This article was originally featured in Heat
Treating Progress, JAN/FEB 2003
Vacuum Carburizing: Process Evolution
New low-pressure processes with enhanced uniformity and
repeatability are helping to make vacuum carburizing more attractive to manufacturers.
SECO/WARWICK vacuum carburizing furnaces were developed for cellular
manufacturing operations in commercial shops or JIT plants. Both
one- and two-zone models are offered. Two-zone systems have a
convection heating chamber and a high-pressure gas-quench
chamber (up to 20 bar nitrogen or nitrogen-helium).
Demands
for higher quality and dissatisfaction with conventional vacuum
carburizing processes provided much of the impetus for the
development of low-pressure vacuum carburizing. Several factors
helped make this new technology possible:
- A better understanding of reactions among ethylene,
acetylene, propane, and methane during carburizing
- The ability to apply computer technology to the control
of heat treating equipment and thermochemical cycles
- Advances in high-pressure gas-quench technology (faster
cooling, more reliable equipment, and use of helium as the
cooling gas
- Decreases in the initial cost of vacuum
equipment
- The development of carburizing steels like Jomasco®* 23
MnCrMo5 that require slower-than-normal cooling rates to
optimize properties, and extend the service life of gears and
other parts made from them.
Low-pressure Processes
Vacuum carburizing using methane (CH4)
as the carburizing gas was developed in the United States in the
1960s and is now used worldwide. Carburizing technology based on
methane requires higher temperatures and pressures up to 500
mbar (375 torr).
This process does not always provide the
uniformity and repeatability needed to meet today’s
specifications for precision parts. Other drawbacks include the
cost of oil disposal and a high furnace maintenance requirement.
To avoid these problems, new technologies have been developed
that use propane (C3H8), ethylene (C2H4),
or acetylene (C2H2) for carburizing at
pressures below 20 mbar (15 torr).
At these low pressures and with a carburizing
gas properly distributed, carbon penetration is increased
throughout the load. Thermal dissociation of the hydrocarbons
takes only a couple of seconds, providing sufficient time to
react with the hot load surface but not long enough to create
soot or tar on furnace parts. This low-pressure vacuum
carburizing process is being applied to a wide range of
components in a variety of industries.
Gas selection:
Propane’s popularity
is due primarily to its ease of transportation and wide
availability. Major drawbacks are a tendency to create soot and
tar because of difficulties in its thermal dissociation, poor
penetration of the gas into holes, and problems with case
uniformity in densely packed loads.
In 1979, the Russian scientists Krilov, Yumatov, and Kubatov developed a vacuum carburizing process that
used acetylene. Soviet Union patent No. 668978 details
carburizing with acetylene at pressures between 9 and 931 mbar
(7 and 700 torr). Professor Ryzhov from the N. E. Bauman Moscow
State Technical University
detailed the ion carburizing process with acetylene in articles
published in Vestnik Mashinostrayeniya in 1985, and in MITOM
in 1992 and 1995. Acetylene has a clear advantage in its ability
to uniformly carburize deep holes. Drawbacks are related to
supply logistics and the possibility of oxidation during
transportation and subsequent contamination of the process.
The problems associated with using either
propane or acetylene as the carburizing gas can be eliminated by
substituting a mixture of acetylene, ethylene, and hydrogen
(SECO/WARWICK patent pending).
C2H2-C2H4-H2
process
To avoid tar formation and still obtain
sufficient carbon potential within the furnace, a vacuum
carburizing process must first be precisely established and then
maintained throughout the cycle. Several parameters influence
the process; major ones being mass flow of carbon (gas flow
rate), carburizing temperature, gas pressure, and the
carburizing (or boost) and diffusion times.
Parts usually are convection heated to 750°C
(1380°F) at 1.5 bar nitrogen pressure, followed by heating in
vacuum to carburizing temperature. Times of heating and holding
depend on part cross section, part weight, and load density. The
carburizing temperature is between 900 and 1050°C (1650 and 1920°F) and depends on the
steel type being treated, the carbon content and the required
case depth. When carburizing temperature is reached, the
carburizing gas is admitted into the heating zone through jets
located throughout the hot zone. A maximum number of jets is
used to ensure proper carburizing gas inflows and uniform charge
penetration.
The carburizing process is computer
controlled. Different chemical reactions may occur in different
sequences but the end result is the same: free atoms of carbon
that react with and penetrate the surface of the steel.
Gas reactions:
Propane, ethylene, and
acetylene decompose rapidly above 800°C (1472°F). At this
temperature, many reactions compete and hydrocarbon radicals (CxHy)
form. As the temperature increases, decomposition of C3H8,
C2H4, and C2H2
accelerates to form atomic carbon. Hydrogen is a by-product that
results in reduction of the metal oxides on the surface of
parts. This reduction of surface oxides facilitates absorption
of carbon into the steel.
The carburizing process can be continuous
(single pulse) or have repeated boost and diffuse steps (a
multipulse process). In the first stage, the inflow of
carburizing gas provides a very high concentration of carbon
that can be absorbed by austenite. In the diffusion stage, the
carburizing gas inflow is cut off, and the carbon is allowed to
diffuse into the surface. Diffusion reduces the surface carbon
concentration, which allows for further carburization. The boost
and diffuse stages are time controlled. The high mass flow of
carbon (gas inflow) and higher process temperature shorten the
carburizing process.
Competing technologies are characterized by different
carburizing agents (acetylene, propane, or an
ethylene-acetylene-hydrogen mixture, for example) and furnace
designs. Selection of the optimum technology requires
consideration of many factors, which we have sorted into three
categories: product or part data, furnace design, and
carburizing agent.
- Geometry and configuration: The number and depth of holes, for example, will affect which carburizing
agent and pressure are chosen.
- Weight and surface area: Part weight, for example,
will influence furnace size and the material used for fixtures.
- Material: The alloy being carburized will
influence the gas-cooling pressure and furnace configuration
(single chamber, multichamber, or tunnel furnace, for example).
- Production rate will influence furnace type and size.
- Case depth and carbon content will influence the
carburizing agent and furnace design.
- Load density influences furnace design and
carburizing pressure and gas. For example, it can affect the
carburizing pressure that will create soot, particularly if the
carburizing agent is propane. (In addition, compared with
propane, acetylene or ethylene mixtures, in gerneral, will
carburize faster, shortening the cycle.)
Furnace Design Considerations
Flexibility - The larger a furnace, the
less flexible it is. A single-chamber furnace is the most
flexible. Tunnel furnaces are the least flexible, but they can
simultaneously process a very large number of identical parts.
Hot zone - Hot zone size and shape will influence both cooling
uniformity and speed — cooling speed decreases as hot zone
size increases. A cylindrical hot zone is more flexible than one
having a rectangular cross section and, size for size, provides
20% greater cooling capability. The number of carburizing
gas inlet jets will influence case uniformity and tar and soot
formation.
Heating - A furnace should be capable of heating by
convection and radiation. Use of convection speeds heating time
for a densely packed load by 20 to 40%.
Quench gas - Different gases have different cooling
capabilities. Hydrogen is the fastest, followed by helium and
then nitrogen. The material being processed dictates which gas
and pressure are needed. Use of nozzles to introduce the gas
generally provides more uniform and faster cooling than bung
designs, where gas enters from openings usually located in the
bottom and top of the hot zone. However, the bung design is
preferable for long parts hung vertically in a hot zone.
Maintenance - Multi-chamber and tunnel
furnaces require more maintenance and associated downtime. In a
tunnel furnace, if one chamber requires maintenance then all
chambers need to be taken out of service. Modular multi-chamber
systems that have separate carburizing and cooling chambers do
not have this drawback.
Type of operation influences equipment selection
and utilization. Cellular and flexible manufacturing operations
require a different type of furnace than that which is optimum
for a mass production application. In the latter case, a high
production rate outweighs the need for flexibility.
Carburizing agent factors
Hole penetration: Acetylene provides better deep
hole penetration due to its ability to rapidly dissociate,
producing free carbon.
Availability: Propane is readily
available and easy to transport and store.
Surface quality: An additional dose of hydrogen
provides surface deoxidation and better surface penetration by
carbon, but it also may cause hydrogen embrittlement.
*Jomasco® is the registered trademark for the family of steel products
manufactured by ASCOMETAL, France.
For more information, contact
Mr. Janusz Kowalewski, HPQ product manager, SECO/WARWICK Corp., +1-814/332-8491;
fax: 814/724-1407; e-mail: jkowalew@secowarwick.com;
Mr. Kowalewski is a member of the ASM Heat Treating Society, and serves on the Heat Treating
Progress Editorial Committee.
R1, 4/7/2003, corrected spelling of Russian Scientist's
names.
R2, 12/7/2004 updated style
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