SUBMERGED ARC WELDING PROCESS SAW PROCESS

SUBMERGED ARC WELDING (S.A.W) PROCESS

            This process uses Constant Voltage Power Supply (or) Flat Characteristic and Polarity used in this Process is D.C.E.P.The arc is struck between a continuous bare wire and the parent metal. The arc, electrode & weld pool is submerged in an agglomerated or fused powdered flux, which turns into a slag in its lower layers when subjected to the heat of arc, thus protecting the weld from contamination. The wire electrode is fed continuously by a feed unit of motor driven rollers.  

SUBMERGED ARC WELDING 

            The flux is fed from a hopper fixed to the welding head. Some of the flux melts to form a protective blanket over the weld ment and the remaining flux can be recovered and can be re-used provided it is not contaminated. It is a high heat input process & prone to Hydrogen Cracking.

PROCESS VARIABLE:

• Welding current
• Arc voltage
• Polarity
• Travel speed
• Electrode size
• Type of flux and particle distribution
• Width and depth of layer of flux

TYPICAL DEFECTS:

• Lack of penetration
• Slag inclusion
• Porosity
• Centre line cracking
• Under-cut
• Excessive penetration

APPLICATION:

• Ship building
• Heavy fabrication
• Circumferential weld
• Longitudinal seams

ADVANTAGES:

• No spatter, high productivity & no visible arc
• Un-melted flux can be re-used.
• Continuous feed of electrode
• Deep penetration & high productive speed

DISADVANTAGES:

• The use of powdered flux limits the usage only for flat position.

TYPE OF FLUX:

• AGGLOMERATED FLUX
• FUSED FLUX.

ADVANTAGES OF FUSED FLUX:

• Good chemical homogeneity
• Less hygroscopic thus handling and storage are easier.
• Allow fine powder to be removed without change in composition.
• They can be easily recycled through the system without significant change in particle size and composition.

DISADVANTAGES OF FUSED FLUX:

• Limitations in composition as some components such as basic carbonate would be unable to withstand the melting process.
• Difficult to add de-oxides and ferrous alloys.

ADVANTAGES OF AGGLOMERATED FLUX:

• De-oxides and alloying elements can easily be added to the flux to adjust the weld metal composition.
• Allow a thicker flux layer when welding
• Can be identified by color coding.

DIS-ADVANTAGES OF AGGLOMERATED FLUX:

• ARE GENERALLY HYGROSCOPIC AND REQUIRE BAKING
• Gas may be evolved from slag as it is melted, leading to porosity.
• There may be changes in weld metal chemical composition from the segeration of fine particles produced by the mechanical handling of granulated flux.

MCQ IMPORTANT POINTS:

1. Agglomerated fluxes are most prone to moisture pick-up.
2. Fused fluxes are resistant to moisture pick-up.
3. There is a risk of hydrogen cracking in agglomerated flux
4. If travel speed is increased then heat input decreases.
5. Effect of increased welding current

• • • Burn through
    • Undercut
    • Narrow bead prone to solidification cracking
• Effect of decreased welding current.

• • Unstable arc
  • Lack of penetration
  • Possibly lack of fusion
• Effect of increased arc voltage with constant current and travel speed

• • Flatter & wider bead
  • Increase flux consumption
  • Reduce porosity
• Effects of excessively increased arc voltage:

• • Wide bead prone to solidification cracking
  • Difficult to remove slag in groove weld
  • Concave shape fillet weld prone to cracking
  • Undercut
• Effect of reducing arc voltage with constant current & travel speed:

• • Stiffer arc which improves penetration
  • Resist arc blow
• Effect of excessively reduced arc voltage:

• • Produce narrow bead
  • Slag removal is difficult.
• Effect of increased travel speed:

• • Heat input per unit length decreases
  • Less filler metal is applied per unit length
  • Less excessive weld metal
  • Penetration decreases

FUSED FLUX Vs AGGLOMERATED FLUX

FUSED FLUX	AGGLOMERATED FLUX
Less hygroscopic	More hygroscopic
Difficult to add de-oxides & ferrous alloys	De-oxides and alloying elements can easily be added
No significant change in particle size or composition	There may be change in weld metal chemical composition
Flux granules are similar in appearance to crushed glass, irregular in shape, hard & have smooth shiny surface.	The granules tends to be more spherical and have dull finish
During re-circulation they have good resistance to breaking down into fine particles	The granule consists of fine powder, weakly held and can be easily broken down into fine powder during re-circulation.
Have resistance to moisture during exposure	Will tend to absorb moisture during exposure
They gives weld bead a good surface finish, profile  de-slag easily	The weld profile is more convex and more effort is required to remove slag.
No risk of Hydrogen Cracking.	Risk of Hydrogen Cracking.