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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 ¼ inch. This forms
the arc, which produces temperatures up to 5550°C. 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
normally set between 50V and 100V, but drops to the arc voltage
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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