Metallurgical Model of Diffusible Hydrogen and Non-Metallic Slag

metals

Article Metallurgical Model of Diﬀusible Hydrogen and Non-Metallic Slag Inclusions in Underwater Wet Welding of High-Strength Steel

Sergey G. Parshin 1,* , Alexey M. Levchenko 2 and Alexey S. Maystro 1 1 Department of Welding and Laser Technologies, Peter the Great St. Petersburg Polytechnic University, 187015 St. Petersburg, Russia; [email protected] 2 Department of Underwater Welding and Technologies, Educational Scientiﬁc and Technical Center “Svarka”, 187015 St. Petersburg, Russia; [email protected] * Correspondence: [email protected]; Tel.: +7-812-552-63-55

Received: 20 October 2020; Accepted: 6 November 2020; Published: 10 November 2020

Abstract: High susceptibility to cold cracking induced by diffusible hydrogen and hydrogen embrittlement are major obstacles to greater utilization of underwater wet welding for high-strength steels. The aim of the research was to develop gas–slag systems for flux-cored wires that have high metallurgical activity in removal of hydrogen and hydroxyl groups. Thermodynamic modeling and experimental research confirmed that a decrease in the concentration of diffusible hydrogen can be achieved by reducing the partial pressure of hydrogen and water vapor in the vapor–gas bubble and by increasing the hydroxyl capacity of the slag system in metallurgical reactions leading to hydrogen fluoride formation and ionic dissolution of hydroxyl groups in the basic fluorine-containing slag of a TiO2–CaF2–Na3AlF6 system.

Keywords: underwater wet welding; diﬀusible hydrogen; slag inclusions; ﬂux-cored wire

1. Introduction Underwater wet welding is a welding technique commonly used for the construction and repair of ocean-going vessels, oil and gas platforms, and offshore wind turbines. The load-bearing structures of such vessels and installations are often made of high-strength steels [1,2], which pose considerable challenges to welders and the welding processes to obtain the welds exhibiting high levels of strength, ductility, and impact toughness [3,4]. A further issue is that underwater wet welding is susceptible to weld defects like hydrogen-assisted cold cracking, porosity, slag inclusions, and delayed hydrogen embrittlement [5–10]. Consequently, underwater wet welding has found only limited use for critical applications, which requires further study of the mechanism of the appearance of defects. The appearance of defects during underwater welding is associated with the formation of diffusible hydrogen, active oxygen, and slag in the welding zone [11]. In underwater wet welding, the welding occurs in a vapor–gas bubble [12–14] in which the hydrogen content reaches 85–96% [15]. When welding with coated electrodes and flux-cored rutile wire, the vapor–gas bubble comprises 93–98% H2, 1.5–6% CO, and 0.5–2% CO2 [16]. Hydrogen from the vapor–gas bubble can be absorbed into the liquid weld, which is the major cause of porosity in underwater wet welding. The gas composition of pores in the weld consists of 62–82 wt.% H2, 11–24 wt.% CO, and 4–6 wt.% CO2, depending on the composition of the electrode coating and the welding parameters used [17]. Before dissolution of hydrogen atoms in the liquid weld pool, dissociation of H2O and H2 occurs [18]. At the arc temperature, water (H2O) dissociates according to the following reaction:

H O = H + 0.5O 260 kJ (at 6000 K) (1) 2 2 2 −

Metals 2020, 10, 1498; doi:10.3390/met10111498 www.mdpi.com/journal/metals Metals 2020, 10, 1498 2 of 17

0.5 PH2PO2 lgKq = lg (2) PH2O where Kq is the reaction equilibrium constant, and PH2, PO2, and PH2O are the partial pressure (Pa) of H2,O2, and H2O, respectively. With further heating of the gas mixture, endothermic reactions of dissociation of the H2 molecules and ionization of the H atoms occur.

H = H + H 463.1 kJ (3) 2 − H = H + H+ + e 1746.7 kJ (4) 2 − Dissolution of molecular hydrogen in the weld pool increases with growth in the partial pressure of the components of the gas mixture according to Sieverts’ law. p [H] = Ks PH2 (5) where [H] is the hydrogen content in the weld in wt.%, PH2 is the partial pressure of molecular hydrogen in the gas phase, and Ks is the solubility constant. With increases in the immersion depth, the pressure in the vapor–gas bubble increases by about 0.1 MPa for every 10 m of immersion depth, and at a depth of 50 m the total pressure reaches 0.6 MPa. The increase in pressure promotes the dissolution of hydrogen in the weld pool and thus porosity increases [19–23]. One mechanism for diffusible hydrogen reduction is a decrease in the hydrogen partial pressure in the vapor–gas bubble atmosphere, for example, by dissociation of carbonates and fluorides, namely Na2CO3, NaF, CaCO3, CaF2, MgCO3, and MgF2, in the flux-cored wire. To reduce porosity, carbonates CaCO3 and MgCO3 can be added into the electrode coatings. The carbonates dissociate in the vapor–gas bubble with the formation of CO2 and CO, which reduces the hydrogen partial pressure above the weld pool [24]. A second mechanism for hydrogen reduction is via the chemical reaction of hydrogen and fluorine with formation of HF compounds in reactions with fluorides NaF, CaF2, MgF2, AlF3, etc. [25,26]. A linear decrease in the content of diffusible hydrogen [H] in the weld metal occurs with increases in the content of CaF2 from 0 to 86 wt.%. Increasing CaF2 is a more effective approach for reducing hydrogen 3 than adding CaCO3 to the electrode coating. For example, [H] content in the weld is 54 cm /100 g when adding 20 wt.% CaCO3 to the electrode coating, and when adding 20 wt.% CaF2, the [H] content decreases to 39 cm3/100 g [26]. A third mechanism for hydrogen reduction is an increase of the oxidation potential of the weld pool and solubility of water vapor and OH hydroxyl groups in the liquid slag, in particular, by the 3 addition of hematite Fe2O3 with a density of 5.3 g/cm [9]. Hematite Fe2O3 decomposes under high-temperature conditions with the formation of wüstite FeO in the molten slag, which increases the basicity index of the slag; in addition, FeO oxidizes the weld pool, which inhibits dissolution of diffusible hydrogen in the weld pool. However, an increase in the oxidizing potential of the slag and the atmosphere of the vapor–gas bubble leads to slag nonmetallic inclusions and oxidation of alloying elements [27–30], which reduces the mechanical properties of the welds [9,31]. The slag basicity index BI is calculated as follows [32]:

CaO + MgO + BaO + CaF2 + Na2O + K2O + 0.5(FeO + MnO) BI = (6) SiO2 + 0.5(Al2O3 + TiO2 + ZrO2) Metals 2020, 10, 1498 3 of 17

The hydroxyl capacity of a COH slag system is determined according to the following equation:

%H2O COH = q (7) 0 PH2O/P where H2O is the content of water vapor in the slag in wt.%, PH2O is the partial pressure of water vapor in the gas phase above the molten slag in the equilibrium state, and P0 is the atmospheric pressure [33]. A ratio for the hydroxyl capacity of COH slag has been proposed in [34,35] as follows:

log C = 12.04 32.63Λ + 32.71Λ2 6.62Λ3 (8) OH − −

An increase in the basicity index of the slag and hydroxyl capacity can be achieved by adding CaF2 and cryolite Na3AlF6, which decrease the melting point, viscosity, hydrogen permeability, and density of the slag system [36–41]. The increase in slag basicity and the presence of ion F– elevate the solubility of water vapor and promote the ionic binding of the hydrogen atom in OH hydroxyl groups, which leads to a decrease in the content of diffusible hydrogen [42]. The subsequent binding of hydroxyl groups 3 – OH is possible in the polymerization of AlF6 − and AlF4 anions and the formation of clusters with the bonds –F–H–F– and –Al–O–Al– [43–45]. Simultaneous implementation of all three mechanisms for the decrease of diffusible hydrogen in underwater wet welding is possible by creating a low-density slag system based on TiO2–CaF2–Na3AlF6. It is known that water vapor dissolves in acidic and basic slags following the following ionic reactions [46–48]: -Si-O-Si- + H2O = 2(-Si-OH-) (acidic slag) (9) 0 O + H2O = 2(OH) (acidic slag) (10) 2+ 2+ 2(-Si-O-Si-) + Fe + H2O = -Si-O-Si + 2(OH−) + Fe (basic slag) (11) 2+ 2 2+ 2(-Si-O-Si-) + Fe + H2O = 2(-Si-OH-) + O − + Fe (basic slag) (12) 2 H2O + 2O− = (O −) + 2(OH) (highly basic slag) (13) 0 H2O + 2O− = (O ) + 2(OH−) (highly basic slag) (14) 2 H2O + O − = 2(OH−) (highly basic slag) (15) The transition of atomic hydrogen into the weld pool and the formation of diffusible hydrogen [H] occurs according to the following ionic equations (proposed by S.G. Parshin):

+ 2 2+ (H ) + e− + (O −) + (Fe ) = [Fe] + [O] + [H] (16)

2+ (OH−) + e− + (Fe ) = [FeO] + [H] (17)

In the molten fluoride slags, an ionic reaction binds hydrogen with the formation of anions (OH−) and gaseous compound HF . Thus, the formation of anions (OH−), the binding of hydrogen in HF, ↑ 3 and formation of network clusters of AlF6 − and AlF4− anions can energetically hinder the transition of atomic hydrogen into the weld pool and the formation of diffusible hydrogen [H]. The aim of the research was to develop a gas–slag system for a flux-cored wire for underwater wet welding that has high metallurgical activity and reduces diffusible hydrogen and non-metallic slag inclusions by removing hydrogen and hydroxyl in the vapor–gas bubble atmosphere and increasing the solubility of water vapor in the slag phase. Metals 2020, 10, 1498 4 of 17

2. Materials and Methods Samples of API X70 pipeline steel (CHTPZ, Chelyabinsk, Russia) with bainitic microstructure having dimensions of 300 mm 200 mm 21.3 mm was welded underwater in butt and lap joint Metals 2020, 10, x FOR PEER REVIEW × × 4 of 17 conﬁgurations, as shown in Figure1.

(a) (b)

Figure 1. AssemblyAssembly and and root pass welding of ( a) butt and ( b) lap joints.

Mechanized underwater wet wet welding welding was was performed performed by by divers divers at at a a depth of 12 m using a NeptunNeptun-4‐4 submersiblesubmersible (Paton (Paton Institute Institute of of Electric Electric Welding, Welding, Kiev, Kiev, Ukraine). Ukraine). Flux-cored Flux‐cored wires wires were were used usedof type of PPS-AN1 type PPS‐ (PatonAN1 (Paton Institute Institute of Electric of Electric Welding, Welding, Kiev, Ukraine) Kiev, Ukraine) (TiO2–Fe (TiO2O3–MnO–iron2–Fe2O3–MnO–iron powder powdercomposition) composition) and PPS-APL2 and PPS (Educational‐APL2 (Educational Scientific andScientific Technical and CenterTechnical “Svarka”, Center St. “Svarka”, Petersburg, St. Petersburg,Russia) (TiO Russia)2–CaF2 –Na(TiO32AlF–CaF6–MnO–iron2–Na3AlF6–MnO–iron powder composition). powder composition). The wires hadThe awires diameter had a of diameter 1.6 mm ofand 1.6 the mm rutile and electrodes the rutile electrodes E7014 and E7014 UW/CS-1 and (Broco, UW/CS ON,‐1 (Broco, USA) (TiOON,2 USA)–CaCO (TiO3–SiO2–CaCO2–Al2O3–SiO3–MnO–iron2–Al2O3– powderMnO–iron composition) powder composition) were 3.2 mm were in diameter. 3.2 mm in Welding diameter. parameters Welding areparameters shown in are Table shown1. in Table 1. Table 1. Welding parameters of underwater wet welding. Table 1. Welding parameters of underwater wet welding. Welding Consumables Voltage, V Current, A Wire Feed Rate, m/min UWWelding/CS-1 electrode Consumables Voltage, 37.5–42.5 V Current, 135–175 A Wire Feed Rate, m/min - PPS-AN1UW/CS flux-cored‐1 electrode wire 37.5–42.5 135–175 ‐ 37.5–43.5 120–300 4 PPS(TiO‐AN1–Fe O flux) ‐cored wire 2 2 3 37.5–43.5 120–300 4 PPS-APL2 flux-cored(TiO2–Fe wire2O3) 40–45 100–240 4 (TiOPPS–CaF‐APL2–Na fluxAlF‐cored) wire 2 2 3 6 40–45 100–240 4 (TiO2–CaF2–Na3AlF6)

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Mechanical tests were conducted in compliance with GOST 6996–66 using a Super L60 machine (Tinius Olsen, Horsham, PA, USA), a PH450 pendulum impact test system (Walter + Bai AG, Löhningen, Switzerland), and an EMCOTEST DuraScan-20 hardness tester (EMCO-TEST PrufmaSchinen GmbH, Kuchl, Austria). The chemical composition was determined with a Bruker Q4 TASMAN optical emission spectrometer (Bruker, Karlsruhe, Germany). A Zeiss Axiovert 200 MAT microscope (Carl Zeiss AG, Oberkochen, Germany) was used to analyze the microstructure, and an ERESCO 42M X-ray Metals 2020, 10, x FOR PEER REVIEW 5 of 17 unit (GE Sensing and Inspection Technologies GmbH, Ahrensburg, Germany) was used for X-ray testingformation in compliance was performed with GOSTby the 7512–82. shadow Researchmethod with of the a vapor–gaslaser system bubble and Phantom formation VEO was performed710L high‐ byspeed the shadowcamera method(Vision Research, with a laser Wayne, system NJ, and USA) Phantom with VEO a frequency 710L high-speed of 8000 Hz. camera Diffusible (Vision hydrogen Research, Wayne,content NJ,was USA) determined with a frequency by the vacuum of 8000 method Hz. Di ﬀaccordingusible hydrogen to GOST content 34061–2017 was determined (ISO 3690: by 2012) the vacuumusing an method accelerated according method to GOST [49] 34061–2017with automatic (ISO 3690:bead 2012)welding using in an water accelerated at the methoddepth of [49 0.8] with m. automaticThermodynamic bead welding calculations in water were at theperformed depth of using 0.8 m. FactSage Thermodynamic (CRCT, Montreal, calculations Canada) were performed and Terra using(Bauman FactSage Moscow (CRCT, State Montreal, Technical Canada)University, and Moscow, Terra (Bauman Russia) Moscowand were State based Technical on thermodynamic University, Moscow,data of individual Russia) and substances were based [50]. on thermodynamic data of individual substances [50].

3.3. Results and Discussion Underwater wet wet welding welding with with a self-shielded a self‐shielded ﬂux-cored flux‐cored wire wire occurs occurs in a vapor–gas in a vapor–gas bubble formedbubble duringformed dissociation during dissociation of water. Theof water. welding The arc welding consists arc of a consists central zone of a (arc central column), zone a (arc boundary column), zone a aroundboundary the zone arc column, around and the aarc molecular column, layer, and a in molecular which water layer, vapor in which dissociates. water A vapor proposed dissociates. model ofA underwaterproposed model wet welding of underwater using ﬂux-cored wet welding wire using is shown flux‐cored in Figure wire2 .is shown in Figure 2.

(a) (b)

Figure 2. Arc model in underwater wet welding (a): 1—arc column; 2—arc boundary; 3—molecular Figure 2. Arc model in underwater wet welding (a): 1—arc column; 2—arc boundary; 3—molecular layerlayer ofof dissociationdissociation aroundaround thethe arcarc boundary;boundary; 4—vapor–gas4—vapor–gas bubble; 5—weld pool; 6—liquid slag; 7—flux-cored7—flux‐cored wire; 8—drop;8—drop; ((b)) vapor–gasvapor–gas bubblebubble shadowshadow photophoto inin flux-coredflux‐cored underwaterunderwater wet arcarc welding:welding: CT—contactCT—contact tube; tube; FCW—flux-cored FCW—flux‐cored wire; VGB—vapor–gaswire; VGB—vapor–gas bubble; SP—steelbubble; SP—steel plate (proposed plate by(proposed S.G. Parshin). by S.G. Parshin).

TheThe formationformation of of a a vapor–gas vapor–gas bubble bubble includes includes several several phases: phases: nucleation, nucleation, volume volume expansion expansion with thewith pulsations the pulsations (or growth), (or growth), and collapse, and collapse, as shown as shown in Figure in Figure3. 3.

Metals 2020, 10, 1498 6 of 17 Metals 2020, 10, x FOR PEER REVIEW 6 of 17 Metals 2020, 10, x FOR PEER REVIEW 6 of 17

(a()a )

(b()b )

(c) (c) FigureFigure 3.3. FormationFormation of of a avapor–gas vapor–gas bubble: bubble: (a) nucleation (a) nucleation phase; phase; (b) volume (b) volume expansion expansion phase; phase;(c) Figure 3. Formation of a vapor–gas bubble: (a) nucleation phase; (b) volume expansion phase; (c) (ccollapse) collapse phase. phase. collapse phase. AA detaileddetailed modelmodel of metallurgical processes processes in in underwater underwater wet wet welding welding is isshown shown in inFigure Figure 4. 4. A detailed model of metallurgical processes in underwater wet welding is shown in Figure 4.

Figure 4. Model of metallurgical processes in underwater wet welding in vapor–gas bubble, liquid slag, and liquid weld pool (proposed by S.G. Parshin). FigureFigure 4.4. ModelModel of of metallurgical metallurgical processes processes in in underwater underwater wet wet welding welding in vapor–gas in vapor–gas bubble, bubble, liquid liquid slag, slag, and liquid weld pool (proposed by S.G. Parshin). andIn the liquid molten weld slag pool occur (proposed the electrochemical by S.G. Parshin). interactions between OH− hydroxyl and AlF63− and AlF4− anions with the formation of bonds –F–H–F– and –Al–O–Al–, as shown in Figure 5. In the molten slag occur the electrochemical interactions between OH− hydroxyl and AlF63− and AlF4− anions with the formation of bonds –F–H–F– and –Al–O–Al–, as shown in Figure 5.

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3 In the molten slag occur the electrochemical interactions between OH− hydroxyl and AlF6 − and Metals 2020, 10, x FOR PEER REVIEW 7 of 17 AlFMetals4− 2020anions, 10, x with FOR PEER the formation REVIEW of bonds –F–H–F– and –Al–O–Al–, as shown in Figure5. 7 of 17

AlF63− = AlF4− + 2F− → AlF4− + AlF4− = Al2F7− + F− → Al2F7− + OH− = Al2 OHF6− + F− AlF63− = AlF4− + 2F− → AlF4− + AlF4− = Al2F7− + F− → Al2F7− + OH− = Al2 OHF6− + F− Figure 5. Electrochemical model of OH hydroxyl binding with the formation of network clusters in Figure 5.5. ElectrochemicalElectrochemical modelmodel of of OH OH hydroxyl hydroxyl binding binding with with the the formation formation of networkof network clusters clusters in the in the interaction of AlF4− ions (proposed by S.G. Parshin). interactionthe interaction of AlF of 4AlF− ions4− ions (proposed (proposed by S.G.by S.G. Parshin). Parshin).

As aa resultresult ofof waterwater dissociationdissociation andand ionizationionization ofof molecules,molecules, thethe atmosphereatmosphere ofof thethe vapor–gasvapor–gas bubble consists consists of of a a gasgas mixturemixture ofof aa complexcomplex phasephase compositioncomposition withwith highhigh metallurgicalmetallurgical activityactivity inin reactionsreactions withwith thethe metalmetal ofof thethe moltenmolten weldweld pool,pool, asas shownshownshown ininin FigureFigure6 6..

(a(a) ) ((bb))

Figure 6. Molar fraction of components during dissociation of H2O: (a) up to 5000 K; (b) at 5000–9000 Figure 6.6. Molar fractionfraction ofof componentscomponents during during dissociation dissociation of of H HO:2O: (a ()a up) up to to 5000 5000 K; K; (b ()b at) at 5000–9000 5000–9000 K. K. Pressure in the system is 0.1 MPa. 2 PressureK. Pressure in thein the system system is 0.1 is 0.1 MPa. MPa. Dissociation of water during underwater welding leads to an increase in the hydrogen partial Dissociation of water during underwater welding leads to an increase in thethe hydrogenhydrogen partialpartial pressure and oxidation of iron and alloying elements Mn, Si, Cr, Ni, etc. The increase in oxidation is pressurepressure andand oxidationoxidation ofof ironiron andand alloyingalloying elementselements Mn,Mn, Si,Si, Cr,Cr, Ni,Ni, etc.etc. TheThe increaseincrease inin oxidationoxidation isis dependent on the pressure in the gas system. Particularly active in reactions with liquid metals are dependent onon thethe pressure pressure in in the the gas gas system. system. Particularly Particularly active active in reactionsin reactions with with liquid liquid metals metals are theare the OH hydroxyl group, which is formed in the arc during dissociation of H2O, and water vapor, OHthe OH hydroxyl hydroxyl group, group, which which is formed is formed in the arcin the during arc during dissociation dissociation of H2O, of and H2 waterO, and vapor, water which vapor, is which is formed by alloying elements by reactions, as shown in Figure 7. formedwhich is by formed alloying by elementsalloying elements by reactions, by reactions, as shown as in shown Figure 7in. Figure 7. Me + OH = MeO + 0.5H2 (for Mn, Fe, Co) (18) Me + OH = MeO + 0.5H2 (for Mn, Fe, Co) (18) Me + OH = MeO + 0.5H2 (for Mn, Fe, Co) (18) 2Me + 3OH = Me2O3 + 1.5H2 (for Fe, Cr, Al) (19) 2Me + 3OH = Me2O3 + 1.5H2 (for Fe, Cr, Al) (19)

2Me + 3OH3Fe= Me+ 4OH2O3 =+ Fe1.5H3O42 +(for 2H2 Fe, Cr, Al)(20) (19) 3Fe + 4OH = Fe3O4 + 2H2 (20)

Me 3Fe+ 2OH+ 4OH = MeO= Fe2 +3 OH42 +(for2H Ti,2 Si) (21)(20) Me + 2OH = MeO2 + H2 (for Ti, Si) (21)

MeMe+ + 2OHH2O = MeOMeO 2+ +HH2 (for2 (for Mn, Ti, Fe) Si) (22) (21) Me + H2O = MeO + H2 (for Mn, Fe) (22)

2Me + 3H2O = Me2O3 + 3H2 (for Fe, Cr, Al) (23) 2Me + 3H2O = Me2O3 + 3H2 (for Fe, Cr, Al) (23)

3Fe + 4H2O = Fe3O4 + 4H2 (24) 3Fe + 4H2O = Fe3O4 + 4H2 (24)

Me + 2H2O = MeO2 + 2H2 (for Ti, Si). (25) Me + 2H2O = MeO2 + 2H2 (for Ti, Si). (25)

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Me + H2O = MeO + H2 (for Mn, Fe) (22)

2Me + 3H2O = Me2O3 + 3H2 (for Fe, Cr, Al) (23)

3Fe + 4H2O = Fe3O4 + 4H2 (24)

Metals 2020, 10, x FOR PEER REVIEWMe + 2H2O = MeO2 + 2H2 (for Ti, Si).8 (25) of 17

(a) (b)

FigureFigure 7. 7.Gibbs Gibbs free free energy energy of of metal metal oxidation oxidation reactions: reactions: ( a(a)) for for the the OH OH hydroxyl hydroxyl group; group; ( b(b)) for for water water

vaporvapor H H2O.2O.

TheThe following elements elements are are particularly particularly active active in oxidation in oxidation reactions reactions with hydroxyl: with hydroxyl:Al (ΔG500K Al= − (1611.9∆G kJ),= Cr 1611.9(ΔG500K kJ),= −1086.5 Cr (∆ kJ),G Ti (Δ=G500K1086.5 = −911.4 kJ), kJ), Ti Fe (∆ inG Fe3O4 =(ΔG911.4500K = − kJ),1070 FekJ), in and Fe FeO2O3 500K − 500K − 500K − 3 4 (∆(ΔGG500K = −7821070 kJ). kJ), Resistance and Fe Oto oxidation(∆G =by water782 kJ). vapor Resistance is shown to by oxidation Ni (ΔG500K by = water +26 kJ), vapor Cu for is 500K − 2 3 500K − shownCu2O ( byΔG Ni500K ( ∆= G+86500K kJ),= +and26 kJ),Co ( CuΔG for500K Cu= +20.32O(∆ kJ).G500K = +86 kJ), and Co (∆G500K = +20.3 kJ). ThermodynamicThermodynamic modeling modeling of of phase phase equilibria equilibria shows shows that that adding adding 20% 20% CaF CaF2 and2 and 20% 20% Na Na3AlF3AlF6 6 intointo the the gas gas systemsystem atat 0.1 MPa and and at at 0.6 0.6 MPa MPa leads leads to to a adecrease decrease in inthe the partial partial pressure pressure of H, of H H,2, Hand2, andthe theOH OH hydroxyl hydroxyl group group due due to the to the formation formation of HF, of HF, as shown as shown in Figure in Figure 8. 8. For example, at 3000 K at a pressure of 0.1 MPa with 20% CaF2 added, the partial pressure of H2, OH, and O decreases by 9.1, 8.7, and 3.2%, respectively, and with 20% Na3AlF6 added, the partial pressure of H2, OH, and O decreases by 15.8, 9.8, and 3.16%, respectively. At the pressure of 0.6 MPa at 3000 K, adding 20% CaF2 reduces the partial pressure of H2, OH, and O by 7.8, 8.57, and 4.4%, respectively, and adding 20% Na3AlF6 reduces the partial pressure of H2, OH, and O2 by 15.68, 8.57, and 0.15%, respectively. Adding CaF2 and Na3AlF6 leads to the formation of HF with a partial pressure of up to 0.01 and 0.058 MPa, respectively, at a system pressure of 0.1 MPa and 0.6 MPa.

Metals 2020, 10, 1498 9 of 17 Metals 2020, 10, x FOR PEER REVIEW 9 of 17

(a) (b) (c)

(d) (e) (f)

FigureFigure 8. 8. ChangeChange in in partial partial pressure pressure of of components components in in the the equilibrium equilibrium gas gas mixture mixture in in the the vapor–gas vapor–gas bubble at 0.1 MPa (a–c) and at 0.6 MPa (d–f): a,d—100% H2O; b,e—when adding 20% CaF2; c,f—when bubble at 0.1 MPa (a–c) and at 0.6 MPa (d–f): a,d—100% H2O; b,e—when adding 20% CaF2; c,f—when adding 20% Na3AlF6. adding 20% Na3AlF6.

ForDuring example, heating, at 3000 the K complex at a pressure ﬂuoride of 0.1 Na MPa3AlF 6within the 20% arc CaF dissociates2 added, the with partial the formation pressure of H NaF2, OH,and and AlF 3O. Evaporation decreases by and 9.1, dissociation 8.7, and 3.2%, of CaF respectively,2 and Na3AlF and6 leads with to 20% the Na formation3AlF6 added, of NaF, the AlF partial3, AlF 2, pressure of H2, OH, and O decreases by 15.8, 9.8, and 3.16%, respectively. At the pressure of 0.6 MPa at 3000 K, adding 20% CaF2 reduces the partial pressure of H2, OH, and O by 7.8, 8.57, and 4.4%, respectively, and adding 20% Na3AlF6 reduces the partial pressure of H2, OH, and O2 by 15.68, 8.57,

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2 3 6 Metalsand 20200.15%,, 10, 1498 respectively. Adding CaF and Na AlF leads to the formation of HF with a 10partial of 17 pressure of up to 0.01 and 0.058 MPa, respectively, at a system pressure of 0.1 MPa and 0.6 MPa. During heating, the complex fluoride Na3AlF6 in the arc dissociates with the formation of NaF AlF,and and AlF CaF3. Evaporation molecules, and which dissociation reduce the of partialCaF2 and pressure Na3AlF of6 leads H2 in to the the vapor–gas formation bubbleof NaF, andAlF3 react, AlF2, withAlF, H and2O andCaF H molecules,2 according which to reactions reduce shownthe partial in Figure pressure9: of H2 in the vapor–gas bubble and react with H2O and H2 according to reactions shown in Figure 9: 1.5H2O + AlF3 =0.5Al2O3 + 3HF (26) 1.5H2O + AlF3 =0.5Al2O3 + 3HF (26) H O + CaF = CaO + 2HF (27) 2H2O + CaF2 2 = CaO + 2HF (27)

0.5H0.5H2 +2 NaF+ NaF= =Na Na+ +HF HF (28) (28)

1.5H1.5H2 +2 AlF+ AlF3 =3 Al=Al+ +3HF 3HF (29) (29) HH+ F+ F= =HF HF (30) (30)

0.5H0.5H2 +2 +F F= =HF HF (31) (31)

(a) (b)

FigureFigure 9. 9.Gibbs Gibbs free free energy energy of of reactions reactions (26)–(34) (26)–(34) of interactionof interaction of ﬂuorides of fluorides and and ﬂuorine fluorine in a vapor–gasin a vapor– bubblegas bubble atmosphere atmosphere at 0.1 at MPa 0.1 (MPaa) and (a) at and 0.6 at MPa 0.6 (MPab). (b).

AtAt high high temperatures temperatures in in the the arc, arc, metallurgical metallurgical reactions reactions occur occur in in the the gas gas phase phase between between the the fluoridesfluorides NaF, NaF, AlF AlF3,3, AlFAlF22,, and AlF and and the the oxide oxide TiO TiO2 2withwith the the formation formation of offluorides fluorides TiF TiF3, TiF3, TiF4, and4, andTiF TiF2, which2, which are arehighly highly reactive reactive to H to2O H and2O andH2, for H2 example,, for example, in reactions in reactions (32)–(34), (32)–(34), as shown as shown in Figure in Figure9. 9. 4TiO2 + 2Na3AlF6 = 4TiF3 + 3Na2O + Al2O3 + O2 (32) 4TiO2 + 2Na3AlF6 = 4TiF3 + 3Na2O + Al2O3 + O2 (32) 1.5H2O + TiF3 = 0.5Ti2O3 + 3HF (33) 1.5H2O + TiF3 = 0.5Ti2O3 + 3HF (33) 1.5H + TiF = Ti + 3HF (34) 1.5H2 2 + TiF3 3 = Ti + 3HF (34) In a TiO –Fe O slag system with 10% H , an increase in the content of the basic oxide Fe O In a TiO2 2–Fe2 2O33 slag system with 10% H22, an increase in the content of the basic oxide Fe22O3 3to to 30% and a decrease in the acidic oxide TiO to 70% results in an increase in the mass fraction 30% and a decrease in the acidic oxide TiO2 to 270% results in an increase in the mass fraction of H2O, of H O, especially at the melting temperature of 1700–1750 K. When adding a mixture of fluorides especially2 at the melting temperature of 1700–1750 K. When adding a mixture of fluorides (CaF2 + (CaF + Na AlF ) of up to 30% into the TiO –CaF –Na AlF slag system, a sharp decrease occurs in Na32AlF6) of3 up6 to 30% into the TiO2–CaF2–Na2 3AlF2 6 slag3 system,6 a sharp decrease occurs in the mass the mass fraction of H O in the slag, especially at the melting temperature of 1350–1550 K. A decrease fraction of H2O in the2 slag, especially at the melting temperature of 1350–1550 K. A decrease in the in the H O fraction with an increase in the fluorides content can be explained by the formation of HF H2O fraction2 with an increase in the fluorides content can be explained by the formation of HF and and TiF , which confirms the possibility of reactions (32)–(34) in the slag phase, as shown in Figure 10. TiF3, which3 confirms the possibility of reactions (32)–(34) in the slag phase, as shown in Figure 10.

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(a) (b) (c) (a) (b) (c) FigureFigure 10. 10. MassMass fraction fraction of of water water vapor vapor H H2O2 Oand and fluorides fluorides in in an an equilibrium equilibrium slag slag system system when when adding adding Figure 10. Mass fraction of water vapor H2O and fluorides in an equilibrium slag system when adding 10%10% H H2: 2(:(a)a H)H2O2 Ocontent content in inthe the TiO TiO2–Fe2–Fe2O32 slagO3 slag with with a Fe a2O Fe3 2contentO3 content of 5–30%; of 5–30%; (b) H (b2O)H content2O content in the in 10% H2: (a) H2O content in the TiO2–Fe2O3 slag with a Fe2O3 content of 5–30%; (b) H2O content in the TiOthe2–CaF TiO22–CaF–Na32AlF–Na6 slag3AlF with6 slag fluoride with fluoride (CaF2–Na (CaF3AlF2–Na6) content3AlF6) contentof 5–30%; of 5–30%;(c) HF, TiF (c) HF,3 content TiF3 contentin TiO2– in TiO2–CaF2–Na3AlF6 slag with fluoride (CaF2–Na3AlF6) content of 5–30%; (c) HF, TiF3 content in TiO2– CaFTiO2–Na2–CaF3AlF2–Na6 slag3AlF with6 slag fluoride with (CaF fluoride2–Na (CaF3AlF26)–Na content3AlF 6of) content5–30%: 1 of—HF 5–30%: for 5%1—HF fluorides; for 5% 2—HF fluorides; for CaF2–Na3AlF6 slag with fluoride (CaF2–Na3AlF6) content of 5–30%: 1—HF for 5% fluorides; 2—HF for 10%2—HF fluorides; for 10% 3—HF fluorides; for 20%3—HF fluorides; for 20% 4 fluorides;—HF for 430%—HF fluorides; for 30% 5 fluorides;—TiF3 for5 —TiF10% fluorides;3 for 10% fluorides;6—TiF3 10% fluorides; 3—HF for 20% fluorides; 4—HF for 30% fluorides; 5—TiF3 for 10% fluorides; 6—TiF3 for6—TiF 20% 3fluorides;for 20% fluorides; 7—TiF3 for7—TiF 30% 3fluorides.for 30% fluorides. for 20% fluorides; 7—TiF3 for 30% fluorides.

TestingTesting of of flux ﬂux-cored‐cored wires wires with with gas–slag systems TiO22–Fe2OO33 andand TiO TiO22–CaF–CaF22–Na–Na3AlF3AlF6 6showedshowed Testing of flux‐cored wires with gas–slag systems TiO2–Fe2O3 and TiO2–CaF2–Na3AlF6 showed thatthat the the presence presence of of Fe Fe2O2O3 3cancan lead lead to to the the formation formation of of slag slag inclusions inclusions and and penetration penetration defects. defects. that the presence of Fe2O3 can lead to the formation of slag inclusions and penetration defects. UtilizationUtilization of of a a TiO TiO2–CaF2–CaF2–Na2–Na3AlF3AlF6 gas–slag6 gas–slag system system provided provided a ahigher higher density density of of deposited deposited metal metal Utilization of a TiO2–CaF2–Na3AlF6 gas–slag system provided a higher density of deposited metal and a decrease in porosity and slag inclusions, as shown in Figure 11. andand a decreasea decrease in in porosity porosity and and slag slag inclusions, inclusions, as as shown shown in in Figure Figure 11. 11.

(a()a ) (b(b) ) ((cc))

FigureFigureFigure 11. 11. 11. View ViewView of of of welds, welds, welds, macrostructure, macrostructure, andand and X-ray X X‐ray‐ray testing: testing: testing: ( a( )a(a UW) )UW/CS UW/CS/CS-1‐ coated‐11 coatedcoated electrode; electrode;electrode; (b) (( PPS-AN1b)) PPS‐ AN1ﬂux-coredAN1 flux flux‐cored‐cored wire; wire; (wire;c) PPS-APL2 (c ()c PPS) PPS‐APL2‐APL2 ﬂux-cored flux flux‐cored‐cored wire. wire. wire.

TheTheThe chemical chemicalchemical composition compositioncomposition and and mechanical mechanical properties properties of of the the the welds welds welds areare are shownshown shown inin in TablesTables Tables 2 and3 3..

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Metals 2020, 10, 1498 12 of 17 Table 2. Chemical composition of steel and deposited metal in underwater wet welding of API X70 steel, wt.%. Table 2. Chemical composition of steel and deposited metal in underwater wet welding of API X70 Material C Si Mn Ni S P steel, wt.%. API X70 steel 0.1–0.12 0.29–0.31 1.7–1.75 0.015–0.02 >0.006 >0.013 UW/CSMaterial‐1 electrode C0.06–0.1 0.3–0.35 Si 0.49–0.65 Mn ‐ Ni>0.008 S >0.017 P PPS‐APIAN1 X70 flux steel‐cored wire 0.1–0.12 0.04–0.12 0.29–0.31 >0.002 1.7–1.750.048–0.12 0.015–0.021.1–1.47 >0.006>0.013 >0.018>0.013 PPSUW‐APL2/CS-1 electrodeflux‐cored wire 0.06–0.1 0.03–0.15 0.3–0.35 >0.018 0.49–0.650.27–0.52 0.8–1.2 - >0.008>0.013 >0.015>0.017 PPS-AN1 flux-cored wire 0.04–0.12 >0.002 0.048–0.12 1.1–1.47 >0.013 >0.018 PPS-APL2 flux-coredTable 3. Mechanical wire 0.03–0.15 properties of> 0.018underwater 0.27–0.52 wet welding 0.8–1.2 welds of API> X700.013 steel. >0.015 Yield Welding Tensile Elongation, Impact Toughness, Weld Table 3. MechanicalStrength, properties of underwater wet welding welds of API X70 steel. Consumables Strength, MPa % KCV+20, J Hardness, HV5 Welding Yield Strength,MPa Tensile Impact Toughness, Weld Elongation, % UW/CSConsumables‐1 electrode 440–468MPa Strength,498–545 MPa 6–12 KCV68–89+20,J Hardness,165–203 HV 5 UWPPS/CS-1‐AN1 electrode flux‐ 440–468 498–545 6–12 68–89 165–203 323–336 371–458 2–11.2 62–73 135–212 PPS-AN1 cored wire 323–336 371–458 2–11.2 62–73 135–212 flux-cored wire PPS‐APL2 flux‐ PPS-APL2 330–356 433–462 4–12.6 67–98 162–200 330–356 433–462 4–12.6 67–98 162–200 flux-coredcored wire wire

Due to oxidation, the content of alloying elements, especially manganese and carbon, Due to oxidation, the content of alloying elements, especially manganese and carbon, significantly significantly decreased in the direction from the root to the cap weld. The lowest transition coefficient decreased in the direction from the root to the cap weld. The lowest transition coefficient for alloying for alloying elements was observed when welding with flux‐cored wire PPS‐AN1, as shown in Figure elements was observed when welding with flux-cored wire PPS-AN1, as shown in Figure 12. 12.

(a) (b)

FigureFigure 12.12. ChangeChange in in the the content content of (ofa) carbon(a) carbon and (andb) manganese (b) manganese in diff erentin different weld zones weld when zones welding when withwelding the coatedwith the electrode coated electrode UW/CS-1 UW/CS and flux-cored‐1 and flux wires‐cored PPS-AN1 wires PPS and‐AN1 PPS-APL2: and PPS‐APL2: RW—root RW—root weld; FW—fillweld; FW—fill weld; CW—capweld; CW—cap weld. weld.

MechanicalMechanical tests showed that welds made with thethe ﬂux-coredflux‐cored wirewire PPS-APL2PPS‐APL2 havehave similarsimilar characteristicscharacteristics as regards impact toughness, ductility, andand hardnesshardness asas weldswelds mademade withwith thethe coatedcoated electrodeelectrode UWUW/CS/CS-1;‐1; however,however, the the ultimate ultimate strength strength of of the the welds welds is is 13–15% 13–15% lower, lower, as as shown shown in in Table Table3 and3 and in in Figure Figure 13 13..

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(a) (b) (c) (a) (b) (c) (a) (b) (c) FigureFigure 13. 13. HardnessHardness in the in the cross cross section: section: (a) butt (a) buttwelds; welds; (b) lap (b welds) lap welds with PPS with‐AN1 PPS-AN1 flux‐cored flux-cored wire; Figure 13. Hardness in the cross section: (a) butt welds; (b) lap welds with PPS‐AN1 flux‐cored wire; (cwire;Figure) lap welds ( c13.) lap Hardness with welds PPS within‐APL2 the PPS-APL2 cross flux‐ coredsection: flux-cored wire. (a) buttBM—base welds; wire. metal; ( BM—baseb) lap HAZ—heat welds metal; with‐ affectedPPS HAZ—heat-a‐AN1 zone; flux WM—weld‐ffcoredected wire; zone; (c) lap welds with PPS‐APL2 flux‐cored wire. BM—base metal; HAZ—heat‐affected zone; WM—weld metal.WM—weld(c) lap welds metal. with PPS‐APL2 flux‐cored wire. BM—base metal; HAZ—heat‐affected zone; WM—weld metal. metal. WeldsWelds made made with with the the flux flux-cored‐cored wire PPS-AN1PPS‐AN1 hadhad poorerpoorer mechanical mechanical characteristics characteristics because because of Welds made with the flux‐cored wire PPS‐AN1 had poorer mechanical characteristics because ofthe the presence Weldspresence made of of elongated elongated with the slag fluxslag inclusions,‐ coredinclusions, wire as PPSas shown shown‐AN1 in hadin Figure Figure poorer 14 14.. mechanical characteristics because of the presence of elongated slag inclusions, as shown in Figure 14. of the presence of elongated slag inclusions, as shown in Figure 14.

(a) (b) (c) (a) (b) (c) (a) (b) (c) Figure 14. Welds fractures after tensile tests: (a) UW/CS‐1 coated electrode; (b) PPS‐AN1 flux‐cored Figure 14. Welds fractures after tensile tests: ( aa)) UW/CS UW/CS-1‐1 coated electrode; ( b)) PPS PPS-AN1‐AN1 flux flux-cored‐cored wire;Figure (c) 14.PPS Welds‐APL2 fractures flux‐cored after wire. tensile Arrows tests: indicate (a) UW/CS slag ‐inclusions.1 coated electrode; (b) PPS‐AN1 flux‐cored wire; ( c) PPS PPS-APL2‐APL2 flux flux-cored‐cored wire. Arrows indicate slag inclusions. wire; (c) PPS‐APL2 flux‐cored wire. Arrows indicate slag inclusions. Welds made with the coated electrode UW/CS‐1 and flux‐cored wire PPS‐APL2 had the highest Welds made with the coated electrode UW/CS UW/CS-1‐1 and flux flux-cored‐cored wire PPS PPS-APL2‐APL2 had the highest densityWelds and amade small with number the coated of small electrode non‐metallic UW/CS inclusions,‐1 and flux as‐cored shown wire in FiguresPPS‐APL2 15 andhad 16.the highest density and a small number of small non-metallic inclusions, as shown in Figures 15 and 16. densitydensity and a small number of small non‐metallic inclusions,inclusions, asas shownshown inin FiguresFigures 1515 andand 16. 16.

(a) (b) (c) (d) ((a) (b) ((cc)) ((dd)) Figure 15. A butt joint when welding with the coated electrode UW/CS‐1: (a) macrostructure of the Figure 15. A butt joint when welding with the coated electrode UW/CS‐1: (a) macrostructure of the weld;Figure (b ) 15. microstructure A butt joint when of the welding primary with withferrite; the the (c coated) coated microstructure electrode electrode ofUW/CS UW the/CS-1: ferrite‐1: (a (a )with )macrostructure macrostructure second phase; of of (thed the) weld; (b) microstructure of the primary ferrite; (c) microstructure of the ferrite with second phase; (d) typicalweld; (non (bb)) microstructure microstructure‐metallic inclusions of of the the in primary primarythe center ferrite; ferrite; of the (c) ( weld.microstructurec) microstructure Microstructure of the of theofferrite the ferrite weldwith with secondcenter. second phase; phase; (d) typical non‐metallic inclusions in the center of the weld. Microstructure of the weld center. (typicald) typical non non-metallic‐metallic inclusions inclusions in the in thecenter center of the of theweld. weld. Microstructure Microstructure of the of weld the weld center. center.

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(a) ((bb)) (c()c ) (d()d )

(e) ((ff)) (g(g) ) (h()h ) Figure 16. A lap joints when welding with PPS‐AN1 (a–d) and PPS‐APL2 (e–h) flux‐cored wires: (a,e) Figure 16. AA lap lap joints joints when when welding welding with with PPS PPS-AN1‐AN1 ( (aa––dd)) and and PPS PPS-APL2‐APL2 ( (ee––hh)) flux flux-cored‐cored wires: ( (aa,,ee)) view of lap joints; (b,f) weld macrostructure; (c,g) macrostructure of the fusion zone; (d,h) view ofof lap lap joints; joints; (b,f )( weldb,f) weld macrostructure; macrostructure; (c,g) macrostructure (c,g) macrostructure of the fusion of zone;the (fusiond,h) microstructure zone; (d,h) microstructure of the primary ferrite. Microstructure of the weld center. microstructureof the primary ferrite.of the primary Microstructure ferrite. Microstructure of the weld center. of the weld center. To determine the content of diffusible hydrogen, measurements were performed by vacuum To determine the content of didiffusibleffusible hydrogen, measurements were performed by vacuum method at a pressure of of 1.5 1.5 Pa Pa for for 72 72 h h for for bead bead welding welding of of 100 100 mm mm × 2525 mm mm × 8 8mm mm samples samples with with method at a pressure of 1.5 Pa for 72 h for bead welding of 100 mm ×× 25 mm × 8 mm samples with flux-coredflux‐cored wires PPS-AN1PPS‐AN1 for the TiOTiO2–Fe2OO3 systemsystem and and PPS PPS-APL2‐APL2 for for the the TiO2–CaF–CaF2–Na–Na3AlFAlF6 flux‐cored wires PPS‐AN1 for the TiO22–Fe22O33 system and PPS‐APL2 for the TiO2 2–CaF22–Na33AlF66 system. Under identical welding conditions, the average content of [H] with the PPS‐AN1 wire was system. Under Under identical identical welding welding conditions, conditions, the the average average content content of of [H] [H] with the PPS PPS-AN1‐AN1 wire was 34.8 mL/100 g, and with the PPS‐APL2 wire, 27.1 mL/100 g, i.e., a decrease of 21.1%, as shown in 34.8 mL/100 mL/100 g, and with the PPS PPS-APL2‐APL2 wire, 27.1 mL/100 mL/100 g, i.e., a decrease of 21.1%, as shown in Figure 17. Figure 17.17.

Figure 17. Content of diffusible hydrogen during welding with the electrodes UW/CS‐1, the flux ‐ Figurecored wires 17. Content PPS‐AN1, ofof di anddiffusibleffusible PPS‐APL2 hydrogen hydrogen in air during andduring under welding welding water. with with the the electrodes electrodes UW /UW/CSCS-1, the‐1, flux-cored the flux‐ coredwires PPS-AN1,wires PPS‐ andAN1, PPS-APL2 and PPS‐ inAPL2 air andin air under and under water. water. 4. Conclusions 4. Conclusions Conclusions (1) This work proposed a model of metallurgical and electrochemical processes in underwater wet (1) ThisweldingThis work work in proposed proposeda vapor–gas a a model model bubble, of of moltenmetallurgical metallurgical slag, and and and liquid electrochemical electrochemical weld pool basedprocesses processes on a in thermodynamic underwater wet weldingmodelingwelding in in for a a vapor–gas vapor–gasthe optimization bubble, bubble, of molten molten the gas–slag slag, slag, and system liquid and weld the pool improvement based on of a thermodynamicthe quality of modelingwelds.modeling Thermodynamic for thethe optimization optimization modeling of of the andthe gas–slag gas–slagexperiments system system showed and and the improvementthethat improvementa complex of mechanism the of quality the quality of based welds. of welds.onThermodynamic reducing Thermodynamic the partial modeling pressure modeling and experiments of Hand2O, experimentsH2, showedH, and OH that showed in a complexthe thatatmosphere a mechanism complex of the mechanism based arc and on reducing in based the onvapor–gasthe reducing partial bubble pressure the partial and of increasing Hpressure2O, H2, of H,the H and hydroxyl2O, OHH2, H, in capacity and the atmosphere OH of in the the basic atmosphere of theslag arc system and of the in can thearc be vapor–gasand used in tothe vapor–gasreducebubble the and diffusiblebubble increasing and hydrogen increasing the hydroxyl content the capacity hydroxyl and slag of capacityinclusions the basic of slagin the underwater systembasic slag can wetsystem be usedwelding can to reducebe of usedhigh the ‐to reducestrengthdiffusible the steel. hydrogendiffusible This solution hydrogen content is and achieved content slag inclusions andby increasing slag inclusions in underwater the metallurgical in underwater wet welding activity wet weldingof the high-strength gas–slag of high ‐ strengthsystem in steel. removal This ofsolution water vapor,is achieved hydrogen, by increasing and hydroxyl the metallurgical in reactions withactivity the of formation the gas–slag of system in removal of water vapor, hydrogen, and hydroxyl in reactions with the formation of

Metals 2020, 10, 1498 15 of 17

steel. This solution is achieved by increasing the metallurgical activity of the gas–slag system in removal of water vapor, hydrogen, and hydroxyl in reactions with the formation of HF and ionic dissolution of water vapor in the form of hydroxyl groups OH in the basic fluorine-containing slag of the TiO2–CaF2–Na3AlF6 system of the flux-cored wire. (2) The oxidizing potential of the atmosphere of the arc and the vapor–gas bubble decreases with an increase in fluorides, which improves the transition coefficient of the alloying elements and the density of the deposited metal and reduces the volume of slag inclusions. As a result of using a flux-cored wire with a TiO2–CaF2–Na3AlF6 system, the average strength and impact toughness of the weld increased by 8 and 22%, respectively, and the diffusible hydrogen content decreased by 21% compared to a flux-cored wire with a TiO2–Fe2O3 system.

Author Contributions: Conceptualization, modelling and analysis, S.G.P.; methodology, administration, A.M.L.; investigation, A.S.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the project “Energy-efficient systems based on renewable energy for Arctic conditions” (EFREA), KS1054, South-East Finland–Russia CBC Programme 2014–2020. Acknowledgments: The authors acknowledge Peter Jones from LAB University of Applied Sciences of Lappeenranta for the technical support for preparation of the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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