5 Direct
Current Welding
In direct
current welding, the welding current circuit may be hooked up as either ”straight-polarity”
or
”reverse-polarity.”
The machine connection for direct current straight-polarity (DCSP) welding is
electrode
negative
and work positive. In other words, the electrons flow from the electrode to
the plate or workpiece,
as shown
in Figure l-l. For direct current reverse-polarity welding (DCRP), the connections
are just the
opposite;
electrons flow from the plate to the electrode, as shown in Figure 1-2.
DCRP
is often used momentarily to prepare (ball the end) a tungsten electrode for
AC welding. The balling
of the
electrode should be done over a separate copper block to avoid contamination
of the weld joint.
In straight-polarity
welding, the electrons exert a considerable heating effect on the plate. In
reverse-polarity
welding,
just the opposite occurs; the electrode acquires this extra heat which then
tends to melt off the end
of the
electrode. Thus, for any given welding current, DCRP requires a larger diameter
electrode than DCSP
does.
For example, a 1/16 in. diameter pure tungsten electrode can handle 125 amperes
of welding current
under
straight polarity conditions. If the polarity were reversed, however, this amount
of current would melt
off
the electrode and contaminate the weld metal. Hence a 1/4-in. diameter pure
tungsten electrode is
required
to handle 125 amperes DCRP satisfactorily and safely.
