M. JURICA et al.: EFFECT OF THE GMAW METAL-TRANSFER MODE ON THE WELD-METAL STRUCTURE ...
449–453
EFFECT OF THE GMAW METAL-TRANSFER MODE ON THE
WELD-METAL STRUCTURE OF HSLA X80
VPLIV NA^INA PRENOSA KOVINE PRI OBLO^NEM VARJENJU
V ZA[^ITNEM PLINU (GMAW) NA STRUKTURO ZVARA NA
JEKLU HSLA X80
Maja Jurica, Zoran Ko`uh, Branko Bauer, Ivica Gara{i}
University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, Ivana Lucica 5, 10002 Zagreb, Croatia
branko.bauer@fsb.hr
Prejem rokopisa – received: 2016-04-01; sprejem za objavo – accepted for publication: 2016-06-22
doi:10.17222/mit.2016.057
This paper presents the influence of the metal-transfer mode during GMAW of 10-mm-thick HSLA steel grade X80 on the
inclusions and acicular ferrite content in the weld metal. Welding of butt joint specimens was carried out by the conventional
short-circuit and CBT metal-transfer mode with different shielding gases and wire feed speeds. An analysis of obtained results
showed that the metal-transfer mode affects the weld-metal structure by influencing the acicular ferrite content in relation to the
number and size of the inclusions found in the weld metal.
Keywords: GMAW CBT welding, non-metallic inclusions, acicular ferrite, HSLA X80
^lanek predstavlja vpliv na~ina prenosa kovine med oblo~nim varjenjem v za{~itnem plinu (GMAW), 10 mm debelega visoko-
trdnostnega malolegiranega (HSLA) jekla vrste X80 na vklju~ke in vsebnost acikularnega ferita v zvaru. Varjenje sole`nih
vzorcev je bilo izvedeno z obi~ajnim kratkosti~nim na~inom in s kontroliranim prenosom kovine (CBT) z razli~nimi za{~itnimi
plini in hitrostmi podajanja `ice. Analiza dobljenih rezultatov je pokazala, da na~in prenosa kovine vpliva na strukturo zvara, z
vplivanjem na vsebnost acikularnega ferita v povezavi s {tevilom in velikostjo vklju~kov v kovini zvara.
Klju~ne besede: GMAW CBT varjenje, nekovinski vklju~ki, acikularni ferit, HSLA X80
1 INTRODUCTION
The HSLA steel grade X80 is more and more in use,
but not so much has been investigated on the subject of
the influence of the metal transfer on the structure of the
weld metal. The most commonly used technology for
joining pipeline steels is GMAW, because it results in
high productivity and efficiency and also because by
changing the welding parameters the metal-transfer
mode can easily be modified to achieve different heat
inputs. When GMAW with a short-circuit metal-transfer
mode is used, the occurrence of spatter is observed.
Spatter generates mostly when re-arcing takes place
immediately after the short circuit causing additional
financial expenses. In order to eliminate spatter, the
critical time period, where the molten metal is squeezed
by the pinch force effect, the voltage just before
re-arcing must be accurately sensed.1 Following this, the
welding current is rapidly decreased immediately after
the re-arcing occurs. This causes the molten metal at the
tip of the wire to be transferred to the weld pool using
only the surface tension, resulting in minimization or
even elimination of the spatter generation.2,3 The
modified metal transfer also affects the mechanical
properties of the weld metal, which directly depend on
the microstructure.4 Micro-alloyed low-carbon steels
with a multiphase microstructure can have a high tensile
strength, good toughness and weldability. This combi-
nation of properties has led to their application in the
manufacturing of large-diameter pipes for gas and oil
transportation with significant reductions of wall thick-
ness. Interest in acicular ferrite (AF) formation is
motivated by recent evidence that pipeline steels with
bainitic microstructure having predominantly AF+MA
(martensite–austenite) microstructures that have superior
combinations of tensile strength and toughness compared
with CB (conventional bainite) + MA microstructures.5
Acicular ferrite is one of the microstructural constituents
that is most commonly observed in the weld deposit of
low-alloy steel and is easily differentiated from other
constituents by its fine interlocking nature.6 As follows
from Figure 1, the ferrite side plate (such as Widman-
stätten ferrite or upper bainite) provides preferential
crack-propagation paths in austenite grains. This is be-
cause the ferrite side plates nucleate at grain boundaries
as parallel plates with the same crystallographic orien-
tation. Therefore, the toughness of the steels decreases
with an increasing amount of these structures. Alternati-
vely, the acicular ferrite laths nucleate intragranularly at
the surface of the inclusions. Then, they have a chaotic
crystallographic orientation, resulting in a retardation of
the propagation path for a cleavage crack in the metal.
Therefore, it can be expected that the steel toughness
Materiali in tehnologije / Materials and technology 51 (2017) 3, 449–453 449
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 620.1:621.791.75:621.791.053 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(3)449(2017)
increases with an increasing amount of acicular ferrite in
the steel.7
From the published literature on acicular ferrite in
weld metals it is well established that non-metallic
inclusions present effective nucleation sites for acicular
ferrite and a variety of mechanisms were proposed.
Therefore, various authors have discussed the effect of
different factors such as: cooling rate in the temperature
range 800 °C to 500 °C, chemical composition of the
steel, size of the austenite grains, and inclusion para-
meters on the formation of intragranular acicular ferrite
in the weld metal.5,7–10 However, in real practice it is very
difficult, if not impossible, to determine the influence of
each of the previously mentioned factors separately.
In the present work, the effect of the metal-transfer
mode during GMAW on the formation of non-metallic
inclusions as well as acicular ferrite in HSLA X80 steel
has been investigated.
2 EXPERIMENTAL PART
The base material used in the experiment was high-
strength, low-alloy steel API 5L X80 and the filler
material was G 62 4 M Mn3NiCrMo (according to EN
ISO 16834) wire, 1.2 mm in diameter. The exact che-
mical composition of the base and the filler material used
in this experiment is shown in Table 1.
Gas metal arc welding of the specimens was carried
out using two different metal-transfer modes: short-
circuit (SC) mode and controlled bridge transfer (CBT).
During the experiment, three different wire feed speeds
were used (4 m/min, 5.5 m/min and 7 m/min).
The maximum wire feed speed was determined by
previous experiments as the upper limit where the con-
trolled bridge transfer mode is still stable. Also, the
minimum value of the wire feed speed is defined as the
speed that does not result in a lack of fusion. The shield-
ing gases used were M21 and C1 according to EN ISO
14175: 2008 standard and a special mixture consisting of
59 % CO2 and 41 % Ar with the flow set at 18 L/min.
The contact tip to work distance (CTWD) was 12 mm,
while the welding speed was 25 cm/min. Welding of the
filler passes was carried out using the neutral technique.
The interpass temperature was controlled with the
infrared thermometer to be under 100 °C. Because of a
very narrow span of welding parameters which result in
good quality root pass (wire feed speed – 3.5 m/min,
welding speed – 11.6 cm/min, shielding gas flow – 16
L/min, CTWD – 15 mm, pull technique, short circuit
metal transfer mode, heat input – 8.4 kJ/cm), all of the
root passes of the specimens were robotically (Almega
OTC AX-V6 robot and Varstroj VPS 400 welding power
source) welded with the same parameters. Filler passes
were welded using the Daihen Varstroj Welbee P500L
welding power source and BUG-O Systems MDS 1002
drive system. Sample designations and varied welding
parameters are shown in Table 2.
Table 2: Sample designation and varied welding parameters
Tabela 2: Oznaka vzorcev in razli~ni parametri varjenja
Sample
nr.
Shielding gas CO2
content, in volume
fractions (X/%)
Metal transfer
mode,
CV/CBT
Wire feed
speed,
m/min
1 18 CV 4
2 18 CBT 4
3 18 CV 5.5
4 18 CBT 5.5
5 18 CV 7
6 18 CBT 7
7 59 CV 4
8 59 CBT 4
9 59 CV 5.5
10 59 CV 5.5
11 59 CV 5.5
12 59 CBT 5.5
13 59 CBT 5.5
14 59 CBT 5.5
15 59 CV 7
16 59 CBT 7
17 100 CV 4
18 100 CBT 4
19 100 CV 5.5
20 100 CBT 5.5
21 100 CV 7
22 100 CBT 7
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450 Materiali in tehnologije / Materials and technology 51 (2017) 3, 449–453
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Figure 1: Schematic illustration of propagation path: a) in predomi-
nantly ferrite side plate and b) acicular ferrite microstructure7
Slika 1: Shematski prikaz poti napredovanja razpoke: a) v prete`no
feritni plo{~i in b) v mikrostrukturi acikularnega ferita7
Table 1: Chemical composition of base and filler materials11
Tabela 1: Kemijska sestava osnovnega materiala in dodanega mate-
riala11
Material API 5L X80 G 62 4 M Mn3NiCrMo(according to EN ISO 16834)
Chemical
composition in
mass fractions,
(w/%)
C 0.07 0.10
Si 0.35 0.75
Mn 1.94 1.65
P 0.015 -
S 0.0 -
Cr 0.15 0.60
Ni 0.01 0.55
Mo 0.001 0.30
Cu 0.01 0.08
Al 0.027 -
After welding of the plates, test specimens were pre-
pared in accordance with the EN ISO 15614-1 standard.
Later on, specimens were polished and etched in 3 %
Nital solution for micrographic analysis and in 7 % Nital
solution for macrograph analysis. The exact chemical
composition of the inclusions found in the weld metal
was analysed by SEM TESCAN VEGA 5136 MM
equipped with Oxford EDS detector. Microstructural
analysis of the weld metal was observed with an Olym-
pus GX51F-5 inverted light microscope.
All of the gathered data on the microstructure of the
weld metal (acicular ferrite content) were statistically
analysed with the Design Expert software program. A
face-centered central composite design with three factors
was used to analyse the influence of the varied factors. In
this design the star points (not centered) are at the center
of each face of the factorial space, so 	 = ±1. Center
points were repeated 3 times in order to define the "Pure
error" and the curvature of the response surface. Accord-
ing to ANOVA of the results of weld-metal micro-
structure, categorical factor metal transfer mode (conven-
tional-CV and CBT) is a significant factor with P-value
< 0.0001. In order to analyse in detail the influence of
the metal-transfer mode as the significant factor, only the
results for one shielding gas (C1) are presented and
analysed. The reason for analysing this specific shielding
gas lies in the fact that it promotes non-axial detachment
of the metal droplet, causing more spatter.11–14 Because
the newly developed CBT metal-transfer mode can
reduce or even eliminate spatter generation, regardless of
the type of shielding gas, 100 % CO2 shielding gas could
be used in short circuit and globular metal transfer para-
meter range. This type of shielding gas is not typically
used when welding HSLA X80, so a detailed analysis of
its effect on the weld-metal properties needs to be
investigated.
3 RESULTS
3.1 Chemical composition and size distribution of
non-metallic inclusions in the weld metal
Inclusions of varying size and type were examined
and the results in Table 3 present the average values of
all the inclusions analysed per one test specimen. The
results indicate that the analysed inclusions are rich in
oxygen, silicon, manganese and aluminium with a small
amount in titanium and sulphur. The smallest inclusions
were observed in sample 17, which was welded with a
wire feed speed of 4 m/min and a short-circuit metal-
transfer mode. The chemical compositions of these
inclusions are in relation to the diameter – it can be seen
that the mass percentage of all the chemical elements are
at the lowest values in comparison to the other inclu-
sions. It can also be noticed that in the samples 17 and
18 no titanium was observed. In samples 19 and 21,
which were welded with higher wire feed speeds (5.5
m/min and 7 m/min) and short-circuit mode, no chro-
mium was found. All the other alloying elements are
noticed to be in a positive linear dependence with the
diameter of the observed inclusions (with the growth of
the inclusions also the growth of the mass percentage of
all the elements is observed). Taking into account the
fact that carbon, iron and chromium are constituent ele-
ments of the base and filler metal, the observed inclusion
can be characterized as complex Si-, Mn- and Al-oxides.
The average values of the size distribution of the
observed non-metallic inclusions are also presented in
Table 3. It can be seen that the average diameter of the
inclusions varied in the range from 0.51 μm to 1.12 μm.
The smallest inclusions are noticed in sample 1 (welded
with 4 m/min wire feed speed and short-circuit, metal-
transfer mode) and the largest inclusions are observed in
sample 19 (welded with 5.5 m/min wire feed speed and
short-circuit transfer mode), as can be seen in Figure 2.
Comparing the chemical composition with the size
distribution of the observed inclusions it can be seen that
the diameter of inclusions containing chromium is
smaller when welding with the short-circuit metal-tran-
sfer mode (compared to the CBT metal-transfer mode)
and larger when no chromium is found. Samples welded
with the globular metal-transfer mode (samples 19 and
21) show significant difference in inclusion size (in-
crease in size is over 100 %) to the sample (Sample 17)
welded with short-circuit metal-transfer mode.
3.2 Microstructure of the weld metal
The microstructure analysis of the weld metal is
given in Table 4. The obtained results show the area
fraction of acicular ferrite and also of inclusions. Mini-
mum acicular content of 60.2 % and minimum content of
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Table 3: Chemical composition and size distribution of the observed inclusions (samples 17-22)
Tabela 3: Kemijska sestava in razporeditev velikosti opa`enih vklju~kov (vzorci 17 – 22)
Sample di μm
Chemical composition, in mass fractions (w/ %)
C O Al Si S Cr Ti Mn Fe
17 0.51 5.49 7.06 0.22 3.55 0.43 0.47 - 6.30 77.00
18 0.88 6.80 10.51 0.33 4.48 0.71 0.69 - 6.38 71.25
19 1.12 7.27 19.10 0.95 6.93 0.66 - 0.18 10.68 54.05
20 0.93 7.10 13.61 0.92 5.96 0.43 0.48 0.17 11.48 60.90
21 1.01 9.73 18.57 0.31 6.55 0.68 - 0.32 12.39 51.23
22 0.87 8.50 12.40 0.34 5.04 0.61 0.46 0.22 11.13 61.48
inclusions of 1.3 % is achieved in sample 18 (Figure 3),
which was welded with the CBT metal-transfer mode
and a wire-feed speed of 4 m/min. Also, the maximum
values of the area fraction for both acicular ferrite
(74.9 %) and inclusions (2.1 %) were observed in the
same sample, sample 21 (which was welded with glo-
bular transfer mode and 7 m/min wire feed speed). From
these results it can be clearly seen that the area fraction
of acicular ferrite is closely related to the area fraction of
inclusions - higher content of inclusion results in a
higher content of acicular ferrite. The biggest difference
in the obtained results between the CBT and globular
transfer mode is observed in samples 19 and 20. From
Table 4 it can be seen that the area fraction of the
acicular ferrite is in sample 19 16 % larger than in
sample 20. Similarly, the area fraction of inclusions is in
sample 19 larger by 36 % than in sample 20. Comparing
the results of the area fraction of the acicular ferrite and
the inclusion diameter no direct dependence could be
observed.
Table 4: Microstructure analysis – area fraction of acicular ferrite and
non-metallic inclusions
Tabela 4: Analiza mikrostrukture – dele` povr{ine acikularnega ferita
in nekovinskih vklju~kov
Sample Area fraction ofacicular ferrite, %
Area fraction of
non-metallic inclusions, %
17 68.2 1.6
18 60.2 1.3
19 72.3 1.9
20 62.5 1.4
21 74.9 2.1
22 66.7 1.6
4 DISCUSSION
Among other steel deoxidants, Al is the most
commonly used alloying element added for fully killed
steel. Therefore, the simple Al2O3 oxides are often found
in different steel grades, but are not considered to be an
effective nucleant for AF. But, in steels containing Mn
and which have been deoxidized by Al, more complex
manganese-oxide inclusion evolve, which then become
effective nucleation sites for AF.7 Unlike simple
Al-oxides, various Ti-oxides are very efficient nuclei for
the formation of AF.
Some of the complex oxides can also nucleate AF.
More specifically, the observed aluminium-manganese
silicates nucleate AF. Their effectiveness can be ex-
plained with their highest thermal expansion difference
from iron (in comparison with other observed inclusions)
resulting in the largest thermal stresses.7 Among the
observed inclusions, in addition to oxides, some sul-
phides (MnS) were also found. Pure MnS inclusions,
unlike some oxides, are not active nucleants for AF,
because they are incoherent in austenite.15 Complex
oxy-sulphides and multi-phase inclusions observed in
analysed specimens are very often stated as nucleants for
AF. The factors that positively influence the nucleation
of AF are: low mismatch strain between the inclusions
and ferrite; positive thermal strains in the surrounding
matrix (due to large difference in thermal coefficients for
inclusion and matrix) and the formation of Mn-depleted
zone in the matrix near the MnS inclusion. Because
simple inclusions affect the nucleation of the AF with
only one of the mentioned factors, they present less
active nucleants for AF compared to the complex inclu-
sions. According to 16, inclusions consist of an oxide
core, which is formed during the primary deoxidation
stage. The chemical composition of deoxidation products
can vary within wide limits, depending on the relative
activities of aluminium, titanium, silicone, manganese
and oxygen in the weld metal. Except for the MnS
spotted in the observed inclusions in the experiment, the
surface of the oxides can be partially covered by TiN.
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452 Materiali in tehnologije / Materials and technology 51 (2017) 3, 449–453
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Figure 3: Weld-metal microstructure with predominantly acicular
ferrite (sample 21)
Slika 3: Mikrostruktura zvara s prevladujo~im acikularnim feritom
(vzorec 21)
Figure 2: SEM micrograph of the sample 19
Slika 2: SEM-posnetek vzorca 19
The precipitation of these phases occurs after the
completion of the weld-metal deoxidation, probably
during solidification, where the reactions are favoured by
solute enrichment in the interdendritic liquid.
Regarding the frequency of the occurrence of the
observed inclusions, it can be concluded that a smaller
amount of inclusions appeared in the samples welded
with the CBT metal-transfer mode, resulting in a micro-
structure with a smaller area fraction of acicular ferrite.
The larger acicular ferrite content in the weld metal
welded with the natural metal-transfer mode could be
explained by the thermal stresses that occur during the
welding. Namely, during CBT welding a smaller heat
input is introduced to the metal, resulting in smaller
thermal stresses, which negatively influence the nucle-
ation of the acicular ferrite. When welding with a higher
heat input (conventional metal transfer mode), because
of the large difference in the thermal coefficients for the
inclusion and austenite matrix, the nucleation of acicular
ferrite is more stimulated. Besides the area fraction of
inclusions in the weld metal, which has proven to influ-
ence the content of acicular ferrite, the size of the inclu-
sion has also been observed to have a great influence, as
can be seen from Table 3 and Table 4. These results
indicate the critical size of the particles for the hetero-
geneous nucleation of acicular ferrite to be around 1 μm,
which is in accordance with previous investigations of
other authors.6,7 This is a critical value because of the
fact that the energy barrier to heterogeneous nucleation
of ferrite at inclusions decreases significantly with an
increased inclusion diameter in the range from 0 to 1 μm.
This is due to the increasing of the particle surface area.
However, it follows from7 that the value of this energy
barrier for particles with diameters larger than about 1
μm decreases only slightly with a further increase in the
inclusion size. Therefore, it is expected that a further
increase of the diameter of the inclusions over 1 μm does
not result in a greater possibility of ferrite nucleation on
an inclusion surface.
5 CONCLUSIONS
Based on the results obtained in this study, the
following main conclusions can be drawn:
• The diameter of the observed inclusions is in direct
correlation with its chemical composition,
• Samples welded with CBT metal-transfer mode
resulted in weld metals with a smaller amount of
inclusions, resulting in a smaller area fraction of
acicular ferrite,
• Samples welded with the globular metal-transfer
mode resulted in a larger inclusion size to the one
welded with short-circuit, metal-transfer mode.
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