The neutral flame, in which a balanced mixture of
oxygen and acetylene is burned, is used for most
welding operations. The oxidizing flame, in which an
excess of oxygen is burned, is used for welding bronze
or fusing brass and bronze. The carburizing flame, in
which an excess of acetylene is burned, is used when
welding nickel alloys.
NEUTRAL FLAME.The neutral flame does not
alter the composition of the base metal to any great
extent; therefore, it is the flame best suited for most
metals. The neutral flame burns at approximately
5,850°F. A balanced mixture of one volume of oxygen
and one volume of acetylene is supplied from the torch
when the flame is adjusted to neutral.
The neutral flame is divided into two distinct zones.
The inner zone consists of a white, clearly defined,
round, smooth cone, 1/1 6 to 3/4 inch in length. The outer
zone, made up of completely burned oxygen and
acetylene, is blue with a purple tinge at the point and
A neutral flame melts metal without changing its
properties, and it leaves the metal clear and clean. If the
mixture of oxygen and acetylene is correct, the neutral
flame allows the molten metal to flow smoothly, and few
sparks are produced when welding most metals.
CARBURIZING FLAME.The carburizing
flame, produced by burning an excess of acetylene, may
be recognized by its three distinct colors. There is a
bluish-white inner core, a white intermediate cone, and
a light-blue outer flame. It may be recognized also by
the feather at the tip of the inner cone. The degree of
carburization can be judged by the length of the feather.
OXIDIZING FLAME.The oxidizing flame is
produced by burning an excess of oxygen. It has the
general appearance of the neutral flame, but the inner
cone is shorter, slightly pointed, and has a purplish tinge.
This flame burns with a hissing sound. When welding
ferrous metals, you can recognize an oxidizing flame by
the numerous sparks that are thrown off as the metal
melts and by the foam that forms on the surface.
FLAME ADJUSTMENT.To adjust the flame,
light the torch by opening the torch acetylene valve
one-fourth to one-half turn. With only the acetylene
valve open, the flame will be yellow in color and give
off smoke and soot.
Now open the torch oxygen valve slowly. The flame
will gradually change in color from yellow to blue, and
it will show the characteristics of the excess acetylene
flame described earlier.
With most torches, there will be a slight excess of
acetylene when the oxygen and acetylene valves are
wide open and the recommended pressures are being
used. Now close the acetylene valve on the torch slowly.
You will notice that the secondary cone gets smaller
until it finally disappears completely. Just at this point
of complete disappearance, the neutral flame is formed.
To see the effect of an excess of oxygen, close the
acet ylene valve still further. A change will be noted,
although it is by no means as sharply defined as that
between the neutral and excess acetylene flames. The
entire flame will decrease in size, and the inner cone will
become much less sharply defined.
Because of the difficulty in making a distinction
between the excess oxygen and neutral flames, an
adjustment of the flame to neutral should always be
made from the excess acetylene side. Always adjust the
flame first so that it shows the secondary cone
characteristic of excess acetylene; then, increase the
flow of oxygen until this secondary cone just disappears.
During actual welding operations, where a neutral
flame is essential, the flame should be checked
occasionally to make certain it is neutral. This is
accomplished by momentarily withdrawing the torch
from the work and increasing the amount of acetylene
until a distinctive feathery edge appears on the inner
cone. Then, slowly decrease the amount of acetylene
until a well-defined cone, characteristic of the neutral
flame, is formed.
With each size of tip, a neutral, oxidizing, or
carburizing flame can be obtained. It is also possible to
obtain a harsh or soft flame by increasing or
decreasing the pressure of both gases.
For most regulator settings, the gases are expelled
from the torch tip at a relatively high velocity, and the
flame is called harsh. For some work it is desirable
to have a soft or low-velocity flame without a reduc-
tion in thermal output. This maybe achieved by using a
larger tip and closing the needle valves until the neutral
flame is quiet and steady. It is especially desirable to use
a soft flame when welding aluminum, to avoid blowing
holes in the metal when the puddle is formed.
BACKFIRE AND FLASHBACK.Improper
handling of the torch may cause the flame to backfire
or, in very rare cases, to flashback. A backfire is a
momentary backward flow of the gases at the torch tip,
causing the flame to go out. Sometimes the flame may
immediately come on again, but a backfire is always
accompanied by a snapping or popping noise. A backfire
may be caused by touching the tip against the work, by
overheating the tip, by operating the torch at other than
recommended pressures, by a loose tip or head, or by