Metallurgical E ﬀ ect of Rare-Earth Lanthanum Fluoride and Boride in the Composite Coating of Wires in the Arc Welding of Bainitic-Martensitic and Austenitic Steel

: For arc welding of high-strength and cold-resistant steels, the author developed an advanced design of steel wire with a micro-composite coating of a nickel matrix and nanoparticles of LaF 3 and LaB 6 , which improves the metallurgical inﬂuence of rare-earth elements (REE) and forms refractory sulphides and oxides of REE, as well as boron nitrideThe addition of 0.1–0.3 wt% La in the weld pool leads to an increase in the content of the refractory compounds La 2 O 3 , LaO 2 , and LaS, and to the reduction in the content of the low-melting and brittle oxides and sulphides SiO 2 , SiO, MnO, MnS, and SiSThe use of steel wire with the composite coating of LaF 3 and LaB 6 allows for microstructural reﬁnement when welding S960QL bainitic-martensitic steel and X70 API bainitic steel, and increases the impact toughness of the welds by 1.17–1.6 times.


Introduction
Development in the Arctic region requires the application of advanced high-strength and cold-resistant steels, the weldability of which is more complicated due to low-temperature embrittlement and hydrogen-assisted cold cracks (HAC)The focus of research on the weldability of advanced steels is welding metallurgy [1,2], including the development of special welding consumables with rare-earth metals (REEs) [3].
REEs reduce sensitivity to HAC and increase the impact toughness of the weld metal due to microstructural refinement, the formation of acicular ferrite, and a high affinity for sulphur and oxygen [4][5][6]. Wang et al. [6] performed arc welding of HSLA steel 10CrNi3MoV, with a 14 mm thickness, using flux-cored wire with a 0.3-1 wt% rare-earth concentrate of the total flux weight in the wire. When the content of the REE-alloy was 0.3 wt%, the tensile strength of the welds increased from 680 to 750 MPa, and the impact energy increased from 25 to 36 J at −40 • C. Similar positive results were achieved by researchers in welding and casting [7].
Yu et al. [8] improved the ductility and refined the microstructure of the high-temperature alloy Fe-43Ni by adding La 2 O 3 with a residual content in the casting of 0.01-0.04 wt% La. The microstructural refinement was promoted by the formation of dispersed inclusions of La 2 O 2 S with an increase in the content of lanthanum up to 0.025 wt%, which provided effective nucleation centers for the crystallization. Similar results have been achieved by other researchers [9,10], who have reported the formation of spherical inclusions of La 2 O 3 , La 2 O 2 S and LaS instead of elongated inclusions of MnS.
Since REEs have a high affinity not only for oxygen, but also for sulphur, the role of refractory oxides and sulphides should be taken into account when considering the microstructural refinement mechanism of high-strength steels. For example, the formation of the complex oxysulphides Ce 2 O 2 S Thermodynamic modelling of the metallurgical reactions and phase composition were determined on the basis of the thermodynamic data of individual substances, using the modelling program "Terra" (Bauman Moscow State Technical University, Moscow, Russia) and FactSage (CRCT, Montreal, Metals 2020, 10, 1334 3 of 13 Canada) [29][30][31]. Mechanical testing was conducted on a Tinius Olsen Model 602 machine (Walter+Bai AG, Löhningen, Switzerland) in accordance with the GOST 6996-66 standard, using the PH450 pendulum impact testing system (Walter+Bai AG, Löhningen, Switzerland) in accordance with the ISO 148-1:2016 standard Charpy V-notch tests and the EMCOTEST DuraScan-20 hardness tester (EMCO-TEST PrufmaSchinen GmbH, Kuchl, Austria) in accordance with the ISO 6507-1:2018 standardThe chemical composition was determined using a Bruker Q4 TASMAN optical emission spectrometer (Bruker, Karlsruhe, Germany)The metallographic analysis was conducted using the optical microscope Reichert-Jung Me F3A (Leica Microsystems, Wetzlar, Germany), Zeiss Axiovert 200 MAT (Carl Zeiss AG, Oberkochen, Germany), and scanning electron microscope SUPRA 55VP WDS (Carl Zeiss, Oberkochen, Germany), SEM TESCAN MIRA 3 (Tescan Orsay Holding, Brno, Czech Republic).

REE Compound Properties
As pure metallic powders of REEs have high chemical activity, it is preferable to use refractory REE compounds with negative Gibbs free energies for microalloying of the weld pool and grain refinement, through the heterogeneous nucleation mechanism of non-metallic inclusions [14,15]. Tables 2 and 3 and Figures 1 and 2 detail the physical properties and Gibbs free energies of the forming refractory and high-density REE compounds.

Metallurgical Reactions in the Weld Pool
The presence of REE La, Ce, Y, and Th in the weld pool promotes strong metallurgical reactions-deoxidation of FeO and desulfurization of FeS by the formation of refractory REE oxides and sulphides according to reactions (1-8), which have high equilibrium constants, as shown in Figure 3.

Metallurgical Reactions in the Weld Pool
The presence of REE La, Ce, Y, and Th in the weld pool promotes strong metallurgical reactions-deoxidation of FeO and desulfurization of FeS by the formation of refractory REE oxides and sulphides according to reactions (1-8), which have high equilibrium constants, as shown in Figure 3.

Metallurgical Reactions in the Weld Pool
The presence of REE La, Ce, Y, and Th in the weld pool promotes strong metallurgical reactions-deoxidation of FeO and desulfurization of FeS by the formation of refractory REE oxides and sulphides according to reactions (1-8), which have high equilibrium constants, as shown in Figure 3.
2FeO + 2Th = ThO 2 + 2Fe (4) 3FeS + 2La = La 2 S 3 + 3Fe 3FeS + 2Ce = Ce 2 S 3 + 3Fe 3FeS + 2Y = Y 2 S 3 + 3Fe (7)  Thermodynamic modelling of the equilibrium phase composition of the weld pool, in accordance with the maximal solubility of the impurity substances S, O, and N in the program "Terra", confirms that the addition of 0.1-0.3% La and 0.01-0.03 B leads to the formation of refractory REE sulphides and oxides, which are I type modifiers of the microstructure [3,23], as shown in Figures     Thermodynamic modelling of the equilibrium phase composition of the weld pool, in accordance with the maximal solubility of the impurity substances S, O, and N in the program "Terra", confirms that the addition of 0.1-0.3% La and 0.01-0.03 B leads to the formation of refractory REE sulphides and oxides, which are I type modifiers of the microstructure [3,23], as shown in Figures  Thermodynamic modelling of the equilibrium phase composition of the weld pool, in accordance with the maximal solubility of the impurity substances S, O, and N in the program "Terra", confirms that the addition of 0.1-0.3% La and 0.01-0.03 B leads to the formation of refractory REE sulphides and oxides, which are I type modifiers of the microstructure [3,23], as shown in Figures

Wires with Composite Coating
Standard wires of G3Si1, Union X96, and 316L were treated using advanced electrochemical technology, in Ni-electrolytes with 60% Ni(BF4)2 and 10% LaF3 or LaB6. This led to the formation of composite coatings of about 6 μm in thickness from a nickel matrix and LaF3 and LaB6 nanodispersed particles less than 0.7 μm in size, giving an overall content of REE compounds of 0.3-0.4 wt% in the solid wire.

Wires with Composite Coating
Standard wires of G3Si1, Union X96, and 316L were treated using advanced electrochemical technology, in Ni-electrolytes with 60% Ni(BF 4 ) 2 and 10% LaF 3 or LaB 6 . This led to the formation of composite coatings of about 6 µm in thickness from a nickel matrix and LaF 3 and LaB 6 nanodispersed particles less than 0.7 µm in size, giving an overall content of REE compounds of 0.3-0.4 wt% in the solid wire. Figure 6 presents the design of the composite electrode wire, with an electronic and optical view of the macrostructure of the wire surface showing the particles of LaB6The microstructure of the wire's composite coating and the distribution of chemical elements in the composite coating are shown in Figure 7.

Wires with Composite Coating
Standard wires of G3Si1, Union X96, and 316L were treated using advanced electrochemical technology, in Ni-electrolytes with 60% Ni(BF4)2 and 10% LaF3 or LaB6. This led to the formation of composite coatings of about 6 μm in thickness from a nickel matrix and LaF3 and LaB6 nanodispersed particles less than 0.7 μm in size, giving an overall content of REE compounds of 0.3-0.4 wt% in the solid wire. Figure 6   According to the X-ray spectral analysis of the composite coatings, the content of La and F was 31.3% and 15.3%, respectively, as shown in Table 4. The investigation of the macrostructure of the deposited metal shows that the weld deposition of the vertical layers leads to microporosity; however, the presence of lanthanum and fluorine vapour in the arc improves solidity and reduces the levels of microporosity in the weld depositions, as shown in Figure 8. Lanthanum fluoride and boride were found to have a significant effect on the microstructure of the weld metal. The analysis of the microstructure of the deposited metal showed that the use of composite wires with nanodispersed particles of LaF3 and LaB6 leads to microstructural refinement, a decrease in the average grain size for the G3Si1 wire from 40-60 μm to 12-28 μm and from 40-80 μm to 20-35 μm for the 316L wire, and improvement in the shape and the distribution of the microstructural phases, as shown in Figures 9 and 10. According to the X-ray spectral analysis of the composite coatings, the content of La and F was 31.3% and 15.3%, respectively, as shown in Table 4. The investigation of the macrostructure of the deposited metal shows that the weld deposition of the vertical layers leads to microporosity; however, the presence of lanthanum and fluorine vapour in the arc improves solidity and reduces the levels of microporosity in the weld depositions, as shown in Figure 8. According to the X-ray spectral analysis of the composite coatings, the content of La and F was 31.3% and 15.3%, respectively, as shown in Table 4. The investigation of the macrostructure of the deposited metal shows that the weld deposition of the vertical layers leads to microporosity; however, the presence of lanthanum and fluorine vapour in the arc improves solidity and reduces the levels of microporosity in the weld depositions, as shown in Figure 8. Lanthanum fluoride and boride were found to have a significant effect on the microstructure of the weld metal. The analysis of the microstructure of the deposited metal showed that the use of composite wires with nanodispersed particles of LaF3 and LaB6 leads to microstructural refinement, a decrease in the average grain size for the G3Si1 wire from 40-60 μm to 12-28 μm and from 40-80 μm to 20-35 μm for the 316L wire, and improvement in the shape and the distribution of the microstructural phases, as shown in Figures 9 and 10. Lanthanum fluoride and boride were found to have a significant effect on the microstructure of the weld metalThe analysis of the microstructure of the deposited metal showed that the use of composite wires with nanodispersed particles of LaF 3 and LaB 6 leads to microstructural refinement, a decrease in the average grain size for the G3Si1 wire from 40-60 µm to 12-28 µm and from 40-80 µm to 20-35 µm for the 316L wire, and improvement in the shape and the distribution of the microstructural phases, as shown in Figures 9 and 10.   Table 5 shows the chemical composition of S960QL welds with Union X96 wires coated in Ni-LaF 3 and Ni-LaB 6 , indicating a transfer of La from the composite coating in the weld at a content of 0.01-0.04 wt%.   Table 5 shows the chemical composition of S960QL welds with Union X96 wires coated in Ni-LaF3 and Ni-LaB6, indicating a transfer of La from the composite coating in the weld at a content of 0.01-0.04 wt%. When using composite coatings of LaF3 and LaB6 particles in the welding of S960QL steel, it is possible to observe the microstructural refinement in different zones of the weld-corresponding to a decrease in the average grain size from 8-28 μm to 6-12 μm-including the weld metal, the transition zone from the weld to the HAZ (heat-affected zone) and in the HAZ, as shown in Figure  11. When using composite coatings of LaF 3 and LaB 6 particles in the welding of S960QL steel, it is possible to observe the microstructural refinement in different zones of the weld-corresponding to a decrease in the average grain size from 8-28 µm to 6-12 µm-including the weld metal, the transition zone from the weld to the HAZ (heat-affected zone) and in the HAZ, as shown in Figure 11.
Mechanical tests showed that the use of composite coatings with LaF 3 and LaB 6 particles in the welding of S960QL steel led to an increase in the yield strength, hardness, and average impact toughness of the weld from 45 to 54-66 J at −40 • C, as shown in Table 6. Table 7 shows the chemical composition of X70 API welds with G3Si1 wire with coatings of Ni-LaF 3 and Ni-LaB 6 , which indicates the transfer of La and Ni from the composite coating in the weld at a content of 0.01-0.03 wt% La and 0.16-0.21 Ni. Mechanical tests showed that the use of composite coatings with LaF3 and LaB6 particles in the welding of S960QL steel led to an increase in the yield strength, hardness, and average impact toughness of the weld from 45 to 54-66 J at -40 °C, as shown in Table 6.  Table 7 shows the chemical composition of X70 API welds with G3Si1 wire with coatings of Ni-LaF3 and Ni-LaB6, which indicates the transfer of La and Ni from the composite coating in the weld at a content of 0.01-0.03 wt% La and 0.16-0.21 Ni.    A similar positive effect of microstructural refinement in different zones of the weld was achieved during X70 API pipe steel welding with G3Si1 wire with coatings of Ni-LaF 3 and Ni-LaB 6 , as shown in Figure 12.

Weld metal
0.08-0.1 0. A similar positive effect of microstructural refinement in different zones of the weld was achieved during X70 API pipe steel welding with G3Si1 wire with coatings of Ni-LaF3 and Ni-LaB6, as shown in Figure 12. The analysis of the microstructure in the field area of 0.014 mm 2 showed an increase in the proportion of acicular and polygonal ferrite in the bainitic microstructure, and a decrease in the grain size in the weld metal. In particular, the average grain area in the weld decreased from 25 to 11-12 μm 2 . In the HAZ, the average grain area decreased from 29 to 14-16 μm 2 .
As a result of the positive effect of microstructural refinement, the mechanical tests showed that the use of composite coatings with LaF3 and LaB6 particles during X70 API steel welding led to an increase in the yield strength from 526 to 572 MPa, the average impact toughness in the weld from 87 to 143 J and in the HAZ from 143 to 174 J at -20 °C, as shown in Table 8.