U.S. patent application number 12/530799 was filed with the patent office on 2010-02-18 for diffusion promoters for low temperature case hardening.
This patent application is currently assigned to Swagelok Company. Invention is credited to Sunniva R. Collins, William H. Glime, Gary W. Henrich, Andrew P. Marshall, Gerhard H. Schiroky, Geroge R. Vraciu, Peter C. Williams.
Application Number | 20100037991 12/530799 |
Document ID | / |
Family ID | 39410513 |
Filed Date | 2010-02-18 |
United States Patent
Application |
20100037991 |
Kind Code |
A1 |
Collins; Sunniva R. ; et
al. |
February 18, 2010 |
DIFFUSION PROMOTERS FOR LOW TEMPERATURE CASE HARDENING
Abstract
Low temperature carburization of a workpiece surface is
accomplished faster by impregnating the surface with a diffusion
promoter prior to or during the low temperature carburization
process.
Inventors: |
Collins; Sunniva R.;
(Cleveland Heights, OH) ; Glime; William H.;
(Painesville, OH) ; Henrich; Gary W.; (Aurora,
OH) ; Marshall; Andrew P.; (University Heights,
OH) ; Schiroky; Gerhard H.; (Aurora, OH) ;
Williams; Peter C.; (Cleveland Heights, OH) ; Vraciu;
Geroge R.; (Canton, OH) |
Correspondence
Address: |
CALFEE HALTER & GRISWOLD, LLP
800 SUPERIOR AVENUE, SUITE 1400
CLEVELAND
OH
44114
US
|
Assignee: |
Swagelok Company
Solon
OH
|
Family ID: |
39410513 |
Appl. No.: |
12/530799 |
Filed: |
March 12, 2008 |
PCT Filed: |
March 12, 2008 |
PCT NO: |
PCT/US08/56558 |
371 Date: |
September 11, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60921935 |
Apr 5, 2007 |
|
|
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60931064 |
May 21, 2007 |
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Current U.S.
Class: |
148/218 ;
148/206; 148/225; 148/230; 148/237; 148/238; 148/240; 148/316;
148/317; 148/318; 148/319; 148/400; 148/95 |
Current CPC
Class: |
C23C 8/22 20130101; C23C
8/32 20130101; C23C 8/02 20130101; C23C 8/26 20130101 |
Class at
Publication: |
148/218 ;
148/319; 148/95; 148/237; 148/238; 148/206; 148/240; 148/230;
148/225; 148/400; 148/316; 148/318; 148/317 |
International
Class: |
C23C 8/00 20060101
C23C008/00; C23C 8/22 20060101 C23C008/22; C23C 8/24 20060101
C23C008/24; C23C 8/26 20060101 C23C008/26; C23C 8/32 20060101
C23C008/32; C23C 8/30 20060101 C23C008/30 |
Claims
1. A metal product produced by subjecting a metal workpiece to a
low temperature diffusion-based surface treatment to produce an
altered surface layer containing an increased concentration of a
diffusing element in an amount sufficient to alter the properties
of the metal forming the surface layer, the altered surface layer
also being free of precipitates of compounds of the diffusing
element, wherein the altered surface layer further contains an
increased concentration of at least one promoter element capable of
enhancing the rate at which the diffusing element diffuses into the
surface of the workpiece to be altered during the low temperature
diffusion-based surface treatment.
2. The metal product of claim 1, wherein the metal workpiece is
made from a carbon or low alloy steel, a nickel-based alloy, a
chromium-steel alloy, a titanium based alloy or an aluminum-based
alloy.
3. The metal product of claim 2, wherein the altered surface layer
is produced by the low temperature carburization of a stainless
steel workpiece to produce an altered surface layer containing
interstitially diffused carbon atoms, the altered surface layer
being harder than the base metal from which the product is formed
and free of carbide precipitates.
4. The metal product of claim 2, wherein the altered surface layer
is produced by carburizing, nitriding or carbo-nitriding a carbon
or low alloy steel, a chromium-steel alloy, stainless steel, or a
nickel-based alloy.
5. The metal product of claim 2, wherein the altered surface layer
is produced by carburizing, nitriding or carbo-nitriding a
titanium-based alloy.
6. The metal product of claim 2, wherein the altered surface layer
is produced by infusing atomic nitrogen, carbon, boron or mixtures
of these elements into an aluminum-based alloy.
7. A low temperature carburized stainless steel product having an
altered surface layer which (a) contains an increased concentration
of carbon atoms relative to the base metal from which the product
is made, (b) is harder than the base metal from which the product
is made, (c) is free of carbide precipitates, and (d) contains an
increased concentration of at least one promoter element selected
from Mo, Ni, Cr, Ti, V and Nb relative to the base metal from which
the product is made.
8. The process of claim 7, wherein the stainless steel product is
made from an AISI 400 series stainless steel.
9. A process for enhancing the rate at which a diffusing element
diffuses into the surface of a metal workpiece in a low temperature
diffusion-based surface treatment to produce an altered surface
layer containing an increased concentration of the diffusing
element in an amount sufficient to alter the properties of the
metal forming the surface layer, the altered surface layer also
being free of precipitates of compounds of the diffusing element,
the process comprising treating the workpiece to increase the
concentration of at least one promoter element in the surface to be
altered, the promoter element being capable of enhancing the rate
at which the diffusing element diffuses into the surface of the
workpiece to be altered during the low temperature diffusion-based
surface treatment.
10. The process of claim 9, wherein the concentration of the
promoter element in the surfaces to be altered is increased (a) by
applying a coating containing the promoter element, or a compound
capable of decomposing to yield the promoter element, to these
surfaces and then heating the applied coating to drive the
diffusing element into these surfaces, or (b) by a gas phase
diffusion process in which the promoter element, or a compound
capable of decomposing to yield this element, is contacted with the
workpiece surface in the form of a gas, or (c) by contacting the
workpiece with a molten bath of a fluoride salt containing the
promoter element, or (d) by treating the surfaces to be altered to
make them porous, impregnating these porous surfaces with the
promoter element, or a compound capable of decomposing to yield
this element, and then heating these impregnated surfaces to
elevated temperature to promote diffusion of the promoter element
into these surfaces.
11. The process of claim 9, wherein the metal workpiece is made
from a carbon or low alloy steel, a nickel-based alloy, a
chromium-steel alloy, a titanium based alloy or an aluminum-based
alloy.
12. A process for altering the physical properties of the surface
layer of a metal workpiece by a low temperature diffusion-based
surface treatment in which the metal workpiece is contacted with a
gas containing a diffusing element at a treatment temperature which
is high enough to cause the diffusing element to diffuse into the
workpiece surfaces thereby producing an altered surface layer, the
treatment temperature also being low enough to prevent formation of
precipitates of compounds of the diffusing element, wherein the
workpiece is treated to increase the concentration of at least one
promoter element in the surface to be altered, the promoter element
being capable of enhancing the rate at which the diffusing element
diffuses into the surface of the workpiece to be altered during the
low temperature diffusion-based surface treatment.
13. The process of claim 12, wherein the low temperature
diffusion-based surface treatment comprises carburizing, nitriding
or carbo-nitriding a titanium-based alloy.
14. The process of claim 12, wherein the low temperature
diffusion-based surface treatment comprises infusing atomic
nitrogen, carbon, boron or mixtures of these elements into an
aluminum-based alloy.
15. The process of claim 12, wherein the low temperature
diffusion-based surface treatment comprises carburizing, nitriding
or carbo-nitriding an iron-based alloy, a chromium-based alloy or a
nickel-based alloy.
16. The process of claim 15, wherein a stainless steel workpiece is
low temperature carburized to produce an altered surface layer
containing interstitially diffused carbon atoms, the altered
surface layer being harder than the base metal from which the
product is formed and free of carbide precipitates.
17. The process of claim 12, wherein the workpiece is treated to
increase the concentration of at least one promoter element in the
surface to be altered prior to the start of the low temperature
diffusion based surface treatment.
18. The process of claim 12, wherein the workpiece is treated to
increase the concentration of at least one promoter element in the
surface to be altered during the low temperature diffusion based
surface treatment by interrupting this surface treatment, treating
the workpiece to increase the concentration of at least one
promoter element in the surface to be altered during this
interruption, and thereafter resuming the low temperature diffusion
based surface treatment.
Description
RELATED APPLICATIONS
[0001] The present application claims the benefit of pending U.S.
provisional application Ser. No. 60/921,935 filed on Apr. 5, 2007,
for DIFFUSION PROMOTERS FOR LOW TEMPERATURE CASE HARDENING and U.S.
provisional application Ser. No. 60/931,064 filed on May 21, 2007,
for DIFFUSION PROMOTERS FOR LOW TEMPERATURE CASE HARDENING, the
entire disclosures of which are 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. Pat. No. 6,165,597, U.S. Pat. No. 6,547,888, 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.
SUMMARY
[0006] In accordance with this invention, it has been found that
low temperature carburization of a workpiece surface can be
accomplished faster by impregnating the surface with a diffusion
promoter prior to or during the low temperature carburization
process.
[0007] Thus, this invention in one embodiment provides a low
temperature carburized stainless steel product having an altered
surface layer which (a) contains an increased concentration of
carbon atoms relative to the base metal from which the product is
made, (b) is harder than the base metal from which the product is
made, (c) is free of carbide precipitates, and (d) contains an
increased concentration of at least one promoter element selected
from Mo, Ni, Cr, Ti, V and Nb relative to the base metal from which
the product is made.
[0008] In addition, this invention in a broader embodiment provides
a metal product produced by subjecting a metal workpiece to a low
temperature diffusion-based surface treatment to produce an altered
surface layer containing an increased concentration of a diffusing
element in an amount sufficient to alter the properties of the
metal forming the surface layer, the altered surface layer also
being free of precipitates of compounds of the diffusing element,
wherein the altered surface layer further contains an increased
concentration of at least one promoter element capable of enhancing
the rate at which the diffusing element diffuses into the surface
of the workpiece to be altered during the low temperature
diffusion-based surface treatment.
[0009] In addition, this invention further provides a process for
enhancing the rate at which a diffusing element diffuses into the
surface of a metal workpiece in a low temperature diffusion-based
surface treatment to produce an altered surface layer containing an
increased concentration of the diffusing element in an amount
sufficient to alter the properties of the metal forming the surface
layer, the altered surface layer also being free of precipitates of
compounds of the diffusing element, the process comprising treating
the workpiece to increase the concentration of at least one
promoter element in the surface to be altered, the promoter element
being capable of enhancing the rate at which the diffusing element
diffuses into the surface of the workpiece to be altered during the
low temperature diffusion-based surface treatment.
[0010] Finally, this invention also provides a process for altering
the physical properties of the surface layer of a metal workpiece
by a low temperature diffusion-based surface treatment in which the
metal workpiece is contacted with a gas containing a diffusing
element at a treatment temperature which is high enough to cause
the diffusing element to diffuse into the workpiece surfaces
thereby producing an altered surface layer, the treatment
temperature also being low enough to prevent formation of
precipitates of compounds of the diffusing element, wherein the
workpiece is treated to increase the concentration of at least one
promoter element in the surface to be altered, the promoter element
being capable of enhancing the rate at which the diffusing element
diffuses into the surface of the workpiece to be altered during the
low temperature diffusion-based surface treatment.
DETAILED DESCRIPTION
[0011] In accordance with this invention, low temperature
carburization of a workpiece surface is accomplished faster by
impregnating the surface with a diffusion promoter prior to or
during the low temperature carburization process.
Low Temperature Carburization
[0012] The primary focus of this invention is on the low
temperature carburization of iron-, nickel- and cobalt-based
alloys, especially stainless steel. In this process, which is
extensively described in the above-noted U.S. Pat. No. 5,556,483,
U.S. Pat. No. 5,593,510, U.S. Pat. No. 5,792,282, U.S. Pat. No.
6,165,597, U.S. Pat. No. 6,547,888, EPO 0787817, Japan 9-14019
(Kokai 9-268364) and Japan 9-71853 (Kokai 9-71853), elemental
carbon diffuses into the metal matrix forming the workpiece without
formation of carbide precipitates.
[0013] Low temperature carburization normally produces an outermost
oxide surface layer on the workpiece being treated about 20-30 nm
thick. See, Japan 9-71853 (Kokai 9-71853). Depending on the
carburizing conditions, this outermost oxide surface layer may also
be covered with soot. In addition, under this oxide surface layer,
an extremely thin outer surface layer of the metal may contain a
small amount of carbide precipitates, especially if the low
temperature carburization conditions are too severe. See, U.S. Pat.
No. 5,556,483, U.S. Pat. No. 5,593,510 and U.S. Pat. No. 5,792,282.
In order for the workpiece to exhibit an attractive metallic
appearance, this soot and outermost oxide surface layer must be
removed. In addition, this extremely thin outermost metal surface
layer must also be removed in order for the workpiece to exhibit
good corrosion resistance, at least if this outermost metal surface
layer contains carbide precipitates. Therefore, as a practical
matter, these unwanted by-products of the low temperature
carburization process (i.e., the soot, oxide surface layer, and
thin outermost metal layer containing carbide precipitates, if any)
are removed before the workpiece is used. Accordingly, in the
context of this disclosure, reference to a workpiece surface layer
which is "free of carbide precipitates" or which is made "without
formation of carbide precipitates" refers to the carbon-hardened
surface layer remaining after the unwanted by-products of the low
temperature carburization process (i.e., the soot, oxide surface
layer, and thin outermost metal layer containing carbide
precipitates, if any) are removed.
[0014] In this invention, low temperature carburization is carried
out in the same way as done in the past by contacting the workpiece
with a carburizing gas at an elevated temperature for a time
sufficient to produce in the primary surface layer of the workpiece
(i.e. the surface layer of the workpiece after the unwanted
by-products of the low temperature carburization process are
removed) an elevated amount of elemental carbon without formation
of carbide precipitates. For this purpose, the carburization
temperature will normally be no greater than about 500.degree. C,
although higher temperatures can be used by following the modified
approach described in commonly assigned U.S. Pat. No. 6,547,888.
Moreover, carburization will normally last 20-50 hours, although
longer or shorter processing times can be used. As a result, a
primary surface layer typically about 20-50.mu. thick and normally
containing about 2-15 atomic %, more typically about 5-10 atomic %
or even 9-12 atomic % atomic carbon will be obtained.
[0015] Incidentally, because low temperature carburization is a
diffusion-based process, the concentration of carbon in the
workpiece surfaces decreases from a maximum at or very near the
outermost surface of the workpiece down to an equilibrium value
(which is the carbon concentration in the "native" or untreated
metal from which the workpiece is made) in accordance with Fick's
law. Thus, it will be understood that the above reference to a
carbon concentration of about 2-15 atomic %, for example, means
that this is the carbon concentration at or near the outermost edge
of the primary surface layer of the workpiece.
Other Low Temperature Diffusion-Based Surface Treatments
[0016] This invention concentrates on low temperature carburization
of iron-, nickel- and cobalt-based alloys. In this context, an
alloy which is "based" on a particular metal means that the alloy
contains at least 35% of that metal. However, this invention is
also applicable to other analogous low temperature diffusion-based
surface treatments as well.
[0017] 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 form
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 normal
carburization carried out at higher temperatures in which the
carbon atoms react to form carbide precipitates, i.e., specific
metal compounds such as M.sub.23C.sub.6 (e.g., Cr.sub.23C.sub.6 or
chromium carbide), M.sub.5C.sub.2 and the like, arranged in the
form of discrete phases separate and apart from the metal matrix in
which they are contained.
[0018] Other analogous processes are known for altering the surface
characteristics of a metal workpiece by interstitial diffusion of
atoms into the workpiece surfaces at relatively low temperatures to
form solid solutions with the metal atoms therein without formation
of new compounds in separate phases. Examples include carburizing
chromium- and nickel-based alloys, nitriding and carbo-nitriding of
iron-, chromium- and/or nickel-based alloys, carburizing, 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. For convenience, all of these
processes will be referred to collectively as "low temperature
diffusion based surface treatments" or "low temperature"
treatments.
[0019] The present invention is applicable to all such low
temperature diffusion-based surface treatments. That is to say, the
speed or rate at which each of these low temperature
diffusion-based surface treatments can be carried out can be
enhanced by adopting the principles of this invention. Thus, it
will be appreciated that, although this invention is described here
in terms of low temperature carburization for convenience, this
invention also applies to such other analogous processes as
well.
Enhancing Diffusion
[0020] In accordance with this invention, it has been recognized
that the presence of certain alloying elements in a metal workpiece
subjected to a low temperature diffusion-based surface treatment
promotes or facilitates the speed or rate at which this surface
treatment occurs. For example, it is very difficult to low
temperature carburize Alloys 440C (S44004) and 410 (S41000), which
are iron-based alloys containing 16-18% Cr and 12-13% Cr,
respectively, and not much else, since the diffusion of carbon
atoms into these alloys is very slow. In contrast, iron-based
alloys M152 (S32550), AM355 \and 17-4 PH (S17400) carburize very
easily, even though they contain essentially the same amount of Cr,
i.e., 15-17%. However, Alloys M152 (S32550), AM355 and 17-4 PH
(S17400) also contain 2-5% Ni, 2-3% Mo, or both, which indicates
that these additional alloying elements, i.e., the Ni and Mo,
promote the rate at which low temperature carburization of these
alloys occurs. Cr, Ti, V and Nb have also been observed to impart a
similar promoting effect on the ability of certain other iron-based
alloys to respond to low temperature carburization. Accordingly,
recognition of this phenomenon is taken advantage of in this
invention by treating a workpiece made from such an alloy, prior to
or during the low temperature carburization process, to cause one
or more of these "promoter elements" to be taken up by and diffuse
into the workpiece surface. Then, when the low temperature
carburization is carried out, it can be completed faster (and/or a
thicker hardened surface can be achieved in the same amount of
time) than would otherwise occur, because the promoter element
facilitates diffusion of carbon into the metal matrix of the
workpiece surface.
[0021] The amount of promoter element that should be added to the
workpiece surfaces to promote the low temperature diffusion-based
surface treatment varies depending on the composition of the
workpiece being treated and the particular promoter element chosen.
For example, it has been found that the addition of about 0.5-5 wt.
% Mo, Ni, Cr, Ti, V and/or Nb significantly enhances the rate with
which elemental carbon is taken up by iron-based workpieces.
However, greater or lesser amounts will still achieve some benefits
and still can be used. For example, amounts as low as 0.1 wt. % and
as high as 5 wt. % can be used. Additions on the order of 0.3 to
2.0 wt. % are more common. In this regard, routine experimentation
can be used to determine the optimal amounts of particular promoter
elements to use on particular alloys. Similar amounts of promoter
elements are believed appropriate for non-iron-based alloys, it
being understood that routine experimentation may be helpful for
determining the appropriate amounts of particular promoter elements
to use on these alloys as well.
[0022] As indicated above, the primary focus of this invention is
on the low temperature carburization of iron-based alloys,
especially stainless steels and the like. Alloys 440C (S44004) and
410 (S41000) mentioned above are particularly interesting. However,
it is also applicable to other low temperature diffusion based
processes as well.
[0023] So, for example, when carbon is diffused into a
chromium-steel alloy, Ni and Mo are believed to be helpful in
promoting the diffusion process.
[0024] Similarly, when carbon is diffused into a nickel-based alloy
or a stainless steel alloy, Mo, Ti, V and Nb are believed to be
helpful in promoting the diffusion process.
[0025] When nitrogen is diffused into a carbon or low alloy steel,
Cr, Ni and Mo are believed to be helpful in promoting the diffusion
process.
[0026] When carbon or nitrogen are diffused into a titanium-based
alloy, V and Nb are believed to be helpful in promoting the
diffusion process.
[0027] When carbon, nitrogen or boron are diffused into an
aluminum-based alloy, Ti, V and Nb are believed to be helpful in
promoting the diffusion process.
[0028] Such promoting elements can be used singly or in
combination.
[0029] In the same way, it is also believed that when other low
temperature diffusion-based surface treatments are carried out with
other metal/diffusing element combinations, appropriate promoter
elements will enhance the rate at which diffusion occurs. Thus,
this invention can be used with respect to all such other
combinations as well. That is to say, while all combinations of all
promoter elements which will enhance the diffusion of a particular
element into a particular metal in a low temperature
diffusion-based processes are not known as of this writing, such
combinations can be easily determined by routine experimentation.
Therefore, even though these particular combination may not be
known today, this invention is applicable to such combinations.
[0030] It should also be appreciated that there is an inherent
limit to the diffusion-enhancing effect of promoter elements in
general and that, as a result, little or no enhancement in the
speed or rate of diffusion may be possible for certain alloys. So,
for example, many stainless steels already contain significant
amounts of Ni and Cr. Thus, the AISI 300 series steels such as AISI
301, 303, 304, 309, 310, 316, 316L, 317, 317L, 321, 347, CF8M,
CF3M, 254SMO, A286 and AL6XN stainless steels, for example, already
contain significant quantities of Ni, Cr and, in some instances,
Mo. Little if any improvement in the diffusion rate of carbon into
these steels by low temperature carburization is achievable by this
invention, relative to standard practice, because these steels
already contain suitable quantities of promoting elements.
[0031] In contrast, the AISI 400 series stainless steels such as
Alloy 410, Alloy 416 and Alloy 440C, which contain far less nickel,
are difficult to process by standard low temperature carburization
due to a low acceptance of carbon. These alloys can be processed
with particular advantage according to this invention, since the
presence of additional amounts of diffused nickel according to this
invention will significantly enhance the rate at which diffusion of
carbon into these alloys occurs. Similarly duplex steels, which are
composed of ferritic regions rich in Cr but poor in Ni as well as
austenitic regions with moderate Cr content but rich in Ni, can be
processed with advantage by this invention, since the addition of a
suitable diffusing agent such as Ni to the steel as a whole will
allow its ferritic regions to carburize more easily. Alloys 2205
and 2507 are good examples of such duplex steels.
Applying the Promoting Elements
[0032] In accordance with this invention, the speed or rate at
which a low temperature diffusion-based surface treatment of a
metal workpiece can be completed (and/or the thickness of the
altered surface layer obtained can be increased) by impregnating
the surface with a diffusion promoter prior to or during the low
temperature carburization process. This can be done by any process
or technique which will increase the concentration of the promoter
element in the surface of the workpiece which is to be altered by
the low temperature diffusion-based surface treatment. In other
words, this can be done by any process or technique which increases
the concentration of the promoter element in the surface of the
workpiece prior to the start of the low temperature diffusion-based
surface treatment.
[0033] For example, the workpiece can be coated with a layer of the
promoter element by any known coating technique and then heated to
elevated temperature to drive the promoter element into the
workpiece surface. For example, a workpiece made from an iron-based
alloy can be provided with a coating of the promoter element by
electroplating, electroless plating techniques, plasma coating,
dipping in molten metal, painting with a paint containing the
promoter element, or any other technique which will provide a layer
of the promoter element in contact with the workpiece surfaces.
Normally, the promoter element will be present in elemental form,
although it can also be present in the form of a compound which
decomposes to yield the promoter element at the conditions employed
for driving the promoter element into the workpiece surfaces.
Thereafter, the coated workpiece can be heated to elevated
temperature, e.g., 1,000.degree. C. (1832.degree. F.) or more, to
drive the promoter element from the coating into the workpiece. A
modification of this technique based on ALD (atomic layer
deposition) in which the workpiece is subjected to repeated
iterations of deposition and diffusion can also be used.
[0034] Other techniques capable of causing promoter elements to
diffuse into metal surfaces can also be used. For example, gas
phase diffusion processes analogous to the low temperature
diffusion-based process described above can be used, i.e.,
diffusion-based processes in which the promoter element, or a
compound capable of decomposing to yield this element, is contacted
with the workpiece surface in the form of a gas. Alternatively, the
so-called GE Metalliting process in which the workpiece is
contacted with a molten bath of a fluoride salt containing the
promoter element can be used.
[0035] In still another approach, a workpiece previously treated to
make its surface porous can be impregnated with the promoter
element, or a compound capable of decomposing to yield this
element, and then heated to elevated temperature to promote
diffusion of this element into its surfaces. Examples of techniques
that can be used for making the workpiece surface porous include
contact with hot, concentrated HCl, aqua regia or the like,
mechanical abrasion, anodizing the workpiece, and growing a porous
oxide layer the workpiece surface in the manner described in the
Background Section above (i.e., by exposing the surface to oxygen
or a compound capable of liberating oxygen under the conditions
encountered by the workpiece).
[0036] Most often, impregnation of the workpiece with the promoter
element will be done prior to the start of the low temperature
diffusion based surface treatment. However, impregnation can also
be done during the low temperature diffusion based surface
treatment, for example, by interrupting (i.e. stopping) this
surface treatment, applying the promoter element in the manner
described above during this interruption, and then resuming the low
temperature diffusion based surface treatment. In any event,
regardless of the particular technique employed, the workpiece is
impregnated with a promoter element such that the altered surface
layer in the metal product ultimately produced by the low
temperature diffusion based surface treatment, in addition to
containing an increased concentration of the diffusing element
responsible for altering the properties of this surface layer,
further contains an increased concentration of at least one
promoter element capable of enhancing the rate at which this
diffusing element diffuses into the workpiece surface layer.
[0037] In this connection, it has already been proposed to activate
a stainless steel workpiece for subsequent low temperature
carburization by providing the workpiece with an electroless nickel
coating. See, U.S. Published Patent Application U.S. 2006/0090817
A1. When such a workpiece is low temperature carburized, it is
believed that some of the nickel in this coating diffuses into the
workpiece surface as a by-product of the low temperature
carburization process. That is to say, it is believed that the
conditions of time and temperature involved in low temperature
carburization are severe enough so that some incidental amount of
nickel atoms in this coating diffuse from atop the workpiece
surface into this workpiece surfaces along with the carbon atoms
that diffuse into this surface during the low temperature
carburization step. The invention described here differs from that
prior practice in that, in this invention, more than incidental
diffusion of nickel or other promoter elements is involved. In
other words, in this invention the amount of promoter element which
diffuses into the workpiece surfaces is greater than the incidental
amount of nickel atoms that may have diffused into the workpiece
surfaces in that earlier technology. Therefore, it will be
appreciated that reference in this document to an "increased
concentration of promoter element" in the surface layer of the
metal product produced by this invention means an increase over and
above the incidental increase that might occur in that earlier
technology as a by-product of the low temperature diffusion based
surface treatment.
Activation
[0038] Some metals form coherent protective oxide coating layers
essentially instantaneously upon contact with air. A good example
is aluminum and its alloys, which form coherent protective coating
layers of aluminum oxide. Another example is stainless steel, which
forms a coherent protective coating layer of chromium oxide. These
protective coherent oxide coating layers are impervious to most
materials, including the diffusing elements typically used in most
low temperature diffusion-based processes. Accordingly, these
workpieces are typically "activated" before or simultaneously with
the diffusion-based surface treatment to make this coating
permeable to the diffusing element being used.
[0039] Activation is normally done by treatment with a
halogen-containing gas such as F.sub.2, Cl.sub.2, HCl, HF, NF.sub.3
and the like. Treatment with acids in liquid form, particularly
aqueous compositions of strong acids, such as aqueous HCl,
H.sub.2SO.sub.4, HNO.sub.3, aqua regia and the like can also be
used. Activating can be also be done mechanically, for example, by
sawing, scraping or sanding the workpiece to expose the "native"
metal of the surface being treated. In addition, activating can
also be done electrochemically, i.e., by contacting the metal with
an electrolyte and subjecting the metal to a sufficient electric
potential to cause anodic decomposition.
[0040] When the workpiece being treated by this invention is made
from a metal which does not form such a coherent protective oxide
coating, no special pretreatment or other procedures is needed for
causing the selected promoter element to diffuse into the workpiece
surfaces. However, where this metal does form a coherent protective
oxide coating, some form of activation may be necessary depending
on the approach used for supplying the promoter element and/or
driving the diffusing element into the workpiece surfaces.
[0041] In this connection, many of the approaches described above
for causing promoter elements to diffuse into a workpiece's surface
inherently depassivate any coherent protective oxide layer that
might be present at the same time. For example, heating a workpiece
to elevated temperature, e.g., 1,000.degree. C. (1832.degree. F.)
or more, for driving a promoter element into the workpiece's
surface will also depassivate the protective, coherent protective
oxide coating formed by most metals. Similarly, electrolysis metal
coating techniques will also depassivate the protective, coherent
oxide coating formed by most metals. Similarly, the GE Metalliting
process in which the workpiece is contacted with a molten bath of a
fluoride salt containing the promoter element will also depassivate
most protective, coherent oxide coatings. So in many instances, no
special activation or pretreatment is required for practicing this
invention.
[0042] However, in those instances where the protective, coherent
oxide coating is not depassivated by the particular
promoter-diffusion approach used, the workpiece can be pretreated
to depassivate this oxide coating before the promoter element is
applied, or at least before the workpiece is treated to drive a
previously-applied promoter element into its surfaces.
[0043] This pretreatment can be done in the same way as described
above for activating the workpiece in traditional low temperature
diffusion based surface treatments such as, for example, by contact
with a halogen-containing gas or strong acid, by mechanically
exposing the workpiece's native metal, or electrochemically. Once
this is done, the workpiece can be subjected to the particular
coating/diffusion approach selected for diffusing the promoter
element into the workpiece surface. Normally, this will be done
without exposing the activated workpiece to the atmosphere to avoid
repassivating the previously-depassivated surfaces. However, it is
normally not necessary to keep the workpiece out of contact with
the atmosphere in those situations in which the particular
coating/diffusion approach selected provides its own coherent
impervious coating of the promoter element, or a compound capable
of decomposing to yield this element. This is because this promoter
element coating will normally prevent repassivation from
occurring.
Product Workpiece
[0044] As indicated above, the metal products produced by earlier
low temperature diffusion-based processes include an altered
surface layer containing an increased concentration, relative to
the "native" metal from which the workpiece is made, of a diffusing
element in an amount sufficient to alter the properties of the
metal forming the surface layer. Nonetheless, this altered surface
layer is still free of precipitates of compounds formed from the
diffusing element. Thus, for example, when a stainless steel
workpiece is low temperature carburized, an altered surface layer
is obtained which is not only free of chromium carbide precipitates
but, in addition, exhibits greater hardness due to the presence of
the diffused carbon atoms.
[0045] This same result is also achieved in this invention.
However, this invention departs from earlier work in that, in this
invention, the surface of the workpiece which is subjected to the
low temperature diffusion-based process further includes an
increased concentration of at least one promoter element capable of
enhancing the rate at which the diffusing element diffuses into the
workpiece's surfaces. Moreover, the amount of this increase is
greater than any incidental increase that may have occurred as a
by-product of the low temperature carburization process described
in U.S. 2006/0090817 A in which the workpiece is activated by means
of an electroless nickel coating. The result is that the low
temperature diffusion based process can be completed faster, or a
deeper altered surface layer can be produced in the same amount of
time, relative to traditional practice.
[0046] 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.
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