ME452
Welding Technologies
Fall (2020-2021)
Lecture 5
Instructor : Asst. Prof. Dr. Samet AKAR
Department of Mechanical Engineering
Çankaya University
In a nutshell
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Magnetic Arc Blow in Welding
Magnetic arc blow, also known as arc wander is the arc deflection
caused by distortion of the magnetic field produced by an electric
arc current.
Arc blow occurs in DC arc welding when the arc stream does not
follow the shortest path between the electrode and the workpiece
and is deflected forward or backward from the direction of travel
or, less frequently, to one side.
The effect of welding past the current return
cable which is bolted to the center of the plate
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Magnetic Arc Blow in Welding
Magnetic arc blow is caused by an unbalanced condition in the
magnetic field surrounding the arc. This unbalanced condition
results from the fact that at most times, the arc will be farther from
one end of the joint than another and will be at varying distances
from the workpiece connection.
Arc blow can cause a number of welding problems, including
excessive spatter, incomplete fusion, porosity and lower quality.
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Magnetic Arc Blow in Welding
The concentric flux fields will remain circular when they can stay in one
medium expansive enough to contain them until they diminish to essentially
nothing . But if the medium changes (such as from steel plate to air), the
circular lines of force are distorted and tend to concentrate in the steel where
they encounter less resistance. At a boundary between the edges of a steel
plate and air, there is a squeezing of the magnetic flux lines, causing
deformation in the circular lines of force. This squeezing can result in a heavy
concentration of flux behind or ahead of a welding arc. The arc then tends to
move in the direction that would relieve the squeezing and restore the
magnetic field balance. It veers away from the side of magnetic flux
concentration. This veering is observed as arc blow.
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Magnetic Arc Blow in Welding
Another "squeezing" phenomenon results from the current
returning back towards the workpiece connection within the
workpiece. As shown in the Figure, a magnetic flux is also set up
by the electrical current passing through the workpiece to the
workpiece lead. The movement of the arc because of this effect
will combine with the movement resulting from the concentration
previously described to give the observed arc blow.
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Magnetic Arc Blow in Welding
The effect of the returning current may diminish or increase the
arc blow caused by the magnetic flux of the arc. In fact, control of
the direction of the returning current is one way to control arc
blow, especially useful with automatic welding processes.
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Magnetic Arc Blow in Welding
Magnetically Susceptible Materials
Some materials, such as 9%nickel steels, have very high
magnetic permeability and are very easily magnetized by external
magnetic fields, such as those from power lines, etc. These
materials can be very difficult to weld due to the arc blow
produced by the magnetic fields in the material. Such fields are
easily detected and measured by inexpensive hand - held Gauss
meters. Fields higher than 20 Gauss are usually enough to cause
welding problems.
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Magnetic Arc Blow in Welding
High Currents
There is less arc blow with low current than with high. Why? Because
the intensity of the magnetic field a given distance from the conductor
of electric current is proportional to the square of the welding current.
Usually, serious arc blow problems do not occur when stick electrode
welding with DC up to approximately 250 amps (but this is not an
exact parameter since joint fit up and geometry could have major
influence.)
DC Currents
The use of AC current markedly reduces arc blow. The rapid reversal
of the current induces eddy currents in the base metal, and the fields
set up by the eddy currents greatly reduce the strength of the
magnetic fields that cause arc blow.
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Thermal Arc Blow
Another type of arc deflection is thermal arc blow. The physics of the
electric arc require a hot spot on both the electrode and plate to
maintain a continuous flow of current in the arc stream. As the
electrode is advanced along the work, the arc will tend to lag behind.
This natural lag of the arc is caused by the reluctance of the arc to
move to the colder plate. The space between the end of the electrode
and the hot surface of the molten crater is ionized and, therefore, is a
more conductive path than from the electrode to the colder plate. When
the welding is done manually, the small amount of "thermal back blow"
due to the arc lag is not detrimental, but it may become a problem with
the higher speeds of automatic welding or when the thermal back blow
is added to magnetic back blow.
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How to reduce arc blow
o If DC current is being used with the shielded metal-arc process -
especially at rates above 250 amps - a change to AC current may
eliminate problems
o Hold as short an arc as possible
o Reduce the welding current
o Angle the electrode with the work opposite the direction of arc blow
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How to reduce arc blow
o Weld toward a heavy tack or toward a weld already made
o Use a back-step welding technique, as shown
o Wrap the work cable around the workpiece so that the current
returning to the power supply passes through it in such a
direction that the magnetic field set up will tend to neutralize
the magnetic field causing the arc blow
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ELECTROSLAG WELDING (ESW)
Electroslag welding (ESW) is a
process that melts and joins
metals by heating them with a
pool of molten slag held
between the metals and
continuously feeding a filler wire
electrode into it.
The weld pool is covered with
molten slag and moves upward
as welding progresses.
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ELECTROSLAG WELDING (ESW)
A pair of water-cooled copper shoes, one in the front of the workpiece and one
behind it, keeps the weld pool and the molten slag from breaking out. Similar
to SAW, the molten slag in ESW protects the weld metal from air and refines it.
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ELECTROSLAG WELDING (ESW)
Strictly speaking, however, ESW is not an arc welding process,
because the arc exists only during the initiation period of the process,
that is, when the arc heats up the flux and melts it. The arc is then
extinguished, and the resistance heating generated by the electric
current passing through the slag keeps it molten. In order to make
heating more uniform, the electrode is often oscillated, especially when
welding thicker sections.
Transverse cross section of electroslag weld in 70-mm-thick steel.
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Advantages and Disadvantages
o Electroslag welding can have extremely high deposition rates,
but only one single pass is required no matter how thick the
workpiece is.
o Unlike SAW or other arc welding processes, there is no
angular distortion in ESW because the weld is symmetrical
with respect to its axis.
o However, the heat input is very high and the weld quality can
be rather poor, including low toughness caused by the coarse
grains in the fusion zone and the heat-affected zone. Generally
post weld heat treatment (PWHT) operations are needed.
o Electroslag welding is restricted to vertical position welding
because of the very large pools of the molten metal and slag.
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Submerged Arc Welding (SAW)
Submerged arc welding (SAW) is a process that melts and joins
metals by heating them with an arc established between a
consumable wire electrode and the metals, with the arc being
shielded by a molten slag and granular flux.
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Submerged Arc Welding (SAW)
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Submerged Arc Welding (SAW)
Do we need a shielding gas in this process?
o No shielding gas is needed because the molten metal is
separated from the air by the molten slag and granular flux.
The arc is submerged and thus invisible. As a result, the process
is relatively free of the intense radiation of heat and light typical of
most open arc welding processes.
The efficiency of transfer of energy from the electrode source to
the workpiece is very high (usually over 90%), since losses from
radiation, convection, and spatter are minimal.
Direct-current electrode positive is most often used. However, at
very high welding currents (e.g., above 900A) AC is preferred in
order to minimize arc blow.
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Advantages and Disadvantages
o The protecting and refining action of the slag helps produce clean
welds in SAW.
o Since the arc is submerged, spatter and heat losses to the
surrounding air are eliminated even at high welding currents.
o Both alloying elements and metal powders can be added to the
granular flux to control the weld metal composition and increase the
deposition rate, respectively.
o Using two or more electrodes in tandem further increases the
deposition rate. Because of its high deposition rate, workpieces
much thicker than that in GTAW and GMAW can be welded by
SAW.
o However, the relatively large volumes of molten slag and metal pool
often limit SAW to flat-position welding and circumferential welding
(of pipes). The relatively high heat input can reduce the weld quality
and increase distortions.
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Deposition rate in arc welding processes
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