Inclusions,microstructure and properties of flux copper backing submerged arc weld metal

Abstract

This paper investigates the effect of Ce on inclusions, microstructure and mechanical properties of the weld metal in flux copper backing submerged arc welding. The results show, with the addition of Ce in weld metal, the inclusions are mainly composed of Al₂O₃, MnO, SiO₂, TiO, Ce₂S₃, CeS and Ce₂O₂S compound, and the proportion of the inclusions of < 1.0 μm in size increased. In competitive nucleation, acicular ferrite nucleates on the oxysulphides of Ce and inhibits the formation of proentectoid ferrite and ferrite side plate. This led to a high proportion of acicular ferrite, good mechanical properties of weld metal. A small lattice disregistry between CeS, Ce₃S₄, Ce₂O₂S and acicular ferrite contributed to low energy for nucleation of acicular ferrite.

Keywords

FCB submerged arc welding Acicular ferrite Inclusions Weld metal Ce

Introduction

Flux copper backing (FCB) submerged arc welding, which has high production efficiency, can achieve one-side welding with back formation of 20–40 mm thick steel plate and is frequently used for critical applications, such as ships, offshore platforms and large diameter pipelines. Figure 1 is the schematic diagram of FCB submerged arc welding.

Schematic diagram of FCB submerged arc welding

Flux copper backing submerged arc welding is a triple-wire co-pool welding process, the heat input of which can exceed 120 kJ cm^− 1. The high heat input results in coarse grain austenite in FCB weld metal, some of which can reach up to 300 μm or more. This coarse grain considerably decreases the toughness of FCB weld metal. In this case, to refine the grains in FCB high input welding process and enhance the toughness of weld metal are some of the key techniques to be solved.¹

In last decades, lots of studies have noticed some beneficial inclusions, such as TiO, TiN, MnS, CuS, etc., promote the nucleation of acicular ferrite (AF), which can refine the grains and improve the strength and toughness of the weld metals.^2–6 However, these beneficial inclusions tend to gather and overgrow in FCB submerged arc welding, which may reduce the promoting effects of the inclusions to the nucleation of AF. New inclusions that can be adapted to the high heat input welding and refine the grains of FCB weld metals are required under such a condition.

Thewlis et al. reported cerium oxysulphide particles, such as CeS, Ce₃S₄ and Ce₂O₂S, can be both stable at liquid metal temperatures and exhibit good lattice coherency with α iron. The inclusions can promote the nucleation of AF and refine the atustenitic grains.⁷ Wen et al. reported CeAlO₃ and Ce₂O₂S are effective in promoting the nucleation of AF in the heat affected zone of high input welding of 16Mn steel.⁸ So, the oxysulphides of Ce have been proven to be beneficial inclusions of the weld metal in high input welding to refine the grains.

It is widely accepted that the higher the heat input is in welding, the larger the weld metal inclusions will be.⁹ Flux copper backing submerged arc welding has a high heat input of ∼120 kJ cm^− 1, which is very conducive for the growth of inclusions like TiO, TiN, MnS and CuS. However, large size inclusions are not able to promote the formation of AF functionally, and the microstructure of the weld metal is mainly proentectoid ferrite (PF) and ferrite side plate (SPF), the toughness of which is pretty low. Yu et al. believed that Ce can suppress the inclusions of weld metal from overgrowing, and oxides/oxysulphides of Ce promote the formation of AF and refine the atustenitic grains.¹⁰ However, there is few research about the effect of Ce in the weld metal of high heat input welding, and more in depth studies are required. This study investigated the effect of Ce on the inclusions, microstructure and mechanical properties of weld metal in FCB submerged arc welding.

Experimental

Triple-wire FCB submerged arc welding was used in the experiment using 20 mm thick DH32 steel plate with single 25° groove as the base metal. The arrangement of three wires (W1–W3) is shown in Fig. 2. The fluxes used on the front and back surfaces were TGF-55E and TGF-B respectively. The chemical compositions of DH32 base metal, wire and flux are shown in Table 1. Wire W2 was flux cored wire, and 0, 2, 4 and 8% CeO₂ was added to the flux cored alloy powder respectively. Operated by the welding technological parameters shown in Table 2, different weld metals with Ce were obtained. Weld metals that sampled from 2.0 mm underneath the surface were made to be specimens for chemical composition and microstructure analysis, transmission electron microscope (TEM) observation and mechanical property tests. Specifications of the experiments are listed in Table 3.

Arrangement of wires

Table 1

Composition of DH32 base metal, wire and flux/wt-%

		C	Si	Mn	S	P	CeO₂ *	Ti	Al
Wire	H10Mn2A	0.09	0.23	1.52	0.004	0.002	0	< 0.02	< 0.02
	Flux cored	0.06	0.22	1.12	0.003	0.001	0, 2, 4, 8	0	0
Base metal	DH32	0.09	0.35	1.51	0.004	0.016	0	< 0.02	< 0.012
TGF-55E	MgO+CaO	Al₂O₃+TiO₂	SiO₂ + ZrO₂	CaF₂	Fe	Mo
	20∼25%	8–10%	10–15%	10–15%	30%	3–5%

Note: *wt-% in flux cored alloy powder

Table 2

Welding parameters

	Welding machine	Wire	Wire diameter/mm	Current/A	Voltage/V	Speed/cm min^− 1	Heat input/kJ cm^− 1
W1	IDEALARC DC-1500	H10Mn2A	4.8	1320	35
W2	IDEALARC AC-1200	Flux cored	4.8	950	40	65	121.3
W3	IDEALARC AC-1200	H10Mn2A	6.4	1050	45

Table 3

Experimental specifications

Experimental facilities and standard	Objectives
Infrared absorption method	Content of oxygen in weld metal
ICP-AES	Content of cerium in weld metal
LWD300LCS optical microscope	Microstructure observation
JEM-2100F TEM	Morphologies of AF and inclusions
EDS	Composition of inclusions
IBAS-2000 image analyser	Size and distribution of inclusions
ASTM-E562-02 standard	Microstructure proportion
High temperature laser scanning confocal microscope	Size of austenitic grains under 1100°C
RFP-08 universal testing machine	Mechanical properties of weld metal
Charpy impact tests under 0°C, − 20°C	Fracture toughness of weld metal
JSM-6480 SEM	Fracture morphology

Results and discussion

Transfer of Ce and composition of weld metal

Under high temperature in FCB submerged arc welding, the following CeO₂ related reactions caused by welding arc may occur 1 2 3 4 As chemical metallurgical processes list above, Ce transferred into weld metal in the form of atoms or ions due to decomposition of Ce₂O₃, the ionisation of CeO₂ and reduction reaction of C.

The oxygen content in weld metal increased as decomposition and ionisation of CeO₂ (as shown in equations (1), 2 and (3)). However, the deoxygenating elements such as Si and Mn would react with released oxygen of CeO₂ and form composite oxides SiO₂·MnO to decrease the content of oxygen in weld metal. Some larger size oxide compounds floated and turned to slag, other smaller ones remained in the weld pool and acted as inclusions in weld metal. The deoxygenation reactions can be expressed as following 5 6 7 After experiencing the physical–chemical metallurgical process mentioned above, the content of C, Si and Mn, as a result, decreased in the weld metal with the increase in CeO₂. Around 8% of CeO₂ was added into the wire W2 of sample WM4, which caused more oxygen were released and more C, Mn and Si were consumed in the welding process as well (Table 4).

Table 4

Chemical compositions of the weld metals/wt-%

	C	Si	Mn	P	S	Ti	Al	Mo	Ce	O/ppm
WM1	0.085	0.340	1.585	0.0169	0.009	0.001	0.063	0.323	0	206
WM2	0.077	0.310	1.459	0.0176	0.008	0.002	0.055	0.324	0.026	219
WM3	0.071	0.273	1.361	0.0169	0.008	0.001	0.066	0.325	0.038	261
WM4	0.065	0.228	1.243	0.0178	0.009	0.001	0.056	0.328	0.072	367

Inclusions in weld metal

A typical inclusion in WM1 without addition of CeO₂ and the related energy dispersive spectroscopy (EDS) spectra are shown in Fig. 3a. From TEM observation, the inclusion, that promoted multiple AF, is spherical in shape with diameter of ∼2.5 μm. Energy dispersive spectroscopy analysis shows that the main elements of the inclusions are Mn, Si, Al, Ti, O and S, which indicates that the main component of the compound inclusions should be Mn, Si, Al, Ti oxides and Mn sulphides.¹¹

a WM1; b WM2; c WM3; d WM4

Figure 3b and d are the typical inclusions in WM2–WM4 respectively with the matching EDS spectra. These inclusions are also spherical in shape, but the geometric size is different; around 1.0–1.5 μm inclusions were formed in WM2 and WM3, while those around 1.0–1.5 μm were observed in WM4. Energy dispersive spectroscopy analysis shows that the main elements of the inclusions are Mn, Si, Al, Ti, O, S and Ce, which indicates the main components of the inclusions are Mn, Si, Al, Ti, Ce oxides and Ce sulphides.¹²

On the basis of the metallurgical rule of welding, the main deoxygenation reactions usually happen in molten drop and pool. Alloying elements Si, Mn and minor elements Al, Ti in the wire, together with transferred Ce would react with oxygen and form oxide compound, which was mainly composed of SiO₂, MnO, Al₂O₃, TiO and Ce₂O₃. As the deoxygenation is in progress, the ratio of oxygen and sulphur in liquid metal reached a critical value, the following desulphurisation reaction occurred¹³ 8 9 10 Because S has a natural characteristic of attaching to the oxides, the desulphurisation actually happened on those oxides surfaces.¹⁴ The Ce₂O₃, Ce₂O₂S, Ce₂S₃ and CeS were likely to combine with the oxides of Al, Si, Mn and Ti and form complex compound in weld metal. As a result, the main phases in the inclusions were Al₂O₃, MnO, SiO₂, TiO, Ce₂S₃, CeS, Ce₂O₂S, Ce₂O₃, etc. Compound inclusions were identified by EDX and shown in Table 5.

Table 5

Compositions and type of inclusions

	Size range/μm	Elements identified	Compounds identified	Possible other compounds present
WM1	0.5–2.5	O, Al, Si, S, Ti, Mn	Al₂O₃, MnO, SiO₂, TiO, MnS	Ti₂O₃, TiO₂
WM2	0.5–1.5	O, Al, Si, S, Ti, Mn, Ce	Al₂O₃, MnO, SiO₂, TiO, Ce₂S₃, CeS, Ce₂O₂S, Ce₂O₃	Ti₂O₃, TiO₂, CeO₂, MnS
WM3	0.5–1.5	O, Al, Si, S, Ti, Mn, Ce	Al₂O₃, MnO, SiO₂, TiO, Ce₂S₃, CeS, Ce₂O₂S, Ce₂O₃	Ti₂O₃, TiO₂, CeO₂, MnS
WM4	0.5–2.0	O, Al, Si, S, Ti, Mn, Ce	Al₂O₃, MnO, SiO₂, TiO, Ce₂S₃, CeS, Ce₂O₂S, Ce₂O₃	Ti₂O₃, TiO₂, CeO₂, MnS

Tables 6 and 7 show the area proportion and size distribution of the inclusions in WM1–WM4 respectively, which was measured and calculated by IBAS-2000 image analyser.

Table 6

Area distribution of the inclusions

	Area proportion A_A/%	Average area A/μm²	Maximum mean average diameter D_max/μm	Area density ρ_A/mm^− 2
WM1	0.334	0.542	0.978	1351
WM2	0.302	0.502	0.827	1864
WM3	0.255	0.456	0.743	2175
WM4	0.287	0.491	0.832	2031

Table 7

Size distribution of the inclusions

	< 1.0 μm	1.0∼2.0 μm	2.0∼3.0 μm	3.0∼4.0 μm	>4.0 μm
WM1	39.33	48.37	10.26	1.53	0.51
WM2	56.35	39.42	3.14	0.89	0.20
WM3	67.26	29.08	2.75	0.74	0.17
WM4	52.74	41.05	5.07	0.91	0.23

From Tables 6 and 7, the area proportion A_A, mean area A and the maximum mean average diameter D_max of the inclusions in WM1 were larger than those of others. Furthermore, the area density ρ_A of WM1 was the smallest, and >60% inclusions were larger than 1.0 μm. When Ce was added (samples WM2–WM4), the A_A, A and D_max decreased simultaneously, while ρ_A increased. More than a half of the inclusions were < 1.0 μm, especially in WM3, >67% of inclusions were < 1.0 μm.

According to the critical nucleation theory of inclusions in the liquid steel, the growth of the inclusions is due to mutual collisions.¹⁵ When the density of the inclusions is less than a certain critical value, inclusion can barely collide with each other in liquid metal. The more nucleation particles are, the smaller inclusion will be. With the addition of CeO₂, on one hand, it increased the content of oxygen in the liquid metal; on the other hand, due to the strong oxidising property of Ce and high melting point of Ce oxides/oxysulphides, the transfer of element Ce caused soaring amount of Ce₂O₃ and Ce₂O₂S particles in the liquid metal, which acted as the nucleation cores for oxides of Si, Ti, Al and Mn. In WM2 and WM3, although Ce did increase the amount of Ce₂O₃, Ce₂O₂S particles, the density of which was still less than the critical value and the inclusions could not grow by collisions and the size of the inclusions were not as big as that of WM4 (Tables 6 and 7). WM4 had 8% CeO₂ added in wire W2, the oxygen content reached up to 367 ppm, which was far beyond that in weld metals of WM2 and WM3. The weld pool with high oxygen content benefited the nucleation of inclusion particles to exceed the critical density and to grow and coarsen by mutual collisions. In this case, the large diameter inclusions exhibited higher proportion in WM4. As a conclusion, Ce has an alternative effect to the inclusions of FCB welding weld metal by increasing the density and controlling the size of the inclusions. However, too much Ce can cause the size of the inclusions become large.

Microstructure of FCB weld metals

Figure 4 shows the optical microstructure of weld metals from WM1 to WM4. It can be seen that, the microstructure in weld metal consisted of PF, SPF, granular ferrite (GF) and AF. There was only 34.4% AF in WM1. For WM2 (with 0.026% of Ce), 62.3% AF were observed. When Ce content rose to 0.038% in WM3, the microstructure contained 76.7% fine AF. After Ce was risen to 0.072% in WM4, AF did not rise but dropped to 61.6%. The proportions of PF, SPF, GF, AF and original size of austenitic grain are listed in Table 8.

a WM1; b WM2; c WM3; d WM4

Table 8

Proportion of PF, SPF, GF and AF

	PF/%	SPF/%	GF/%	AF/%	Austenite width/μm
WM1	27.1	20.4	18.1	34.4	153–185
WM2	17.6	8.8	11.3	62.3	52–64
WM3	11.2	7.9	4.2	76.7	42–48
WM4	15.4	7.2	15.8	61.6	56–67

From the optical observations on the microstructure of weld metal and Table 8, it is clear that the microstructure and the austenitic grain size have been modified by adding a certain amount of Ce in the weld metal. Ce can suppress the formation of PF, SPF and GF, and on the contrary, promote the formation of AF and decrease austenitic grain size in FCB weld metal.

According to the lattice mismatch formula¹⁶: δ = Δα/α, where δ is the disregistry, Δα is the difference of lattice constant and α is the lattice constant. Ce added liquid weld metal consists non-metallic inclusions like Ce₂S₃, CeS and Ce₂O₂S, all of which have a good coherency with δ-Fe. In addition, these Ce₂S₃, CeS and Ce₂O₂S inclusions are also convenient for the nucleation of δ-Fe as well. Little lattice mismatch attributed to the hindering of the δ-Fe from growing too large in FCB weld metal, and hence, small δ-Fe grains resulted in small austenite and AF grains when cooling down. Furthermore, the high density of tiny oxides/oxysulphide inclusions of Ce, the diameter of which is < 1.0 μm, in samples WM2–WM4 promoted the nucleation of AF and finally caused the increase in AF proportion in weld metal. All in all, Ce can prohibit the overgrowing of austenite grains and the AF grains as well.

Mechanical properties of weld metal

From the results of mechanical test in Table 9, the tensile strength, elongation rate and the charpy impact energy under 0 and − 20°C are much higher of samples WM2–WM4 than those of WM1. Ce can not only refine the austenite and AF grains but also effectively increase the proportion of AF and suppress the formation of PF and SPF in the microstructure. All of these characteristics of Ce can improve the tensile strength and toughness of FCB weld metal. However, when the content of Ce reached 0.072% in WM4, its elongation rate and impact energy did not rise but became much lower than WM2 and WM3. It can be explained by the excessive Ce could not only coarse the inclusions and austenitic grains but also increase the formation of PF and SPF instead of AF (Fig. 4d and Table 8).

Table 9

Mechanical properties of weld metals

	Yield strength/MPa	Tensile strength/MPa	Elongation/%	Average impact energy at 0°C/J	Average impact energy at − 20°C/J
WM1	534	615	12.4	67	41
WM2	533	642	22.8	84	63
WM3	531	649	26.8	109	82
WM4	515	631	18.2	75	59

Figure 5 shows the SEM images on fracture morphologies of impact tested weld metals of samples WM1–WM4 at − 20°C. Both cleavage fracture and ductile fracture can be observed on the fracture surface of WM1 (Fig. 5a), which caused the relatively low impact energy in the test. A typical ductile fracture morphology can be found on samples WM2–WM4, and this can increase the impact energy to certain extent, especially on WM3. From Fig. 5c, the size of the dimples formed on WM3 is much smaller than others (Fig. 5a, b and d ). Most of the dimples are < 1 μm in size, which give rise to the favorable ductile property of WM3 under impact loading. In addition, WM3 has >76% of AF, which has a high angle boundaries property and high dislocation densities in grain, and it will take high energy for microcracks to cross AF grains. As a result, the impact energy tested on WM3 was the highest, comparing to the other three.

a WM1; b WM2; c WM3; d WM4

Nucleation mechanism of AF

As mentioned in the section on ‘Inclusions in weld metal’, there are many 0.5–1.5 μm inclusions that are composed of Ce₂O₂S, Ce₂S₃ and CeS in FCB submerged arc welding metal with Ce. According to theory of small lattice disregistry,^{12, 17}smaller mismatch between inclusions and α-Fe will lower the distortion energy for AF to nucleate on inclusions. The lattice constant of α-Fe is 0.2886 nm, while Ce₃S₄ has three times as much as α-Fe with a constant of 0.8625 nm, and CeS has twice as much as α-Fe with 0.5778 nm lattice constant. Ce₂O₂S is a CeO·CeS complex, the lattice of which is also twice as much as α-Fe. In this case, the degree of mismatch is relatively low between Ce₂O₂S, Ce₂S₃, CeS inclusions and α-Fe. The distortion energy decreased as well and this benefit the nucleation of AF on those inclusions.

Conclusions

In this study, the effect of Ce on the inclusions, microstructure and mechanical properties in FCB weld metal was investigated. The results can be summarised as follows.

Under the influence of the welding arc, CeO₂ was decomposed and ionised, released oxygen reacted with C, Si and Mn in welding process. With the addition of Ce, the proportion of the inclusions, the diameter of which are < 1.0 μm, considerably increased and the average diameter of the inclusions decreased in the welding metal. Energy dispersive spectroscopy spectra show the inclusions of the Ce added weld metal are mainly composed of Al₂O₃, MnO, SiO₂, TiO, Ce₂S₃, CeS, Ce₂O₂S and Ce₂O₃.

With the addition of Ce, AF nucleated on the oxides and oxysulphides of Ce in austenite, suppressed the formation of PF and SPF at the austenitic grain boundaries in the competitive nucleation. The tests in this study show the proportion of AF can approach 76.7%, when the content of Ce in weld metal is 0.038% (WM3).

Ce refined the grains of the high heat input FCB weld metal, increased the proportion of AF and improved the toughness of weld metal.

The mechanism of nucleation of AF in Ce added high heat input FCB welding metal is a small lattice disregistry between CeS, Ce₃S₄, Ce₂O₂S and α-Fe, which leads to relative low energy for AF to nucleate on the these inclusions. As a result, CeS, Ce₃S₄ and Ce₂O₂S can induce AF to be generated in large amounts.

Acknowledgements

This research is supported by the National Natural Science Foundation of China and Baoshan Iron & Steel Co., Ltd (no. U1260103) and the Provincial Key Laboratory of Advanced Welding Technology, Jiangsu University of Science & Technology.

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