Figure 4-17.MAPP-gas flames.
After checking the fuel-gas adjustment, you can
adjust the oxygas flame to obtain the desired charac-
teristics for the work at hand, by further manipulating
the oxygen and fuel-gas needle valves according to the
torch manufacturers direction.
There are three types of gas flames commonly used
for all oxygas processes. They are carburizing, neutral,
and oxidizing. To ensure proper flame adjustment, you
should know the characteristics of each of these three
types of flame. Figure 4-17 shows how the three differ-
ent flames look when using MAPP gas as the fuel.
A pure fuel-gas flame is long and bushy and has a
yellowish color. It takes the oxygen it needs for combus-
tion from the surrounding air. The oxygen available is
not sufficient enough to burn the fuel gas completely;
therefore, the flame is smokey and consists of soot. This
flame is not suitable for use. You need to increase the
amount of oxygen by opening the oxygen needle valve
until the flame takes on a bluish white color, with a
bright inner cone surrounded by a flame envelope of a
darker hue. It is the inner cone that develops the required
CARBURIZING FLAME. The carburizing
flame always shows distinct colors; the inner cone is
bluish white, the intermediate cone is white, the outer
envelope flame is light blue, and the feather at the tip of
the inner cone is greenish. The length of the feather can
be used as a basis for judging the degree of carburiza-
tion. The highly carburizing flame is longer with yellow
or white feathers on the inner cone, while the slightly
carburizing flame has a shorter feather on the inner cone
and becomes more white. The temperature of carburiz-
ing flames is about 5400°F.
Strongly carburizing flames are not used in cutting
low-carbon steels because the additional carbon they
add causes embrittlement and hardness. These flames
are ideal for cutting cast iron because the additional
carbon poses no problems and the flame adds more heat
to the metal because of its size.
Slightly carburizing flames are ideal for cutting
steels and other ferrous metals that produce a large
amount of slag. Although a neutral flame is best for most
cutting, a slightly carburizing flame is ideal for produc-
ing a lot of heat down inside the kerf. It makes fairly
smooth cuts and reduces the amount of slag clinging to
the bottom of the cut.
NEUTRAL FLAME. The most common preheat
flame for oxygas cutting is the neutral flame. When you
increase the oxygen, the carburizing flame becomes
neutral. The feather will disappear from the inner flame
cone and all that will be left is the dark blue inner flame
and the lighter blue outer cone. The temperature is about
The neutral flame will not oxidize or add carbon to
the metal you are cutting. In actuality, a neutral flame
acts like the inert gases that are used in TIG and MIG
welding to protect the weld from the atmosphere. When
you hold a neutral preheat flame on one spot on the metal
until it melts, the molten puddle that forms looks clear
and lies very quietly under the flame.
OXIDIZING FLAME. When you add a little
more oxygen to the preheat flame, it will quickly be-
come shorter. The flame will start to neck down at the
base, next to the flame ports. The inner flame cone
changes from dark blue to light blue. Oxidizing flames
are much easier to look at because they are less radiant
than neutral flames. The temperature is about 6000°F.
The oxidizing flame is rarely used for conventional
cutting because it produces excessive slag and does not
leave square-cut edges. Oxidizing flames are used in
conjunction with cutting machines that have a high-low
oxygen valve. The machine starts the cut with a oxidiz-
ing flame then automatically reverts to a neutral flame.
The oxidizing flame gives you fast starts when using
high-speed cutting machines and is ideal for piercing
holes in plate. Highly oxidizing flames are only used in
cutting metal underwater where the only source of oxy-
gen for the torch is supplied from the surface.
CUTTING MILD-CARBON STEEL
To cut mild-carbon steel with the oxygas cutting
torch, you should adjust the preheating flames to neutral.