A Case for Acetylene Based

Low Pressure Carburizing of Gears

By Daniel H. Herring

A catalytically decomposable hydrocarbon with a high average carbon flux, Acetylene

is a strong candidate for the preferred carbon gas choice.

Low pressure carburizing (LPC) is the as the hydrocarbon gas of choice helps and Development for C. I. Hayes, Inc.,
technology of choice for the precise us achieve this. Cranston, RI. Commercialization followed
carburizing of high-performance gearing in early 1969 (Figure 2).
[1]. To achieve absolute process HISTORICAL PERSPECTIVE Full acceptance of the process
repeatability and the highest quality The history of vacuum carburizing is by industry, however, involved three
outcome, it should come as no surprise a fascinating one. The process was decades of work and contributions from
that one of our goals is to fix as many of invented in late 1968 and subsequently all over the world including the discovery
the process parameters (e.g. hydrocarbon patented (U. S. Patent No. 3,796,615, and patenting of acetylene technology
gas type, pressure, flow rate, etc.) as U. S. Patent RE 29,881) by Mr. Herbert in the former Soviet Union (USSR
possible (Figure 1). Selecting acetylene W. Westeren, Director of Research Patent No. 668978) by V. S. Krylov, V.

40 | Thermal Processing for Gear Solutions

 Figure 1: 
Variables Influencing the Carburizing Process.

•D
 evelopment of high pressure gas and
oil quenching technology;
•
Availability of low cost carburizing
alloys specifically designed to take
advantage of vacuum carburizing –
including high temperature capability.

Modern-day carburizing installations

(Figure 3) have taken advantage of
these developments.

THE LPC PROCESS EXPLAINED Figure 2: 
First Commercial Heat Treat Load, February
LPC is a recipe-controlled boost/diffuse
1969.
 Gears Carburized at 930°C (1700°F), 13 mbar
process. By contrast, atmospheric gas (10 torr), Methane (CH4). (Photograph Courtesy of C. I. Hayes)
carburizing is controlled via carbon
potential. In vacuum carburizing,
process-related parameters such as One method of recipe development
temperature, carburizing gas-flow, involves solving the following three (3)
time, and pressure are adjusted and equations:
controlled to achieve the desired case
A. Yumatov and V. V. Kurbatov in 1977 profile in the parts. (1)
and culminating in the application and
patenting (U. S. Patent No. 5,702,540)
of acetylene based carburizing by Mr. K.
Kubota, JH Corporation (formerly Japan
Hayes Corporation), Nagoya, Japan.
Since that time, a significant number of
individuals and companies have made
patentable inventions that have helped
advance the technology [1].

The worldwide technology advances

needed to make LPC a viable technology
included:

• Improvements in the design and

construction of vacuum furnaces;
• D evelopment of low pressure (< 20
torr) carburizing methods;
• P rocess optimization – especially the
selection of hydrocarbon gas;
• D evelopment of optimized gas injection
methods and flow/pressure controls;
• C reation of empirical data bases and Figure 3: Present Day Commercial Heat Treat Load, February 2012.
Gears Carburized at 930°C (1700°F),
design of process simulators; 14.5 mbar (11 torr), Acetylene (C2H2). (Photograph Courtesy of ALD Thermal Treatment)

thermalprocessing.com | 41
Figure 4: 
Screen Shot – LPC Simulation Program. (Photograph Courtesy of ALD Figure 6: Acetylene Decomposition – RGA Analysis. [3]
Vacuum Technologies GmbH)

Figure 7: Propane Decomposition – RGA Analysis. [3]

as a function of depth can be predicted. In addition, work is

underway to use the software to predict such parameters as
hardness values and residual stress levels versus depth below
the surface.
Figure 5: 
Comparison of Simulated and Measured Carbon Potential for 20MnCr5 Simulation programs are available from any number of
Material. (Photograph Courtesy of ALD Vacuum Technologies GmbH) suppliers of vacuum carburizing equipment and are designed
to create recipes and test scenarios for process development.
These programs are based on a mathematical description
(2) of the carbon dissociation and adsorption of the carbon at
the surface of the parts and equations, which describe the
(3) diffusion of the carbon into the material. While the carbon
transport to the surface in low pressure vacuum carburizing
Where: differs significantly from that in atmospheric gas carburizing,
D = e ffective case depth (50 HRC equivalent) the same diffusion laws apply for the carbon transport within
k = carburizing constant (c.f. Table 1) the material.
t = total time, in hours
c = carburizing time, in hours TYPICAL INPUT PARAMETERS OF THE SOFTWARE
r = boost/diffuse ratio (c.f. Table 1) INCLUDED:
d = diffusion time, in hours • Material
• Carburizing temperature
While this method can be used effectively, recipe development • Targeted carburizing depth
by means of a simulation program has become quite popular • Targeted surface carbon content
in the industry. The recipe allows us to determine a sequence • Surface carbon content limit
of carburizing and diffusion pulses in which carbon profile • Load surface area

42 | Thermal Processing for Gear Solutions

 Carburizing Constant Boost/
Diffuse Ratio
Temperature, k value r value
°C (°F)
D D
(mm) (inches)
840 (1550) 0.25 0.010 0.75
870 (1600) 0.33 0.013 0.65
900 (1650) 0.41 0.016 0.55
930 (1700) 0.51 0.020 0.50 Figure 8: Vacuum Hot Zone Contamination Due to Hydrocarbon Gas Choice.
950 (1750) 0.64 0.025 0.45
980 (1800) 0.76 0.030 0.40 time thereby increasing the productivity NOTES:
of the system. As soon as the required [a] D MF Acetylene (without acetone)
1010 (1850) 0.89 0.035 0.35
profile is achieved, the recipe is fixed preferred, though not mandatory
1040 (1900) 1.02 0.040 0.30 and not changed. [b] Typical dilutions up to 50%
Table 1: Carburizing Parameters. [2] Furthermore the simulation program [c]  Typical dilution 7:1 (US Patent
is a powerful tool to achieve the wanted 7,514,035 Solar Atmospheres Inc.)
microstructure after vacuum carburizing. [d]  Typically ratios of acetylene to
The simulation program can be used The program shows the formation of the ethylene to hydrogen are 3:2:1 or
in two modes. The first mode allows carbon profile as a function of time for 2:2:1 (US Patent 7,550,049 SECO/
one to enter a desired surface carbon different surface distances. Therefore it WARWICK Corporation)
content and case depth. Then the is possible to create recipes that meet [e] C yclohexane (US Patent 7,267,793
program calculates the required recipe microstructural specification, i.e. the Surface Combustion, Inc.)
consisting of different carburizing and absence of large quantities of carbides [f] Temperatures above 955°C (1750°F)
diffusion pulses (Figure 4). Furthermore or avoidance of excessive amounts of recommended unless plasma
the program shows diagrams of carbon retained austenite. assisted
flux versus time and carbon profile A further result of the simulation [g] Typical dilution: 40/60 to 60/40
versus time and distance from surface. program is the calculation of the correct (methane/propane)
The second mode is to enter a hydrocarbon flux, which depends on the
certain recipe consisting of carburizing actual load surface area and the carbon WHY ACETYLENE?
and diffusion pulses and the program yield of the carburizing gas that is used. Acetylene is a catalytically decomposable
calculates the resulting carburizing The carbon yield defines the amount hydrocarbon, which essentially means
profile. The first mode is generally of carbon transferred into the parts in that it will break down into its elemental
preferred. These simulations have been relation to the amount of carbon supplied constituents (Equation 1, Figure 6)
found to be quite accurate (Figure 5). to the treatment chamber by injecting in the presence of an iron catalyst.
The simulated carbon profile is very the carburizing gas. For acetylene the Other hydrocarbons (e.g. methane,
similar to the measured carbon profile. carbon yield is in the range of 60% to propane) are thermally decomposable,
Some simulation programs also include almost 80%. This tool allows the user to which means that they will break down
a quenching module for calculation of reduce the amount of carburizing gas to immediately upon entry into the hot
the case hardening depth instead of the a minimum, which is both economically zone of the vacuum furnace negating
carburizing depth. and ecologically beneficial. the ability of the carbon to react with
The use of a simulation program As the chemistry of the materials to the surface of the steel and creating
to create a carburizing recipe is well be carburized has an influence on the unwanted hydrocarbon byproducts
accepted for LPC applications in all carburizing profile to be achieved, it is (Figure 7) and ultimately contaminating
manufacturing industries including possible for a user to enter the exact the hot zone. In addition, acetylene has
automotive and aerospace. Using these chemical composition of their own a higher average carbon flux, about
simulators, a test load is run and the material into the program and store this 150 g/m2-h, then other hydrocarbons.
parts are checked for correct case data in the already existing material data For these technical reasons, acetylene
profile. Normally, the achieved case bank. is a strong candidate for the preferred
profile is within the specified range. If hydrocarbon gas choice.
not, the parameters are adjusted slightly HYDROCARBON CHOICES
and a second cycle is run. Since the Over the years a number of hydrocarbon (1) C 2H 2 —> 2C + H 2
LPC process offers consistent case gases and liquids (Table 2) have been
uniformity, it might be advantageous to used to supply a source of carbon for ACETYLENE DECOMPOSITION –
perform additional simulation runs so LPC. While all of these choices are RGA ANALYSIS
as to adjust the case depth towards the possible, acetylene and acetylene In general, carburizing gases must have
lower end of the case-specification. This mixtures have become the dominant high purity (99.95% or better) and
is done to reduce the overall process choice in the industry. have the ability to be effective at low

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(< 20 torr) pressure. The use of high avoids these issues and dramatically transported in a solvent, either acetone
pressures coupled with impure gases reduces maintenance time and cost. or DMF (dimethyl formamide). DMF
(i.e. the presence of so-called “heavy” has a boiling point about 100˚C (212˚F)
hydrocarbons) has in the past resulted DMF ACETYLENE higher than acetone, and acetylene
in the formation of excessive deposits of Gas consistency plays an important and similar solubility (Table 3). Thus
soot and tar (Figure 8) and unacceptably role in the hydrocarbon choice for DMF is less likely to volatilize and
high equipment maintenance. Acetylene LPC. Chemically produced acetylene is enter the vacuum furnace reducing the
risk of introducing oxygen (leading to
Family Combinations concerns over intergranular oxidation)
or other unwanted constituents into the
carburizing process.
Acetylene & Acetylene Mixtures 100% Acetylene (C2H2) [a]
Acetylene + Nitrogen [b] APPLICATION EXAMPLE
[c] Full production loads (Figure 9) of
Acetylene + Hydrogen
several types of SAE 8620 transfer
Acetylene + Ethylene (C2H4) + Hydrogen [d] pinion gears and clutch hubs (Figure
Acetylene + Cyclohexane 10) were run using two (2) different
carburizing methods (gas atmosphere
and LPC). In the case of the acetylene
Cyclohexane & Cyclohexane Mixtures 100% Cyclohexane (C6H12) [e]
vacuum carburized gears, oil and high
Cyclohexane + Acetylene gas pressure quenching methods were
employed. In the case of the atmosphere-
Methane & Methane Mixtures 100% Methane (CH4) [f] carburized gears, traditional plug
quenching methods were used.
Methane + Propane [g] Low pressure carburizing was
performed at 960°C (1760°F) for 3.34
Propane & Propane Mixtures 100% Propane (C3H8) hours (boost/diffuse time) with acetylene
(2200 nL/h, 10.5 mbar) followed by
Propane + Methane [f]
either oil quenching (70% agitator
Propane + Hydrogen speed) or high pressure gas quenching
Propane + Butane (C4H10) (11 bar, nitrogen). Targeted effective
(50 HRC) case depth was 1.25 mm
Table 2: Hydrocarbon Choices for LPC. (0.050”) with a surface carbon content
of 0.72%C. Gas quenching utilized four
Property Acetone Dimethyl Formamide (DMF) (4) changes in speed and pressure
made through the critical transformation
range of the material.
Boiling Point (˚C) 56.5 152 Atmosphere carburizing (Endothermic
gas, natural gas additions) was
Acetylene Solubility [a] 425 400
performed at 960°C (1760°F) for 4.0
Table 3: Solubility Comparison. hours with a carbon potential of 1.3%

Figure 9: 
Typical Furnace Load, 385 kg (850 lbs.). Figure 10: Vacuum Carburizing of Highly Distorted Prone Gearing.

44 | Thermal Processing for Gear Solutions

followed by slow cooling, reheating to 830°C (1525°F), CONCLUSION
stabilizing and plug quenching (45 seconds) in oil. Targeted While many choices for hydrocarbon gases are possible in
effective (50 HRC) case depth was 1.25 mm (0.050”) with a LPC, and although special circumstances may necessitate
surface carbon content of 0.80 -0.90%C. an alternative choice, acetylene and acetylene mixtures have
All gears were subsequently tempered at 150°C - 175°C clearly separated themselves as the preferred hydrocarbon
(300°F - 350°F) for two (2) hours at temperature. choice from a process, equipment and quality standpoint. In
the heat treatment process, every choice matters. Remember that
The result of processing this family of gears the right hydrocarbon choice will save time and money, as well as
found that: improve the final product.
• L ow pressure vacuum carburizing in combination with high
pressure gas quenching produced the most consistent REFERENCES
repeatability. [1] H erring, Daniel H., Vacuum Heat Treatment, BNP Custom
• The degree of dimensional change is capable of being Media Group, 2012.
compensated for in standard post manufacturing processes. [2] A SM Handbook, Volume 4: Heat Treating, ASM International,
• Low pressure vacuum carburizing in combination with high 1991, p. 350.
pressure gas quenching allowed for the replacement of [3] J ones, William R., Low Torr Range Vacuum Carburizing in an
atmosphere carburizing and plug quenching on all gears in Experimental Vacuum Furnace, IIT/TPTC Vacuum Carburizing
this family. Symposium, November 2004.
• The depth of high hardness (> 58 HRC) was greatest in the [4] E sper, Bob, Acetylene: The Right Carbon Source for Low-
low-pressure vacuum carburized samples. Pressure Carburizing, Industrial Heating, 2009.
• The root-to-pitch line case depth ratio was 93% for vacuum [5] O tto, Frederick J., and Daniel H. Herring, Improvements in
carburizing compared to 63% for atmosphere carburizing. Dimensional Control of Heat Treated Gears, Gear Solutions,
• A tmosphere carburizing resulted in unacceptable levels of June 2008.
retained austenite.
• Gear charts indicated an average movement of 0.08 mm
(0.003”). The involute form remained intact after low About the author: Daniel H. Herring is “The Heat Treat Doctor” with
pressure vacuum carburizing and gas quenching, as did the THE HERRING GROUP, INC. For more information, call 630-834-3017 or email
lead on the gear teeth and splines. dherring@heat-treat-doctor.com, or visit www.heat-treat-doctor.com.

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