17 Graph
B presents the metal deposition rate
versus AC amps for several wire sizes. The same deviations
when using DC power, changing wire extension,
and different fluxes mentioned for Graph A will also
hold. Weld penetration
is also affected by the polarity of the DC arc. DCEN provides from 15 to 25 percent
less penetration
than DCEP. This can be used effectively for handling poorly fitted joints. DCEN
arcs are, however,
less stable than DCEP power and weld edges may become irregular.
The use of rusted welding wire should
be avoided; rusted wire makes poor contact with the contact tip or jaw
resulting in irregular current flow
and arcing. It also wears the jaws excessively. Wire
extension is the distance between the contact tip and plate. Generally this is
set at approximately 8 times
the wire diameter. Increases
in extension increase the resistance heating of the wire as it passes from the
contact tip to the plate
- increasing metal deposition rate per ampere. However, excessive extensions lead
to improper wire positioning
and unstable arcs causing irregular weld beads, reduced and uneven weld penetration,
and poor quality
welds.
