gmaw.ppt
DESCRIPTION
gas metal arc weldingTRANSCRIPT
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Gas Metal Arc Welding
INDIABULLS
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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• Thyrester – DC• Motor Generator – DC• Rectifier – DC
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Characteristic Of GMAW Power Source
Constant V / Linier Characteristic
Appx. Horizontal Curve
V1V2
A1 A2 A
V
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Current & Polarity
DC- Electrode +VeStable ArcSmooth Metal TransferRelatively Low SpatterGood Weld Bead Characteristics
DC- Electrode – Ve, Seldom UsedAC- 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 Clam
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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 Mt & Al
• MIG: Ar + 1 to 2 % O2, Wire With Add. Mn & Si For SS, CS, LAS & Non-ferrous Mt & Al
• MIG: Ar + 5 to 20 % Co2 Wire With Add. Mn & Si For SS, CS, LAS & Non-ferrous Mt & Al
• MAG: Co2 With Solid Wire For CS & LAS
• 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 WireER 70 S – 2, ER 70 S – 3ER 70 S – 6, ER 70 S – 7
• SFA 5.20 :- CS Flux Cored WireE 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
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Metal Transfer In MIG
Dip/Short Circuiting Globular Spray
CS Solid Wire 1.2 mm ΦAbove230A
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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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/Hr
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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/Hr
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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 Wire2) Rusted / Unclean BM / Groove 3) Rusted wire4) Inadequate Shielding Gas
1) Use High Mn & Si Wire2) Clean & warm the BM3) Replace the Wire4) Check & Correct Flow Rate
Porosity . .
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SpattersCause Remedy
1) Low Voltage2) Inadequate Inductance 3) Rusted BM surface4) Rusted Core wire5) Quality Of Gas
1) Increase Voltage2) Increase Inductance3) Clean BM surface4) Replace By Rust Free wire5) Change Over To Ar + Co2
Spatters• • •
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Lack Of FusionCause Remedy
1) Inadequate Current2) Inadequate Voltage3) Wrong Polarity4) Slow Travel Speed5) Excessive Oxide On Joint
1) Use Right Current2) Use Right Voltage3) Connect Ele. + Ve4) Increase Travel speed5) Clean Weld Joint
Lack Of Fusion
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Undercut
Cause Remedy1) Excess Voltage2) Excess Current3) Improper Torch angle4) Excess Travel Speed
1) Reduce Voltage2) Reduce Current3) Train & Qualify the Welder4) Reduce Travel Speed
Under cut
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Overlap
Cause Remedy1) Too Long Stick Out
2) Inadequate Voltage
1) Reduce Stick Out
2) Increase the Voltage
Overlap
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SlagCause Remedy
1) Inadequate Cleaning2) Inadequate Current3) Wrong Torch angle4) Improper bead placement
1) Clean each bead2) Use Right Current3) Train / Qualify welder4) Train / Qualify Welder
Slag
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Crack
Cause Remedy1) Incorrect Wire Chemistry 2) Too Small Weld Bead3) Improper Preheat
4) Excessive Restrain
1) Use Right Wire2) Increase wire Feed3) Preheat Uniformly4) Post heating or ISR
crack
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Lack Of Penetration*Cause Remedy
1) Too Narrow Groove Angle2) Inadequate Root opening3) Too Low Welding current 4) Wrong Torch angle5) Puddle Roll In Front Of Arc6) Long Stick Out
1) Widen The Groove2) Increase Root Opening3) Increase Current4) Train / Qualify Welder5) Correct Torch Angle6) Reduce Stick Out
LOP
* Applicable to SSFPW
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Burn through*Cause Remedy
1) Excess Current2) Excess Root opening3) Inadequate Root face4) Too Low Travel Speed5) Quality Of Gas
1) Reduce the Current2) Reduce root opening3) Increase root face4) Increase Speed5) Use Ar + Co2
Burn trough*Applicable to root pass
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Convex Bead FinishCause Remedy
1) Low Current2) Low Voltage3) Low Travel Speed 4) Low Inductance5) Too Narrow Groove
1) Increase Current2) Increase Voltage3) Increase Travel Speed4) Increase Inductance5) Increase Groove Width
Uneven bead finish
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Unstable arc
Cause Remedy1) Improper Wire Feed2) Improper Gas Flow3) Twisted Torch Conduit
1) Check Wire Feeder2) Check Flow Meter3) Straighten Torch Cab
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Wire Stubbing
Cause Remedy1) Too Low Voltage2) Too High Inductance3) Excess Slope 4) Too Long Stick Out
1) Increase Voltage2) Reduce Inductance3) Adjust Slope4) 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 thicknessCarbon 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 toThicker hardenable low alloy steelsRestrained 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 toHighly 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 toStainless SteelCarbon 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 grainsImproves the notch toughness propertiesMinimize the loss of alloying elements in
weldsReduces 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 toAll 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