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Fused Fluxes versus Bonded Fluxes

Among the various types of fluxes use in Submerged Arc Welding are the fused flux and the bonded flux.  Each of these fluxes offers some advantages and some disadvantages.

Fused Fluxes

When making fused fluxes, the raw materials are dry mixed together, and then they are fused or melted into a liquid state by using a high temperature furnace.  After fusion is complete, the fluxes are cooled.  This may be accomplished by using a stream of water or with big chill blocks.

Once the fluxes are cooled, they are crushed or ground into particles.  A variety of particle sizes are made to ensure optimal performance for different applications. 

Advantages of fused fluxes include:

The non-hygroscopic flux particles do not absorb moisture and, therefore, any surface moisture can be eliminated merely by drying the particles at a low temperature oven setting of 300 degrees F.

  • Low temperature drying of condensation on the fused flux particles provides better protection against hydrogen cracking.
  • Flux particles create welds that are chemically consistent.
  • Recycling of fused flux particles through the flux recovery systems can be achieved without losing sizing or composition.

A disadvantage of fused fluxes is that the high temperature used during the manufacture process makes it difficult to add alloys and deoxidizers.

Bonded Fluxes

The manufacture of bonded fluxes involves combining the dry ingredients, then using a liquid binder such as sodium silicate or potassium silicate to glue the ingredients together.  After the bonded mix is made into pellets, the pellets are baked at a low oven temperature.  Once the drying of the pellets is complete, the pellets are broken up by using a sieve to attain the desired particle size.  The particles are then packaged for shipping.

Advantages of bonded fluxes include:

  • Deoxidizers are present in bonded fluxes, protecting against rust and mill scale.  These deoxidizers also help to prevent welds from becoming porous.
  • Alloys can be added to bonded fluxes.  Alloy elements may improve chemical and mechanical properties of the flux.
  • Bonded fluxes allow for a thicker flux layer when welding.
  • Bonded fluxes can be identified by color.
  • Bonded fluxes typically provide better peeling properties than fused fluxes.

There are at least two disadvantages of using bonded fluxes.  These are:

  • They absorb moisture.
  • They can change in composition due to segregation or loss of fine particle size.


Miller Submerged Arc Welding
ESAB Welding and Cutting, U.S.



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