Carburizing is a case-hardening process in which carbon is dissolved in the surface
layers of a low-carbon steel part at a temperature sufficient to render the steel
austenitic, followed by quenching and tempering to form a martensitic microstructure.
The resulting gradient in carbon content below the surface of the part causes a
gradient in hardness, producing a strong, wear-resistant surface layer on a material,
usually low-carbon steel, which is readily fabricated into parts.
In gas carburizing, commercially the most important variant of carburizing, the source
of carbon is a carbon-rich furnace atmosphere produced either from gaseous hydrocarbons,
for example, methane (CH4), propane (C3H3), and butane
(C4H10), or from vaporized hydrocarbon liquids.
Carbon Sources
Low-carbon steel parts exposed to carbon-rich atmospheres derived from a wide variety
of sources will carburize at temperatures of 850°C (1560°F) and above.
In the most primitive form of this process, the carbon source is so rich that the
solubility limit of carbon in austenite is reached at the surface of the steel and some
carbides may form at the surface. Such atmospheres will also deposit soot on surfaces
within the furnace, including the parts. While this mode of carburizing is still
practiced in parts of the world in which resources are limited, the goal of current
practice in modern manufacturing plants is to control the carbon content of furnace
atmospheres so that:
- The final carbon concentration at the surface of the parts is below the solubility
limit in austenite.
- Sooting of the furnace atmosphere is minimized.
A carrier gas similar in composition to Endogas produced from methane can be formed
from a nitrogen-methanol blend. The proportions of nitrogen and methanol (CH3OH)
are usually chosen to give the same nitrogen-to-oxygen ratio as that of air, that is,
about 1.9 volumes of nitrogen for each volume of gaseous methanol.
Carburizing Equipment
Gas carburizing furnaces vary widely in physical construction, but they can be divided
into two major categories, batch and continuous furnaces.
In a batch-type furnace, the workload is charged and discharged as a single unit or
batch. In a continuous furnace, the work enters and leaves the furnace in a continuous
stream. Continuous furnaces are favored for the high-volume production of similar parts
with total case depth requirements of less than 2 mm (0.08 in.).
Carburizing Process Variables
The successful operation of the gas carburizing process depends on the control of three
principal variables:
- Temperature
- Time
- Atmosphere composition.
Temperature. The maximum rate at which carbon can be added to steel
is limited by the rate of diffusion of carbon in austenite. This diffusion rate
increases greatly with increasing temperature; the rate of carbon addition at
925°C (1700°F) is about 40% greater than at 870°C (1600°F).
The temperature most commonly used for carburizing is 925°C (1700°F). This
temperature permits a reasonably rapid carburizing rate without excessively rapid
deterioration of furnace equipment, particularly the alloy trays and fixtures. The
carburizing temperature is sometimes raised to 955°C (1750°F) or 980°C
(1800°F) to shorten the time of carburizing for parts requiring deep cases.
Conversely, shallow case carburizing is frequently done at lower temperatures because
case depth can be controlled more accurately with the slower rate of carburizing
obtained at lower temperatures.
Therefore, for best results, the workload should be heated to the carburizing
temperature in a near-neutral furnace atmosphere. In batch furnaces, parts can be
heated in Endogas until they reach the furnace temperature; then carburizing can
commence with the addition of the enriching gas. Many new continuous furnaces are being
built with separate preheat chambers to ensure that the load is at a uniform temperature
before entering the carburizing zone. In continuous furnaces that lack positive
separation between heating and carburizing stages, the best that can be done is to:
- Add only Endogas to the front of the furnace.
- Establish a front-to-back internal flow of atmosphere gases by adjusting flow rates
and orifice size in the effluent lines at either end of the furnace.
Carbon Potential. The carbon potential a furnace atmosphere at a
specified temperature is defined as the carbon content pure iron that is in
thermodynamic equilibrium with the atmosphere. The carbon potential of the furnace
atmosphere must greater than the carbon potential of the surface of the work pieces in
order for carburizing to occur. It is the difference in carbon potential that provides
the driving force for carbon transfer to the parts.
Carbon Diffusion. The combined effects of time, temperature, and carbon
concentration on the diffusion of carbon in austenite can be expressed by Fick’s
laws of diffusion.
Fick’s first law states that the flux of the diffusing substance perpendicular to
plane of unit cross-sectional area is proportional to the local carbon gradient
perpendicular to the plane. The constant of proportionality is the diffusion coefficient
D, which has the units (distance)2/time. Fick’s second law is a material
balance within elemental volume of the system; the flux carbon into an elemental volume
of iron minus the flux of carbon out of the elemental volume equals the rate of
accumulation of carbon within the volume. Combining the two laws leads to a partial
differential equation that describes the diffusion process.
Alloy Effects. The various alloying elements found in carburizing
steels have an influence on the activity of carbon dissolved in austenite. A definition
of carbon activity (ac) is:
ac = (wt% C)Γ
where Γ, the activity coefficient, is chosen so that ac=1 for an amount
of carbon in solution that is in equilibrium with graphite.
Chromium tends to decrease the activity coefficient, and nickel tends to raise it. As a
consequence, foils of a chromium-bearing steel equilibrated with a specific furnace
atmosphere will take on more carbon than pure iron, and nickel-bearing steels will take
on less carbon. It is also true that carbides are produced at lower carbon potentials
in chromium-bearing steels than in carbon steels.
The primary effect of alloying elements on the diffusion of carbon is due to their
effect on the driving force for the surface reaction. To obtain the true driving force,
the surface carbon content in an alloy must be converted into the equivalent carbon
content in pure iron. Methods of correcting the activity coefficient of carbon for
alloy content are available. However, the quantity of experimental data upon which
such correlations are based is rather limited. Therefore, predictions should be
verified by experiments, particularly when an alloy contains substantial amounts
of more than one alloying element.
Process Planning
Designers usually specify the case hardness, case depth, and core hardness required
to meet the service loads they anticipate, for a particular part. It is the task of fat
the process engineer to develop the carburiziring treatment that will produce the
properties desired. Some of the considerations involved in setting up processes
include:
- Case microstructure
- Residual stress
- Alloy selection
- Operating schedules
- Quenchants
- Reheating for quenching
- Tempering
- Selective carburizing.
For a particular alloy, the amount of retained austenite in the case increases as the
case carbon content increases. An appreciable decrease in case hardness is usually
found when the amount of retained austenite exceeds about 15%, but for applications
involving contact loading, such as rolling element bearings, the best service life is
found when the retained austenite content is quite high, for example, 30 to 40%. In
other applications, especially when dimensional stability is critical, the retained
austenite content should be low.
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