U.S. patent application number 11/327011 was filed with the patent office on 2006-07-13 for carburization of ferrous-based shape memory alloys.
Invention is credited to Peter C. Williams.
Application Number | 20060151069 11/327011 |
Document ID | / |
Family ID | 36192133 |
Filed Date | 2006-07-13 |
United States Patent
Application |
20060151069 |
Kind Code |
A1 |
Williams; Peter C. |
July 13, 2006 |
Carburization of ferrous-based shape memory alloys
Abstract
The shape memory effect of an article made from a ferrous-based
shape memory alloy is enhanced by carburizing the article's
surfaces.
Inventors: |
Williams; Peter C.;
(Cleveland Heights, OH) |
Correspondence
Address: |
CALFEE HALTER & GRISWOLD, LLP
800 SUPERIOR AVENUE
SUITE 1400
CLEVELAND
OH
44114
US
|
Family ID: |
36192133 |
Appl. No.: |
11/327011 |
Filed: |
January 6, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60642932 |
Jan 10, 2005 |
|
|
|
Current U.S.
Class: |
148/225 ;
148/319 |
Current CPC
Class: |
C23C 8/22 20130101; C22C
38/02 20130101; C22C 38/04 20130101; C22C 38/38 20130101; C23C 8/04
20130101 |
Class at
Publication: |
148/225 ;
148/319 |
International
Class: |
C23C 8/22 20060101
C23C008/22 |
Claims
1. The process comprising carburizing at least a portion of the
surface of a shaped article formed from a carbon-enhanceable
ferrous-based shape memory alloy in the austenitic phase to produce
a concentration of interstitial carbon in the carburized surface
portion greater than in the body of the article.
2. The process of claim 1, wherein carburization is continued until
the concentration of interstitial carbon in the carburized surface
portion provides a noticeable improvement in the shape memory
effect exhibited by the article.
3. The process of claim 1, wherein carburization is continued until
the concentration of interstitial carbon in the carburized surface
portion is at least 2 atomic %.
4. The process of claim 3, wherein carburization is continued until
the concentration of interstitial carbon in the carburized surface
portion is at least about 4 atomic %.
5. The process of claim 4, wherein carburization is continued until
the concentration of interstitial carbon in the carburized surface
portion is at least about 6 atomic %.
6. The process of claim 1, wherein carburization is accomplish by
low temperature carburization.
7. The process of claim 6, wherein the alloy is a stainless
steel.
8. The process of claim 7, wherein all surfaces of the shaped
article are carburized.
9. A shaped article formed from a carbon-enhanceable ferrous-based
shape memory alloy, at least a portion of the surface of the
article carrying a carburized surface layer having a concentration
of interstitial carbon greater than in the body of the article.
10. The shaped article of claim 9, wherein the concentration of
interstitial carbon in the carburized surface portion is sufficient
to provide a noticeable improvement in the shape memory effect
exhibited by the article.
11. The process of claim 9, wherein the concentration of
interstitial carbon in the carburized surface portion is at least 2
atomic %.
12. The shaped article of claim 11, wherein the concentration of
interstitial carbon in the carburized surface portion is at least
about 4 atomic %.
13. The shaped article of claim 12, wherein the concentration of
interstitial carbon in the carburized surface portion is at least
about 6 atomic %.
14. The shaped article of claim 9, wherein the carburized surface
portion is substantially free of carbide precipitates.
15. The shaped article of claim 14, wherein the alloy is a
stainless steel.
16. The shaped article of claim 9, wherein all surfaces of the
shaped article are carburized.
17. The shaped article of claim 9, wherein the carbon-enhanceable
ferrous-based shape memory alloy is in the austenitic phase.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
patent application Ser. No. 60/642,932 filed on Jan. 10, 2005 for
CARBURIZATION OF FERROUS-BASED SHAPE MEMORY ALLOYS, the entire
disclosure of which is fully incorporated herein by reference.
BACKGROUND AND SUMMARY
[0002] In an article by Kate Ireland entitled "Iron-based Shape
Memory Alloys" found at www. uow.edu.au/eng/mm/shapememoryalloys on
Nov. 21, 2004, it is indicated that the effectiveness of the shape
memory phenomenon in iron-based shape memory alloys "is also
enhanced by the creation of an interstitial solution of carbon in
austenite." See, also, Tsuzaki et al., Scr. Metall., Vol. 27, p.
471 .COPYRGT. 1992 and Maki et al., Shape Memory Effect in Ferrous
Alloys, Proceedings of the Institute of Metals, Nara, Japan,
1986.
[0003] In accordance with the present invention, the shape memory
phenomenon in iron-based shape memory alloys is enhanced to a
degree greater than possible in, or contemplated by, earlier work
by surface carburizing articles made from the alloy, while in
austenitic form, so as to achieve an increased surface
concentration of interstitial carbon. In a preferred embodiment,
the article is subjected to low temperature carburization so that
carburization is accomplished without substantial formation of
carbide precipitates.
[0004] Thus, the present invention provides a new process for
providing an article formed from an iron-based shape memory alloy
having superior shape memory properties, the process comprising
carburizing at least a portion of the article's surfaces while in
austenitic form so as to achieve an increased surface concentration
of interstitial carbon.
[0005] In addition, the present invention provides a new article of
manufacture comprising a shaped article formed from an iron-based
shape memory alloy in austenitic form, the concentration of
interstitial carbon in at least a portion of the article's surfaces
being greater than in its body.
DETAILED DESCRIPTION
Ferrous-Based Shape Memory Alloys
[0006] Shape-Memory Alloys are metals that, after being strained,
revert back to their original shape when heated to an appropriate
temperature, referred to as the "reverse transformation
temperature." See, Chapter 1 of Otsuka et al., Shape Memory
Materials, Cambridge University Press, .COPYRGT. 1998.
Ferrous-based shape-memory alloys contain a substantial amount of
iron. Normally in the austenitic phase, they undergo martensitic
transformation (i.e. transform into the martensite phase) when
strained and then return to the austenitic phase when heated to
their reverse transformation temperatures. See, Chapter 5 of Otsuka
et al.
[0007] Many ferrous-based shape-memory alloys are known. See,
Chapter 5 and especially Table 5.1 of Otsuka et al. See, also, U.S.
Pat. No. 4,933,027 to Yutaka Moriya et at. and U.S. Pat. No.
5,199,497 to Richard Ross, the disclosures of which are
incorporated herein by reference.
[0008] For example, Japanese Patent Provisional Publication No.
61-201,761 dated Sep. 6, 1986, describes ferrous-based shape-memory
alloys composed of: [0009] 20 to 40 wt. % Mn, [0010] 3.5 to 8.0 wt.
% Si, [0011] at least one element selected from the group
consisting of: [0012] up to 10 wt. % Cr, [0013] up to 10 wt. % Ni,
[0014] up to 10 wt. % Co, [0015] up to 2 wt. % Mo, [0016] up to 1
wt. % C, [0017] up to 1 wt. % Al, [0018] up to 1 wt. % Cu, and
[0019] the balance being iron and incidental impurities.
[0020] Meanwhile, U.S. Pat. No. 4,933,027 to Yutaka Moriya et at.
describes ferrous-based shape-memory alloys composed of: [0021]
from 5.0 to 20.0 wt. % Cr, [0022] from 2.0 to 8.0 wt. % Si, [0023]
at least one element selected from the group consisting of: [0024]
from 0.1 to 14.8 wt. % Mn, [0025] from 0.1 to 20.0 wt. % Ni, [0026]
from 0.1 to 30.0 wt. % Co, [0027] from 0.1 to 3.0 wt. % Cu, and
[0028] from 0.001 to 0.400 wt. % N, [0029] with the balance being
iron and incidental impurities, [0030] where
Ni+0.5Mn+0.4Co+0.06Cu+0.002N..gtoreq.0.67(Cr+1.2Si)-3.
Carbon-Enhanceable Ferrous-Based Shape Memory Alloys
[0031] As indicated in the Ireland, Tsuzaki et al. and Maki et al.
articles mentioned above, the shape memory effect of at least some
of these ferrous-based shape memory alloys can be enhanced by
increasing their concentrations of interstitial carbon atoms, i.e.,
carbon atoms not chemically bound as carbides to other elements
present. For convenience, such alloys will be referred to in this
disclosure as "carbon-enhanceable ferrous-based shape memory
alloys" or "carbon-enhanceable alloys" for short.
[0032] Normally, increasing the interstitial carbon content of
these alloys is done by alloying, i.e. by including carbon-yielding
materials in the ingredients used to form the alloys. However,
there are practical limits to the amounts of interstitial carbon
that can be incorporated into such alloys in this way.
Carburization
[0033] In accordance with the present invention, additional amounts
of interstitial carbon are incorporated into shaped articles made
from carbon-enhanceable ferrous-based shape memory alloys while in
austenitic form by carburization, i.e. by contacting the article
with a carburizing gas at elevated temperature whereby carbon atoms
diffuse into the article's surfaces.
[0034] Carburization of steels and other ferrous-based alloys has
traditionally been done for improving the surface hardness of the
alloy. This process, which is known in industry as "case
hardening," is normally done at 1700.degree. F. (950.degree. C.) or
above. At these temperatures, and with the steel or other alloy is
in the austenitic phase, carbon atoms rapidly diffuse into the
article's surfaces. At these temperatures, this diffused carbon
forms carbide precipitates, which are specific chemical compounds
such as iron carbide, chromium carbide and the like suspended in a
matrix of the surrounding metal. Carbide precipitates are very
hard, and so the resultant carburized surface or "case" is also
very hard. See Stickels, "Gas Carburizing", pp 312 to 324, Volume
4, ASM Handbook, copyright 1991, ASM International. Nonetheless, at
least some of the carbon atoms which diffuse into the article's
surfaces remains chemically uncombined and hence present in the
carburized surface in interstitial form.
[0035] Accordingly, in one embodiment of the present invention, the
concentration of interstitial carbon atoms in some or all of the
surfaces of shaped articles made from carbon-enhanceable
ferrous-based shape memory alloys while in the austenitic phase is
increased by traditional case hardening techniques, i.e. by
contacting the surfaces to be carburized with a carbon-containing
gas at elevated temperature under conditions such that carbide
precipitates form in these surfaces.
Low Temperature Carburization
[0036] Although carbide precipitates enhance surface hardness, they
also can promote corrosion.
[0037] In commonly-assigned U.S. Pat. Nos. 6,093,303, 6,165, 597
and 6,547,888 B1, we describe techniques for case hardening
stainless steel in which the workpiece is gas carburized below
1000.degree. F. See, also, U.S. Pat. No. 5,792,282, EPO 0787817 and
Japanese Patent Document 9-14019 (Kokai 9-268364). The disclosures
of all of these documents are incorporated herein by reference. At
these temperatures, and provided that carburization does not last
too long, the workpiece will carburize with little or no formation
of carbide precipitates. As a result, the workpiece surfaces not
only become hardened but also retain the inherent corrosion
resistance of the stainless steel. Most significantly,
comparatively large amounts of interstitial carbon, e.g. 2-12
atomic % and even greater, can be incorporated into the article's
surfaces in this way.
[0038] In accordance with another embodiment of the invention, the
concentration of interstitial carbon atoms in some or all of the
surfaces of shaped articles made from carbon-enhanceable
ferrous-based shape memory alloys while in the austenitic phase is
increased by low temperature carburization, i.e. by contacting the
surface to be carburized with a carbon-containing gas at elevated
temperature under conditions so that elemental carbon diffuses into
these surfaces without substantial formation of carbides. By this
approach, the beneficial effects of increasing interstitial carbon
atom concentration can be realized without compromising corrosion
resistance to any appreciable degree. Moreover, the concentration
of interstitial carbon atoms in the article's surfaces can be
increased to levels not obtainable at more elevated
temperatures.
[0039] For convenience, the carburization processes described in
this section are referred to as "low temperature carburization."
Note also that, as described in the references cited in this
section, it is typically necessary to activate the surfaces of
austenitic stainless steels before carburization to enable carbon
diffusion to occur.
The Carburized Product
[0040] In accordance with the present invention, a shaped article
made from a carbon-enhanceable ferrous-based shape memory alloy in
the austenitic phase is provided with one or more carburized
surfaces having a concentration of interstitial carbon atoms which
is greater than the concentration of interstitial carbon atoms in
the metal forming the body of the article. As a result, the shape
memory effect exhibited by the article as a whole is believed to be
even more pronounced than an otherwise identical article without a
carburized surface due to the stronger shape memory effect
occurring in the article's carburized surfaces.
[0041] The amount by which the concentration of interstitial carbon
atoms in the article's carburized surfaces should be increased
relative to its body can vary widely, and essentially any amount
will be effective. Normally, however, the amount interstitial
carbon concentration in the article's surfaces should be sufficient
to create a noticeable improvement on the shape memory effect
exhibited by the article. This can be easily determined by
subjecting two otherwise identical sample articles, one with a
carburized surface in accordance with the invention and the other
without, to the same shape memory cycle of deformation and reverse
transformation and measuring the force generated by the two
articles during the reverse transformation portion of the cycle.
Where this "return" force is greater in the carburized article, the
amount of carburization has been sufficient to generate a
"noticeable improvement" in the shape memory effect.
[0042] The concentration of interstitial carbon atoms in the
article's carburized surfaces can also be determined by known
analytical techniques including ESCA (Electron Spectroscopy for
Chemical Analysis) and X-ray diffraction. Carbon-enhanceable
ferrous-based shape memory alloys typically have carbon
concentrations on the order of less than about 1 atomic %, more
typically less than about 0.5 atomic %. Carburized articles in
accordance with the present invention normally will have case
hardened surfaces layers on the order of about 10 to 30 microns
thick with carbon concentrations of as little as 2 atomic % to as
high as 18 atomic % or higher, more typically about 4-12 atomic %
or even 6-12, atomic %.
[0043] That is to say, the carburized surface layer of increased
carbon concentration formed in accordance with the present
invention extends from the very outside surface of the article down
to a depth which is normally between about 10 to 30 microns. The
concentration of carbon at the very surface of the article, as
measured by ESCA and/or X-ray diffraction, will normally range from
as little as 2 atomic % to as high as 18 atomic %. More typically,
this surface carbon concentration will be between about 4-16 atomic
%, more typically about 6-12 atomic %. This elevated carbon
concentration decreases as the distance from the very surface of
the article increases, with the carbon concentration decreasing to
the same level as that of the body of the article at depths
typically between about 10 to 30 microns. In any event,
interstitial carbon concentrations of at least about 2 atomic %,
more typically at least about 4 atomic % and even a least about 6
atomic % as measured by ESCA and X-ray diffraction are readily
obtainable in accordance with the present invention.
[0044] Finally it should be understood that, for the purposes of
this disclosure, "shaped article" is intended to refer to an
article of any shape other than in the shape in which the bulk
alloy is received from the mill. When an alloy is manufactured, a
molten mass of its ingredients is poured into a mold and
solidified. The ingot so made is then usually formed into a
convenient shape for delivery to the customer in bulk. Wire, sheet,
rods and bars of various thicknesses and indeterminate lengths are
examples. These bulk products are then shaped into useful articles
by some type of forming operation which may involve subdividing the
bulk product into subsections and then imparting a useful shape to
the subsection by cutting, bending, hot and/or cold working,
extruding, forging or other metal-working operation. In the context
of this disclosure, "shaped article" is intended to exclude the
bulk products delivered from the mill but to include any products
made therefrom, i.e., any product of commerce which is obtained by
imparting a useful shape to a bulk alloy by any type of metal
shaping operation.
[0045] Although only a few embodiments of the present invention
have been described above, many modifications can be made without
departing from the spirit and scope of the invention. All such
modifications are intended to be included within the scope of the
present invention, which is to be limited only by the following
claims:
* * * * *
References