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

Abstract

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 LaF₃ and LaB₆, which improves the metallurgical influence of rare-earth elements (REE) and forms refractory sulphides and oxides of REE, as well as boron nitride. The addition of 0.1–0.3 wt% La in the weld pool leads to an increase in the content of the refractory compounds La₂O₃, LaO₂, and LaS, and to the reduction in the content of the low-melting and brittle oxides and sulphides SiO₂, SiO, MnO, MnS, and SiS. The use of steel wire with the composite coating of LaF₃ and LaB₆ 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.

1. 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 La₂O₃ 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 La₂O₂S 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 La₂O₃, La₂O₂S 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 Ce₂O₂S and La₂O₂S, which have a high melting temperature, density, and thermodynamic stability, and a low Gibbs free energy [11,12,13,14]. Non-metallic inclusions in high-strength steel welding promote the intragranular nucleation of acicular ferrite, increasing the fatigue crack propagation energy and decreasing the critical temperature of the ductile–brittle transition [15].

It is known that the addition of boron, leading to the formation of borides and carboborides, also has a positive effect on the properties and microstructure of cold-resistant austenitic steel 316L with a low residual nitrogen content of up to 0.04% and boron content of up to 0.004%. This effect is associated with boron solubility in γ-Fe up to 0.02% and in α-Fe up to 0.0081%, and the formation of the borides Me₂B, Me₅B₃, Me₃(C, B), Me₂₃(C, B)₆, and FeMo₂B₄ [16,17]. The addition of REEs in the welding of the flux-cored wire and solid wires also makes it possible to improve the technological properties of the welding arc and the arc stability, and to reduce spattering of the CO₂ shielding gas [18,19,20,21].

The transition of REEs from the welding consumable into the weld pool is accompanied by loss of REEs through evaporation and oxidation; therefore, to achieve an effective residual content of REE in the molten pool, the content of REE in the electrode wire should be at least 0.3–1.6 wt% [3]. The content of REE in the molten weld pool undergoes a further reduction as a result of the REE metallurgical reactions with sulphur, oxygen, and nitrogen, which are characterized by the transition coefficient of an alloying element from the weld pool in the weld metal [22]. In the work of Goldstein and Mizin [23], it is noted that the residual content of REE in the weld metal is 0.02% and 0.05% when adding 0.1% and 0.25% of the REE content to the weld pool, respectively; that is, the transition coefficient from weld pool in the weld metal is 0.2. The effect of grain refinement is observed when the residual content of REEs is 0.02–0.16 wt% in the weld [24,25,26,27]. For example, Zhu and Yan [28] improved the microstructure and reduced sensitivity to cracks in 441 steel welding by adding 0.024% cerium. Therefore, a positive effect was observed with an initial content of 0.1–0.8% REEs in the weld pool, before metallurgical reactions had commenced [3].

2. Materials and Methods

The arc welding described here was focused on using samples of X70 API pipe steel (CHTPZ, Chelyabinsk, Russia) of 300 mm × 150 mm × 21.3 mm with G3Si1 wire (ESAB, Gothenburg, Sweden) of 1.2 mm diameter, and S960QL steel (SSAB, Stockholm, Sweden) of 350 mm × 100 mm × 8 mm with Union X96 wire (Boehler Welding, Hamm, Germany) of 1.0 mm diameter, with multilayer vertical weld depositions that were 60–80 mm high and 180 mm long, with 316L wire (ESAB, Sweden) of 1.0 mm and G3Si1 of 1.2 mm diameter. In the arc welding and deposition of samples in a shielding mixture of 82% Ar and 18% CO2, the authors used a Comau Smart NC-16-1.65 robot (Comau S.p.A., Grugliasco, Italy) with a Fronius TransPuls Synergic 4000 CMT source (Fronius, Wels, Austria), according to the welding parameters shown in

Table 1. Welding parameters for S960QL, X70 API welding and the multilayer weld deposition.

Root Pass	S960QL with Union X96 1.0 mm			X70 API with G3Si1 1.2 mm
	I, A	U, V	V, m/min	I, A	U, V	V, m/min
	170	24	7.8	160	20	5.5
Filling passes	210	26	8.5	200	25	6.5
Weld deposition	Multilayer deposition with 316L 1.0 mm (45 layers)			Multilayer deposition with G3Si1 1.2 mm (45 layers)
	I, A	U, V	V, m/min	I, A	U, V	V, m/min
	64	12.9	2.2	92	13.3	2.1

.

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, 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 standard. The 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).

3. Results and Discussion

3.1. 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].

Table 2. Physical properties of refractory oxides of REEs [32,33,34,35].

REE Compound	Melting Temperature, °C	Density, g/cm3	REE Compound	Melting Temperature, °C	Density, g/cm3
La2O3	2304	6.51	Ce2O3	2250	6.2
Y2O3	2439	5.03	CeO2	2480	7.21
Nd2O3	2233	7.24	ThO2	3350	10

and

Table 3. Physical properties of refractory sulphides of REEs [32,33,34].

REE Compound	Melting Temperature, °C	Density, g/cm3	REE Compound	Melting Temperature, °C	Density, g/cm3
LaS	2300	5.61	Y2S3	1925	3.87
La2S3	2110	4.9	NdS	2200	6.23
CeS	2445	5.9	Nd2S3	2207	5.46
Ce2S3	2450	5.02	ThS	2335	9.56
YS	2060	4.92	Th2S3	2300	7.88

and Figure 1 and Figure 2 detail the physical properties and Gibbs free energies of the forming refractory and high–density REE compounds.

3.2. 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.

3FeO + 2La = La₂O₃ + 3Fe

(1)

2FeO + 2Ce = CeO₂ + 2Fe

(2)

3FeO + 2Y = Y₂O₃ + 3Fe

(3)

2FeO + 2Th = ThO₂ + 2Fe

(4)

3FeS + 2La = La₂S₃ + 3Fe

(5)

3FeS + 2Ce = Ce₂S₃ + 3Fe

(6)

3FeS + 2Y = Y₂S₃ + 3Fe

(7)

3FeS + 2Th = Th₂S₃ + 3Fe

(8)

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 Figure 4 and Figure 5. At the same time, the content of S, O, N, MnO, MnS, SiO, SiO₂, SiS, and CrS decreases. The presence of these substances leads to the formation of low-melting eutectics at the grain boundaries, and reduces the impact toughness.

3.3. 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 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 LaB6. The microstructure of the wire’s composite coating and the distribution of chemical elements in the composite coating are shown in Figure 7.

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. X–ray spectral analysis of the composite coatings, wt%.

Coating	Ni	La	F	B
Ni–LaF3	58.08	23.57	15.30	–
Ni–LaB6	61.9	31.3	–	16.5

.

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 Figure 9 and Figure 10.

Table 5. Chemical composition of S960QL welds, wt%.

Base metal	C	Si	Mn	Cr	Mo	Ni	Al
	0.13–0.16	0.4–0.5	1.7–1.8	1.2–1.36	0.46–0.6	0.8–1.1	<0.07
	V	Cu	Ti	La	B	P	S
	<0.16	0.4–0.51	<0.01	–	<0.001	<0.008	<0.003
Weld metal	C	Si	Mn	Cr	Mo	Ni	Al
	0.08–0.11	0.6–0.63	1.94–2.1	0.49–0.7	0.5–0.56	1.1–1.4	<0.05
	V	Cu	Ti	La	B	P	S
	<0.016	0.13–0.18	<0.02	0.01–0.04	<0.005	<0.01	<0.008

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.

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. The mechanical properties of the welds of S960QL steel with Union X96 wire.

Wire	Yield Strength, MPa	Tensile Strength, MPa	Impact Toughness, KCV−40, J	Hardness, HV10 Test Force: 98,07 N Test Time: 4 s
			Weld Metal	Base Metal	Weld Metal	HAZ
Union X96 standard wire	930–970 950	1048–1152 1100	38–52 45	297–335 316	367–385 376	252–281 266
Union X96 wire with Ni-LaF3 coating	950–990 970	1035–1125 1080	50–58 54	302–355 328,5	365–404 384,5	269–286 277,5
Union X96 wire with Ni-LaB6 coating	1033–1059 1046	1133–1199 1166	62–70 66	308–358 333	392–403 397,5	282–292 287

.

Table 7. Chemical composition of X70 API welds, wt%.

Base metal	C	Si	Mn	Cr	Mo	Ni	Al
	0.09–0.12	0.28–0.32	1.62–1.7	0.03–0.04	<0.01	0.02–0.04	0.04–0.05
	V	Cu	Ti	La	B	P	S
	0.03–0.05	0.16–0.19	0.01–0.02	–	–	<0.007	<0.006
Weld metal	C	Si	Mn	Cr	Mo	Ni	Al
	0.08–0.1	0.8–1.1	1.3–1.4	0.01–0.03	<0.01	0.16–0.21	<0.005
	V	Cu	Ti	La	B	P	S
	<0.008	0.01–0.03	<0.002	0.01–0.034	<0.005	<0.007	<0.02

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-LaF₃ and Ni-LaB₆, as shown in Figure 12.

The analysis of the microstructure in the field area of 0.014 mm² 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 μm². In the HAZ, the average grain area decreased from 29 to 14–16 μm².

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. The mechanical properties of the welds of X70 API steel with G3Si1 wire.

Wire	Yield Strength, MPa	Tensile Strength, MPa	Elongation, %	Impact Toughness, KCV–20, J		Hardness, HV10 Test Force: 98,07 N Test Time: 4 s
				Weld Metal	HAZ	Base Metal	Weld Metal	HAZ
G3Si1 standard wire	519–533 526	574–589 581.5	10.9–11.3 11.1	84–90 87	116–170 143	193–212 202	189–193 191	208–241 224
G3Si1 wire with Ni-LaF3 coating	546–574 560	600–620 610	11.8–15.5 13.6	100–104 102	134–174 154	204–217 210	199–214 206	223–236 229
G3Si1 wire with Ni-LaB6 coating	560–584 572	609–670 639.5	12.5–19.5 16	127–160 143.5	150–198 174	201–224 212	202–210 206	213–247 230

.

4. Conclusions

(1)

The advanced design of steel wire with a micro-composite coating of a nickel matrix and nanoparticles of REE compounds allows improvement of the microstructure and the properties of the welds during arc welding or wire-arc additive manufacturing of high-strength and cold-resistant steels. Adding LaF₃ and LaB₆ into the matrix of the composite coating of the wire at a content of 0.4 wt% increases the transition coefficient of lanthanum from the wire to the weld pool, and the metallurgical effect of REE on the nucleation mechanism of the nonmetallic inclusions.

(2)

The REE and boron transition from the wire composite coating to the weld pool leads to strong metallurgical reactions, the formation of refractory sulphides and oxides of REEs and boron nitride, and a decrease in the content of low-melting and brittle sulphides and oxides. The maximum solubility of O, N and S, and the addition of 0.3 wt% La in the weld pool of G3Si1 pearlitic steel or 316 L austenitic steel, leads to an increase in the content of the refractory compounds La₂O₃, LaO₂, and LaS to 0.0035 wt% each, which allows reduction of the content of SiO₂, SiO, MnO, and MnS to 0.001–0.002 wt%, or by 3.3–3.5 times, and the SiS content by 1.6–2.2 times.

(3)

As a result, the pearlitic, bainitic, and austenitic microstructural refinement enables a reduction of the average grain size by 1.3–2 times, decreasing the width of the grain boundaries. The application of wires with a composite coating of LaF₃ and LaB₆ particles in the welding of S960QL bainitic-martensitic steel offers an increase in the impact toughness of the weld by 1.2–1.4 times, and in the welding of X70 API bainitic steel by 1.17–1.6 times.