Automatic flashback arrester
This is an automatic safety device (Figure 9.10)
for use in the high-pressure system with either
oxygen or acetylene cylinders. An arrester is
fitted to the pressure regulator outlet of a cylinder
with its opposite end connected to the hose leading
to the welding torch. If excess pressure builds
up, the arrester acts by completely sealing off the
gas supply. Only when the pressure has subsided
to normal working level via the built-in escape
valve will the arrester allow the main supply of
gas to continue flowing. The increase in safety
which this piece of equipment affords makes it an
essential part of the high-pressure system of
welding.
Gas hose
Gas hoses are of a special non-porous rubber,
reinforced with canvas. They are designed to
withstand the working pressures which transmit
gas from the regulator to the welding torch.
Hoses are available with internal diameters of
5–20 mm and are coloured red for acetylene and
blue for oxygen to prevent the risk of hose interchange.
The nut on the acetylene hose is distinguished
from the nut on the oxygen hose by a
groove that runs around its centre, indicating
that it has a left-hand thread. A ferrule is used
to squeeze the hose around the union shank to
prevent it from working loose. The life of the
hose may be prolonged by reasonable care in use,
e.g. by keeping it away from heat, sparks, oil and
Figure 9.9Acetylene regulator (Murex Welding
Products Ltd)
Figure 9.10Flashback arresters for oxygen and
acetylene regulators (Murex Welding Products Limited )
Gas welding, gas cutting and plasma arc cutting 255
grease and by preventing it from being crushed.
Hose fittings are shown in Figure 9.11.
Hose check valve
This is a safety device in the form of a nonreturnable
valve which is fitted behind the welding
torch and connected to the hose (Figure 9.12).
There are check valves for both oxygen and
acetylene connections. If the pressure or gas flow
tries to reverse, the hose check valve immediately
seals off the torch from the main supply, thus
eliminating the possibility of flashback and backfiring
of the welding torch through the hose to the
regulator and cylinder.
Cylinder key
This key is an essential part of the equipment, and
is used for turning the valves on the gas cylinders
on and off. It should be always attached to the
welding equipment or plant so that it is readily
available in case of emergency.
Welding goggles
Goggles fitted with tinted glass of an approved
type complying with the requirements set out in
British Standard specification 679 should be
worn to protect the eyes from the intense light of
the flame as well as from sparks and small
metal particles thrown up from the weld. As the
goggles become pitted in time and obscure
the work, it is usual to protect the tinted glass
with a plain one which can be replaced at low
cost. Typical welding goggles are shown in
Figure 9.13.
Figure 9.11Hose fittings: nut, nipple, and O-clips
(Murex Welding Products Ltd )
Figure 9.12Hose check valves (Murex Welding
Products Ltd )
Figure 9.13Types of welding goggles (Murex
Welding Products Limited )
Cylinder trolley
It is advisable to have all welding equipment
portable and mobile when in use in a body repair
shop, and for this purpose a cylinder trolley can be
used. Both cylinders must be securely fastened but
at the same time easy to replace. The trolley itself
should be of sturdy construction and should be as
narrow as possible so that it can pass through
restricted spaces.
256Repair of Vehicle Bodies
Assembly of high-pressure
Welding equipment
The cylinders must be kept upright and all metal
jointing surfaces should be free from oil or grease.
Before fitting the regulators, blow out the cylinder
valves to remove any dirt or obstruction. To ensure
gas-tight connections between cylinder valves and
regulators, first screw down the hexagon or wing
nuts by hand, then give the regulators a twist to
bed them down on their seats, and finally tighten
the nuts properly, but without the use of excessive
force. New hoses in use for the first time should
be blown through to remove any grit that may be
present. Attach the appropriate hoses to the regulators
and the welding torch, and fit the latter with a
suitable welding tip. Next slacken off the pressure
regulating screws, open the regulator outlet valves
and turn on the gases with the cylinder key; this
must be done slowly to avoid damage to the regulators.
The cylinder valves must be opened at least
two full turns to ensure that the flow of gases to
the regulators is unrestricted. Next set the regulators
to the required pressure, open the acetylene
control valve on the welding torch, check working
pressures when pure acetylene is coming out of
the end of the nozzle, and then light the gas.
Reduce or increase the acetylene supply by the
welding torch valve until the flame just ceases to
smoke, then turn on the oxygen by the welding
torch control valve until the white inner cone is
sharply defined, and check working pressures
again. The welding torch is now ready and is burning
with a neutral flame, which is used on most
welding operations.
On completion of the welding operation the
following procedure must be carried out to render
the equipment safe. Turn off the acetylene first by
the welding torch control valve, then the oxygen.
Close the cylinder valves. Open the welding torch
valves one at a time to release the pressure in
the hose; open the oxygen valve and shut it; and
open the acetylene valve and shut it. Unscrew the
pressure regulating screws on the oxygen and
acetylene regulators.
9.4 Definitions of welding terms
The material of the parts to be welded is described
as the parent metal. Any material that it may be
necessary to add to complete the weld is known as
filler metal which, if in rod or wire form, is
obtained from a welding rod. The surface to be
welded is called the fusion face. That part of the
weld where the parent metal has been melted, if
filler is used and interdiffusion has taken place, is
called the fusion zone, the depth of which is termed
the weld penetration. Bordering on the fusion zone
is the zone of thermal disturbance, consisting of
that portion of the parent metal which, although
not melted by the flame, has been heated sufficiently
to disturb the grain structure; where the
fusion zone and zone of thermal disturbance meet
is known as the junction (Figure 9.14). A bead is a
single longitudinal deposit of weld metal laid on a
surface by fusion welding; a local deposit laid on a
surface is termed a pad, and is usually formed by a
series of overlapping beads. Tack welds are local
welds used to hold parts in their correct relative
positions ready for welding.
Figure 9.14Section through a welded joint
Most welded assemblies are made by butt joints
in which the ends or edges directly face each other.
In some cases the joining weld is made between
these edges. Additionally or alternatively fillet
welds may be used, which are of approximately triangular
cross-section and lie externally to the parts
joined. These fillet welds may consist of T-joints or
lap joints; in the former the parts are usually set at
right angles to one another, and in the latter the
weld is made in the angle formed by the overlap.
An alternative to the butt joint is the edge joint, in
which the two metals are put together to form a
corner.
Various terms are used to indicate the different
parts of a weld. The weld face is the exposed
surface of any weld, a leg is the fusion face of a
fillet weld, and the toe is a border line where the
weld face adjoins a welded part; along this line
undercut or wastage of the parent metal in the
Gas welding, gas cutting and plasma arc cutting 257
form of a grooving may occur. The root is the
zone at the bottom of a space provided for or
occupied by a fusion weld, and the throat is the
minimum depth of the weld measured along a line
passing through the root. The condition which
arises when the filler metal flows on to heated but
unfused joint surfaces, and the interfusion of the
filler and parent metals does not take place, is
known as adhesion.
9.5 Welding rods and fluxes
Filler rods
Filler rods for use in oxy-acetylene welding are
available in the following metals: mild steel,
wrought irons, high-carbon steel, alloy steel, stainless
steel, cast iron, copper, copper alloys, aluminium,
aluminium alloys, hard facing alloys,
zinc-based die cast alloys.
The metal from the rod has considerable influence
on the quality of the finished weld. Good
welding rods are designed to give deposited metal
of the correct composition, and have allowance in
their chemistry for changes which take place in the
welding process. Rods are obtainable in sizes ranging
from 1.6 mm to 5.0 mm diameter. Some have a
copper coating to keep their surfaces free from
oxides or rust, but uncoated rods are equally efficient
provided that they are clean. Sound welds,
comparable in strength with the material welded,
can be produced with satisfactory filler rods, but
similar results cannot be expected with dirty or
rusty filler rods or with any odd piece of wire that
may come to hand.
Welding fluxes
It is impracticable to incorporate in welding rods
all the elements necessary to overcome oxidation.
Therefore it is necessary to use with them certain
chemical compounds to act as deoxidizing agents
or fluxes, which must be of the correct composition
to ensure perfect welds. A flux must be used
with the following metals: cast iron, high-carbon
steel, stainless steel, copper, copper alloys, aluminium,
aluminium alloys, magnesium alloys. In
the majority of cases it is essential that the flux
residues should be removed from the surface of
the metal after the welding operation has been
completed.
9.6 Flame control and types of flame
Temperature of flame
When acetylene is burned with an equal volume of
oxygen, a maximum flame temperature of over
3200 °C (about 2.5 times hotter than the melting
point of cast iron and steel) is obtained just beyond
the inner luminous cone (Figure 9.15). Variations
of the proportions of oxygen and acetylene can be
made at the welding torch, and three distinct types
of flame are obtained (Figure 9.16) as follows.
Figure 9.15Flame temperature
Figure 9.16Regulation of welding flame
Carbonizing, carburizing or reducing flame
For this flame the gases are adjusted so that all the
acetylene gas is not completely burned and there
is an excess of acetylene or an insufficiency of
258Repair of Vehicle Bodies
oxygen. When the welding torch is lit, the acetylene
is turned on first and ignited, giving a very
smoky yellow flame of abnormal size which
shows two cones of flame in addition to an outer
envelope; this is an exaggerated form of the carbonizing
flame, but gives out comparatively little
heat and is useless for welding. The oxygen is
turned on and the supply is gradually increased
until the flame, though still of abnormal size, contracts
towards the welding torch tip, where an
inner white cone of great luminosity commences
to make its appearance. The increase in the supply
of oxygen is stopped before the cone becomes
clearly defined and while it is still an inch or
so long; this results in a carburizing flame, which
is characterized by a dullish white feather surrounding
a brilliant, clearly defined white cone.
The flame can be used to advantage in the welding
of high-carbon steel, aluminium, Inconel and
Monel metal in the technique of stelliting, for
stainless steel, and wherever excess of oxygen
on metals would cause detrimental oxidation.
However, in some cases the flame is detrimental
owing to the fact that it deposits carbon; for example,
if it is used on mild steel the weld deposit will
be higher in carbon and therefore becomes hardenable
material, and cracks may result.
Neutral flame
As the supply of oxygen to the welding torch is
increased beyond the point at which the carbonizing
flame is formed, the flame contracts and the
white cone assumes a definite rounded shape. At
this stage approximately equal quantities of acetylene
and oxygen are being used and combustion is
complete, all the carbon supplied by the acetylene
being consumed and the maximum heat given out.
This flame is known as the neutral flame, and is
the one most extensively used by the welder. It
does not oxidize or carburize the material, and it is
used on mild steel, copper and magnesium.
Oxidizing flame
A further increase in the oxygen supply will produce
an oxidizing flame in which there is more
oxygen than is required for complete combustion.
The inner cone will become shorter and sharper
and the flame will turn a deeper purple colour
and emit a characteristic slight hiss. With this, the
oxidizing flame, the molten metal will be less
fluid and tranquil during welding and excessive
sparking will occur. An oxidizing flame is only
used for special applications, and should always
be avoided when welding steel, because it causes
rapid oxidation of the metal. It is, however, used
in the welding of brasses, bronzes, copper-silicon
alloys and in the process of bronze welding, the
degree of oxidation required being determined
experimentally.
Power of flame
The power or heat value of the flame is governed by
the size of the orifice in the tip used. The power
required depends on the thickness, mass, melting
point and heat conductivity of the metal to be
welded. Manufacturers of welding torches provide
information regarding the size and rated gas consumption
of tips for different thicknesses of metals
and alloys. The power of a welding torch is measured
by the number of litres or cubic feet of acetylene
which is consumed in an hour with the flame
perfectly regulated, and this figure is sometimes
marked on the welding torch itself. The heat value
of the flame must be adjusted by changing the tip
and not by unduly increasing or decreasing the pressure
and volume of gases used. The pressure of
gases used for any size of tip determines the length
of the inner cone, which should be 3 to 3.5 times as
long as its breadth at the base. Too low a pressure
gives too short a cone, which may cause lack of
penetration and fusion; it also causes frequent backfiring
and will be deflected by particles of metal and
slag thrown up from the weld. On the other hand,
too high a gas pressure giving too long a cone
causes overheating and lack of control of the molten
metal, resulting in adhesion and over-penetration.