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What You Should Know about SMAW
In shielded arc welding, the intense heat from the electric
an electric arc is used to melt and fuse metals to form a weld. It’s one of the
the oldest and most widespread welding procedures. Although used
mainly for joining iron and mild steel, shielded metal arc
welding is well suited to maintenance tasks because
the equipment is relatively cheap, easy to operate and can
used for welding various types of metals.
Below you will find explanations that describe the protected
metal arc welding process and how welding machines and
plugins are installed and used. You will also find information
when choosing an electrode. Personal protective equipment i
precautions are also described.
How the process works
Typical SMAW equipment consists of an electric welding machine,
two welding cables, grounding clamp, electrode holder ia
covered metal electrode. Electric current from welding
the machine is used to create an electric arc between the tip
electrode and work.
Welding starts by touching the end of the electrode
base metal, then raise the electrode about ¼ inch. This forms
arc, which produces temperatures up to 5550°C. Intensively
the heat in the arc area instantly melts the base metal and begins
burn the covering off the electrode and melt the core.
The molten core becomes the filler metal for welding and
the breakdown of the flux creates a protective gaseous atmosphere
around the port area. The gas forms a shield against contamination
from oxygen and nitrogen in the surrounding air. Additional
protection is provided by the current of the electrode, which forms a
a deposit called slag.
The shielding gas is ionized, conducts electricity and
maintains the stability of the arch.
Welding voltage and current
Direct current (DC) or alternating current (AC) is used.
Arc voltage or operating voltage is the voltage present in the
welding circuit while the arc is lit and welding is performed.
The arc voltage ranges from 15V to 40V depending on the arc
lenght.
Open circuit voltage is the voltage generated by welding
machine when not welding. The open circuit voltages are
normally set between 50V and 100V, but drops to arc voltage
level when the arc is ignited and welding begins.
Arc length
In every electrical circuit there is a correlation between
voltage, current and resistance. The best results are
it is usually obtained with an arc length of about the diameter
electrode.
When the arc length increases, less current flow occurs
due to increased resistance. The result is a cooler onion
and greater tendency to splash. It will be less
weld penetration, increased exposure to oxidation and
contamination and irregular, unstable arc.
When the arc length decreases, the lower the resistance, the higher the current
flows with a lower voltage and the arc becomes hotter. With thin
material, heat can melt the weld hole, porosity and
trimming the adjacent base metal.
Polarity
For DC machines this is important. When the electrode
negative, and the workpiece is positive, this is called flat
Polarity. The opposite of this is reverse polarity.
DCSP or direct current polarity is characterized
faster melting of the electrode, where the welded vessel is wide and
penetration into the base metal is relatively shallow. This is
it is used when there are high welding speeds and high deposition rates
required.
DCRP or reverse direct current polarity results in a hotter arc,
creating a deeper, narrower welded vessel. This is used for structural
welding, multi-pass welding and applications requiring deep
penetrations.
Most electrodes are designed for use with only one polarity.
Energy sources
Most AC power supplies contain a step-down transformer
voltage to the level required for welding (usually less than
100 V)
Welding machines
Many types and sizes of welding machines are used for protection
metal arc welding. Current of the constant current type
does not change significantly in arc length variations. This is
preferred for manual welding operations.
The constant potential welding machine adjusts
current according to arc length. If used in the manual
operation, will produce unavoidable variations in arc length
large current fluctuations, resulting in unstable,
uneven arc. For automatic operations, constant electrode
feed rate establishes a stable arc and uniform arc length.
The current rating of the machine is its maximum current output.
A rated current of 400 amps means that the machine can deliver up to
400 amperes of welding current.
The rating of the working cycle of the machine is safe operation
capacity for non-stop welding. This is expressed as a
percentage during the 10-minute period during which the machine can
delivers its rated maximum welding current output without damage
or overheating. An 80 percent 400 amp machine is one that can
deliver 400 amps of welding current for a total of 8 minutes
out of every 10, and must be idle for at least 2 minutes out of every 10
for cooling.
Tools and accessories
Tools for SMAW are: cables, holders, clamps, breaker hammer,
wire brush. Protective equipment is a face shield, glasses,
apron, gloves, shoes, long-sleeved shirt. Protective
equipment is to avoid eye injuries and burns. There is also an i
it is necessary to prevent electric shocks, especially in wet areas.
Electrode selection
Electrodes are classified according to core material: mild steel,
high carbon steel, special alloy steel, cast iron and
non-ferrous. Most often, mild steel electrodes are used.
In general, the electrode core material is equally matched
which is possible with the composition of the base metal. Electrode
the size varies depending on the thickness of the base metal.
Size and characteristics of the selected electrode
determine the arc current settings on the welding machine.
Normally, the recommended amperage range for the electrode
provided by the manufacturer.
Installation and operation
Observe fire precautions before starting welding. Not
flammable should be near the work area.
While the electricity is still out; attach the ground wire to
the workpiece and the electrode on the holder. Turn it on
power. Place the end of the electrode to ignite the arc
about 1 inch above the initial weld point. Put your face down
shield and touch with a quick tap or scratching motion
metal-based electrode. Right after
contact, lift the tip of the electrode slightly to make contact
an arc whose length is approximately equal to the electrode
diameter.
If you don’t lift the electrode fast enough, it will stick
to work. Twist or bend the electrode to release it. If
the electrode does not release, release it quickly
bracket. With a little practice, you can learn to fire a bow
without gluing the electrode.
When the current settings and arc length are correct
if maintained, a continuous popping sound is heard during welding.
A humming sound indicates that the arc length is too long or
the current is too high. Bows that are too short create a snapping sound
and may flash on and off, indicating that the electrode has stuck
and a short circuit to the base metal.
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