Obstacles in welding and solution

Obstacles to welding

Such obstructions can be of various kinds: 
– geometrical surface irregularities,
– pollution of the surface (oxides, grease, moisture, etc.),
– chemical elements brought in by the surrounding air.

Mitigation of welding obstructions

To mitigate them, surface preparation(grinding, machining etc.) is to be done before welding.
All pollutants present on the surfaces to be welded are eliminated by

• Mechanical Actions (sanding, grinding etc.)
• Chemical Means (solvents, scouring, drying)

Harmful affects of atmosphere on weld

Welding carried in atmosphere is susceptible to contamination by oxygen, nitrogen, hydrogen (H2 formed by dissociation of water vapors). These are elaborated below.

Affects of oxygen on weld

Oxygen may interact with weld in one or more of the following ways

• Fumes may be produced by reaction of oxygen with volatilized elements from base metal, consumables or atmosphere.
• Metallic oxides may form which constitute inclusions in solidified metal 
•  Porosities in the molten metal due to the drop in solubility at lower temperatures while cooling.

Protection of welds - from oxygen

Protection from oxygen can be achieved by

• Using a neutral gas for shielding
•  Introduction of oxygen hungry element like silicon whose oxides for slag

Affects of nitrogen on weld

Nitrogen can dissolve in molten metal and can contribute to

• Porosities in the molten metal due to the drop in solubility at lower temperatures while cooling.
• Formation of metal nitrides which, according to the conditions in which
  they  appear,  constitute  inclusions  or  precipitates  more  or  less  hardening  and
  weakening in nature
• The part which remains as solution in metal results in gradual weakening of metal by the process of ageing

Protection of welds - from nitrogen

Protection from nitrogen can be achieved by

• Using a neutral gas for shielding

Affects of hydrogen on weld

Hydrogen dissolves even up to saturation level if preventive measures are not taken. The solubility decreases with temperature drop which results in

• Porosities during solidification
• Cracks are developed when hydrogen concentrations get oversaturated and hydrogen molecules gather at structural defects

Protection of welds - from hydrogen

This can be done by

• Limiting the presence of hydrogen, by ensuring that surface is free of water and other materials which may contain hydrogen for e.g. greases, coatings etc.
• Using a neutral gas for shielding