STEEL - Materials for Civil and Construction Engineers - page 126

Current conductor Solid electrode wire

Shielding gas in

Wire guide and
contact tube

Gas nozzle

Welding electrode

Gaseous shield

Arc

Weld metal

Penetration depth

Base metal

Direction of welding

FIGURE 3.25 Schematic drawing of gas welding.

pool and provides the desired arc characteristics (Figure 3.25). This welding
process is normally used for small welds due to the lack of slag formation.
Care must be taken during welding to consider the distortion that is the
result of the nonuniform heating of the welding process. When the molten
weld metal cools, it shrinks, causing deformation of the material and intro-
ducing residual stresses into the structure. Frequently, these residual stress-
es cause cracks outside the weld area. The distortion produced by welding
can be controlled by proper sequencing of the welds and predeforming the
components prior to welding. Finally, care must be taken by the welder and
the inspector to protect their eyes and skin from the intensive ultraviolet ra-
diation produced during welding (Frank and Smith 1990).
The relative ease with which steel can be welded is related to the hard-
ness of the steel. In general, harder steels are more difficult to weld. Winter-
ton developed the concept of using a carbon equivalent formula for estimating
the carbon equivalent of steels and an associated zone chart for determining
the need to preheat steel to control the development of hydrogen in the
welded steel. There are several different carbon equivalent formulas. The
one used for structural steels is

1 Mn + Si 2 1 Cr + Mo + V 2 1 Ni + Cu 2

CE = C + + + (3.4)

6 5 15

Figure 3.26 shows the zones associated with the different combinations
of carbon and carbon equivalent. The zones are used as a guide to deter-
mining the method used to determine preheat requirements (Hassett, 2003):

Zone I—Cracking is unlikely, but may occur with high hydrogen or
high restraint. Use hydrogen control method to determine preheat.

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Zone II—The hardness control method and selected hardness shall be
used to determine minimum energy input for single-pass fillet welds
without preheat.

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Zone III—The hydrogen control method shall be used.

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