792 Agricultural Mechanics and Technology Systems
Copyright Goodheart-Willcox Co., Inc.
by power companies in the United States. Modern SMAW welding power
supplies provide constant current (CC) that can be set to either AC or DC,
depending on the choice of electrode. DC welding currents are designated
as direct current electrode negative (DCEN) or direct current electrode
positive (DCEP). The smoothest welding characteristics are obtained by
using the correct polarity for the chosen electrode.
In a welding circuit, current fl ows from the negative terminal of the
welding machine, through the arc, and back to the positive terminal of the
welding machine. The useful effect of this is that more of the heat follows the
direction of current fl ow—about 30% of the heat is distributed to the positive
side of the circuit and the other 70% is released on the negative side. Under
most circumstances, DCEP results in deeper penetration. DCEN results in
faster melting of the electrode and yields a faster deposition rate. Different
chemicals in the fl ux covering generate exceptions to this rule for some
electrodes.
Electrode Selection
Choosing the proper electrode type and size for the job greatly affects the
quality of the fi nished joint. There are hundreds of electrodes to choose from.
For most agricultural needs, however, a collection of a half dozen is adequate.
In fact, for most agricultural repair and construction needs, a stock of E6011s
and E7018s is suffi cient.
The fi rst step in choosing an electrode is to
determine the composition of the base metal. The
electrode type must be matched to the base metal
composition to ensure a strong weld. Begin by
considering the source of the metal. Assess the
appearance of metals from questionable sources.
If the base metal is magnetic, it is likely a carbon
steel or alloy steel. Standard electrodes, such as E6011
or E6013 with a mild steel core, will work with these
base metals. If the base metal has a coarse and grainy
internal surface, it is likely a cast metal requiring a
specialty electrode. If the base metal is not magnetic,
the material could be stainless steel, manganese steel,
or a nonferrous alloy such as titanium. A chisel bites
into softer metals, such as mild steel or aluminum, and
bounces off or chips against harder metals, such as high-
carbon steel, chrome-molybdenum alloy, or cast iron. A
spark test with a grinder may give some indication as
to the type of steel. More fl are in the sparks indicates a
higher carbon content in steel, Figure 29-23.
High-carbon steels require low-hydrogen electrodes. These electrodes
prevent the formation of oxides in the weld and limit the formation of
hydrogen compounds in the steel that lead to cracking in the completed
bond. Low-hydrogen electrodes are recommended for steels in which
cracking can be a problem due to low alloy, high carbon, or high sulfur
Nagy-Bagoly Arpad/Shutterstock.com
Figure 29-23. Spark-testing steel to determine its
relative carbon content.