Page 87 - Ship Construction.DJ Eyres 6Ed

Ch09-H8070.fm Page 76 Wednesday, October 18, 2006 7:36 AM
76 Ship Construction
Gas Welding

A gas flame was probably the first form of heat source to be used for fusion
welding, and a variety of fuel gases with oxygen have been used to produce
a high temperature flame. The most commonly used gas in use is acetylene
which gives an intense concentrated flame (average temperature 3000°C)
when burnt in oxygen.
An oxy-acetylene flame has two distinct regions, an inner cone, in which
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the oxygen for combustion is supplied via the torch, and a surrounding
envelope in which some or all the oxygen for combustion is drawn from
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the surrounding air. By varying the ratio of oxygen to acetylene in the gas
mixture supplied by the torch it is possible to vary the efficiency of the com-
bustion and alter the nature of the flame (Figure 9.1). If the oxygen supply
is slightly greater than the supply of acetylene by volume, what is known as
an ‘oxidizing’ flame is obtained. This type of flame may be used for welding
materials of high thermal conductivity, e.g. copper, but not steels as the
steel may be decarburized and the weld pool depleted of silicon. With equal
amounts of acetylene and oxygen a ‘neutral’ flame is obtained, and this
would normally be used for welding steels and most other metals. Where
the acetylene supply exceeds the oxygen by volume a ‘carburizing’ flame is
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obtained, the excess acetylene decomposing and producing sub-microscopic
particles of carbon. These readily go into solution in the molten steel, and can
produce metallurgical problems in service.
The outer envelope of the oxy-acetylene flame by consuming the surround-
ing oxygen to some extent protects the molten weld metal pool from the sur-
rounding air. If unprotected the oxygen may diffuse into the molten metal
and produce porosity when the weld metal cools. With metals containing
refractory oxides, such as stainless steels and aluminium, it is necessary to
use an active flux to remove the oxides during the welding process.
Both oxygen and acetylene are supplied in cylinders, the oxygen under pres-
sure and the acetylene dissolved in acetone since it cannot be compressed.
Each cylinder which is distinctly coloured (red—acetylene, black—oxygen)
has a regulator for controlling the working gas pressures. The welding torch
consists of a long thick copper nozzle, a gas mixer body, and valves for
adjusting the oxygen and acetylene flow rates. Usually a welding rod is used
to provide filler metal for the joint, but in some cases the parts to be joined
may be fused together without any filler metal. Gas welding techniques are
shown in Figure 9.1.
Oxy-acetylene welding tends to be slower than other fusion welding
processes because the process temperature is low in comparison with the
melting temperature of the metal, and because the heat must be transferred
from the flame to the plate. The process is therefore only really applicable
to thinner mild steel plate, thicknesses up to 7 mm being welded with this
process with a speed of 3 to 4 metres per hour. In shipbuilding oxy-acetylene