U.S. patent application number 10/509156 was filed with the patent office on 2006-03-16 for nitrided mo alloy worked material having high corrosion resistance, high strength and high toughness and method for production thereof.
Invention is credited to Masahiro Nagae, Makoto Nakanishi, Jun Takada, Tomohiro Takida.
Application Number | 20060054247 10/509156 |
Document ID | / |
Family ID | 28671934 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060054247 |
Kind Code |
A1 |
Takada; Jun ; et
al. |
March 16, 2006 |
Nitrided mo alloy worked material having high corrosion resistance,
high strength and high toughness and method for production
thereof
Abstract
The present invention provides an innovative material which has
properties which cannot be achieved with conventional materials,
i.e., having satisfactory high corrosion resistance and high
strength in very severe corrosive conditions, for example, a 75%
sulfuric acid (H.sub.2SO.sub.4) aqueous solution (180.degree. C.)
in addition to high strength at high temperatures and high
toughness at low temperatures, and provides a method for
effectively manufacturing the innovative material. A worked
molybdenum-alloy material, subjected to nitriding, which has high
corrosion resistance, high strength, and high toughness, includes
fine nitride particles formed by subjecting a nitride-forming-metal
element dissolved to form a solid solution in an untreated worked
molybdenum-alloy material to internal nitriding, the fine nitride
particles being dispersed inside the worked molybdenum-alloy
material subjected to nitriding; and a molybdenum nitride layer
formed by subjecting a worked structure or a recovered structure at
the surface of the untreated worked molybdenum-alloy material to
external nitriding, the molybdenum nitride layer being provided at
the surface of the worked molybdenum-alloy material subjected to
nitriding. A method for manufacturing a worked molybdenum-alloy
material subjected to nitriding includes the steps of subjecting an
untreated worked alloy material in which at least any one of
titanium, zirconium, hafnium, vanadium, niobium, and tantalum is
dissolved to form a solid solution to multi-step internal nitriding
treatment including a stepwise increase of the treatment
temperature, and then subjecting the worked alloy material to
external nitriding treatment.
Inventors: |
Takada; Jun; (Okayama,
JP) ; Nagae; Masahiro; (Okayama, JP) ;
Nakanishi; Makoto; (Okayama, JP) ; Takida;
Tomohiro; (Toyama, JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW
SUITE 700
WASHINGTON
DC
20036
US
|
Family ID: |
28671934 |
Appl. No.: |
10/509156 |
Filed: |
March 27, 2003 |
PCT Filed: |
March 27, 2003 |
PCT NO: |
PCT/JP03/03912 |
371 Date: |
November 1, 2005 |
Current U.S.
Class: |
148/237 ;
148/317 |
Current CPC
Class: |
C23C 8/24 20130101; C23C
26/00 20130101; C23C 8/02 20130101; C22C 27/04 20130101 |
Class at
Publication: |
148/237 ;
148/317 |
International
Class: |
C23C 8/24 20060101
C23C008/24 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2002 |
JP |
2002-98039 |
Claims
1. A worked molybdenum-alloy material, subjected to nitriding,
which has high corrosion resistance, high strength, and high
toughness, comprising: fine nitride particles formed by subjecting
a nitride-forming-metal element dissolved to form a solid solution
in an untreated worked molybdenum-alloy material to internal
nitriding, the fine nitride particles being dispersed inside the
worked molybdenum-alloy material subjected to nitriding; and a
molybdenum nitride layer formed by subjecting a worked structure or
a recovered structure at the surface of the untreated worked
molybdenum-alloy material to external nitriding, the molybdenum
nitride layer being provided at the surface of the worked
molybdenum-alloy material subjected to nitriding.
2. The worked molybdenum-alloy material subjected to nitriding
according to claim 1, wherein the molybdenum nitride layer at the
surface of the worked molybdenum-alloy material subjected to
nitriding comprises at least any one of .delta.-MoN,
.gamma.-Mo.sub.2N, and .beta.-Mo.sub.2N.
3. The worked molybdenum-alloy material subjected to nitriding
according to claim 1 or 2, wherein a layer between the molybdenum
nitride layer and the matrix in the inside of the worked
molybdenum-alloy material subjected to nitriding has a worked
structure or recovered structure.
4. The worked molybdenum-alloy material subjected to nitriding
according to claim 1 or 2, wherein the inside of the worked
molybdenum-alloy material subjected to nitriding has a
recrystallized structure.
5. A method for manufacturing a worked molybdenum-alloy material
subjected to nitriding according to claim 1 or 2, comprising the
steps of: subjecting an untreated worked alloy material in which at
least any one of titanium, zirconium, hafnium, vanadium, niobium,
and tantalum is dissolved to form a solid solution in a molybdenum
matrix to multi-step internal nitriding treatment including a
stepwise increase of the treatment temperature, and then subjecting
the worked alloy material to external nitriding treatment.
6. The method for manufacturing a worked molybdenum-alloy material
subjected to nitriding according to claim 5, wherein the internal
nitriding treatment is performed with a nitrogen gas, and then the
external nitriding treatment is performed with an ammonia gas.
Description
TECHNICAL FIELD
[0001] The present invention relates to a worked molybdenum-alloy
material, which is subjected to nitriding, having improved
strength, toughness, and corrosion resistance as a result of a
combination treatment of internal nitriding and external nitriding,
and a method for manufacturing the worked molybdenum-alloy material
subjected to nitriding.
BACKGROUND ART
[0002] Molybdenum (Mo) that has, for example, a high melting point
(about 2600.degree. C.), relatively high mechanical strength
superior to other metals having high melting points, a low thermal
expansion coefficient, excellent electrical conduction and thermal
conduction properties, and a high corrosion resistance to a melted
alkali metal and hydrochloric acid, can be applied to, for example,
electrodes, components for vessels, components for semiconductors,
components for heat-resistant structures, and materials for nuclear
reactors.
[0003] A worked material having a worked structure exhibits high
toughness due to suppressed crack growth. However, in a material
recrystallized by heating (about 1050.degree. C. or more), strength
at high temperatures is not satisfactory because a crack readily
grows to cause embrittlement. Therefore,
Mo--Ti(0.5)-Zr(0.08)-C(0.03) (TZM) alloy and
Mo--Nb(1.5)-Ti(0.5)-Zr(0.03)-C(0.03) (TZC) alloy have been
developed as molybdenum alloys having improved strength at high
temperatures.
[0004] The inventors found that, in a worked refractory-metal-alloy
such as an ultrafine-nitride-containing molybdenum alloy formed by
multi-step internal nitriding treatment, high toughness and high
strength are achieved by maintaining a worked structure in at least
the surface region of the worked material (patent document 1,
non-patent documents 1 to 3).
[0005] Molybdenum has excellent properties as described above.
However, molybdenum has no corrosion resistance against oxidizing
acids such as nitric acid and hot concentrated sulfuric acid.
Regarding the improvement of the corrosion resistance, the
inventors developed a highly corrosion-resistant molybdenum-based
composite material having a molybdenum nitride (MO.sub.2N) with a
thickness of 0.5 to 10 .mu.m produced by nitriding molybdenum and a
molybdenum alloy (patent document 2). [0006] Patent document 1:
Japanese Unexamined Patent Application Publication No. 2001-73060.
[0007] Patent document 2: Japanese Unexamined Patent Application
Publication No. 11-286770. [0008] Non-patent document 1: Masahiro
Nagae, Jun Takada, Yoshito Takemoto, Yutaka Hiraoka, and Tetsuo
Yoshio. J. Japan Inst. Metals, 64(2000)747-750. [0009] Non-patent
document 2: Masahiro Nagae, Jun Takada, Yoshito Takemoto, Yutaka
Hiraoka, and Tetsuo Yoshio. J. Japan Inst. Metals, 64(2000)751-754.
[0010] Non-patent document 3: Masahiro Nagae, Jun Takada, Yoshito
Takemoto, and Yutaka Hiraoka. Materia Japan, 40(2001)666-667.
DISCLOSURE OF THE INVENTION
[0011] Only the metal tantalum (Ta) is useful as a material for use
in very severe corrosive conditions (for example, a boiling
concentrated sulfuric acid solution). However, tantalum has low
strength, in particular, its strength is low at high temperatures;
hence, it is inappropriate for an apparatus and a structural
material which require high strength. The above-described highly
corrosion-resistant molybdenum-based composite material which is
developed as an alternative to tantalum by the inventors has a
disadvantage in that a base material is recrystallized during the
manufacturing process to cause the embrittlement of the entire
material.
[0012] Accordingly, it is an object of the present invention to
provide an innovative material, which has properties which cannot
be achieved with conventional materials, i.e., having satisfactory
high corrosion resistance and high strength in very severe
corrosive conditions, for example, a 75% sulfuric acid
(H.sub.2SO.sub.4) aqueous solution (180.degree. C.), in addition to
high strength at high temperatures and high toughness at low
temperatures, and to provide a method for effectively manufacturing
the innovative material.
[0013] The inventors found that a worked molybdenum-alloy material
having excellent corrosion resistance against oxidizing acids in
addition to high strength and high toughness was effectively and
inexpensively produced by subjecting a worked molybdenum material
to a combination treatment of internal nitriding and external
nitriding.
[0014] That is, a worked molybdenum-alloy material, subjected to
nitriding, which has high corrosion resistance, high strength, and
high toughness, includes fine nitride particles formed by
subjecting a nitride-forming-metal element dissolved to form a
solid solution in an untreated worked molybdenum-alloy material to
internal nitriding, the fine nitride particles being dispersed
inside the worked molybdenum-alloy material subjected to nitriding;
and a molybdenum nitride layer formed by subjecting a worked
structure or a recovered structure at the surface of the untreated
worked molybdenum-alloy material to external nitriding, the
molybdenum nitride layer being provided at the surface of the
worked molybdenum-alloy material subjected to nitriding.
[0015] In the above-described worked molybdenum-alloy material
subjected to nitriding, the molybdenum nitride layer at the surface
of the worked molybdenum-alloy material subjected to nitriding is
composed of at least any one of .delta.-MoN, .gamma.-MO.sub.2N, and
.beta.-Mo.sub.2N.
[0016] In the above-described worked molybdenum-alloy material
subjected to nitriding, a layer between the molybdenum nitride
layer and the matrix in the inside of the worked molybdenum-alloy
material subjected to nitriding has a worked structure or recovered
structure.
[0017] In the above-described worked molybdenum-alloy material
subjected to nitriding, the inside of the worked molybdenum-alloy
material subjected to nitriding has a recrystallized structure.
[0018] A method for manufacturing a worked molybdenum-alloy
material subjected to nitriding includes the steps of subjecting an
untreated worked alloy in which at least any one of titanium,
zirconium, hafnium, vanadium, niobium, and tantalum is dissolved to
form a solid solution in a molybdenum matrix to multi-step internal
nitriding treatment including a stepwise increase of the treatment
temperature, and then subjecting the worked alloy to external
nitriding treatment.
[0019] In the method for manufacturing a worked molybdenum-alloy
material subjected to nitriding, the internal nitriding treatment
is performed with a nitrogen gas, and then the external nitriding
treatment is performed with an ammonia gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic cross-sectional view of a worked
molybdenum-alloy material subjected to nitriding of the present
invention.
[0021] FIG. 2 is a schematic view showing the structures of a
worked material at each step (1) to (3) of the internal nitriding
treatment in a manufacturing process of a worked molybdenum-alloy
material subjected to nitriding.
[0022] FIG. 3 is a graph showing the results of a corrosion test of
a worked molybdenum-alloy material, which is subjected to
nitriding, produced in EXAMPLE 1 and EXAMPLE 2 and also showing the
result of a pure molybdenum material in COMPARATIVE EXAMPLE.
[0023] FIG. 4 shows a photograph (a), which is an alternative to a
drawing, of the cross-sectional structure of a worked
molybdenum-alloy material subjected to nitriding, and also shows a
macro photograph (b), which is an alternative to a drawing, after a
specimen of a worked molybdenum-alloy material subjected to
nitriding was tested by bending. The worked alloy shown in the
photographs (a) and (b) are produced in EXAMPLE 2.
BEST MODE FOR CARRYING OUT THE INVENTION
[0024] FIG. 1 is a schematic view showing an example of the
cross-sectional structure of a worked molybdenum-alloy material
subjected to nitriding of the present invention. The worked
molybdenum-alloy material subjected to nitriding shown in FIG. 1
has a triple-layer structure including a layer having nano-size
nitride particles 2 dispersed in the surface region of a worked
alloy material 1; a molybdenum nitride (MO.sub.2N) surface layer 4
produced by subjecting a worked structure or a recovered structure
3 to external nitriding; and a molybdenum recrystallized layer 5.
When a worked material composed of an alloy is relatively thin, a
worked structure can be completely maintained through the entire
worked material. In this case, a double layer structure is produced
without the molybdenum recrystallized layer 5.
[0025] A worked material is produced by processing, for example,
rolling a dilute alloy which has a matrix composed of molybdenum
and in which at least any one of titanium (Ti), zirconium (Zr),
hafnium (Hf), vanadium (V), niobium (Nb), or tantalum (Ta) is
dissolved to form a solid solution. The term "dilute alloy" means
an alloy in which the content of the solute element(s) in a solid
solution alloy is about 5 percent by weight or less.
[0026] A worked molybdenum-alloy material, which is subjected to
nitriding, having high corrosion resistance, high strength, and
high toughness according to the present invention is manufactured
by an internal nitriding treatment including steps (1) to (3) and
an external nitriding treatment (4) described below. FIG. 2 shows
schematic views (1) to (3) illustrating the structures of a worked
material at each step (1) to (3), respectively, of the internal
nitriding treatment including a stepwise increase of the heating
temperature.
[0027] (1) First nitriding step: A worked material is heated in a
nitriding atmosphere between a temperature 200.degree. C. lower
than the lower limit temperature of recrystallization and the upper
limit temperature of recrystallization to nitride a
nitride-forming-metal element. As a result, a worked material in
which ultrafine nitride particles are dispersed is formed. In this
first nitriding step, nitrogen is diffused into a worked
dilute-alloy material while maintaining a worked structure X1 in
the worked material. As a result, the nitride-forming-metal element
that is dissolved to form a solid solution in a matrix is subjected
to preferential nitriding to form subnano nitride particles, which
have diameters of about 1 nm to about 2 nm, in the form of plates,
the subnano nitride particles being dispersed in the matrix. The
term "preferential nitriding" means a phenomenon in which a
nitride-forming-metal element alone is preferentially nitrided but
a metal constituting a matrix is not nitrided. A recrystallization
temperature is increased due to the pinning effect of the particles
precipitated during this nitriding step.
[0028] (2) Second nitriding step: The worked alloy produced by the
first nitriding step is heated at equal to or more than the lower
limit temperature of recrystallization of the worked material in a
nitriding atmosphere, thus leading to the grain growth and the
stabilization of the ultrafine nitride particles. The grain growth
and the stabilization of the precipitated particles induced by this
second nitriding step further increase the recrystallization
temperature. In nitriding, recrystallization occurs inside a worked
material but a worked structure X2 still remains. When a worked
material is relatively thin (3 mm or less), a worked structure can
be completely maintained through the entire worked material.
[0029] (3) Third nitriding step and steps following the third step:
The worked material produced by the previous steps is heated in a
nitriding atmosphere at equal to or more than the lower limit
temperature of recrystallization of the worked material, thus
leading to the grain growth and the stabilization of the nitride
particles. An object of the third step and steps following
nitriding in the third step is to further grow and to further
stabilize the nitride particles while retaining a worked structure
X3. Bar-shaped nitride particles having a thickness of about 10 nm
and having a length of about 50 nm are uniformly dispersed in the
molybdenum matrix. For example, fourth and fifth nitriding steps
after the third nitriding step can be performed, if necessary.
[0030] (4) External nitriding treatment: A molybdenum nitride layer
is formed by a strong nitriding treatment. An ammonia gas
atmosphere, a nitrogen gas atmosphere, a forming gas atmosphere
(the ratio of hydrogen gas to nitrogen gas is 1:9 to 5:5), and an
atmosphere produced by subjecting each gas to plasma discharge, may
be used as a nitriding atmosphere. Molybdenum nitride formed is at
least any one of .delta.-MoN, .beta.-MO.sub.2N, or
.beta.-MO.sub.2N. The external nitriding treatment is performed
such that a worked structure or a recovered structure remains
between the molybdenum nitride surface layer and the matrix of the
inside of the worked material.
[0031] Table 1 shows the relationship between the temperature of
heating treatment and the thickness of the surface layer of a
Mo--Ti-alloy (Ti content: 0.5 percent by weight). The layer
thickness increases with the increase in heating temperature. It is
better to increase the layer thickness in view of corrosion
resistance. However, it was found that toughness (bending
properties) was reduced with the increase in layer thickness.
Therefore, striking a balance between toughness and corrosion
resistance requires that the external nitriding treatment (about 3
mm or less of layer thickness) be performed at 900.degree. C. or
less. TABLE-US-00001 TABLE 1 Material subjected (Internal nitriding
to internal up to third step) + nitriding up to (external
nitriding) Pure Mo third step (2.8 .mu.m) Yield 550 MPa 1190 MPa
1280 MPa strength Maximum 750 MPa 1020 MPa 1870 MPa strength
[0032] A worked molybdenum-alloy material subjected to nitriding of
the present invention is useful for, for example, supporting plates
for semiconductors, ceramics, and metals; heaters for
high-temperature furnaces; components for high-temperature
furnaces; structural materials for chemical equipment and
apparatuses used in corrosive atmospheres (including
high-temperature incinerators); and materials for reactors with
supercritical solutions and/or subcritical solutions. In addition,
the worked molybdenum-alloy material subjected to nitriding is also
useful for, for example, acid-resistant vessels and tubes for
oxidizing acids such as sulfuric acid and nitric acid; materials
for apparatuses used in very severe corrosive conditions (for
example, a boiling concentrated sulfuric acid solution);
ultra-high-temperature heaters; injection molds for metals; and
injection nozzles for diesel engines.
EXAMPLES
Example 1
[0033] A worked Mo--Ti-alloy (Ti content: 1.0 percent by weight) in
the form of a plate having a side of 10 mm and a thickness of 1 mm
was subjected to internal nitriding up to the fourth step at
predetermined heating temperatures in a nitrogen gas flow (1 atm).
The profile of the heating temperature was set as follows:
900.degree. C..fwdarw.950.degree. C..fwdarw.1200.degree.
C..fwdarw.1500.degree. C.
[0034] By this multi-step nitriding treatment, the surface region
of the worked material (up to about 200 .mu.m in depth from the
surface) maintained a worked structure or a recovered structure
(the inside of the worked material consisted of a recrystallized
structure). In addition, fine titanium nitride particles were
precipitated and dispersed in the surface region. Subsequently,
external nitriding treatment was performed at 1000.degree. C. for 4
hours in an ammonia (NH.sub.3) gas flow (1 atm) to form a
molybdenum nitride (for example, .gamma.-MO.sub.2N) layer having a
thickness of 14.0 .mu.m at the surface of the worked material.
[0035] This worked material had a triple layer structure as
follows: The surface of the worked material was composed of a
molybdenum nitride layer. The inside of the molybdenum nitride
layer was composed of a nitride layer of an element which is
dissolved to form a solid solution in a molybdenum matrix of a
worked structure or a recovered structure in which fine titanium
nitride (TiN) particles are precipitated and dispersed. The inside
of the nitride layer is composed of a molybdenum-alloy layer having
a structure with isometric and coarse recrystallized grain.
[0036] FIG. 3 shows the results of a corrosion test in a boiling
75% concentrated sulfuric acid solution at 185.degree. C. in order
to evaluate corrosion resistance in severe corrosive conditions.
FIG. 3 also shows the results of pure molybdenum as a reference. As
shown in FIG. 3, the pure molybdenum was heavily corroded and
exhibited a high corrosion rate of 8 mm/year, while the worked
material (EXAMPLE 1) of the present invention was hardly corroded
and exhibited a corrosion rate of 0.076 mm/year. That is, it was
found that the worked material of the present invention exhibited
substantially complete corrosion resistance ((corrosion
rate)<0.05 mm/year).
Example 2
[0037] A worked Mo--Ti-alloy material (Ti content: 0.5 percent by
weight) was subjected to internal nitriding up to the third step at
predetermined heating temperatures in a nitrogen gas flow (1 atm).
The profile of the heating temperature was set as follows:
900.degree. C..fwdarw.1200.degree. C..fwdarw.1500.degree. C. The
resulting Mo alloy subjected to the internal nitriding up to the
third step was further heated (external nitriding treatment) at
900.degree. C. for 4 hours in an ammonia gas flow (1 atm) to
uniformly form a molybdenum nitride (.delta.-MoN,
.gamma.-MO.sub.2N) layer at the surface of the worked material. The
internal nitrided layer composed of a worked structure or a
recovered structure, in which fine titanium nitride particles were
precipitated and dispersed by the multi-step nitriding treatment,
had a thickness of 310 .mu.m. The external nitrided layer composed
of molybdenum nitride had a thickness of 2.8 .mu.m. An X-ray
diffraction pattern showed that .delta.-MoN and .gamma.-MO.sub.2N
were formed at the external nitrided layer.
[0038] FIG. 3 shows the results of a corrosion test in a boiling
75% concentrated sulfuric acid solution at 185.degree. C. The
worked material of EXAMPLE 2 was hardly corroded and exhibited a
corrosion rate of 0.046 mm/year. That is, the worked material
exhibited complete corrosion resistance ((corrosion rate)<0.05
mm/year).
[0039] Table 2 shows the bending strength at room temperature
(yield strength and maximum strength) of a worked material
subjected to internal nitriding up to the third step (900.degree.
C..fwdarw.1200.degree. C..fwdarw.1500.degree. C.) and a worked
material subjected to external nitriding treatment (at 900.degree.
C. for 4 hours) after internal nitriding up to the third step. FIG.
4 shows a photograph (a) of the cross-sectional structure and a
macro photograph (b) of a specimen subjected to the bending test.
TABLE-US-00002 TABLE 2 1100 .degree. C. 1000.degree. C. 940.degree.
C. 900.degree. C. 850.degree. C. 800.degree. C. 4 h 4 h 4 h 4 h 4 h
4 h 30 .mu.m 14.0 .mu.m 4.7 .mu.m 2.8 .mu.m 1.7 .mu.m 1.1 .mu.m
[0040] As shown in Table 2, it was found that both yield strength
and maximum strength of the worked material (having a molybdenum
nitride layer thickness of about 2.8 .mu.m) subjected to external
nitriding treatment at 900.degree. C. for 4 hours in EXAMPLE 2
represented high stress values at the same level as those of the
material (highly strengthened and highly toughened) subjected to
only internal nitriding up to the third step.
[0041] That is, it was proved that a worked molybdenum-alloy
material subjected to nitriding of the present invention had very
high strength in addition to high corrosion resistance.
INDUSTRIAL APPLICABILITY
[0042] The present invention provides a worked molybdenum-alloy
material, which is subjected to nitriding, having high strength and
high toughness in addition to high corrosion resistance against
oxidizing acids and thus can be used in the most extreme corrosive
conditions. The worked molybdenum-alloy material is effectively and
inexpensively produced by only nitriding. The worked
molybdenum-alloy material subjected to nitriding contributes to
enabling the practical use of molybdenum materials in various
applications such as materials for apparatuses used in very severe
corrosive conditions (for example, a boiling concentrated sulfuric
acid solution), ultra-high-temperature heaters, injection molds for
metals, and injection nozzles for diesel engines, as well as
various applications of conventional molybdenum or molybdenum
alloys.
* * * * *