The temperature of the arc in SMAW can be over
9000°F (5000°C). With the correct diameter electrode,
the heat created by the arc is enough to melt any
weldable metal. Mild carbon steel melts at slightly
above 2800°F (1540°C).
Shielded metal arc welding is used to construct
buildings, ships, truck chassis, pipelines, and other
weldments. SMAW equipment is fairly inexpensive. It
is widely used on farms and in small repair shops.
SMAW Current and Polarity
In SMAW, electrical current flows across an air
gap between the covered electrode and base metal. An
electric arc is formed as the current flows across the
air gap. The arc creates the heat required for welding.
The electrical current for welding is supplied by
an arc welding machine. Welding machines, also
called power sources, produce two types of current:
direct current (dc) and alternating current (ac).
An electrical current is actually the flow of elec-
trons within a circuit. The direction that the electrons
flow is referred to as the polarity. Electrons flow from
a negatively charged (polarized) body to a positively
charged body. Direct current may flow from the
electrode to the base metal. When the current flows
in this direction, the electrode has negative polarity
and the base metal has positive polarity. It is called
direct current electrode negative (DCEN). Direct current
electrode negative is also known as direct current
straight polarity (DCSP). See Figure 5-2. The current
direction may be reversed to flow from the base metal
to the electrode. When the current flows in this
direction, the base metal has negative polarity, and the
electrode has positive polarity. It is called direct current
electrode positive (DCEP). Direct current electrode
positive is also known as direct current reverse polarity
(DCRP). See Figure 5-3.
The direction selected is determined by the
metal thickness, joint position, and type of electrode
used. The selection of polarity is covered later in
this chapter.
Welding Technology Fundamentals 52
Electrode
wire
Direction
of travel
Shielding
gas
Arc flame
Slag
Base metal
Depth of
penetration
Flux
covering
Welding arc
Weld pool
Figure 5-1. In shielded metal arc welding, slag covers the weld
bead while it cools. Shielding gas is formed as the flux covering
is burned.
Electrode
holder
Electrode
(–)
Arc
gap
Workpiece
(+)
Figure 5-2. A diagram for a direct current electrode negative
(DCEN) arc welding circuit. Notice the current is traveling
from the negative electrode to the positive base metal.
Electrode
holder
Electrode
(+)
Arc
gap
Workpiece
(–)
Figure 5-3. A diagram for a direct current electrode positive
(DCEP) arc welding circuit. In DCEP, the current travels
from the negative base metal to the positive electrode.