somenath ghosh
TRANSCRIPT
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Gas Tungsten Arc WeldingEmphasis on Stainless Steel
Presented By :
Sekhar Basu Roy ChaudhuariOn 19 th. January, 2013.
With active support of
Somenath Ghosh
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Major Arc welding processes
MMAW / SMAWManual Metal Arc Welding / Shielded Metal Arc Welding
GMAW / FCAW *Gas Metal Arc Welding ( MIG / MAG )Flux Cored Arc welding
GTAW *Gas Tungsten Arc weldingPlasma Arc welding
SAWSubmerged Arc WeldingElectro slag
*Gas shielded processes
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Welding processes Areas of application
MMAW Most versatile process, low investmentDeposition rate upto 4 kg/hr. Offersflexibility with higher productivity.
GMAW Deposition rate upto 6 kg/hr. for manual
Upto 20 kg/hr. for mechanised twin wire
GTAW High quality low productivity processDeposition rate upto 2kg/hr. Maybemechanised for higher productivity
SAW Very high productivity but restricted toStraight welds in downhand & H-V filletDeposition rates upto 25 kgs / hr.
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Gas Tungsten Arc
Welding Process
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Development of GTAW process
On February 24th 1942 a patent was issued to RussellMeredith of the Northrop Aircraft Company concerning
an invention 'relating to welding magnesium by means
of an electric arc'. After acquiring the rights to the
process the Linde Corporation renamed the process
'Heliarc'.
The process is now widely known as Tungsten Inert
Gas, or TIG, although it may sometimes be called'Argonarc', or, less frequently now, 'Heliarc', while in
the USA it is known as Gas Tungsten Arc Welding or
GTAW.
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Gas Tungsten Arc ( TIG ) Welding
Uses Tungstenelectrode
Inert gas
shielded Manual process
but maybeautomated
Very high quality
Low productivity
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GTA welding advantages and
disadvantages
Advantages:
Very good control of the weld pool and penetration
Heat transfer is easily controlled
No spatterWelding without filler material possible
Thin material welding with very low currents is possible
Can weld all types of metals and alloys
The weld seam forming is good and slag free
Weld root side is clean and slag-free (with root gas)
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Disadvantages:
Slow speed in filling large grooves
Sensitive to drafts and wind
Sensitive to impurities
Gas purity is a critical factor
High need for root protection
Welder skill requirement is high
Cost of process is comparitively higher.
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Application of TIG welding
Welding of critical pipes and pipelines, forexample pressurized pipes, Manufacturing heat exchanger pipes andcombinations, Manufacturing stainless pipe and tubes, Thin materials from ~ 0,1 mm and higher, Thin stainless steel welding, Aluminum welding,
Special materials welding, eg. Titanium, Railway and Wagan Industry, Fixing and repairing minor works, Space and aircraft industry.
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The Tungsten Inert Gas, or TIG process, uses the heat generated by anelectric arc struck between a non-consumable tungsten electrode and
the workpiece to fuse metal in the joint area and produce a molten weldpool. The arc area is shrouded in an inert or reducing gas shield toprotect the weld pool and the non-consumable electrode. The processmay be operated autogenously, that is, without filler, or filler may beadded by feeding a consumable wire or rod into the established weldpool.
Schematic showing the principle of the TIG welding process
How the TIG process works
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GTAW process
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SHIELDING GAS
WATER COOLERCONSTANT CURRENT POWER SUPPLY
WORK PIECE
TIG TORCH REGULATOR/FLOWMETER
(AIR or WATER COOLED)
(OPTIONAL)
TIG welding set-up
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Power Source Characteristics
Power Sources ofConstant Current type havingdrooping characteristics are used for
- MMAW process
- GTAW process
- Plasma processes
Power sources ofconstant voltage type havingflat characteristics are used for
- GMAW & FCAW processes
- SAW process
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Designed to give stable operation where the electrode moves up
and down with the welders hand eg MMAW and GTAW processes.
Variation in arc voltage with movement of the welders hand resultsin very little change in current
Stable current gives consistent arc heat and weld pool
Drooping characteristics power source
ISO line of the power
source is V= 20+ 0.04xI
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DCENCathode
DCEPAnode
AC
1/3 heat
1/16 1/4 3/32
_
_
2/3 heat 50% heat
Effect of polarity on heat input
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(Positive Half Cycle)(Negative Half Cycle)
DCEN DCEP
Penetration Cleaning
Oxide Layer
Electrode
Arc
Melt Zone
AC Cleaning
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Polarity for arc processes
GTAW
Electrode DC ve for mild steel, stainless steel, Ni
and Ni alloys, Cu and Cu alloys
AC for aluminium, magnesium & Al-Bronze.
Sometimes electrode DC +ve used for thin sheet
GMAWElectrode DC +ve for all materials
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Some, more sophisticated, power sources allow the weldingcurrent to start at a low level and build-up and also allowthe current to decay slowly when welding ceases. This'slope-up' and 'slope-down' facility prevents burn throughon thin material and allows crater filling to take place.
Layout of typical TIG welding equipment
Development of Power sources
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Gas post flow The solenoid valve in the shielding gas flow
path opens immediately after the command
from the start switch. When welding is stopped the solenoid valve
shuts after a certain adjustable post flow time.
This delay protects in cooling the weld andespecially the cooling of the electrode from
oxidation. When the post flow time is long enough the
electrode tip remains shiny and clear.
Too short a post gas time oxidises the Tungsten,and darkens its colour. The electrode wear iseven 30 % higher, causing also faults in the
weld. Post gas time is adjusted according to the
electrode cooling time, which is long whenwelding with high currents.
Usually the setting is 5 - 15 s.
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GTA Welding Machines
For GTAW Welding we can have
DC TIG welding Machines
DC Pulsed welding machines
AC TIG welding machines,
AC Square wave pulsing machines
AC/DC TIG Welding Machines
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Many welding sets now are capable of giving a pulsedarc in which high current pulses are provided over alower background current. The pulse frequency,duration, and maximum current and the backgroundduration and current can be varied to suit theapplication. With some equipment the pulse shapecan be varied too.
Use of pulsing allows greater control over heat inputof the arc and enables consistent welds to be made invery thin materials, in dissimilar thickness joints, andunsupported root runs, and when welding in position.
Pulsed welding Power Sources
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Pulsed welding
PULSEON /OFF
PULSE
RATIO
BACK.
FREQUENCY
t c
t s
I s
I t
t
I
In pulse-TIG welding the current changes according to the settings
of frequency and pulse ratio, between pulse and pause currents.Iave. = Average current
t c = Cycle time / Frequency
t s = Pulse time
I s = Pulse current
I t = Pause current
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Pulsed TIG welding modesPulse welding can be divided to three different classes, according to
frequency:
Long pulse welding
Frequency 0.2 - 10Hz
Considerable pause time, during which the molten metal has time tocool, so the weld is easier to control
.
Quick pulse
Frequency 50 - 500Hz
Frequency is so high, that pulse cycles are no longer distinguished
Usually the penetration is better and weld narrowerThis feature is typical for ordinary inverter power sources.
High frequency pulse
Frequency 1kHz- 20kHz
Only in special machines
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AC waveforms
Sine wave Traditional AC - transformer (50Hz)
Low noise,half cycle ignition bad
Square wave
Traditional or inverter power source High noise, good oxide cleaning
AC frequency adjustable
Formed square wave
Used in modern power sources Combined sine- and square wave
Optimum AC properties
AC frequency adjustable
+
0-
+
0-
+
0-
t
t
t
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AC - balance adjustment
Maximum positive balance Low penetration
Good oxide cleaning
A hotter electrode, tip wears round
Maximum negative balance
Deep penetration Bad oxide cleaning
A colder electrode, stays sharper
70%
+
-
30%
0
30%
0
+
-
70%t
+
0
-t
Adjustment depends on:Material thickness
Oxide layer thicknessBase material
Usually in AC welding the arc
ignition is done with DC+. This
function is automatic
BALANCE
2.43.2
4.0
AC
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Pulsed TIG applications
Pulse TIG is a versatile welding process for all
positions and material thicknesses
Uses: Positional welding
Pipe welding
Welding without filler material
Welding of materials of different
thicknessesSpecial materials eg SS & Ni alloys
Advantages:
Greater control on fusion & penetration
Welding is quicker (quick pulse)
Deeper penetrationEasier welding
Stabile arc
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TIG Welding Quality
Shielding GasesARGON
Best arc stability
Best shielding
Best cleaning action
HELIUM
Increases penetration and heat
Increases welding speed
ARGON / HELIUM MIXTURES
Combines benefits of both gases
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Shielding gas flow rates
Flow rates : 6 10 litres per minute, depending on
application, using argon gas. A low flow rate can easily cause defects in the welding.
Too high a flow rate can also cause defects in the
welding due to turbulence, along with high costs in gas.
Shield gas type : For example, when using light heliumas shielding gas, the flow rate must be 1.5 - 3 times
more than with argon.
Welding speed: In mechanized welding the weld pool
can become larger than usual. Then a larger gas nozzleand flow rate can be used
When needed, a gas lens can be used in the torch.
Longer gas shield distances are possible when using a
gas lense.
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When TIG welding certain materials, such as
stainless steel or nickel alloys, it is necessary toprevent the reverse side of the weld area
oxidising, particularly in pipework when post-
weld cleaning of the root is impossible.This is
usually achieved by passing a stream of inertgas through the pipe, or over the reverse side of
a plate, to exclude, or purge, oxygen from the
area.
The gases most frequently used for purging are
argon or nitrogen. Which gas is chosen will
depend on technical, practical or economic
reasons.
Back Purging
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TIG Torch and electrodes
Tungsten Electrodes
2% Thoriated for DC
welding of MS, SS, Cu &Ni alloys - Red Tip
Pure tungsten for ACwelding of Aluminium,Magnesium Green Tip
Electrode Identification
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Electrode Identification
Tungsten electrodes are identified in some countries by coloured rings or theends being painted according to their alloy content. These colour codes areoften country specific.
Tungsten Electrode Compositions and Colour Code as per EuropeanStandard EN 26848, 1991
* Composite electrodes also have a pink identification ring in additionto the above
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Application ofTungsten electrodes
Code Oxide Color Polarity
WP Pure Green AC
WT20 Thorium 2% Red DC/AC
WZ 8 Zirconium 0.80% White AC
WL10 Lanthanoid 1% Black DC/AC
WL15 Lanthanoid 1.50% Gold DC/AC
WC20 Cerium 2% Gray DC/AC
P ti f T t El t d
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It is necessary to prepare the tip of the tungsten electrode, usually bygrinding, to aid arc initiation, stability, and control. For DCENoperation a point is ground with an included, or vertex, angleaccording to the application and the current that is to be used.
It should be noted that grinding should be carried out along thelength of the electrode not round the tungsten. Grinding round thetungsten can cause the arc to wander up the electrode creating anunstable or rotating arc.
For AC operation the tip of the electrode should be rounded off andnot pointed. This is because when using AC the tungsten tends toform a ball on the end of the electrode rather than keeping a point.
Preparation of Tungsten Electrode
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Touching a tungsten electrode into a molten weld pool
will often result in some tungsten being melted off andtaken into the weld metal. Similarly, if the tip of atungsten electrode for AC operation is ground to a point,this will be removed on initiation of the AC arc and end
up in the weld metal.The resultant tungsten inclusions will be classified as adefect and will have to be ground out and the weldrepaired.
Welders should be careful not to allow the electrodes tocome into contact with the weld pool and to ensure alltungsten electrodes are correctly ground and preparedfor welding.
Tungsten Inclusions
AWS S ifi ti f Fill M t l
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AWS Specifications for Filler Metals
Suitable for GTAWSpecification
No.
A 5.2 Iron and Steel gas welding rods.
A 5.7 Copper and Copper alloy bare welding rods and electrodes
A 5.9 Corrosion resistant Chromium-Nickel steel bars and
composite metal cored and standard arc welding electrodes
and welding rods
A 5.10 Aluminium and Aluminium alloy welding rods and bare
electrodes
A 5.13 Surfacing welding rods and electrodes
A 5.14 Nickel and nickel alloy bare welding rods and electrodesA 5.16 Titanium and titanium alloy bare welding rods and electrodes
A 5.18 Mild steel electrodes for Gas Metal Arc Welding
A 5.19 Magnesium alloy welding rods and bare electrodes
A 5.24 Zirconium and Zirconium alloy bare welding rods and
electrodes
R d d C t R f T t El t d
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Recommended Current Ranges for Tungsten Electrodes
Based upon European Standard, EN 26848, 1991.
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The current and polarity to be used will determine thediameter of the tungsten electrode. The relationship
between current and electrode diameter for tungstenelectrodes with additions of thorium oxide, ceriumoxide, lanthanum oxide or zirconium oxide is shown in
the next table
If using pure tungsten electrodes the currents quotedshould be reduced to 75-90% of these values for DCEN
operation and to 60-75% for AC operation.
It is advised that national standards be consulted forsuitable current ranges.
GTAW Welding Parameters
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Selecting the right parameters
Gas nozzle number is defined by the marking 1/16 ( 1,5875mm )
Example: number 5 is 5 x 1,5875 mm = 7,9 mm
ELECTRODE
DIAMETER
GAS NOZZLE no. GAS FLOW
l / min
CURRENT RANGE
A
1,6 4 - 5 5 - 7 20 - 130
2,4 5 - 6 6 - 8 100 - 250
3,2 6 - 7 7 - 9 150 - 350
4,0 7 - 8 8 - 10 200 - 500
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HF - Ignition Unit High frequency (HF) unit ignites the arc without contact between the
electrode and workpiece.
Usually TIG welding equipment are compact, so that the high frequencyunit is mounted inside the machine.
Separate units are mainly used with multiprocess power sources.
HF has been known to create difficulties and interference with computersystems, telephones, and other electronic items, including heartpacemakers
For example at nuclear power plant constructions the use of HF - spark isprohibited.
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DC Suppressor in AC
l Function of DC Suppressor
n Additional accessory to remove unwanted DC from output
- provides only AC output for welding
l Why DC Suppressor?
n In the positive half cycle, the oxide layer formed on the
surface of the job behaves as a PN junction diode in the
reverse bias and does not conduct current
n A DC suppressor removes the DC component andfacilitates AC output free from unwanted DC
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Electrode stick-out and arc length
in DC (-)ve welding
3 - 5 mm
1 - 5 mm
Electrode stick-out depends on the current and joint type used
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The ActivatedFlux (A-TIG)process
Gas Tungsten Arc Welding is widely used in nuclear,aerospace, power, oil, shipbuilding & host of otherindustries.
Fundamental to applications requiring high qualitythru control of bead shape & metallurgical
characteristics. It is, however, a low productivity process,
A-TIG welding process, developed by Paton WeldingInstitute in the 1960s, is a feasible alternative toincrease the process productivity
A-TIG uses a thin layer of an active flux that isapplied to the surface before welding.
The activated flux causes constriction in the arc-,increasing current density at the anode root and thearc force acting on weld pool.
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Advantages Of A-TIG Process
A-TIG can enhance weld penetration by up to 300%
A-TIG gives weld metal properties similar to parent
metal
Less sensitive to cast to cast material variation
Higher welding speed and productivity
Cost of Welding can be reduced by as much as 50 %
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Weld Penetration Profile
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(Automation) Mechanised wire feeding
in TIG to increase Productivity
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Boiler Tube to Plate Welding Machine Head
Typical equipment for tube and tube-plate welding for heatexchangers has been developed. These systems mayoperate from the outside or inside depending on tubediameter and the size of the welding head.
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Special thanks to Mr. Somenath Ghosh
for his support and help.
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THANK YOU ALL