U.S. patent application number 11/880597 was filed with the patent office on 2008-01-31 for metal article with high interstitial content.
Invention is credited to Sunniva R. Collins, Steven V. Marx, Peter C. Williams.
Application Number | 20080023110 11/880597 |
Document ID | / |
Family ID | 38606706 |
Filed Date | 2008-01-31 |
United States Patent
Application |
20080023110 |
Kind Code |
A1 |
Williams; Peter C. ; et
al. |
January 31, 2008 |
Metal article with high interstitial content
Abstract
A thin metal workpiece is subjected to a low temperature
diffusion-based surface treatment to produce a thin metal product
in which at least one property of the thin metal product, as a
whole, is enhanced by at least 10% as compared with an otherwise
identical product not subjected to such surface treatment.
Inventors: |
Williams; Peter C.;
(Cleveland Heights, OH) ; Collins; Sunniva R.;
(Cleveland Heights, OH) ; Marx; Steven V.;
(University Heights, OH) |
Correspondence
Address: |
CALFEE HALTER & GRISWOLD, LLP
800 SUPERIOR AVENUE
SUITE 1400
CLEVELAND
OH
44114
US
|
Family ID: |
38606706 |
Appl. No.: |
11/880597 |
Filed: |
July 23, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60832844 |
Jul 24, 2006 |
|
|
|
Current U.S.
Class: |
148/225 ;
148/206; 148/316; 148/319; 148/95 |
Current CPC
Class: |
C23C 8/20 20130101; C23C
8/22 20130101; C23C 8/00 20130101 |
Class at
Publication: |
148/225 ;
148/206; 148/316; 148/319; 148/095 |
International
Class: |
C23C 8/00 20060101
C23C008/00 |
Claims
1. The process comprising subjecting a thin metal workpiece to a
low temperature interstitial diffusion-based surface treatment to
produce a thin metal product in which at least one property of the
thin metal product, as a whole, is enhanced by at least 10% as
compared with an otherwise identical product not subjected to such
surface treatment.
2. The process of claim 1, wherein the property being enhanced is
one or more of a mechanical property, an electrical property and a
magnetic property.
3. The process of claim 2, wherein the property being enhanced is
at least one of hardness, yield strength, ultimate tensile
strength, elastic limit, electrical resistance and magnetic
susceptibility.
4. The process of claim 1, wherein the thin metal workpiece is
about 0.01 to 0.25 mm thick.
5. The process of claim 4, wherein the metal workpiece is a wire,
powder, platelet, other particulate or foil.
6. The process of claim 1, wherein the low temperature
diffusion-based surface treatment is low temperature
carburization.
7. The process of claim 6, wherein the metal is an iron-, nickel-,
cobalt-based or manganese-based alloy.
8. The process of claim 1, wherein the metal is stainless
steel.
9. A thin metal product produced by subjecting a thin metal
workpiece to a low temperature diffusion-based surface treatment,
the thin metal product as a whole exhibiting at least one property
which is enhanced by at least 10% as compared with an otherwise
identical product not subjected to such surface treatment.
10. The thin metal product of claim 9, wherein the thin metal
product is produced by subjecting a thin metal workpiece made from
an iron-, nickel- or cobalt-based or manganese-based alloy to low
temperature carburization.
11. The thin metal product of claim 10, wherein the thin metal
product has a yield strength at least 100% greater than an
otherwise identical product not subjected to low temperature
carburization, the thin metal product exhibiting a ductility in
terms of elongation at fracture of at least 20%.
12. A shaped article produced by forming a mass of the thin metal
product of claim 11 into a desired shape and sintering.
13. The shaped article of claim 12, wherein the thin metal product
is produced by subjecting a thin metal workpiece made from an
iron-, nickel- or cobalt-based or manganese-based alloy to low
temperature carburization.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based on, and claims priority to, prior
U.S. Provisional Patent Application Ser. No. 60/832,844, filed Jul.
24, 2006, the disclosure of which is 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 natural
gas or propane 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 carbon
monoxide and hydrogen 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
liberated by decomposition of the carbon monoxide 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 OF THE INVENTION
[0006] In accordance with this invention, very thin workpieces are
low temperature carburized so that diffused carbon reaches a
substantial portion of the product's core. The result is that new
products are obtained which, as a whole, contain higher levels of
interstitial (diffused) carbon and exhibit better combinations of
properties than seen in the past.
[0007] Thus, this invention provides a process for producing a thin
metal product in which at least one property of the thin metal
product, as a whole, is enhanced by at least 10% as compared with
an otherwise identical untreated product, the process comprising
subjecting a thin metal workpiece to a low temperature
diffusion-based surface treatment, preferably low temperature
carburization. Most commonly, yield strength is substantially
increased while ductility is substantially retained.
[0008] In addition, this invention also provides a thin metal
product produced by subjecting a thin metal workpiece to a low
temperature diffusion-based surface treatment, the thin metal
product as a whole exhibiting at least one property which is
enhanced by at least 10% as compared with an otherwise identical
product not subjected to such surface treatment, preferably low
temperature carburization.
[0009] Finally, this invention also provides a shaped article which
is produced by forming a mass of the thin metal product described
above into a desired shape and sintering.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present invention may be more readily understood by
reference to the drawings wherein:
[0011] FIGS. 1 and 2 show the effect on the yield strength and
ductility of AISI 316 stainless steel foil low temperature
carburized in accordance with this invention, FIG. 1 illustrating
the raw load vs. displacement data and FIG. 2 showing the data
normalized to a standard stress/strain curve.
Low Temperature Carburization
[0012] As indicated above, the primary focus of this invention is
on the low temperature carburization of iron-, nickel-, cobalt-,
and/or chromium-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. In this
context, reference to carburizing stainless steel "without
formation of carbide precipitates" will be understood to mean
"without formation of the types and amounts of carbide precipitates
which adversely affect the corrosion resistance of the stainless
steel."
[0013] In accordance with the present invention, low temperature
carburization is carried out in the same way as done in the past so
as to produced carburized workpieces whose treated surfaces or
"case" contain elevated amounts of elemental carbon, normally about
2-15 atomic %, more typically about 5-10 atomic % or even 9-12
atomic %. Because low temperature carburization is a
diffusion-based process, the concentration of carbon in the
workpiece's surface 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 reference to a carbon
concentration of about 2-15 atomic % means that this is the carbon
concentration at or near the workpieces surface, with this
concentration falling off to the equilibrium value at depth which
can be as little as 5.mu. from the workpiece's outer surface, but
is more typically on the order of 20-50.mu. from the workpiece's
outer surface. Greater depths of diffused carbon, e.g., as deep as
75.mu. or even 100.mu. are possible, however.
Other Low Temperature Diffusion-Based Surface Treatments
[0014] Although this invention concentrates on low temperature
carburization of iron-, nickel- and cobalt-based alloys, other
analogous diffusion-based surface treatments can also be used.
[0015] 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 corrosion-promoting 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.
[0016] 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 nitriding of
iron, chromium and/or nickel based alloys, carbo-nitriding of iron,
chromium and/or nickel based alloys, and nitriding of
titanium-based alloys, to name a few. For convenience, all of these
processes will be referred to collectively as "low temperature
interstitial diffusion based surface treatments."
[0017] All such low temperature interstitial diffusion-based
surface treatments can be used in accordance with the present
invention. That is to say, each of these low temperature
interstitial diffusion-based surface treatments can be applied to
thin metal workpieces using the technology of this invention to
make new products with greater concentrations of the diffused atoms
and better properties than available in the past.
Alloys
[0018] The present invention will normally be carried on workpieces
made from iron 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).
[0019] 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. As of this writing, the invention has not been successfully
practiced on 400 series stainless steels, which is believed due to
the fact that appropriate conditions for depassivating the steel in
preparation for low temperature carburization have not yet been
determined. Nonetheless, the AISI 400 series stainless steels and
especially Alloy 410, Alloy 416 and Alloy 440C are also of special
interest.
[0020] 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.
[0021] In addition to iron- and 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.
[0022] 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.
Thin Workpieces and Products
[0023] In accordance with the present invention, a low temperature
interstitial diffusion-based surface treatment is carried out on a
"thin" workpiece to produce a "thin" surface-treated product.
[0024] A workpiece that has been subjected to a low temperature
interstitial diffusion-based surface treatment can be considered as
having an internal core surrounded by a diffusion-enriched surface
or "case". When the interstitial diffusion treatment is low
temperature carburization, this carburized surface will normally
extend down to a depth of about 20.mu. to about 40.mu. or even
50.mu. from the outermost surface, although greater depths are
possible. Because this case depth is extremely thin compared with
the overall thickness of the workpiece, the vast majority and
indeed essentially all of the article is composed of native metal
i.e., metal not infused with additional amounts of interstitial
carbon. As a result, the case exerts no noticeable effect on the
mechanical properties of the workpiece as a whole.
[0025] In accordance with the invention, however, the workpiece
being processed is very thin, normally on the order of 0.0004 to
0.01 inch thick (.about.0.01 to .about.0.25 mm; .about.10 to
.about.250.mu.), more typically about 0.001 to 0.003 inch thick
(.about.0.025 to .about.0.08 mm; .about.25 to .about.75.mu.). At
these small workpiece thicknesses, case thickness becomes
significant relative to core thickness, the result of which is that
the properties of the workpiece as a whole are indeed influenced by
the case. Thus it is possible, in accordance with the present
invention, to produce new materials having properties not
previously seen before.
[0026] This is illustrated in the FIGS. 1 and 2 which shows the
stress/strain relationships exhibit by a number of different AISI
316 stainless steel foils 0.002 in (.about.0.048 mm; .about.50.mu.)
thick which have been low temperature carburized to produce a
"thin" surface-treated foil product in accordance with the present
invention. As can be seen from this figure, the untreated foil
represented by Curve A reached its elastic limit at a stress of
about 300 MPa (megaPascals). In contrast the foils treated in
accordance with the present invention, which are represented by
Curve B, did not reach their elastic limits until the applied
stress was about 1200 MPa. This represents a four-fold increase in
yield strength with ductility being substantially retained, thereby
showing that these treated foils are considerably different
materials from the untreated foils from which they were made.
[0027] As indicated above, the workpieces processed by the present
invention are thin, normally on the order of 0.0004 to 0.01 inch
thick (.about.0.01 to .about.0.25 mm; .about.10 to .about.250.mu.),
more typically about 0.001 to 0.003 inch thick (.about.0.024 to
.about.0.08 mm; .about.25 to .about.75.mu.). However, workpieces of
greater or lesser thickness can also be processed if desired. What
is important is that these workpieces are thin enough, and the low
temperature diffusion-based surface treatment carried out long
enough, so that the case produced by this surface treatment imparts
a not-insignificant change (i.e. .gtoreq.10%) to at least one
property of the product as a whole as compared with an otherwise
identical workpiece from which it is made.
[0028] So in the context of this disclosure, "thin" in relation to
a workpiece which is subjected to a low temperature interstitial
diffusion surface treatment will be understood to mean a thickness
which is small enough so that at least one property of the product
produced by the treatment is enhanced by at least 10% as compared
with an otherwise identical product made without the surface
treatment.
[0029] Particular examples of "thin" workpieces and products for
the purposes of this invention include foils, wires, powder,
platelets and other particulates, for example. Other shapes are
also possible.
Property Enhancements
[0030] Various different mechanical, electrical and magnetic
properties of thin metal workpieces can be enhanced by this
invention. Examples include, but are not limited to hardness, yield
strength, ultimate tensile strength, elastic limit, electrical
resistance and magnetic susceptibility. Moreover, while the above
disclosure refers to enhancing at least one of these properties by
at least 10%, it should be appreciated that far greater
enhancements are possible. For example, electrical resistance can
be increased by as much as 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%
or even 100%, typically 15% to 60%. Similarly, yield strength
increases by as much as 100%, 200%, 300%, 400% and even 500% are
possible. Most significantly, these remarkable enhancement can be
achieved without significant reduction in other properties such
ductility, etc., as a practical matter.
[0031] This is further illustrated in the FIGS. 1 and 2 which shows
that the foils low temperature carburized in accordance with this
invention, Curve B, remained ductile until they ruptured at an
elongation of about 20% or more. Although this elongation at
rupture is not as great as that exhibited by the untreated foils of
Curve A (about 35% elongation at rupture), it is far greater than
that of the conventionally carburized foils (about 5% elongation at
rupture), represented by Curve C. Thus the present invention not
only achieves a substantial increase in yield strength, electrical
resistance and corrosion resistance, but also does so without
substantial sacrifice in ductility.
Sintered Articles
[0032] In accordance with another feature of this invention, shaped
metal articles are made by sintering processes using masses of the
diffusion-treated thin metal products of this invention as their
raw materials. Powder metallurgy techniques for forming shaped
metal articles are well known, and any such technique can be used
to form shaped metal articles from the diffusion-treated powders,
platelets and other particulate thin products of this invention.
Analogous sintering processes can also be used to form the thin
metal foil products of this invention into shaped articles.
[0033] Such sintering processes typically involve forming a mass of
metallic particles into a desired shape, optionally compacting the
mass to desired density (with respect to theoretical) and heating
the mass to cause the particles to melt and fuse to one another at
their surfaces. Analogous sintering processes can be used to form
shaped metal articles from foils. These same processes can be used
to form shaped articles of any desired shape from the
diffusion-treated thin metal products of this invention regardless
of shape, i.e., whether in the form of powder, platelet, other
particulate, wire or foil. Such products are unique because they
are made from new materials not previously known.
Magnetic Susceptibility
[0034] Another feature of the thin metal products of this invention
when made from austenitic stainless steels, including shaped metal
articles made by sintering masses of such thin metal products, is
that they exhibit significant magnetic susceptibility. Magnetic
susceptibility is the degree of magnetization of a material in
response to an applied magnetic field. The dimensionless volume
magnetic susceptibility, represented by the symbol
.chi..sub..upsilon. (also represented in the literature by .kappa.
or K), is defined by the relationship M=.chi..sub..upsilon.H where
[0035] M is the magnetization of the material (the magnetic dipole
moment per unit volume), measured in amperes per meter, and [0036]
H is the applied field, also measured in amperes per meter.
[0037] Ferritic and martensitic stainless steels exhibit good,
inherent magnetic susceptibility. In contrast, austenitic stainless
steels exhibit essentially no magnetic susceptibility. However, the
carbon hardened surfaces produced when austenitic stainless steels
and other metals having face centered cubic lattice structures are
low temperature carburized do. Accordingly, when a "thin" workpiece
made from an austenitic stainless steel or other metal having a
face centered cubic lattice structure is low temperature
carburized, the "thin" product obtained exhibits significant
magnetic susceptibility since its carbon hardened surfaces
represent a significant portion of its entire mass. In the same
way, a shaped metal article made from a sintered mass of such a
product, as described above, also exhibits significant magnetic
susceptibility as a whole since the portion of its mass which has
been low temperature carburized is significant with respect to its
entire mass.
[0038] 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.
* * * * *