To prevent oxidation of the weld zone inside the pipe, high-quality welding of stainless steel pipe requires gas purging. Gas tungsten arc (GTA) welding of the 304H pipes commonly used in refinery applications was done with and without purging gas. For purging gases argon (Ar), nitrogen (N-2), Ar+N-2, and N-2+10% hydrogen (H-2) were used, respectively. The aim was to determine the effects of purging gases on the microstructural, corrosion, tensile, bend, and impact toughness properties of the welded joints. Macro sections of the welds were investigated as well as microstructures. Chemical composition of the weld metal of the joints was obtained by glow discharge optical emission spectroscopy (GDOES). Leco analyzers were used to obtain the weld root N-2, O-2, and H-2 contents. The ferrite content of the beads was measured with a Ferritscope, and Vickers hardness (HV10) values were measured. Intergranular and pitting corrosion tests were applied to determine the corrosion resistance of the welds. The various purging gases affected corrosion properties as well as the amount of the heat tints that occurred at the roots of the welds. As obtained by Leco N-2-O-2-H-2 analysis, a significant increase occurred in the root bead N-2 content from 480 to 820 ppm for no-purged and N-2-purged welds, respectively. As a result, the ferrite content of the root beads decreased to about 6 Ferrite Number when changing the purge gas to N-2 instead of no purging. However, mechanical properties were not considerably affected due to purging. 304H steel and 308H consumable compositions would permit use of N-2, including gases for purging, without a significantly increased risk of hot cracking.
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APRIL 2014, VOL. 93
124-s
WELDING RESEARCH
Introduction
For the high-quality stainless steel pipe
welds required for power plants, petro-
chemical facilities, pharmaceutical, brew-
ery, and food-processing factories, the gas
tungsten arc welding (GTAW) process is
preferred (Ref. 1). Weld root quality of
stainless steel pipe and tubes can be en-
sured by removing the air from the fusion
zone using an inert purging gas. Unsatis-
factory purging results in formation of fer-
rochromium layers of colored oxide films
commonly referred to as "heat tints."
Oxygen (O
2
) contamination in stain-
less steel welding causes dross or "sugar-
ing," referred to in the sanitary industry as
an oxide layer on the root surface of the
weld bead. This is a rough, pitted, and
porous layer that can trap organic matter
that may lead to contamination, weakened
mechanical properties, and compromised
in-service corrosion resistance of the
weldments (Refs. 1–6). In particular, pit-
ting in weld heat-affected zone (HAZ) has
often been reported for standard
austenitic stainless steels such as EN
1.4301/1.4401 (AISI 304/316).
In practice, the high-temperature ox-
ides formed during welding are removed
by pickling using a bath or paste followed
by repassivation. This is considered the
best method to restore the pitting corro-
sion resistance of an already oxidized
weld. However, postweld cleaning using
mechanical or chemical procedures is
often complex or too expensive (Refs. 1,
3). It is therefore essential to use a proper
purging technique to shield the root side
of the joint from atmospheric contamina-
tion. Common root shielding (purging)
gases are argon (Ar), nitrogen (N
2
), and
nitrogen mixed with hydrogen (H
2
, typi-
cally 10%). Other gases such as helium
(He), Ar/He, and H
2
mixtures are also
used. Hydrogen provides a reducing at-
mosphere that counteracts oxide forma-
tion more effectively than Ar, but is gen-
erally only recommended for austenitic
stainless steels. For N
2
alloyed austenitic
grades or superduplex stainless steels, it is
often recommended to use N
2
-containing
mixes to counteract losses of N
2
from the
weld pool. Variation in purge gas quality
may arise during welding and it may be de-
sirable to apply continuous gas monitor-
ing, in particular to control O
2
and mois-
ture content (Refs. 1, 6–10).
Pure Ar is the most commonly used
purging gas in GTA welding of standard
austenitic stainless steels such as 304 and
316. Since Ar is in short supply worldwide
and prices are rising, there is a cost and
availability incentive in changing to alter-
native purge gases such as pure N
2
or mix-
tures (Ref. 1).
The oil and gas plants must select the
most cost-effective and reliable materials
due to their diverse applications and con-
ditions. Much oil and gas technology is
mature practice. In large part, the stainless
steels are employed in plant and associ-
ated equipment where the corrosion re-
sistance of plain carbon- or low-alloy
steels is inadequate. The austenitic grades
find applications where their excellent el-
evated-temperature or cryogenic mechan-
ical properties are of advantage (Ref. 11).
There is little published literature
about the effects of purging gases on the
microstructural, corrosion, and mechani-
cal properties of 304H austenitic stainless
steel that is applicable to the oil and gas in-
dustry and refinery applications where se-
vere corrosion conditions are common.
Thus, the aim of this study was to detect
the effects of purging gases on the mi-
crostructural properties, ferrite content,
corrosion resistance, and mechanical
properties of the GTA welded joints of
304H stainless steel pipes used in refinery
applications.
Effect of the Purging Gas on Properties of
304H GTA Welds
Experiments measured the effects of various purging gases on the microstructural, corrosion,
tensile, bend, and impact toughness properties of stainless steel weld metal
BY E. TABAN, E. KALUC, AND T. S. AYKAN
KEYWORDS
304H
GTAW
Purging
Microstructure
Corrosion
E. TABAN (emel.taban@yahoo. com) and
E. KALUC are with Kocaeli University
(KOU) Welding Research, Education and
Training Center, and the KOU Engineering
Faculty, Dept. of Mechanical Engineering,
Kocaeli, Turkey. T. S. AYKAN is with
Turkish Petroleum Refineries Corp., Izmit
Refinery, Technical Control and R & D
Dept., Kocaeli, Turkey.
ABSTRACT
To prevent oxidation of the weld zone inside the pipe, high-quality welding of
stainless steel pipe requires gas purging. Gas tungsten arc (GTA) welding of the 304H
pipes commonly used in refinery applications was done with and without purging gas.
For purging gases argon (Ar), nitrogen (N
2
), Ar+N
2
, and N
2
+10% hydrogen (H
2
)
were used, respectively. The aim was to determine the effects of purging gases on the
microstructural, corrosion, tensile, bend, and impact toughness properties of the
welded joints. Macro sections of the welds were investigated as well as microstruc-
tures. Chemical composition of the weld metal of the joints was obtained by glow dis-
charge optical emission spectroscopy (GDOES). Leco analyzers were used to obtain
the weld root N
2
, O
2
, and H
2
contents. The ferrite content of the beads was meas-
ured with a Ferritscope®, and Vickers hardness (HV10) values were measured. In-
tergranular and pitting corrosion tests were applied to determine the corrosion re-
sistance of the welds. The various purging gases affected corrosion properties as well
as the amount of the heat tints that occurred at the roots of the welds. As obtained
by Leco N
2
-O
2
-H
2
analysis, a significant increase occurred in the root bead N
2
con-
tent from 480 to 820 ppm for no-purged and N
2
-purged welds, respectively. As a re-
sult, the ferrite content of the root beads decreased to about 6 Ferrite Number when
changing the purge gas to N
2
instead of no purging. However, mechanical properties
were not considerably affected due to purging. 304H steel and 308H consumable
compositions would permit use of N
2
, including gases for purging, without a signifi-
cantly increased risk of hot cracking.
TABAN Research Supplement_Layout 1 3/13/14 10:48 AM Page 124
Table 1 presents the chemical analysis
and transverse tensile properties for AISI
304H stainless steel pipe with 0.236-in. (6-
mm) wall thickness and 4-in. (101.6-mm)
diameter.
Welding Setup
Gas tungsten arc welding was used for
root and hot pass welding the 304H pipe
using Ar shielding gas. The welded joint
referred to as 04H NP (no purge) was pro-
duced without using any purge gas for the
root pass. For the following joints, purging
gases of pure Ar, pure N
2
, Ar+5%N
2
, and
N
2
+10%H
2
, respectively, were used as
root shielding gases. In addition to the
root pass, five more fill passes were used
to finish the welds.
The welded pipes were referred to as
04H A (Ar purging), 04H N (N
2
purging),
04H AN (Ar+N
2
purging), 04H NH
(N
2
+H
2
purging), respectively. During
the whole period of welding, purging was
maintained until cooling the root pass as
well as all fill passes. ER 308H GTA weld-
ing rods were used as filler metal. The
chemical composition of the filler metal is
given in Table 2.
A fixed root opening was maintained
by tack welding before producing the
welds. Welding started after the measure-
ment of O
2
content by an O
2
analyzer and
after obtaining an oxygen content of max-
imum 10 ppm within the pipe, before
welding started. The details of welding
procedures are presented in Table 3.
Cross sections taken from the pipe
welds were prepared according to stan-
dard metallographic techniques; grinding
on composite discs then polishing on tex-
tile discs with diamond suspensions of in-
creasingly finer diamond-grain sizes. The
samples were electrolytically etched in ox-
alic reagent for evaluation of the mi-
crostructure. In addition, a second series
of samples was prepared and color etched
in Lichtenegger solution to permit identi-
fication of the solidification mode.
The ferrite phase content of the root
bead and cap pass as well as base metal
were determined using a Fisher Fer-
ritscope® by measuring at the root bead
surface. A minimum of 15 measuring
points were taken on each sample to de-
termine the Ferrite Number (FN).
The chemical composition of the weld
metal of the joints was obtained by glow
discharge optical emission spectroscopy
(GDOES). In addition, Leco combustion
equipment was used to obtain the weld
root N
2
, O
2
, and H
2
contents.
The Vickers measurements were de-
termined with a 10-kg load using an In-
stron hardness machine over the weld
cross sections of each weld in accordance
with EN standard practices. Three meas-
urements were taken at the surfaces for
weld metal roots (≤ 2 mm deep from the
root surface) and base metal.
Intergranular and pitting corrosion
tests were applied in accordance with TS
EN 3157/EN ISO 3651-2 and ASTM G48,
respectively. Intergranular corrosion test-
ing was conducted as a sulfuric acid-cop-
per sulfate test for 20 h in boiling solution,
then bending of the boiled samples. Pit-
ting corrosion testing samples were kept in
ferric chloride solution at 50°C for 72 h.
Weight losses were measured. Pho-
tomacrographs of the test samples were
obtained after corrosion testing both for
intergranular and pitting corrosion.
Transverse tensile specimens from all
joints were prepared in accordance with the
API 1104 standard, then tested at room
temperature by a hydraulically controlled
test machine. Transverse face and root bend
test specimens with nominal specimen
width of 25 mm were prepared then tested
125-s
WELDING JOURNAL
WELDING RESEARCH
Fig. 1 — Photographs showing roots of welded
joints of 304H stainless steel pipes after root and
five fill passes. A — 04H NP; B — 04H A; C —
04H N; D — 04H AN; E — 04H NH, after only
root pass; F — 04H NP; G — 04H N.
Table 1 — Chemical Composition and Mechanical Properties of the 304H Pipe
CSi MnCr MoNi AlCoCuNb Ti VWFe
0.07 0.22 1.45 18.00 0.32 9.05 0.06 0.07 0.35 0.02 0.01 0.06 0.02 Bal.
Transverse tensile properties
Yield strength (R
P0.2
) MPa UTS (R
m
) MPa % Elongation
205 515 40
A
B
C
D
E
F
G
TABAN Research Supplement_Layout 1 3/13/14 10:48 AM Page 125
with a 20-mm mandrel diameter and 180-
deg bending angle. Subsized notch impact
test samples were extracted transverse to
the welds and through thickness with
notches positioned at the weld metal center
(WM), at the HAZ; 2 mm away from the
weld interface (FL+2 mm) and at base
metal (BM). The impact toughness testing
was done at 20°, –20°, –40°, and –60°C.
Results and Discussion
Weld Cross Sections
Representative photographs of the
weld roots showing the chromium oxide
layers (heat tints) of the welded 304H
pipes are presented in Fig. 1. The 304H
pipes with 6-mm wall thickness were
welded in six passes in total. Depending on
the total heat input applied, the material
revealed a wide range of heat tints, in par-
ticular for the weld without purging. Fig-
ure 1A–E refers to the roots of the welds
after welding of one root pass + five fill-
passes. To give a better idea and correla-
tion with AWS D18.1 (Ref. 12), photo-
graphs of the roots after only the root pass
were also given for the weld without purg-
ing (Fig. 1F), and for the weld with N
2
purging (Fig. 1G). The heat tint formation
after one root pass was less than that after
welding of a root pass + five fill passes
(Fig. 1A–G). However, the differences be-
tween the no-purged and purged welds are
clearly observed by the wideness of the
heat-tint areas. The purged welds are
much cleaner with brighter colors com-
pared to the no-purge weld. The joint
without any purge gas (04H NP) revealed
a wide chromium-oxide layer (Fig. 1A, F).
This oxide layer (heat tints) decreases the
corrosion resistance since it contains
chromium that has been taken from the
metal immediately beneath this layer.
However, for the joints with a purge gas:
04H A (Fig. 1B), 04H N (Fig. 1C, G), 04H
AN (Fig. 1D), 04H NH (Fig. 1E), the
widths of the heat tints decreased as well
as the color is lighter compared to 04H NP
(no purged joint). In particular, for the
weld purged with H
2
-containing gas (04H-
NH) the bleaching effect of the H
2
could
clearly be observed.
Figure 2 shows the photomacrographs
of the welds. The differences between the
root shapes of the no purged weld (04H
NP) and the other welds can be seen. The
04H NP root needs cleaning, incurring an
additional expense compared to the other
joints with better root shapes. The me-
chanical properties of welds are affected
significantly by their shape and composi-
tion. Particularly at the weld root, a posi-
tive reinforcement combined with a
smooth transition from the weld to base
metal are prerequisites to achieve opti-
mum mechanical strength.
In fusion arc welding, an important part
is the type of shielding gas used since it af-
fects the shape, material transfer mode, and
energy distribution in the arc. For instance,
thermal conductivity of Ar is very low, which
affects both the arc shape and the weld
shape, thus mainly a wine-glass-shaped
weld is obtained. However, due to the high
thermal conductivity of H
2
, the arc gets nar-
rower and energy concentration in it in-
creases, which leads to deeper penetration.
APRIL 2014, VOL. 93
126-s
WELDING RESEARCH
Fig. 2 — Photomacrographs of welded 304H pipes. A — 04H NP; B — 04H A; C — 04H N; D — 04H
AN; E — 04H NH.
Table 2 — Chemical Composition of the Filler Metal Used in this Study
Filler Type Chemical Composition (wt-%)
C Si Mn Cr Ni Mo Other
ER308H 0.04–0.08 0.5 1.7 20.1 9.8 — —
Fig. 3 — Photomicrographs of 304H root passes. A
— 04H NP; B — 04H A; C — 04H N; D — 04H
AN; E — 04H NH with 200× magnification (ox-
alic etching).
A
A
B
B
C
C
D
E
D
E
TABAN Research Supplement_Layout 1 3/13/14 10:48 AM Page 126
Hydrogen as a reducing gas hinders oxide
formation on the surface of the weld. Thus,
the weld appearance is nicer. However, H
2
solubility in steel is very high, which may
produce porosity and cracks mainly in du-
plex stainless steel welds (Refs. 4, 13).
Microstructure
The microstructure consisted in all cases
of austenite with some ferrite, including
both root and cap passes of all joints ob-
tained by using various purging gases — Fig.
3. Color etching with Lichtenegger etchant
shows that the weld metal mainly solidified
primarily as ferrite — Fig. 4.
Ferrite Content
The chemical composition and thermal
history affect the amount of ferrite in
stainless steel weld metals. The ferrite
content affects toughness properties, cor-
rosion resistance, long-term high-temper-
ature stability, and in particular, resistance
to hot cracking. Thus, it is important to
control the ferrite level within specified
limits. Typically, a minimum ferrite con-
tent of 3 FN is desired to ensure solidifi-
cation with ferrite as the primary phase to
provide good resistance to hot cracking.
The WRC-92 diagram can be used to pre-
dict the FN of weld metal with reasonable
accuracy (Ref. 14) and also give guidance
on the type of expected solidification.
Plotting the wire and the pipe composi-
tions of this study, the diagram predicts
ferrite content of the weld metal approxi-
mately to be 9 FN.
The results of the FN measurements
are summarized in Table 4.
Addition of N
2
to the purging (root
shielding) gas clearly resulted in a lower
root pass ferrite content. The joint without
any purging revealed an average root pass
FN of 8, while a minimum average ferrite
content of 1.5 was obtained, for the weld
with N
2
purging. The maximum difference
is about 6.5 FN for these welds. For the Ar-
purged weld (04H A), an average of 4.3 FN
was obtained which is also quite low com-
pared to the weld without purging (04H
NP). Purging with N
2
+10% H
2
(04H NH)
also presented an average ferrite content of
3.3 FN. This is important and desired in par-
ticular for refinery applications where hot
cracking is a problem. The predicted ferrite
content of 9 FN is in good agreement with
127-s
WELDING JOURNAL
WELDING RESEARCH
Table 3 — Welding Details of 304H Pipes
Welded Position, Consumable Shielding Gas, Purging Gas, Welding Welding Total Heat Interpass
Pipe Code Number of Shielding Gas Purge Gas Parameters Speed Input Temp. (° C )
Total Layers Flow Rate Flow Rate (V/A) (mm/s) (kj/mm)
Purging time
04H NP Not available 8.9–10.5/ 80–120 0.70–1.00 7.37
04H A 100% Ar
2
, 9.2–10.7/85–120 0.95–1.81 5.05
5 L/min for root,
2 L/min for filler
passes
100% N
2
, 8.8–10.9/85–120 0.90–1.72 5.5
PA, Ø 2.4 mm 100% 5 L/min for root, 100–150
04 N Ar
2
, 2 L/min for filler
root + 5 308H passes
filler passes (Avesta) 9 L/min Ar
2
+ 5% N
2
, 8.2–10.6/85–120 0.6–1.65 5.7
04 AN 5 L/min for root,
2 L/min for filler
passes
04 NH N
2
+10%H
2
, 8.9–11.9/85–120 0.70–1.81 5.38
3 L/min for root,
2 L/min for filler passes
Fig. 4 — Photomicrographs of 304H root passes.
A — 04H NP; B —04H A; C — 04H N; D — 04H
AN; E — 04H NH with 200× magnification
(Lichtenegger etching).
A
B
D
E
C
TABAN Research Supplement_Layout 1 3/13/14 10:48 AM Page 127
the measured values of the no-purged weld.
Most measured and predicted FNs fall well
within the F+A region in the diagram pre-
dicting solidification with ferrite as the lead-
ing phase. Primary ferritic solidification was
also confirmed by color etching — Fig. 4.
Chemical Composition
The chemical compositions of the root
beads were determined by chemical analy-
sis and listed in Table 5.
The N
2
, O
2
, and H
2
contents (in ppm)
of the root beads, determined by Leco
chemical analysis, are shown in Table 6.
The use of pure N
2
and N
2
-rich,
N
2
+10% H
2
as purging gas resulted in sta-
ble levels of about 820 and 795 ppm in the
root beads, whereas about only 485 and 445
ppm were measured for the non-purged
and Ar-purged welds, respectively. This
agrees with the ferrite content data, where
the FN was decreased to about 1.5 and 3.3
FN in pure N
2
and N
2
-rich, N
2
+10% H
2
gas
as purging.
Hardness Properties
The Vickers hardness values for the
weld roots and base metal are given in
Table 7. No significant differences were
observed for the hardness properties of
the weld roots of all joints.
Intergranular Corrosion Properties
Due to the relatively high carbon con-
tent of the 304H base metal compared to
304L grade, the risk for intergranular cor-
rosion of the 304H grade would be higher
compared to that of the 304L grade. Thus,
to detect the possible susceptibility to in-
tergranular corrosion, testing in accor-
dance with TS EN 3157/EN ISO 3651-2
was applied by immersing the samples in a
boiling solution of sulfuric acid-copper
sulfate for 20 h followed by bending the
samples.
Nitrogen was added to the shielding
and/or purging gas mainly to improve pit-
ting corrosion resistance but also to im-
prove mechanical strength to some extent.
Corrosion resistance at the root side was
also increased by using pure N
2
or N
2
with
5–10% H
2
in the purging gas. Higher N
2
lev-
els and exposure to higher temperature for
extended periods would end up affecting
the properties of the welds, since N
2
in
austenitic stainless steels plays a role similar
to that of carbon in increasing the mechan-
ical strength but without the associated dis-
advantages related to precipitation of car-
bides and carbon nitrides. No failures were
detected after bending, which indicated
good corrosion resistance. Photomacro-
graphs of the corrosion test samples are pre-
sented in Fig. 5, including root sides of the
joints.
Pitting Corrosion Properties
Pitting corrosion testing was applied in
accordance with ASTM G48. Samples were
immersed in ferric chloride solution at 50°C
for 72 h. Materials used in the oil and gas in-
dustry are affected by several different types
of corrosion, often caused by seawater and
spray. In marine environments, pitting and
crevice corrosion occur, and for austenitic
grades, stress corrosion cracking also occurs
if the material temperature is above 60°C.
High temperatures, high chloride contents,
and low pH values increase the risk of lo-
calized attacks in any chloride-containing
environment.
The electrochemical corrosion potential
is also very important. This potential is af-
fected by biological activities on the steel
surface. Since seawater and related envi-
ronments are, in a sense, living corrosive en-
vironments, it is sometimes difficult to de-
fine exactly what the service conditions will
be. At temperatures above 40°C, the bio-
logical activity will cease and the corrosion
potential would change. The use of contin-
uous chlorination, to stop marine growth,
may increase the corrosion potential. Nor-
mally, the CPT of 304H grade is much lower
than 50°C. However, it should be kept in
mind that for refinery applications where
highly corrosive fluids are carried in the
pipes, the pipes could deteriorate during
service conditions. Here, the maximum
time and temperature conditions were used
in the test to extrapolate the excessive con-
APRIL 2014, VOL. 93
128-s
WELDING RESEARCH
Fig. 5 — Photomicrographs of the root side dispa-
lying the intergranular corrosion test samples of the
welds. A — 04H NP; B — 04H A; C — 04H N; D
— 04H AN; E — 04H NH.
Table 4 — Ferrite Content of the 304H Pipe GTAW Welds
Location of the Welded joint code
Measurement 04H-NP 04H-A 04-N 04-AN 04-NH
Base metal 0.13-0.16
Root pass Min: 7.3; Min: 3.8; Min: 0.97; Min: 4.1; Min: 2.8;
Max: 8.7 Max: 4.8 Max: 2.0 Max: 5.1 Max: 3.7
(Average 8) (Average 4.3) (Average 1.5) (Average 4.6) (Average 3.3)
Cap pass 8.0–11.5 7.5–9.5 7.4–8.8 7.3–8.2 7.7-9.1
Table 5 — Chemical Analysis of the Weld Metal of GTAW Welded 304H Pipes
Welded Joint Code C Si Mn Cr Mo Ni Al Co Cu Nb Ti V W Fe
4H NP 0.10 0.15 1.64 19.80 0.06 9.50 0.05 0.03 0.12 0.01 0.01 0.04 0.03 Bal.
4H A 0.02 0.22 1.69 19.80 0.09 9.93 0.06 0.03 0.12 0.02 0.01 0.04 0.05 Bal.
4H N 0.07 0.16 1.68 19.20 0.05 9.17 0.04 0.03 0.12 0.01 0.01 0.04 0.02 Bal.
4H AN 0.07 0.20 1.63 19.70 0.08 9.39 0.05 0.03 0.12 0.01 0.01 0.04 0.02 Bal.
4H NH 0.05 0.28 1.68 19.90 0.06 9.54 0.04 0.03 0.10 0.01 0.01 0.04 0.02 Bal.
A
B
D
E
C
TABAN Research Supplement_Layout 1 3/13/14 10:48 AM Page 128
ditions to receive accelerated data and serve
as a ranking test (Ref. 15). Weight losses
were measured and are presented in Table
8. Photomacrographs of the samples were
obtained after corrosion testing and are
shown in Fig. 6. The minimum weight loss,
thus the least corrosion, was observed for
the samples welded with Ar purging. The
maximum weight losses seem to be from the
welds purged with N
2
; however, looking at
the photographs, most of the pits are ob-
served to be from the base metal and not in
the weld metal.
Transverse Tensile Test Results
All transverse tensile specimens
demonstrated, without exception, the ac-
tual overmatching strength of the weld vs.
the base metal, and fracture occurred at
the base metal. The tensile strength varied
from 619 to 675 MPa (Table 9).
Bend Test Results
None of the face and root bend test
samples of 304H welded pipes failed dur-
ing bending.
Impact Toughness Results
The impact toughness results of the
joints with various purging gases and ef-
fect of the backing gas is also shown in a
graph in Fig. 7. The impact toughness re-
sults do not present serious differences
due to the purging gas except for the joints
purged with N
2
+10%H
2
.
Conclusions
This study dealing with the effect of the
purging (root shielding) gas on the mi-
crostructural, corrosion, and mechanical
properties of GTA welded 304H pipes re-
vealed the following conclusions:
The joints welded without any purge
gas revealed a wide area of chromium-
oxide layer (heat tints) that decreased the
corrosion resistance. For the joints welded
with a purge gas, the width of the heat tints
decreased and the color was lighter com-
pared to the welds with no purging. In par-
ticular, for the welds purged with H
2
-con-
taining gas, the bleaching effect was
clearly observed.
The use of N
2
-containing purge gas can
introduce significant amounts of N
2
into
the root bead and is more likely to occur
with larger root openings and manual weld-
ing. This affected the ferrite content of the
weld metal with a decrease of up to 6 FN.
For welds produced with 304H steel and
308H consumable wire compositions, any
hot cracking problems are not predicted as
the welds will still solidify with ferrite as the
primary phase. However, it is recom-
mended to check steel and wire composi-
tions against the WRC-92 diagram to verify
there is sufficient safemargin. It is also ad-
vised to measure root bead ferrite content,
if accessible, because this gives an indication
of whether the weld metal has solidified
with ferrite or austenite as the leading
phase. It should be kept in mind though that
heavily reheated root beads can have a
lower ferrite content than predicted by
WRC-92 and that this is not necessarily an
indication of increased risk of hot cracking.
A significant decrease in the ferrite con-
tent of root beads was found when changing
the root shielding gas from pure Ar to mixed
N
2
/H
2
(90%N
2
+10%H
2
) or pure N
2
. The
root bead ferrite content decreased up to 6
FN compared to the no-purged welds.
However, no indication of hot cracking was
found and all root beads solidified as pre-
dicted by the WRC-92 diagram with ferrite
as the leading phase.
The use of pure N
2
and N
2
-rich,
N
2
+10% H
2
as purging gas resulted in sta-
ble levels of about 820 and 795 ppm in the
root beads, whereas about only 485 and 445
ppm were measured for the non-purged
and Ar-purged welds, respectively. These
data are very much related and in accor-
dance with the ferrite-content data, where
the FN was decreased to about 1.5 and 3.3
FN in pure N
2
and N
2
-rich, N
2
+10%H
2
purging gases.
129-s
WELDING JOURNAL
WELDING RESEARCH
Fig. 6 — Photomicrographs featuring the
root side of the pitting corrosion test sam-
ples of 304H welds. A — 04H NP; B —
04H A; C — 04H N; D — 04H AN; E —
04H NH.
A
B
D
E
C
Table 6 — Leco Analysis of the Root Beads of GTA Welded 304H Pipes
Welded joint code N
2
(ppm) O
2
(ppm) H
2
(ppm)
4H NP 480 487 484 260 265 260 2.02 1.87 1.92
4H A 448 442 445 88 89 80 1.90 3.80 3.11
4H N 819 820 819 73 75 71 1.03 1.90 1.53
4H AN 470 478 474 66 64 62 1.95 2.0 2.18
4H NH 793 790 797 54 50 58 2.43 3.08 3.29
Table 7 — HV10 of the 304H Pipe GTAW Welds without and with Various Purge Gases
Location of the Welded Joint Code
Measurement 04H–NP 04H–A 04–N 04–AN 04–NH
Root pass 179–181 168–170 185–187 189–193 183 –186
Base metal 193 191 193 181 181
TABAN Research Supplement_Layout 1 3/13/14 10:48 AM Page 129
Pitting corrosion testing has shown that
minimum pitting corrosion products were
observed for the Ar-purged weld, compared
to no-purged welds. Mechanical testing
showed no significant change according to
the purge gas.
The 304H steel and 308H consumable
compositions would permit use of N
2
-rich
gases for root shielding without a signifi-
cantly increased risk of hot cracking. How-
ever, the increased N
2
level must be consid-
ered in the choice of steel and consumable.
It is recommended to use the WRC-92 dia-
gram to verify there is a sufficient safety
margin for actual compositions and, if pos-
sible, check the root bead ferrite content to
avoid the risk of hot cracking.
Acknowledgments
The authors would like to acknowledge
the financial and technical support of Turk-
ish Petroleum Refineries Co., Izmit Refin-
ery, in scope of R&D project (Project No.
2010/07). In addition, the support of col-
leagues at the Inspection, R&D, and Chem-
istry Departments of the Refinery and con-
tributions of IWE Mehmet Bilgen and Asil
Celik Corp. are very much appreciated.
References
1. Bergquist, E. L., Huhtala, T., and Karlsson,
L. 2011. The effect of purging gas on 308L TIG
root pass ferrite content. Welding in the World
3/4, 55: 57–64.
2. Saggau, R. 2005. Investigation of the effect
of yellow heat tints on the pitting corrosion be-
havior of welded stainless steels. PhD thesis,
Technical University Carolo Wilhelmina at
Braunschweig.
3. Ödegard, L., and Fager, S. A. 1993. The pit-
ting resistance of stainless steel welds. Aus-
tralasian Welding Journal, Second quarter, 24–26.
4. Fletcher, M. 2006. Gas purging optimizes
root welds. Welding Journal 85(12): 38–40.
5. Sewell, R. A. 1997. Gas purging for pipe
welding. Welding and Metal Fabrication.
6. Andersen, N. E. Welding stainless pipes,
key technology for process industry applications.
Svetsaren 1/2; 53–56.
7. Petersens, A. F., and Runnerstam, O. 1993.
Selecting shielding gases for welding of stainless
steels. Svetsaren 47: 2; 11–15.
8. Cuhel, J., and Benson, D. 2012. Maintain-
ing corrosion resistance when welding stainless
tube and pipe. Welding Journal 91(11): 47–50.
9. Li, L. J., and Davis, T. 2007. Effect of purg-
ing gas oxygen levels on surface structure and me-
chanical properties of GTA welded Type 304
stainless sanitary tube. Journal of Advanced Ma-
terials 39(4): 14–19.
10. McMaster, J. 2008. Using inert gases for
weld purging. Welding Journal 87(5): 40–44.
11. Gooch, T. G. 2010. Welding new stainless
steels for the oil and gas industry. The Welding In-
stitute, March, 16 pages.
12. D18.1/D18.1M:2009, Specification for
Welding of Austenitic Stainless Steel Tube and Pipe
Systems in Sanitary (Hygienic) Applications.
American Welding Society, Miami, Fla.
13. Tusek, J., and Suban, M. 2000. Experi-
mental research of the effect of hydrogen in
argon as a shielding gas in arc welding of high
alloy stainless steel. Int'l Journal of Hydrogen En-
ergy 25: 369–376.
14. Lippold, J. C., and Kotecki, D. J. 2005.
Welding Metallurgy and Weldability of Stainless
Steels. John Wiley & Sons, New Jersey.
15 MIG welding stainless steel gas mixes.
www.weldreality.com/stainlesswelddata.htm. Vis-
ited on May 19, 2010.
APRIL 2014, VOL. 93
130-s
WELDING RESEARCH
Fig. 7 — Impact toughness test results of GTAW welded 304H pipes.
Table 8 — Weight Loss after Pitting Corrosion Test
Welded First Measurement Weight Measurement Weight Measurement Weight Measurement Total Loss
Joint Code before Testing after 24 h after 48 h after 72 h
04H-NP 128.176 126.574 125.241 125.092 3.084
04H-A 126.425 125.105 124.084 123.953 2.472
04H-N 124.953 122.408 121.685 121.414 3.538
04H-AN 120.187 118.278 117.563 117.348 2.839
04H-NH1 125.798 123.782 123.045 122.868 2.929
Table 9 — Transverse Tensile Test Results of the GTA Welded 304H Pipes with Various Purging Gases
Welded Joint Code ReH (N/mm
2
) Rp 0,2 (N/mm
2
) Rm (N/mm
2
) % Elongation Fracture Location
04H NP 431–459 436–466 645–675 36,70 Base metal
04H A 427–429 435–440 644–649 42 Base metal
04H N 405–428 408–440 619–621 32 Base metal
04H AN 412–455 435–461 636–657 32 Base metal
04H NH 423–461 430–463 629–648 36 Base metal
TABAN Research Supplement_Layout 1 3/13/14 10:48 AM Page 130
... In metallic materials, there is a constant search for improvement in its mechanical properties and corrosion resistance [1]. This is also similar in the field of welding of metals [2][3][4][5][6]. The behavior of materials during welding can be significantly affected by modifications in the fabrication processes. ...
... In this line of thought, it is the objective of the present study to evaluate the properties and behavior of the face and root of a weld while protected with various gas purging conditions. This work will benefit the continuously increasing demand of these types of studies in the welding industry in the past several years [2,3]. ...
... During welding of stainless steel tubular components, it is a common practice to use gas purging to protect the weld during melting and solidification processes [3,6]. The presence of gas minimizes the high-temperature reaction of the weld with air and other atmospheric impurities, which affects the quality of the weld. ...
The primary objective of the present work is to evaluate the influence of various commercial gases on the microstructure of tubular AISI 304 austenitic stainless during welding. This was carried out using CO2, Ar, N2, Ar + 25%CO2 and Ar + 2%O2 gases, which are common inert gases or mixture specified in numerous technical standards associated with welding. These five gases were evaluated using flow rate range of 6 to 18 L/min. Different welding speed, wire feed speed, shield gas (Ar + 2% O2), distance nozzle contact-piece, voltage and current were employed to validate the present observations. The microstructures of the samples were evaluated along the cross-section of the face of the weld using optical microscopy. The samples were further analyzed by means of magnetic testing, which could provide information related to the evolution of ferrite. The estimated phase fractions were then compared to the predictions provided by the Welding Research Council (WRC-92) diagram. The optical microscopy images showed small microstructural variations between the samples with different gas purge condition, even when using the maximum gas flow rate. However, these observations were inconsistent with the magnetic response of the material, which provided significant differences in the phase fractions between the face and the root of the weld. The discrepancies between these two methods were analyzed to evaluate the phases and consistently track the phase fractions after welding.
... In general, the corrosion properties of weldments could be improved by either filler metals or gas mixtures for shielding and backing. Many pieces of research have been performed on austenitic stainless steel, mostly grade 304 and 316 [4][5][6][7][8][9][10]. Filler metal with Nitrogen mixed into the backing gas would be beneficial to improve the corrosion properties. ...
... In the sample with 2.0 mm thickness, the ferrite content decreased from 8.9%, 8.7%, and 8.5% as the N 2 addition to Argon increased from 5%, 10% and 15%, respectively. These results conformed to other works in literature, in which Nitrogen addition would be helpful for austenite promotion during solidification altering of the weld solidification modes and microstructures [4,6,16]. In addition, the weld heat input required for creating the weldment increased as the sample thickness increased, according to Table 2. ...
- Picha Panmongkol
- Isaratat Phung-on
Pitting corrosion would pose difficulty for the cleaning of equipment used in food processing, even when using the Cleaning in Place (CIP) process. It tends to occur at a location requiring welding. In general, Gas Tungsten Arc Welding (GTAW) is the most applicable process for food and drug equipment. To improve the properties of the weldment, filler metals are commonly used. However, the use of thin-sheet stainless steel in the application of filler metals would be difficult in food processing. In this research, the weld metal properties would be improved only by using N2 mixing to the backing gas during welding. Experiments were conducted using various mixtures of N2 on 1.5, 2.0, and 3.0 mm thicknesses of 304 stainless steel. The roots of weld metal taken from the samples were evaluated for corrosion properties in 3.5%NaCl from the polarization curves. The results showed the improvement of pitting corrosion from 379 mV to 757 mV, while the general corrosion rate decreased from 0.00477 mm/y to 0.00099 mm/y with 100%Ar and 100%N2, respectively. Both Nitrogen content and solidification rate together affected the solidification modes and ferrite contents that would affect the corrosion properties. At least 15%N2 mixing would be sufficient for weld metal property improvement. Due to the decrease of ferrite content from N2 addition, solidification cracking susceptibility test by fishbone test was acceptable with no cracking observed, showing the potential to recommend N2 mixtures for actual production.
... Taban E. y otros [13] reportan que las juntas soldadas sin algún tipo de purga revela un área con capas de óxido de cromo que reduce la resistencia a la corrosión y que en las juntas soldadas con gas de purga, la profundidad de los tintes de calor se reducen y los colores son claros comparados con las soldaduras sin purga. ...
... Las juntas soldadas sin algún tipo de purga comúnmente revelan un área con capas de óxido de cromo que reduce la resistencia a la corrosión, pero varios estudios han demostrado que en las juntas soldadas con gas de purga se reduce la defectología al controlar el oxígeno atmosférico en la cámara de purga para la realización del proceso de soldadura. Se evidencia que en las juntas soldadas con gas de purga, la profundidad de los tintes de calor se reduce y los colores son claros [10][11][12][13]. ...
TIG welding was performed on AISI 304 stainless steel pipes purged with argon, the amounts of atmospheric oxygen in the purge chamber analyzed in this work were 999, 500, 200, 100, 50, 25 and 10 ppm that used to determine as the amount of oxygen affects the superficial quality of the heat affected zone (HAZ) in the internal part of the pipe. In this study, mechanical tests, metallographic analysis and morphological study were carried out. We determined that the levels of discoloration in the HAZ are directly influenced by the amount of atmospheric oxygen present in the purge, with amounts between 50 to 10 ppm the discoloration is according to AWS D18.1 and AWS D18. 2 standards. While was evidenced good resistance in the welded joint because the failure occurs in the base metal and not in the HAZ which has not influence of surface discoloration corresponding to 10 ppm.
... Unsatisfactory purging results in formation of ferrochromium layers of colored oxide films commonly referred to as "heat tints." Pure Ar is the most commonly used purging gas in GTA welding of several grades of steels [4]. In addition, at least 2% nitrogen would be recommended in pure argon mixture for shielding gas for optimizing the phase balance and impact toughness do DSS [5]. ...
... Taban et. al. [4] also observed the differences between the no-purged and purged welds in surface of 304H pipes. The purged welds were much cleaner with brighter colors compared to the no-purge weld. ...
... Ferrite fraction is an important microstructure variable, which influences weld characteristics [41]. Ferrite content of weld is significantly influenced by the chemical composition and thermal exposure in the multipass process, which in turn affects toughness, corrosion, and hot cracking resistance [42]. Ferrite number gives a fair estimation of weld's ferrite content, and it can be theoretically calculated using Schaeffler and WRC-1992 diagram. ...
In the present experimental work, an attempt has been made to investigate the effect of filler metal on solidification, microstructure, and mechanical properties of the dissimilar weld between super duplex stainless 2507 and high strength low alloy API X70 pipeline steel. This joint is widely used in offshore hydrocarbon drilling risers and oil-gas transportation pipelines. Welds have been fabricated by the gas tungsten arc welding process using super duplex 2594 and austenitic 309L grade filler. A comparative assessment of both the fillers has been done by investigating multiple aspects of weld's structural integrity. 309L fillers weld has skeletal ferrite morphology, whereas 2594 filler solidifies with precipitation of multiple reformed austenites in the ferrite matrix. Lower width unmixed zone is formed on the API X-70 side in 2594 filler as compared to 309L. The tensile and impact strength of 309L filler weld is superior to 2594 filler. Precipitation of reformed austenite in the weld zone leads to lower hardness in 2594 as compared to skeletal ferrite dominated 309L weld zone. High chromium concentration in the weld zone results in superior pitting corrosion resistance of 2594 filler.
... Hence, inert gas such as argon or helium gas purging technique are commonly used to prevent oxidation. Two to five percent nitrogen addition in argon gas [8] improves and assist phase balance. ...
- Prabhu Paulraj
-
![Rajnish Garg]()
Duplex Stainless Steel (DSS) and Super Duplex Stainless Steel (SDSS) pipes were welded by Gas Tungsten Arc Welding (GTAW) process. The effect of welding parameters such as heat input, cooling rate, shielding/purging gas composition and interpass temperature on tensile strength, hardness and impact toughness were studied. The microstructure analysis revealed presence of intermetallic phases at root region of the weldments. All mechanical properties were improved at lower heat input and high cooling rate due to grain refinement and balanced microstructure [ferrite and austenite]. All weldments exhibited higher strength than base materials. Weld root region was harder than centre and cap region. SDSS is more susceptible to sigma phase formation due to higher alloying elements and weld thermal cycles, which lead to considerable loss of toughness. Higher nitrogen contents in shielding and purging gas resulted strengthening of austenite phase and restriction of dislocations, which ultimately improved mechanical properties. Higher interpass temperature caused reduction in strength and toughness because of grain coarsening and secondary phase precipitation.
... While the oxidizing gases formed on the opposite side of the weld can introduce porosity in the weld [4,10,11] , which results into decrease in the weld strength. Therefore, to remove these oxidizing gases, a uniform flow of purging gas is required [10,13,14] . Thus, gas flow rate and purging gas flow rate are also considered as probable variables. ...
-
![Mukundraj Patil]()
In the fabrication of a pressure vessel, the successful bending operation (after welding) demands higher tensile strength of weld bead. Therefore, to achieve typical tensile strength and hardness is the primary objective of this paper. Stainless steel 304 is widely used material in almost all the industrial applications, hence it is selected as a candidate material for study of tungsten inert gas (TIG) welding process. In order to produce, a high quality and reliable welding, the welding process needs to be robust in performance. A recently developed popular experimental approach known as definitive screening design (DSD) is used for process improvement. These optimum variable settings necessary for consistent welding are obtained through the application of simulation by using central composite design. The typical values of tensile strength and hardness are obtained at a low value of purging gas flow rate, filler rod dia.; intermediate values of root gap, plate thickness; and at high values of electrode dia., current, and gas flow rate.
Purpose: The purpose of this study is to improve the quality of permanent stainless-steel connections produced by manual gas tungsten arc welding. Design/methodology/approach: The impact of welding conditions was studied for the ranges of welding amperage and shielding gas (argon) consumption, and electrode grind angle recommended by the operating procedure (PI 1.4.75 - 2000). The quality of the completed welds was determined and evaluated by visual and radiographic inspection. Ferrite content in the weld metal was measured with a ferrite meter; mechanical properties were determined with the Instron universal testing machine and the Shimadzu microhardness tester for standard coupons; principal alloying element distribution across the cross section of the weld was identified with the Hitachi scanning electron microscope. Findings: The study statistically analyzed defects in the tungsten inert gas welds (TIG, Procedure 141) of structural elements made of aviation light-gauge stainless steel. The TIG welding conditions were found to affect the risk of internal defects, structure, and mechanical properties of stainless steel welds. A linear regression model capturing a correlation between weld strength and the TIG welding conditions was generated. Recommendations for the fabrication of welded piping elements from aviation stainless steel were made. Originality/value: Correlations between weld strength and the TIG welding conditions that can optimize process parameters were identified through experiments. Selection principles for welding conditions to develop structural fabrication processes for light-gauge stainless steel were recommended.
- Philip Jordan
-
![Chris Maharaj]()
This study aims to develop an asset management strategy to address weld cracking on 304H austenitic stainless-steel piping welded with 308H filler metal. The 304H NPS 16 outlet line from the primary reformer fired heater to the primary reformer which operates in high temperature service at around 620 °C was the principal case study. A metallurgical evaluation characterized the failure as sigma-phase and creep embrittlement in the heat-affected zone; however, sigma-phase embrittlement was the more predominant damage mechanism. Sigma-phase is the most common intermetallic phase precipitating in austenitic stainless steels. The cracks due to embrittlement were exacerbated by a significant amount of sigma-phase formation, which initiated at the fusion line and propagated at the coarse-grained heat-affected zone which were interconnected with creep voids. Welding procedures were developed using two alternative filler metals, ER16-8-2 and ER19-10H, as a repair option and for new fabrication. The philosophy was to reduce the sigma-phase formation by controlling the heat input; thereby limiting the ferrite; and the probability for embrittlement. The results from the mechanical and metallurgical examination of the welded specimens proved that the ER16-8-2 filler metal possessed superior resistance to sigma-phase formation. Acoustic emission testing and a combination of field metallographic replication and ferrite testing were identified as the most effective predictive technologies to detect the incipient cracking.
The retained δ-ferrite content in austenitic stainless steel welds is a crucial parameter to control, since several properties of the weld depend on this. It is conventionally measured using a Ferrite Number (FN) scale, which has been defined in the internationally accepted standards such as ISO 8249 and AWS A4.2 M. A FeritScope® is commonly used to measure the FN of the welds, which takes advantage of the fact that the retained δ-ferrite phase at room temperature is magnetic whereas austenite is not. The readings obtained from the FeritScope® are known to be influenced by several constraints such as the sheet thickness, weld clad thickness, curvature of the surface, etc. Some correction factors have been introduced by the FeritScope® manufacturer to account for these influential conditions. However, when the corrected FNs of the welds made on thin 304 L stainless steel sheets (≤ 2.4 mm thick) were plotted against the cooling rates, the trends obtained contradicted those available in the literature. The FN was found to increase with an increase in the cooling rate, whereas it was expected to decrease. Moreover, addition of nitrogen as an alloying element, which is known to be a strong austenite promoting element, was not found to influence the measured FN of the welds, again contradicting the literature. On further investigation it was found that some additional geometrical features, apart from those mentioned in the FeritScope® user manual must be considered while measuring the FN. This lead to the definition of a new scale – Ferrite Density Number (FDN) – which was found to be a better indicator of the amount of retained δ-ferrite in the welds as against the conventionally used FN scale. Using the FDN scale could eliminate the contradictions to the literature and explain the strong austenite-promoting behaviour of nitrogen.
- J. Tus̆ek
-
![Marjan Suban]()
The paper treats the effect of hydrogen in argon as a shielding gas in arc welding of austenitic stainless steel. The studies were carried out in TIG (Tungsten Inert Gas) welding with a non-consumable electrode and MIG (Metal Inert Gas) welding with a consumable electrode, in both cases with different volume additions of hydrogen to the argon shielding gas, i.e., 0.5, 1.0, 5.0, 10 and 20%.The studies showed that hydrogen addition to argon changes the static characteristic of the welding arc. The hydrogen addition to argon increases arc power and, consequently, the quantity of the material melted. In TIG welding a 10% addition of hydrogen to argon increases the quantity of the parent metal melted by four times. The hydrogen addition increases thermal and melting efficiencies of the welding arc too. The process stability in TIG welding in the mixture of hydrogen and argon is very good.Also in MIG welding, the hydrogen addition to argon increases melting rate and melting efficiency of the arc, but the increase is much smaller than in TIG welding.Since hydrogen is a reducing gas, the weld surface produced by hydrogen addition to argon is in both cases very clean and without oxides.
- J. Cuhel
- D. Benson
The basics of welding stainless steel tube and pipe for applications ranging from high-purity food and beverage, pharmaceuticals, and petrochemicals, are discussed. When prepping stainless steel, dedicated brushes, files, and grinders should be used that never touch carbon steel or aluminum. Filler metals with an 'L' designation, such as ER3O8L, provide a lower maximum carbon content, which can help retain corrosion resistance in low-carbon stainless alloys. Straight argon is recommended for gas tungsten arc welding (GTAW) of stainless steel tube and pipe. Precisely controlled metal transfer with modified short-circuit GMAW provides uniform droplet deposition and makes it easier for the welder to control the weld pool and, thus, heat input and welding speeds. A modified short circuit GMAW process presents an improvement over traditional short-circuit GMAW in that the welding system anticipates and controls the short circuit, then reduces the welding current to create a consistent metal transfer.
- M. Fletcher
Gas purging plays an important role in the removal of air from the fusion zone thereby ensuring the weld root quality. Specialization applications, purging techniques using argon-hydrogen and helium-argon mixtures and nitrogen have been developed. The materials being joined and the welding process used are the two main factors in the selection of the optimum gas or gas mixture. Purge gas flow rate and pressure also need to be established and should be included in the formal welding procedure. Variation in gas quality may arise during welding, and it is desirable to apply continuous gas monitoring, especially to control oxygen and moisture content. An efficient purge gas contaminant method is to use inflatable dams such as the Argweld system which has been developed to provide a reusable solution to gas purging.
- Richard Af Petersens
- I. Ballingal
The authors describe the effects that various gases have upon the arc characteristics and the resultant welds in different types of stainless steels. The optimum selection for each type, austenitic, ferritic, martensitic, and duplex, is discussed together with possible causes of contamination.
-
![Leijun Li]()
- Timothy Davis
Oxygen contamination in argon-purge gas used in welding stainless steels causes what is referred to in the sanitary industry as "sugar" or "dross", an oxide layer on the root surface of the weld bead that is rough, pitted, and porous. This oxide layer can trap organic matter that leads to contamination, weakens the mechanical properties, and compromises the corrosion resistance of the joints. This study involved GTA welding square-groove butt joints of Type 304 stainless steel tube. Argon with a varying percentage of oxygen content was used to purge the inside surface of the tube. Weld discoloration comparisons, surface analysis, and tensile tests of the weld joints were performed. The result shows that to prevent excessive oxidation and maintain acceptable mechanical properties in the welded joints, the argon purge should contain less than 0.1% by volume of oxygen.
Argon is commonly used as root shielding gas in TIG-welding of stainless steels. As argon world-wide is in short supply and prices are rising, it is of interest to investigate what effects changing to alternative root shielding gases might have on weld metal microstructure. Manual TIG welding was used for root and hot pass welding in 304L pipe and plate material using Ar2 as shielding gas and Ar2, N2+10%H2 or N2 as root shielding gas. A significant increase in the nitrogen content of root beads was found when changing the root shielding gas from pure argon to a nitrogen-rich gas. Typically the root bead nitrogen content increased from 500–600 ppm for argon shielding to 1 100 ppm with nitrogen as root shielding gas. As a result the ferrite content decreased with up to 5 FN from about 8–9 FN to 3–5 FN. However, no indication of hot cracking was found and all root beads solidified as predicted by the WRC-92 diagram with ferrite as the leading phase. It is suggested that typical 304L steel and 308L consumable compositions will permit use of nitrogen-rich gases for root shielding without a significantly increased risk of hot cracking. However, the increased nitrogen level must be considered in the choice of steel and consumable. It is advised to use the WRC-92 diagram to make sure there is sufficient safety margin for actual compositions and, if possible, check root bead ferrite content.
- John C. Lippold
-
![Damian Kotecki]()
Welding Metallurgy and Weldability of Stainless Steels, the first book in over twenty years to address welding metallurgy and weldability issues associated with stainless steel, offers the most up-to-date and comprehensive treatment of these topics currently available. The authors emphasize fundamental metallurgical principles governing microstructure evolution and property development of stainless steels, including martensistic, ferric, austenitic, duplex, and precipitation hardening grades. They present a logical and well-organized look at the history, evolution, and primary uses of each stainless steel, including detailed descriptions of the associated weldability issues.
Investigation of the effect of yellow heat tints on the pitting corrosion behavior of welded stainless steels The pitting resistance of stainless steel welds Gas purging optimizes root welds
- R L Saggau
- S A Fager
Saggau, R. 2005. Investigation of the effect of yellow heat tints on the pitting corrosion behavior of welded stainless steels. PhD thesis, Technical University Carolo Wilhelmina at Braunschweig. 3. Ödegard, L., and Fager, S. A. 1993. The pitting resistance of stainless steel welds. Australasian Welding Journal, Second quarter, 24–26. 4. Fletcher, M. 2006. Gas purging optimizes root welds. Welding Journal 85(12): 38–40.
New Jersey. 15 MIG welding stainless steel gas mixes
- Welding Metallurgy
- Weldability Of
- Stainless Steels
Welding Metallurgy and Weldability of Stainless Steels. John Wiley & Sons, New Jersey. 15 MIG welding stainless steel gas mixes. www.weldreality.com/stainlesswelddata.htm. Visited on May 19, 2010.
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