Gas Metal Arc Welding Poyyara Consultants 1 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 Poyyara Consultants 2 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 Poyyara Consultants 3 Power Source For MIG / MAG • • • • Inverter- DC Thyrester – DC Motor Generator – DC Rectifier – DC Poyyara Consultants 4 Characteristic Of GMAW Power Source Constant V / Linier Characteristic V Appx. Horizontal Curve V1 V2 A1 A A2 Poyyara Consultants 5 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 Poyyara Consultants 6 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 Poyyara Consultants 7 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 Poyyara Consultants 8 GMAW Torch On / Off Switch Shielding Gas Torch Handle Spring Conduit Gas Cup Arc Nozzle Tip Filler Wire - Electrode Job Poyyara Consultants 9 Equipment & Accessories Pressure Regulator Flow Meter Shielding Gas Heater (Only For Co2) Solenoid Valve Switch Shielding Gas Cylinder Copper Cup Electrode / Wire Arc – Welding Torch Wire Inside Spring Lining Contact Tip Argon / Co2 Shielding Work Wire Feeder Wire Spool Power Source Torch With Cable Max. 3Mtr With Inductance Poyyara Consultants + – 10 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 Poyyara Consultants 11 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 Poyyara Consultants 12 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 Poyyara Consultants 13 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) Poyyara Consultants 14 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 Poyyara Consultants 15 Metal Transfer In MIG • Short-Circuiting / Dip Transfer • Globular Transfer • Spray Transfer Poyyara Consultants 16 Metal Transfer In MIG Up to 120A CS Solid Wire 1.2 mm Φ 120 to 250A 14 – 22V Dip/Short Circuiting Co2 or Ar 16 – 24 V Globular Co2 or Ar Poyyara Consultants Above230A 24 – 35 V Spray Only Ar / Ar+O2 17 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 Poyyara Consultants 18 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 Poyyara Consultants 19 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 Poyyara Consultants 20 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 Poyyara Consultants 21 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 Poyyara Consultants 22 GMAW Process Variables • • • • • • • • Current Voltage Travel Speed Stick Out / Electrode Extension Electrode Inclination Electrode Size Shielding Gas & Flow Rate Welding Position Poyyara Consultants 23 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 Poyyara Consultants 24 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 Poyyara Consultants 25 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 Poyyara Consultants 26 Common Defects In GMAW 1. Porosity 3. Lack Of Fusion 5. Over Lap 7. Crack 9. Burn Through 11. Unstable Arc 2. Spatters 4. Under Cut 6. Slag 8. Lack Of Penetration 10. Convex Bead 12. Wire Stubbing Poyyara Consultants 27 Porosity Cause Remedy 1) 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 . . Poyyara Consultants 28 Spatters Cause 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 • •• Poyyara Consultants 29 Lack Of Fusion Cause Remedy 1) Inadequate Current 1) Use Right Current 2) Inadequate Voltage 3) Wrong Polarity 4) Slow Travel Speed 5) Excessive Oxide On Joint 2) Use Right Voltage 3) Connect Ele. + Ve 4) Increase Travel speed 5) Clean Weld Joint Lack Of Fusion Poyyara Consultants 30 Undercut Cause 1) Excess Voltage 2) Excess Current 3) Improper Torch angle 4) Excess Travel Speed Remedy 1) Reduce Voltage 2) Reduce Current 3) Train & Qualify the Welder 4) Reduce Travel Speed Under cut Poyyara Consultants 31 Overlap Cause Remedy 1) Too Long Stick Out 1) Reduce Stick Out 2) Inadequate Voltage 2) Increase the Voltage Overlap Poyyara Consultants 32 Slag Cause 1) Inadequate Cleaning 2) Inadequate Current 3) Wrong Torch angle 4) Improper bead placement Remedy 1) Clean each bead 2) Use Right Current 3) Train / Qualify welder 4) Train / Qualify Welder Slag Poyyara Consultants 33 Crack Cause Remedy 1) Incorrect Wire Chemistry 1) Use Right Wire 2) Increase wire Feed 2) Too Small Weld Bead 3) Preheat Uniformly 3) Improper Preheat 4) Post heating or ISR 4) Excessive Restrain crack Poyyara Consultants 34 Lack Of Penetration* Cause 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 Remedy 1) Widen The Groove 2) Increase Root Opening 3) Increase Current 4) Train / Qualify Welder 5) Correct Torch Angle 6) Reduce Stick Out * Applicable to SSFPW LOP Poyyara Consultants 35 Burn through* Cause 1) Excess Current 2) Excess Root opening 3) Inadequate Root face 4) Too Low Travel Speed 5) Quality Of Gas Burn trough Remedy 1) Reduce the Current 2) Reduce root opening 3) Increase root face 4) Increase Speed 5) Use Ar + Co2 *Applicable to root pass Poyyara Consultants 36 Convex Bead Finish Cause 1) Low Current 2) Low Voltage 3) Low Travel Speed 4) Low Inductance 5) Too Narrow Groove Remedy 1) Increase Current 2) Increase Voltage 3) Increase Travel Speed 4) Increase Inductance 5) Increase Groove Width Uneven bead finish Poyyara Consultants 37 Unstable arc Cause 1) Improper Wire Feed 2) Improper Gas Flow 3) Twisted Torch Conduit Remedy 1) Check Wire Feeder 2) Check Flow Meter 3) Straighten Torch Cab Poyyara Consultants 38 Wire Stubbing Cause 1) Too Low Voltage 2) Too High Inductance 3) Excess Slope 4) Too Long Stick Out Remedy 1) Increase Voltage 2) Reduce Inductance 3) Adjust Slope 4) Reduce Stick Out Poyyara Consultants 39 Important Terminology used in Critical Welding • • • • • Preheating Post Heating or Dehydrogenation Intermediate Stress leaving Inter pass Temperature Post Weld Heat Treatment Poyyara Consultants 40 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 Poyyara Consultants 41 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 Poyyara Consultants 42 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 Poyyara Consultants 43 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 Poyyara Consultants 44 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 Poyyara Consultants 45 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 Poyyara Consultants 46 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 Poyyara Consultants 47 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 Poyyara Consultants 48 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 Poyyara Consultants 49 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 Poyyara Consultants 50 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 Poyyara Consultants 51 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 Poyyara Consultants 52 Thanks Poyyara Consultants 53
