Figure 2-1.Progressive hardening torch tip.
Tips are produced that can be used for hardening flats,
rounds, gears, cams, cylinders, and other regular or
In hardening localized areas, you should heat the
metal with a standard hand-held welding torch. Adjust
the torch flame to neutral (see chapter 4) for normal
heating; however, in corners and grooves, use a slightly
oxidizing flame to keep the torch from sputtering. You
also should particularly guard against overheating in
comers and grooves. If dark streaks appear on the metal
surface, this is a sign of overheating, and you need to
increase the distance between the flame and the metal.
For the best heating results, hold the torch with the
tip of the inner cone about an eighth of an inch from the
surface and direct the flame at right angles to the metal.
Sometimes it is necessary to change this angle to obtain
better results; however, you rarely find a deviation of
more than 30 degrees. Regulate the speed of torch travel
according to the type of metal, the mass and shape of the
part, and the depth of hardness desired.
In addition, you must select the steel according to
the properties desired. Select carbon steel when surface
hardness is the primary factor and alloy steel when the
physical properties of the core are also factors. Plain
carbon steels should contain more than 0.35% carbon
for good results inflame hardening. For water quench-
ing, the effective carbon range is from 0.40% to 0.70%.
Parts with a carbon content of more than 0.70% are
likely to surface crack unless the heating and quenching
rate are carefully controlled.
The surface hardness of a flame-hardened section is
equal to a section that was hardened by furnace heating
and quenching. The decrease in hardness between the
case and the core is gradual. Since the core is not
affected by flame hardening, there is little danger of
spalling or flaking while the part is in use. Thus flame
Figure 2-2.Progressive hardening.
hardening produces a hard case that is highly resistant
to wear and a core that retains its original properties.
Flame hardening can be divided into five general
methods: stationary, circular band progressive, straight-
line progressive, spiral band progressive, and circular
STATIONARY METHOD. In this method the
torch and the metal part are both held stationary.
CIRCULAR BAND PROGRESSIVE METHOD.
This method is used for hardening outside surfaces of
round sections. Usually, the object is rotated in front of
a stationary torch at a surface speed of from 3 to 12
inches per minute. The heating and quenching are done
progressively, as the part rotates; therefore, when the
part has completed one rotation, a hardened band encir-
cles the part. The width of the hardened band depends
upon the width of the torch tip. To harden the full length
of a long section, you can move the torch and repeat the
process over and over until the part is completely hard-
ened. Each pass or path of the torch should overlap the
previous one to prevent soft spots.
STRAIGHT-LINE PROGRESSIVE METHOD.
With the straight-line progressive method, the torch
travels along the surface, treating a strip that is about the
same width as the torch tip. To harden wider areas, you
move the torch and repeat the process. Figure 2-2 is an
example of progressive hardening.
SPIRAL BAND PROGRESSIVE METHOD.
For this technique a cylindrical part is mounted between
lathe centers, and a torch with an adjustable holder is
mounted on the lathe carriage. As the part rotates, the
torch moves parallel to the surface of the part. This travel
is synchronized with the parts rotary motion to produce
a continuous band of hardness. Heating and quenching
occur at the same time. The number of torches required
depends on the diameter of the part, but seldom are more
than two torches used.
CIRCULAR BAND SPINNING METHOD.
The circular band spinning method provides the best