To see the effect of an excess of oxygen, close the
extent; therefore, it is the flame best suited for most
acetylene valve still further. A change will be noted,
metals. The neutral flame burns at approximately
5,850°F. A balanced mixture of one volume of oxygen
although it is by no means as sharply defined as that
between the neutral and excess acetylene flames. The
and one volume of acetylene is supplied from the torch
entire flame will decrease in size, and the inner cone
when the flame is adjusted to neutral.
will become much less sharply defined.
The neutral flame is divided into two distinct zones.
Because of the difficulty in making a distinction
The inner zone consists of a white, clearly defined,
between the excess oxygen and neutral flames, an
round, smooth cone, 1/16 to 3/4 inch in length. The
adjustment of the flame to neutral should always be
outer zone, made up of completely burned oxygen and
made from the excess acetylene side. Always adjust the
acetylene, is blue with a purple tinge at the point and
flame first so that it shows the secondary cone
characteristic of excess acetylene; then, increase the
A neutral flame melts metal without changing its
flow of oxygen until this secondary cone just
properties, and it leaves the metal clear and clean. If the
mixture of oxygen and acetylene is correct, the neutral
During actual welding operations, where a neutral
flame allows the molten metal to flow smoothly, and
flame is essential, the flame should be checked
few sparks are produced when welding most metals.
occasionally to make certain it is neutral. This is
CARBURIZING FLAME.--The carburizing
accomplished by momentarily withdrawing the torch
flame, produced by burning an excess of acetylene,
from the work and increasing the amount of acetylene
may be recognized by its three distinct colors. There is
until a distinctive feathery edge appears on the inner
a bluish-white inner core, a white intermediate cone,
cone. Then, slowly decrease the amount of acetylene
and a light-blue outer flame. It may be recognized also
until a well-defined cone, characteristic of the neutral
by the feather at the tip of the inner cone. The degree of
flame, is formed.
carburization can be judged by the length of the feather.
With each size of tip, a neutral, oxidizing, or
OXIDIZING FLAME.--The oxidizing flame is
carburizing flame can be obtained. It is also possible to
produced by burning an excess of oxygen. It has the
obtain a "harsh" or "soft" flame by increasing or
general appearance of the neutral flame, but the inner
decreasing the pressure of both gases.
cone is shorter, slightly pointed, and has a purplish
For most regulator settings, the gases are expelled
tinge. This flame burns with a hissing sound. When
from the torch tip at a relatively high velocity, and the
welding ferrous metals, you can recognize an oxidizing
flame is called "harsh." For some work it is desirable to
flame by the numerous sparks that are thrown off as the
have a "soft" or low-velocity flame without a reduction
metal melts and by the foam that forms on the surface.
in thermal output. This may be achieved by using a
FLAME ADJUSTMENT.--To adjust the flame,
larger tip and closing the needle valves until the neutral
light the torch by opening the torch acetylene valve
flame is quiet and steady. It is especially desirable to
one-fourth to one-half turn. With only the acetylene
use a soft flame when welding aluminum, to avoid
valve open, the flame will be yellow in color and give
blowing holes in the metal when the puddle is formed.
off smoke and soot.
BACKFIRE AND FLASHBACK.--Improper
Now open the torch oxygen valve slowly. The
handling of the torch may cause the flame to backfire
flame will gradually change in color from yellow to
or, in very rare cases, to flashback. A backfire is a
blue, and it will show the characteristics of the excess
momentary backward flow of the gases at the torch tip,
acetylene flame described earlier.
causing the flame to go out. Sometimes the flame may
immediately come on again, but a backfire is always
With most torches, there will be a slight excess of
accompanied by a snapping or popping noise. A
acetylene when the oxygen and acetylene valves are
backfire may be caused by touching the tip against the
wide open and the recommended pressures are being
work, by overheating the tip, by operating the torch at
used. Now close the acetylene valve on the torch
other than recommended pressures, by a loose tip or
slowly. You will notice that the secondary cone gets
head, or by dirt or slag in the end of the tip. A backfire is
smaller until it finally disappears completely. Just at
rarely dangerous, but the molten metal may be
this point of complete disappearance, the neutral flame
splattered when the flame pops.