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The
Oxy-fuel Gas Cutting Process What is really happening?
By J. Jones
J. Jones is Training Specialist,
 Equipment
Company (www.victorequip.com)
| Oxy-fuel gas cutting is nothing new. It
has been used for cutting ferrous steel since the late
1800’s, but the alloying elements influence the ease with
which steels may be cut. This is due to the fact that the
steel is being “burned” by the reaction of 99% oxygen at
elevated temperatures. For example, the presence of
manganese assists the cutting process while chromium does
the opposite.
The composition of some steel makes it
hardenable and the cutting process may lead to the formation
of hardened zones adjacent to the cut surface. These zones
are called “the heat affected zone”. So, oxy-fuel cutting
the steel, particularly the alloy types may do more damage
than good. |
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Ferrous metals must be brought to their
kindling temperature before almost pure oxygen will
accelerate their burning. The kindling temperature is
achieved buy using the pre-heat flames of the torch. The
cutting torch has three control valves, one for fuel, one
for oxygen to adjust the pre-heat flames, and one for the
cutting oxygen. Once the fuel gas is lit, the cutting
torch’s valves are adjusted to mix the fuel gas with the
proper amount of oxygen to produce the pre-heat flames. The
oxygen stream is directed through the cutting tip’s center
orifice at the pre-heated portion of the metal where the
burning will begin. The burned metal becomes slag and the
void produced is called the kerf. The tip’s movement and
cutting orifice size control the width of the kerf. Slag and
rust are both products of oxidized metal. This is why
oxy-fuel will not burn through a heavy layer of rust or
slag, there is nothing there for the oxygen to burn. |
The metal being burned during
this process also produces heat. This is referred to as an
exothermal process and assists with maintaining the kindling
temperature of the metal. The metal itself becomes the fuel and
will continue to burn in the presents of pure oxygen. The oxygen
is the tool that burns the metal. To oxidize one pound of steel,
4.6 cubic feet of oxygen is required. Once the burn is started,
the fuel gas only assists with maintaining kindling temperature.
The fuel gas DOES NOT CUT the metal and it has no affect on the
cutting oxygen consumption!
| The type of cutting tip chosen and how it
is being used are major factors in producing a quality cut.
The fuel gas DOES NOT increase cutting speed, produce more
or less slag nor add any characteristics to the cut’s
surface. The flame’s adjustment, coupling distance, angle
and travel rate all contribute to the cut quality. These
variables can be closely controlled with a cutting machine
and produce higher quality cuts than those made with a hand
torch. |
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