U.S. patent application number 11/959825 was filed with the patent office on 2008-06-26 for method for oxygen free carburization in atmospheric pressure furnaces.
Invention is credited to Karen Anne Connery, Robert Marcel De Wilde, Amitabh Gupta, Simon Ho, Richard A. Novak.
Application Number | 20080149225 11/959825 |
Document ID | / |
Family ID | 46330132 |
Filed Date | 2008-06-26 |
United States Patent
Application |
20080149225 |
Kind Code |
A1 |
Connery; Karen Anne ; et
al. |
June 26, 2008 |
METHOD FOR OXYGEN FREE CARBURIZATION IN ATMOSPHERIC PRESSURE
FURNACES
Abstract
A method of oxygen-free heat treatment of a steel part in an
atmospheric pressure furnace is disclosed. The present batch
treatment process employs an oxygen-free controlled gas atmosphere
including hydrogen gas in concentrations between about 1.0 percent
to 10.0 percent, a hydrocarbon gas, such as propylene, in
concentrations of between about 0.1 percent and 10.0 percent that
varies as a function of time, with the balance of the gas
atmosphere being nitrogen. The present continuous furnace treatment
process employs a plurality of zones with each zone including an
oxygen-free controlled gas atmosphere including hydrogen gas in
concentrations between about 1.0 percent to 10.0 percent that
varies across the different zones, a hydrocarbon gas, such as
propylene, in concentrations of between about 0.1 percent and 10.0
percent that varies across the different zones, with the balance of
the gas atmosphere within each zone being nitrogen. The presently
disclosed oxygen-free heat treatment processes, preferably
carburization processes, use a precisely controlled atmosphere to
minimize inter-granular oxidation, eliminate the formation of soot
and cementite or other metallic carbides, and avoid hydrogen
embrittlement.
Inventors: |
Connery; Karen Anne;
(Westmont, IL) ; Novak; Richard A.; (Naperville,
IL) ; Ho; Simon; (Naperville, IL) ; Gupta;
Amitabh; (Naperville, IL) ; De Wilde; Robert
Marcel; (St. Niklaas, BE) |
Correspondence
Address: |
PRAXAIR, INC.;LAW DEPARTMENT - M1 557
39 OLD RIDGEBURY ROAD
DANBURY
CT
06810-5113
US
|
Family ID: |
46330132 |
Appl. No.: |
11/959825 |
Filed: |
February 14, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11645447 |
Dec 26, 2006 |
|
|
|
11959825 |
|
|
|
|
Current U.S.
Class: |
148/206 |
Current CPC
Class: |
C23C 8/20 20130101; C21D
1/74 20130101; C21D 1/76 20130101 |
Class at
Publication: |
148/206 |
International
Class: |
C23C 8/20 20060101
C23C008/20 |
Claims
1. A method of treating a metal part in an atmospheric pressure
furnace comprising the steps of: heating the metal part in the
atmospheric pressure furnace having a gas atmosphere substantially
free of oxygen, the gas atmosphere comprising a carbon containing
gas, a reducing gas, and an inert gas; controlling the gas
concentrations of the carbon containing gas or reducing gas to
inhibit formation of metal-oxides on the metal part and to maintain
a carbon saturation factor of between about 0.75 and 1.10 with
negligible cementite or other metallic carbide formation; and
removing the treated metal part from the atmospheric pressure
furnace.
2. The method of claim 1 wherein the atmospheric pressure furnace
is a continuous-type furnace having a plurality of atmospheric
zones and the step of controlling the gas concentrations further
comprises maintaining the gas concentrations of the carbon
containing gas in each atmospheric zone at the same or different
levels to maintain a carbon saturation factor of between about 0.75
and 1.10 with negligible cementite or other metallic carbide
formation.
3. The method of claim 1 wherein the atmospheric pressure furnace
is a batch-type furnace and wherein the step of controlling the gas
concentrations further comprises varying the volume concentration
of the carbon containing gas as a function of time to maintain the
carbon saturation factor in the metal part between about 0.75 and
1.10 with negligible cementite or other metallic carbide
formation.
4. The method of claim 1 wherein the carbon containing gas is a
hydrocarbon gas and the reducing gas is hydrogen.
5. The method of claim 4 wherein hydrocarbon gas is selected from
the group consisting of: propylene, butene, butadiene, ethylene,
ethane, propane, and acetylene.
6. The method of claim 1 wherein the carbon containing gas has a
free energy of formation per gram-mole of carbon between 64 and 85
kJ over the temperature range of about 850 degrees Centigrade to
about 1000 degrees Centigrade.
7. The method of claim 1 wherein a volume concentration of the
carbon containing gas is between about nil percent and 10.0
percent.
8. The method of claim 1 wherein the volume concentration of the
reducing gas is between about nil and 10.0 percent.
9. The method of claim 1 further comprising the step of cooling or
quenching the metal part in an atmosphere that is substantially
free of oxygen.
10. The method of claim 1 wherein the metal part contains metals
selected from the group of chromium, manganese, titanium or silicon
and any inter-granular oxidation of such metals occurs at a depth
of less than or equal to 0.0007 inches.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of co-pending
U.S. patent application Ser. No. 11/645,447 filed Dec. 26, 2006,
the disclosure of which is incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present invention relates to methods of carburizing
metal parts in an oxygen-free gas atmosphere, and more particularly
to a method of varying in a controlled way the hydrocarbon and
hydrogen gas concentrations in the furnace atmosphere to carburize
metal parts with very low levels of inter-granular oxidation, and
without formation of cementite or other metallic carbides in and/or
soot on the carburized metal part.
BACKGROUND
[0003] Carburizing is a well-known process by which carbon atoms
are adsorbed on the surface of a metal part and are introduced into
the metal part via diffusion and is intended to make the surface of
the metal part harder and more abrasion resistant. Carburization of
steel generally involves a heat treatment of the metallic surface
using a gaseous or plasma source of carbon within a furnace.
Current carburization practice typically includes creating an
endo-gas type of atmosphere containing nitrogen, hydrogen, a
hydrocarbon, and an oxygen-containing compound such as carbon
monoxide.
[0004] Unfortunately, endo-gas atmospheres are somewhat difficult
to generate and precisely control, are typically more costly to
operate, and involve additional safety risks due to the gas
atmosphere flammability and toxicity. In addition, because the
endo-gas atmosphere includes oxygen-laden gases, the resulting
formation of inter-granular oxidation (IGO) within the metal part
cannot be avoided.
[0005] Optimization of the carburization process requires careful
control of both the ambient gas composition and the furnace
temperature. Clearly, temperature control of the furnace is
required as the heat within the furnace impacts the microstructure
of the metal part and the reaction equilibriums and kinetics.
Precisely controlling the ambient gas composition is also required
so as to optimize the carbon content of the ambient gas
composition, as too great a concentration of carbon may make metal
part brittle and unworkable. Likewise, control of the ambient gas
composition within the furnace has been used in efforts to try to
limit undesirable effects of carburization, namely the formation of
inter-granular oxidation, cementite or other metallic carbides, and
soot, as well as avoiding hydrogen embrittlement. Unfortunately,
the current methods of controlling endo gas type atmospheres during
the carburization process in an atmospheric pressure furnace have
often resulted in a trade-off or balancing of the undesired effects
in the carburized metal part and are not able to avoid IGO.
[0006] Previously, where greater control over the ambient gas
composition is desired, carburization may take place under very low
pressures in a vacuum chamber. Such vacuum carburization process
allows for the use of less complex gas atmospheres when compared to
endo-gas atmosphere systems and can produce parts with low
inter-granular oxidation, but require expensive and complex vacuum
furnaces. In addition, such vacuum carburization process typically
operate in a very narrow range of low pressure, as too low of a
pressure yields inadequate or inefficient carburization rates, and
too high a pressure (above about 0.1 atmosphere) produces
undesirable sooting of the metal parts. Also, the low atmosphere
density within a vacuum carburization process and system makes it
difficult to carburize blind holes and recesses.
[0007] What is needed, therefore, is an efficient carburization
technique for use in atmospheric pressure furnaces that minimizes
formation of inter-granular oxidation, and avoids undesirable
effects such as hydrogen embrittlement, cementite or other carbide
formation, and soot formation.
SUMMARY OF THE INVENTION
[0008] The present invention may be characterized as a method of
treating a metal part in an atmospheric pressure furnace comprising
the steps of: (i) heating the metal part into the atmospheric
pressure furnace having a gas atmosphere substantially free of
oxygen, the gas atmosphere comprising a carbon containing gas, a
reducing gas, and an inert gas; (ii) controlling the gas
concentrations of the carbon containing gas, reducing gas, and
inert gas to inhibit formation of metal-oxides on the metal part
and to maintain a carbon saturation factor of between about 0.75
and 1.10 with negligible cementite or other metallic carbide
formation; and (iii) removing the treated metal part from the
atmospheric pressure furnace.
[0009] In one embodiment, the atmospheric pressure furnace is a
continuous-type furnace having a plurality of atmospheric zones and
the step of controlling the gas concentrations further comprises
maintaining the gas concentrations of the carbon containing gas and
reducing gas in each atmospheric zone at different levels to
inhibit formation of metal-oxides on the metal part and to maintain
a carbon saturation factor of between about 0.75 and 1.10 with
negligible cementite or other metallic carbide formation.
[0010] In an alternate embodiment, the atmospheric pressure furnace
is a batch-type furnace and the step of controlling the gas
concentrations further comprises varying the volume concentration
of the carbon containing gas in the furnace as a function of time
to maintain the carbon saturation factor in the metal part between
about 0.75 and 1.10 with negligible cementite or other metallic
carbide formation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The above and other aspects, features, and advantages of the
present invention will be more apparent from the following, more
detailed description thereof, presented in conjunction with the
following drawings, wherein:
[0012] FIG. 1 is a schematic representation of the oxygen-free
carburization system in accordance with the present invention;
[0013] FIG. 2 is a graph that depicts the amount of oxygen required
for the formation of chromium oxide compared to the equilibrium
concentration of oxygen in the furnace atmosphere as a function of
added hydrogen at about 900 degrees Centigrade;
[0014] FIG. 3, FIG. 4, and FIG. 5 depict various hydrocarbon gas
concentrations as a function of time in accordance with batch-type
furnace embodiments of the present invention;
[0015] FIG. 6 is a graph depicting the free energy of formation for
selected hydrocarbons; and
[0016] FIGS. 7A and 7B are charts depicting hydrocarbon content as
a function of time as a part to be treated passes through three
different zones of a continuous type furnace having a plurality of
atmospheric zones.
DETAILED DESCRIPTION
[0017] Referring now to FIG. 1, there is shown a schematic
representation of the present oxygen-free carburization control
system 10. The oxygen-free carburization control system 10 employs
a gas control panel 12 to control the flow and mixing of a specific
gas mixture and deliver that specific gas to an atmospheric
pressure furnace 20. For purposes of this document atmospheric
pressure furnace broadly means any non-vacuum furnace and more
particularly means furnaces operating at pressures between 10 psig
and slightly below atmospheric.
[0018] The controlled gas atmosphere includes a variable
combination of: a reducing gas; and a carbon containing gas with
the balance being an inert gas, all such constituent gases being
substantially free of oxygen. In the preferred embodiment, the
specific gas mixture 60 includes a variable combination of nitrogen
gas 14, hydrogen gas 16, and hydrocarbon gas 18 in prescribed
concentrations and is delivered to the atmospheric pressure furnace
20 as a function of time. The gas atmosphere within the furnace is
thus substantially oxygen free, with very small quantities of
oxygen present as a result of leakage, impurities, etc.
[0019] A series of user inputs 22 defining the desired process
parameters are collected via the control panel 12. The desired
process parameters help characterize the final carbon profile
desired in the metal part to be carburized, and would preferably
include the desired case depth, desired carburization temperature,
initial carbon content, target saturation factor, maximum
inter-granular oxidation (IGO) allowable, etc. Other user inputs 22
may include results or analysis from test samples from the furnace
during the calibration and operation of the furnace. Preferably
such results or analysis would include actual carbon uptake
realized in the furnace for given steel alloys at given atmosphere
conditions. In addition, a series of sensed or measured parameters
including furnace temperature 24 as measure with a temperature
sensor, and furnace oxygen content 26 as measured with an oxygen
probe, are also preferably input to the gas control panel 12.
[0020] The control panel 12 then uses a carburization model 30 to
calculate a profile of nitrogen gas 14, hydrogen gas 16, and
hydrocarbon gas 18 concentrations as a function of time. The model
30 is preferably a software routine that uses selected inputs,
including user inputs 22, furnace temperature 24, furnace oxygen
content 26, as well as known parameters such as alloy composition,
furnace type, etc. to calculate or ascertain the desired gas
concentrations as a function of heat treatment time.
[0021] Metal parts 70 to be carburized are loaded into an
atmospheric pressure furnace 20 and contacted with the prescribed
gas mixtures 60 for the prescribed durations. Once heat treated,
the treated metal parts 72 are removed from the atmospheric
pressure furnace 20 and placed in a cooling or quench chamber 40.
The cooling or quench chamber 40 also comprises an atmosphere that
is substantially free of oxygen so as to further minimize
oxidation. The carburized metal part 72 is cooled or quenched for a
prescribed duration under controlled conditions after which the
heat-treated metal part 74 is removed. The gas atmosphere can be
released or exhausted via a vent 42, as appropriate.
[0022] Flow of the nitrogen gas 14, hydrogen gas 16 and hydrocarbon
gas 18 are controlled to the atmospheric pressure furnace 20 using
control valves 34, 36, and 38 which are controlled or adjusted by
the gas control panel 12 in response to the model 30 and feedback
or inputs from the corresponding flow meters 44, 46, and 48. The
individually metered gas streams 54, 56, and 58 are then mixed and
the resulting gas mixture 60 is introduced to the atmospheric
pressure furnace 20 in accordance with the model 30.
[0023] The preferred oxygen-free heat treatment or carburization
process involves the following typical sequence of steps: (1)
Seasoning the atmospheric pressure furnace with a nitrogen,
hydrogen, and hydrocarbon gas atmosphere to remove or reduce the
oxygen and moisture content within the furnace and to equilibrate
the furnace materials with the carburizing atmosphere; (2)
Calibrating the carburization recipe or model for the nitrogen,
hydrogen, and hydrocarbon gas atmosphere; (3) Establishing user
inputs including case depth, treatment temperature, initial carbon
content, saturation factor, etc.; (4) Generating an initial profile
of desired hydrocarbon gas in the furnace as a function of time;
(5) Changing the furnace atmosphere to a substantially nitrogen and
hydrogen gas atmosphere; (6) Loading the metal parts into the
furnace atmosphere until such metal parts achieve the desired
treatment or process temperature; (7) Adjusting the hydrocarbon gas
concentration, hydrogen gas concentration, and nitrogen gas
concentration in the furnace in accordance with the model or
recipe; (8) Removing the treated metal parts when the treatment
cycle is complete and placing the treated metal parts into a quench
or cooling chamber that, preferably, is substantially free of
oxygen; and (9) Cooling (e.g. quenching) the treated metal parts in
the substantially oxygen free atmosphere.
[0024] Seasoning of a furnace is characterized as a process that
uses an inert gas plus a carbon source to purge or reduce the
oxygen and moisture content within the furnace as well as to
eliminate or reduce the carbon uptake variability within the
furnace prior to the start of the heat treating process. In the
preferred embodiment, the seasoning atmosphere within the furnace
includes a small concentration of hydrocarbon gas (e.g.
approximately 0.5% to 2.0% propylene gas), about 5.0% hydrogen gas,
and the balance of the introduced gas atmosphere being nitrogen
gas. The seasoning step should proceed for a time sufficient to
allow the furnace to equilibrate at the desired process or
treatment temperature. This is usually determined by steady
measurements from an oxygen probe disposed within the furnace at
the desired process or treatment temperature.
[0025] Calibration of the oxygen-free carburization model for the
nitrogen, hydrogen, and hydrocarbon gas atmosphere is preferably
accomplished using one or more test samples or shims. The test
sample or shim is introduced into the seasoned furnace atmosphere
comprising constant volumes of nitrogen gas, hydrogen gas a small
percentage of hydrocarbon gas. The shim remains in the furnace
atmosphere for about 30 minutes or a time sufficient to soak at the
desired process or treatment temperature. The test sample or shim
is then removed from the furnace and the actual carbon uptake is
determined and with other user inputs entered into the oxygen-free
carburization model or recipe.
[0026] After calibration of the oxygen-free carburization model is
complete, user inputs are received and an initial profile of the
ambient gas atmosphere of the furnace as a function of time is
produced. Prior to the actual heat treating process of the metal
parts, the hydrocarbon gas flow is briefly interrupted and the
furnace atmosphere is changed to a nitrogen and hydrogen gas
atmosphere. The metal parts are loaded into the nitrogen and
hydrogen gas atmosphere and heated until such parts attain the
desired treatment or process temperature. Upon attaining the
desired treatment or process temperature, adjustment of the
hydrogen gas, hydrocarbon gas, and nitrogen gas concentrations in
the furnace atmosphere is initiated.
[0027] As explained in more detail below, the step of adjusting the
hydrocarbon gas concentration in accordance with the model
typically involves varying the hydrocarbon gas flow to achieve a
desired concentration of hydrocarbon gas in the furnace atmosphere
as a function of time. The desired range of hydrocarbon gas
concentration is from about nil or 0.0% to about 10.0% and more
preferably in the range of between about nil or 0.0% to about 5.0%.
The actual hydrocarbon concentration is calculated using a carbon
saturation factor within the process model.
[0028] A minimum concentration of hydrocarbon gas in the furnace
atmosphere has been found to be preferred in order to achieve
reasonable levels of carbon flux on the metal parts to be treated.
It will be understood that some of the hydrocarbon gas is consumed
through reacting with oxidizing species that are continuously
introduced into the furnace through leaks, door openings, etc, and
this can vary from one furnace and implementation to another. Upon
initiation of hydrocarbon gas flow, the preferred minimum
concentration of hydrocarbon gas within the furnace is likely in
the range of nil or 0.0% to 2.0%, and more preferably in the range
of nil or 0.0% to 1.5%. This minimum level of hydrocarbon gas
within the furnace atmosphere is easily determined by shim tests as
the present model is calibrated for a given furnace.
[0029] Similarly, a maximum hydrocarbon gas concentration within
the furnace atmosphere may be set due to practical flow rate
limitations of the control valves and lines employed as well as
economic or safety considerations. Maximum hydrocarbon
concentrations in the furnace atmosphere are typically in the range
of about 4.0% to about 10.0%. For optimum process performance, the
preferred hydrocarbon is propylene and the gas concentration range
of the propylene is preferably between about nil or 0.0% to about
3.5% concentration of the substantially oxygen-free gas atmosphere
in the furnace.
[0030] The step of adjusting the hydrogen gas concentration in the
furnace in accordance with the model or recipe comprises setting
the hydrogen gas flow to achieve a desired concentration of
hydrogen gas in the furnace atmosphere. The desired concentration
of hydrogen gas is calculated within the model and is preferably
between a prescribed threshold minimum concentration and a
prescribed threshold maximum concentration. As explained in more
detail below, the minimum concentration of hydrogen gas depends on
several factors including the alloy elements within the metal part
to be treated with potential for oxide formation at the process or
treatment temperature; and the quantity of oxygen, moisture, and
other impurities present in the furnace. The maximum concentration
of hydrogen gas is preferably determined based on consideration of
several other process factors, such as flammability of the gas,
potential for decarburization, potential for hydrogen
embrittlement, and hydrogen gas cost. In the described embodiment,
the preferred range of hydrogen gas is between about 1.0% to about
10.0% and more preferably in the range of between about 1.5% to
about 5.25% concentration of hydrogen gas in each zone and/or
within the entire furnace. For many continuous furnace
applications, however, the plurality of zones are not separated and
the hydrogen gas from one zone may "spill-over" into adjacent
zones. Thus, in continuous furnace applications, the concentrations
of hydrogen gas introduced into any given zone may require further
adjustment to account for the "spill-over" effect. In such cases,
the preferred range of hydrogen gas in any given zone is between
about 0.0% (nil) to about 10.0%.
[0031] The final step of cooling (e.g. quenching or tempering) the
carburized metal parts preferably in a substantially oxygen free
atmosphere also helps to minimize the formation of inter-granular
oxidation. Upon completion of the quenching step, the quenching
atmosphere is vented and the heat treatment process is complete. It
should be noted that the quenching process may utilize various
quenching techniques including intensive quenching with water or a
cryogen or standard quenching with oil. Alternatively, the cooling
may be done more gradually at ambient temperatures in a nitrogen
atmosphere or other oxygen free atmosphere for a prescribed period
of time.
Calculation of Hydrogen Gas Concentration
[0032] For certain steel alloys, particularly those containing Cr,
Ti, Mn, and/or Si, the presence of oxygen in the gas atmosphere
within the furnace promotes the formation of inter-granular
oxidation (IGO) within the treated part. Introduction of hydrogen
gas in sufficient concentrations into the gas atmosphere within the
furnace negates the formation of the likely metal oxides in favor
of reactions between hydrogen and oxygen. The actual concentration
of hydrogen gas necessary to reduce or negate the formation of
inter-granular oxidation (IGO) is ascertained or calculated based
on the thermodynamic equilibrium between the potential oxidized
products and reactants within the system. The oxidation potential
observed with hydrogen varies based on the concentration of
hydrogen gas in the furnace. The reactants can include not only
hydrogen, water, and oxygen, but also include species containing
carbon, such as hydrocarbons, carbon monoxide, and carbon dioxide.
For simplicity, the procedure described herein is limited to
reactants including hydrogen, water, and oxygen.
[0033] Oxidation of the metals of interest, including Cr, Ti, Mn,
and/or Si, are well known and documented processes that depend, in
part, on given furnace temperatures and operating conditions. The
partial pressure of oxygen (pO2) in equilibrium with the metal
oxide is given by the following equation:
pO.sub.2=exp(.DELTA.G.degree./RT);
[0034] where `T` is the furnace temperature in Kelvin (K), `R` is
the gas constant (=8.3 J/(mole*K) and .DELTA.G.degree. is the Gibbs
free energy (J/mole) for the reaction a
Me+O.sub.2>>>Me.sub.aO.sub.2 (assuming ideal behavior and
pure metal). The partial pressure of oxygen available in the
furnace atmosphere is calculated based on the equilibrium
relationship between hydrogen, oxygen and water
2H.sub.2O=2H.sub.2+O.sub.2.
[0035] A comparison is made between the partial pressure of oxygen
required to form oxides of the metals of interest and the
equilibrium concentration of oxygen in the furnace atmosphere with
varying amounts of hydrogen gas in a given furnace under prescribed
conditions. Included in these comparisons and calculations are the
quantity of O.sub.2 and moisture (i.e. H.sub.2O) initially present
in the seasoned furnace atmosphere as adjusted to account for
additional concentrations of O.sub.2 and H.sub.2O attributable to
impurities subsequently introduced in the furnace. The adjusted
quantity of O.sub.2 and moisture (i.e. H.sub.2O) in the furnace at
the given operating conditions is preferably estimated from past
experience with the furnace or determined through experimentation
or calibration testing of the furnace.
[0036] The minimum concentration of hydrogen gas to be used in the
ambient gas atmosphere in the furnace is the calculated amount of
hydrogen that results in a lower partial pressure of oxygen than is
required to form oxides of the metals of interest found in the
alloy of the part to be carburized. It should be understood that
the minimum amount of hydrogen gas needed to limit inter-granular
oxidation (IGO) formation may vary depending not only on the gas
composition, but also physical construction and operation of the
furnace and the actual composition of the steel alloy. For example,
a furnace that allows a greater flow rate of outside atmosphere
containing oxygen and/or moisture into the furnace would require a
higher concentration of hydrogen gas to achieve the desired low
value of oxygen partial pressure (pO.sub.2). From a practical
standpoint, minimizing inter-granular oxidation (IGO) to 0.0007
inches or below meets the SAE Aerospace Material Specification
2759/1C for heat treatment of carbon and low-alloy steel parts.
[0037] On the other hand, the maximum concentration of hydrogen gas
used in the ambient gas atmosphere in the furnace is determined
based on consideration of several other factors, such as
flammability of the gas, potential for decarburization, potential
for hydrogen embrittlement, and economic impacts of such hydrogen
gas use. It has been shown that hydrogen gas concentrations within
a carburizing furnace up to about 40% can increase the
carburization rate of hydrocarbons in nitrogen by several fold.
However, when using the present model and oxygen-free carburization
process, it has been found that the increase in carburization rate
lessens above hydrogen gas concentrations of about 10%. Also, at
hydrogen concentrations substantially above 10%, other adverse
effects become apparent including hydrogen embrittlement of the
metal part. Limiting the hydrogen gas concentration to a maximum of
about 10% avoids such adverse effects.
[0038] From a safety standpoint, limiting the hydrogen gas
concentration within the furnace is also advisable. The National
Fire Protection Association (NFPA) has set a 10% hydrogen gas
concentration in ambient atmosphere conditions as constituting an
explosive concentration, which makes 10% hydrogen gas concentration
a reasonable maximum concentration for safety reasons. Still
further, the lower explosive limit (LEL) for hydrogen gas in air at
ambient conditions is about 4.0%, and the minimum oxygen
concentration required for hydrogen flammability is about 5.0%.
Thus, if the oxygen-free carburizing atmosphere within the furnace
contains a hydrogen gas concentration of 5.25% or less, then no
mixture of furnace atmosphere and ambient air (from outside the
furnace) would produce both a hydrogen gas concentration and oxygen
concentration above the minimums required for flammability.
Therefore, from a safety standpoint, a more preferable range of
hydrogen gas concentration is between about 1.5% and about
5.25%.
[0039] FIG. 2 depicts a graph that compares the partial pressure of
oxygen that is required to form chromium oxide in a gas atmosphere
furnace to the partial pressure of oxygen available in the furnace
as a function of added hydrogen. As seen therein, the partial
pressure of oxygen required to form the oxide of pure chromium (Cr)
at 900 degrees C. is about 6.0.times.10.sup.-25 and is constant as
the concentration of hydrogen gas within the furnace varies.
Conversely, the partial pressure of oxygen available in the furnace
as a function of hydrogen (H.sub.2) at 900 degrees C. ranges from
about 2.59.times.10.sup.-23 at about a 0.4% concentration of
hydrogen gas within the furnace to about 4.14.times.10.sup.-26 at
about a 10% concentration of hydrogen gas. In this example, the gas
atmosphere of the seasoned furnace is estimated to have about 1
parts per million (ppm) O.sub.2 and about 1 ppm of H.sub.2O, as
adjusted for impurities. Using the graph in FIG. 2 and assuming the
metal part to be carburized includes significant amounts of
chromium and no significant amounts of other metals of interest
having high oxidation potentials, the amount of hydrogen
concentration in the furnace atmosphere required to substantially
prevent oxidation of chromium in the metal part is about 3% or more
concentration of hydrogen gas.
[0040] Steel alloy compositions of the most prevalent forms of
steel alloy are presented in Table 1 below. As seen therein,
chromium is often the element of interest with oxidation potential
in the Steel alloys SAE 8620, SAE 9310, and SAE 4140 at the typical
carburization temperatures (1700 degrees F.).
TABLE-US-00001 TABLE 1 Steel Alloy Element Compositions Element SAE
1010 SAE 4140 SAE 4820 SAE 9310 SAE 8620 C (%) 0.08-0.13 0.38-0.43
0.18-0.23 0.08-0.13 0.18-0.23 Mn (%) 0.30-0.60 0.75-1.00 0.50-0.70
0.45-0.65 0.70-0.90 P.sub.max (%) 0.04 0.035 0.035 0.035 0.035
S.sub.max (%) 0.05 0.040 0.04 0.04 0.04 Si (%) 0.15-0.35 0.15-0.30
0.15-0.35 0.15-0.35 0.15-0.35 Ni (%) . . . . . . 3.25-3.75
3.00-3.50 0.40-0.70 Cr (%) . . . 0.80-1.10 . . . 1.00-1.40
0.40-0.60 Mo (%) . . . 0.15-0.25 0.20-0.30 0.08-0.15 0.15-0.25
Calculation of Hydrocarbon Gas Concentration
[0041] The primary objective of varying the concentration of
hydrocarbon gas in the furnace is to maintain a desired net carbon
flux from the furnace atmosphere that is less than the carbon flux
required to saturate the surface of the metal part with carbon. By
"saturate," we mean that the carbon concentration in the metal is
at a specified level. For example, the specified level may be the
maximum carbon concentration at a given temperature that would
avoid formation of a certain phase, such as cementite or other
metallic carbide. Alternately, the specified level may be a desired
surface carbon concentration, for example the starting bulk carbon
concentration in the case of a neutral hardening heat treatment.
The specified level denoting "saturation" may change depending on
composition of the metal, temperature, or time in the heat treating
process. Moreover, as carbon transport occurs across the gas and
solid interface, it is desirable for the net carbon flux from the
gas flow to the surface of the metal part and carbon diffusion of
atoms within the metal part to match or approximate one another.
Adjusting the concentration of hydrocarbon gas within the furnace
allows control of the carburization process such that: (1) the
carbon flux from the atmosphere gas flow to the metal surface
matches or approximates the diffusion of carbon atoms within the
metal part; and (2) the carbon flux from the atmosphere gas flow to
the metal part is less than the carbon flux that would saturate the
surface of the metal part with carbon. Controlling both aspects
results in avoidance or minimization of soot and cementite
formation (or other undesirable carbide) at or near the surface of
the treated metal part.
[0042] As the oxygen-free carburization process continues, both the
carbon flux required to saturate the surface of the metal part with
carbon as well as the carbon diffusion of atoms within the metal
part change as a function of time. Accordingly, the concentration
of hydrocarbon gas within the furnace should likewise be adjusted
as a function of time.
[0043] In the present embodiment, the desired hydrocarbon gas
concentration is determined based on the actual measured carbon
flux data from the furnace together with an estimated or calculated
diffusivity of carbon in the treated metal part. Measuring the
carbon flux is preferably accomplished using an appropriate methods
such as shim testing, resistivity measurements of a thin wire
disposed within the furnace, or other methods known to those
skilled in the art and used to determine rate of carburization or
carbon content of the metal parts being treated at known operating
conditions.
[0044] The calculated diffusivity of carbon or carbon diffusion
profile in the metal part is calculated based on Fick's second law,
presented below, which is used to determine the flux of carbon with
time into a metal part:
.differential. C .differential. t = D .differential. 2 C
.differential. x 2 ##EQU00001##
[0045] where `C` is the carbon atom concentration of the part, `x`
is the distance from the surface of the part that carbon atoms must
diffuse into the part, `D` is the diffusion coefficient of carbon
atoms into the metal, and `t` is time. As is well known in the art,
the diffusion coefficient generally increases exponentially with
the negative reciprocal of the temperature.
[0046] The calculation of the carbon diffusion profile in the metal
can be performed by assuming that a carbon source is supplied
directly to the metal surface, as is the case in the carburization
furnace, and the metal part is treated as a semi-infinite slab. The
approximated solution of Fick's second law thus becomes:
(C.sub.s-C.sub.x)/(C.sub.s-C.sub.o)=erf{x/[2(Dt).sup.1/2]}
[0047] where C.sub.s is the carbon concentration at the surface,
C.sub.O is the original carbon concentration in the metal, and
C.sub.x is the carbon concentration at the distance x, from the
surface of the metal part into the body of the metal part, and the
value `erf {x/[2(Dt).sup.1/2]}` is the Gaussian Error Function.
Note, the carbon concentration at the surface, C.sub.s, is the
specified level or saturation concentration. Using this estimate of
diffusive carbon flux, it is possible to calculate the carbon
profiles at any time during the treatment process and used as a
benchmark for process control.
[0048] The carbon concentration at the surface of the metal part is
determined empirically by taking carbon flux measurements of
samples in known furnace atmosphere conditions. The carbon flux
measurements are used to determine the amount of carbon (i.e.
hydrocarbon gas) that is delivered to the surface of the metal part
over time, at different atmosphere compositions, and different
furnace temperatures. Carbon flux measurements are taken with foil
samples of known surface area that are exposed to a known
atmosphere compositions.
[0049] A carbon saturation factor .PHI., is then applied to assure
that the amount of carbon delivered to the surface of the part is
below that which would produce cementite, other metallic carbides,
soot or, in the case of neutral hardening for example, would
prevent decarburization on the surface of the metal part. The
carbon saturation factor .PHI., is represented by the equation:
.PHI. = ( Flux Fuel ) ( Flux Metal ) ##EQU00002##
[0050] where Flux.sub.Fuel is the carbon flux rate from the
hydrocarbon gas to the surface of the metal part and is determined
empirically from the measurements taken from the samples exposed to
the known atmosphere compositions; and Flux.sub.Metal is the sum of
the calculated diffusive flux of carbon away from the surface of
the metal part taken from the above carbon diffusivity estimate,
and the carbon flux leaving the surface due to any decarburizing
effect of reactions with the gas atmosphere. In carburizing
applications the diffusive flux is usually the critical component
of the flux away from the metal surface. In neutral hardening
applications, the diffusive flux is usually negligible due to a
uniform carbon concentration throughout the metal, such that the
decarburizing flux is the critical component of the flux away from
the metal surface. In applications where the decarburizing flux is
significant, it can be either calculated by methods known in the
art based on estimates or measurements of decarburizing specie
concentration in the gas atmosphere, or determined empirically by
test.
[0051] The theoretical ideal carbon saturation factor, .PHI., is
between about 0.75 and 0.99. However, given variation in accuracy
of sample measurements and in precision of flow and temperature
control, a practical range for the saturation factor is preferably
between about 0.75 and 1.25, and more preferably between about 0.90
and 1.10. Using the preferred saturation factor range and the
Flux.sub.Metal value calculated from the above diffusivity model,
an optimal or target value of Flux.sub.Fuel is determined. The
preferred hydrocarbon gas concentration as a function of time is
then determined based on the optimal or target value of
Flux.sub.Fuel so that the carbon saturation factor .PHI., is
maintained in the desired range during any time interval of the
process treatment. The relationship between Flux.sub.Fuel and
hydrocarbon gas concentration is determined empirically and
periodically updated using actual foil sample data from the actual
furnace.
[0052] The preferred hydrocarbons useful with the present
embodiments of the invention include those high purity hydrocarbons
having intermediate stability at typical carburization temperatures
(e.g. 1550 to 1700 degrees F.). Hydrocarbons of intermediate
stability can be selected based the value of free energy of
formation per gram-atom of carbon.
[0053] As shown in FIG. 6, hydrocarbons with a free energy of
formation per gram-mole carbon between about 64 and 85 kJ/gmol over
the carburization temperature range of 850-1000.degree. C. provide
the best balance of carburization rate and avoidance of soot or
carbide formation. This includes hydrocarbons such as propylene
101, butadiene 102, ethylene 103, butene (not shown), ethane 105,
propane 106, and acetylene 107. Also shown in FIG. 6, are hexane
111, Cyclohexane 112, methane 113 and benzene 114. Even more
preferred sources of hydrocarbon for use with the present methods
include unsaturated hydrocarbons and propylene is the most
preferred hydrocarbon. The molecular orbitals of carbon-carbon
double bonds found in unsaturated hydrocarbons are known to
interact with the molecular orbitals of transition metals such as
iron. In the proposed process, this effect promotes the pyrolysis
reaction at the surface of the metal parts as opposed to reactions
in the bulk gas phase. Such promotion of reaction at the metal
surface helps to minimize soot formation and improve process
efficiency and control. In the case of propylene, an important
reaction pathway for pyrolysis involves the breakdown of the
propylene molecule into methane and acetylene. The thermodynamic
stability of the methane byproduct makes this pathway kinetically
favored, and the instability of acetylene causes a fast subsequent
breakdown to carbon at the surface, resulting in favorable
carburization rates for propylene. The higher stability of
propylene in the gas phase as compared with acetylene serves to
minimize soot formation and improve control.
[0054] Application of the above-described diffusive carbon flux
model to the present oxygen-free carburization process is
preferably accomplished in the following manner: (1) Ascertain the
known parameters C.sub.s, C.sub.o, D, D.sub.o, R, and T from the
properties of the metal part and carburization process targets
and/or constraints; (2) Specify the desired final carbon profile or
case depth, x; (3) Use the model to calculate the target carbon
flux as a function of time, t, or the target carbon concentration
at the surface of the metal part, C.sub.s as a function of time, t;
and (4) Establishing the hydrocarbon gas content that achieves the
desired carbon concentration at the surface of the metal part,
C.sub.s using the actual measured carbon flux data from the shim
tests or other test methods together with the user defined
saturation factor .PHI.; (5) Set the furnace processing conditions
to achieve the hydrocarbon gas concentration as a function of time.
Upon completion and execution of the carbon diffusivity model, the
oxygen-free carburization process, as set forth above is continued.
The final result of the oxygen-free carburization process performed
in accordance with the model should achieve the desired case depth
and carbon profile in the treated metal part with little or no
inter-granular oxidation (IGO), soot formation or cementite or
other metallic carbide formation. Some testing and adjustment of
operating conditions may be used to adapt the model and associated
procedures to the conditions and variability of a particular
furnace, material, surface finish, etc.
[0055] An optimum hydrocarbon gas concentration profile may involve
three or four phases including: (a) an initial high hydrocarbon
concentration phase; (b) a moderate hydrocarbon concentration
phase; (c) a lower hydrocarbon concentration phase; and (d) zero or
nil hydrocarbon concentration phase. For carburizing applications,
the most preferred profile would be a continuously varying
hydrocarbon concentration, beginning with the high concentration
phase and ending with the low or zero concentration phase. This
would provide the closest match to the diffusive flux and minimize
carburization time.
[0056] The initial high hydrocarbon phase is intended or adapted to
bring the carbon content at the surface of the part rapidly up to
at or near the constraint level (i.e. 1.2% C, or as high as
practical so as not to form carbides on the surface of the metal
part). The moderate hydrocarbon concentration phase is intended or
adapted to maintain the carbon content at the surface at the
desired percent (e.g. about 1.2% C) while the carbon atoms diffuse
into the body of the metal part. Put another way, the moderate
hydrocarbon concentration phase attempts to match the rate of
carbon delivery to the surface of the metal part to the rate of
carbon diffusion into the body of the metal part. The lower
hydrocarbon concentration phase is intended or adapted to allow the
carbon content at the surface of the metal part to decline to a
lower level, for example where the process conditions or
carburization requirements call for a surface carbon content of
less than 1.2% C. Finally, the zero or nil hydrocarbon
concentration phase is intended or adapted to allow the carbon
profile to reach the desired end state. This zero or nil phase may
occur near the end of the carburization process to remove any trace
of free carbon from the surface of the treated metal part or may
also be used during the cooling or quenching operations.
[0057] A minimum concentration of hydrocarbon gas in the furnace
atmosphere has been found to be needed to achieve reasonable rates
of carbon flux on the metal parts to be treated. Some of the
hydrocarbon gas feed is consumed by reacting with oxidizing species
that are continuously introduced into the furnace through leaks,
door openings. This minimum concentration of hydrocarbon gas varies
depending on rates of impurities in the furnace, but is likely in
the range of nil or 0.0% to 2.0%, and more probably in the range of
nil or 0.0% to 1.5%. The level is easily determined by shim tests
when the model is calibrated for a given furnace or with an oxygen
probe.
[0058] A maximum hydrocarbon concentration may be set due to
practical flow rate limitations of the hardware utilized, economic
considerations, or more preferably safety considerations. For
example, the lower explosive limit (LEL) for propylene in air is
about 2.4%, and the minimum oxygen concentration required for
flammability is about 5.0%. If the carburizing atmosphere contains
propylene at about 3.15% or less, then no mixture of furnace
atmosphere and ambient air (i.e. outside the furnace) would produce
a propylene concentration above the LEL coincident with an oxygen
concentration above the minimum required for flammability.
Therefore from a safety aspect, the preferred embodiment would set
a maximum propylene concentration of 3.15% to ensure a
non-flammable atmosphere during operation. Broadly speaking,
hydrocarbon concentrations in the range of nil or 0% to about 10%
are recommended, more preferably in the range of about nil or 0% to
about 5%.
[0059] FIG. 3 shows a graph depicting the model hydrocarbon
concentrations 90 as a function of time based on the outputs of the
model compared with the actual hydrocarbon concentrations 92
employing the practical or prescribed threshold minimum
concentrations 94 and maximum concentrations 96 as discussed
herein.
[0060] Turning now to FIGS. 7A and 7B, there is shown charts
generally depicting application of the present oxygen-free
carburization treatment in a continuous type-furnace having a
plurality of identified zones or regions. Preferably, each zone or
region has the ability to independently and precisely manage the
concentration of atmosphere gases in such zone, including
hydrocarbon gas concentration, reducing gas concentration and inert
gas concentration using one or more gas injectors disposed in or
proximate to such zone. As seen in FIGS. 7A and 7B, the hydrocarbon
concentrations are set at a prescribed concentration for each zone.
The following paragraphs discuss a preferred process to ascertain
the prescribed concentration of hydrocarbon gas in each zone during
an oxygen-free carburization treatment in a continuous-type
furnace.
[0061] First, samples of steel foil are introduced to one or more
identified zones of the furnace atmosphere for a fixed amount of
time at constant hydrocarbon gas flow to determine the uptake of
carbon that occurs within the furnace or each zone thereof. This
carbon uptake information is used as an input to the oxygen-free
carburization recipe program (e.g. similar to the calculations used
in a batch-type furnace) and to ascertain the desired overall
concentration of hydrocarbon gas as a function of time as
represented by the target hydrocarbon profile 150. In some
continuous furnace applications, it may be advantageous to adjust
the target hydrocarbon profile 150 in Zone 1 or in the early stages
of the heat treating process, as shown in FIG. 7B. Such adjustments
are made to reflect the time needed to heat the parts to be treated
such that the highest hydrocarbon concentration is not delivered
when the parts are too cool to absorb it.
[0062] Each zone within the continuous-type heat treating furnace
is represented on FIGS. 7A and 7B as discreet duration of times
during which a treated part would traverse through the different
treating zones. FIGS. 7A and 7B depict three carburizing zones
152,154,156 of generally equal size and equal total gas flow rate.
For each of the three carburizing zones identified, the amount of
carbon delivered to the zone is determined, based in part on the
area under the target hydrocarbon profile 150 in each zone. A
calculated constant flow of hydrocarbon gas 162,164,166 needed to
deliver the same amount of carbon to each respective zone is then
calculated or otherwise ascertained. A correction factor (x) 170 is
then applied to the calculated constant flow of hydrocarbon gas
162,164,166 to produce a corrected constant flow of hydrocarbon gas
172,174,176. The correction factor 170 is preferably a calculated
or empirically determined parameter intended to assure that the
amount of hydrocarbon gas delivered in each zone maximizes the
driving force for carbon diffusion while not forming unwanted
carbides on the surface of the treated part.
[0063] Once the corrected constant flow of hydrocarbon gas
172,174,176 for each zone is identified, suitable calculations for
the concentrations of the reducing gas and inert gas in each zone
are identified. The corresponding gas flows for the hydrocarbon gas
(e.g. propylene) flow, reducing gas (e.g. hydrogen) flow and inert
gas (e.g. nitrogen) flow in each zone are set. Shim testing can be
periodically performed to verify the carbon uptake in each zone and
to account for other process variables occurring within or between
zones such as oxygen infiltration, hydrocarbon "spillover," and
local temperature changes.
[0064] Clearly, the more zones that are identified, the more
closely the calculated constant flow of hydrocarbon gas 162,164,166
and corrected constant flow of hydrocarbon gas 172,174,176 are to
the target hydrocarbon profile 150 in each zone. However,
increasing the number of zones adversely affects the cost and
complexity of the heat treating process.
EXAMPLES
Example 1
[0065] This example involved the carburization of a piece of
3/4''.times.2'' SAE 9310 steel alloy rod to a desired effective
case depth of about 0.050 inches using the oxygen-free
carburization model. A nitrogen atmosphere containing 0.6%
propylene and 5.0% hydrogen was equilibrated in a laboratory tube
furnace atmosphere at about 1700 degrees F. until the O.sub.2
concentration stabilized. The carbon uptake in the tube furnace was
empirically determined to be about 1.0% based on a SAE 1010 shim
sample (0.001'' thick) placed in the furnace and contacted with the
oxygen-free gas atmosphere for about 15 minutes.
[0066] The relevant process inputs were entered into the
oxygen-free carburization model and a desired hydrocarbon gas
concentration profile as a function of time was established to
achieve the desired effective case depth. The hydrocarbon gas
concentration profile 98 used is depicted in FIG. 4. Hydrogen gas
concentration during the carburization process was maintained at
about 5.0%. The sample was placed in a loading vestibule and purged
with nitrogen to remove residual oxygen. The sample was then
introduced to the furnace atmosphere at about 1700 degrees F. and
allowed to come to temperature. The propylene gas flow was then
introduced according to the predicted hydrocarbon gas profile
98.
[0067] After about 8 hours, the hydrogen and propylene were turned
off, the sample was removed from the furnace and water quenched.
Analysis of the sample showed an effective case depth of about
0.058 inches and there was no visible sooting (see Table 2). Trace
amounts of inter-granular oxidation (IGO) was measured at one of
four points. IGO was not detected in the other three measured
points. These results compare beneficially with the expected IGO
level of about 0.0006 inches in a conventional endo-gas atmosphere
and a case depth of 0.050 inches.
Example 2
[0068] This example involved the carburization of a SAE 4140 steel
alloy sample to a desired effective case depth of about 0.047
inches in an operating atmosphere pressure carburization furnace
using the oxygen-free carburization model. A nitrogen, propylene
and hydrogen gas atmosphere was equilibrated in an operating
atmosphere pressure carburization furnace at about 1700 degrees F.
until the O.sub.2 concentration stabilized.
[0069] The relevant process inputs were entered into the
oxygen-free carburization model and a desired hydrocarbon gas
concentration profile as a function of time was established to
achieve the desired effective case depth. The upper hydrocarbon
limit was constrained by the hydrocarbon injection equipment. The
hydrocarbon gas concentration profile 99 used is depicted in FIG.
5. Hydrogen gas concentration during the carburization process was
maintained between about 1.19% and about 2.99%. The sample was
placed in a loading vestibule and purged with nitrogen to remove
residual oxygen. The sample was then introduced to the furnace
atmosphere at about 1700 degrees F. and allowed to come to
temperature. The propylene gas flow was then introduced according
to the predicted hydrocarbon gas profile 99.
[0070] After about 8 hours, the hydrogen gas and propylene gas were
turned off, the sample was removed from the furnace and cooled.
Analysis of the sample showed an effective case depth of about
0.048 inches and there was no visible sooting (see Table 2). Trace
amounts of inter-granular oxidation (IGO) was measured and was less
than or equal to 0.0002 inches. These results compare beneficially
with the expected IGO level of about 0.0006 inches in a
conventional endo-gas atmosphere and a case depth of 0.050
inches.
Examples 3 and 4
[0071] These examples involved the carburization of a SAE 9310
steel alloy sample (Example 3) and a SAE 4820 steel alloy sample
(Example 4) to desired effective case depths of about 0.034 inches
in an operating atmosphere pressure carburization furnace using the
oxygen-free carburization environment but keeping the hydrocarbon
gas profile constant as a function of time. A nitrogen atmosphere
containing about 1.7% propylene and 5.0% hydrogen was equilibrated
in an operating atmosphere pressure carburization furnace at about
1700 degrees F. until the O.sub.2 concentration stabilized.
[0072] As with the earlier described examples, the samples were
placed in a loading vestibule and purged with nitrogen to remove
residual oxygen. The samples were then introduced to the furnace
atmosphere at about 1700 degrees F. and allowed to come to
temperature. The propylene gas flow and hydrogen gas flows were
then introduced into the furnace. The hydrocarbon gas concentration
profile was kept constant at about 1.7% throughout the 4 hour
oxygen-free carburization treatment. Hydrogen gas concentration
during the carburization process was also maintained at a constant
concentration of about 5.0%.
[0073] After about 4 hours, the hydrogen and propylene were turned
off, the samples were removed from the furnace and quenched.
Analysis of the sample in Example 3 showed an effective case depth
of about 0.034 inches, no visible sooting, and an IGO level of less
than or equal to 0.0001 inches (See Table 2). Analysis of the
sample in Example 4 showed an effective case depth of about 0.034
inches, no visible sooting, and no detectable level of IGO (See
Table 2). These results compare beneficially with the expected IGO
level of about 0.0004 inches in a conventional endo-gas atmosphere
and a case depth of 0.035 inches.
Example 5
[0074] This example involved the carburizing of a SAE 1010 steel
alloy sample (1/8'' thick.times.2'') to a desired effective case
depth of about 0.030 inches in a laboratory tube furnace using the
oxygen-free carburization model. A nitrogen atmosphere containing
0.3% ethane and 5.0% hydrogen was equilibrated in an operating
atmosphere pressure carburization furnace at about 1700 degrees F.
until the O.sub.2 concentration stabilized.
[0075] The relevant process inputs were entered into the
oxygen-free carburization model and a constant hydrocarbon gas
concentration (e.g. about 0.3% ethane) was selected to achieve the
desired treatment. Hydrogen gas concentration during the treatment
process was maintained at about 5.0%. The sample was placed in a
loading vestibule and purged with nitrogen to remove residual
oxygen. The sample was then introduced to the furnace atmosphere at
about 1700 degrees F. and allowed to come to temperature. The
ethane gas flow was then introduced according to the predicted
hydrocarbon gas profile.
[0076] After about 2 hours, the hydrogen gas and ethane gas were
turned off and the sample was removed from the furnace and
quenched. Analysis of the sample showed an effective case depth of
about 0.030'' and there was no visible sooting (see Table 2).
Because the sample does not contain appreciable levels of alloying
elements known to promote IGO, the IGO level was not measured.
TABLE-US-00002 TABLE 2 Oxy-Free Carburizing Test Results Time
Case-Depth Steel Alloy HC (type) HC (%) H.sub.2 (%) (hrs) Temp (F.)
(in) IGO (in) Sooting Ex. 1 SAE 9310 Propylene 0.44-0.80 5.0 8 1700
0.058 .ltoreq.0.0003 None Ex. 2 SAE 4140 Propylene 1.19-2.99 5.0 8
1700 0.048 .ltoreq.0.0002 None Ex. 3 SAE 9310 Propylene 1.7 5.0 4
1700 0.034 None None Detected Ex. 4 SAE 4820 Propylene 1.7 5.0 4
1700 0.034 0.0001 None Ex. 5 SAE 1010 Ethane 0.3 5.0 2 1700 0.030
Not None Measured
INDUSTRIAL APPLICABILITY
[0077] As can be appreciated from the foregoing description, the
presently disclosed oxygen-free carburization process uses a
precisely controlled, low-oxygen, nitrogen-based, carbonaceous
atmosphere in an atmosphere pressure furnace to minimize the
formation of soot, unwanted carbides, and inter-granular oxides,
and avoid hydrogen embrittlement. Utilization of the
above-described carburization process provides a high-quality and
cost-effective process for the carburization of many steel alloys
that offers many features and advantages when compared to
commercially available prior art carburization processes and
systems.
[0078] The present carburization process employs a substantially
oxygen-free furnace atmosphere with a prescribed minimum
concentration of hydrogen gas to reduce oxidation potential with
trace metals in most steel alloys and to minimize the formation of
inter-granular oxides (IGO). Hydrogen gas is also used as the
energizer to promote dissociation of hydrocarbon fuel near and on
the metal surface. As indicated above, the hydrogen gas
concentration is preferably kept low to avoid hydrogen
embrittlement and to improve operational safety of the
carburization process by eliminating the possibility of gas
explosion with low oxygen and low hydrogen concentrations. In
addition, the hydrocarbon concentrations within the nitrogen-based
furnace atmosphere are continuously varied to match the carbon-iron
diffusion flux within the treated part for optimized carburization.
The required fuel concentration to provide proper carbon flux is
estimated using a flux/diffusion model together with empirically
derived data obtained during calibration and operation of the
oxygen-free carburization process. Moreover, the disclosed
oxygen-free carburization process advantageously provides a means
or technique to optimize speed of carburization through control of
carbon flux.
[0079] The above-identified oxygen-free carburization methods and
the features associated therewith can be utilized alone or in
conjunction with other heat treatment processes and variations.
Moreover, each of the specific steps involved in the carburization
process, described herein, and each of the inputs, elements, or
variables in the preferred carburization control system are easily
modified or tailored to meet the specific treatment requirements of
the particular part to be carburized or the peculiar requirements
of the heat treating furnace with which it is used or other
operating environment restrictions.
[0080] For example, the hydrocarbon gas could comprise propylene,
propane, methane or other suitable hydrocarbon. Likewise, helium or
argon gas may be used as a substitute for the nitrogen gas and
dissociated ammonia may be used as a source of nitrogen gas or
hydrogen gas. Also, the heat treating process can be performed in
any atmospheric pressure furnace including, for example, continuous
type furnaces, batch furnaces, or other furnace types (e.g. rotary
retort, humpback, bell, annealing, pit, etc.) and the furnace may
have a single or multiple control zones.
[0081] From the foregoing, it should be appreciated that the
present invention thus provides a method of oxygen-free
carburization in an atmospheric pressure furnace. While the
invention herein disclosed has been described by means of specific
embodiments and processes associated therewith, numerous
modifications and variations can be made thereto by those skilled
in the art without departing from the scope of the invention as set
forth in the claims or sacrificing all its material advantages. For
example, similar such methods could be employed for
nitrocarburizing or ferritic nitrocarburizing by including an
additional source of nitrogen such as ammonia. In addition, the
present process can be adapted for use with other heat treating
processes such as neutral hardening, ferritizing, annealing,
normalizing, spheroidizing, tempering, sintering and sinter
hardening.
* * * * *