Porosity is referred to as cavities produced in a weld
due to the entrapment of gas. The introduction of gas in a weld is supported by some mistakes that ought to be eliminated. In weld metal porosity is categorized according to shape and location of occurrence. Some gas cavities occurs in spherical shape, some in elongated (like in case of worm holes) while on the other hand some cavities occur throughout weld metal and some on the weld surface. These gas cavities appear either singly called voids or uniformly distributed throughout the weld and sometime developed in clustered form. These voids and cavities are produced primarily by gas entrapment during the solidification procedure of weld. So making prediction about mistakes that may cause porosity and their removal is essential part before welding. Given below is the complete detail about common porosity reasons and their solution.
It is recommended to use baked electrodes in Stick welding process
in order to avoid involvement of moisture content in weld metal. In worst, steam can be driven out, during heat of welding which creates small cavities in the weld metal. Porosity eventuates when stainless steel electrodes or low hydrogen electrodes are not properly kept in dry condition. Although in low hydrogen electrodes some moisture is required within a limit
, for better performance but is some case if moisture exceeds from the limit, the weld metal will prone to porosity. For prevention following are the steps to follow;
- Parent metal must be preheated in order to remove any moisture.
- The electrodes must be stored in dry conditions.
- Apply final backing before use.
- Electrodes must not be oven baked before use, because a little amount of moisture is required for better performance and weld-ability of electrode.
Parent metal or electrodes may be subjected to atmospheric contamination. Any grease, oil or moisture content on surface contribute gas formation when exposed to welding temperature and may cause porosity problems in your weld. In order to fix it, clean the material by specified surface preparation procedures.
The possibility of air entrapment due to inconsistent gas shield may also cause porosity problems. For this the welder must check gas hose attachment with the equipment and remove hose contamination if any. Correct connection prevents any loss of gas shield and consequently no air entrapment. Moreover air flow and draughts in shop can cause restriction in gas flow. Some sort of screening is required to avoid such kinds of restrictions in gas flow.
High flow rate of gas shield may cause turbulence due to which air can be drawn into the weld metal. Optimize gas flow, provides quality welds with no gas loss. Moreover it saves cost on the other side.
5. Inadequate electrode deoxidant
During solidification, the excess oxygen comes out from weld metal due to reduced solubility and may undergo carbon monoxide formation that may cause porosity too. For this some deoxidants are added in electrodes, filler metals and even in parent metals to remove the oxygen content (which is so called deoxidation). Therefore inadequate deoxidants may result in poor deoxidation and are responsible for weld defects. In order to prevent this, use electrodes with adequate deoxidants.
Too much longer arc length (in other words high voltage) is also a cause of weld porosity. If the welding gun is held away from the joint keeping arc length longer, the magnitude of shielding is reduced which may lead to air entrapment from atmosphere. Correct arc length is recommended primarily to avoid draughts/breezes.
7. Incorrect surface treatments
Release of gases, during painting/surface treatments, can impair weld characteristics. Treatments like zinc coating or galvanizing produce gas and other unwanted particles. In an endeavor to fix this problem, make predictions about the outcome of chemical reactions before specific surface treatments.
Any surface which is open to atmospheric air can be contaminated. Air introduction from back side through root opening, gets entrap in the weld puddle. Make sure that weld joint is protected from such kinds of air crevices.
Laminated surface also creates disturbance in welding. In case of welding a laminated work piece, prepare un-laminated parent metal to conduct quality weld procedure.
Welding flux must be treated with great care as they have capability to absorb moisture (granular SAW flux). They must be supplied in dry condition. On the other hand, use of low activity flux results in surface porosity. That is why it is recommended to use high activity flux which offers quality welds with no porosity.