Other methods of carburizing use gases or liquids
from distortion and quenching cracks, and (2) normal
that have a high carbon content. Although these
tempering is not required after the process.
methods also use the principle of increasing the carbon
content to harden the outer shell or case of the steel, they
use different equipment and different procedures.
MARTEMPERING
CYANIDING can be used to obtain a hard
MARTEMPERING is the quenching from the
superficial case on low-carbon steels. The process
normal austenitizing temperature in a molten salt bath
involves the introduction of both carbon and nitrogen
maintained at approximately the start of the martensite
into the surface layers of the steel. Steels to be cyanided
temperature. The part is held at this temperature long
normally are heated in a molten bath of cyanide-
carbonate-chloride salts and then quenched in brine,
the piece, but not long enough to allow any
water, or mineral oil. The temperature of operation is
transformation to take place. The material being
generally within the range of 1,550 to 1,600F. The
heat-treated is then removed from the hot bath and
depth of the case is a function of time, temperature,
allowed to air cool through the martensite temperature
range, followed by tempering to obtain the desired
immersion is quite short as compared with
mechanical properties. This two-step cooling process
carburizing, usually varying from 15 minutes to 2
has the same advantage as austempering--freedom
hours. The maximum case depth is rarely more than
from distortion and quenching cracks.
about 0.020 inch and the average depth is
considerably less.
CASE HARDENING
NITRIDING is still another method by which a case
or skin of hardened steel can be produced. The piece to
CASE HARDENING is a process by which a steel
be case hardened is put into a furnace and heated to
can be given a hard, wear-resistant surface while
between 950 and 1,200F and, at the same time, is
retaining a softer but tougher interior than would be
exposed to ammonia gas. The heat of the furnace causes
possible if the whole piece were hardened. Steels may
the ammonia to break down into nitrogen and hydrogen.
be case hardened by carburizing, cyaniding, nitriding,
Some of the nitrogen combines with the elements in the
and various other processes.
steel to form chemical compounds called nitrides in the
outer layer of the steel. These nitrides give the surface
CARBURIZING is a term applied to several case-
its hard, wear-resistant qualities.
hardening processes in which carbon is introduced into
the surface layer of the steel. The steel is heated in
contact with a substance that has a high carbon content;
CARBON TOOL STEELS
it is held at a temperature above the upper
transformation temperature for a suitable length of time
and is then quenched rapidly to produce a hardened
The carbon content of plain-carbon steels ranges
outer layer or "case" over a softer, tougher core. As a
from about 0.20 to 1.13 percent. To heat-treat these
rough indication, a carburized depth of about 0.030 to
steels, heat them uniformly to 1,450 to 1,550F, using
0.050 inch can be obtained in about 4 hours at 1,700F,
the lower end of the temperature range for the higher
depending upon the type of carburizing agent, which
carbon content. Use brine or water as the quenching
may be a solid, liquid, or gas.
medium, and agitate the part vigorously to prevent the
accumulation of bubbles on the surface of the metal. Do
Sometimes the steel to be carburized is packed in a
not remove these steels from the quench until they have
substance such as charcoal and is then heated in a
reached a temperature of less than 200 to 250F. To
furnace. The length of time the piece is left in the
prevent cracking, do not allow the steel to cool below
furnace determines the depth to which the carbon will
125F and follow the cooling immediately with
be absorbed. For many pieces the procedure is to
tempering. Tools made from these steels do not have to
carburize the material, allow it to cool slowly, reheat it,
withstand severe usage or high cutting speeds. Some
and then harden it by quenching. Small pieces are
sometimes dumped into the quenching medium as soon
examples are wrenches, hammers, pliers, knives, simple
as they are removed from the carburizing furnace.
dies, and low-speed cutting tools.
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