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What You Should Know about SMAW!
In shielded metal-arc welding, the intense heat from an
electric arc is used to melt and fuse metals to form a weld. It
is one of the oldest and most widely used welding processes.
Although used chiefly for joining iron and mild steels, shielded
metal-arc welding is well suited to maintenance tasks because
the equipment is relatively inexpensive, simple to operate, and
can be used for welding many different kinds of metals.
Below, you will find explanations describing the shielded
metal-arc welding process and how the welding machines and
accessories are set up and used. You will also find information
on selecting an electrode. The personal safety equipment and
precautions are also described.
How the Process Works
A typical SMAW outfit consists of an electric welding machine,
two welding cables, a ground clamp, an electrode holder, and a
covered metal electrode. Electric current from the welding
machine is used to form an electric arc between the tip of the
electrode and the work.
Welding is started by touching the end of the electrode to the
base metal, then lifting the electrode about inch. This forms
the arc, which produces temperatures up to 5550C. The intense
heat at the arc area instantly melts the base metal and begins
to burn the covering off the electrode and melt the core.
The melted core becomes filler metal for the weld and the
decomposition of the flux forms a protective gaseous atmosphere
around the arc area. The gas forms a shield against
contamination from oxygen and nitrogen in the surrounding air.
Additional shielding is provided by the electrode flux, which
forms a deposit called slag.
The shielding gas is ionized, and conducts electricity and
maintains the stability of the arc.
Welding Voltage and Current
Either direct current (DC) or alternating current (AC) is used.
The arc voltage or working voltage is the voltage present in the
welding circuit while an arc is struck and welding is being
done. The arc voltage ranges from 15V to 40V depending on the
arc length.
The open circuit voltage is the voltage generated by the welding
machine when no welding is being done. Open-circuit voltages are
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level when an arc is struck and welding begin.
Arc Length
In any electrical circuit, there is a correlation between the
voltage, current and the resistance. The best results are
normally obtained with an arc length about the diameter of the
electrode.
When the arc length is increased, less current flow occurs
because of the increased resistance. The result is a cooler arc
and a greater tendency to spatter. There will be less
penetration of the weld, increased exposure to oxidation and
contamination, and an erratic, unstable arc.
When the arc length is reduced, less resistance more current
flows with less voltage and the arc becomes hotter. With thin
material, the heat can melt a hole in the welding, porosity, and
undercutting of the adjacent base metal.
Polarity
For DC machines, this is important. When the electrode is
negative and the work piece is positive, this is called Straight
Polarity. The opposite of this is Reverse Polarity.
DCSP or direct current straight polarity is characterized by
faster melting of the electrode, the weld puddle being broad and
penetration into the base metal is relatively shallow. This is
used when fast welding speeds and high deposition rates are
required.
DCRP or direct current reversed polarity results in a hotter
arc, making deeper, narrower weld puddle. This is used for
structural welding, multi pass welds, and applications requiring
deep penetrations.
Most electrodes are designed to be used with only one polarity.
Power Sources
Most AC power sources contain a transformer that steps down line
voltage to the level required for welding (normally less than
100V)
Welding Machines
Many types and sizes of welding machines are used for shielded
metal-arc welding. The current of a Constant-Current type does
not change significantly in variations in arc length. This is
preferred for manual welding operations.
The Constant-Potential type of welding machine adjusts the
current according to the arc length. If used in a manual
operation, the unavoidable variations in arc length will produce
large fluctuations in the current, resulting in an unstable,
non-uniform arc. For automatic operations, the constant
electrode feed rate establishes a stable arc and uniform arc
length.
The current rating of the machine is its maximum current output.
A current rating of 400 amp means the machine can deliver up to
400 amps of welding current.
The duty cycle rating of the machine is the safe operating
capacity for non-stop welding. This is expressed as a percentage
over a 10-minute period over which a machine can deliver its
rated maximum welding current output without damage or
overheating. An 80 percent 400-amp machine is one, which can
deliver 400 amps of welding current for a total of 8 minutes out
of every 10, and must idle at least, 2 minutes out of every 10
for cooling.
Tools and Accessories
The tools for SMAW are: cables, holders, clamps, chipping
hammer, wire brush. The protective equipment are face shield,
goggles, apron, gloves, shoes, long sleeved shirt. The
protective equipment is to avoid eye injuries, and burns. Also
there is a need to prevent electric shocks, especially in wet
areas.
Selecting an Electrode
Electrodes are classified by their core material: mild steel,
high-carbon steel, special alloy steel, cast iron, and
non-ferrous. Mild steel electrodes are the most commonly used.
In general, the electrode core material is matched as closely as
possible with the composition of the base metal. Electrode size
varies with the thickness of the base metal.
The size and characteristics of the selected electrode determine
the arc current settings on the welding machine. Normally, the
range of recommended amperage for the electrode is given by the
manufacturer.
Set up and Operation
Observe fire precautions before starting to weld. No combustible
should be near the work area.
With the electrical power still off; clamp the earth wire to the
work piece and the electrode on the holder. Turn on the power.
To strike an arc, position the end of the electrode about 1 inch
above the weld start point. Lower your face shield and with a
rapid tapping of scratching motion, touch the electrode to the
base of the metal. Immediately after the contact, raise the end
of the electrode slightly to establish an arc whose length is
approximately equal to the electrode diameter.
If you do not raise the electrode fast enough, it will stick to
the work. Twist or bend the electrode to break it free. If the
electrode does not break free, quickly release it from the
holder. With a little practice, you can learn to strike an arc
without the electrode sticking.
When the current settings and a proper arc length is maintained,
a continuous cracking sound is heard while welding. A humming
sound indicates that the arc length is too long or the current
is too high. Arcs too short make a popping sound and may flash
on and off, indicating the electrode is sticking and short
circuiting to the base metal.
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