Chapter 29 Shielded Metal Arc Welding 783
Copyright Goodheart-Willcox Co., Inc.
Other parts of the fl ux are mixed into the weld
pool. Some of these parts work within the molten pool,
like soap in water, to fl oat out impurities and bind
them to the waste that forms on top of the bead. This
waste is called slag. The slag covering prevents the
welded bead from cooling too quickly, which would
result in brittleness. Slag is easily removed from a
properly executed weld after cooling, Figure 29-9.
Some electrode types have iron powder and
alloying elements included in the fl ux to improve the
physical properties of the metal in the completed weld.
Most steel electrodes have the same mild steel fi ller wire
in their core but differ in the composition of the fl ux.
The heat of the electric arc causes the fi ller metal
to melt faster than the fl ux can burn. This results in
a cupped formation of the fl ux around the arc as it
extends from the end of the electrode, Figure 29-10. The
insulating cup has a stabilizing effect on the arc and
helps to keep the arc directed at the work.
Electrode Classifi cations
Electrodes are classifi ed under a numbering system
developed by the American Welding Society (AWS).
In accordance with AWS Specifi cation A5.1, codes are
stamped on all covered electrodes or their packaging to
indicate the electrode type. See Figure 29-11.
Each code begins with an E for electrode. The
fi rst two digits of a 4-digit number and the fi rst three digits of a 5-digit
number indicate minimum tensile strength of the completed weld in ksi
(thousands of psi). The next-to-last digit indicates the welding position for
which the electrode was designed. (Welding positions are explained later
in this chapter.) A 1 (EXX1X) indicates an all-position
electrode. An EXX2X electrode indicates horizontal and
fl at positions only, an EXX3X electrode is fl at position
only, and an EXX4X electrode can be used in the fl at,
overhead, horizontal, vertical-down positions. The last
digit, together with the next-to-last digit, signifi es the
type of covering and current to be used.
An E6010 electrode, for example, has 60,000 psi
tensile strength, may be used to weld in all positions,
and works best with DC electrode positive current to
provide deep penetration and a thin slag covering.
An E7024 has 70,000 psi tensile strength and can
only be used in the fl at and horizontal positions. This
electrode type works with any polarity to produce a
soft arc, producing light penetration welds with a high
deposition rate of fi ller material.
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Figure 29-9. Slag easily separates from a cooling
bead due to a difference in rates of contraction
between the bead metal and the slag.
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Figure 29-10. Filler metal melts faster than the flux
on the electrode, creating a cupped effect at the
electrode tip. The slag formation on the core must
be chipped away to restart an arc.