Stainless Steel Welding OR Gas Metal Arc Welding
- Slides: 23
Stainless Steel Welding OR Gas Metal Arc Welding BY J. K. TOMAR, Chief Instructor
Gas Metal Arc Welding the Dominant Process • Improved productivity with flexibility • High weld quality with low hydrogen deposit • Suitable for semi-automatic and automatic welding • Increased penetration and deposition rates • Amenable to mechanization and robotic applications • Adaptable to microprocessor based feed back control
Gas Metal Arc ( MIG ) Welding Uses continuous wire 0. 6 – 2. 0 mm as electrode Gas shielded, inert or active gas Manual, automatic or semi-automatic process High productivity
GMAW process
MIG/MAG WELDING SYSTEM • • Regulator / Flowmeter • • High Pressure Gas Cylinder, • Shielding Gas • • Filler Metal, (Electrode)- Wire Reel • • Wire Feeder • • Constant Voltage Power Source • • Welding Torch/MIG Gun
Power Source Characteristics • Power Sources of Constant Current type having drooping characteristics are used for • - MMAW process • - GTAW process • - Plasma processes • Power sources of constant voltage type having flat characteristics are used for • - GMAW & FCAW processes • - SAW process
Automatic arc length regulation
Wire Feed Speed / Current • Constant potential power sources are used for GMAW and have no built in means of changing the current. The current adjusts itself to burn off the quantity of wire delivered. If the wire feed speed is increased more current is drawn to burn it off. In this way adjusting the wire feed speed also adjusts the current supplied. • The current dictates the amount of heat generated by the arc. Increasing the current increases the arc energy and therefore the heat input. This in turn increases fusion and penetration, wire deposition rate and travel speed.
Shielding Gases Shielding gases provide a protection to the weld metal from the atmosphere and have a pronounced effect on: • Arc characteristics • Mode of metal transfer • Penetration and weld bead profile • Speed of welding • Undercutting tendency • Cleaning action • Weld metal mechanical properties
Types of shielding gases used in GMAW • Carbon Dioxide (CO 2) • Argon (Ar) • Helium (He) • Ar + O 2 • Ar + CO 2 + O 2 • Ar + He • Ar + H 2 • Ar + N 2
GMAW Filler Wire GMAW filler wire for welding carbon-manganese and low alloy steels require additional quantities of silicon and Manganese as de-oxidisers. Some stainless steel wires may also have higher silicon, otherwise chemistry of GMAW wire generally match the plate material AWS specifications covering GMAW wire • SFA-5. 7 for copper and copper alloys • SFA-5. 9 for stainless steels • SFA-5. 10 for aluminium and aluminium alloys • SFA-5. 14 for nickel and nickel allos • SFA-5. 18 for carbon – manganese steels • SFA-5. 28 for low alloy steels
Modes of metal transfer The mode of transfer is determined by a number of factors: – Magnitude, type and polarity of welding current – Electrode diameter – Electrode composition – Electrode extension and – Shielding gas composition
MODES OF METAL TRANSFER
Influence of welding current & gas on metal transfer mode in GMAW
Problems in using pure Argon as Shielding gas • Stable and Soft arc with a tendency to wander • Finger shaped penetration profile • Poor fusion and penetration due to low heat input • Comparatively high bead profile
Benefits of using gas mixtures • Improved arc stability leading to lower spatter loss • Improved weld bead geometry leading to weld metal saving • Faster welding speeds leading to higher productivity and reduced labour costs • Improved weld quality leading to lesser rejects • Reduced clean up time • Lower distortion • No heaters required for individual cylinders
Gas Metal Arc Welding Parameters • Current ( amps ) • Voltage ( volts ) • Shielding gas flow rate ( liters / min ) • Stick out • Torch angle • Welding speed
Balancing the wire feed speed
Balancing the wire feed speed • Two options are available to balance the wire feed rate • Adjusting the arc voltage to increase or decrease the burn off rate - used when the current is OK for the job • Adjusting the wire feed speed if the current is too high or low.
Influence of wire angle The wire angle influences • penetration • weld convexity • tendency to undercutting • porosity. Backhand gives high penetration, narrow and high weld convexity, and relatively high risk for undercutting Vertical welding gives optimum performance Forehand gives low penetration, wide and low weld convexity, and relatively slight risk for undercutting
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