M. PRIJANOVI^ TONKOVI^, J. LAMUT: PHASE ANALYSIS OF THE SLAG AFTER SUBMERGED-ARC WELDING
101–107
PHASE ANALYSIS OF THE SLAG AFTER SUBMERGED-ARC
WELDING
ANALIZA FAZ V @LINDRI PRI OBLO^NEM VARJENJU POD
PRA[KOM
Marica Prijanovi~ Tonkovi~1, Jakob Lamut2
1High Mechanical Engineering School, [egova 112, 8000 Novo Mesto, Slovenia
2University of Ljubljana, Faculty of Natural Sciences and Engineering, A{ker~eva cesta 12, 1000 Ljubljana, Slovenia
marica.prijanovic-tonkovic@guest.arnes.si
Prejem rokopisa – received: 2015-01-16; sprejem za objavo – accepted for publication: 2015-03-04
doi:10.17222/mit.2015.014
The quality of a weld depends, to a large extent, on the filler material and type of welding. Welds and surfacing welds were
produced with submerged-arc welding. The welding current was varied. The flux with a fineness of 0.2–1.8 mm was used.
During the welding process, the welding flux melted and the liquid slag was formed. After the input of heat was stopped, the
solidification of the slag began and different mineral phases started to precipitate. We found out that besides the basic
constituents listed by the manufacturer of the welding flux, the alloying elements and deoxidizers from the flux and from the
melt are also present in the slag. Based on these results, it can be concluded that in the case of reusing the welding slag for the
production of welding flux, it is important to consider the composition of the welding slag.
Keywords: submerged-arc welding, welding flux, welding slag
Na kakovost zvara ima velik vpliv dodajni material ter postopek varjenja. Zvari in navari so bili izdelani po postopku varjenja
pod pra{kom. Tok varjenja se je spreminjal. Uporabljen je bil varilni pra{ek, zrnatosti od 0,2 do 1,8 mm. Med varjenjem se je
varilni pra{ek stalil in nastala je teko~a `lindra. Po kon~anem dovajanju toplote se je za~elo strjevanje `lindre in v `lindri so se
izlo~ale razli~ne mineralne faze. Ugotovili smo, da na mineralno sestavo `lindre vplivajo poleg osnovnih sestavin, ki jih navaja
proizvajalec varilnega pra{ka, tudi legirni elementi in dezoksidanti iz pra{ka in iz taline. Na osnovi rezultatov preiskav
sklepamo, da je pri ponovni uporabi varilne `lindre za izdelavo varilnega pra{ka pomembno, da se upo{teva tudi sestavo varilne
`lindre.
Klju~ne besede: varjenje pod pra{kom, varilni pra{ek, `lindra po varjenju
1 INTRODUCTION
The quality of welds and surfacing welds depends on
the chemical composition of the filler and base material
as well as on the method of welding, determining the
heat input which influences the development in the heat-
affected zone and in the melt during the welding process.
The slag, formed during the process of flux melting,
plays an important role.1 The slag accompanies the metal
during the wire fusion and it covers the melt and protects
it until it solidifies. The welding slag can be used again
for the flux preparation.2
The important welding parameters are the welding
current, the voltage and the speed of welding.3 If the
current is too high, degassing in the weld is weak and
cracks occur. Raising the current increases the depth of
remelting the base material.4 Surfacing with a low
welding current (450 A) is recommended in the case of
using the alloyed agglomerated fluxes.5
Slag is formed during the melting of the fluxes that
lead to the ionisation of the arc atmospere and deoxi-
dation of the melt, enable the passage of the alloying
elements into the melting bath and prevent oxidation of
carbon, manganese and silicon. Many types of welding
fluxes are in use and they differ by the main mineral
content and the presence of metal additives.6–10 Recycled
SAW slag can be used as a welding flux.8
For the investigations, the agglomerated flux labelled
as FB 12.2,11 suitable for reaching a high toughness in
multipass welds, was used. The width of the heat-
affected zone (HAZ) of steel OCR12 VM12,13 welded
with the submerged-arc welding method depends on the
welding current.14 Figure 1a shows the microstructure at
the HAZ/weld border at a current of 470 A, and Figure
1b shows the effect of a current of 610 A. The mineral
composition of the slag, formed during the welding
process, was determined.
2 EXPERIMENTS
Four different welds were produced, two welds and
two surfacing welds. Figure 2a presents a steel sample,
used for welding. Figure 2b presents a surfacing weld.
The welding was carried out on a device for submerged-
arc welding made by Iskra. The chemical compositions
of the welding pieces (steel OCR12 VM) and the filler
material are presented in Table 1.
The applied basic welding flux11 is used for automa-
tic welding and surfacing of construction steels.
Materiali in tehnologije / Materials and technology 50 (2016) 1, 101–107 101
UDK 621.791.793:66.046.58 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(1)101(2016)
The first joining of the welding pieces was performed
with the MAG procedure. Table 2 shows the welding
parameters. The current and the voltage were varied
during the welding process. The time of welding and the
mass of the used welding flux were measured.
Table 2: Welding parameters
Tabela 2: Parametri varjenja
Specimen
Wire
diameter
(mm)
Voltage
(V)
Current
(A)
Specimen 1 − weld 3.2 28 470
Specimen 2 − weld 3.2 27 610
Specimen 3 −
surfacing weld 3.2 29 450
Specimen 4 −
surfacing weld 3.2 28 628
The samples of the flux and slags were examinated
with metallography, scanning electron microscopy
(SEM) of type JEOL JSM 5610 and analysed with elec-
tron dispersive spectroscopy (EDS) of type JEOL JSM
5610 and X-ray diffraction (XRD) of type PANalitical
X’pert PRO.
3 WELDING-FLUX COMPOSITION
The agglomerated commercial basic welding flux of
type FB 12.2 was used for the welding. The chemical
composition of the welding flux (Table 3) and the
basicity of the flux according to the Boniszewski index
of 1.70 were taken from the catalogue.11
For the microscopic investigation of the welding flux
and welding slag, metallographic samples were prepared.
After grinding and polishing, the samples were ready for
the metallographic analysis. Figure 3 shows a SEM
image of a welding flux composed of non-metallic and
metallic materials.
M. PRIJANOVI^ TONKOVI^, J. LAMUT: PHASE ANALYSIS OF THE SLAG AFTER SUBMERGED-ARC WELDING
102 Materiali in tehnologije / Materials and technology 50 (2016) 1, 101–107
Figure 1: Microstructure of the weld on the HAZ/weld border with a
current of: a) 470 A and b) 610 A
Slika 1: Mikrostruktura zvara na meji CTV/zvar pri toku varjenja: a)
470 A in b) 610 A
Table 1: Chemical composition of base steel and filler material
Tabela 1: Kemijska sestava osnovnega jekla in dodajnega materiala
Type of material SIST EN 10027-2
Chemical composition (w/%)
C Si Mn P S Cr Mo V
Welding piece: OCR12 VM (1.2379) 1.50 0.4 0.4 0.03 0.03 11.5 0.8 0.85
Filler material 0.08 0.35 1.4 / 0.03 5.0 0.85 /
Figure 2: Macro-shot of the welding piece for: a) weld and b) sur-
facing weld
Slika 2: Makroposnetek varjenca za: a) zvar in b) navar
Table 3: Chemical composition of welding flux11
Tabela 3: Kemijska sestava varilnega pra{ka11
Welding
flux
Chemical composition (w/%)
SiO2 +
TiO2
CaO +
MgO
Al2O3 +
MnO CaF2
FB 12.2 20 30 25 20
The distribution of the elements in the welding flux
found with EDS is presented in Figure 4. The area
where, on the EDS chart, aluminium overlaps with tita-
nium aluminium oxide, which contains mass fractions of
1.5 % of titanium oxide, is presented.
The region where only calcium can be found indi-
cates the presence of calcium fluorite CaF2. Calcium
overlapping with silicon is typical for calcium silicon
(CaSi) and wollastonite (CaO·SiO2). As expected, three
elements – aluminium, silicon and potassium – overlap
with the presence of alumosilicate containing potassium. The presence of silico manganese (SiMn) in the
welding powder indicates a distribution of manganese
and silicon.
Figure 5 depicts the grains of calcium silicon (CaSi)
that contain mass fractions of 46 % of calcium and mass
fractions of 54 % of silicon and an inclusion of ferro-
silicon with minimum amounts of w(Si) = 45 %, w(Al)
= 2.7 % and about w(Fe) = 52 %. The calcium silicon
grains have a slag inclusion from the production of
calcium silicon (CaSi) (Figure 6).
Figure 7 presents an XRD diagram of the welding
flux that shows the presence of aluminium and magne-
sium oxides (periclase) and calcium fluorite.
M. PRIJANOVI^ TONKOVI^, J. LAMUT: PHASE ANALYSIS OF THE SLAG AFTER SUBMERGED-ARC WELDING
Materiali in tehnologije / Materials and technology 50 (2016) 1, 101–107 103
Figure 3: Microstructure of SAW flux
Slika 3: Mikrostruktura EPP pra{ka
Figure 6: Microstructure of calcium silicon with included slag
Slika 6: Mikrostruktura kalcij silicija z vklju~kom `lindre
Figure 4: EDS distribution of elements; SEM; EDS
Slika 4: EDS-prikaz porazdelitve elementov; SEM; EDS
Figure 5: Microstructure of calcium silicon with a ferro-silicon
inclusion
Slika 5: Mikrostruktura kalcij silicija z vklju~kom fero silicija
Figure 7: XRD of the welding flux FB 12.2
Slika 7: XRD-diagram varilnega pra{ka FB 12.2
4 RESULTS AND DISCUSSION
4.1 Influence of the welding current on the welding-
flux usage
During the process of welding, the welding flux
melts and a liquid slag is formed. The arc is hidden in
the liquid slag, which means that the welding procedure
is welder friendly. The welding current influences the
time of welding and the consumption of the welding
flux. Different currents were applied during the welding
procedures; the surfacing weld was welded at currents of
450 A and 628 A, while the weld was welded at currents
of 470 A and 610 A. Figure 8 represents the used flux in
correlation with the welding current. Increasing the
current power for the surfacing welds leads to a larger
welding-flux consumption, while the time of the welding
is decreased to a small degree.
4.2 Slag of the weld at the current of 470 A
The welding of specimen 1 was carried out at the
current of 470 A. The slag was formed on the weld toe in
the shape of a circular arch in the center, 2–3 cm wide
and 3–5 mm thick. The weld/slag border is smooth, but
some unmelted grains of the welding flux remained on
its top.
Figure 9 shows the slag microstructure on the weld
of steel OCR12 VM after the application of agglome-
rated welding flux FB 12.2. During the welding process,
a liquid slag was formed, which solidified during the
cooling. Aluminium and manganese oxides included in
the welding flux reacted and formed a solid-solution
spinel structure. The welding flux contained manganese
oxide and, after the melting, it reacted with magnesium
oxide which was also included in the flux.
During the welding, chromium from the base mate-
rial and the welding wire is oxidized. Chromium oxide
together with aluminium, magnesium and manganese
oxides forms a spinel structure with a solid-solution
composition (MgO,MnO)·(Al2O3,Cr2O3). After the weld-
ing, spinel contains mass fractions of 2.8 % MnO and
mass fractions of 3.3 % Cr2O3.
On the left side of Figure 9, there is magnesium
oxide, encircled with spinel. Solid magnesium oxide
reacts with the liquid slag and thus spinel (MgO, MnO)·
(Al2O3,Cr2O3) is formed on its surface. The resulting
spinel prohibits a dessolution of magnesium oxide in the
liquid slag.
The solidified welding slag has the following compo-
sition: magnesium oxide, spinel, a lamelar phase
(w(MgO) = 21.1 %, w(Al2O3) = 18.8 %, w(SiO2) = 39.8
% and w(K2O) = 10.2 %) and the matrix (w(CaO) = 46.2
%, w(MgO) = 11.8 %, w(SiO2) = 28.9 %, w(Al2O3) = 2.8
%, w(MnO) = 4.2 % and w(F) = 4.5 %).
Figure 10 shows an XRD diagram of the slag formed
at the welding current of 470 A. The diagram shows the
peaks for aluminium oxide, spinel and calcium fluorite.
In the sample, the undissolved flux from the surface of
the circular arch of the slag is also observed.
4.3 Slag of the weld at the current of 610 A
Figure 11 shows a SEM image of the slag formed
during the welding at the current of 610 A. During the
welding with a higher current, the unreacted leftovers of
magnesium oxide in the welding slag are surrounded
with spinel. The spinel generated during the welding
with the current of 610 A contains mass fractions of 3.9
% MnO, mass fractions of 24.3 % MgO, mass fractions
of 5.2 % Cr2O3 and mass fractions of 66.6 % Al2O3. In
the slag, tiny drops of metal (white particles) are located
at the border between the spinel and magnesium oxide
and in the lamelar phase.
M. PRIJANOVI^ TONKOVI^, J. LAMUT: PHASE ANALYSIS OF THE SLAG AFTER SUBMERGED-ARC WELDING
104 Materiali in tehnologije / Materials and technology 50 (2016) 1, 101–107
Figure 10: XRD diagram of the slag formed at the welding current of
470 A
Slika 10: Rentgenogram `lindre, nastale pri varjenju s tokom 470 A
Figure 8: Welding current and flux consumption
Slika 8: Varilni tok in poraba varilnega pra{ka
Figure 9: Microstructure of the welding slag of specimen 1
Slika 9: Mikrostruktura varilne `lindre preizku{anca 1
Figure 12 shows a SEM image of the slag, having a
metalic particle with a size of 50 μm and a composition
of mass fractions of 27.6 % Si, mass fractions of 4.8 %
Ti, mass fractions of 4.6 % Cr, mass fractions of 33.8 %
Mn (label 4), while the rest is iron. The particle was
formed during the metling of silico manganese and the
welding wire. Its composition is not similar to the com-
positions of the filler or the base material. In the slag ma-
trix, a lamellar phase and small spinel crystals containing
manganese and chromium oxides are embedded.
Welding with a higher current increases the amount
of MnO by mass fractions of 1.1 % and Cr2O3 by mass
fractions of 1.9 % in the spinel, compared to the welding
with the current of 470 A.
Figure 13 presents an XRD diagram of the slag
formed at the welding with current of 610 A. The
diagram shows that the slag is composed of aluminium
oxide, spinel, calcium fluorite and periclase (MgO).
4.4 Slag of the surfacing weld at the current of 628 A
The specimens listed under numbers 3 and 4 are the
slags of the surfacing welds. Figure 14 presents the slag
formed during the surfacing with the current of 628 A. In
the base (matrix) of the slag, there are spinel, a lamelar
phase and the leftovers of a free magnesium oxide. Com-
pared to the surfacing with 450 A, during the surfacing
with 628 A, the slag has more aluminium oxide and less
calcium oxide. The matrix of the slag (label 2) is com-
posed of mass fractions of 32.7 % CaO, 12.3 % MgO,
25.6 % SiO2, 16.5 % Al2O3, 5.7 % MnO, 2.7 % F and 3.2
% of K2O. In the slag, there are drops of metal and most
of them are located close to the lamelar phase.
4.5 Composition of the spinel and matrix of the slag
Figure 15 gives a graphical presentation of the con-
tents of the oxides in the slag base (the matrix), in which
the phases with higher melting points are precipitated.
From the diagram it is evident that specimens 1 (470 A)
and 3 (450 A) were welded with lower currents and have
the same contents of silicon oxide and different contents
of calcium oxide and alumina. Specimens 2 and 4,
welded with higer currents (610 A, 628 A), also have
different contents of calcium oxide. The content of
calcium oxide in a slag decreases with the increasing
current, while the content of aluminium oxide increases
(Figure 15). At lower welding currents, the slag includes
phases with lower melting points, while at higher
welding currents more liquid slag is formed due to the
melting of the oxides with higher melting points such as
aluminium and magnesium oxides as well as manganese
and chromium oxides.
M. PRIJANOVI^ TONKOVI^, J. LAMUT: PHASE ANALYSIS OF THE SLAG AFTER SUBMERGED-ARC WELDING
Materiali in tehnologije / Materials and technology 50 (2016) 1, 101–107 105
Figure 14: Micrograph of the phases in slag of specimen 4
Slika 14: Mikroposnetek faz v `lindri preizku{anca 4
Figure 12: SEM image of welding slag; place of analysis 2
Slika 12: SEM-posnetek varilne `lindre; mesto analize 2
Figure 13: XRD diagram of slag formed during the welding at the
current of 610 A
Slika 13: Rentgenogram `lindre, nastale pri varjenju s tokom 610 A
Figure 11: SEM image of welding slag; place of analysis 1
Slika 11: SEM-posnetek varilne `lindre; mesto analize 1
The content of the formed spinel depends on the
welding current. With an increase in the welding current
the content of manganese oxide increases from 2.8 to 3.9
% of mass fractions and chromium oxide also increases
from 3.3 to 7.3 % of mass fractions. Figure 16 presents
the change in the composition of the spinel in relation to
the welding current.
We calculated the basicity15 of the main components
in the slag with Equation (1):
B =
+ + + + + + ⋅ +
+ ⋅
CaO MgO BaO CaF Na O K O (MnO FeO)
SiO
2 2 2
2
0 5
0 5
.
. (Al O TiO ZrO2 3 2 2+ + )
(1)
The content of the base (matrix) of a slag and its
basicity also vary with respect to the welding current.
The basicity is calculated with Equation (1) and it
changes with the welding current, as presented in Table
4.
Lower welding currents also diminish the loss of the
alloying elements. The calculated values of the basicity
show that the basicity of the matrix of a slag is higher at
lower welding currents (Figure 17) because of the
content of Al2O3 in the slag melt.
5 CONCLUSIONS
During the submerged-arc welding of steel OCR 12
VM, slag solidifies above the weld in the form of a cir-
cular arch. The contact area with the slag has a smooth
glassy shine. On the contrary, at the contact with the slag
surface, the leftovers of undissolved welding flux are
observed.
The agglomerated welding flux is in the form of
pellets with a size of 0.2–1.8 mm. During the welding,
the slag is liquid. The solidified weld slag contains alu-
minum and magnesium oxides, spinel, a lamellar phase
and the matrix. The composition of the matrix is similar
to that of mineral cuspidin16 (3CaO·2SiO2·CaF2) but it
also contains aluminium, potassium and magnesium
oxides. The magnesium oxide in the welding slag is
surrounded by spinel.
Spinel16 with a complex composition (MgO,MnO)·
(Al2O3,Cr2O3) is formed from magnesium and aluminium
oxides, with smaller amounts of manganese and chro-
mium oxides.
The composition of the welding-slag matrix depends
on the welding current. At higher welding currents more
aluminium oxide is formed.
The welding slag generated during the submerged
welding can be a valuable raw material for the produc-
tion of a new welding flux.
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M. PRIJANOVI^ TONKOVI^, J. LAMUT: PHASE ANALYSIS OF THE SLAG AFTER SUBMERGED-ARC WELDING
106 Materiali in tehnologije / Materials and technology 50 (2016) 1, 101–107
Figure 17: Basicity of the base (matrix) of the welding slag
Slika 17: Bazi~nost osnove (matice) varilne `lindre
Figure 15: Relative contents of oxides and fluorine in the bases
(matrices) of welding slags
Slika 15: Relativna vsebnost oksidov in fluora v osnovi (matici)
varilnih `linder
Table 4: Basicity of the base (matrix) of a slag
Tabela 4: Bazi~nost osnove (matice) `lindre
Welding flux Current (A) Matrixbasicity
Specimen 1 − weld 470 2.14
Specimen 2 − weld 610 1.68
Specimen 3 − surfacing weld 450 1.94
Specimen 4 − surfacing weld 628 1.50
Figure 16: Composition of the spinel in relation to the welding
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Slika 16: Sestava spinela v odvisnosti od toka pri varjenju
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M. PRIJANOVI^ TONKOVI^, J. LAMUT: PHASE ANALYSIS OF THE SLAG AFTER SUBMERGED-ARC WELDING
Materiali in tehnologije / Materials and technology 50 (2016) 1, 101–107 107