U.S. patent application number 12/530794 was filed with the patent office on 2010-05-13 for hybrid carburization with intermediate rapid quench.
This patent application is currently assigned to Swagelok Company. Invention is credited to Sunniva R. Collins, Peter C. Williams.
Application Number | 20100116377 12/530794 |
Document ID | / |
Family ID | 39561887 |
Filed Date | 2010-05-13 |
United States Patent
Application |
20100116377 |
Kind Code |
A1 |
Collins; Sunniva R. ; et
al. |
May 13, 2010 |
HYBRID CARBURIZATION WITH INTERMEDIATE RAPID QUENCH
Abstract
A carbon hardened surface is produced in a metal workpiece
without forming carbide precipitates by carburizing the workpiece
at high temperature carburization conditions, rapidly quenching the
workpiece, and then carburizing the workpiece under low temperature
carburization conditions.
Inventors: |
Collins; Sunniva R.;
(Cleveland Heights, OH) ; Williams; Peter C.;
(Cleveland Heights, OH) |
Correspondence
Address: |
CALFEE HALTER & GRISWOLD, LLP
800 SUPERIOR AVENUE, SUITE 1400
CLEVELAND
OH
44114
US
|
Assignee: |
Swagelok Company
Solon
OH
|
Family ID: |
39561887 |
Appl. No.: |
12/530794 |
Filed: |
March 12, 2008 |
PCT Filed: |
March 12, 2008 |
PCT NO: |
PCT/US08/56559 |
371 Date: |
September 11, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60922174 |
Apr 6, 2007 |
|
|
|
Current U.S.
Class: |
148/225 ;
148/240 |
Current CPC
Class: |
C21D 1/10 20130101; Y02P
10/25 20151101; C23C 8/22 20130101; C23C 8/20 20130101; Y02P 10/253
20151101 |
Class at
Publication: |
148/225 ;
148/240 |
International
Class: |
C23C 8/22 20060101
C23C008/22; C23C 8/00 20060101 C23C008/00 |
Claims
1. A process for fanning a carbon hardened surface in a metal
workpiece without forming carbide precipitates, the process
comprising subjecting the workpiece to both high temperature
carburization and low temperature carburization, wherein
immediately after high temperature carburization, the workpiece is
rapidly quenched to a temperature below which carbide precipitates
form.
2. The process of claim 1, wherein the process comprises
carburizing the workpiece at high temperature carburization
conditions, rapidly quenching the workpiece, and then carburizing
the workpiece under low temperature carburization conditions.
3. The process of claim 2, wherein the workpiece is rapidly
quenched through a region of conditions where carbide precipitates
can form without forming such carbide precipitates.
4. The process of claim 2, wherein during carburization of the
workpiece under low temperature carburization conditions, the
instantaneous rate of carburization is held essentially
constant.
5. The process of claim 2, wherein during carburization of the
workpiece under low temperature carburization conditions, the
instantaneous rate of carburization is reduced from a higher value
during earlier stages of carburization to a lower value during
subsequent stages of carburization.
6. The process of claim 5, wherein the instantaneous rate of
carburization is reduced by reducing the carburization
temperature.
7. The process of claim 5, wherein the instantaneous rate of
carburization is reduced by reducing the concentration of carbon in
the carburizing gas.
8. The process of claim 1, wherein the process comprises
carburizing the workpiece at low temperature carburization
conditions, rapidly heating the workpiece to a temperature above
which carbide precipitates can form, further carburizing the
workpiece under high temperature carburization conditions, and
rapidly quenching the workpiece to a temperature below which
carbide precipitates can form.
9. The process of claim 8, wherein the workpiece is rapidly
quenched through a region of conditions where carbide precipitates
can form without forming such carbide precipitates.
10. The process of claim 1, wherein the metal is stainless
steel.
11. A process for altering the surface of a metal workpiece by
diffusing an element into the workpiece without forming
precipitates of the diffused element in the altered surface, the
process comprising contacting the workpiece with a diffusion gas
containing the element at a first elevated temperature which is
above the temperature at which such precipitates can form and, in
addition, contacting the workpiece with a diffusion gas at a second
elevated temperature which is lower than the first elevated
temperature and which is also below a temperature at which such
precipitates can form, wherein immediately after the workpiece is
contacted with the diffusion gas at the first elevated temperature,
the workpiece is rapidly quenched to a temperature below which such
precipitates can foam.
12. The process of claim 11, wherein the process comprises
contacting the workpiece with a diffusion gas containing the
element at a first elevated temperature which is above the
temperature at which such precipitates can form, rapidly quenching
the workpiece through a region of conditions where such
precipitates can form, and then completing diffusion of the element
into the workpiece by contacting the workpiece with another
diffusion gas at a second elevated temperature lower than the first
elevated temperature under conditions which avoid formation of such
precipitates.
13. The process of claim 12, wherein during completion of diffusion
at the second elevated temperature, the instantaneous rate of
diffusion is held essentially constant.
14. The process of claim 21, wherein during completion of diffusion
at the second elevated temperature, the instantaneous rate of
diffusion is reduced from a higher value during earlier stages of
diffusion to a lower value during subsequent stages of
diffusion.
15. The process of claim 14, wherein the instantaneous rate of
diffusion is reduced by reducing the diffusion temperature,
reducing the concentration of diffusing element in the diffusion
gas, or both.
16. The process of claim 11, wherein the process comprises
contacting the workpiece with a diffusion gas at a second elevated
temperature below which such precipitates can form, rapidly heating
the workpiece to a temperature above which such precipitates can
form, contacting the workpiece with a diffusion gas at a first
elevated temperature above which such precipitates can form, and
rapidly quenching the workpiece to a temperature below which such
precipitates can form.
17. The process of claim 11, wherein the metal is aluminum or an
alloy of aluminum.
18. The process of claim 11, wherein the metal is titanium or an
alloy of titanium.
Description
RELATED APPLICATIONS
[0001] The present application claims the benefit of pending U.S.
provisional application Ser. No. 60/922,174 filed on Apr. 6, 2007,
for HYBRID CARBURIZATION WITH INTERMEDIATE RAPID QUENCH, the entire
disclosure of which is fully incorporated herein by reference.
BACKGROUND
[0002] Case hardening is a widely used industrial process for
enhancing the surface hardness of shaped metal articles. In a
typical commercial process, the workpiece is contacted with a
gaseous carbon compound at elevated temperature whereby carbon
atoms liberated by decomposition of the carbon compound diffuse
into the workpiece's surface. Hardening occurs through the reaction
of these diffused carbon atoms with one or more metals in the
workpiece thereby forming distinct chemical compounds, i.e.
carbides, followed by precipitation of these carbides as discrete,
extremely hard, crystalline particles in the metal forming the
workpiece's surface. See, Stickels, "Gas Carburizing", pp 312 to
324, Volume 4, ASM Handbook, .COPYRGT. 1991, ASM International.
[0003] Carbide precipitates not only enhance surface hardness, they
can also promote corrosion. For this reason, stainless steel is
rarely case hardened by conventional gas carburization, since the
corrosion resistance of the steel is compromised.
[0004] In the mid 1980's, a technique for case hardening stainless
steel was developed in which the workpiece is contacted with a
carburizing gas at low temperature, typically below 500.degree. C.
(932.degree. F.). At these temperatures, and provided that
carburization does not last too long, carbon atoms diffuse into the
workpiece surfaces, typically to a depth of 20-50.mu., without
formation of carbide precipitates. Nonetheless, an extraordinarily
hard case (surface layer) is obtained, which is believed due to the
stress placed on the crystal lattice of the metal by the diffused
carbon atoms. Moreover, because carbide precipitates are absent,
the corrosion resistance of the steel is unimpaired, even
improved.
[0005] This technique, which is referred to a "low temperature
carburization," is described in a number of publications including
U.S. Pat. No. 5,556,483, U.S. Pat. No. 5,593,510, U.S. Pat. No.
5,792,282, U.S. 6,165,597, EPO 0787817, Japan 9-14019 (Kokai
9-268364) and Japan 9-71853 (Kokai 9-71853). The disclosures of
these documents are incorporated herein by reference. For
convenience, this process will be referred to herein as
"conventional low temperature" carburization.
[0006] Conventional high temperature carburization and conventional
low temperature carburization are schematically illustrated in FIG.
1. Curve QQ in this figure is a time/temperature transformation
diagram illustrating the conditions at which carbide precipitates
form in a carbon-containing AISI 316 stainless steel workpiece
heated to an elevated temperature. In particular, Curve QQ shows
that carbide precipitates form in this workpiece when it encounters
conditions inside the envelope defined by Curve QQ.
[0007] Curve A in FIG. 1 illustrates the time/temperature profile
encountered in conventional high temperature carburization. As can
be seen from this curve, carburization is normally carried out at a
temperature of about 950.degree. C. or above in order to drive
carbon atoms into the workpiece as fast as possible. At these high
temperatures, carbide precipitates do not form at all, because
carbon is fully soluble in the metal.
[0008] High temperature carburization normally takes about 2-10
hours to complete, as represented by Curve Segment A(a) in this
figure. Thereafter, the workpiece is cooled to room temperature, as
represented by Curve Segment A(b) in this figure. As this occurs,
carbide precipitates readily form since the workpiece must
necessarily traverse a region where carbide precipitates form,
i.e., inside the envelope formed by Curve QQ. So, for example, when
the workpiece reaches point B where Curve Segment A(b) crosses
Curve QQ, carbide compounds start forming. In addition, these
carbide compounds form precipitates which continue growing as the
workpiece continues cooling along Curve Segment A(b) within Curve
QQ. Once the workpiece cools below point C, however, carbide
precipitates no longer form or grow but simply remain suspended in
the metal matrix in which they are formed.
[0009] As indicated above, conventional high temperature
carburization is not used for surface hardening stainless steel,
because its corrosion resistance is completely lost when this is
done. Stainless steel is corrosion resistant because of the
chromium metal in the steel. When stainless steel is high
temperature carburized, the diffused carbon atoms react with this
chromium metal to form chromium carbide precipitates. This robs the
surrounding metal of its chromium content, thereby destroying its
corrosion resistance.
[0010] Curve D in FIG. 1 illustrates the time/temperature profile
of the conventional low temperature carburization process. As can
be seen from this curve, the carburization temperature in this
process is maintained at a constant temperature of 500.degree. C.
or less throughout the carburization process. This temperature is
below Curve QQ, and so carbide precipitates do not form at all.
Surface hardening does occur, however, through the stress placed on
the crystal lattice of the metal forming the workpiece. Moreover,
because no chromium carbide precipitates have formed, the corrosion
resistance of the stainless steel is preserved, even improved. Note
that, because a much lower carburization temperature is used
compared to conventional high temperature carburization, low
temperature carburization typically takes much longer to complete,
typically 25-50 hours rather than 5-10 hours. Also, the thickness
of the surface hardened layer produced is much thinner than in
conventional high temperature carburization, e.g., 20-50.mu. rather
than 1000-2000.mu..
[0011] In commonly assigned U.S. Pat. No. 6,547,888, the disclosure
of which is also incorporated herein by reference, a modified low
temperature carburization processes is described in which the
severity of the carburization conditions (and hence the
instantaneous rate of carburization) is lowered from an initial
higher value at earlier stages of carburization to a subsequent
lower value at later stages of carburization. The overall result is
that the time for completing the carburization process can be
shortened relative to conventional low temperature carburization,
since faster carburization is accomplished at earlier stages of
carburization when the workpiece is less sensitive to formation of
carbide precipitates. For convenience, this process will be
referred to as "modified" low temperature carburization.
[0012] This modified low temperature carburization process is
illustrated in Curve E in FIG. 1. As shown by this curve, the
carburization temperature of this modified process starts
considerably higher than the maximum carburization temperature used
in conventional low temperature carburization, i.e. about
600.degree. C.+. After less than an hour or so, the carburization
temperature is then reduced in a manner so that it always remains
below, but eventually follows, the lower aim of Curve QQ as the
carburization process goes to completion. The overall result is
that carburization can be completed faster than in conventional low
temperature carburization, because the workpiece is held at higher
temperature for most if not all of the carburization process.
SUMMARY
[0013] In accordance with this invention, it has been found that
carbon hardened surfaces can be produced in metal workpieces
without forming carbide precipitates even faster (and/or deeper)
than prior low temperature carburization processes by combining low
temperature carburization and high temperature carburization in the
same process. In particular, it has been found that high
temperature carburization can be used to augment low temperature
carburization, without formation of carbide precipitates, provided
that immediately after high temperature carburization the workpiece
is rapidly quenched through the region where carbide precipitates
can form.
[0014] Thus, this invention in one embodiment provides a process
for forming a carbon hardened surface in a metal workpiece without
forming carbide precipitates, the process comprising subjecting the
workpiece to both high temperature carburization and low
temperature carburization, wherein immediately after high
temperature carburization, the workpiece is rapidly quenched to a
temperature below which carbide precipitates can form. In this
embodiment, low temperature carburization can occur before or after
high temperature carburization, or both, provided that in either
case rapid quench occurs immediately after high temperature
carburization.
[0015] In addition, this invention in a more general embodiment
provides a process for altering the surface of a metal workpiece by
diffusing an element into the workpiece without forming
precipitates of the diffused element in the altered surface, the
process comprising contacting the workpiece with a diffusion gas
containing the element at a first elevated temperature which is
above the temperature at which such precipitates can form and, in
addition, contacting the workpiece with a diffusion gas at a second
elevated temperature which is lower than the first elevated
temperature and which is also below a temperature at which such
precipitates can form, wherein immediately after the workpiece is
contacted with the diffusion gas at the first elevated temperature,
the workpiece is rapidly quenched to a temperature below which such
precipitates can form.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] This invention may be more easily understood by reference to
the following drawings wherein
[0017] FIG. 1 is a time/temperature phase diagram illustrating the
conditions under which an AISI 316 stainless steel workpiece forms
carbide precipitates as well as the time/temperature profiles
experienced in conventional high temperature carburization,
conventional low temperature carburization and the modified low
temperature carburization process of commonly assigned U.S. Pat.
No. 6,547,888;
[0018] FIG. 2 is a view similar to FIG. 1, illustrating a first
embodiment of the invention in which the workpiece is subjected to
high temperature carburization first, the workpiece then rapidly
quenched through the region where carbide precipitates can form,
and then carburization is completed using low temperature
carburization conditions; and
[0019] FIG. 3 is a view similar to FIG. 2, illustrating a second
embodiment of the invention in which the workpiece is subjected to
low temperature carburization before high temperature
carburization.
DETAILED DESCRIPTION
[0020] According to this invention, a workpiece to be carburized is
subjected to both high temperature carburization and low
temperature carburization, wherein immediately after high
temperature carburization, the workpiece is rapidly quenched (i.e.,
rapidly cooled) to a temperature below which carbide precipitates
can form.
Alloys
[0021] The present invention is normally used for carburizing
workpieces made from iron, chromium and/or nickel-based alloys.
Such materials are well known and described for example in the
above-noted U.S. Pat. No. 5,792,282, U.S. Pat. No. 6,093,303, U.S.
Pat. No. 6,547,888, EPO 0787817 and Japanese Patent Document
9-14019 (Kokai 9-268364).
[0022] Particular alloys of interest are steels, especially steels
containing 5 to 50, preferably 10 to 40, wt. % Ni. Preferred alloys
contain 10 to 40 wt. % Ni and 10 to 35 wt. % Cr. More preferred are
the stainless steels, especially the AISI 300 series steels. Of
special interest are AISI 301, 303, 304, 309, 310, 316, 316L, 317,
317L, 321, 347, CF8M, CF3M, 254SMO, A286 and AL6XN stainless
steels. The AISI 400 series stainless steels and especially Alloy
410, Alloy 416 and Alloy 440C are also of interest.
[0023] Particular nickel-based alloys which can be low temperature
carburized in accordance with this invention include Alloy 600,
Alloy 625, Alloy 825, Alloy C-22, Alloy C-276, Alloy 20 Cb and
Alloy 718, to name a few examples.
[0024] In addition to iron, chromium and/or nickel-based alloys,
low temperature carburization in accordance with the present
invention can also be practiced on cobalt-based alloys as well as
manganese-based alloys. Examples of such cobalt-based alloys
include MP35N and Biodur CMM, while examples of such
manganese-based alloys include AISI 201, AISI 203EZ and Biodur
108.
[0025] The particular phase of the metal being processed in
accordance with the present invention is unimportant, as the
invention can be practiced on metals of any phase structure
including, but not limited to, austenite, ferrite, martensite,
duplex metals (e.g., austenite/ferrite), etc.
Carburizing Gas
[0026] When a workpiece is carburized by this invention, any known
carburizing gas can be used for this purpose.
[0027] The carburizing gases used in both high temperature
carburization and low temperature carburization include a
"carburizing specie," i.e., one or more compounds containing
carbon. In addition, they normally also contain an inert diluent,
most commonly nitrogen, in order to maintain a desired
concentration of the carburizing specie. Hydrogen gas may also be
included, especially if the carburizing specie also contains
oxygen, in order to capture the oxygen atoms liberated when the
carburizing specie decomposes.
[0028] High temperature carburization is normally accomplished to
drive carbon atoms into the workpiece surfaces as fast as possible.
Therefore, the carburizing gas often contains "carburization
enhancers" to provide more carbon atoms in the system.
C.sub.1-C.sub.5 alkanes and alkenes, and particularly butane, are
often used for this purpose.
[0029] In contrast, low temperature carburization is done under
much milder conditions so as to avoid formation of carbide
precipitates. Accordingly, carburization enhancers are normally
avoided. In addition, the carburizing specie is usually CO,
CO.sub.2 or a mixture thereof, since these oxides decompose to
yield elemental carbon at relatively low temperatures and, in
addition, yield only one such carbon atom per molecule when they
decompose. In addition, the concentration of the carburizing specie
is usually maintained at about 50% or less, more typically about
25% or less in conventional low temperature carburization. In the
modified low temperature carburization process of commonly assigned
U.S. Pat. No. 6,547,888, the concentration of carburizing specie
typically starts somewhat higher and ends somewhat lower.
[0030] Each of these carburizing gases is useful for the relevant
carburizing step of the inventive process. That is, carburizing
gases heretofore useful in conventional high temperature
carburization processes are useful in the high temperature
carburization step of the inventive process. Similarly, carburizing
gases heretofore useful in prior low temperature carburization
processes, both conventional and the modified process of commonly
assigned U.S. Pat. No. 6,547,888, are useful in the low temperature
carburization step of the inventive process. However, it is also
possible to use the same carburizing gas in the low temperature
carburization step and the high temperature carburization step of
this invention, if desired.
Quenching Techniques
[0031] According to this invention, the workpiece is subjected to
both high temperature carburization and low temperature
carburization, wherein immediately after high temperature
carburization, the workpiece is rapidly quenched (i.e., rapidly
cooled) to a temperature below which carbide precipitates form.
[0032] For this purpose, rapid quench can be accomplished by any
known technique. For example, immersion (or other contact) of the
workpiece in water, oil or other cooling medium such as a gas,
molten salt or the like can be used. Regardless of which approach
is adopted, however, rapid quenching should continue until the
temperature of the workpiece surface containing diffused carbon
atoms drops below the minimum temperature at which carbide
precipitates can form, e.g., below the lower arm of Curve QQ, as
this prevents unwanted precipitates from forming.
Hybrid Carburization--High Temperature Carburization First
[0033] As indicated above, the workpiece in this invention is
subjected to both high temperature carburization and low
temperature carburization, with the workpiece being rapidly
quenched immediately after high temperature carburization.
Normally, high temperature carburization will occur before low
temperature carburization. However as further discussed below, high
temperature carburization can occur after low temperature
carburization, as well. Indeed, in still another embodiment, high
temperature carburization can also occur between two separate low
temperature carburization steps, if desired.
[0034] Thus, in a first embodiment of this invention, high
temperature carburization is accomplished before low temperature
carburization by subjecting the workpiece to high temperature
carburization conditions during the early stages of carburization,
then rapidly quenching the workpiece through the region where
carbide precipitates can form, and then completing carburization
under low temperature carburization conditions. This is illustrated
by Curve F in FIG. 2 which shows that, at the start of
carburization, the workpiece is subjected to conventional high
temperature carburization conditions, as represented by Curve
Segment F(a) in this figure. Then after only a short period of
time, for example about 5 minutes (<0.1 hr.) in the particular
embodiment illustrated in this figure, the workpiece is rapidly
quenched (i.e. rapidly cooled) to a temperature below Curve QQ, as
represented by Curve Segment F(b) in this figure. Thereafter, the
workpiece is subjected to low temperature carburization conditions
to complete the carburization process. This can be done by
subjecting the workpiece to the carburization conditions of the
modified low temperature carburization process of commonly assigned
U.S. Pat. No. 6,547,888, as represented by Curve Segment F(c) in
this figure. Alternatively, this can be done by subjecting the
workpiece to the carburization conditions of conventional low
temperature carburization, i.e., using a carburizing gas having a
constant carbon concentration and maintaining a constant
carburization temperature such as 500.degree. C., for example.
[0035] In accordance with this first embodiment of the invention,
the workpiece is subjected to the more severe conditions normally
encountered in conventional high temperature carburization during
the early stages of carburization. In addition, carburization under
these conditions is allowed to proceed in time to point G which is
passed the point H where carbide precipitates would form at lower
temperature. In other words, carburization at high temperature
carburization conditions is allowed to proceed to point above Curve
QQ so that the workpiece must pass through the region within Curve
QQ where carbide precipitates form in order to reach room
temperature. Because the carburization temperature is so high,
infusion of carbon atoms into the workpiece is very rapid. This in
turn shortens the overall time it takes to complete carburization,
because a relatively large concentration of carbon has already
infused into the workpiece by the time the workpiece is quenched
and the low temperature carburization step begins.
[0036] As explained above in connection with FIG. 1, carbide
precipitates form and grow in conventional high temperature
carburization as the workpiece passes between points B and C along
Curve Segment A(b). In accordance with this embodiment of the
invention, the workpiece does enter the region within Curve QQ
where carbide precipitates can form. However, in this invention,
the workpiece is quenched (cooled) very rapidly so that the time
the workpiece (or at least the surface of the workpiece) remains
within this region is very short. The result is that essentially no
carbide precipitates form, because the time involved is simply too
short to allow this to happen.
[0037] In this connection, part of the process that occurs when
carbide precipitates form is that chromium metal, which is
uniformly distributed in the metal matrix forming the workpiece
surfaces, migrates to a central location, i.e. to sites where
individual carbide precipitates nucleate and grow. This phenomenon
does not occur instantaneously but rather takes some finite period
of time to accomplish. In accordance with this invention, the
workpiece is quenched rapidly so that the time the workpiece (or at
least the surface of the workpiece) is capable of forming carbide
precipitates form is very short. In other words, the time the
workpiece (or at least the surface of the workpiece) remains within
Curve QQ is very short. The net result is that essentially no
carbide precipitates form. This is because, while there may be some
formation of chromium carbide compounds while the workpiece
traverses Curve QQ along Curve Segment F(b), there is insufficient
time for crystalline precipitates of these compounds to nucleate
and grow. Thus, any chromium carbides which might formed are
essentially "frozen" in place before they can nucleate and grow
into a separate phase, i.e., before they can precipitate out as
distinct crystalline materials.
[0038] Accordingly, it is possible by this approach to combine the
advantages of conventional high temperature carburization, i.e.,
rapid infusion of carbon, with the benefits of low temperature
carburization, i.e., surface hardening of stainless steel without
formation chromium carbide precipitates, in the same process. As a
result, it is possible to complete carburization faster and/or
produce a deeper carburized surface layer than possible in prior
low temperature carburization processes, including the modified
process of commonly assigned U.S. Pat. No. 6,547,888, while still
preserving the corrosion resistance of the steel.
[0039] In a modification of this first embodiment, rapid quench
occurs at a point in time before point H is reached so that the
region within Curve QQ where carbide precipitates can form is
entirely avoided during the rapid quench step. Although this slows
the overall carburization process relative to the unmodified first
embodiment as described above, it still speeds the overall
carburization process relative to earlier low temperature
carburization technology, since a significant amount of carbon is
still infused into the workpiece as a result of the shortened high
temperature carburization step.
Hybrid Carburization--Low Temperature Carburization First
[0040] As indicated above, high temperature carburization can also
occur after low temperature carburization in accordance with this
invention. Thus, in a second embodiment of this invention, the
workpiece is subjected to low temperature carburization conditions
during early stages of carburization, then subjected to high
temperature carburization conditions for completing carburization
(i.e., completing the uptake of diffused carbon atoms), and then
rapidly quenched through the region where carbide precipitates can
form to produce the product carburized workpiece.
[0041] This is illustrated by Curve J in FIG. 3 which shows that,
at the start of carburization, the workpiece is subjected to
conventional low temperature carburization conditions, in the
manner described above in, as represented by Curve Segment J(a) in
this figure. Then after carburization has proceeded in time to a
point L which is passed the point H where carbide precipitates
would form at higher temperature, for example about 30 minutes
(.about.0.5 hr.) in the particular embodiment illustrated in this
figure, the workpiece is rapidly heated to a temperature above
Curve QQ, as represented by Curve Segment J(b) in this figure.
Then, the workpiece is subjected to high temperature carburization
conditions for a suitable period of time to complete the desired
uptake of carbon atoms, as represented by Curve Segment J(c) in
this figure. Immediately thereafter, the workpiece is rapidly
quenched to a temperature below Curve QQ, as represented by Curve
Segment J(d) in this figure.
[0042] As in the first embodiment, this embodiment also allows the
overall carburization process to be accomplished faster than in
prior low temperature carburization processes, since a significant
amount of the infused carbon is provided by high temperature
carburization which occurs at a much faster rate than low
temperature carburization. This second embodiment also uses rapid
quench to eliminate or at least minimize formation of carbide
precipitates. However, this second embodiment differs from the
first embodiment in that, in this second embodiment, the workpiece
has received its full complement of diffused carbon before rapid
quench occurs. The effect of this difference is that it takes more
time for the workpiece to traverse the region where carbide
precipitates form, i.e., within the envelope defined by Curve QQ,
in the second embodiment relative to the first embodiment, because
this envelope is thicker at this point in time as can be seen by
comparing the lengths of Curve Segments J(d) (FIG. 3) and F(b)
(FIG. 2) inside Curve QQ.
[0043] This means that, for a workpiece of a given size, there is a
greater risk that carbide precipitates will form using the second
embodiment of this invention relative to the first embodiment.
Accordingly, it may be beneficial to use the second embodiment
primarily on workpieces which cool faster. Thus, this second
embodiment is desirably used on smaller workpieces, i.e., workpiece
in which the minimum thickness dimension is smaller, as well as
workpieces in which the outside surface area to volume ratio is
greater.
[0044] This second embodiment of the invention also differs from
the first embodiment in that, in the second embodiment, the
workpiece crosses into the region where carbide precipitates form a
second time i.e., during rapid heating along Curve Segment J(b).
However, because carbon is fully soluble in the metal at the
elevated temperatures involved in high temperature carburization
along Curve Segment J(c), any carbide precipitates that may form
during rapid heating will tend to resolubilize during high
temperature carburization. Of course, it is desirable to carry out
rapid heating as quickly as possible to prevent or at least
minimize formation of carbide precipitates as much as possible
during this step.
[0045] Note, also, that this second embodiment of the invention can
be modified by beginning rapid heating at a point in time before
point H is reached so that the region within Curve QQ where carbide
precipitates can form is entirely avoided during the rapid heating
step. Although this slows the overall carburization process
relative to the unmodified second embodiment as described above, it
still speeds the overall carburization process relative to earlier
low temperature carburization technology, since a significant
amount of carbon is still infused into the workpiece as a result of
the high temperature carburization step.
Immediate Rapid Quenching and Immediate Heating
[0046] As indicated above, rapid quenching occurs in the inventive
process immediately after high temperature carburization step. In
this context, "immediately after" means that rapid quenching occurs
before the temperature of the workpiece is allowed to drift into
the region where carbide precipitates can form, i.e., inside the
envelope defined by Curve CC. In other words, rapid quenching
occurs without allowing the temperature of the workpiece to drop
below the upper arm of Curve QQ for any significant time before
rapid quenching starts.
[0047] In the same way, rapid heating of the workpiece "immediately
after" low temperature carburization in the second embodiment of
this invention means that rapid heating occurs without allowing the
temperature of the workpiece to increase above the lower arm of
Curve QQ for any significant time before rapid heating occurs.
Thus, rapid heating "immediately after" low temperature
carburization in this embodiment includes the situation, for
example, in which the workpiece is cooled to room temperature after
the low temperature carburization step and before rapid
heating.
[0048] Initiating rapid quenching immediately after high
temperature carburization, and initiating rapid heating immediately
after low temperature carburization in the second embodiment of
this invention, help to insure that formation of carbide
precipitates is avoided, or at least minimized to the greatest
degree possible, since it minimizes the time when the workpiece is
exposed to conditions where carbide precipitates can form.
Subsurface Layer with Carbide Precipitates
[0049] As well understood in the art, the rate at which an article
cools depends on both its size and its shape. In particular, large
compact articles inherently cool more slowly than smaller
less-compact articles. Furthermore, the temperature of the article
can vary considerably from its interior to exterior as the article
cools. For example, when a hot, large, compact article is rapidly
quenched by contact with water, its exterior temperature can drop
off very rapidly while its interior temperature can remain high for
a much longer time.
[0050] In the context of this invention, this phenomenon can come
into play when relatively large, compact workpieces are being
processed. In particular, it is possible when such workpieces are
being processed to produce carburized products having a hardened
surface layer which is free of carbide precipitates, in the manner
described above, but which also contains a subsurface layer
containing carbide precipitates beneath this carbide
precipitate-free surface layer. This can occur, for example, if the
metal forming this subsurface layer cools too slowly during rapid
quench to prevent carbide precipitates from forming.
[0051] In this case, the carburized product formed (after removal
of the oxide surface layer) has a main or primary surface layer
which is free of carbide precipitates, as described above. In
addition, it further includes a subsurface layer below this primary
carburized surface layer which contains carbide precipitates.
Although these carbide precipitates may foster corrosion, the
precipitate-free primary surface layer shields this subsurface
layer from contact with water, the atmosphere, or other
corrosion-causing media, and so this is not a problem.
Thickness of Precipitate-Free Primary Surface Layer
[0052] The thickness of the corrosion resistant, carbon hardened
surface layer produced by low temperature carburization, both
conventional as well as the modified process of commonly assigned
U.S. Pat. No. 6,547,888, is typically about 20.mu.-50.mu., although
thicknesses as low as 5.mu., and as high as 70.mu., and even
100.mu. have been reported. The corrosion resistant, carbide
precipitate free, carbon hardened surface layers produced by this
invention can have essentially the same thicknesses. And this is so
whether the carburized workpiece includes a subsurface layer
containing carbide precipitates, as discussed above, or not.
[0053] In addition, however, these carbon hardened surface layers
can also be considerably thicker, since the rapid infusion of
carbon atoms during the high temperature carburization step of the
inventive process allows the low temperature carburization step of
the inventive process to start with a substantial amount of
diffused carbon atoms already taken up by the workpiece. The result
is that the low temperature carburization step of the inventive
process finishes with a greater amount of diffused carbon atoms
driven deeper into the workpiece interior than the products
produced by prior low temperature carburization processes which
start with "fresh" or "virgin" workpieces containing no diffused
carbon atoms. That is, because the low temperature carburization
step of this invention starts with a greater amount of diffused
carbon atoms driven deeper into the workpiece interior, it ends
with a greater amount of diffused carbon atoms driven deeper into
the workpiece interior, as well.
[0054] Moreover as further described below, an additional feature
of this invention is that the diffused carbon atoms stabilize the
metal of the workpiece against formation of carbide precipitates,
at least if this metal has an austenitic phase structure. This
phenomenon also allows greater amounts of diffused carbon atoms to
be driven deeper into the workpiece relative to prior
technology.
[0055] Thus, it is contemplated that the carbon hardened,
precipitate free surface layers produced by this invention can be
>100.mu., .gtoreq.125.mu., .gtoreq.150.mu., .gtoreq.175.mu., and
even .gtoreq.200.mu. thick or more.
Changing the Carbon Concentration in the Carburizing Gas
[0056] As explained in commonly assigned U.S. Pat. No. 6,547,888,
the sensitivity of a stainless steel workpiece to formation of
carbide precipitates in low temperature carburization is a function
of the instantaneous rate of carburization at any time T.
Therefore, conventional low temperature carburization can be
completed faster by starting carburization at a higher
instantaneous rate of carburization than previously thought
possible and then reducing the instantaneous rate of carburization
from this higher value to a lower value at later stages of
carburization. As also explained in commonly assigned U.S. Pat. No.
6,547,888, the instantaneous rate of carburization at any time T is
a function of both the carburization temperature and the
concentration of the carburizing specie (or carbon) in the
carburizing gas. Therefore, the instantaneous rate of carburization
can be reduced from its higher value to its lower value by reducing
the carburizing temperature, the carbon concentration in the
carburizing gas, or both.
[0057] It will therefore be appreciated that, when carrying out the
inventive process, the concentration of carbon in the carburizing
gas can also be changed during carburization to foster faster
carburization (i.e. greater instantaneous rate of carburization) at
earlier stages of the process and slower carburization at later
stages of the process.
[0058] For example, during the high temperature carburization step,
the carburizing gas can contain carburization enhancers, i.e.,
additional carbon containing compounds such as methane, ethane,
propane, butane, etc., for fostering very rapid infusion of carbon
into the workpiece at this time. Since such enriching gases are
typically not involved in low temperature carburization, they will
not normally be present in the carburizing gas in the low
temperature carburization step of the inventive process either.
[0059] Changing the concentration of carbon in the carburizing gas
can also be done as part of the low temperature carburization step
of the inventive process. For example, the approach described in
connection with FIG. 2 can be repeated except that, rather than
reducing the carburization temperature during the low temperature
carburization step as illustrated by Curve Segment F(c) in this
figure, the carburization temperature can be maintained at a
constant value during this step such as 500.degree. C., for
example, while the concentration of carbon in the carburizing gas
can be reduced from a higher to a lower value during this low
temperature carburization step so as to achieve the desired
decrease in the instantaneous rate of carburization during this
step.
[0060] In still another approach, both the carburizing temperature
and the concentration of carbon in the carburizing gas can be
reduced from higher to lower values during the low temperature
carburization step to achieve the desired decrease in the
instantaneous rate of carburization during this step.
[0061] In yet another approach, neither the carburizing temperature
nor the concentration of carbon in the carburizing gas are reduced
from higher to lower values during the low temperature
carburization step. In other words, both the carburizing
temperature and the concentration of carbon in the carburizing gas
are held constant during the low temperature carburization step, as
accomplished in conventional low temperature carburization.
Although this approach does not enjoy the advantages of the
modified low temperature carburization technology of commonly
assigned U.S. Pat. No. 6,547,888, it does enjoy the benefits of
combining high and low temperature carburization steps, with
intermediate rapid quench, in the same process.
Other Diffusion-Based Surface Treatments
[0062] The primary focus of this invention is on the carburization
of iron-, nickel- and cobalt-based alloys, i.e., processes for
surface hardening workpieces made from iron-, nickel- and
cobalt-based alloys by infusing carbon atoms into these surfaces in
a manner so that carbide precipitates do not final. However, this
invention is also applicable to other analogous diffusion-based
surface treatments as well.
[0063] In low temperature carburization, as indicated above, atomic
carbon diffuses interstitially into the workpiece surfaces, i.e.,
carbon atoms travel through the spaces between the metal atoms
without significant substitutional diffusion of the metal atoms.
Because the processing temperature is low, these carbon atoms fowl
a solid solution with the metal atoms of the workpiece surfaces.
They do not react with these metal atoms to form other compounds.
Low temperature carburization is therefore different from high
temperature carburization in which the carbon atoms react to form
carbide precipitates, i.e., specific metal compounds such as
M.sub.23C.sub.6, M.sub.5C.sub.2 and the like, arranged in the faun
of discrete phases separate and apart from the metal matrix in
which they are contained.
[0064] Other analogous processes are known for altering the surface
characteristics of a metal workpiece by interstitial diffusion of
atoms into the workpiece surfaces to form solid solutions with the
metal atoms therein without formation of new compounds in separate
phases. Examples include nitriding and carbo-nitriding of iron,
chromium and/or nickel based alloys, nitriding and carbo-nitriding
of titanium-based alloys, and infusing atomic nitrogen, carbon,
boron or mixtures of these elements into aluminum and its alloys,
to name a few.
[0065] In addition to carburization, this invention is also
applicable to all such other interstitial diffusion-based surface
treatments, whether previously known or developed in the future.
That is to say, each of these other interstitial diffusion-based
surface treatments can be also be modified by this invention to
achieve faster results and/or deeper altered surface layers,
without forming precipitates of the diffused element, than possible
in the past.
[0066] So, for example, when the inventive process is used for
infusing atomic nitrogen, atomic carbon, atomic boron or mixtures
of these elements into aluminum and its alloys, it can be used on
pure aluminum metal as well as alloys of aluminum which also
exhibit a face centered cubic crystal lattice structure. More
specifically, the present invention is applicable to any aluminum
alloy whose surface exhibits contiguous regions or "domains"
exhibiting a face centered cubic crystal structure such that
diffused atoms can penetrate into these contiguous regions
sufficient to form a noticeable hardening effect on the surface
being treated. Aluminum alloys of particular interest are those
containing one or more of Cu, Mg, Mn, Si, Fe, Cr, Zn and Ni, while
aluminum alloys containing one or more of Cu, Mg, Mn, Si and Fe are
of particular interest. Other elements can also be included.
Particular aluminum alloys of interest are AA 7075, AA 3003, AA
6061, AA 6063, AA 2026, AA 2024, AA 2017, AA 2011, AA 5029, AA
5052, AA 5053 and AA 1100.
[0067] Or, for example, when this invention is used for nitriding
or carbo-nitriding of titanium or its alloys, any titanium metal
and alloy exhibiting at least about 30% of the .alpha.-phase
structure can be processed. Thus, this invention applies to
titanium metal (i.e. essentially pure titanium) as well as to
titanium alloys composed substantially completely of the
.alpha.-phase. In addition, the present invention also applies to
duplex and other titanium alloys containing somewhat less than 100%
.alpha.-phase such as 90%, 80%, 70%, 60%, 50%, 40% and even 30%
.alpha.-phase. Generally, such alloys will contain at least about
90 wt. % titanium, although alloys containing as little as 65 wt.
%, 50 wt. % or even 35 wt. % titanium can also be used. Titanium
alloys containing aluminum, vanadium and molybdenum are
interesting. Alloys of special interest are Ti-6A1-4V (6 wt. % Al,
4 wt. % V, balance Ti), which is known as "Titanium 64," and
Ti-8A1-1V-1Mo (8 wt. % Al, 1 wt. % V, 1 wt. % Mo, balance Ti),
known as "Titanium 811."
[0068] Thus, it will be appreciated that, although this invention
is described for convenience in this document in terms of surface
hardening by carburization, this invention also applies to such
other analogous processes as well.
[0069] Although only a few embodiments of this technology have been
described above, it should be appreciated that many modifications
can be made. All such modifications are intended to be included
within the scope of this disclosure, which is to be limited only by
the following claims.
* * * * *