Welded Connections
Dr John Zhang
Welding process
• Structural welding is a process by which
the parts that are to be connected are
heated and fused, with supplementary
molten metal at the joint.
• A relatively small depth of material will
become molten, and upon cooling, the
structural steel and weld metal will act as
one continuous part where they are joined.
Welding processes
• Metal arc-welding (MMA or SMAW) with
covered electrodes
• MAG (metal active gas) welding
• MIG (metal inert gas) welding or
• flux-cored arc welding
• Submerged arc welding
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In the shielded metal arc welding (SMAW) process, current arcs
across a gap between the electrode and the base metal, heating the
connected parts and depositing part of the electrode into the molten
base metal.
A special coating on the electrode vaporizes and forms a protective
gaseous shield, preventing the molten weld metal from oxidizing
before it solidifies.
The electrode is moved across the joint, and a weld bead is
deposited, its size depending on the rate of travel of the electrode.
As the weld cools, impurities rise to the surface, forming a coating
called slag that must be removed before the member is painted or
another pass is made with the electrode.
Shielded metal arc welding is usually done manually and is the
process universally used for field welds.
• For shop welding, an automatic or semi
automatic process is usually used. Foremost
among these is the submerged arc welding
(SAW),
• In this process, the end of the electrode and the
arc are submerged in a granular flux that melts
and forms a gaseous shield. There is more
penetration into the base metal than with
shielded metal arc welding, and higher strength
results.
• Other commonly used processes for shop
welding are gas shielded metal arc, flux
cored arc, and electro-slag welding.
MAW (SMAW)
Fillet weld
Fillet weld
MAG (MIG-Metal inert gas)
Choice of welding process
Fusion of metal
Type of joints
Types of welds
• Fillet weld
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–
–
–
Simple preparation
Low skill
Simple equipment
-low cost-especially
single run FW
• Butt weld
– More preparation work
– More skill
Example of welded connection
Fillet weld
Throat = a x cos45o
= 0.707 a
a
a
L
Failure Plane
• Lap joint, in which the
parts welded are in
parallel planes,
• Tee or cruciform joint, in
which the parts welded
are more or less
perpendicular to one
another,
• Corner joint, in which the
parts are also more or
less perpendicular to one
another,
Shape of FW
Fillet Weld
• Size of the fillet weld is specified by the leg
length tw
• Minimum size of weld is around 40% of the
thickness of the plate being welded
• Maximum size is
– same thickness as plate for plates < 6mm
– 1mm less than plate thickness for plates > 6mm
• Plug and slot welds, fillet weld around the
circumference of a hole.
Fillet weld
Fillet Weld
Details
and
Defects
Butt weld
• Used as full penetration butt weld.
• Where fusion exists between the weld
and the parent metal throughout the full
depth of the joint.
• Full thickness of parent metal being
welded
• The effective length is full length of the
continuous full size weld.
Butt weld
• full penetration butt weld in which there is
a complete penetration and fusion of weld
and parent metal throughout the thickness
of the joint.
• partial penetration butt weld, in which
there is a weld penetration less than the
full thickness of the joint.
Butt weld
Full butt weld illustrations
Edge preparation in butt weld
Butt weld Details and Defects
Butt Weld v’s Fillet Weld
• Butt weld requires edge preparation
• The volume in a Butt weld depends o the
type of preparation and the depth of
penetration.
• Fabricators equipment
Plug and Slot Welds
Spot Welds
DESCRIPTION OF WELDS DEFINITIONS
• The parent metal or the base metal is the
steel material which is welded.
• The filler metal or the weld metal is the
electrode material used in the welding
process.
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•
•
•
The root is the point of the joint line
reached by the penetration of the
filler metal.
The face is the outside surface of
the weld.
The toe is the line on the weld face
between the filler metal and the
parent metal.
The heat affected zone is a part of
the parent metal which has not
been melted with the filler metal,
but which undergoes fast heating
and cooling during the passage of
the welding arc. In this zone, the
parent metal is subject to a
hardening treatment and can
consequently become brittle.
• The throat thickness, a, is the
minimum distance from the
root to the weld face ignoring
the convex zone.
• The length is the longitudinal
weld dimension in the direction
of the welding arc
displacement.
Weldability
• The parent metal must have the
weldability properties defined by the steel
qualities of each steel grade. The
weldability of a steel depends on its
chemical composition and metallurgical
characteristics
Welding consumables
• Welding consumables shall be appropriate to the
welding process used, the parent metal welded and the
welding procedure adopted.
• Welding consumables must be stored and handled with
care and in conformity with the recommendations of the
manufacturer.
• Electrodes for manual arc welding shall be stored in their
original containers in a dry and warm place, efficiently
protected from adverse effects of weather.
• Flux shall be stored and transported in containers which
protect it from absorbing moisture.
Welding strength
Welding
positions
Example of
welding
positions
Welding with multiple runs
• The welding
conditions,
particularly the
current limitation of
the welding
equipment, constitute
a limit to the depth of
penetration into the
parent plate
Butt weld edge preparation
Weld quality
• Quality control of welded connections is
particularly difficult, because defects below
the surface, or even minor flaws at the
surface, will escape visual detection.
Welders must be properly certified, and for
critical work, special inspection techniques
such as radiography or ultrasonic testing
must be used.
Code of practice
• General, shall comply with
AS1554.1,AS1554.2 or AS1554.5
• Weld Quality
– SP, structural purpose
– GP, general purpose
Inspection of Welding
• Common internal defects in steel welds are
– Gas porosity
– Lack of fusion
– Slag inclusion
– Cracking in the weld or parent metal
• Ways of inspection
– Visual inspection
– Non destructive testing
Weld defect
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•
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Undercut. The thickness of the
parent metal is reduced near
the weld toe.
Porosity or gas inclusions. Air
or gas bubbles are
incorporated in the melted
metal, where they remain after
cooling.
Insufficient throat. The throat
thickness is smaller than the
design thickness. The
resistance of the joint might be
insufficient.
Incomplete penetration. The
throat thickness is smaller than
the design thickness. The
resistance of the joint might be
insufficient.
Non destructive testing
• X-ray and gamma ray photography, for
pressure vessels
• Ultrasonic testing
• Magnetic particle testing
• Dye penetration testing
• Disadvantage is that it disrupts flow of
work and is usually very expensive
Weld strength and behaviour
Stresses in
FW
Transfer of
tension/compress
ion
Tension member to
gusset plates
Transfer of shear