcrack tip, the thicker is the layer. Energy neces-sary for macrocrack growth consists from twoparts, namely energy of formation of free sur-faces, and energy of near-surface plastically de-formed metal. In metal containing no hydrogenspecific energy necessary for formation of such alayer is several orders higher than the specificenergy of free surfaces of crack [63]. Stress nec-

Figure 12. Scheme of growth of macrocrack in metal con-taining hydrogen: 1 – macrocrack; 2 – area of plasticdeformations; 3 – subsurface layer of plastically deformedmetal; 4 – microdefect; 5 – dislocation cluster; Ep –energy necessary for plastic strain of layer of metal of λplthickness; γs – specific surface energy

Figure 11. Dependence of degree of reduction of brittlestrength of iron δH = σH/σ0 on grain size d at T = 250 (1),275 (2); 300 (3), 325 (4) and 350 (5) K (σ0, σH – fracturestress of iron grain containing no hydrogen and containing, itrespectively), and dependence σH of on d at T = 300 K (6)

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essary for formation of microdefect in front oftip of macrocrack significantly reduces due toeffect of hydrogen-enhanced localized plasticity.Effect of hydrogen-enhanced localized plasticityreduces the most energy-consuming constituentof marcocrack growth, i.e. formation of near-sur-face layer. Therefore, development of macrocrackshould take place more brittle, with lower energyconsumption in hydrogen-containing metal withBCC lattice that is observed in experiments [2,12, 64].

Conclusions

1. Physical model of saturation of metals by gasesbeing in contact with low-temperature plasmawas developed. The model is build on the basisof kinetics theory of gases and considers move-ment of ions, atoms and molecules in plasma vol-ume, adsorption and desorption of gas on metalsurface as well as diffusion transfer of dissolvedgas in metal melt.

2. It is shown that activation of molecules inplasma (excitation, dissociation, ionization) in-creases speed of dissolution by several orders incomparison with equilibrium conditions.

3. It is determined that entering of fluorinecompounds in composition of welding consu-mables results in HF formation. Thermal-dy-namic analysis showed that HF is substantiallydissociated in larger part of arc section (high-temperature). Bonding of fluorine by hydrogentakes place in arc periphery that results in reduc-tion of hydrogen absorbed by weld pool. Presenceof HF in arc zone was experimentally proved.

4. New chromatographic methods for analysisof hydrogen in metal of welds, welding consu-mables and their components were proposed.Chromatographic method for analysis of diffu-sion hydrogen with degassing temperature up to150 °C was entered in GOST 23338—91.

5. Method for reduction of hydrogen contentin coated electrode welding and submerged arcwelding providing extremely low concentrationsof diffusion hydrogen in weld metal was pro-posed.

6. It is stated based on experiment-calculationinvestigations of kinetics of degassing of hydro-gen from the weld metal that dependence of co-efficient of diffusion on concentration of hydro-gen is character for undeformed metal whereasit is not observed in deformed specimen. This iswell settled in scope of ideas about dislocationsas hydrogen traps.

7. It is determined that formation of hydro-gen-induced cold cracks in welded joints is rep-resentation of HE under specific conditions of

thermal-deformation welding cycle. Therefore,solving of problem of HICC should be based onaccurate knowledge of mechanism of HE of metalof the welded joint.

8. It is shown with high reliability that inter-action of hydrogen with dislocations makes a ba-sis of mechanism of HE. Hydrogen influences onnucleation and growth of microcracks in metalfacilitating coalescence of dislocations that resultin localized plastic strain under effect of hydro-gen.

9. Effect of hydrogen on nucleation of mi-crocrack in macrolevel appears in a form of re-duction of normal tensile stresses necessary forits nucleation. Further growth of microcracktakes place on quasi-brittle mechanism due toformation of new microdefect in its tip underhydrogen effect.

10. It is shown with the help of computercalculations that quantity of hydrogen trans-ferred by dislocations to the place of microdefectnucleation depends on speed of dislocation move-ment, metal temperature and has maximum inarea of room temperatures. This is agreed withexperimentally stated temperature-speed de-pendence of reversible HE having minimum inarea of room temperature.

11. Modeling of growth of submicrocrack inmetal grain considering effect of hydrogen-en-hanced localized plasticity showed that metal be-comes more brittle and sensitive to HE with re-duction of size of grain, however, absolute valueof fracture stress increases.

12. It is shown that removal of hydrogen frommetal as well as its redistribution between dif-fusion and residual ones due to presence in metalof hydrogen traps take place in welded joint cool-ing. Increase of bond energy of traps and hydro-gen reduces diffusion of the latter and increasesquantity of residual hydrogen.

1. http://www.worldsteel.org2. Pokhodnya, I.K., Yavdoshchin, I.R., Paltsevich,

A.P. et al. (2004) Metallurgy of arc welding. Kiev:Naukova Dumka.

3. Beloglazov, S.M. (1975) Hydrogenation of steel inelectrochemical processes (Review). Leningrad:LGU.

4. Kasatkin, O.G. (1994) Peculiarities of hydrogen em-brittlement of high-strength steels in welding (Re-view). Avtomatich. Svarka, 1, 3—7.

5. Yukhnovsky, P.I., Tkachev, V.I. (1987) About stateof hydrogen in metal. Fiz.-Khimich. Mekhanika Ma-terialov, 4, 107—108.

6. Shvachko, V.I. (2002) Reversible hydrogen brittle-ness of bcc iron alloys – structural steels: Syn. ofThesis for Dr. of Phys.-Math. Sci. Degree. Kharkiv.

7. Teus, S.M., Shivanyuk, V.N., Shanina, V.D. et al.(2007) Effect of hydrogen on electronic structure offcc iron in relation to hydrogen embrittlement ofaustenite steels. Phys. Status Solids A, 204(12),4249—4258.

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8. Gavriljuk, V.G., Shivanyuk, V.N., Shanina, B.D.(2005) Change in the electron structure cause by C, Nand H atoms in iron and its effect on their interactionwith dislocations. Acta Materialia, 53, 5017—5024.

9. Pokhodnya, I.K., Shvachko, V.I. (1997) Physical na-ture of hydrogen-induced cold cracks in welded jointsof structural steels. Avtomatich. Svarka, 5, 3—12.

10. Pokhodnya, I.K. (1998) Problems of welding ofhigh-strength low alloy steels. In: Current materialsscience of 21st century: Transact. Kiev: NaukovaDumka, 31—69.

11. Geld, P.V., Ryabov, R.A. (1974) Hydrogen in met-als and alloys. Moscow: Metallurgiya.

12. Kolachev, B.A. (1985) Hydrogen brittleness of met-als. Moscow: Metallurgiya.

13. Yavdoshchin, I.R. (1969) Investigation and develop-ment of universal electrodes with rutile coating:Syn. of Thesis for Cand. of Techn. Sci. Degree. Kiev.

14. Pokhodnya, I.K., Shvachko, V.I., Portnov, O.M.(2000) Mathematical modelling of absorption ofgases by metal during welding. The Paton WeldingJ., 7, 11—16.

15. Pokhodnya, I.K. (2003) Mathematical modelling ofprocesses of interaction of metal with gases in arcwelding. Ibid., 2, 2—9.

16. Pokhodnya, I.K., Portnov, O.M. (2003) Mathemati-cal modelling of absorption of gases by electrodedrop metal. Ibid., 6, 2—5.

17. Pokhodnya, I.K., Portnov, O.M., Shvachko, V.I.(2001) Computer modeling of hydrogen absorptionby electrode metal drop under its intensive evapora-tion. In: Proc. of 6th Seminar on Numeric Analysisof Weldability (Graz, Oct. 2001). Graz: TU of Graz,895—902.

18. Pokhodnya, I.K., Shvachko, V.I., Utkin, S.V.(1998) Calculated assessment of hydrogen behaviorin arc discharge. Avtomatich. Svarka, 9, 4—7.

19. Pokhodnya, I.K., Tsybulko, I.I., Orlov, L.N. (1993)Influence of slag composition on hydrogen content inliquid metal during CO2 welding. Ibid., 11, 8—14.

20. Tsibulko, I.I. (1993) Calculation of thermodynamicequilibrium in metallurgical system gas-slag-metal.In: Proc. of 2nd Int. Seminar on Numeric Analysisof Weldability (Graz-Segau, 10—12 Sept. 1993).Graz: TU of Graz.

21. Pokhodnya, I.K., Shvachko, V.I., Ustinov, V.G. etal. (1972) Mass-spectrometric examinations of fluo-rides emitted in arc welding. Avtomatich. Svarka, 6,10—12.

22. Pokhodnya, I.K., Shvachko, V.I. (1981) Formationof hydrogen fluoride in arc discharge. Ibid., 2, 11—13.

23. Pokhodnya, I.K., Paltsevich, A.P., Yavdoshchin,I.R. (1986) Influence of methods of weld metal sam-pling for determination of diffusion-mobile hydrogencontent in it. Ibid., 1, 24—28.

24. Pokhodnya, I.K., Paltsevich, A.P., Yavdoshchin,I.R. (1988) Influence of welding conditions on hy-drogen content in welds made with electrodes of ba-sic type coatings. Ibid., 3, 19—22.

25. Paltsevich, A.P. (1999) Chromatographic method ofhydrogen content evaluation in electrode coatingcomponents. Ibid., 6, 46—48.

26. Pokhodnya, I.K., Paltsevich, A.P. (2003) Examina-tion of potential content of hydrogen. In: Abstr. forInt. Conf. on Current Problems of Welding and Re-source of Structures (Kiev, 24—27 Nov. 2003). Kiev:PWI, 67

27. Panasyuk, V.V. (1991) Mechanics of quasi-brittlefracture of materials. Kiev: Naukova Dumka.

28. Paltsevich, A.P. (1988) Development of methods ofhydrogen content reduction in welds for new coatedelectrodes and flux-cored wires of basic type: Syn. ofThesis for Cand. of Techn. Sci. Degree. Kiev.

29. Pokhodnya, I.K., Demchenko, L.I., Paltsevich, A.P.et al. (1976) Kinetics of diffusion redistribution ofhydrogen between weld metal and base metal in arcwelding. Avtomatich. Svarka, 8, 1—5.

30. Pokhodnya, I.K., Demchenko, V.F., Demchenko,L.I. (1979) Mathematical modeling of gas behaviorin welds. Kiev: Naukova Dumka.

31. Pokhodnya, I.K., Pavlyk, V.A., Shvachko, V.I.(1993) Effect of heat treatment and deformation onhydrogen diffusion and permeability of 10KhN3DMtype steel. In: Metallurgy of welding and consu-mables. St.-Petersburg: StPGTU, 158—160.

32. (2011) Development of materials for welding oftechnological equipment of mining and smelting andfuel and energy complexes. Pt 1: Study by methodsof experimental and numerical modeling of hydrogenbehavior in weld metal of higher strength under con-ditions of thermal-deformation cycle of welding: Fi-nal report No. 0107U0022787. Kiev: PWI.

33. Sinyuk, V.S., Stepanyuk, S.N. (2009) Interaction ofhydrogen with dislocation structure of structural steelwelded joints. In: Proc. of Sci. Conf. on Mechanics ofFracture Materials and Strength of Structures (23—27June, 2009, Lviv). Lviv: FMI, 999—1002.

34. Stepanyuk, S.M. (2001) Reversible hydrogen embrit-tlement in welding of high strength low-alloy steels:Syn. of Thesis for Cand. of Techn. Sci. Degree. Kyiv.

35. Shvachko, V.I., Ignatenko, A.V. (2007) Model oftransportation of hydrogen with dislocations. The Pa-ton Welding J., 2, 24—26.

36. Ignatenko, A.V. (2007) Mathematical model oftransportation of hydrogen by edge dislocation. Ibid.,9, 23—27.

37. Pokhodnya, I.K., Shvachko, I.V., Kotrechko, S.A. etal. (1999) A new method for quantitative determina-tion of sensitivity of steels to hydrogen embrittle-ment. Int. J. Mater. Sci., 34(4), 538—543.

38. Shvachko, V.I., Stepanyuk, S.M., Pokhodnya, I.K.(2000) The evaluation methods of HLSA steels sus-ceptibility to hydrogen embrittlement. In: Proc. of4th Int. Conf. on HSLA Steels (Xi’an, China, Oct.30—Nov. 2, 2000). Beijing: Metallurg. IndustryPress, 453—458.

39. Pokhodnya, I.K., Meshkov, Yu.Ya., Shvachko, V.I.et al. Method of quantitative determination of levelof hydrogen embrittlement of structural steels andwelds. Appl. 5040067. Int. Cl. G 01 n 17/00. Fil.01.07.91.

40. Meshkov, Yu.Ya. (1981) Physical principles ofstrength of steel structures. Kiev: Naukova Dumka.

41. Meshkov, Yu.Ya., Pakharenko, G.A. (1985) Struc-ture of metal and brittleness of steel products. Kiev:Naukova Dumka.

42. Meshkov, Yu.Ya., Serditova, T.N. (1989) Fractureof wrought steel. Kiev: Naukova Dumka.

43. Kotrechko, S.A., Meshkov, Yu.Ya., Mettus, G.S.(1990) To problem of tough and brittle states ofpolycrystalline metals. Metallofizika, 12(6), 3—13.

44. Sinyuk, V.S., Pokhodnya, I.K., Paltsevich, A.P. etal. (2012) Experimental study of the mechanism ofhydrogen embrittlement of metals with the bcc lat-tice. The Paton Welding J., 5, 8—11.

45. Tsaryuk, A.K., Brednev, V.I. (1996) Problem of coldcrack prevention. Avtomatich. Svarka, 1, 36—40.

46. Vladimirov, V.I. (1984) Physical nature of metalfracture. Moscow: Metallurgiya.

47. Birnbaum, H.K., Sofronis, P. (1994) Hydrogen-en-hanced localized plasticity – a mechanism for hydro-gen-related fracture. Mat. Sci. and Eng. A, 174,191—202.

48. Kotrechko, S.A., Meshkov, Yu.Ya. (2008) Ultimatestrength. Kiev: Naukova Dumka.

49. Pokhodnya, I.K., Shvachko, V.I., Utkin, S.V.(2002) Effect of hydrogen on equilibrium of disloca-tion submicrocrack in α-iron. Fiz.-Chimich. Mekha-nika Materialiv, 1, 7—14.

50. Beachem, C.D. (1972) A new model for hydrogen-as-sisted cracking (hydrogen embrittlement). Metallurg.Transact., 3, 259—273.

51. Gedeon, S.A., Eagar, T.W. (1990) Assessing hydro-gen-assisted cracking fracture modes in high-strengthsteel weldments. Welding J., 6, 213—219.

12 5/2013

52. Sofronis, P., Liang, Y., Aravas, N. (2001) Hydrogeninduced shear localization of the plastic flow in met-als and alloys. Eur. J. Mech. – A: Solids, 20, 857—872.

53. Liang, Y., Sofronis, P., Aravas, N. (2003) On the ef-fect of hydrogen on plastic instabilities in metals.Acta Materialia, 51, 2717—2730.

54. http://www.icf.11.com/proceeding/EXTENDED/5638.pdf

55. Liang, Y., Sofronis, P., Dodds, R.H. (2004) Interac-tion of hydrogen with crack-tip plasticity: Effect ofconstraint on void growth. Mat. Sci. and Eng. A,366, 397—411.

56. Ahn, D.C., Sofronis, P., Dodds, R.H. (2007) On hy-drogen-induced plastic flow localization during voidgrowth and coalescence. Int. J. Hydrogen Energy,32, 3734—3742.

57. Ignatenko, O.V., Pokhodnya, I.K. (2011) Influenceof hydrogen-enhanced localized plasticity and grainsize on the strength of bcc metal. In: Proc. of 2ndUkrain.-Greek Symp. on Fracture Mechanics of Ma-terials (3—7 Oct. 2011, Lviv). Lviv: FMI.

58. Ignatenko, A.V., Pokhodnya, I.K., Paltsevich, A.P.et al. (2012) Dislocation model of hydrogen-en-

hanced localizing of plasticity in metals with bcc lat-tice. The Paton Welding J., 3, 15—19.

59. http://dfmn2011.imetran.ru/2011/index.php60. Ignatenko, O.V., Pokhodnya, I.K., Stepanyuk, S.M.

et al. (2010) Principles of hydrogen cracking ofwelded joints of high strength low alloy steels. In:Fundamental problems of hydrogen energy. Kyiv:KIM, 340—360.

61. Ostash, O.P., Vitvittsky, V.I. (2011) Duality of hy-drogen effect on mechanical behavior of steels andstructural optimization of their hydrogen resistance.Fiz.-Chimich. Mekhanika Materialov, 4, 4—19.

62. Ignatenko, A.V., Sinyuk, V.S. (2012) Influence ofhydrogen-enhanced localized plasticity and complex-stressed state on strength of metal. In: Proc. of 6thInt. Conf. on Mathematical Modelling and Informa-tion Technologies in Welding and Related Processes(Katsiveli, 29 May—1 June 2012). Kiev: PWI, 35—36.

63. Panasyuk, V.V. (1968) Ultimate equilibrium of brit-tle bodies with cracks. Kiev: Naukova Dumka.

64. Morozov, L.S., Chechulin, B.B. (1967) Hydrogenbrittleness of metals. Moscow: Metallurgiya.

Received 27.03.2013

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