What is submerged arc welding (SAW) process?

As one of the many arc welding methods, submerged arc welding is for the joint of butt welding, fillet welding, lap welding. Maintained between the tip of wire and base metal, a stable arc is under the cover of granular agglomerated flux or fused flux. Thus, less sparks and splatter is leaked. Working condition is much better compared with open arc welding processes. Additionally, welding fluxes provide welding puddle a protection from harm caused atmospherical Oxyggen and Nitrogen. Submerged arc releases heat to melt fluxes, wire and base metal. Sometimes, with alloying element transfered, weld metal is at least equivalent to base metal in regards to mechanical properties.

By automatic/semi-automatic welding system, submerged arc welding gives high deposition rate and consistant weld quality. Human factors are minimized to the lowest level. Two disadvantages limit the applications of SAW: A mainly for flat position welding. B unable to the arc and puddle in reaching the root of a groove weld and properly filling or sizing.

The classification of submerged arc welding flux (SAW flux)

According to the manufacturing process, there are four types of flux: fused fluxes, bonded fluxes, agglomerated fluxes, mixed fluxes.

Fused flux: Raw materials are mixed, and melt in a high temperature electric furnace. When the fusion is over, homogeneous liquid is quenched and cooled in water to get glass-like solids. Then they are ground and milled to desired size.

Bonded flux: Various ingredients are dry mixed, then glued together by a liquid binder. Sodium silicate (water glass) is the most used binder. After the agglomeration process, big particles are dried at 400-500℃, and then sieved by vibration screen. Bigger particles are ground by roll crusher, while smaller particles are collected and used as raw materials.

Agglomerated flux: Manufacturing process is much the same way as bonded flux. However, agglomerated flux would go through additional sintering process at 800-900℃.

Mixed flux: It's more than a combination of two ore more fluxes mentioned above. It combines the advantages of different fluxes for special purpose.

What's the function of submerged arc welding flux (SAW flux)?

SAW fluxes play several roles in metal joining. Welding fluxes would obviously influence weld bead, mechanical properties, anti-porosity, anti-cracking, etc.

A  provide a cover on the welding area. Protect weld puddle from contamination of atmospheric Oxygen and Nitrogen.

B  molten fluxes turn into slag in the end. Slag layer slows down the cooling rate of weld metal. This allows more time for gases to flee from weld metal.

C  get the arc free from airflow disturbance. Make it stable.

D  alloy transfer.

E  a compensation for elements that are burnt-off.

F  when electricity passes through slag pool, vast resistance heat is generated.

Agglomerated flux VS fused flux

Agglomerated flux advantages:

A  less pullution to environment during production compared with fused flux.

B  less energy consumption during production

C  higher mechanical properties

D  alloy transfer is possible. Fusion process would damage alloying elements at high temperature above 1300℃

E  easy slag removal

Fused flux advantages:

A  slag could be recycled & reused. It means higher economic benefit 

B  excellent weld bead, higher resistance to porosity

C  non-hygroscopic. No need for additional drying

D  consistent, homogenous chemical composition of weld metal 

Basic flux VS acid flux

Basic fluxes take obvious advantages in high mechanical properties, while acid fluxes perform better in slag removal, weld metal appearance.

Useful links for SAW welding and flux:

http://www.thefabricator.com/article/arcwelding/3-ways-to-boost-submerged-arc-welding-performance    3 ways to boost submerged arc welding performance
http://www.weldpedia.com/2015/03/pipeline-submerged-arc-welding-closeup.html    pipeline submerged arc welding closeup