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Steel Designers' Manual - 6th Edition (2003)
Gouging
The gouging process is the removal of metal at the underside of butt welds. Gouging
techniques are also used for the removal of defective material or welds. The various
forms of gouging are flame gouging, air-arc gouging, oxygen-arc gouging and metalarc
gouging.
Flame gouging is an oxy fuel gas cutting process and uses the same torch but with
a different nozzle. It is important that a proper gouging nozzle is adopted so that
the gouging profile is correct.
Air-arc gouging uses the same equipment as manual metal arc welding. The
process differs in that the electrode is made of a bonded mixture of carbon and
graphite encased in a layer of copper, and jets of compressed air are emitted from
the specially designed electrode holder. These air jets blow away the parent metal
from beneath the arc.
A special electrode holder is used in the oxygen-arc gouging process. A special
tubular coated steel electrode controls the release of a supply of oxygen. Oxygen is
released once the arc is established, and when the gouge is being made the oxygen
flow is increased to a maximum.
Standard manual metal arc welding equipment is used for metal-arc gouging. The
electrodes, however, are specially designed for cutting or gouging.This process relies
on the metal being forced out of the cut by the arc and not blown away as in the
other processes.
32.5.2 Surface preparation
Cutting 961
Structural sections from the rolling mills may require surface cleaning prior to fabrication
and painting. Hand preparation, such as wire brushing, does not normally
conform to the requirements of modern paint or surface protection systems.
Blast cleaning is the accepted way of carrying out surface preparation. It involves
blasting dry steelwork with either shot or grit at high velocity to remove rust, oil,
paint, mill scale and any other surface contaminants. The most productive form of
blast cleaning plant has special equipment comprising infeed conveyor, drying oven,
blast chamber, spray chamber and outfeed conveyor.
Steel is loaded on to the infeed conveyor either as separate bars or side by side
depending on the blast chamber passage opening. It then travels through the drying
oven, which ensures that any surface moisture is removed prior to blasting.The blast
chamber receives the steel and passes it over racks and between the blasting turbines,
which are impellers fed centrally with either shot or grit. The material is
thrown out from the edge of the turbine and impacts against the steelwork. The
speed of the turbine, its location and aperture determine the velocity and direction
of the blasting medium.
The blasting medium is retrieved from within the blast chamber and recycled until
it is exhausted. Shot provides a good medium for steel which is to be painted; grit