hardening of Carbon steel - Carburizing
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 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
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
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
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
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 steel.
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 specimen.
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
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
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, or sooting.
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
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 charcoal packs
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
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 layer.
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.