Transcript Shielding Gas Selection

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February 2008
CWA Toronto Chapter conference
Effect of Gas selection on arc stability,
chemistry, mechanical properties and diff. H2
contents of FCAW, MCAW, GMAW weldmetals
Viwek Vaidya
February 12th 2008
February 2008
The GMAW Set-up
Wire
Wire Feeder
Power Source
Water Cooler
(optional)
Regulator / Flow meter
Shielding Gas
Welding Gun
Work Ground Clamp
February 2008
Work piece (Base
Material)
FCAW, MCAW, GMAW
Contact tube
Electrode wire
Gun Nozzle
Electrode stick out
Shielding gas
Welding Arc
Arc length
February 2008
Base metal
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!
February 2008
The functions of shielding gases are
 Protect
the weld pool from atmosphere
 Provide a gas plasma - ionized gas
 Support metal transfer and bead wetting
February 2008
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
February 2008
Argon
Thermal conductivity and plasma shape : Globular transfer




February 2008
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!
Penetration profiles


February 2008
Argon has a finger nail
penetration profile
consistent with spray
transfer
CO2 and He have elliptical
penetration consistent with
the globular transfer
Thermal conductivity and plasma shape : Spray Transfer

Low thermal conductivity
 Expanded plasma column
 Electron condensation
heating
February 2008
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!
February 2008
Penetration profiles


February 2008
Argon has a finger nail
penetration profile
consistent with spray
transfer
CO2 and He have elliptical
penetration consistent with
the globular transfer
Addition of Oxygen to argon increases arc speed by 20%
Introduction of oxygen through the contact tip in GMAW Aluminium
Dark deposited removed with rag
Annular gas:
Argon + 1,5%O2
February 2008
or by brushing or final degreasing
+ 20 %
Annular gas: Argon +
contact tip: +0,3 l/min O2
NICKEL BASE ALLOYS GMAW
Ar
Ar+% CO2
Ar+ He+ CO2
Ar+He+  % CO2
Ar+H2+  % CO2
Appearance of the weld and stability of the pulsed transfer greatly improved with
CO2 additions
February 2008
NICKEL BASE ALLOYS GMAW
Ar+ H2 +  CO2
Influence of  CO2 addition on the pulse transfer stability
D U peak
DU
droplet
detachment
Argon
February 2008
Argon+  CO2
NICKEL BASE ALLOYS GMAW
Influence of  CO2 addition on Welding speed
+26%
40
35
+12%
30
+17%
energy distribution &
transfer stability
25
20
transfer stability
Welding speed (cm/mn)
45
15
10
5
0
Ar
February 2008
stability
of the pulse transfer
+H2+  %CO2
+  CO2
+He+  CO2
welding speed
NICKEL BASE ALLOYS GMAW
Ar+ H2 +  CO2
improvement in bead appearance
INCONEL 625
February 2008
INCONEL 600
GMAW Dual wire process
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
February 2008
Tandem Technique
FCAW & MCAW wire cross section
Metal sheath - outer
envelope
Joint
Metallic and non
Metallic Fluxes &
powders
February 2008
FCAW weld with slag formation
February 2008
Observation of the welding arc
 Video
of Ar-CO2 systems - FCAW
To see above video, click here
February 2008
•Improved weld
profile with
FCAW+GMAW
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.
February 2008
GMAW chemistry variation with Ar-O2 mixes.
Wire
Chemistry :
C=0.1%,
Si=0.9%,
1
GMAW weldmetal chemistry
Wire Chemistry : C=0.1%, Si=0.9%, Mn=1.48%
0.9
0.8
Chemstry variations %
0.7
Wire Chemistry : C=0.1%, Si=0.9%, Mn=1.48%
% Carbon
0.6
% Silicon
% Manganese
0.5
0.4
0.3
0.2
0.1
0
0%
2%
4%
6%
8%
10%
Ar-O2 ( O2 in %)
February 2008
12%
14%
16%
18%
20%
GMAW chemistry variations : Ar-CO2 system
GMAW chemistry variation Ar-CO2
%Carbon
%Manganese
%Silicon
1.2
1
% Mn, Si and C
0.8
Wire: Mn=1.25%, Si=0.73% C =0.08%,
0.6
0.4
0.2
0
0
5
10
15
20
% CO2
February 2008
25
30
35
40
Mechanical properties : 1% Ni MCAW all tests with same lot
Shielding gas
UTS
MPa
YS
MPa
%E
Impacts Cv
J @ -51ºC
100% CO2
554
497
30
71,62,64,49,69
Argon +15% CO2
613
577
32.5
75,62,68,82,45
Argon+10% He +
15% CO2
616
557
30
61,72,95,92,79
February 2008
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-9H8/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
February 2008
Weldmetal chemistries – E491 C6-H4
Shielding gas
Oxidation
potential
% Carbon
% Manganese
% Silicon
Ar+2%O2
2%
0.06
1.13
0.56
Ar+5%O2
5%
0.05
1
0.47
Ar+10%CO2
5%
0.05
1.37
0.77
Ar+25%CO2
12.5%
0.05
1.3
0.66
Ar+4%O2+
5%CO2
6.5%
0.04
1.25
0.67
CSA
W48
= %O2 + ½ %
CO2
N/R
1.75 max
0.90 max
February 2008
Weldmetal mechanical property variation – E491 C6-H4
Shielding gas
UTS MPa
YS Mpa
%E
Impacts Cv
J @ -30ºC
Ar+2%O2
514
450
27.5
78
Ar+5%O2
499
430
29
77
Ar+10%CO2
542
467
29
92
Ar+25%CO2
514
435
25.5
112
Ar+4%O2+
5%CO2
533
456
30
58
CSA
W48
500 min
410 min
22 min
27
February 2008
Carbon pick up in stainless steel weld deposits Ar-CO2
Carbon pick up - GMAW : Ar-CO2
30%
25%
Wire Carbon = 0.012%
%CO2 in Ar
20%
Series1
15%
10%
5%
0%
0
0.01
0.02
0.03
% Carbon in deposit
February 2008
0.04
0.05
0.06
FCAW wire storage conditions and worm tracking
February 2008
FCAW wire storage conditions and worm tracking
February 2008
Typical FCAW/MCAW wire cross sections
Wire closing seam configuration
February 2008
FCAW wires – Hydrogen pick up susceptibility
FCAW wires - Hydrogen pick up
wire A
wire B
30
Diffusible H2 : ml/100g
25
20
15
10
5
0
As received
Exposed to 80'F/80%RH for 1 week
Exposure condition
February 2008
Variation of diffusible hydrogen content
and shielding gases
Parameters
100% CO2
Wire dia.
1/16"
1/16"
1/16"
Amps
299
312
323
Volts
28.5
28.5
27.5
E.S.O
3/4"
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
February 2008
Argon+15%CO2 Argon + 5% CO2
Diffusible Hydrogen variation with oxidation potential
Diffusible hydrogen content variation with Oxidiation potential
GMAW
MCAW
FCAW
12
Diffusible hydrogen: ml/100g deposited weldmetal
Gas
Oxidation Potential
100% Argon
Ar-2% O2
Ar-5% CO2
Ar-15% CO2
Ar-20% CO2
100% CO2
10
8
0%
2%
2.5%
7.5%
10 %
50%
6
4
2
0
0
10
20
30
Oxidation potential O2% + 0.5*CO2%
February 2008
40
50
60
FCAW/diffusible hydrogen and electrical stick
out
February 2008
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
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
February 2008
Deleterious effect of Nitrogen on impact energy: carbon steels
120
Impact: Joules at -40'C
Energy: Cv Joules at -40'C
100
80
60
40
20
0
0
50
100
150
Weldmetal N2 content, ppm
February 2008
200
250
300
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

February 2008
Beyond 6% Nitrogen in the
gas will produces weld
porosity..
Choice of Shielding gases
 Too
many to choose from
 Too complex for users
 Too complex for producers
 ALMIG
 ALTIG
 ALFLUX
February 2008
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 Argon+He 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.
February 2008
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 Argon+Helium+CO2
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.
February 2008
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.
February 2008
Thank you!
February 2008