Page 103 - Ship Construction.DJ Eyres 6Ed

Ch09-H8070.fm Page 92 Wednesday, October 18, 2006 7:36 AM
92 Ship Construction
Cutting Processes

Steel plates and sections were mostly cut to shape in shipyards using a gas
cutting technique, but the introduction of competitive plasma-arc cutting
machines has led to their widespread use in shipyards today.

GAS CUTTING Gas cutting is achieved by what is basically a chemical/
--- ใช้เพื่อการศึกษาเท่านั้น---
thermal reaction occurring with iron and iron alloys only. Iron or its alloys
may be heated to a temperature at which the iron will rapidly oxidize in an
atmosphere of high purity oxygen.
(see Figure 9.10). งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
The principle of the process as applied to the cutting of steel plates
and sections in shipbuilding is as follows. Over a small area the metal is
preheated to a given temperature, and a confined stream of oxygen is
then blown onto this area. The iron is then oxidized in a narrow band,
and the molten oxide and metal are removed by the kinetic energy of the
oxygen stream. A narrow parallel sided gap is then left between the cut
edges. Throughout the cutting operation the preheat flame is left on to
heat the top of the cut since most of the heat produced by the reaction
at the cutting front is not instantaneous, and tends to be liberated at the
lower level of the cut only. Alloying elements in small amounts are dis-
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solved in the slag and removed when cutting steel. However, if they are
present in large quantities, alloying elements, especially chromium, will
retard and even prevent cutting. The reason for this is that they either
decrease the fluidity of the slag or produce a tenacious oxide film over
the surface which prevents further oxidation of the iron. This may be
overcome by introducing an iron rich powder into the cutting area, a
process often referred to as ‘powder cutting’. When cutting stainless
steels which have a high chromium content ‘powder cutting’ would be
employed.
Generally acetylene is used with oxygen to provide the preheat flame
but other gases can be used: propane for example or hydrogen which is
used for underwater work because of its compressibility. Apart from the
torch, the equipment is similar to that for gas welding. The torch has
valves for controlling the volume of acetylene and oxygen provided for the
preheat flame, and it has a separate valve for controlling the oxygen jet

The oxy-acetylene cutting process has been highly automated for use in
shipyards; these developments are considered in Chapter 13. Hand burning
with an oxy-acetylene flame is used extensively for small jobbing work, and
during the fabrication and erection of units.

PLASMA-ARC CUTTING Plasma in this sense is a mass of ionized gas
which will conduct electricity. An electrode is connected to the negative
terminal of a DC supply and a gas shield is supplied for the arc from a