WELDING PROCESSES
LARSEN & TOUBRO LTD. WELDING PROCESS & DEFECTS
WELDING PROCESSES
Welding plays an important role in fabrication of heat
exchangers and pressure vessels, as it is the major process in
manufacturing of heat exchangers. Here in L & T only arc
welding processes are used for welding. The processes that are used
for fabrication are listed below:1) Gas Tungsten Arc Welding
(GTAW)
2) Shielded Metal Arc Welding (SMAW)
3) Submerged Arc Welding (SAW)
4) Flux Cored Arc Welding (FCAW)
5) Gas Metal Arc Welding (GMAW)
6) Strip Cladding (ESSC)1) GAS TUNGSTEN ARC WELDING (GTAW)
TUNGSTEN INERT GAS WELDING (TIG):
GTAW is an arc welding process where in which coalescence is
produced by heating the job with an electric arc struck between a
tungsten electrode & the job. A shielding gas (argon, helium,
nitrogen etc.) is used to avoid atmospheric contamination of the
molten weld pool. A filler metal may be added if required.
ADVANTAGES: No flux is used; hence there is no danger of flux
entrapment.
Because of visibility of the arc & job, the operator can
exercise better control.
This process can weld in all positions & produce smooth and
sound welds.
It is a very good process for welding non-ferrous
metals.LIMITATIONS: Tungsten if it is transferred to molten weld
pool can contaminate the same.
By chance if the filler rod comes out of the inert gas shield it
can cause weld metal contamination.
Equipment cost is high. 2) SHIELDED METAL ARC WELDING:
SMAW is an arc welding process where in coalescence heating
produces with an electric arc between a covered metal electrode
& the work piece.
Shielding is obtained from decomposition of the electrode
covering. Pressure is not used and the filler metal is obtained
from the electrode.
ADVANTAGES: This is the simplest of the entire arc welding
processes.
The equipment can be portable & the cost is fairly low.
Welding can be carried out in any position with highest weld
capacity.
A big range of metals & their alloy can be welded.
LIMITATIONS: Because of the limited length of each electrode
& brittle flux coating on it, Mechanization is difficult.
In welding long seams as one electrode finishes, the welding has
to be progressed with the next electrode, unless properly cared, a
defect like (slag inclusion or insufficient penetration) may occur
at the place where welding is re started with the new electrode.
Because of flux-coated electrode, the chances of slag entrapment
& other related defects are more.
3) SUBMERGED ARC WELDING: SAW is an arc welding process wherein
heating with an electric set produces coalescence up between a bare
metal electrode and the job. The arc end of the electrode and the
molten pool remain completely hidden & is invisible being
submerged under a blanket of granular flux. The continuously fed
bare metal electrode melts & acts as filler rod. No pressure is
applied for welding purposes. In submerged arc welding instead of a
flux covered electrode granular flux & a bare metal electrode
is used.
ADVANTAGES: Molten flux provides very suitable conditions for
current to flow.
Because of high heat concentration considerably higher welding
speeds can be achieved.
Weld distortion is very less.
LIMITATIONS: Since the operator cannot see the welding being
carried out, he cannot judge accurately the process of welding.
The process is limited to welding in flat position.
Flux is subjected to contamination that may cause welding
porosity.
The process requires edge preparation & accurate fit up on
the joint otherwise flux may spill through he gap and arc may burn
the work piece edge.
4) FLUX CORED ARC WELDING (FCAW):
CO2 WELDING:
FCAW is a process in which coalescence is produced by heating
with an arc between a continuous filler metal (consumable)
electrode and the work. Shielding is obtained from flux contained
within the electrode. Additional shielding may be or may not be
obtained from an externally supplied gas or gas mixture. This
welding process produces high quality of welds & mostly used
for over lay purpose.
ADVANTAGES: High qualities weld metal deposit.
Excellent weld metal appearance smooth, uniform welds.
Relatively high travel speed.
Visible arc- easy to use.
High deposition rate- high current density.LIMITATIONS: FCAW is
used only to weld ferrous metals primarily steels.
The process produces a slag covering, which must be removed.
Flux cored wire is more expensive.
5) GAS METAL ARC WELDING (GMAW):
GMAW which is often called MIG (Metal Inert Gas) welding, is an
arc welding in which coalescence of metals by heating them with an
arc established between a continuous filler metal (consumable)
electrode & the work. Shielding of the electrode, the molten
weld pool, arc, & the adjacent area is obtained from an
externally supplied gas or a mixture of gases through the welding
gun. This process requires the use of direct current reverse
polarity (electrode positive). This type of connection yields a
stable arc, smooth metal transfer, relatively low spatter cost,
& good weld bead characteristics for the entire range of
welding current.
ADVANTAGES: GMAW overcomes the restriction of using an electrode
of limited length, as in SMAW, & overcomes the inability to
weld in various positions, which is a limitation of SAW.
High productivity, wide area of application, ease of use, &
low cost, these all qualities are inherent in GMAW.
All ferrous and non-ferrous materials can be joined using
GMAW.
A visible arc and virtually no slag formation makes it easy for
an operator to place the weld metal where it is needed.LIMITATIONS:
Equipment for GMAW is more complex & consequently is more
costly & less transportable.
In GMAW the welding gun must be close to the work it makes the
process less adaptable than SMAW since it is difficult to reach
areas.6) ELECTRO-SLAG STRIP CLADDING:
Economic factors as rule will not permit fabricating components
from solid high-alloyed material. As a consequence, it is necessary
to surface non-alloyed or low alloy base material with high alloy
cladding. The submerged arc (SAW) and electro slag welding (ESSC)
process is suitable for applying weld deposits over large surface
areas using strip electrodes
. Both the processes use a granular flux material. A strip
electrode, fed continuously, is melted and fused to the substrate.
In width, which are melted by applying current of appropriate
strength, while using an auxiliary magnetic field, which improves
the geometry of the bead.
DESCRIPTION OF THE PROCESS:
Electro-slag strip cladding is the modified version of submerged
arc strip cladding process. The heat generation in the case of ESSC
is due to the current flowing through the electro conductive slag.
Molten weld pool will be visible during welding. No ultraviolet
radiation because of the absence of the arc. Flux is fed from front
side only. Slag crust is automatically removed. Very regular,
finely ripped bead, without any slag adherence.
STRIP CLADDING PROCESS
PROCESS PRINCIPLE: In electro slag strip cladding the heat
required to melt the base metal and the strip electrode is not
generated by an electric arc; the Joule effect is utilized instead.
The current flows through the strip and into a layer of
electrically conductive slag; the resistance of this material
generates the heat and keeps the welding process going (slag
temperature approximately 2300 C).DEFECTS IN WELDSThe significance
of defects is of paramount importance, as it causes damage to
property and loss of human lives. This makes it necessary to study
the various defects in weld joints and analyze their causes.
Defects can occur either in the weld metal or in the heat affected
zones. THE SIGNIFICANCE OF WELDING DEFECTS DEPENDS ON THE
FOLLOWING:
The microstructure in which the defects occurs. The type of
general loading (static, cyclic or shock). The environment
(corrosive or non-corrosive). Section thickness. Type and size of
defects. The stress pattern local to the defect.
Following are the defects in welds:
1. Cracks. 6. Lack and fusion.
2. Distortion. 7. Undercutting.
3. Incomplete penetration. 8. Poor weld bead appearance.
4. Inclusions. 9. Spatter.
5. Porosity and blowholes. 10.Overlapping. 1.Cracks:
Cracks may appear in the base metal, base metal-weld metal
boundary and in the weld metal. They may occur either on weld
surface or under the weld bead.
Reasons:
1. Rigidity of joint, i.e. joint members not free to expand or
contract when subjected to welding heat and subsequent cooling.
2. Poor ductility of base metal.
3. Harden ability, high S and C percentage of base metal.
4. Fast arc travel speed.
5. Concave weld bead.
2. Distortion:
While welding a job, base metal under the arc melts, base metal
ahead gets preheated and the base metal portion already welded
starts cooling. There is a good amount of temperature difference at
various points along the joint at any instant certain areas of the
base metal expand and others including weld bead contract. This
phenomena leads to distortion. Distortion is the change in shape
and difference in the positions of the two plates before welding
and after welding. Reasons:
1. More number of passes.
2. Slow arc travel speed, fast arc travel speed.
3. Large root face, small root gap.
4. Less arc current.
5. Large electrode diameter.
3. Incomplete penetration:
Penetration is the distance from base plate top surface to the
maximum extent of the weld nugget. Reasons:
1. Faster arc travel speed.
2. Wrongly held electrode.
3. Long arc length and large electrode diameter.
5. Small bevel angle.4. Inclusions:
Inclusions may be in the form of slag or any other foreign
material, which does not get a chance to float on the surface of
the solidifying weld metal and thus gets entrapped inside the same.
Inclusions lower the same. Inclusions lower the strength of the
joint and make it weaker.
Reasons:
1. Insufficient chipping and cleaning of previous passes in
multi pass welding.
2. Under-cutting (it can entrap slag).
5. Porosity and blowholes:
Porosity is a group of small voids, whereas blow or gas pocket
is a comparatively bigger isolated hole or cavity. They occur
mainly due to the entrapped gasses. The parent metal melted under
the arc tends to absorb gases like hydrogen, carbon monoxide,
nitrogen and oxygen if they are present around the molten weld
metal pool. These gases spread throughout the molten pool by
diffusion and convention phenomena.
Reasons:1. Improper electrode.
2. Electrode with damp and damage coating.
3. Scale, rust, oil grease, moisture, etc if present on the job
surface, 6. Poor fusion:
Sometimes the molten metal deposited by the electrodes does not
fuse properly with cold metal and the two do not unite properly and
completely. The fusion may be lacking at the root, sides, or
between the runs in the multi-run weld.
Reasons:
1. Faster arc travel speed.
2. Lower arc current.
3. Presence of oxides, rust, scale, and other impurities, which
do not permit the deposited metal to fuse properly with the base
metal.
4. Incorrect electrode manipulation.
5. Incorrect joint preparation and presence of oxides, rust, and
other impurities.7. Under-Cutting:
In undercutting, a groove is formed in the parent metal along
the sides of the weld bead. Groove reduces the thickness of the
plate and thus the area along the bead, which in turn weakens the
weld.
Reasons:
1. Wrong manipulation and inclination of electrode
2. Rusty and scaly job surfaces.8. Poor weld bead
appearance:
A weld bead appearance to be poor if it is not deposited
straight, the bead width varies from place to place, if the spatter
is present, etc. Reasons:
1. Limited practice on the part of the welder.
2. Arc length being not constant.
3. Poor earth and electrode holder (electric) connection.
4. Non-concentric and damaged electrode coating.
9. Spatter:
Spatter are the small metal particles which are thrown out of
the arc during welding and get deposited on the base metal around
the weld bead along its length.
Reasons:
1. Excessive arc current.
2. Longer arcs.
3. Damp electrodes.
4. Electrodes being coated with improper flux ingredients.
5. Arc blow making the arc uncontrolled.
10.Overlapping:
Overlapping is just reverse to undercutting. An overlap occurs
when the molten metal from the electrode flows over the parent
metal surface, and remains there without getting fused and united
with the same.
Reasons:
1. Lower arc current.
2. Slower arc travel speed.
3. Longer arcs.
4. Improper joint geometry (I.e. root gap)
5. Incorrect electrode diameter.
The defects mentioned above can be located by different Non
Destructive tests of welding.
EMBED PBrush
GOVT. POLYTECHNIC MUMBAI 60 OF 85
