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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