Page 225 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

210 Chapter Three

problem with higher currents. AC is less prone to art blow, and can
sometimes be used to overcome this phenomenon.
Heat input is proportional to the welding amperage, times the arc
voltage, divided by the travel speed. Higher heat inputs relate to larger
weld cross-sectional areas and larger heat-affected zones, which may
negatively affect mechanical properties in that region. Higher heat
input generally results in slightly decreased yield and tensile
strength in the weld metal, and generally lowers notch toughness
because of the interaction of bead size and heat input.
Current density is determine by dividing the welding amperage by
the cross-sectional area of the electrode. For solid electrodes, the cur-
2
rent density is therefore proportional to I/d . For tubular electrodes
where current is conducted by the sheath, the current density is related
to the area of the metallic cross section. As the current density
increases, there will be an increase in deposition rates, as well as pen-
etration. The latter will increase the amount of admixture for a given
joint. Notice that this may be accomplished by either the amperage or
decreasing the electrode size. Because the electrode diameter is a
squared function, a small decrease in diameter may have a significant
effect on deposition rates and plate penetration.
Preheat and interpass temperature are used to control cracking
tendencies, typically in the base materials. Regarding weld metal
properties, for most carbon-manganese-silicon systems, a moderate
interpass temperature promotes good notch toughness. Preheat and
interpass temperatures greater than 550°F may negatively affect
notch toughness. Therefore, careful control of preheat and interpass
temperatures is critical.

3.5.2 Purpose of welding procedure
specifications (WPSs)
The particular values for the variables discussed previously have a
significant effect on weld soundness, mechanical properties, and pro-
ductivity. It is therefore critical that those procedural values used in
the actual fabrication and erection be appropriate for the specific
requirements of the applicable code and job specifications. Welds that
will be architecturally exposed, for example, should be made with pro-
cedures that minimize spatter, encourage exceptional surface finish,
and have limited or no undercut. Welds that will be covered with fire-
proofing, in contrast, would naturally have less restrictive cosmetic
requirements.
Many issues must be considered when selecting welding procedure
values. While all welds must have fusion to ensure their strength, the
required level of penetration is a function of the joint design in the

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