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Jon S. Lee and Tom J.
Landon, CB&I, USA, discuss
how to improve the toughness of
high heat input welded vertical joints.
reprinted from HydrocarbonEngineering
June2007
www.hydrocarbonengineering.com
W
hen it comes to welding the vertical joints of atmospheric flat bottom storage tanks, it is
common practice to use a high heat input electrogas welding (EGW) process to achieve
maximum productivity while maintaining acceptable quality.
The EGW process can be used to weld joints up to 1 in. (25 mm) thick in a single pass, and takes
about 1.2 hours to weld an 8 ft (2438 mm) vertical seam. Compared with the 5.3 hours it takes to
weld the same seam using a conventional flux cored arc welding (FCAW) process, this represents a
400% improvement in productivity. The trade off for using the EGW process, however, is a reduction in
notch toughness, which is a result of the high welding heat input. Traditional industry practice shows
that the EGW process is capable of achieving adequate joint toughness down to 0 ˚F (-18 ˚C). At test
temperatures below this point, typical joint details and welding consumables may not consistently
provide code minimum toughness, particularly in the weld metal.
In an effort to expand process utilisation, CB&I, a global engineering, procurement and construction
contractor, has developed and field tested a new EGW technique, based on lower joint volume and
better welding materials, that improves weld metal and heat affected zone toughness at temperatures
down to -40 ˚F (-40 ˚C). The new technique is based on electrogas welding equipment that CB&I
specifically developed for API-650 storage tanks in cold weather environments (Figure 1). The unit
includes a mobile power supply that can be moved with the EGW delivery platform
from one vertical seam to the next on each ring of the tank. This
combination of power supply and mechanised
delivery results in a safe and
efficient means of implementing
mobile
ment with
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the EGW process while
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eliminating the constraint of
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welding cables going to the
bottom of the tank.
In developing the new EGW
technique, an evaluation of
various joint designs and welding
consumables was conducted to
ensure adequate joint toughness
at -40 ˚F (-40 ˚C). Joint details and
welding techniques were likewise
evaluated to reduce joint volume
and heat input. Welding consumable
formulations were studied to identify
the optimum chemistry and weld
metal microstructure. Based on these
evaluations and the subsequent
results, it was concluded that the
toughness in the weld metal and heat
affected zone (HAZ) can be improved
using lower heat input welding
techniques and properly selected
welding consumables.
Joint detail
The most critical factor in improving
toughness in the weld metal and HAZ
is reducing the joint volume, which is
Figure 2. Joint detail. Conventional EGW process. 1 in. (25 mm) thick
especially critical for plate
material.
thicknesses nearing 1 in. (25
mm). As the joint is being
welded in a single pass, a
small joint volume results in
significantly lower welding
heat input. Table 1 provides a
comparison of typical
welding parameters between
conventional electrogas
welding using a standard
joint configuration and
Figure 3. Joint detail. 'Narrow groove' EGW process. 1 in. (25 mm) thick material.
electrogas welding
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reprinted from June2007
HydrocarbonEngineering
Table 1. Welding parameters and heat input
Process
Bevel
angle
(˚)
AWS
electrode
class
Electrode
diameter, in.
(mm)
Current
(amps)
Voltage
Travel speed,
in./min
(mm/min)
Heat input,
KJ/in
(KJ/cm)
EGW conventional
EGW narrow groove
55
EG72T-1
3/32 (2.4)
560
47
3.7 (94)
425 (1080)
25
EG72T-G
1/16 (1.6)
440
44
3.8 (97)
308 (782)
Table 2. CVN toughness of HAZ. CSA G40.21 Grade 38WT (as rolled) steel. 1 in. (25 mm) thick
Process
CSA G40.21
Gr.38WT
Notch
location
Test
temperature,
˚F (˚C)
CVN Spec,
1 ft-lbs
(joules)
CVN Spec,
2 ft-lbs
(joules)
CVN Spec,
3 ft-lbs
(joules)
CVN AVG,
ft-lbs
(joules)
Base metal
-20 (-29)
73 (99)
15 (20)
16 (22)
35 (47)
EGW conventional
HAZ
0 (-18)
93 (126)
115 (156)
89 (121)
99 (134)
EGW conventional
HAZ
-10 (-23)
23 (31)
19 (26)
17 (23)
20 (27)
20 (27)
RGW conventional
HAZ
-20 (-29)
7 (9)
39 (53)
13 (18)
CSA G40.21 Gr.38WT
Base metal
-40 (-40)
21 (28)
10 (14)
33 (45)
21 (28)
EGW - narrow groove
HAZ
-20 (-29)
95 (129)
75 (102)
90 (122)
87 (118)
EGW - narrow groove
HAZ
-40 (-40)
24 (33)
34 (46)
37 (50)
32 (43)
EGW - narrow groove
HAZ
-40 (-40)
54 (73)
97 (132)
78 (106)
76 (103)
EGW - narrow groove
HAZ
-60 (-51)
55 (75)
72 (98)
65 (88)
64 (87)
1. All specimens taken within 1/16 in. surface of first side (bevel side) of test plate
2. All specimens taken longitudinal direction to plate primary rolling direction (PRD)
3. All CVN specimens per ASTM A370 (V-notch) of 10 x 10 mm size
Table 3. CVN toughness of welds. CSA G40.21 grade 38WT (as-rolled) steel 1 in. (25 mm) thick
Process
Notch
location
Test temp,
˚F (˚C)
CVN spec 1,
ft-lbs (joules)
CVN spec 2,
ft-lbs (joules)
CVN spec 3,
ft-lbs (joules)
CVN AVG,
ft-lbs (joules)
EGW conventional
Weld metal
-20 (-29)
20 (27)
21 (28)
21 (28)
21 (28)
EGW conventional
Weld metal
-30 (-34)
19 (26)
19 (26)
19 (26)
19 (26)
EGW conventional
Weld metal
-40 (-40)
19 (26)
21 (28)
25 (34)
22 (30)
EGW - narrow groove
Weld metal
-20 (-29)
86 (117)
94 (127)
90 (122)
90 (122)
EGW - narrow groove
Weld metal
-40 (-40)
44 (60)
38 (52)
47 (64)
43 (58)
EGW - narrow groove
Weld metal
-40 (-40)
65 (88)
69 (94)
69 (94)
68 (92)
EGW - narrow groove
Weld metal
-60 (-51)
62 (84)
56 (76)
55 (75)
58 (79)
reprinted from HydrocarbonEngineering
June2007
Conversely, the
‘narrow groove’ EGW
process utilises both a
smaller diameter (1/16 in.
or 1.6 mm) electrode and
an oscillating system to
achieve full joint fusion in
the reduced volume detail.
Oscillating the electrode
enables the EGW process
to achieve full fusion and
provides the added benefit
of stirring the molten weld
puddle, which results in a
measure of grain refinement.
As with joint volume,
optimum oscillation is a key
component for improving
toughness in high heat
input EGW welds. CB&I
experimented with different
oscillation characteristics
during the development of
the process and found that
the oscillation range, or
stroke length with respect
to the midpoint of the joint
thickness, has a significant
influence on the front and
backside weld profiles.
In addition, the speed of
oscillation, measured in
cycles per minute, has a
substantial impact on the
weld metal grain structure
due to the stirring effect.
Materials
It was important that CB&I
improve joint toughness
EGW - narrow groove Weld metal
-80 (-62)
42 (57)
49 (66)
59 (80)
50 (68)
(as measured by Charpy
1. All specimens taken within 1/16 in. (1.6 mm) from the surface of first side (bevel side) of test plate
V-Notch Impact Testing)
2. All welding completed in CSA G40.21 Gr.38 WT Plate 1 in. (25 mm) thickness
down to -40 ˚F (-40 ˚C) so
3. All CVN specimens per ASTM A370 (V-notch) of 10 x 10 mm size
that the EGW process could
be used on API-650 storage
tanks in Canada. Use of
with a joint configuration for narrow groove (low volume)
the process in Canada is driven not only by the improved
welding (basic joint details are provided in Figures 2 and 3
productivity that can be obtained, but also by the need to find
and resulting weld cross-sections in Figures 4 and 5). When
alternative welding resources.
properly executed, the ‘narrow groove’ EGW process reduces
As with much of the world, Canada is experiencing a
the welding heat input in a 1 in. (25 mm) thick joint by
significant increase in the demand for qualified welding
approximately 27%.
personnel; however, available resources are currently
strained. The opportunity to use the EGW process in an area
Oscillation
where it was not previously qualified for use can provide
The EGW process used for both conventional and ‘narrow
some relief on these demands. Understanding this driver was
groove’ weld details uses a solid copper backup bar on the
instrumental in determining the materials to be specified for
back side of the joint and a sliding water cooled copper
the welding applications.
shoe on the front side of the joint, allowing the weld metal
Improving the toughness capabilities of the EGW process
to essentially be cast in place. In the conventional EGW
requires that both the weld metal and HAZ toughness
application, the electrode is held in a fixed position in the
meet code or contract acceptance criteria. To meet these
centre of the joint without any oscillation. When using this
requirements in the HAZ, material selection and heat input
technique, a relatively large diameter electrode (3/32 in. or
reduction are both important factors to consider. The material
2.4 mm) and an increased bevel angle is required to achieve
of choice for storage tanks in Canada with a minimum design
complete fusion with adequate buildup on both plate surfaces.
metal temperature of -40 ˚F (-40 ˚C) is CSA G40.21 Grade 38WT
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Figure 4. Weld bead profile on cross-section. Conventional EGW
process. 1 in. (25 mm) thick material.
fusion line between the weld metal and base material, where
minimal grain refinement from the heat of welding had taken
place. To evaluate the most critical zone, the HAZ notch was
machined in the coarse grain region near the edge of the
fusion line, as shown in Figure 6.
Electrode
Figure 5. Weld bead profile on cross-section. 'Narrow groove' EGW
process. 1 in. (25 mm) thick material.
Figure 6. Typical notch location for HAZ impact specimen.
carbon steel, typically furnished in ‘as rolled’ condition. This
material is very similar to ASTM A36 carbon steel.
When the EGW process is anticipated, the tank material
must be controlled by additional purchasing specifications,
including restrictions on maximum carbon content,
minimum manganese content and the maximum amount of
low melting point elements such as sulfur and phosphorus.
Table 2 provides typical data on HAZ toughness for
joints welded with the EGW process using a single pass.
The data compares HAZ impact results for both the
conventional process and the ‘narrow groove’ welding
process. For comparison purposes, the required toughness
for this material, as specified in API-650, is an average of
15 ft-lbs (20 joules) for three specimens, with one specimen
allowed to be below the average, but no less than 10 ft-lbs
(14 joules).
HAZ specimens were machined in accordance with
API-650 requirements, with the notch positioned to include
as much of the HAZ material as possible in the fracture zone.
Fracture initiation was deemed to be most critical at the
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Meeting toughness requirements at the design metal
temperatures necessary for the storage of petroleum products
in low temperature environments is also a challenge for the
deposited weld metal. Most commercially available welding
consumables used in the EGW process are classified as
AWS EG72T-1 electrodes and are designed to meet Charpy
V-Notch impacts of 20 ft-lbs (27 joules) average at -20 ˚F
(-29 ˚C) per AWS specification A5.26/A5.26M-97. To achieve
acceptable weld metal toughness at lower test temperatures,
CB&I turned to Devasco International of Houston, Texas.
Devasco is a manufacturer of custom made and
proprietary welding consumables that have special mechanical
properties. A number of test electrodes were manufactured
by Devasco and shipped to CB&I’s welding laboratory for
welding and mechanical testing. Chemistry modifications
and batch testing progressed until an EG72T-1­ electrode
formulation was derived that would consistently meet needed
impact properties at -40 ˚F (-40 ˚C). From this initial formulation,
additional variations have been developed that are able to
achieve both good low temperature toughness and weld metal
hardness below 225 BHN. The electrode and its variations
are produced exclusively for CB&I as an AWS EG72T-G
classification.
Using the ‘narrow groove’ EGW process, the Devasco
electrode will produce weld metal Charpy V-notch impacts
exceeding 20 ft-lbs (27 joules) average at -40 ˚F (-40 ˚C).
Table 3 provides information on weld metal toughness,
comparing results for the EG72T-1 electrodes using
the conventional EGW process with proprietary
EG72T-G electrodes using the ‘narrow groove’ EGW
process.
It should be noted that the base material has a
significant influence on impact properties because
dilution of the base metal accounts for 30 - 40% of the
weld metal composition in single-pass EGW welding.
EGW consumables are formulated to account for this
dilution, but welding on different grades of carbon steel
materials can produce significantly different weld metal
toughness results. The improvement in weld metal
toughness is also aided by the reduction in heat input,
which is a result of the narrow groove and smaller weld
volume. And finally, the electrode oscillation also results
in better weld metal toughness, compared with no
oscillation in conventional EGW welding, by facilitating
a more rapid solidification of the weld nugget and finer
grains in the weld deposit.
Conclusion
Based on this study, CB&I has concluded that by using
enhanced techniques and materials, the weld metal
and HAZ toughness of vertical welds in carbon steel flat
bottom storage tanks made with the EGW process can be
significantly improved. The narrow groove low volume weld
detail, electrode oscillation, careful material selection and
specially developed welding consumables are all instrumental
in achieving acceptable impact results at test temperatures at
or below -40 ˚F (-40 ˚C).
reprinted from June2007
HydrocarbonEngineering