- Porosity in welding can be
defined as small gas holes or cavities or pores which are present in the
weld bead. These gas holes can vary in size and appear to be scattered all
over the weld bead in a random manner. These holes or pores can occur
either over or under the weld surface.
- Porosity in welds does not
normally present a problem from the strength standpoint, unless the weld
is extremely porous. Uniformly distributed porosity that is microscopic in
size may be beneficial under some service conditions. In weld metal
porosity is categorized according to shape and location of occurrence.
- The gas cavities can be of
spherical shape or elongated like worm holes which occur all over the weld
metal.
- Surface holes in the weld bead
are undesirable from an appearance standpoint and can be starting points
for fatigue cracks or result in other defective weld conditions. Weld
imperfections closely related to porosity are blow holes, gas pockets and
slag inclusions. These conditions tend to weaken the weld much more than
ordinary porosity.
- The various reason for the
formation of pores are mentioned below
- Contaminated electrodes,
filler rods, wires, consumables, tungsten rod, work surface etc.
- Inadequate electrode
de-oxidant, In-appropriate flux.
- Moisture, paint, oil, grease,
chalk marks, chemicals used for NDT on open work surface.
- Improper Gas shield or too
high gas flow.
- Laminations and other defects
in base metal.
- Too high arc length.
- Incorrect surface treatments.
- Improper welding procedure.
The causes and cures for
the above weld defects are mentioned below,
Contaminated surface:
Cause
|
Cure
|
It generally occurs on the parent
metal or electrodes used during the welding process. Porosity can be formed
if the presence of grease, oil or moisture which tends to form a gas during
welding temperature.
|
The metal wire, rod, powder or electrode which is used during the welding process must be cleaned and made
free from oil, grease or moisture. Base metal and material to be welded to it
should be free from grease, paints, moisture, oil, grinding dust or any other
contaminant.
|
Inadequate electrode
deoxidant:
Cause
|
Cure
|
It is formed when excess amount of
oxygen comes out in from the weld metal which tends to form carbon monoxide
and leads to formation of porosity. Lack of deoxidation causes porosity.
|
Before welding proper deoxidation
process must be done which removes all the oxygen content from the weld metal
and prevents the metal from reaction with oxygen during the process of
welding. Proper deoxidant electrodes must be used for welding purpose.
|
Open work surface:
Cause
|
Cure
|
Whenever the metal is exposed to
air, it gets absorbed through the root opening and finally entrapped by the
welded surface and as a result porosity is formed. Excessive hydrogen or
oxygen in welding atmosphere.
|
Weld metal must not be exposed to
air to ensure that it doesn’t enter into the weld metal easily in order to
avoid porosity. Change electrode to avoid oxidation.
|
Improper Gas Shield:
Cause
|
Cure
|
The gas shielding is one of most
important reason for porosity. If gas shielding is improper
during the process of welding air easily gets entrapped into the weld metal
and results in formation of oxidation. Too much of air flow and draughts in
the area of welding is may also be a reason for porosity.
|
Proper connection of gas pipelines
or hoses must be ensured before starting the welding. In order to avoid too
much of air flow during welding some kind of shielding must be given during
the time of welding or the process can be done inside a place where the flow
of outer air is less.
|
Too high gas flow:
Cause
|
Cure
|
Due to high flow of gas during the
time of welding some amount of turbulence is formed which sucks in external
air enabling it to react with the weld metal and finally results in porosity.
Ensure that there are no holes or loose fittings in the gas hose from the gas
cylinders to the power source / welding torch / Gun. This is because air is
sucked into the gas hose which will create porosity in the weld pool.
|
Flow meters must be checked and
optimized before starting the process. The proper pressure and flow setting
of gas will v ensure proper shielding of weld from the external atmosphere.
|
In-appropriate flux:
Cause
|
Cure
|
Generally welding fluxes , base
metal , filler rode , electrodes wires or powders have the tendency to
absorb moisture very easily . If moisture is absorbed it may be the cause of
weld defects. Application of low activity flux also results in porosity of
the weld metal.
|
In order to avoid weld defects the
fluxes and base metal, filler rod, electrodes wires or powders must be
ensured to be moisture free and in dry condition. Usage of fluxes with high
activity to ensure quality weld with no weld defects especially with the
absence of porosity.
|
Too high arc length:
Cause
|
Cure
|
Improper arc length, current or
manipulation
|
Use correct arc
distance (within recommended voltage range), control welding technique
|
Incorrect surface
treatments:
Cause
|
Cure
|
Porosity happens when the surface
of the metal is subjected to chemical treatment or painting such as zinc
coating or galvanizing produces gas which can lead to change in requirements
of weld parameters and procedure.
|
In order to avoid these problems
the chemical reactions of the material are to be predicted in prior so that
the defect can be avoided. Use E6010 electrode.
|
Moist Electrodes:
Cause
|
Cure
|
Excessive moisture in electrode or
joint
|
Use dry electrodes and dry
materials
|
Base Metal:
Cause
|
Cure
|
High sulphur base metal
|
Improper welding
procedure:
Cause
|
Cure
|
Porosity may occur due to improper
WPS decided before welding. Due to improper setting of parameters in the
machine and welding procedure weld defects can occur easily.
|
The WPS for welding a particular
job must be taken care. All the machine parameters are to be checked prior of
welding. Ensure machine is calibrated for current and voltage readings on the
display panel.
.
|
General solution for
prevention of porosity:
- Be sure the base metal is one
that will produce a porosity free weld. High Sulfur, phosphorus
and silicon steel sometimes produce gaseous combinations which tend
to make blow holes and gas pockets. Non-ferrous material high in oxygen
also tends to result in porous welds. Base metal
containing segregation's and impurities contribute to porosity.
- The welding procedure has much
to do with the soundness of finished welds. Do not use excessive currents
but be sure that each layer of weld metal is completely free of slag and
flux before depositing another layer. Puddle the weld, keeping the metal
molten sufficiently long to allow entrapped gases to escape. Use a weaving
pass in making the weld since a series of stringer beads is apt to contain
minute pin holes.
- Since some electrodes have a tendency to produce sounder welds than others, try to use such electrodes in making welds. Most low-hydrogen electrodes will be found helpful in eliminating porosity. When using the submerged arc process, be sure the flux is suited to scavenging gaseous elements. When using bare electrodes, thorough drying of them or selection of a type that has a copper flash coating will eliminate oxygen or hydrogen being introduced into the weld from moisture on the surface of the electrode.
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