hardening of Carbon steel - Carburizing
or case hardning is the addition of carbon to the surface of low-carbon
steels at temperatures generally between 850 and 950°C (1560
and 1740°F), at which austenite, with its high solubility for
carbon, is the stable crystal structure. Hardening is accomplished
when the high-carbon surface layer is quenched to form martensite
so that a high-carbon martensitic case with good wear and fatigue
resistance is superimposed on a tough, low-carbon steel core.
hardness of carburized steels is primarily a function of carbon
content. When the carbon content of the steel exceeds about 0.50%
additional carbon has no effect on hardness but does enhance hardenability.
Carbon in excess of 0.50% may not be dissolved, which would thus
require temperatures high enough to ensure carbon-austenite solid
hardening depth of carburized steel is a function of carburizing
time and the available carbon potential at the surface. When prolonged
carburizing times are used for deep case depths, a high carbon potential
produces a high surface-carbon content, which may thus result in
excessive retained austenite or free carbides. These two microstructural
elements both have adverse effects on the distribution of residual
stress in the case-hardened part. Consequently, a high carbon potential
may be suitable for short carburizing times but not for prolonged
steels for case hardening usually have base-carbon contents of about
0.2%, with the carbon content of the carburized layer generally
being controlled at between 0.8 and 1% C. However, surface carbon
is often limited to 0.9% because too high a carbon content can result
in retained austenite and brittle martensite.
steels that are carburized are killed steels (deoxidized by the
addition of aluminum), which maintain fine grain sizes to temperatures
of about 1040°C. Steels made to coarse grain practices can be
carburized if a double quench provides grain refinement. Double
quenching usually consists of a direct quench and then a requench
from a lower temperature.
alloy steels for case hardening are now specified on the basis of
core hardenability. Although the same considerations generally apply
to the selection of uncarburized grades, there are some peculiarities
in carburizing applications.
in a case-hardened steel, the hardenability of both case and core
must be considered. Because of the difference in carbon content,
case and core have quite different hardenabilities, and this difference
is much greater for some steels than for others.
the two regions have different in-service functions to perform.
Until the introduction of lean alloy steels such as the 86xx series,
with and without boron, there was little need to be concerned about
case hardenability because the alloy content combined with the high
carbon content always provided adequate hardenability. This is still
generally true when the steels are direct quenched from carburizing,
so that the carbon and alloying elements are in solution in the
case austenite. In parts that are reheated for hardening and in
heavy-sectioned parts, however, both case and core hardenability
requirements should be carefully evaluated.
relationship between the thermal gradient and the carbon gradient
during quenching of a carburized part can make a measurable difference
in the case depth as measured by hardness. That is, an increase
in base hardenability can produce a higher proportion of martensite
for a given carbon level, yielding an increased measured case depth.
Therefore, a shallower carbon profile and shorter carburizing time
could be used to attain the desired result in a properly chosen
Hardness. A common mistake is to specify too narrow a range of core
hardness. When the final quench is from a temperature high enough
to allow the development of full core hardness, the hardness variation
at any location will be that of the hardenability band of the steel
at the corresponding position on the end-quenched hardenability
standard steels purchased to chemical composition requirements rather
than to hardenability, the range can be 20 or more HRC points; for
example, 8620 may vary from 20 to 45 HRC at the 4/16 in.(6.35mm)
position. The 25-point range emphasizes the advantage of purchasing
to hardenability specifications to avoid the intolerable variation
possible within the ranges for standard chemistry steels. Another
way to control core hardness within narrow limits without resorting
to the use of high-alloy steels is to use a final quench from a
lower temperature so that full hardness in the case will be developed
without the disadvantage of excessive core hardness.
are almost always oil quenched because distortion must be held to
the lowest possible level. Therefore, alloy steels are usually selected,
with much debate about which particular alloy. The lower-alloy steels
such as 4023, 5120, 4118, 8620, and 4620, with a carbon range between
0.15 and 0.25%, are widely used and generally satisfactory. Usually,
the first choice is one of the last two steels mentioned, either
of which should be safe for all ordinary applications. The final
choice, based on service experience or dynamometer testing, should
be the least expensive steel that will do the job. For heavy-duty
applications, higher-alloy grades such as 4320, 4817, and 9310 are
justifiable if based on actual performance tests. The life testing
of gears in the same mountings used in service to prove both the
design and the steel selection is particularly important.
other applications, when distortion is not a major factor, the carbon
steels described above, water quenched, can be used up to a 50 mm
(2 in.) diameter. In larger sizes, low-alloy steels, water quenched,
such as 5120, 4023, and 6120 can be used, but possible distortion
and quench cracking must be avoided.
Methods. While the basic principle of carburizing has remained unchanged
since carburizing was first employed, the method used to introduce
the carbon into the steel has been a matter of continuous evolution.
its earliest application, parts were simply placed in a suitable
container and covered with a thick layer of carbon powder (pack
carburizing). Although effective in introducing carbon, this method
was exceedingly slow, and as the demand for greater production grew,
a new process using a gaseous atmosphere was developed.
gas carburizing, the parts are surrounded by a carbon-bearing atmosphere
that can be continuously replenished so that a high carbon potential
can be maintained. While the rate of carburizing is substantially
increased in the gaseous atmosphere, the method requires the use
of a multicomponent atmosphere whose composition must be very closely
controlled to avoid deleterious side effects, for example, surface
and grain-boundary oxides. In addition, a separate piece of equipment
is required to generate the atmosphere and control its composition.
Despite this increased complexity, gas carburizing has become the
most effective and widely used method for carburizing steel parts
in large quantities.
efforts required to simplify the atmosphere, carburizing in an oxygen-free
environment at very low pressure (vacuum carburizing) has been explored
and developed into a viable and important alternative. Although
the furnace enclosure in some respects becomes more complex, the
atmosphere is greatly simplified. A single-component atmosphere
consisting solely of a simple gaseous hydrocarbon, for example methane,
may be used. Furthermore, because the parts are heated in an oxygen-free
environment, the carburizing temperature may be increased substantially
without the risk of surface or grain-boundary oxidation. The higher
temperature permitted increases not only the solid solubility of
carbon in the austenite but also its rate of diffusion, so that
the time required to achieve the case depth desired is reduced.
vacuum carburizing overcomes some of the complexities of gas carbunzing,
it introduces a serious new problem that must be addressed. Because
vacuum carburizing is conducted at very low pressures, and the rate
of flow of the carburizing gas into the furnace is very low, the
carbon potential of the gas in deep recesses and blind holes is
quickly depleted. Unless this gas is replenished, a great nonuniformity
in case depth over the surface of the part is likely to occur. If,
in an effort to overcome this problem, the gas pressure is increased
significantly, another problem arises, that of free-carbon formation,
in order to obtain cases of reasonably uniform depth over a part
of complex shape, the gas pressure must be increased periodically
to replenish the depleted atmosphere in recesses and then reduced
again to the operating pressure. Clearly, a delicate balance exists
in vacuum carburizing: The process conditions must be adjusted to
obtain the best compromise between case uniformity, risk of sooting,
and carburizing rate.
method that overcomes both of these major problems, yet retains
the desirable features of a simple atmosphere and permissible operating
temperature is plasma or ion carburizing.
summarize, carburizing methods include:
Salt bath carburizing
These methods introduce carbon by the use of gas (atmospheric-gas,
plasma, and vacuum carburizing), liquids (salt bath carburizing),
or solid compounds (pack carburizing). All of these methods have
limitations and advantages, but gas carburizing is used most often
for large-scale production because it can be accurately controlled
and involves a minimum of special handling.
Vacuum carbunzing and plasma carburizing have found applications
because of the absence of oxygen in the furnace atmosphere. Salt
bath and pack carburizing arc still done occasionally, but have
little commercial importance today.
characteristics of the above-mentioned carburizing methods fall
into two general groups:
methods, which introduce carbon by gas atmospheres, salt baths or
Plasma methods, which impinge positive carbon ions on the surface
of a steel part (the cathode)
The main difference between conventional and plasma methods is the
reduced carburizing times achieved in plasma-assisted methods. The
quickly attained surface saturation also results in faster diffusion
kinetics. Furthermore, plasma carburizing produces very uniform
case depths, even in parts with irregular surfaces.
With the conventional methods, carburization always takes place
by means of a gaseous phase of carbon monoxide; however, each method
also involves different reaction and surface kinetics, producing
different case-hardening results.
general, with conventional methods, carbon monoxide breaks down
at the steel surface:
? CO2 + C
liberated carbon is readily dissolved by the austenite phase and
diffuses into the body of the steel. For some process methods (gas
and pack carburizing), the carbon dioxide produced may react with
the carbon atmosphere or pack charcoal to produce new carbon monoxide
by the reverse reaction.
is most frequently performed between 850 and 950°C (1550 and
1750°F), but sometimes higher temperatures are used to reduce
cycle times and/or produce deeper depths of the high-carbon surface
comprehensive model of gas carburization must include algorithms
Kinetics of the surface reaction
Kinetics of the reaction between endogas and enriching gas
Purging (for batch processes)
The atmosphere control system.