G. R. KUMAR et al.: EFFECT OF ACTIVATED FLUX AND NITROGEN ADDITION ON THE BEAD GEOMETRY ...
357–364
EFFECT OF ACTIVATED FLUX AND NITROGEN ADDITION ON
THE BEAD GEOMETRY OF BORATED STAINLESS-STEEL GTA
WELDS
VPLIV AKTIVIRANEGA TOPILA IN DODATKA DU[IKA NA
GEOMETRIJO KOPELI PRI GTA ZVARIH BORIRANEGA
NERJAVNEGA JEKLA
Guttikonda Raja Kumar1, Gabbita Durga Janaki Ram2, Sajja Rama Koteswara Rao3
1Swarna Barathi Institute of Science and technology, Khammam, T. S., 507002, India
2IIT Madras, Department of Metallurgical & Materials Engineering, Chennai 600036, India
3S.S.N. College of Engineering, Department of Mechanical Engineering, Chennai 603110, India
rajkumardotcom@yahoo.com
Prejem rokopisa – received: 2015-03-05; sprejem za objavo – accepted for publication: 2015-05-14
doi:10.17222/mit.2015.052
Borated stainless steels (304B) are used in nuclear power plants as control rods, shielding material, spent-fuel storage racks and
transportation casks as they have a high capacity to absorb thermal neutrons. In this study, bead-on-plate welds were made on
10-mm-thick 304B plates using gas tungsten arc welding with Ar and Ar+2% nitrogen as the shielding gases, activated-flux
GTA and electron-beam welding processes. The effects of the activated flux and nitrogen addition to the weld metal through the
shielding gas, on the microstructure, bead geometry and mechanical properties were investigated. Activated-flux GTA welding
and electron-beam welding substantially enhanced the depth of penetration and the aspect ratio compared to the other processes.
Full-penetration welds were obtained in a single pass using activated-flux GTA and EB welding. The fusion-zone (FZ)
microstructure of an activated GTA weld exhibits a columnar dendritic structure with eutectic borides in interdendritic regions,
while a fine equiaxed dendritic structure was noticed in EB welds. GTA, nitrogen-added GTA and activated-flux GTA welds
exhibited a partially melted zone adjacent to the fusion zone, with the activated-flux GTAW process resulting in a significantly
thinner partially melted zone (PMZ). No PMZ was noticed in the EB welds. All the welds exhibited a high joint efficiency and
impact toughness equal to those of the base material. It is concluded that the activated-flux GTA and EB welding processes are
advantageous due to the use of a low heat input and failure location.
Keywords: borated stainless steels, bead geometry, activated flux, GTAW, partially melted zone, nitrogen
Borirana nerjavna jekla (304B) se uporabljajo v nuklearnih elektrarnah kot kontrolne palice, za{~itni material, nosilci shranjeval
za izrabljene palice in za transportne sode, saj imajo sposobnost velike absorbcije termi~nih nevtronov. V tej {tudiji so bili
izdelani okrogli in plo{~ati zvari na 10 mm debelih plo{~ah iz 304B, z uporabo TIG varjenja v za{~itni atmosferi iz Ar + 2 %
du{ika, z aktiviranim talilom GTA in varjenjem z elektronskim curkom. Preiskovan je bil vpliv aktiviranega talila in dodatka
du{ika v zvar s pomo~jo za{~itnega plina, na mikrostrukturo, geometrijo kopeli in mehanske lastnosti. Varjenje z aktiviranim
talilom GTA in varjenje z elektronskim curkom, ob~utno pove~ata globino penetracije in razmerje, v primerjavi z drugimi
procesi. Z uporabo aktiviranega talila GTA in z varjenjem z elektronskim curkom je bila dose`ena popolna penetracija zvarov.
Mikrostruktura cone zlivanja (FZ) pri zvaru z aktiviranim talilom GTA, ka`e stebrasto dendritno strukturo z evtekti~nimi boridi
v meddendritnih podro~jih, medtem ko je bila pri zvarih z elektronskim curkom opa`ena struktura z drobnimi enakoosnimi
dendriti. Zvari GTA, GTA z dodanim du{ikom in GTA z aktiviranim talilom, so kazali delno raztaljena podro~ja okrog podro~ja
zlivanja ter z mo~no stanj{ano delno staljeno podro~je (PMZ) pri postopku z aktiviranim talilom GTA. V zvarih z elektronskim
curkom ni bilo opaziti PMZ. Vsi zvari so pokazali veliko u~inkovitost spoja in udarno `ilavost, ki je enaka kot pri osnovnem
materialu. Ugotovljeno je, da imata varjenje z aktiviranim talilom GTA in varjenje z elektronskim snopom, prednost zaradi
uporabe majhnega vnosa toplote in zaradi mo`nosti lokacije napak.
Klju~ne besede: borirano nerjavno jeklo, geometrija kopeli, aktivirano talilo, GTAW, delno staljeno podro~je, du{ik
1 INTRODUCTION
Borated stainless steel 304B is an austenitic-type
stainless steel containing 0.2 % to 2.25 % boron; it is
widely used in nuclear industries for various applica-
tions. These applications involve storage of spent nuclear
fuel in the forms of long-term storage tanks or caskets,
transportation baskets and control rods.1,2 The objective
of developing these steels is to absorb radiation from the
spent fuel. Due to the presence of the 10B isotope, these
stainless steels have superior neutron-absorption capabi-
lities.3 Spent fuel rods are stored in dry casks, made up
of borated stainless steel for long-term thermal neutron
irradiation. Despite the fact that boron allows an ade-
quate neutron absorption, it has an adverse influence on
mechanical properties, particularly on the fracture
toughness. Very low solubility of boron in an austenitic
matrix results in the formation of hard, brittle (Cr, Fe)2B
precipitating phases, which are low-melting eutectics.4
The size, shape and distribution of these borides also
deteriorate the mechanical properties.5 Park et al.6 con-
ducted post-weld heat treatments and reported that sphe-
roidized accicular eutectic phases were found to enhance
the ductility.
Earlier, borated SSs were typically used as bolts on
additions to a structural framework. However, due to the
Materiali in tehnologije / Materials and technology 50 (2016) 3, 357–364 357
UDK 62-4:621.791.053:669.14.018.8 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(3)357(2016)
slow and non-automatic riveting process, welding was
adopted. The solidification cracking susceptibility is
high at 0.2 % of mass fractions of boron due to a wider
solidification range.7 In later studies, it was found that a
boron content of 0.5 % of mass fractions or more, i.e.,
above its solid-solubility limit causes a reduction in the
coefficient of thermal expansion and constricts the
solidification range, which, in turn, causes the
crack-healing phenomenon and decreases the cracking
susceptibility.8,9 The lower solidification-cracking
susceptibility of high-boron steels is mainly due to the
healing of cracks by the abundant amounts of the
low-melting eutectic liquid of (Cr,Fe)2B.10
ASTM A887-89 covers eight different types of
borated stainless steels, and each type has two grades,
i.e., A and B with different boron contents.11 Specifi-
cation designates Grade A for the materials produced
with powder metallurgy containing finer and more
uniformly distributed borides compared to Grade B made
with ingot metallurgy. The borated SSs fabricated with
the hot-rolling technique, employing the powder-sinter-
ing method have sufficient ductility and other
properties.12
The GTAW is the welding process normally used in
the fabrication of borated stainless steels. However, its
disadvantage is a relatively shallow penetration during a
single-pass welding operation on thicker plates. The
thickness of the austenitic stainless steels that can be
welded in a single pass with argon as the shielding gas is
normally restricted to 3 mm. In this connection, a novel
variant of the GTA welding process called the acti-
vated-flux GTA welding process was initially developed
for the welding of titanium at the Paton Institute of
Electric Welding.13,14 Several investigations were carried
out by various researchers to develop suitable flux com-
ponents to extend this technique to other alloys. In the
activated-flux GTA welding process, a thin coating of the
activated flux is applied onto the surface of a joint and
the electrons outside the arc are captured by the flux
when it is vaporized during welding. Thus, the arc gets
constricted which, in turn, leads to an increase in the
penetration. Sun and Pan reported that the penetration
capability can be increased by as much as 300 % due to
the activated-flux GTA welding when compared with the
conventional GTA welding.15,16
The main reasons for the significant improvement in
the penetration in the steels are reported to be the arc
constriction and the reversed Marangoni convection.17
However, several researchers stated that the increase in
the penetration is due to the positive surface-tension
gradient in a molten-weld pool, which causes the fluid to
flow towards the bottom of the weld.18,19 Thus, based on
the productivity, the activated GTA welding process is
more viable than the conventional GTA welding process.
The activated GTA welding of austenitic stainless steels
was investigated by various authors to understand the
effect of the flux on the depth of penetration, the aspect
ratio, the microstructure and the mechanical beha-
vior.20–26
Furthermore, the effect of the activated flux on the
GTA welding of austenitic stainless steel 316L was
studied by Kuang Hung et al.27 It was reported that the
activated TIG welding increases the joint penetration and
the aspect ratio, which, in turn, reduce the angular dis-
tortion of the weldments. A great deal of research was
carried out to develop various flux combinations and
understand their beneficial influence on the GTA weld-
ing of austenitic stainless steels. Several investigations
showed that an addition of nitrogen to the shielding gas
influences the weld-bead shape, the depth of penetration
and the weld metal microstructure.28,29
However, not much information pertaining to the
effect of the activated flux and nitrogen addition on the
GTA welding of borated SS has been reported so far.
Therefore, this work presents the microstructure and
mechanical behavior of GTAW, activated-flux GTAW,
nitrogen-added GTAW and electron-beam welding
(EBW) of 10-mm-thick borated stainless steel.
2 EXPERIMENTAL WORK
The base material used in this study was borated
stainless steel (BSS) 304B4 and its chemical compo-
sition is given in Table 1. As-received rolled plates of 40
mm were cut, using an electrical discharge machine to
prepare plates of 300 mm × 110 mm × 10 mm. The
plates were cleaned using acetone to remove surface con-
tamination before the welding.
Table 1: Chemical composition of the base material
Tabela 1: Kemijska sestava osnovnega materiala
Material Cr Ni Mn B Si P C
304B4 19.3 13.4 2.0 1.05 0.74 0.045 0.08
Bead-on-plate welds were produced using the auto-
matic GTAW process. A tungsten electrode (AWS classi-
fication EWTh-2) with a diameter of 3.2 mm and the
shielding gas of pure argon were used in the conven-
tional GTAW process with direct-current electrode
negative (DCEN) polarity. In addition to argon-shielded
bead-on-plate welds, welds were produced using a
shielding-gas mixture containing 2 % (volume) nitrogen
along with argon. Also, autogenous full-penetration
electron-beam welds were also made.
For the activated GTA welding, the activated flux was
prepared, with its constituents in the required propor-
tions, and then mixed with ethanol and the resulting
mixture was stirred until it turned into paste. The flux
used in this process is commercially available and its
composition was reported in the patent by M.Vasu-
devan.30 A thin layer of this paste was applied manually
on the surface to be welded and dry powder remained
there after the evaporation of ethanol. Using this acti-
vated flux, several bead-on-plate experiments were
G. R. KUMAR et al.: EFFECT OF ACTIVATED FLUX AND NITROGEN ADDITION ON THE BEAD GEOMETRY ...
358 Materiali in tehnologije / Materials and technology 50 (2016) 3, 357–364
carried out in order to achieve a single-pass full-pene-
tration weld in the activated GTAW process.
The parameters used in various welding processes are
presented in Table 2. Using standard metallographic pro-
cedures, specimens were prepared for a microstructural
investigation. After polishing, the specimens were etched
with Kalling’s 1 solution containing 5 g of cupric chlo-
ride, 100 mL of hydrochloric acid and 100 mL of
ethanol. The welds were initially examined with a stereo
microscope and then bead-geometry measurements were
taken using image-analysis software. The microstruc-
tures of different zones of interest such as FZ, PMZ and
the base metal (BM) were observed with a light micro-
scope. Micro-hardness tests were carried out using a
Vickers digital micro-hardness tester along the weld
joint. A load of 500 g was applied for a duration of 10 s.
In accordance with ASTM E8, tensile specimens were
machined with an EDM-wire-cutting machine so that the
weld metal was located in the center of gauge length.
The tensile properties of weldments, i.e., the ultimate
tensile strength, the proof strength and the percent
elongation were evaluated on the basis of the results of
the tensile tests conducted at room temperature. The
tensile-test results are listed in Table 3 and all the values
presented are average values of at least three specimens.
Charpy-impact test specimens were prepared in accor-
dance with ASTM standard E23. Identical V-notches
were machined on three specimens, at the weld center,
using a broaching machine. Impact testing was con-
ducted at room temperature using a pendulum-type
machine with the maximum capacity of 300 J.
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Materiali in tehnologije / Materials and technology 50 (2016) 3, 357–364 359
Table 2: Common used welding parameters
Tabela 2: Uporabljeni splo{ni parametri varjenja
Cross-sections of the
welds
Depth of
penetration (mm)
Aspect ratio
(d/w)
Weld area
(mm2) Welding parameters
Heat input
(KJ/mm)
GTAW
3.8 0.27 51.45
Weld current: 235 A
Arc voltage: 17 V
Travel speed:
120 mm/min
Gas flow rate:
25,3 Pa m3 s–1
2
Nitrogen-added GTAW
5.77 0.4 57.54
Weld current: 235 A
Arc voltage: 17 V
Travel speed:
120 mm/min
Gas flow rate:
25,3 Pa m3 s–1
2
Activated GTAW
10 0.8 92.01
Weld current: 235 A
Arc voltage: 17 V
Travel speed:
120 mm/min
Gas flow rate:
25,3 Pa m3 s–1
2
EBW
10 2.82 17.02
Acceleration voltage:
60KV
Beam current: 70 mA
Travel speed:
700 mm/min
0.36
3 RESULTS
Cross-sections of the welds made using different
welding processes are presented in Table 2, along with
the bead-profile parameters and corresponding welding
parameters. The microstructure of the base metal is pre-
sented in Figure 1. The microstructures of FZ and PMZ
for the welds made using various welding processes are
shown in Figures 2a to 2d and Figures 3a to 3d, res-
pectively. The microhardness surveys carried out along
the various weld zones such as FZ, PMZ and BM of
different welds are shown in Figures 4a to 4d. The ten-
sile properties for the different welds considered in the
present study are presented in Table 3. The results of the
Charpy-impact test conducted at room temperature for
the welds produced using various welding processes are
presented in Table 3.
4 DISCUSSION
4.1 Weld bead geometry
From Table 2, it can be seen that the three variants of
the GTA process use the same welding parameters and
heat input, i.e., 2 kJ/mm. The use of Ar+2%N shielding
gas in place of Ar significantly increases the depth of
penetration, from 3.8 mm to 5.7 mm; and the aspect ratio
of the weld bead also increases. This was observed by
earlier researchers for other materials.28,29 For the same
heat input, the effect of the activated flux on the bead
geometry is much more significant and a full penetration
of 10 mm is observed. It can also be seen from the
macrograph presented in Table 2 that the activated flux
causes the aspect ratio of the weld bead to reach 0.8,
which is a significant change. Electron-beam welds
exhibit the keyhole type of the bead geometry and a full
penetration can be achieved using a much smaller heat
input of 0.36 kJ/mm, showing the process to be greatly
enhanced.
It was observed in this study that the bead shape
becomes relatively wider and shallower with an increase
in the welding current for both GTA and nitrogen-added
GTA welding. Generally, the surface tension () on the
pool surface, formed by the cohesive forces of liquid me-
tal, decreases with an increase in the temperature. Thus,
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360 Materiali in tehnologije / Materials and technology 50 (2016) 3, 357–364
Figure 2: Microstructures of the FZs after various welding processes: a) GTAW, b) activated GTAW, c) nitrogen-added GTAW, d) EBW
Slika 2: Mikrostrukture FZ pri razli~nih postopkih varjenja: a) GTAW, b) aktiviran GTAV, c) du{ik dodan GTAW, d) EBW
Figure 1: Microstructure of the base metal showing a boride network
in an austenitic matrix
Slika 1: Mikrostruktura osnovnega materiala, ki ka`e mre`o boridov v
avstenitni osnovi
the temperature gradient becomes negative, i.e., d/dT < 0
which, in turn, generates centrifugal Marangoni convec-
tion in the molten pool. This constitutes one of the main
reasons for the shallow penetration and lower aspect
ratio in the conventional GTA welding. However, in the
case of activated GTA welds, the presence of the acti-
vated flux changes the temperature gradient to a positive
value, i.e., d/dT > 0. This positive temperature gradient
causes centripetal Marangoni convection, which, in turn,
directs the flow toward the pool center resulting in a
deeper and narrower weld pool. By and large, it was
noticed that the activated flux beneficially influenced the
Marangoni convection mode by changing the tempera-
ture gradient. Furthermore, the aspect ratio was in-
creased due to the variation in the temperature gra-
dient.21,22
4.2 Microstructure
As it can be noticed, irregular boride particles
(Fe,Cr)2B, seen as a dark phase, are dispersed in the
austenitic matrix. Boron is insoluble in austenite vir-
tually at all temperatures. The insolubility is more
significant in the case of steels with high boron levels,
which, in turn, results in a continuous network of boride
eutectics such as Fe2B and Cr2B in an austenitic matrix.
The boride eutectics strengthen the austenitic matrix but
adversely affect the toughness and ductility of these
steels. Furthermore, the shape of these eutectic phases is
also one of the factors affecting the mechanical behavior
of a weld.6
In order to have full-penetration welds for micro-
structural studies and an evaluation of mechanical
properties, the welds were made using two passes in the
case of the GTA and nitrogen-added GTA processes and
a single pass in the case of the activated-flux and EB
processes. The fusion zone of the activated-flux-GTA
weld was characterized by a columnar, austenite den-
dritic structure with eutectic borides solidified in the
interdendritic regions (Figure 2b). However, the GTA
and nitrogen-added-GTA welds exhibited both equiaxed
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Materiali in tehnologije / Materials and technology 50 (2016) 3, 357–364 361
Figure 3: Microstructures of the weldments after various welding processes: a) GTAW, b) activated GTAW, c) nitrogen-added GTAW, d) EBW
Slika 3: Mikrostrukture zvarov pri razli~nih procesih varjenja: a) GTAW, b) aktiviran GTAW, c) GTAW z dodanim du{ikom, d) EBW
Table 3: Mechanical properties of 304B stainless-steel welds made with various GTA welding processes
Tabela 3: Mehanske lastnosti zvarov nerjavnega jekla 304B, izdelanih po razli~nih GTA varilnih postopkih
Process Proof strength Ultimate tensilestrength % Elongation
Joint efficiency
in terms of
tensile strength
Failure location Impacttoughness
Base metal 384 576 12 – – 7
GTAW 379 545 10 94.62 PMZ 7
Nitrogen added GTAW 390 550 11 96.32 PMZ 8
Activated GTAW 400 569 14 98.78 BM 7
EBW 415 570 12 98.95 BM 10
and columnar dendritic structures, as can be seen in Fig-
ures 2a and 2c. A short interaction span with an acute
energy density causes a low heat input in EBW. As a
result, the fusion zone in EBW cools faster resulting in a
finer equiaxed dendritic structure.
Typical appearances of the PMZ for various weld
types are presented in Figure 3. As it can be noticed, the
representative PMZ consists of localized regions of
austenite that remain solid during the welding,
surrounded by irregular boride eutectics. However, the
width of PMZ was found to be larger in the case of the
GTA and nitrogen-added-GTA welds than that of the
activated-flux-GTA welds. This is attributed to slow
cooling rates associated with high heat inputs prevailing
in the GTA and nitrogen-added-GTA welds. The low
heat input associated with high cooling rates almost
eliminated the PMZ in electron-beam welding and no
localized region of austenite was noticed in Figure 3d.
4.3 Mechanical properties
4.3.1 Microhardness
Hardness profiles revealed that FZ exhibited a higher
hardness for all the welds. The increase in the hardness
of FZ is attributed to the presence of a dendritic micro-
structure with boride eutectics in the interdendritic
regions (Figure 2). It was observed that the nitrogen
addition in the GTA welds significantly enhances the FZ
hardness. It was also noticed that there is a sudden fall in
the hardness of PMZ in the case of both the GTA and
nitrogen-added GTA welds, while there is no such trend
in the activated GTA and EB welding. The significant
reduction in the PMZ hardness is attributed to the
difference in the cooling rate, which results in a variation
in the size and shape of the eutectic borides formed in
PMZ as can be seen from Figures 3a to 3d. It can be
seen from Figure 5 that the fracture during the tensile
test in the case of the GTA and nitrogen-added GTA
welds, occurs in PMZ, whereas in the case of the other
two welds the fracture occurs in the base materials far
away from the weld metal.
4.3.2 Tensile properties
In order to obtain joint properties, automatic GTAW
was carried out in two passes, one from each side.
Though there is no significant variation in the joint
efficiencies of the welds, a marginal improvement in the
yield strength for the electron-beam and activated-GTA
welds was noticed. The GTA and nitrogen-added-GTA
welds were found to fracture at PMZ, as shown in Fig-
ure 5, the region where a loss in the hardness can be
clearly noticed due to an irregular distribution of boride
eutectics. It was also observed that the activated-GTA
and EB welds failed in BM. Low-heat-input welding
processes exhibited a significant improvement in the
tensile strength compared to high-heat-input welding
process.
4.3.3 Impact toughness
It was observed that the welds exhibit the same
toughness as that of BM, irrespective of the welding
G. R. KUMAR et al.: EFFECT OF ACTIVATED FLUX AND NITROGEN ADDITION ON THE BEAD GEOMETRY ...
362 Materiali in tehnologije / Materials and technology 50 (2016) 3, 357–364
Figure 4: Microhardness profiles for various welding processes
Slika 4: Profil mikrotrdote pri razli~nih procesih varjenja
process employed. The lower toughness of BM and FZ is
mainly due to brittle (Cr,Fe)2B borides.
5 CONCLUSIONS
1. Defect-free welds of 304B4 borated stainless steels
can be easily made using GTA, activated-flux GTA,
nitrogen-added GTA and EB welding processes.
2. The activated-flux GTA and EB welding processes
substantially enhance both the depth of penetration
and the aspect ratio.
3. The fusion-zone microstructure of the activated-flux
GTA welds reveals a columnar, austenite dendritic
structure with eutectic borides solidified in the
interdendritic regions. However, the EB welds exhibit
a fine equiaxed dendritic structure.
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Materiali in tehnologije / Materials and technology 50 (2016) 3, 357–364 363
Figure 5: Fracture locations of the welds made with different welding processes: a) tensile-test samples, b) cross-sections
Slika 5: Lokacija preloma zvarov, izdelanih z razli~nimi postopki varjenja: a) vzorci za natezne preizkuse, b) preseki
4. Low-heat-input EBW inhibits the formation of PMZ.
A significant loss in the hardness occurs in the PMZs
of the GTA and nitrogen-added GTA welds, which is
mainly due to the slow cooling rates associated with
high heat inputs.
5. A tensile failure occurs in the PMZ area of the GTA
and nitrogen-added GTA welds, while the
activated-flux GTA and EB welds failed in the base
material.
Acknowledgments
The authors would like to acknowledge the technical
support of IIT Madras, Defence Research & Develop-
ment Laboratory (DRDL), Hyderabad and Ador Welding
Pvt. Ltd., Chennai.
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364 Materiali in tehnologije / Materials and technology 50 (2016) 3, 357–364