Earlier, the overhead fillet weld of offshore deck
structures were carried out using either SMAW or
FCAW-SS process
for the plate to beam section of
production deck modules. Recently, fabricators have started
automation in many
applications to improve productivity with
consistent weld quality ensuring health & safety measures. In
our article,
manual welding/semi-automatic processes were
used for plate to beam section were not efficient. So, there
is
requirement of mechanization to improve the welding
efficiency by changing from manual/semi-automatic to
mechanization.
In this article, the mechanized welding technique was
developed using FCAW-GS process using a welding carriage
unit (Korea make) with modified torch.
Welding trials were carried out using welding carriage
units using various permutation and combinations of welding
processes, consumable & fabrication variables of primer
coating & fit-up tolerances etc. In addition to that, comparison
of welding processes between FCAW -SS and mechanized
FCAW-GS were carried out with respect to health & safety,
quality and productivity.
Development of the mechanized welding technique has led to the following key benefits
when compared to the
FCAW SS welding processes.
• Health & safety – reduced neck straining & eye problems
for the welders
• Deposition rate – 70% more than the FCAW
SS processes
• Consumable saving – 80% compared to FCAW
SS processes
• Welding procedure was qualified successfully on primed
plates without porosity issues
• Cosmetic grinding /dressing is not required before
painting due to high quality weld bead appearance
EXPERIMENTAL PLAN & WORK CARRIED OUT
The objective of our research is summarized as follows
• Development of efficient processes requires selection
of welding processes,Filler materials& shielding gas
considering the
mechanized technique.
• Trails are conducted to resolve the effect of primer and
fabrication tolerances.
• Comparison of welding processes with respect to
productivity, health & safety and quality aspects to know
the benefits of
mechanization.
The plate material used for research was structural steel
grade of quality 355K2G4 , EN 10025 and10mm thickness.
Trials
were carried out with and without primer. Consumables
brand NST FC-3 1 Ni, ESAB OK 12.62, NST SF-1E & ESAB
OK
15.13C were utilized for trails with GMAW or FCAW-GS.
Majority flux cored wires use Ar-CO2 gas mixes for shielding
purpose, with the proportion of CO2 varying from 5% up to
25%, often with 2% oxygen. Some wires can be used CO2
as
shielding gas, which can offer a worthwhile cost-saving,
compared with mixed gases.
During the trials, the fillet weld size was not achieved as
per requirement due to torch angle. It is difficult to position
exactly
45° angle to the root of the fillet joint by using
conventional guns. Specific torch was designed and used for
above trials
purpose and details as below
Developed special torch for mechanized process
The welding carriage used throughout this trial is
manufactured by Koweld, Korea. The welding carriage
was capable of
welding horizontal fillet joints as per
manufacturer’s recommendation. Since the welding carriage is
small & has light weight,
planned to utilize for overhead weld
positions with necessary set-up.
Operator has to control the welding carriage by using
push button system available in the unit for starting the
machine. The
carriage is designed in such a way to seam track
automatically with the help of guide arm & roller the fillet joint
and ends up
when the end limit is pushed on. The carriage has
a magnet block inside the machine & can be easily attached
or detached
from the welding materials by controlling the
magnet intensity button. Skilled personnel are not required to
operate the welding
carriage.
Operating instructions (courtesy :Koweld ,Korea)
The power source used for the trials is Miller XMT350 series throughout the trials.This is an
multi process inverter based
single or three phase input power source and can generate
conventional DCEP and DCEN output power, can operate in
both CC and CV.
Experimental
Plates are cut into required size & fitted to form lap joint kept on the beam for welding trials
as shown below sketch. In
real scenario, the trials should be carried between the beam &
plate and this set-up was selected to avoid the wastage
of beams. Welding trails were carried
out using GMAW & FCAW-GS processes with NST FC-3Ni1, ESAB OK Tigrod
12.62,ESAB OK
Tigrod 15.13 and NST SF-1E.The overhead fillet welds were carried out on plates without
primer
application with various probability of welding parameters. Once the welding
parameters are established, then the
trails are carried out using primer coated plates &
investigated for the visual requirements.
The fillet weld was examined using macro section & physical measurements. The
measurements are recorded in the report. Fit-up gap was maintained between 0.5 – 1.5mm
throughout the experiment.
Below variables were kept constant throughout the experiments.
Base metal grade, Thickness of base metal ,Heat No. / Carbon equivalent of base metal,
Preheat temperature, Diameter of electrode, Polarity of current, Electrode angle & Stick -out
length
Experimental set-up for welding trials
Examination of welds
The welds were examined visually in accordance with
AWS D1.1 standard.
After visual examination, the welds were cut & assessed for
weld bead profile & base materials penetration.Further Macro
examinations were carried out on the welds to assess the quality & check any defects at required magnification by the
code. Almost 90 trials were carried out for the development
of this mechanization process. Trials are grouped in a
table (Refer Page 6 of this article) with a range for
better understanding.
Process selection
Upon comparing all trials, below was concluded with respect
to process selection
Note :
• Polarity used for the above processes are DCEP or DCRP
• Fit –up gap maintained between 0.5 – 1.5 mm
• Electrode extension in GMAW is 12 -15 mm & FCAW is 14 -17mm
Measurement of fillet size from the Macro photograph of the trial samples
Comparison of welding with FCAW-SS, FCAW-GS & Mechanized FCAW-GS process
Once the welding process & consumables are finalized, then the comparison was carried out and found below data.
RESULTS:
The mechanized welding process using FCAW-GS with 100 CO2 shielding
gas was established on the
primer coated plates with below precise welding
parameters.
Comparison of fillet leg size between GMAW & FCAW-GS
Porosity resistance of consumables:
Amount of spatter deposited on nozzles:
Effect of porosity on fillet weld:
Comparison of weld processes for the weld length carried out
per shift:
Comparison on deposition rate of FCAW SS & mechanized
FCAW-GS process with respect to man-hour
Comparison on deposition rate of FCAW SS & mechanized
FCAW-GS process with respect to arc-hour.
Comparison on cost saving of FCAW SS & mechanized FCAW-GS
with respect to arc-hour & man-hour.
Cost saving per arc hour : 100 $
Cost saving per Man hour : 24 $
DISCUSSIONS:
Welding process:
During GMAW welding trials FW1 to FW8, quality problems were below observed
• Spatter observed almost all fillet welds
• Irregular fillet size
• Lack of sidewall fusion defect due to deep finger
penetration
GMAW fillet weld with lack of fusion
Decision was made to change the process from GMAW to FCAW-GS. FCAW-GS trials FW 15 & FW 16 proved
satisfied results with NST SF-1E consumable due to below observations
• Spray arc transfer with minimum spatter formation
• Excellent bead appearance
• Flat bead with good wetting characteristics
• Little cleaning & pickling
The butt weld was welded and tested to confirm the strength
as well as charpy requirements of application
intended & found
meeting the engineering design requirements.
Trials for fit-up gap requirements
From the graph effect of porosity on fillet weld is clear that
porosity were found zero fit-up gap trials. Then fit-up
gap
varied slowly from 0.5 -1.5 mm using GTAW consumable wires
& observed the trials. The porosity was not
observed when the
f
it-up gap was kept between 0.5 to 1.5 mm.The fit-up gaps are
maintained during site
fabrication by placing the GTAW wires of
known diameter between the place & beam.
Primer coating thickness requirement
Initially trials were carried out on blasted plates (without
primer coating) & maintaining fit-up gap between
0.5 -1.5mm.
The process was established with consumable & shielding gas.
Then the trials are tried with
the same welding parameter
for primer coated plates & porosity were observed. Later the
primer thickness
coating was decreased on various scales to know the effect of porosity & found the requirement of coating
successfully.
More trials were carried out on the same thickness of primer
coating to confirm the stability of
weld quality.Thickness of the
coating shall be less than 10 microns for better control & not
exceeding 25microns.
Tack weld causing over fillet size
To maintain the fit-up gap between the plate & beam, tack welds
are carried at regular intervals. After completion
of welding, the
f
illet size was unacceptable because of oversize (>6mm) above
the tack weld area . Then tack
welds were carried out using
SMAW process of E7018 electrodes to keep thin passes & fillet
welding using
mechanized process were carried above the tack
weld. The size was acceptable as per AWS D1.1 requirements.
Sometimes, the tack weld was cracked due to thin passes &
necessary training has been given to the fit-up team
to maintain
the quality of tack weld. Finally, it was successfully with
repeated trails.
Welding procedure implementation
Once the welding procedures are established as per
construction code & the mechanized welding was utilized in
many projects in the yard. Welders were educated about the
knowledge of welding set-up & trained for 3 to 4 days.
They
were qualified as per ASME SEC IX & BS standards & tested
by NDT examination (Magnetic particle testing)
followed by
mechanical testing .
Since the earlier processes were giving more problems like
neck & body pain tiredness & eye straining. The welders
were
interested to adopt the developed process quickly. Since the
welding are carried out in a constraint space &
fumes are not
extracted from the area & advised to use extractor to avoid the
fumes inhaling.
OPPORTUNITIES FOR FURTHER DEVELOPMENT WORK
The fabricated deck have beam height ranges from 250 to
1100 mm. The developed mechanized technique are used
for
beams range from 220 to 750 mm & the maximum fabrication
welding are covered. But the developed welding
technology
cannot be applied more than 750mm beam.
Reason is that the welding carriage was not able to hold on
welding materials because of low magnetic force. So
Koweld
has been informed to design the carriage with more hauling
force ( from 16 to 25 Kgf )which can attach to
the welding
materials along with torch. Inaddition to this, intersection of
beam & corners are welded using FCAW
GS process manually.
Management is planned to develop the mechanized process using the vertical carriage with
oscillations which can meet
the above requirement and the proposed plan is under
development. By developing
above areas,the maximum welding for deck
fabrication area deck plate to beams will be mechanized almost
97%
in future.
Area of Deck area where FCAW-GS welded carried out manually.
CONCLUSION:
Based on the experiments & results, the following conclusions
were drawn
1. The FCAW GS process was mechanized using Koweld
welding carriage /Specially designed torch with NST SF
1E
consumable & 100% CO2 as shielding gas. So finally semi
automatic process was changed to mechanized process.
2. Primer coating thickness requirements was finalized after
many trials & maintained especially for this work to control
the porosity issues during fabrication.
3. Tack welded using SMAW electrodes E7018 by thin passes at
required intervals to maintain the final fillet size as per
quality
requirement.
4. Fit-up gap has to be maintained between 0.5 - 1.5 mm to
avoid porosity issues during the mechanization process.
5. Welding procedure has been established as per construction
code (ASME /BS) & implemented effectively at fabrication
site.
6. Comparative study of welding processes of FCAW SS &
Mechanized FCAW GS with respect to productivity, Health &
safety aspects & quality has been carried.
The benefits of mechanization are
• Health & safety – reduced neck straining & eye problems
for the welders when compared to FCAW SS semi
automatic
process.
• Deposition rate – 70% more than the FCAW SS processes
• Consumable saving – 80% compared to FCAW SS processes
• Cosmetic Grinding /dressing is not required before painting
because of high quality weld bead appearance.
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