gas metal arc welding

PRESENTATION BY: HARDISH TRIVEDI

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Gas Metal Arc Welding

By Hardish Trivedi


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What Is GMAW ?• A Fusion Welding Process – Semi Automatic• Arc Between Consumable Electrode &Work• Arc Generated by Electric Energy From a Rectifier

/ Thyrester / Inverter• Filler Metal As Electrode Continuously fed From

Layer Wound Spool. • Filler Wire Driven to Arc By Wire Feeder through

Welding Torch• Arc & Molten Pool Shielded by Inert Gas through

Torch / Nozzle


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Gas Metal Arc Welding• MIG – Shielding Gas Ar / Ar + O2 / Ar + Co2

• MAG – Shielding Gas Co2

• FCAW – Shielding Gas Co2 With Flux cored Wire

Note:- Addition of 1 – 5% of O2 or 5 – 10% of Co2 in Ar. increases wetting action of molten metal


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Power Source For MIG / MAG

• Inverter- DC

• Thyristor – DC

• Diesel Generator – DC

• Rectifier – DC


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Characteristic Of GMAW Power Source

Constant V / Linier Characteristic

Appx. Horizontal Curve

V1V2

A1 A2A

V


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Current & Polarity

DC- Electrode +Ve

Stable Arc

Smooth Metal Transfer

Relatively Low Spatter

Good Weld Bead Characteristics

DC- Electrode – Ve, Seldom Used

AC- Commercially Not In use


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Accessories Of GMAW

• Power Source• Wire Feed Unit• Shielding Gas Cylinder, Pressure gauges/

Regulator, Flow meter ( Heater For Co2 )

• Welding Torch • Water Cooling System (For Water cooled Torch)• Earthing Cable With Clamp


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Tools For GMAW • Head Screen With DIN 13 / 14 Dark Glass• Hand Wire Brush / Grinder With Wire Wheel • Cutting Pliers• Hand Gloves • Chipping Hammer / Chisel & hammer • Spanner Set• Cylinder Key• Anti-spatter Spray• Earthing Cable With Clamp


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GMAW Torch

Torch HandleSpring Conduit

JobArc

Gas Cup

Shielding Gas

Filler Wire - ElectrodeNozzle Tip

On / Off Switch


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Equipment & Accessories

+ –

Wire Inside Spring Lining

Flow Meter

Welding Torch Wire Feeder

Shielding Gas Cylinder

Pressure Regulator

Argon / Co2 Shielding

Power Source

With Inductance

Work

Arc–

Solenoid Valve

Copper Cup

Wire Spool

Electrode / Wire

Shielding GasHeater

(Only For Co2)

Contact Tip

Switch

Torch With Cable Max. 3Mtr


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Types Of Wire Feeding In GMAW

• Push Type– Wire fed in to The torch by Pushing through Flexible

Conduit From A Remote Spool

• Pull Type– Feed Rollers Mounted on The Torch Handle Pulls the

Wire From A Remote spool

• Self Contained– Wire Feeder & The Spool On the Torch


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Function Of Shielding Gas In GMAW

• Prevents Air contamination of weld Pool• Prevents Contamination During Metal

Transfer• Increases fluidity of molten metal• Minimizes the spatter generation• Helps in even & uniform bead finish


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Shielding Gases For GMAW

• MIG: Argon Or Helium For SS, CS, LAS & Non-ferrous

• MIG: Ar + 1 to 2 % O2, Wire With Add. Mn & Si For SS, CS & LAS

• MIG: Ar + 5 to 20 % Co2 Wire With Add. Mn & Si For SS, CS & LAS

• MAG: Co2 With Solid Wire For CS

• FCAW: Co2 With Flux Cored Wire For CS, LAS & SS Overlay


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ASME Classification For CS GMAW Wire

• SFA 5.18 : - CS Solid Wire

ER 70 S – 2, ER 70 S – 3

ER 70 S – 6, ER 70 S – 7

• SFA 5.20 :- CS Flux Cored Wire

E 71 T-1, E 71 T-2 ( Co2 Gas )

E 71 T-1M, E 71 T-2M ( Ar + Co2 Mix)


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GMAW CS Wire

• Generally Copper Coated– Prevents Oxidation / rusting in Storage– Promotes Electric Conductivity in Arcing

• Available In Solid & Flux Cored– Size in mm 0.8, 1, 1.2, 1.6, 2, 2.4, 3

• Manganese & Silicon ( Mn 1 – 2 %, Si Max 1%)– Act As Deoxidizing Agents– Eliminate Porosity– Increase Wetting Of Molten Pool


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Metal Transfer In MIG

• Short-Circuiting / Dip Transfer

• Globular Transfer

• Spray Transfer / Free Flight Mode


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Metal Transfer In MIG

Dip/Short Circuiting Globular Spray / Free Flight

CS Solid Wire 1.2 mm Φ

Above230A

24 – 35 V

120 to 250A

16 – 24 V

Up to 120A

14 – 22V

Co2 or Ar Co2 or Ar Only Ar / Ar+O2


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Short-Circuiting / Dip Transfer• Wire In Contact With Molten Pool 20 to 200 times per Second• Operates in Low Amps & Volts – Less Deposition• Best Suitable for Out of Position Welding• Suitable for Welding Thin Sheets• Relatively Large opening of Root Can be Welded• Less Distortion• Best Suitable for Tacking in Set up • Prone to Get Lack of Fusion in Between Beads


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Globular Transfer

• Metal transferred in droplets of Size grater than wire diameter

• Operates in Moderate Amps & Volts – Better Deposition

• Common in Co2 Flux Cored and Solid Wire

• Suitable for General purpose Welding


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Free Flight / Spray Transfer• Metal transferred in multiples of small droplets• 100 to 1000 Droplets per Second• Metal Spray Axially Directed• Electrode Tip Remains pointed• Applicable Only With Inert Gas Shielding

– Not With Co2

• Operates in Higher Amps & Volts – Higher Deposition Rate

• Not Suitable for Welding in Out of Position.• Suitable for Welding Deep Grooves


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Pulsed Spray Welding

• Power Source Provides Two different Current Levels“Background” and “Peak”at regular interval

• “Background” & “Peak” are above and below the Average Current

• Best Suitable for Full Penetration Open Root Pass Welding

• Good Control on Bead Shape and Finish


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Synergic Pulse GMAW

• Parameters of Pulsed Current (Frequency, Amplitude, Duration, Background Current) Related to Wire feed Rate

• One Droplet detaches with each pulse• An Electronic Control unit synchronizes wire feed

Rate with Pulse Parameters • Best Suitable for Most Critical Full Penetration

Open Root Pass Welding• Good Control on Open Root penetration, Bead

Shape and Finish


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GMAW Process Variables• Current• Voltage• Travel Speed• Stick Out / Electrode Extension • Electrode Inclination • Electrode Size• Shielding Gas & Flow Rate• Welding Position


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Parameter For 1.2 ф FC Wire• Current – 200 to 240 A

• Voltage – 22-24

• Travel Speed 150 to 250 mm / min

• Stick Out / Electrode Extension – 15 to 20 mm

• Electrode Inclination – Back Hand Technique

• Shielding Gas – Co2, 12 L/Min


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Parameter For 1.2 ф Solid Wire• Current – 180 to 220 A

• Voltage – 20-22

• Travel Speed 150 to 200 mm / min

• Stick Out / Electrode Extension – 10 to 20 mm

• Electrode Inclination – Back Hand Technique

• Shielding Gas – Co2 – 12 L/Min


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Results In Change Of Parameters• Increase In Current

– More deposition, More Penetration, More BM Fusion• Increase In Voltage

– More Weld Bead Width, Less Penetration, Less Reinforcement, Excess Spatter

• Increase In Travel Speed– Decrease in Penetration, Decrease in Bead Width,

• Decrease In Gas Flow rate– Results In porosity

• Long Stick Out / Electrode Extension– Excess Weld Deposit With Less Arc intensity, Poor Bead Finish,

Shallow Penetration


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Common Defects In GMAW

1. Porosity 2. Spatters

3. Lack Of Fusion 4. Under Cut

5. Over Lap 6. Slag

7. Crack 8. Lack Of Penetration

9. Burn Through 10. Convex Bead

11. Unstable Arc 12. Wire Stubbing


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Porosity

Cause Remedy1) Less Mn & Si In Wire

2) Rusted / Unclean BM / Groove

3) Rusted wire

4) Inadequate Shielding Gas

1) Use High Mn & Si Wire

2) Clean & warm the BM

3) Replace the Wire

4) Check & Correct Flow Rate

Porosity . .


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SpattersCause Remedy

1) Low Voltage

2) Inadequate Inductance

3) Rusted BM surface

4) Rusted Core wire

5) Quality Of Gas

1) Increase Voltage

2) Increase Inductance

3) Clean BM surface

4) Replace By Rust Free wire

5) Change Over To Ar + Co2

Spatters• • •


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Lack Of FusionCause Remedy

1) Inadequate Current

2) Inadequate Voltage

3) Wrong Polarity

4) Slow Travel Speed

5) Excessive Oxide On Joint

1) Use Right Current

2) Use Right Voltage

3) Connect Ele. + Ve

4) Increase Travel speed

5) Clean Weld Joint

Lack Of Fusion


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Undercut

Cause Remedy

1) Excess Voltage

2) Excess Current

3) Improper Torch angle

4) Excess Travel Speed

1) Reduce Voltage

2) Reduce Current

3) Train & Qualify the Welder

4) Reduce Travel Speed

Under cut


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Overlap

Cause Remedy

1) Too Long Stick Out

2) Inadequate Voltage

1) Reduce Stick Out

2) Increase the Voltage

Overlap


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SlagCause Remedy

1) Inadequate Cleaning

2) Inadequate Current

3) Wrong Torch angle

4) Improper bead placement

1) Clean each bead

2) Use Right Current

3) Train / Qualify welder

4) Train / Qualify Welder

Slag


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Crack

Cause Remedy

1) Incorrect Wire Chemistry 2) Too Small Weld Bead

3) Improper Preheat

4) Excessive Restrain

1) Use Right Wire

2) Increase wire Feed

3) Preheat Uniformly

4) Post heating or ISR

crack


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Lack Of Penetration*Cause Remedy

1) Too Narrow Groove Angle

2) Inadequate Root opening

3) Too Low Welding current

4) Wrong Torch angle

5) Puddle Roll In Front Of Arc

6) Long Stick Out

1) Widen The Groove

2) Increase Root Opening

3) Increase Current

4) Train / Qualify Welder

5) Correct Torch Angle

6) Reduce Stick Out

LOP

* Applicable to SSFPW


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Burn through*Cause Remedy

1) Excess Current

2) Excess Root opening

3) Inadequate Root face

4) Too Low Travel Speed

5) Quality Of Gas

1) Reduce the Current

2) Reduce root opening

3) Increase root face

4) Increase Speed

5) Use Ar + Co2

Burn trough*Applicable to root pass


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Convex Bead FinishCause Remedy

1) Low Current

2) Low Voltage

3) Low Travel Speed

4) Low Inductance

5) Too Narrow Groove

1) Increase Current

2) Increase Voltage

3) Increase Travel Speed

4) Increase Inductance

5) Increase Groove Width

Uneven bead finish


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Unstable arc

Cause Remedy

1) Improper Wire Feed

2) Improper Gas Flow

3) Twisted Torch Conduit

1) Check Wire Feeder

2) Check Flow Meter

3) Straighten Torch Cab


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Wire Stubbing

Cause Remedy

1) Too Low Voltage

2) Too High Inductance

3) Excess Slope

4) Too Long Stick Out

1) Increase Voltage

2) Reduce Inductance

3) Adjust Slope

4) Reduce Stick Out


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Important Terminology used in Critical Welding

• Preheating

• Post Heating or Dehydrogenation

• Intermediate Stress leaving

• Inter pass Temperature

• Post Weld Heat Treatment


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What Is Preheating?

• Heating the base metal along the weld joint to a predetermined minimum temperature immediately before starting the weld.

• Heating by Oxy fuel flame or electric resistant coil• Heating from opposite side of welding wherever

possible• Temperature to be verified by thermo chalks prior to

starting the weld


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Why Preheating?

• Preheating eliminates possible cracking of weld and HAZ• Applicable to

Hardenable low alloy steels of all thickness

Carbon steels of thickness above 25 mm.

Restrained welds of all thickness

• Preheating temperature vary from 75°C to 200°C depending on hardenability of material, thickness & joint restrain


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How does Preheating Eliminate Crack?

• Preheating promotes slow cooling of weld and HAZ

• Slow cooling softens or prevents hardening of weld and HAZ

• Soft material not prone to crack even in restrained condition


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What Is Post Heating?

• Raising the pre heating temperature of the weld joint to a predetermined temperature range (250° C to 350° C) for a minimum period of time (3 Hrs) before the weld cools down to room temperature.

• Post heating performed when welding is completed or terminated any time in between.

• Heating by Oxy fuel flame or electric resistant coil• Heating from opposite side of welding wherever possible• Temperature verified by thermo chalks during the period


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Why Post Heating?

• Post heating eliminates possible delayed cracking of weld and HAZ

• Applicable to

Thicker hardenable low alloy steels

Restrained hardenable welds of all thickness• Post heating temperature and duration depends on

hardenability of material, thickness & joint restrain


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How does Post Heating Eliminate Crack?

• SMAW introduces hydrogen in weld metal• Entrapped hydrogen in weld metal induces

delayed cracks unless removed before cooling to room temperature

• Retaining the weld at a higher temperature for a longer duration allows the hydrogen to come out of weld


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What Is Intermediate Stress Relieving? • Heat treating a subassembly in a furnace to a

predetermined cycle immediately on completion of critical restrained weld joint / joints without allowing the welds to go down the pre heat temperature. Rate of heating, Soaking temperature, Soaking time and rate of cooling depends on material quality and thickness

• Applicable to

Highly restrained air hardenable material


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Why Intermediate Stress Relieving?

• Restrained welds in air hardenable steel highly prone to crack on cooling to room temperature.

• Cracks due to entrapped hydrogen and built in stress

• Intermediate stress relieving relieves built in stresses and entrapped hydrogen making the joint free from crack prone


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What Is Inter- Pass Temperature?

• The temperature of a previously layed weld bead immediately before depositing the next bead over it

• Temperature to be verified by thermo chalk prior to starting next bead

• Applicable to

Stainless Steel

Carbon Steel & LAS with minimum impact


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Why Inter Pass Temperature?

• Control on inter pass temperature avoids over heating, there by

Refines the weld metal with fine grains

Improves the notch toughness properties

Minimize the loss of alloying elements in welds

Reduces the distortion


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What Is Post Weld Heat Treatment?

• Heat treating an assembly on completion of all applicable welding, in an enclosed furnace with controlled heating/cooling rate and soaking at a specific temperature for a specific time.

• Rate of heating, Soaking temperature, Soaking time and rate of cooling depends on material quality and thickness

• Applicable to

All type of CS & LAS


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Why Post Weld Heat Treatment?

• Welded joints retain internal stresses within the structure

• HAZ of welds remains invariably hardened

• Post Weld Heat Treatment relieves internal stresses and softens HAZ. This reduces the cracking tendency of the equipment in service


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Thanks

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