Carbon steels are usually quenched in brine or
water, and alloy steels are generally quenched in oil.
When hardening carbon steel, remember that you must
cool the steel to below 1000°F in less than 1 second.
When you add alloys to steel, the time limit for the
temperature to drop below 1000°F increases above the
l-second limit, and a slower quenching medium can
produce the desired hardness.
Quenching produces extremely high internal
stresses in steel, and to relieve them, you can temper the
steel just before it becomes cold. The part is removed
from the quenching bath at a temperature of about 200°F
and allowed to air-cool. The temperature range from
200°F down to room temperature is called the cracking
range and you do not want the steel to pass through it.
In the following paragraphs, we discuss the differ-
ent methods of hardening that are commercially used.
In the Seabees, we use a rapid surface hardening com-
pound called Case that can be ordered through the
Navy supply system. Information on the use of Case
is located in the Welding Materials Handbook, P-433.
Case hardening produces a hard, wear-resistant sur-
face or case over a strong, tough core. The principal
forms of casehardening are carburizing, cyaniding, and
nitriding. Only ferrous metals are case-hardened.
Case hardening is ideal for parts that require a
wear-resistant surface and must be tough enough inter-
nally to withstand heavy loading. The steels best suited
for case hardening are the low-carbon and low-alloy
series. When high-carbon steels are case-hardened, the
hardness penetrates the core and causes brittleness. In
case hardening, you change the surface of the metal
chemically by introducing a high carbide or nitride
content. The core remains chemically unaffected. When
heat-treated, the high-carbon surface responds to hard-
ening, and the core toughens.
CARBURIZING. Carburizing is a case-harden-
ing process by which carbon is added to the surface of
low-carbon steel. This results in a carburized steel that
has a high-carbon surface and a low-carbon interior.
When the carburized steel is heat-treated, the case be-
comes hardened and the core remains soft and tough.
Two methods are used for carburizing steel. One
method consists of heating the steel in a furnace con-
taining a carbon monoxide atmosphere. The other
method has the steel placed in a container packed with
charcoal or some other carbon-rich material and then
heated in a furnace. To cool the parts, you can leave the
container in the furnace to cool or remove it and let it
air cool. In both cases, the parts become annealed during
the slow cooling. The depth of the carbon penetration
depends on the length of the soaking period. With to-
days methods, carburizing is almost exclusively done
by gas atmospheres.
CYANIDING. This process is a type of case
hardening that is fast and efficient. Preheated steel is
dipped into a heated cyanide bath and allowed to soak.
Upon removal, it is quenched and then rinsed to remove
any residual cyanide. This process produces a thin, hard
shell that is harder than the one produced by carburizing
and can be completed in 20 to 30 minutes vice several
hours. The major drawback is that cyanide salts are a
NITRIDING. This case-hardening method pro-
duces the hardest surface of any of the hardening proc-
esses. It differs from the other methods in that the
individual parts have been heat-treated and tempered
before nitriding. The parts are then heated in a furnace
that has an ammonia gas atmosphere. No quenching is
required so there is no worry about warping or other
types of distortion. This process is used to case harden
items, such as gears, cylinder sleeves, camshafts and
other engine parts, that need to be wear resistant and
operate in high-heat areas.
Flame hardening is another procedure that is used
to harden the surface of metal parts. When you use an
oxyacetylene flame, a thin layer at the surface of the part
is rapidly heated to its critical temperature and then
immediately quenched by a combination of a water
spray and the cold base metal. This process produces a
thin, hardened surface, and at the same time, the internal
parts retain their original properties. Whether the proc-
ess is manual or mechanical, a close watch must be
maintained, since the torches heat the metal rapidly and
the temperatures are usually determined visually.
Flame hardening may be either manual or automat-
ic. Automatic equipment produces uniform results and
is more desirable. Most automatic machines have vari-
able travel speeds and can be adapted to parts of various
sizes and shapes. The size and shape of the torch de-
pends on the part. The torch consists of a mixing head,
straight extension tube, 90-degree extension head, an
adjustable yoke, and a water-cooled tip. Practically any
shape or size flame-hardening tip is available (fig. 2-1).