Page 222 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 207

The welding procedure is somewhat analogous to a cook’s recipe.
It outlines the steps required to make a quality weld under specific
conditions.

3.5.1 Effects of welding variables
The effects of the variables are somewhat dependent on the welding
process being employed, but general trends apply to all the processes.
It is important to distinguish the difference between constant current
(CC) and constant voltage (CV) electrical welding systems. Shielded
metal arc welding is always done with a CC system. Flux-cored weld-
ing and gas metal arc welding generally are performed with CV sys-
tems. Submerged arc may utilize either.
Amperage is a measure of the amount of current flowing through
the electrode and the work. It is a primary variable in determining
heat input. Generally, an increase in amperage means higher deposi-
tion rates, deeper penetration, and more admixture. The amperage
flowing through an electric circuit is the same, regardless of where it
is measured. It may be measured with a tong meter or with the use of
an electric shunt. The role of amperage is best understood in the con-
text of heat input and current density considerations. For CV weld-
ing, an increase in wire-feed speed will directly increase amperage.
For SMAW on CC systems, the machine setting determines the basic
amperage, although changes in the arc length (controlled by the
welder) will further change amperage. Longer arc lengths reduce
amperage.
Arc voltage is directly related to arc length. As the voltage increases,
the arc length increases, as does the demand for arc shielding. For CV
welding, the voltage is determined primarily by the machine setting,
so the arc length is relatively fixed in CV welding. For SMAW on CC
systems, the arc voltage is determined by the arc length, which is
manipulated by the welder. As arc lengths are increased with SMAW,
the arc voltage will increase and the amperage will decrease. Arc volt-
age also controls the width of the weld bead, with higher voltages
generating wider beads. Arc voltage has a direct effect on the heat-
input computation.
The voltage in a welding circuit is not constant, but is composed of
a series of voltage drops. Consider the following example: Assume
the power source delivers a total system voltage of 40 V. Between the
power source and the welding head or gun, there is a voltage drop of
perhaps 3 V associated with the input-cable resistance. From the
point of attachment of the work lead to the power source work termi-
nal, there is an additional voltage drop of, say, 7 V. Subtracting the
3 V and the 7 V from the original 40 V, this leaves 30 V for the arc.

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