778 Agricultural Mechanics and Technology Systems Copyright Goodheart-Willcox Co., Inc.
T
he shielded metal arc welding (SMAW) process, also known as stick
welding, involves using a consumable electrode, or metal rod, coated in a
fl ux to form the welded bead. Electricity from a welding machine creates
an electric arc between the electrode and the base metals. Welding current
is the fl ow of electricity that results in the welding arc. The amount of
heat generated by the arc is directly proportional to the amount of current
produced by the power source. When the welder establishes the arc, the
base metal and the electrode melt to form the weld pool, Figure 29-1. As
the molten pool cools, it resolidifi es to form a bonded joint. The fl ux coating
on the electrode chemically reacts to the heat of the arc, using up the oxygen
in the vicinity of the weld to create an oxygen-free shield of gas around the
molten pool. Residue from the fl ux forms a layer of slag that protects the
weld from atmospheric contamination as it cools. See Figure 29-2.
Development of
Electric Arc Welding
The art of blacksmithing was developed during the
Middle Ages. Blacksmiths, through forging processes,
formed and welded items of iron by hammering red-
hot pieces together until they bonded, Figure 29-3.
Developed near the close of the nineteenth century,
carbon arc welding (CAW) was the fi rst arc welding
process. In this process, an arc is created between a
copper-clad carbon electrode and the base metal. The
intense heat of the arc melts the surfaces of the base
metal to be joined. A separate fi ller rod, which carries
no electrical current, is often added to the weld, similar
to the way fi ller metal is added in the oxyacetylene
welding process.
The fi rst US patent for an arc welding
process using a metal electrode was awarded
in 1890 to C. L. Coffi n of Detroit, who used
an uncoated, electrically conductive wire
that melted to become part of the weld. The
fi rst coated metal electrodes, introduced in
about 1900, had a thin coating of clay or
lime, providing a more stable arc. During
the 1920s, electrode coatings were developed
that not only stabilized the arc but also
improved the physical characteristics of the
completed weld joint.
The shielded metal arc welding process
is best suited to bonding ferrous (iron-
containing) metals, including mild steel,
Goodheart-Willcox Publisher
Figure 29-1. Welders use electric energy from a
welding machine to form an electric arc between
the electrode and the base metal.
Filler metal
Electrode
Flux
Weld pool
Molten
droplets
Oxygen-free
shield
Slag
Bead
Arc
Base metal
Goodheart-Willcox Publisher
Figure 29-2. The SMAW process.