U.S. patent application number 10/528362 was filed with the patent office on 2005-11-10 for method of nitriding metal ring and apparatus therefor.
Invention is credited to Narita, Naoki, Saruyama, Masaomi, Suzuki, Teiji, Takagaki, Masashi.
Application Number | 20050247375 10/528362 |
Document ID | / |
Family ID | 32045727 |
Filed Date | 2005-11-10 |
United States Patent
Application |
20050247375 |
Kind Code |
A1 |
Suzuki, Teiji ; et
al. |
November 10, 2005 |
Method of nitriding metal ring and apparatus therefor
Abstract
A metal ring made of maraging steel is heated in the presence of
a halogen compound gas, so as to eliminate an oxide film from the
surface of the metal ring and to form a halogenous compound film.
Thereafter, the metal ring is heated under a vacuum or reduced
pressure atmosphere to eliminate the halogenous compound film, and
then it is maintained under an atmosphere comprising ammonia at a
processing temperature ranging between 450.degree. C. and
500.degree. C. for a processing time ranging between 30 and 120
minutes, so as to carry out a nitriding processing. The above
nitriding processing comprises the steps of: placing the above
metal ring into a heating furnace and raising the temperature
inside the heating furnace to the above processing temperature;
introducing a first mixed gas consisting of 50% to 90% ammonia
byvolume, 0.1% to 0.9% oxygen by volume, and a residual volume
consisting of nitrogen into the heating furnace, and maintaining
the above processing temperature, so as to form a nitrided layer on
the surface of the metal ring; and when the one-third to one-half
of the processing time has passed, replacing the atmosphere inside
the heating furnace by a second mixed gas consisting of 0% to 25%
ammonia by volume and a residual volume consisting of nitrogen, and
maintaining the above processing temperature until the remaining
processing time passes.
Inventors: |
Suzuki, Teiji; (Sayama-shi,
JP) ; Saruyama, Masaomi; (Sayama-shi, JP) ;
Takagaki, Masashi; (Saitama, JP) ; Narita, Naoki;
(Sayama-shi, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
32045727 |
Appl. No.: |
10/528362 |
Filed: |
March 18, 2005 |
PCT Filed: |
September 22, 2003 |
PCT NO: |
PCT/JP03/12081 |
Current U.S.
Class: |
148/217 ;
148/230 |
Current CPC
Class: |
C23C 8/02 20130101; C23G
5/00 20130101; F16G 5/16 20130101; C23C 8/26 20130101 |
Class at
Publication: |
148/217 ;
148/230 |
International
Class: |
B21C 037/30; C23C
008/30 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 24, 2002 |
JP |
2002-277898 |
Apr 15, 2003 |
JP |
2003-109727 |
Apr 15, 2003 |
JP |
2003109728 |
Claims
1. A method of nitriding a metal ring made of maraging steel,
comprising the steps of: heating the metal ring made of maraging
steel in the presence of a halogen compound gas, so as to eliminate
an oxide film from the surface of the metal ring and to form a
halogenous compound film; heating the metal ring made of maraging
steel on which said halogenous compound film is formed under a
vacuum or reduced pressure atmosphere, so as to eliminate said
halogenous compound film; and heating the metal ring made of
maraging steel from which said halogenous compound film is
eliminated in the presence of an ammonia gas, so as to carry out
nitriding.
2. The method of nitriding a metal ring made of maraging steel
according to claim 1, wherein the elimination of said halogenous
compound film is carried out by heating the metal ring made of
maraging steel, on which said halogenous compound film is formed,
under a vacuum or reduced pressure atmosphere at a temperature
ranging from 450.degree. C. to 490.degree. C. for 5 to 10
minutes.
3. The method of nitriding a metal ring made of maraging steel
according to claim 1, wherein said halogen compound gas is one type
of gas selected from a group consisting of a fluorine compound gas,
a chlorine compound gas, and a bromine compound gas.
4. The method of nitriding a metal ring made of maraging steel
according to claim 1, wherein said nitriding is carried out by
maintaining the metal ring made of maraging steel under an
atmosphere comprising at least ammonia at a processing temperature
ranging from 450.degree. C. to 500.degree. C. for a processing time
ranging from 30 to 120 minutes.
5. The method of nitriding a metal ring made of maraging steel
according to claim 4, wherein said nitriding comprises the steps
of: placing said metal ring into a heating furnace and raising the
temperature inside the heating furnace to said processing
temperature; introducing a first mixed gas consisting of 50% to 90%
by volume of ammonia, 0.1% to 0.9% by volume of oxygen, and the
residual volume substantially consisting of nitrogen into said
heating furnace at the raised processing temperature, and
maintaining the furnace inside at the processing temperature, so as
to form a nitrided layer on the surface of the metal ring; and when
one-third to one-half of the processing time has passed, replacing
the atmosphere inside said heating furnace by a second mixed gas
consisting of 0% to 25% by volume of ammonia and a residual volume
consisting of nitrogen, and maintaining the furnace inside at the
processing temperature until the remaining processing time
passes.
6. The method of nitriding a metal ring made of maraging steel
according to claim 5, wherein said first mixed gas consists of 50%
to 90% by volume of ammonia, 0.5% to 4.5% by volume of air, and a
residual volume consisting of nitrogen.
7. A nitriding apparatus for nitriding a metal ring made of
maraging steel, which heats a metal ring made of maraging steel in
the presence of a halogen compound gas to eliminate an oxide film
from the surface of the metal ring, and then maintains the metal
ring under an atmosphere comprising at least ammonia at a
processing temperature ranging from 450.degree. C. to 500.degree.
C. for a processing time ranging from 30 to 120 minutes, so as to
carry out nitriding, said nitriding apparatus, comprising a
preheating chamber and a nitriding chamber, said preheating chamber
comprising first heating means for heating the inside of said
preheating chamber to said processing temperature when the chamber
accommodates the metal ring, halogen compound gas introducing means
for introducing a halogen compound gas into said preheating
chamber, and first exhausting means for exhausting the atmosphere
inside said preheating chamber, wherein the metal ring is heated by
said first heating means in the presence of a halogen compound gas
introduced by said halogen compound gas introducing means, so as to
eliminate an oxide film from the surface of the metal ring and to
form a halogenous compound film, and after the formation of said
halogenous compound film, the atmosphere inside said preheating
chamber is exhausted by said first exhausting means to reduce the
pressure in said preheating chamber, the metal ring is then heated
by said first heating means under a vacuum or reduced pressure
atmosphere, so as to eliminate the halogenous compound film, and
after the elimination of said halogenous compound film, the inside
of said preheating chamber is heated to the processing temperature
by said first heating means, and said nitriding chamber comprising
second heating means for heating the inside of said nitriding
chamber to said processing temperature and maintaining the furnace
inside at the temperature, nitrogen gas introducing means for
introducing a nitrogen gas into said nitriding chamber, ammonia gas
introducing means for introducing an ammonia gas into said
nitriding chamber, oxygen gas introducing means for introducing an
oxygen gas into said nitriding chamber, and second exhausting means
for exhausting the atmosphere inside said nitriding chamber,
wherein a first mixed gas consisting of 50% to 90% by volume of
ammonia, 0.1% to 0.9% by volume of oxygen, and a residual volume
substantially consisting of nitrogen is generated from the nitrogen
gas introduced by said nitrogen gas introducing means, the ammonia
gas introduced by said ammonia gas introducing means, and the
oxygen gas introduced by said oxygen gas introducing means, and the
metal ring is transferred from said preheating chamber into said
nitriding chamber under the first mixed gas atmosphere, so as to
form a nitrided case on the surface of the metal ring, when
one-third to one-half of the processing time has passed, while the
first mixed gas is exhausted by said second exhausting means, a
second mixed gas consisting of 0% to 25% by volume of ammonia and a
residual volume consisting of nitrogen is generated from nitrogen
gas introduced by said nitrogen gas introducing means and ammonia
gas introduced by said ammonia gas introducing means, and the
atmosphere inside said nitriding chamber is replaced by the second
mixed gas, thereafter, the metal ring is maintained in said
nitriding chamber under the second mixed gas atmosphere until the
remaining processing time passes.
8. The nitriding apparatus according to claim 7, comprising a first
combustion device for burning the atmosphere removed from said
preheating chamber by said first exhausting means.
9. The nitriding apparatus according to claim 7, comprising a
second combustion device for burning the atmosphere removed from
said nitriding chamber by said second exhausting means.
10. The nitriding apparatus according to claim 7, wherein said
preheating chamber comprises an door that is established between
said preheating chamber and said nitriding chamber and flexibly
moves up and down, and that said preheating chamber can be
connected to said nitriding chamber through said door.
11. The nitriding apparatus according to claim 7, comprising
transferring means for intermittently transferring the metal ring
from said preheating chamber to said nitriding chamber.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method of nitriding a
metal ring made of maraging steel, which is used for a belt for
continuously variable transmission and the like, and an apparatus
therefor.
BACKGROUND ART
[0002] There is known a belt for CVT, which is obtained by
laminating multiple metal rings to form a laminated ring and
combining the laminated ring with a certain shaped element. It is
conventionally used for power transmission of continuously variable
transmission (CVT) of an automobile or the like.
[0003] A metal ring forming the above laminated ring consists of
low-carbon steel comprising Ni and Mo, such as maraging steel. The
metal ring is produced, for example, by cutting a thin plate of the
above low-carbon steel into a rectangle, rounding the thin plate
along the long side, welding one edge on the short side to the
other edge on the other short side to form a cylindrical drum, and
cutting the drum to a certain width, so as to obtain an endless
ring. The endless ring is rolled to a thickness of approximately
0.1 to 0.3 mm, and it is then subjected to circumference correction
to have a certain circumference, so that the above metal ring can
be produced.
[0004] The above maraging steel is ultra strength steel, which has
both high strength and high toughness, when the maraging steel is
heated at an appropriate temperature and is subjected to age
hardening in a martensite state. Accordingly, it is appreciated to
use the maraging steel for the above metal ring. However, when a
belt for CVT consisting of a laminated ring obtained by laminating
the above metal rings is used for power transmission of CVT, the
belt for CVT is stretched between a pair of pulleys, which can
flexibly change V-groove pitch. Accordingly, when the above belt
for CVT travels between the above pulleys, the above metal ring
takes a linear shape, and when the belt travels along the pulleys,
it takes a curved shape. Thus, the metal ring is exposed to severe
bending deformation due to repetition of the above linear and
bending states.
[0005] Thus, the above metal ring is required to also have abrasion
resistance and fatigue resistance strength, as well as high
strength and high toughness obtained by age hardening of the above
maraging steel. For this purpose, the metal ring is subjected to a
surface hardening processing after the age hardening
processing.
[0006] The surface hardening processing is generally carried out by
performing nitriding processing to the metal ring, so as to form a
nitrided layer on the surface thereof. Examples of such a nitriding
processing include a gas nitriding and a gas nitrocarburizing.
[0007] By performing the above gas nitriding or gas
nitrocarburizing, nitrogen generated as a result of the
decomposition of ammonia penetrates into the metal tissues of
maraging steel. As a result, a nitrided layer is formed and
hardened on the surface of the metal ring, thereby improving the
abrasion resistance and fatigue resistance strength of the metal
ring.
[0008] In order that nitrogen penetrates into the metal tissues of
the maraging steel, it is required that the surface of the metal
ring is activated. However, there is a problem in that when an
oxide film is present on the surface of the metal ring, the above
nitrided layer is not uniformly formed. To solve this problem,
Japanese Patent Publication No. 8-9766 discloses a technique of
replacing the oxide film with a fluoride film by heating the metal
ring under an atmosphere containing a fluorine compound or fluorine
prior to the gas nitriding or gas nitrocarburizing.
[0009] According to the above technique, when the metal ring on
which the fluoride film is formed is subjected to gas nitriding or
gas nitrocarburizing, the surface of the metal ring is activated,
while the fluoride film is decomposed and eliminated. Then,
nitrogen penetrates into the activated metal surface, so that a
uniformed nitrided layer is formed.
[0010] However, in the above gas nitriding or gas nitrocarburizing,
ammonia is decomposed into hydrogen and nitrogen, and the hydrogen
is reacted with fluorine generated as a result of the decomposition
of the above fluoride film, so as to form hydrogen fluoride having
strong oxidizing ability. As a result, the surface of the metal
ring is damaged by hydrogen fluoride, and it becomes difficult to
laminate the above metal rings and to use the laminated ring for a
belt for CVT.
[0011] In order to promote the decomposition of ammonia during the
above gas nitriding or gas nitrocarburizing so as to form a
nitrided layer with an excellent hardness, for example, Japanese
Patent Laid-Open No. 62-270761 discloses a technique of maintaining
a steel material at a temperature of 500.degree. C. to 600.degree.
C. for 120 to 360 hours under an atmosphere containing 40% to 90%
ammonia by volume, 0.2% to 3% oxygen by volume, and a residual
volume consisting of nitrogen. This technique is applied for
nitriding steel materials such as those used for a metal mold for
plastic molding. It is claimed that a nitrided layer is formed to
0.25 mm deep from the surface by nitriding them under the above
conditions, thereby improving the hardness thereof.
[0012] However, since it is desired that the metal ring has an
excellent hardness on the surface thereof and maintains high
toughness even after the above nitrided layer is formed thereon,
the metal ring is required to comprise a non-nitrided portion in
the nitrided layer. Accordingly, if the above technique is applied
to the above metal ring, it could cause a problem that the nitrided
layer is formed through the full thickness of the metal ring so
that it becomes difficult to maintain high toughness after
formation of the nitrided layer.
[0013] In order to solve this problem, the present applicant has
already filed a patent application regarding a technique of
controlling the thickness of the nitrided layer formed on the
surface of the metal ring to from 20% to 40% of the full thickness
of the metal belt by carrying out gas nitriding or gas
nitrocarburizing as described above (refer to Japanese Patent
Laid-Open No. 2000-337453). According to this technique, the metal
ring is maintained under an atmosphere consisting of ammonia and
nitrogen or atmosphere consisting of ammonia and an RX gas, for
example, at a temperature ranging between 450.degree. C. and
500.degree. C. for 60 to 90 minutes. According to this technique,
anon-nitrided portion remains within the metal ring, and the metal
ring can have high toughness maintained in this portion.
[0014] However, it is desired to further improve the toughness of
the above metal ring.
DISCLOSURE OF THE INVENTION
[0015] It is an object of the present invention to solve such an
inconvenience and to provide a nitriding processing method, which
enables to eliminate an oxide film from the surface of a metal ring
made of maraging steel so as to activate the metal surface without
damaging the surface, and which also enables to impart excellent
hardness and toughness to the metal ring.
[0016] It is another object of the present invention to provide a
nitriding apparatus that is suitable for carrying out the above
nitriding processing method.
[0017] To achieve the above objects, the nitriding processing
method of the present invention comprises the steps of: heating a
metal ring made of maraging steel in the presence of a halogen
compound gas, so as to eliminate an oxide film from the surface of
the metal ring and to form a halogenous compound film; heating the
metal ring made of maraging steel on which the above halogenous
compound film is formed under a vacuum or reduced pressure
atmosphere, so as to eliminate the above halogenous compound film;
and heating the metal ring made of maraging steel from which the
above halogenous compound film is eliminated in the presence of an
ammonia gas, so as to carry out a nitriding processing.
[0018] In the nitriding processing method of the present invention,
first, a metal ring made of maraging steel is heated in the
presence of a halogen compound gas. By this step, an oxide film
present on the surface of the metal ring made of maraging steel is
reacted with the halogen compound and eliminated therefrom, and
instead, a halogenous compound film is formed on the metal ring
surface. Examples of an available halogen compound gas may include
a fluorine compound gas, a chlorine compound gas, and a bromine
compound gas.
[0019] Thereafter, the metal ring made of maraging steel on which
the above halogenous compound film is formed is heated under a
vacuum or reduced pressure atmosphere. By this step, the above
halogenous compound film is decomposed and eliminated. In the
nitriding processing method of the present invention, ammonia is
not present during this step, and therefore, halogen generated as a
result of the decomposition of the halogenous compound film is
eliminated as a halogen gas, without generating acid. As a result,
either the above oxide film or a halogenous compound film is not
present on the surface of the metal ring, and the surface becomes
extremely clean and activated.
[0020] Thus, thereafter, the metal ring made of maraging steel from
which the halogenous compound film is eliminated is heated in the
presence of an ammonia gas to carry out a nitriding processing, so
that a uniformed nitrided layer can be formed without damaging the
surface of the metal ring. The above nitriding processing that is
carried out in the presence of an ammonia gas may be either a gas
nitriding or gas nitrocarburizing.
[0021] In the nitriding processing method of the present invention,
the elimination of the above halogenous compound film is preferably
carried out by heating the metal ring made of maraging steel, on
which the above halogenous compound film is formed, under a vacuum
or reduced pressure atmosphere at a temperature ranging between
450.degree. C. and 490.degree. C. for 5 to 10 minutes.
[0022] If the temperature is lower than 450.degree. C., it may
become difficult to carry out the nitriding processing after the
elimination of the halogenous compound film. In contrast, if the
temperature exceeds 490.degree. C., residual stress may become
excessively large. Moreover, if the heating time is shorter than 5
minutes, it becomes difficult to exhaust the generated halogen gas.
In contrast, if it exceeds 10 minutes, residual stress decreases
and it becomes difficult to obtain sufficient strength.
[0023] In addition, in the nitriding processing method of the
present invention, the nitriding processing is preferably carried
out by maintaining a metal ring made of maraging steel under an
atmosphere comprising at least ammonia at a processing temperature
ranging between 450.degree. C. and 500.degree. C. for a processing
time ranging between 30 and 120 minutes.
[0024] The nitriding processing cannot be carried out at a
processing temperature of lower than 450.degree. C. On the other
hand, if the processing temperature exceeds 500.degree. C., aging
further progresses by heating in the nitriding processing, and it
becomes overaging. As a result, martensite is deposited, and
hardness decreases. Moreover, if the processing time is shorter
than 30 minutes, the nitriding processing cannot be carried out. If
it exceeds 120 minutes, however, aging further progresses by
heating in the nitriding processing, and it becomes overaging. As a
result, martensite is deposited, and hardness decreases.
[0025] The above nitriding processing preferably comprises the
steps of: placing the above metal ring into a heating furnace and
raising the temperature inside the heating furnace to the
processing temperature; introducing a first mixed gas consisting of
50% to 90% ammonia byvolume, 0.1% to 0.9% oxygen by volume, and a
residual volume substantially consisting of nitrogen into the above
heating furnace having the raised processing temperature, and
maintaining the process temperature, so as to form a nitrided layer
on the surface of the metal ring; and when the one-third to
one-half of the processing time has passed, replacing the
atmosphere inside the above heating furnace by a second mixed gas
consisting of 0% to 25% ammonia by volume and a residual volume
consisting of nitrogen, and maintaining the processing temperature
until the remaining processing time passes.
[0026] In the above nitriding processing method, first, a metal
ring made of maraging steel is placed into a heating furnace, and
the temperature inside the heating furnace is raised to a
processing temperature ranging between 450.degree. C. and
500.degree. C. If the temperature inside the heating furnace is
lower than 450.degree. C., the nitriding processing cannot be
carried out. If it exceeds 500.degree. C., aging further progresses
by heating in the nitriding processing, and it becomes overaging.
As a result, martensite is deposited, and hardness decreases.
[0027] Thereafter, when the temperature inside the heating furnace
reaches the above range of processing temperature, the first mixed
gas consisting of 50% to 90% ammonia by volume, 0.1% to 0.9% oxygen
by volume, and a residual volume substantially consisting of
nitrogen is introduced into the above heating furnace, and the
above process temperature is maintained.
[0028] By this step, nitrogen generated as a result of the
decomposition of the above ammonia is diffused in the metal ring,
so that a nitrided layer is formed on the surface of the metal
ring. At the same time, hydrogen as well as nitrogen is generated
as a result of the decomposition of the above ammonia. If the
partial pressure of hydrogen increases, the decomposition of
ammonia is inhibited, or hydrogen binds to nitrogen again, thereby
inhibiting nitriding.
[0029] Thus, in the above nitriding method, the first mixed gas
comprises oxygen in the above range of amount, so that the hydrogen
is bound to the oxygen to be eliminated as water from the system.
As a result, the decomposition of ammonia can be promoted.
[0030] If the amount of ammonia contained in the first mixed gas is
less than 50% by volume, a nitrided layer cannot be sufficiently
formed, thereby not imparting a desired hardness to the surface of
the metal ring. On the other hand, if the amount of ammonia exceeds
90% by volume, an excessive amount of nitrogen is generated,
compounds such as iron nitride are formed in the metal tissues of
the metal ring, and nitriding cannot be carried out uniformly. This
is not preferable.
[0031] Moreover, if the amount of oxygen contained in the first
mixed gas is less than 0.1% by volume, the effect of eliminating
hydrogen by biding oxygen to hydrogen cannot be sufficiently
obtained. On the other hand, if the amount of oxygen exceeds 0.9%
by volume, oxide is formed in the metal tissues of the metal ring,
or the decomposition of ammonia is excessively promoted, thereby
generating an excessive amount of nitrogen. This is not
preferable.
[0032] When one-third to one-half of the processing time ranging
from 30 to 120 minutes has passed, it is determined that the
nitrided layer should have been formed on the metal ring. Then, the
atmosphere inside the above heating furnace is replaced by the
second mixed gas consisting of 0% to 25% ammonia by volume and a
residual volume consisting of nitrogen, and the processing
temperature is maintained until the remaining processing time
passes.
[0033] By replacing the atmosphere inside the heating furnace by
the above second mixed gas, a portion of nitrogen contained in the
above nitrided layer evaporates into the atmosphere and so it is
eliminated from the metal ring, and another portion of nitrogen is
diffused further inside the metal ring. As a result, there is
formed on the surface of the metal ring a nitrided layer, which has
a structure such that the hardness decreases at a certain constant
level depending on the depth from the surface and which has an
excellent toughness as well as an excellent hardness.
[0034] If the atmosphere inside the above heating furnace is
replaced by the second mixed gas before at least the one-third of
the processing time passes, the formation of the above nitrided
layer becomes insufficient, and a desired hardness might not be
obtained on the surface of the metal ring. On the other hand, if
the above replacement is carried out after the one-half of the
processing time has passed, the above nitrided layer is excessively
formed in the metal ring, and a sufficient toughness might not be
obtained.
[0035] If the amount of ammonia contained in the second mixed gas
exceeds 25% by volume, either the effect of eliminating a portion
of nitrogen contained in the nitrided layer from the metal ring or
the effect of diffusing another portion of nitrogen further inside
the metal ring cannot be attained.
[0036] By performing the above step, in addition to toughness
maintained at a non-nitrided portion in the metal ring, toughness
can be obtained from the nitrided layer in the above structure.
Thus, the toughness of the metal ring can be further improved.
[0037] As oxygen contained in the above first mixed gas, an oxygen
containing gas such as air can be used instead of pure oxygen, and
thus, cost can be reduced. The above air consists of 20% oxygen by
volume, a very small volume of other ingredients, and a residual
volume substantially consisting of nitrogen, and so it is
conveniently used.
[0038] When air is used instead of pure oxygen, the above first
mixed gas may consist of 50% to 90% ammonia by volume, 0.5% to 4.5%
air by volume, and a residual volume consisting of nitrogen.
[0039] The nitriding apparatus nitrides the metal ring made of
maraging steel by heating a maraging steel in the presence of a
halogen compound gas to remove an oxide film formed on the surface
of the metal ring, and maintaining the metal ring under an
atmosphere comprising at least ammonia at a processing temperature
ranging from 450.degree. C. to 500.degree. C. for a processing time
ranging from 30 to 120 minutes. The nitriding apparatus comprises a
preheating chamber and a nitriding chamber. The preheating chamber
comprises a first heating means for heating the interior of the
preheating chamber after accommodating the metal ring therein, a
halogen compound gas introducing means for introducing a halogen
compound gas into the preheating chamber, and a first exhaust means
for exhausting the atmosphere inside the preheating chamber. The
preheating chamber is heated by the first heating means in the
presence of the halogen compound gas introduced by the halogen
compound gas introducing means so as to remove an oxide film from
the surface of the metal ring and form a halogen compound film on
the metal ring, is discharged of the atmosphere therein by the
first exhaust means so as to reduce the pressure therein after the
oxide film is formed on the surface of the metal ring, is heated by
the first heating means under a vacuum or reduced pressure
atmosphere so as to remove the halogen compound film from the metal
ring, and is heated by the first heating means to the processing
temperature after the halogen compound film is removed from the
metal ring. The nitriding chamber comprises a second heating means
for heating the interior of the nitriding chamber to the processing
temperature and maintaining the temperature therein, a nitrogen gas
introducing means for introducing nitrogen gas into the nitriding
chamber, an oxygen gas introducing means for introducing oxygen gas
into the nitriding chamber, and a second exhaust means for
exhausting the atmosphere inside the nitriding chamber. First, the
nitriding chamber accommodates the metal ring transferred into the
nitriding chamber from the preheating chamber under the atmosphere
of a first mixed gas consisting of 50% to 90% by volume of ammonia,
0.1% to 0.9% by weight of oxygen, and a residual volume
substantially consisting of nitrogen, the first mixed gas generated
from the nitrogen gas introduced by the nitrogen gas introducing
means, the ammonia gas introduced by the ammonia gas introducing
means and the oxygen gas introduced by the oxygen gas introducing
means, so as to form a nitride layer on the surface of the metal
ring. Then, the nitriding chamber is discharged of the first mixed
gas by the second exhausting means after 1/3 to 1/2 of the
processing period has elapsed and is provided with the atmosphere
of a second mixed gas consisting of 0 to 25% by volume of ammonia,
and a residual volume consisting of nitrogen, the second mixed gas
generated from the ammonia gas introduced by the ammonia gas
introducing means and nitrogen gas introduced by the nitrogen gas
introducing means, so as to replace the atmosphere of the first
mixed gas inside the nitriding chamber with the second mixed gas.
Thereafter, the nitriding chamber maintains the metal ring therein
under the atmosphere of the mixed gas until the remaining
processing time has elapsed.
[0040] In the nitriding apparatus of the present invention, first,
the above metal ring made of maraging steel is placed in the above
preheating chamber. In the above preheating chamber, a halogen
compound gas has already been introduced by the above halogen
compound gas introducing means. The preheating chamber is heated by
the above first heating means under the halogen compound gas
atmosphere, so that an oxide film is eliminated from the surface of
the metal ring and that a halogenous compound film is formed
instead.
[0041] Thereafter, the atmosphere is exhausted from the above
preheating chamber by the above first exhausting means to reduce
the pressure in the above preheating chamber to a vacuum or reduced
pressure atmosphere, the inside of the preheating chamber is heated
by the above first heating means to a temperature ranging between
450.degree. C. and 490.degree. C., and the temperature is
maintained for 5 to 10 minutes, so as to eliminate the above
halogen compound. As a result, the surface of the metal ring
becomes extremely clean and activated, having neither oxide film
nor halogenous compound film.
[0042] Thereafter, the inside of the preheating chamber is heated
by the first heating means. After the temperature is raised to a
processing temperature ranging between 450.degree. C. and
500.degree. C., the metal ring is transferred from the preheating
chamber into the nitriding chamber.
[0043] In the above nitriding chamber, a nitrogen gas, an ammonia
gas, and an oxygen gas have already been introduced by the above
nitrogen gas introducing means, the above ammonia gas introducing
means, and the above oxygen gas introducing means, respectively. As
a result, in the nitriding chamber, a first mixed gas consisting of
50% to 90% ammonia by volume, 0.1% to 0.9% oxygen by volume, and a
residual volume substantially consisting of nitrogen is generated
from each of the above gases. Moreover, the inside of the nitriding
chamber is heated by the above second heating means and it is
maintained at the processing temperature ranging between
450.degree. C. and 500.degree. C.
[0044] Thus, the metal ring transferred from the preheating chamber
into the nitriding chamber is maintained at the above range of
processing temperature under the first mixed gas atmosphere. The
above oxygen gas introducing means may introduce either a pure
oxygen gas or gas containing oxygen such as air, as long as the gas
is capable of generating the above first mixed gas.
[0045] Thereafter, in the nitriding chamber, a nitrided layer is
formed on the surface of the metal ring. When the one-third or
one-half of the above processing time has passed, the first mixed
gas is exhausted by the above second exhausting means. Thereafter,
a nitrogen gas and an ammonia gas are newly introduced into the
nitriding chamber by the above nitrogen gas introducing means and
the above ammonia gas introducing means, respectively, and then, a
second mixed gas consisting of 0% to 25% ammonia by volume and a
residual volume consisting of nitrogen is generated from these
gases. As a result, the atmosphere in the nitriding chamber is
replaced by the above second mixed gas.
[0046] Thereafter, the metal ring is maintained in the above
nitriding chamber under the second mixed gas atmosphere until the
remaining processing time passes.
[0047] As a result, the nitriding processing is advantageously
carried out using the nitriding apparatus of the present
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is an illustrative view, which schematically shows
the production process of a metal ring used for a belt for CVT;
[0049] FIG. 2 is a process diagram showing the nitriding method in
the present embodiment;
[0050] FIG. 3 is a graph showing the relationship between a depth
from the surface of the metal ring and hardness; and
[0051] FIG. 4 is a system block diagram showing the configuration
of the nitriding apparatus in the present embodiment.
BEST MODE FOR CARRYING OUT THE INVENTION
[0052] To produce the above metal ring, first, as shown in FIG. 1,
a thin plate 1 made of maraging steel is bended in a loop, and one
edge of the plate is welded to the other edge, so as to form a
cylindrical drum 2. The above maraging steel is low-carbon steel
comprising 0.03% or less C, 0.10% or less Si, 0.10% or less Mn,
0.01% or less P, and 0.01% or less S. It is what is called 18% Ni
steel comprising 18% to 19% Ni, 4.7% to 5.2% Mo, 0.05% to 0.15% Al,
0.50% to 0.70% Ti, and 8.5% to 9.5% Co. Since the above maraging
steel shows age hardening by the heat of welding, both sides of the
welded portion 2a of the drum 2 become hardened.
[0053] Hence, the drum 2 is then placed in a vacuum furnace 3 and
maintained at a temperature of 820.degree. C. to 830.degree. C. for
20 to 60 minutes, so that the first solution treatment is carried
out and uneven hardness is eliminated. After completion of the
above first solution treatment, the drum 2 is carried away from the
vacuum furnace 3, and it is cut to a certain width, so as to form a
metal ring W.
[0054] The thus formed metal ring W is then rolled at a draft of
40% to 50%. The metal ring W is rolled to a thickness of 0.2 mm by
the above rolling, and rolled tissues are formed to a thickness of
approximately 30 .mu.m from the surface. Thus, the rolled metal
ring W is placed in a heating furnace 4 and subjected to the second
solution treatment, so that the above rolled tissues are
eliminated, and that uniform metal crystal grains are formed.
[0055] Thereafter, the solution-treated metal ring W is subjected
to circumference correction, and it is then placed in a heating
furnace 5, where it is maintained at a temperature ranging between
440.degree. C. and 480.degree. C. for 60 to 120 minutes to carry
out aging treatment. After completion of the aging treatment, the
metal ring W is cooled in the heating furnace 5 and then
transferred into a heating furnace 6 that is a nitriding apparatus,
so as to carry out a nitriding processing.
[0056] In the nitriding processing in the present embodiment,
first, the above aging-treated metal ring W is placed in the
heating furnace 6, and it is heated in the presence of a chloride
gas such as methylene chloride at a temperature ranging between
440.degree. C. and 490.degree. C. for 1 to 10 minutes. In many
cases, an oxide film is formed on the surface of the metal ring W.
However, when the oxide film is heated in the presence of the above
chloride gas, the film is reacted with chloride and eliminated, and
instead, a chloride film is formed on the surface of the metal ring
W.
[0057] After completion of the heating in the presence of the
chloride gas, the chloride gas is exhausted, and further, the
pressure in the heating furnace 6 is reduced to a vacuum or reduced
pressure atmosphere of less than 10.sup.-3 Pa. Then, the metal ring
W on which the above chloride film is formed is heated under the
above vacuum or reduced pressure atmosphere at a temperature
ranging between 450.degree. C. and 490.degree. C. for 5 to 10
minutes. It should be noted that air is always exhausted from the
heating furnace 6 so as to maintain the above vacuum or reduced
pressure atmosphere.
[0058] By the above step, a chloride gas is generated as a result
of the decomposition of the chloride film, and the chloride gas is
exhausted. At the same time, the surface of the metal ring W
becomes clean and activated by decomposition and elimination of the
chloride film.
[0059] After completion of the heating under the above vacuum or
reduced pressure atmosphere, an atmosphere containing at least an
ammonia gas is introduced into the heating furnace 6. Thereafter,
the metal ring W is nitrided by heating under the above atmosphere
at a temperature ranging between 440.degree. C. and 490.degree. C.
for 30 to 120 minutes.
[0060] In the present embodiment, when the metal ring W is heated
under an atmosphere containing the above ammonia gas, no chloride
film is present on the surface of the metal ring W, and as stated
above, the surface is clean and activated. Accordingly, the
generation of hydrogen chloride as a result of the reaction between
hydrogen generated as a result of the decomposition of ammonia and
the above chloride film does not take place. Thus, the surface of
the metal ring W is prevented from being damaged by hydrogen
chloride, and so it is free from the resulting defects. Moreover,
since the surface of the metal ring W is clean and activated, a
uniform nitrided layer can be formed.
[0061] In the present embodiment, a case where an oxide film is
eliminated from the surface of the metal ring W using a chloride
gas is explained. However, a halogen compound gas such as a
fluorine gas or bromine gas may also be used instead of a chloride
gas.
[0062] In the above nitriding processing, as shown in FIG. 2,
first, the inside of the heating furnace 6 is heated from room
temperature to a certain nitriding processing temperature for a
predetermined time t.sub.1. The inside of the heating furnace 6 is
set under a nitrogen atmosphere, and the above nitriding processing
temperature is set within the range between 450.degree. C. and
500.degree. C.
[0063] After the temperature in the heating furnace 6 reaches the
above nitriding processing temperature, the first mixed gas is
introduced into the furnace 6. The first mixed gas consists of 50%
to 90% ammonia by volume, 0.5% to 4.5% air by volume, and a
residual volume consisting of nitrogen. The air contains
approximately 20% oxygen by volume, and thus, the above first mixed
gas consists of 50% to 90% ammonia by volume, 0.1% to 0.9% oxygen
by volume, and a residual volume substantially consisting of
nitrogen. The metal ring W is maintained under an atmosphere
containing the above first mixed gas for a predetermined time
t.sub.2 at the above nitriding processing temperature, so as to
form a nitrided layer on the surface thereof.
[0064] After formation of the nitrided layer on the surface of the
metal ring W, the second mixed gas is introduced into the heating
furnace 6, and the above first mixed gas is replaced by the second
mixed gas during a predetermined time t.sub.3. The second mixed gas
consists of 0% to 25% ammonia by volume and a residual volume
consisting of nitrogen. After the first mixed gas in the atmosphere
in the heating furnace 6 is replaced by the second mixed gas, the
metal ring W is further maintained under an atmosphere containing
the above second mixed gas for a predetermined time t.sub.4 at the
above nitriding processing temperature, so as to complete nitriding
of the metal ring W.
[0065] Herein, the processing time (t.sub.2+t.sub.3+t.sub.4) to
maintain the metal ring at the above nitriding processing
temperature to carry out the above nitriding processing is set
within the range between 30 to 120 minutes. Moreover, after the
time t.sub.2 to maintain the metal ring at the above nitriding
processing temperature under the above first mixed gas atmosphere
has passed, the time to introduce the second mixed gas is set
within the range from the one-third to one-half of the above
processing time. As a result, there can be obtained a metal ring W,
which has an improved hardness because of a nitrided layer formed
on the surface and a toughness maintained in a non-nitrided portion
thereinside. Moreover, the above metal ring W has a structure such
that the hardness of the nitrided layer decreases at a certain
level depending on the depth from the surface, and because of such
a nitrided layer, the metal ring W can obtain a further excellent
toughness.
[0066] Thereafter, the heating furnace 6 is left to cool for a
predetermined time t.sub.5, so that the above nitriding processing
temperature is cooled to room temperature, and the nitriding
processing is terminated. Thereafter, the metal ring W on the
surface of which a nitrided layer is formed is carried away from
the heating furnace 6.
[0067] FIG. 3(a) shows the relationship between a depth from the
surface and hardness of the metal ring W obtained by the method in
the present embodiment. In addition, FIG. 3(b) shows the
relationship between a depth from the surface and hardness of the
metal ring W, which is obtained by the conventional method in which
the heating furnace 6 is filled with the first mixed gas throughout
the above processing time (t.sub.2+t.sub.3+t.sub.4), and the first
mixed gas is not replaced by the second mixed gas.
[0068] As shown in FIG. 3(a), the outermost of the metal ring W
obtained by the method in the present embodiment has an excellent
hardness of 800 or more based on Vickers hardness (Hv.sub.0.3).
Moreover, the metal ring comprises a non-nitrided portion with a
constant hardness deeper than 40 .mu.m from the surface. At the
same time, the metal ring comprises a nitrided layer formed until a
depth 40 .mu.m from the surface, wherein hardness decreases at a
certain level. Accordingly, the metal ring W obtained by the method
in the present embodiment has toughness provided by the nitrided
layer with the above described structure, as well as toughness
provided by the non-nitrided portion thereinside. Thus, it is clear
that the metal ring has an excellent toughness.
[0069] On the other hand, as shown in FIG. 3(b), the metal ring W
obtained by the conventional method has the same structure as the
metal ring W obtained by the method in the present embodiment, in
that it has a hardness of 800 or more based on Vickers hardness
(Hv.sub.0.3) outermost sureface, and in that it comprises a
non-nitrided portion with a constant hardness deeper than 40 .mu.m
from the surface.
[0070] However, in the metal ring W obtained by the conventional
method, hardness sharply decreases within a depth range of 10 .mu.m
from the surface, and therefore the metal ring does not have a
structure such that hardness decreases at a certain level until a
depth 40 .mu.m from the surface. Accordingly, it is clear that the
metal ring W obtained by the conventional method cannot obtain
toughness other than the toughness provided by the non-nitrided
portion thereinside.
[0071] The nitriding processing method in the present embodiment
can be carried out using a nitriding apparatus 6 as shown in FIG.
4.
[0072] As shown in FIG. 4, the nitriding apparatus 6 in the present
embodiment comprises a first replacement chamber 11, a preheating
chamber 12, a nitriding chamber 13, a cooling chamber 14, and a
second replacement chamber 15. The above each chamber comprises an
door that is established at the border of each chamber and freely
moves up and down (not shown), and the chambers can be connected to
each other by opening or closing the door. Moreover, the first
replacement chamber 11 comprises a carrying-in door that flexibly
moves up and down (not shown) on the opposite side of the
preheating chamber 12, whereas the second replacement chamber 15
comprises a carrying-out door that flexibly moves up and down (not
shown) on the opposite side of the cooling chamber 14. It is noted
that all of the above doors are closed in an ordinary state.
[0073] Moreover, the nitriding apparatus 6 comprises transferring
means for intermittently transferring the metal ring W in the
direction from the first replacement chamber 11 to the second
replacement chamber 15, while the processing is carried out in the
above each chamber (not shown). An example of the above
transferring means may include a belt conveyor, which is
established through the first replacement chamber 11, the
preheating chamber 12, the nitriding chamber 13, the cooling
chamber 14, and the second replacement chamber 15. Furthermore,
each of the preheating chamber 12 and the nitriding chamber 13
comprises heating means for heating the inside thereof (not
shown).
[0074] The nitriding apparatus 6 further comprises a nitrogen gas
source 16, a halogen compound gas source 17, an ammonia gas source
18, and an air (oxygen gas) source 19. The nitrogen gas source 16
is connected to a nitrogen gas pipe 20. A nitrogen gas branch pipe
20a branching from the nitrogen gas pipe 20 is connected to the
first replacement chamber 11. A nitrogen gas branch pipe 20b is
connected to the preheating chamber 12, a nitrogen gas branch pipe
20c is connected to the nitriding chamber 13, a nitrogen gas branch
pipe 20d is connected to the cooling chamber 14, and a nitrogen gas
branch pipe 20e is connected to the second replacement chamber
15.
[0075] Moreover, the halogen compound gas source 17 is connected to
the preheating chamber 12 through a halogen compound gas pipe 21,
and the ammonia gas source 18 is connected to the nitriding chamber
13 through an ammonia gas pipe 22. Furthermore, the air source 19
is connected to an air pipe 23, an air branch pipe 23a branching
from the air pipe 23 is connected to the nitriding chamber 13, and
an air branch pipe 23b is connected to the cooling chamber 14.
[0076] The first replacement chamber 11 comprises an exhaust pipe
25 connected to an atmosphere releasing unit 24. On the exhaust
pipe 25, a high-vacuum pump 26 for discharging the atmosphere in
the first replacement chamber 11 and an oxygen sensor 27 for
measuring the oxygen level in the exhausted air are
established.
[0077] The preheating chamber 12 comprises an exhaust pipe 29
connected to an exhaust gas combustion device 28. On the exhaust
pipe 29, a low-vacuum pump 30 for discharging the atmosphere in the
preheating chamber 12 and an oxygen sensor 31 for measuring the
oxygen level in the exhausted air are established. The exhaust gas
combustion device 28 is connected to the atmosphere releasing unit
24 through a combustion gas pipe 32.
[0078] The nitriding chamber 13 comprises an exhaust pipe 33
connected to the exhaust gas combustion device 28. On the exhaust
pipe 33, a low-vacuum pump 34 for discharging the atmosphere in the
nitriding chamber 13 and an oxygen sensor 35 for measuring the
oxygen level in the exhausted air are established. The exhaust pipe
33 comprises an air exhaust branch pipe 33a, which branches from
the exhaust pipe 33 to connect to the exhaust gas combustion device
28. On the air exhaust branch pipe 33a, an ammonia analyzer 36 for
measuring the ammonia level in the exhausted air is
established.
[0079] The cooling chamber 14 comprises an exhaust pipe 37
connected to the atmosphere releasing unit 24, and the second
replacement chamber 15 comprises an exhaust pipe 38 connected to
the atmosphere releasing unit 24. On the exhaust pipe 38, a
high-vacuum pump 39 for discharging the atmosphere in the second
replacement chamber 15 and an oxygen sensor 40 for measuring the
oxygen level in the exhausted air are established.
[0080] In the present embodiment, the preheating chamber 12 and the
nitriding chamber 13 both comprise the common exhaust gas
combustion device 28. However, it may also be possible that each of
the preheating chamber 12 and the nitriding chamber 13 comprises an
exhaust gas combustion device, independently.
[0081] Next, the actuation of the nitriding apparatus 6 in the
present embodiment will be explained.
[0082] In the nitriding apparatus 6, first, a carrying-in door of
the first replacement chamber 11 that is not shown in the figure is
opened, and the metal ring W is then carried in the first
replacement chamber 11 by transferring means not shown in the
figure. Thereafter, the above carrying-in door is closed, and the
high-vacuum pump 26 is actuated, so that the atmosphere in the
first replacement chamber 11 is exhausted to the atmosphere
releasing unit 24 through the exhaust pipe 25. Simultaneously,
nitrogen is introduced from the nitrogen gas source 16 into the
first replacement chamber 11 through the nitrogen gas pipe 20 and
the nitrogen gas branch pipe 20a. When the oxygen level detected by
the oxygen sensor 27 becomes less than a certain value, it is
determined that the atmosphere in the first replacement chamber 11
is replaced by a nitrogen gas, and the action of the high-vacuum
pump 26 is terminated.
[0083] Thereafter, an door located between the first replacement
chamber 11 and the preheating chamber 12, which is not shown in the
figure, is opened, and the metal ring W is transferred into the
preheating chamber 12 by the above transferring means. When the
metal ring W is accommodated in the preheating chamber 12, the door
located between the first replacement chamber 11 and the preheating
chamber 12 is closed, and the inside of the preheating chamber 12
is then heated by heating means not shown in the figure.
[0084] At this time, a nitrogen gas is introduced into the
preheating chamber 12 from the nitrogen gas source 16 through the
nitrogen gas pipe 20 and the nitrogen gas branch pipe 20b, and a
halogen compound gas such as methylene chloride is also introduced
therein from the halogen compound gas source 17 through the halogen
compound gas pipe 21. Accordingly, the atmosphere in the preheating
chamber 12 is replaced by an atmosphere containing a mixed gas
consisting of 1.0% to 20.0% halogen compound gas by volume and the
balance of nitrogen.
[0085] To replace the atmosphere in the preheating chamber 12,
first, the low-vacuum pump 30 is actuated, and thereby the
atmosphere in the preheating chamber 12 is released to the
atmosphere releasing unit 24 through the exhaust pipe 29, the
exhaust gas combustion device 28, and the combustion gas pipe 32.
When the oxygen level detected by the oxygen sensor 31 becomes less
than a certain value, the action of the low-vacuum pump 30 is
terminated, and a nitrogen gas and a halogen compound gas are
introduced into the preheating chamber 12 from the nitrogen gas
source 16 and the halogen compound gas source 17, respectively.
[0086] When the metal ring W is heated under the above atmosphere
at a temperature ranging between 440.degree. C. and 490.degree. C.
for 1 to 10 minutes, an oxide film formed on the surface of the
metal ring W is reacted with the halogen compound and eliminated
therefrom, and instead, a halogenous compound film is formed. After
completion of the heating in the presence of the above chloride
gas, the low-vacuum pump 30 is actuated, and the atmosphere in the
preheating chamber 12 is exhausted to the atmosphere releasing unit
24 to further reduce the pressure, so that the pressure in the
preheating chamber 12 is reduced to a vacuum or reduced pressure
atmosphere of less than 10.sup.-3 Pa. Thereafter, the metal ring W
on which the above halogenous compound film is formed is heated
under the above vacuum or reduced pressure atmosphere at a
temperature ranging between 450.degree. C. and 490.degree. C. for 5
to 10 minutes. It is noted that air is always exhausted from the
preheating chamber 12 so as to maintain the above vacuum or reduced
pressure atmosphere. By the above step, the above halogenous
compound film is decomposed and exhausted as a halogen compound
gas, whereas the surface of the metal ring W becomes clean and
activated.
[0087] After completion of the heating under the above vacuum or
reduced pressure atmosphere, the action of the low-vacuum pump 30
is terminated, and a nitrogen gas is introduced into the preheating
chamber 12 from the nitrogen gas source 16. Then, the temperature
inside the preheating chamber 12 is raised to a temperature ranging
between 450.degree. C. and 500.degree. C. during a predetermined
time t.sub.1.
[0088] When the inside of the preheating chamber 12 is raised to a
temperature ranging between 450.degree. C. and 500.degree. C., an
door located between the preheating chamber 12 and the nitriding
chamber 13, which is not shown in the figure, is opened, and the
metal ring W is transferred into the nitriding chamber 13 by the
above transferring means. After the metal ring W is accommodated in
the nitriding chamber 13, the door located between the preheating
chamber 12 and the nitriding chamber 13 is closed. The metal ring W
is maintained in the nitriding chamber 13 until a processing time
of 30 to 120 minutes passes, so as to carry out a nitriding
processing.
[0089] At this time, a nitrogen gas is introduced into the
nitriding chamber 13 from the nitrogen gas source 16 through the
nitrogen gas pipe 20 and the nitrogen gas branch pipe 20c, an
ammonia gas is introduced therein from the ammonia gas source 18
through the ammonia gas pipe 22, and air is introduced therein from
the air source 19 through the air pipe 23 and the air branch pipe
23a. Accordingly, the atmosphere in the nitriding chamber 13 is
replaced by an atmosphere containing the first mixed gas consisting
of 50% to 90% ammonia gas by volume, 0.1% to 0.9% oxygen by volume,
and a residual volume consisting of nitrogen. The nitriding chamber
is heated to a temperature ranging between 450.degree. C. and
500.degree. C. by heating means that is not shown in the
figure.
[0090] To replace the atmosphere in the nitriding chamber 13,
first, the low-vacuum pump 34 is actuated, and thereby the
atmosphere in the nitriding chamber 13 is released to the
atmosphere releasing unit 24 through the exhaust pipe 33, the air
exhaust branch pipe 33a, the exhaust gas combustion device 28, and
the combustion gas pipe 32. Simultaneously, a nitrogen gas, an
ammonia gas, and air are introduced into the nitriding chamber 13
from the nitrogen gas source 16, the ammonia gas source 18, and the
air source 19, respectively. Thereafter, when each of the oxygen
level detected by the oxygen sensor 35 and the ammonia level
detected by the ammonia analyzer 36 reaches within the range to
constitute the above first mixed gas, the action of the low-vacuum
pump 34 is terminated.
[0091] To carry out the above nitriding processing, first, the
inside of the nitriding chamber 13 is maintained under the first
mixed gas atmosphere for a predetermined time t.sub.2 at a
temperature ranging between 450.degree. C. and 500.degree. C., so
that a nitrided layer is formed on the surface of the metal ring W.
When the above predetermined time t.sub.2 has passed, the
low-vacuum pump 34 is actuated again, so that the first mixed gas
filled in the nitriding chamber 13 is released to the atmosphere
releasing unit 24 through the exhaust pipe 33, the air exhaust
branch pipe 33a, the exhaust gas combustion device 28, and the
combustion gas pipe 32. Simultaneously, a nitrogen gas and an
ammonia gas are introduced into the nitriding chamber 13 from the
nitrogen gas source 16 and the ammonia gas source 18, respectively.
When the ammonia level detected by the ammonia analyzer 36 reaches
within a range between 0% to 25% ammonia by volume, the action of
the low-vacuum pump 34 is terminated. As a result, the atmosphere
filled in the nitriding chamber 13 is replaced by the second mixed
gas atmosphere consisting of 0% to 25% ammonia gas by volume and a
residual volume consisting of nitrogen.
[0092] The replacement of the first mixed gas atmosphere by the
second mixed gas atmosphere is carried out during a predetermined
time t.sub.3. Moreover, after the predetermined time t.sub.2 has
passed, the time to start to introduce the second mixed gas is set
within the range from the one-third to one-half of the above
processing time.
[0093] For the above nitriding processing, thereafter, the inside
of the nitriding chamber 13 is maintained under the above second
mixed gas atmosphere at the above temperature ranging between
450.degree. C. and 500.degree. C. for a predetermined time t.sub.4
until the above processing time passes, so as to complete the
nitriding of the metal ring W.
[0094] Thereafter, when the above predetermined time t.sub.4 has
passed and the above nitriding processing has completed, an door
located between the nitriding chamber 13 and the cooling chamber
14, which is not shown in the figure, is opened, and the metal ring
W is transferred into the cooling chamber 14 by the above
transferring means. After the metal ring W is accommodated in the
cooling chamber 14, the door located between the nitriding chamber
13 and the cooling chamber 14 is closed, followed by a cooling
treatment.
[0095] The cooling treatment is carried out using a nitrogen gas
introduced from the nitrogen gas source 16 through the nitrogen gas
pipe 20 and the nitrogen gas branch pipe 20d, and air introduced
from the air source 19 through the air pipe 23 and the air branch
pipe 23b. As a result, the inside of the cooling chamber 14 is
cooled to room temperature during a predetermined time t.sub.5. The
above nitrogen gas and air are exhausted to the atmosphere
releasing unit 24 through the exhaust pipe 37.
[0096] When the inside of the cooling chamber 14 is cooled to room
temperature, an door located between the cooling chamber 14 and the
second replacement chamber 15, which is not shown in the figure, is
opened, and the metal ring W is transferred into the second
replacement chamber 15 by the above transferring means. After the
metal ring W is accommodated in the second replacement chamber 15,
the door located between the cooling chamber 14 and the second
replacement chamber 15 is closed, and the atmosphere filled in the
second replacement chamber 15 is replaced by a nitrogen
atmosphere.
[0097] The replacement of the atmosphere filled in the second
replacement chamber 15 is carried out by actuating the high-vacuum
pump 39 and exhausting the atmosphere filled in the second
replacement chamber 15 to the atmosphere releasing unit 24 through
the exhaust pipe 38, and at the same time, introducing nitrogen
therein from the nitrogen gas source 16 through the nitrogen gas
pipe 20 and the nitrogen gas branch pipe 20e. When the oxygen level
detected by the oxygen sensor 40 becomes less than a certain value,
it is determined that the atmosphere in the second replacement
chamber 15 is replaced by a nitrogen gas, and the action of the
high-vacuum pump 39 is terminated.
[0098] When the atmosphere in the second replacement chamber 15 is
replaced by a nitrogen atmosphere, a carrying-out door not shown in
the figure is opened, and the metal ring W is carried-away from the
nitriding apparatus 6 by the above transferring means.
[0099] In the nitriding apparatus 6 in the present embodiment, the
operations of the above transferring means, the doors, the vacuum
pumps 26, 30, 34 and 39, the nitrogen gas source 16, the halogen
compound gas source 17, the ammonia gas source 18, the air source
19, and others may be controlled manually, or they may be
controlled by controlling means such as a computer comprising CPU,
ROM, RAM, etc. In the use of a computer, the operations may also be
controlled by the detection values of the oxygen sensors 27, 31, 35
and 40 and that of the ammonia analyzer 36.
[0100] In the present embodiment, the halogen compound gas source
17 is established to introduce a halogen compound gas into the
preheating chamber 12, but it may also be possible not to introduce
the halogen compound gas into the preheating chamber 12.
[0101] Moreover, in the present embodiment, the air source 19 is
established to introduce air into the nitriding chamber 13 and to
form the first mixed gas. However, it may be possible to introduce
other types of oxygen-containing gas instead of air, or it may also
be possible to introduce a pure oxygen gas.
[0102] The atmosphere in the preheating chamber 12 contains a
halogen compound and the atmosphere in the nitriding chamber 13
contains an ammonia gas. Accordingly, if they are exhausted as are,
they are considered to cause air pollution. However, in the
nitriding apparatus 6 in the present embodiment, both the
atmosphere in the preheating chamber 12 and the atmosphere in the
nitriding chamber 13 are burned in the exhaust gas combustion
device 28, and they are rendered harmless before releasing to the
atmosphere releasing unit 24, and thus, they can be prevented from
causing air pollution.
INDUSTRIAL APPLICABILITY
[0103] The present invention can be used for the nitriding
processing of a metal ring made of maraging steel, which is used
for a belt for continuously variable transmission and the like.
* * * * *