Shielding Gas Selection

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Effect of Gas selection on arc stability,
chemistry, mechanical properties and diff. H2
contents of FCAW, MCAW, GMAW weldmetals
CWA Toronto Chapter conference

• Viwek Vaidya
• February 12th 2008

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The GMAW Set-up
Wire
Wire Feeder Power Source Water Cooler
(optional)
Regulator / Flow meter Shielding Gas Welding
Gun Work Ground Clamp Work piece (Base Material)
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FCAW, MCAW, GMAW
Electrode wire
Contact tube
Gun Nozzle
Shielding gas
Electrode stick out
Arc length
Welding Arc
Base metal
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Observation of the welding arc

• Video of metal transfers in GMAW steel
• Please note
• Members will receive above video by e-mail
  request.
• It include other processes as well.
• (SAW, SMAW, FCAW, GMAW, PULSE MIG)
• Thank You for Your Support!

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The functions of shielding gases are

• Protect the weld pool from atmosphere
• Provide a gas plasma - ionized gas
• Support metal transfer and bead wetting

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Thermal conductivity and plasma shape

• Thermal Conductivity is the ease with which the
  gas will dissipate heat
• Argon has low thermal conductivity
• It is used for superior R-Value windows
• Helium has high thermal conductivity, CO2 also
  has high thermal conductivity than Argon

Argon
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Thermal conductivity and plasma shape Globular
transfer

• Consider energy flow through He and CO2, both
  characterised with Higher thermal conductivity
  than Argon
• Narrow plasma column
• CO2 and Helium produce globular transfer
• cannot produce spray transfer!

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Penetration profiles

• Argon has a finger nail penetration profile
  consistent with spray transfer
• CO2 and He have elliptical penetration consistent
  with the globular transfer

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Thermal conductivity and plasma shape Spray
Transfer

• Low thermal conductivity
• Expanded plasma column
• Electron condensation heating

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Thermal conductivity and plasma shape Spray
Transfer

• Wire melts in a fast fine droplet stream
• Wire end becomes pointed
• Spray transfer results in high deposition and
  good penetration
• Argon gives spray transfer!

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Penetration profiles

• Argon has a finger nail penetration profile
  consistent with spray transfer
• CO2 and He have elliptical penetration consistent
  with the globular transfer

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GMAW single wire deposition rates ( 0.045
0.035 )
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GMAW Aluminum welding modulated pulse
10 drops /pulse
1 drop /pulse
Pulsed arc
Spray modulated
(interrupted spray)
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Addition of Oxygen to argon increases arc speed
by 20
Introduction of oxygen through the contact tip in
GMAW Aluminium
or by brushing or final degreasing
Dark deposited removed with rag
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Annular gas Argon contact tip 0,3 l/min O2
Annular gas Argon 1,5O2
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NICKEL BASE ALLOYS GMAW
Ar
Ar He CO2
ArH2 ? CO2
Ar? CO2
ArHe ? CO2
Appearance of the weld and stability of the
pulsed transfer greatly improved with ?CO2
additions

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NICKEL BASE ALLOYS GMAW
Ar H2 ? CO2
Influence of ? CO2 addition on the pulse
transfer stability
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NICKEL BASE ALLOYS GMAW
Influence of ? CO2 addition on Welding speed
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stability of the pulse transfer
Welding speed (cm/mn)
energy distribution transfer stability
welding speed
transfer stability
H2 ? CO2
? CO2
Ar
He ? CO2
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NICKEL BASE ALLOYS GMAW
Ar H2 ? CO2
improvement in bead appearance
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Two-wire GMAW welding process can double
productivity!

• Extremely fast response power sources needed
• Two wires fed simultaneously into the same weld
  pool
• Wires powered to operate with peak pulses
• Perfectly in phase twin wire technique
• Perfectly out of phase tandem wire technique

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Automatic GMAW with dual wires thickness 1.5 -
6mm Carbon steel, stainless steels and aluminium
alloys
2 wires connected at the same electrical
potential
Each wire connected at the different electrical
potential
Twin wire
Tandem Technique
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FCAW MCAW wire cross section
Joint
Metal sheath - outer envelope
Metallic and non Metallic Fluxes powders
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FCAW weld with slag formation
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Observation of the welding arc

• Video of Ar-CO2 systems - FCAW
• To see above video, click here

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• Improved weld profile with FCAWGMAW combination,
  due to better wetting.
• Presence of oxidizing species through the FCAW
  wire
• 5/16 inch single pass fillet weld 35 ipm dual
  wire as opposed to 16 ipm with single wire
  systems.

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GMAW chemistry variation with Ar-O2 mixes.
Wire Chemistry C0.1, Si0.9, Mn1.48
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GMAW chemistry variations Ar-CO2 system
Wire Mn1.25, Si0.73 C 0.08,
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Mechanical properties 1 Ni MCAW all tests with
same lot
Shielding gas UTS MPa YS MPa E Impacts Cv J _at_ -51ºC
100 CO2 554 497 30 71,62,64,49,69
Argon 15 CO2 613 577 32.5 75,62,68,82,45
Argon10 He 15 CO2 616 557 30 61,72,95,92,79

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Classification of metal cored and FCAW wires in
Canada and US

• METAL CORED
• CSA W48-01/W48-06, CLASS E491C-6-H4/E491C-6M-H4
• AWS A5.18-95/ASME SFA 5.18, Class
  E70C-6-H4/E70C-6M-H4
• FLUX CORED
• CSA W48-01/W48-06, Class E491T-1-H8/T-1M-H8,
  E491T-9-H8/T-9M-H8
• AWS A5.20-95/ASME SFA 5.20, Class
  E71T-1-H8/T-1M-H8, E71T-9-H8/T-9M-H8
• CSA W48-01/W48-06, Class E492T-9-H8/T-9M-H8
• AWS A5.20-95/ASME SFA 5.20, Class
  E70T-1-H8/T-1M-H8, E70T-9-H8/T-9M-H8

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Weldmetal chemistries E491 C6-H4
Shielding gas Oxidation potential Carbon Manganese Silicon
Ar2O2 2 0.06 1.13 0.56
Ar5O2 5 0.05 1 0.47
Ar10CO2 5 0.05 1.37 0.77
Ar25CO2 12.5 0.05 1.3 0.66
Ar4O2 5CO2 6.5 0.04 1.25 0.67
CSA W48 O2 ½ CO2 N/R 1.75 max 0.90 max
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Weldmetal mechanical property variation E491
C6-H4
Shielding gas UTS MPa YS Mpa E Impacts Cv J _at_ -30ºC
Ar2O2 514 450 27.5 78
Ar5O2 499 430 29 77
Ar10CO2 542 467 29 92
Ar25CO2 514 435 25.5 112
Ar4O2 5CO2 533 456 30 58
CSA W48 500 min 410 min 22 min 27
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Carbon pick up in stainless steel weld deposits
Ar-CO2
Wire Carbon 0.012
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Effect of ambient humidity on diffusible H2
contents-SMAW
Same electrode lot, sealed in vacuum packed
condition was shipped to various locations below
and tested at different times of the year!
Location (Diff H2 ml/100g) Scotland Zurich Tokyo Rio New Orleans Cape Town
January 2 1.7 1.7 4.4 2.8 3.4
August 3.6 3.1 4.5 3.7 4.6 2.9
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FCAW wire storage conditions and worm tracking
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FCAW wire storage conditions and worm tracking
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Typical FCAW/MCAW wire cross sections
Wire closing seam configuration
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FCAW wires Hydrogen pick up susceptibility
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• Variation of diffusible hydrogen content and
  shielding gases

Parameters
100 CO2
Argon15CO2
Argon 5 CO2
Wire dia.
1/16"
1/16"
1/16"
299
312
323
Amps
Volts
28.5
28.5
27.5
3/4"
E.S.O
3/4"
3/4"
Diffusible Hydrogen
7.5ml/100g
9.5ml/100g
10.4ml/100g
R.H/Temp
45/22.6'C
45/22.6'C
45/22.6'C
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Diffusible Hydrogen variation with oxidation
potential
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• FCAW/diffusible hydrogen and electrical stick out

Wire A
Wire A
Wire B
Wire B
1.2mm dia.
1.2mm dia.
1.6mm dia.
1.6mm dia.
230 amps
230 amps
285 amps
285 amps
26 volts
26 volts
28 volts
28 volts
14 ipm
14 ipm
14 ipm
14 ipm
ESO 10mm
ESO 20mm
ESO 10 mm
ESO 20 mm
8.1ml/100g
5.5ml/100g
10.0ml/100g
9.0ml/100g
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FCAW wire storage conditions and worm tracking

• To avoid worm tracking and porosity store the
  wire properly
• Use shielding gas with higher oxidation potential
• Reduce welding amperage
• Weld with a longer stick out to preheat the wire
• Discard two layers of the spool and retry
• If possible recondition the wire not generally
  recommended

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Deleterious effect of Nitrogen on impact energy
carbon steels
250 ppm
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Nitrogen additions to shielding gas for Duplex
stainless

• Up to 2 additions of N2 advantageous for duplex
  stainless steel GMAW welding
• Reduction of 10-15 ferrite improving
  ferrite/austenite balance
• 10 improvement in strength
• Better performance against pitting corrosion
• Beyond 6 Nitrogen in the gas will produces weld
  porosity..

Arcal 129 Ar5 He2CO2 2 N2 for
Duplex stainless steels
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Choice of Shielding gases

• Too many to choose from
• Too complex for users
• Too complex for producers
• ALMIG
• ALTIG
• ALFLUX

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Conclusions

• Video imaging of the welding arc shows that
  progressive increase in oxidation potential of
  the shielding gas, stabilizes the arc for GMAW
  welds in stainless and mild steel welds
• Fumes also increase with increasing CO2 content
  of the shielding gases
• Addition of 1-2 Oxygen to Argon seems to improve
  arc stability and arc speeds for Aluminum GMAW
  process
• Micro additions of CO2 to Argon H2 or ArgonHe
  mixtures improves stability of the GMAW welding
  of Inconel 625 alloys
• GMAW, FCAW, MCAW deposits in mild steel loose
  strength and alloying elements with increasing
  oxidation potential of the shielding gases
• Increasing CO2 content of the shielding gas may
  contribute to increased pick up of carbon in
  extra low carbon stainless steels GMAW deposits.

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Conclusions - continued

• Diffusible hydrogen of a FCAW weld deposit
  increases with higher levels of Argon contents in
  the shielding gas
• Improper storage of FCAW consumable can result in
  substantial increase in diffusible hydrogen
  content, causing worm tracking porosity. Some
  remedies have been suggested
• An addition of up to 2 Nitrogen to an
  ArgonHeliumCO2 mixture shows improved control
  on ferrite content of the weldmetal, about 10
  increase in strength and improved pitting
  corrosion resistance in case of duplex stainless
  steel GMAW welds.

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Acknowledgements

• The author would like to thank the research staff
  at the Air Liquide World Headquarters in Paris
  for providing guidance and stimulating
  discussions while the manuscripts were being
  drawn up. Thanks are also due to technical
  experts at Air Liquide Canada and data obtained
  from the certification center in Boucherville.
  Photographic support came from several CAP Audit
  reports, performed at various customer locations
  in Canada.
• Dr. Christian Bonnet, Dr. P. Rouault, Mr. J. M.
  Fortain, Mr. Pierre Geoffroy, Mr. Joe Smith and
  Mr. Jean Venne provided valuable technical
  support for this paper and are being recognized
  for their contribution.

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• Thank you!