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WELDING
DEFINITION OF WELDING
“Welding is the process of joining together two pieces of
metal so that bonding takes place at their original
boundary surfaces”. When two parts to be joined are
melted together, heat or pressure or both is applied and
with or without added metal for formation of metallic
bond.
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TYPES OF WELDING PROCESS
Welding process can also be classified as follows:
1. Gas Welding
2. Arc Welding
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Defects in welding
The lack of training to the operator or careless application of
welding technologies may cause discontinuities in welding.
The Common weld defects include
1. Porosity
2. Lack of fusion
3. Inclusions
4. Cracking
5. Undercut
6. Lamellar tearing
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Porosity
Porosity occurs, when the solidifying weld metal has gases trapped in
it. The presence of porosity in most of the welded joints is due to dirt
on the surface of the metal to be welded or damp consumables. It is
found in the shape of sphere or as elongated pockets. The region of
distribution of the porosity is random and sometimes it is more
concentrated in a certain region. By storing all the consumables in dry
conditions and degreasing and cleaning the surface before welding,
porosity can be avoided.
Lack of Fusion
Due to too little input or too slow traverse of the welding torch, lack
of fusion arises. By increasing the temperature, by properly cleaning
the weld surface before welding and by selecting the appropriate joint
design and electrodes, a better weld can be obtained. On extending the
fusion zone to the thickness of the joints fully, a good quality joint can
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Inclusions
Due to the trapping of the oxides, fluxes and electrode coating
materials in the weld zone the inclusions are occurred.
Inclusions occur while joining thick plates in several runs using
flux cored or flux coated rods and the slag covering a run is not
totally removed after every run and before the next run starts.
By maintaining a clean surface before the run is started,
providing sufficient space for the molten weld metal between
the pieces to be joined, the inclusions can be prevented.
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Cracking
Due to thermal shrinkage, strain at the time of phase change,
cracks may occur in various directions and in various locations
in the weld area. Due to poor design and inappropriate procedure
of joining high residual stresses, cracking is observed. A stagewise pre-heating process and stage-wise slow cooling will
prevent such type of cracks. This can greatly increase the cost of
welded joints. Cracks are classified as hot cracking and hydrogen
induced cracking.
Undercutting
The undercut is caused due to incorrect settings or using
improper procedure. Undercutting can be detected by a naked
eye and the excess penetration can be visually detected.
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Lamellar
Tearing Due to non metallic inclusions, the lamellar
tearing occurs through the thickness direction. This is
more evidently found in rolled plates. As the fusion
boundary is parallel to the rolling plane in T and corner
joints, the lamellar tearing occur. By redesigning the joint
and by buttering the weld area with ductile material, the
lamellar tearing can be minimized.
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Welded joints and their advantages:
A welded joint has following advantages:
(i) Compared to other type of joints, the welded joint has
higher efficiency. An efficiency > 95 % is easily possible.
(ii) Since the added material is minimum, the joint has lighter
weight.
(iii) Welded joints have smooth appearances.
(iv) Due to flexibility in the welding procedure, alteration and
addition are possible.
(v) It is less expensive.
(vi) Forming a joint in difficult locations is possible through
welding.
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Types of Welded Joints
• Butt joint
• Corner joint
• Edge joint
• Lap joint
• T joint
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Square Butt Joints
• Used to butt weld light sheet metal.
• 1/16 to 3/16 thick metal.
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Butt Joints
• Butt joints: formed by placing the plates edge to edge and
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•
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•
welding them. Grooves are sometimes cut (for thick plates)
on the edges before welding. According to the shape of the
grooves, the butt joints may be of different types, e.g.,
􀂾 Square butt joint
􀂾 Single V-butt joint, double V-butt joint
􀂾 Single U-butt joint, double U-butt joint
􀂾 Single J-butt joint, double J-butt joint
􀂾 Single bevel-butt joint, double bevel butt joint
• Version
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Butt Joints
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Bevel Butt Joints
• Used to butt weld heavier pieces of metal together.
• 3/8 to ½ inch metal can welded using a single V or U joint.
• ½ Inch metal and up can be welded using a double V or U
joint.
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Butt Joints
• .
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Lap or fillet joint
• obtained by overlapping the plates and
welding their edges. The fillet joints may
be single transverse fillet, double
transverse fillet or parallel fillet joints
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Lap or fillet joint
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Lap Joints
• Used to join two overlapping pieces of
metal.
• Single lap joint welded from one side.
• Single lap joint welded from two sides
develops full strength.
• Off set lap joint is used when two pieces of
metal need to be joined in the same plain.
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Lap Joints Cont.
• A- single lap joint, one
weld.
• B- single lap joint, two
welds.
• C- offset lap joint.
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OTHER TYPES OF WELDING
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Corner Joints
• Used to join to pieces of metal that are
approximately right angles to each other.
• Closed corner joint is used on light sheet
metal were strength is not a required at the
joint.
• Half open corner joint is used on heavier
metal when welding can only be done on
one side. Used when load is not severe.
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Corner Cont.
• Open corner joint is used on heavy material.
It is the strongest of the corner joints.
• Corner joints on heavy material are welded
on both sides. The outside first them
reinforced on the inside.
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Corners Cont.
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Edge Joints
• Used to join two parallel or nearly parallel
pieces of metal. Not very strong.
• Used mainly to join edges of sheet metal,
reinforce flanges of I beams, and mufflers.
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Tee Joints
• Used to join two pieces of metal that are
approximately 90 degrees to each other, but
the surface of one piece of metal is not in
the same plain as the other metal.
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Tee Joints Cont.
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•
A- plain tee
B- single beveled
C- double beveled
D- single J
E- double J
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Fillet Welds
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Groove Welds
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Basic weld types and their symbols Basic
weld types and their symbols
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