Page 223 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

208 Chapter Three

This example illustrates how important it is to ensure that the volt-
ages used for monitoring welding procedures properly recognize any
losses in the welding circuit. The most accurate way to determine arc
voltage is to measure the voltage drop between the contact tip and
the workpiece. This may not be practical for semiautomatic welding,
so voltage is typically read from a point on the wire feeder (where the
gun and cable connection is made) to the workpiece. For SMAW,
welding voltage is not usually monitored, since it is constantly
changing and cannot be controlled except by the welder. Skilled
workers hold short arc lengths to deliver the best weld quality.
Travel speed, measured in inches per minute, is the rate at which
the electrode is moved relative to the joint. All other variables being
equal, travel speed has an inverse effect on the size of the weld beads.
As the travel speed increases, the weld size will decrease. Extremely
low travel speeds may result in reduced penetration, as the arc
impinges on a thick layer of molten metal and the weld puddle rolls
ahead of the arc. Travel speed is a key variable used in computing
heat input; reducing travel speed increases heat input.
Wire-feed speed is a measure of the rate at which the electrode is
passed through the welding gun and delivered to the arc. Typically
measured in inches per minute (in/min), the wire-feed speed is directly
proportional to deposition rate and directly related to amperage. When
all other welding conditions are maintained constant (for example, the
same electrode type, diameter, electrode extension, arc voltage, and
electrode extension), an increase in wire-feed speed will directly lead
to an increase in amperage. For slower wire-feed speeds, the ratio of
wire-feed speed to amperage is relatively constant and linear.
For higher levels of wire-feed speed, it is possible to increase the
wire-feed speed at a disproportionately high rate compared to the
increase in amperage. When these conditions exist, the deposition
rate per amp increases but at the expense of penetration.
Wire-feed speed is the preferred method of maintaining welding pro-
cedures for constant-voltage wire-feed processes. The wire-feed speed
can be independently adjusted, and measured directly, regardless of
the other welding conditions. It is possible to utilize amperage as an
alternative to wire-feed speed although the resultant amperage for a
given wire-feed speed may vary, depending on the polarity, electrode
diameter, electrode type, and electrode extension. Although equipment
has been available for 20 years that monitors wire-feed speed, many
codes such as AWS D1.1 continue to acknowledge amperage as the pri-
mary method for procedure documentation. D1.1 does permit the use
of wire-feed speed control instead of amperage, providing a wire-feed
speed–amperage relationship chart is available for comparison.

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