U.S. patent application number 14/428823 was filed with the patent office on 2015-08-27 for method of manufacturing machine component.
This patent application is currently assigned to NTN CORPORATION. The applicant listed for this patent is Takumi FUJITA, Kazuhiro YAGITA. Invention is credited to Takumi Fujita, Kazuhiro Yagita.
Application Number | 20150240342 14/428823 |
Document ID | / |
Family ID | 50341315 |
Filed Date | 2015-08-27 |
United States Patent
Application |
20150240342 |
Kind Code |
A1 |
Fujita; Takumi ; et
al. |
August 27, 2015 |
METHOD OF MANUFACTURING MACHINE COMPONENT
Abstract
A method of manufacturing a machine component includes the steps
of preparing a member made of steel containing 0.1 mass % or more
of vanadium, forming a film containing vanadium at a surface of the
member, and forming a nitrogen-enriched layer by heating the member
having the film formed in a heat treatment gas atmosphere
containing nitrogen gas and absent of ammonia gas. In the step of
forming a film, the member is heated to and oxidized in a
temperature range not lower than 500.degree. C. and lower than
A.sub.1 transformation point of steel.
Inventors: |
Fujita; Takumi; (Kuwana-shi,
JP) ; Yagita; Kazuhiro; (Kuwana-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FUJITA; Takumi
YAGITA; Kazuhiro |
Mie
Mie |
|
JP
JP |
|
|
Assignee: |
NTN CORPORATION
Osaka
JP
|
Family ID: |
50341315 |
Appl. No.: |
14/428823 |
Filed: |
September 12, 2013 |
PCT Filed: |
September 12, 2013 |
PCT NO: |
PCT/JP2013/074671 |
371 Date: |
March 17, 2015 |
Current U.S.
Class: |
148/218 |
Current CPC
Class: |
C22C 38/24 20130101;
C23C 8/32 20130101; C22C 38/00 20130101; C23C 8/34 20130101; C21D
9/40 20130101; C23F 17/00 20130101; C22C 38/02 20130101; C21D 1/06
20130101; C22C 38/12 20130101; C21D 1/74 20130101; C21D 1/18
20130101; C23C 8/80 20130101; C21D 9/36 20130101; C23C 8/02
20130101; C22C 38/04 20130101 |
International
Class: |
C23C 8/32 20060101
C23C008/32; C21D 1/18 20060101 C21D001/18; C22C 38/02 20060101
C22C038/02; C21D 9/36 20060101 C21D009/36; C22C 38/24 20060101
C22C038/24; C22C 38/04 20060101 C22C038/04; C23F 17/00 20060101
C23F017/00; C21D 9/40 20060101 C21D009/40 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 19, 2012 |
JP |
2012-205627 |
Claims
1. A method of manufacturing a machine component comprising the
steps of: preparing a member made of steel; forming a film
containing vanadium at a surface of said member; and forming a
nitrogen-enriched layer by heating said member having said film
formed in an atmosphere of heat treatment gas containing nitrogen
gas and absent of ammonia gas, in said step of preparing a member,
a member made of steel containing 0.1 mass % or more of vanadium
being prepared, and in said step of forming a film, said member
being heated to and oxidized in a temperature range not lower than
500.degree. C. and lower than A.sub.1 transformation point of said
steel.
2. The method of manufacturing a machine component according to
claim 1, wherein said heat treatment gas includes endothermic
converted gas.
3. The method of manufacturing a machine component according to
claim 1, wherein said heat treatment gas is a gas mixture of the
nitrogen gas and reducing gas.
4. The method of manufacturing a machine component according to
claim 1, wherein said heat treatment gas includes the nitrogen gas
and has an oxygen partial pressure less than or equal to 10.sup.-16
Pa.
5. The method of manufacturing a machine component according to
claim 4, wherein said heat treatment gas includes reducing gas so
that it has the oxygen partial pressure less than or equal to
10.sup.-16 Pa.
6. The method of manufacturing a machine component according to
claim 5, wherein said reducing gas is hydrogen gas.
7. The method of manufacturing a machine component according to
claim 1, wherein said step of forming a nitrogen-enriched layer is
performed such that said member heated to said temperature range is
not cooled to a room temperature in said step of forming a
film.
8. The method of manufacturing a machine component according to
claim 1, wherein in said step of forming a film, said member is
heated in a heat treatment chamber of an oxidative atmosphere, and
in said step of forming a nitrogen-enriched layer, an atmosphere in
said heat treatment chamber is replaced with said heat treatment
gas and then said member is heated in said heat treatment chamber
to thereby form the nitrogen-enriched layer.
9. The method of manufacturing a machine component according to
claim 1, further comprising the step of quench-hardening said
member by cooling said member having a nitrogen-enriched layer
formed from a temperature greater than or equal to the A.sub.1
transformation point down to a temperature less than or equal to
M.sub.S point, wherein in said step of forming a film, said film is
formed as said member is oxidized in an oxidation apparatus, in
said step of forming a nitrogen-enriched layer, said member having
said film formed is conveyed by a conveyance apparatus into a
nitrogen-enriched layer formation apparatus connected to said
oxidation apparatus with said conveyance apparatus being interposed
and then said nitrogen-enriched layer is formed in said
nitrogen-enriched layer formation apparatus, and in said step of
quench-hardening said member, said member is quench-hardened in a
quenching apparatus connected to said nitrogen-enriched layer
formation apparatus.
10. The method of manufacturing a machine component according to
claim 1, wherein said machine component is a component constituting
a rolling bearing.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method of manufacturing a
machine component, more particularly, a method of manufacturing a
machine component having a nitrogen-enriched layer at a surface
layer.
BACKGROUND ART
[0002] For the purpose of improving the fatigue strength and/or
wear resistance of a machine component, a nitrogen-enriched layer
having a high nitrogen concentration as compared to the interior
may be formed at the surface layer of the machine component by
means of carbonitriding or the like. In a general carbonitriding
process, atmosphere gas is often used, which is based on producing
carrier gas by mixing propane, butane or city gas with air at a
high temperature greater than or equal to 1000.degree. C.
(endothermic converted gas; hereinafter, referred to RX gas),
having a small amount of propane, butane, and ammonia added. By
heating a workpiece in this atmosphere gas, a nitrogen-enriched
layer is formed at the surface layer of the workpiece. During a
carbonitriding process using RX gas as the carrier gas, a nitriding
response occurs by undecomposed ammonia (for example, refer to
"Improvement of Wear Resistance by Heat Treatment for Carburizing
Steel" by Nobuyuki Mouri et al., NTN TECHNICAL REVIEW, 2008, No.
76, pp. 17-22 (NPD 1)).
CITATION LIST
Non Patent Document
[0003] NPD 1: "Improvement of Wear Resistance by Heat Treatment for
Carburizing Steel" by Nobuyuki Mouri et al., NTN TECHNICAL REVIEW,
2008, No. 76, pp. 17-22
SUMMARY OF INVENTION
Technical Problem
[0004] Decomposition of ammonia gas generally proceeds as the
temperature becomes higher. Therefore, the nitriding process by
undecomposed ammonia is not often executed at a temperature region
not lower than 900.degree. C. As a result, it was difficult to
increase the treatment temperature to shorten the carbonitriding
time even in the case of treating a product that requires a thick
nitride layer. There was a problem that the processing time is
lengthened. The carbonitriding process using ammonia gas also had
the problem that the facility maintenance management cost is
increased due to the requirement of installing a facility to
introduce ammonia gas into the heat treatment furnace and rapid
consumption of components employed in the heat treatment furnace
(for example, the basket for product transportation).
[0005] The present invention was made to solve the problems as
described above, and an object thereof is to provide a machine
component manufacturing method allowing a machine component having
a nitrogen enriched layer at a surface layer to be manufactured by
rapid heat treatment not using ammonia gas.
Solution to Problem
[0006] A method of manufacturing a machine component according to
the present invention includes the steps of preparing a member made
of steel, forming a film containing vanadium at a surface of the
member, and forming a nitrogen-enriched layer by heating the member
having the film formed in an atmosphere of heat treatment gas
containing nitrogen gas and absent of ammonia gas. In the step of
preparing a member, a member made of steel containing 0.1 mass % or
more of vanadium is prepared. In the step of forming a film, the
member is heated to and oxidized in a temperature range not lower
than 500.degree. C. and lower than A.sub.1 transformation point of
the steel.
[0007] During various studies in association with heat treatment of
steel, the inventors found out that, by forming a film containing
vanadium at the surface of a member made of steel, followed by
heating in an atmosphere including nitrogen gas, a
nitrogen-enriched layer is formed at the surface layer of the
member even if the atmosphere is absent of ammonia gas, thus
conceiving of the present invention. In the method of manufacturing
a machine component in the present invention, a member made of
steel, having a film containing vanadium formed at the surface, is
heated in an atmosphere including nitrogen gas and absent of
ammonia gas, leading to formation of a nitrogen-enriched layer at
the surface layer of the machine component. Since the formation of
a nitrogen-enriched layer is not advanced by undecomposed ammonia
in the present manufacturing method, heat treatment at a higher
temperature is allowed. Accordingly, the period of time for heat
treatment can be shortened. Moreover, since ammonia is not used in
the manufacturing method, consumption of the components employed in
the heat treatment furnace can be suppressed to reduce the facility
maintenance management cost. According to the machine component
manufacturing method of the present invention, a machine component
having a nitrogen-enriched layer at a surface layer can be
manufactured by rapid heat treatment not using ammonia gas.
[0008] By adopting steel containing 0.1 mass % or more of vanadium
as steel making up a machine component and subjecting the steel to
oxidation, a film containing vanadium can readily be formed. Here,
by performing oxidation in a temperature range lower than the
A.sub.1 transformation point of steel, phase transformation does
not occur during oxidation and change in dimension or deformation
due to heat treatment can be suppressed. In addition, by performing
oxidation in a temperature range lower than the A.sub.1
transformation point of steel, a mother phase of steel is
maintained in a ferrite state in which a solid solubility limit of
carbon is low, and occurrence of decarburization can be
suppressed.
[0009] Heat treatment gas absent of ammonia gas implies not
including ammonia gas substantially, and does not exclude the
mixture of ammonia gas at the impurity level.
[0010] In the step of forming a film in the method of manufacturing
a machine component set forth above, the member may be forged.
[0011] When the manufacturing process of a machine component
includes a forging step, a film containing vanadium can be formed
efficiently by subjecting the machine component to oxidation in the
forging step.
[0012] In the method of manufacturing a machine component set forth
above, the heat treatment gas may include endothermic converted
gas. Accordingly, formation of a nitrogen-enriched layer can be
achieved while readily adjusting the carbon potential in the
atmosphere.
[0013] In the method of manufacturing a machine component set forth
above, the heat treatment gas may be a gas mixture of the nitrogen
gas and reducing gas.
[0014] Thus, a nitrogen-enriched layer can be formed with reducing
heat treatment gas containing nitrogen which is inexpensive and
readily available as a nitrogen supply source. Consequently, cost
for heat treatment can be reduced. For example, hydrogen gas,
methane gas, propane gas, butane gas, or carbon monoxide gas can be
adopted for the reducing gas.
[0015] In the method of manufacturing a machine component set forth
above, the heat treatment gas may include the nitrogen gas and have
an oxygen partial pressure less than or equal to 10.sup.-16 Pa.
[0016] Thus, heat treatment gas containing nitrogen which is
inexpensive and readily available as a nitrogen supply source and
having oxidizing capability suppressed to a low level can be
employed. Consequently, cost for heat treatment can be reduced.
[0017] In the method of manufacturing a machine component set forth
above, the heat treatment gas may include reducing gas so that it
has the oxygen partial pressure less than or equal to 10.sup.-16
Pa. By adopting the heat treatment gas containing the reducing gas,
the oxygen partial pressure can readily be lowered to 10.sup.-16 Pa
or less.
[0018] In the method of manufacturing a machine component set forth
above, the reducing gas may be hydrogen gas. The hydrogen gas which
is readily available is suitable as the reducing gas.
[0019] The method of manufacturing a machine component set forth
above may further include the step of quench-hardening the member
by cooling the member having a nitrogen-enriched layer formed from
a temperature greater than or equal to A.sub.1 transformation point
to a temperature less than or equal to M.sub.S point. Accordingly,
a machine component of high durability, having a nitrogen-enriched
layer formed and quench-hardened, can be readily manufactured.
[0020] In the method of manufacturing a machine component set forth
above, the step of forming a nitrogen-enriched layer may be
performed such that the member heated to the temperature range is
not cooled to a room temperature in the step of forming a film. By
doing so, energy required for heat treatment can be lowered and a
time period for heat treatment can be shortened.
[0021] In the method of manufacturing a machine component set forth
above, in the step of forming a film, the member may be heated in a
heat treatment chamber of an oxidative atmosphere, and in the step
of forming a nitrogen-enriched layer, an atmosphere in the heat
treatment chamber may be replaced with the heat treatment gas and
then the member may be heated in the heat treatment chamber to
thereby form the nitrogen-enriched layer. By doing so, a
nitrogen-enriched layer can efficiently be formed on a machine
component with the use of a batch furnace.
[0022] The method of manufacturing a machine component set forth
above may further include the step of quench-hardening the member
by cooling the member having a nitrogen-enriched layer formed from
a temperature greater than or equal to the A.sub.1 transformation
point down to a temperature less than or equal to M.sub.S point. In
the step of forming a film, the film may be formed as the member is
oxidized in an oxidation apparatus. In the step of forming a
nitrogen-enriched layer, the member having the film formed may be
conveyed by a conveyance apparatus into a nitrogen-enriched layer
formation apparatus connected to the oxidation apparatus with the
conveyance apparatus being interposed, and then the
nitrogen-enriched layer may be formed in the nitrogen-enriched
layer formation apparatus. In the step of quench-hardening the
member, the member may be quench-hardened in a quenching apparatus
connected to the nitrogen-enriched layer formation apparatus. By
doing so, a nitrogen-enriched layer can efficiently be formed on a
machine component with the use of a continuous furnace and the
machine component can be quench-hardened.
[0023] In the method of manufacturing a machine component set forth
above, the machine component may be a component constituting a
rolling bearing.
[0024] A component such as a bearing ring and a rolling element
constituting a rolling bearing is often required to have high
fatigue strength and wear resistance. Therefore, the method of
manufacturing a machine component of the present invention in which
a nitrogen-enriched layer is formed is suitable for a method of
manufacturing a component constituting a rolling bearing.
Advantageous Effects of Invention
[0025] As is clear from the description above, according to the
method of manufacturing a machine component in the present
invention, a machine component having a nitrogen-enriched layer at
a surface layer can be manufactured by rapid heat treatment not
using ammonia gas.
BRIEF DESCRIPTION OF DRAWINGS
[0026] FIG. 1 is a flowchart showing overview of a method of
manufacturing a machine component in a first embodiment.
[0027] FIG. 2 is a schematic diagram for illustrating one example
of the method of manufacturing a machine component.
[0028] FIG. 3 is a schematic diagram for illustrating another
example of the method of manufacturing a machine component.
[0029] FIG. 4 is a flowchart showing overview of a method of
manufacturing a machine component in a second embodiment.
[0030] FIG. 5 represents concentration distribution of nitrogen in
a nitrogen-enriched layer when a vanadium-containing film is formed
at different oxidation temperatures.
[0031] FIG. 6 represents concentration distribution of nitrogen in
a nitrogen-enriched layer formed without cooling to a room
temperature after oxidation.
DESCRIPTION OF EMBODIMENTS
[0032] Embodiments of the present invention will be described
hereinafter with reference to the drawings. In the drawings below,
the same or corresponding elements have the same reference
characters allotted, and description thereof will not be
repeated.
First Embodiment
[0033] A first embodiment that is one embodiment of the present
invention will initially be described. Referring to FIG. 1, in a
method of manufacturing a machine component according to the first
embodiment, a steel member preparation step is performed as a step
(S10). At this step (S10), a steel member that is a member made of
steel and formed in substantially the shape of a machine component
is prepared. Specifically, for example, a steel material of AMS2315
that is steel containing vanadium greater than or equal to 0.1 mass
% or a steel material having such composition that 0.1 mass % or
more of vanadium has been added to SUJ2 complying with JIS is
prepared and subjected to working such as forging, turning, and the
like to produce a steel member.
[0034] Then, an oxidation step is performed as a step S20. At this
step (S20), the steel member prepared at the step (S10) is
subjected to oxidation. Specifically, the steel member is heated at
a temperature range greater than or equal to 500.degree. C. and
lower than A.sub.1 transformation point of steel making up the
steel member in an oxidative atmosphere such as in the air, whereby
the surface layer of the steel member is oxidized. At this stage,
the reaction of vanadium in the steel with carbon in the steel and
nitrogen in the atmosphere causes a film containing vanadium to be
formed at the surface of the steel member. Specifically, this film
is a V (vanadium)-N (nitrogen) film, a V--C (carbon) film, a
V--C--N film, or the like.
[0035] Then, a carbonitriding step is performed as a step (S30). At
this step (S30), the steel member subjected to oxidation at the
step (S20) is subjected to carbonitriding. Specifically, in an
atmosphere adjusted to the desired carbon potential by adding
propane gas or the like as enrich gas into RX gas that is
endothermic converted gas obtained by mixing propane gas and air in
a reforming furnace and heating to a temperature greater than or
equal to 1000.degree. C. under the presence of a catalyst, the
steel member is heated at a temperature range greater than or equal
to A.sub.1 transformation point. At this stage, ammonia gas is not
added to the atmosphere. Accordingly, an amount of carbon at the
surface layer of the steel member attains to a value corresponding
to carbon potential in the atmosphere. Since the surface of the
steel member has a film containing vanadium formed at the step
(S20) and the nitrogen gas in the air is included in the RX gas,
nitrogen invades the surface layer of the steel member. As a
result, the steel member is carbonitrided, forming a
nitrogen-enriched layer at the surface layer of the steel
member.
[0036] Then, a quench-hardening step is performed as a step (S40).
At this step (S40), the steel member subjected to carbonitriding at
the step (S30) is quench-hardened. Specifically, the steel member
subjected to carbonitriding at the temperature range greater than
or equal to A.sub.1 transformation point at the step (S30) is
quench-hardened by being cooled down to the temperature range less
than or equal to M.sub.S point from the temperature range greater
than or equal to A.sub.1 transformation point. Accordingly, the
entire steel member including the nitrogen-enriched layer is
quench-hardened, thus providing high fatigue strength and wear
resistance to the steel member.
[0037] Then, a tempering step is performed as a step (S50). At this
step (S50), the steel member subjected to quench-hardening at the
step (S40) is tempered. Specifically, at the step (S50), the steel
member subjected to quench-hardening at the step (S40) is heated to
a temperature less than or equal to A.sub.1 transformation point,
and then cooled for the tempering process.
[0038] Then, a finishing step is performed as a step (S60). At this
step (S60), the steel member obtained by performing the steps (S10)
to (S50) is subjected to a finishing work to complete a machine
component such as a bearing component. Specifically, at the step
(S60), the tempered steel member is polished and the like for the
completion of a machine component. By the process set forth above,
the method of manufacturing a machine component of the present
embodiment is completed to produce a completed machine
component.
[0039] In the method of manufacturing a machine component of the
present embodiment, a steel member having a film containing
vanadium formed at the surface is heated in an atmosphere
containing nitrogen gas and absent of ammonia gas to manufacture a
machine component having a nitrogen-enriched layer. In the method
of manufacturing a machine component of the present embodiment, the
formation of a nitrogen-enriched layer is not advanced by
undecomposed ammonia. Therefore, heat treatment at high temperature
is allowed without having to take into account the decomposition of
ammonia. As a result, in the method of manufacturing a machine
component of the present embodiment, the process of forming a
nitrogen-enriched layer is performed at high temperature, allowing
the period of time for the heat treatment to be shortened.
Furthermore, since ammonia is not used in the manufacturing method,
consumption of components employed in the heat treatment furnace is
suppressed to allow the facility maintenance management cost to be
reduced. Thus, according to the method of manufacturing a machine
component of the present embodiment, a machine component having a
nitrogen-enriched layer at the surface layer can be manufactured by
rapid heat treatment not using ammonia gas.
[0040] By preparing a steel member made of steel containing 0.1
mass % or more of vanadium in the step (S10) and subjecting the
steel member to oxidation in the step (S20), a film containing
vanadium can readily be formed. Here, by performing oxidation in a
temperature range lower than the A.sub.1 transformation point,
phase transformation does not occur during oxidation and change in
dimension or deformation due to heat treatment can be suppressed.
In addition, by performing oxidation in a temperature range lower
than the A.sub.1 transformation point of steel, a mother phase of
steel is maintained in a ferrite state in which a solid solubility
limit of carbon is low, and occurrence of decarburization can be
suppressed. By performing oxidation at 500.degree. C. or higher, a
film containing vanadium can efficiently be formed. In order to
further efficiently form a film containing vanadium, a temperature
for oxidation in the step (S20) may be set to 600.degree. C. or
higher or to 650.degree. C. or higher.
[0041] Here, the heat treatment gas adopted in the step (S30) may
be a gas mixture of nitrogen gas and reducing gas. Thus, a
nitrogen-enriched layer can be formed with reducing heat treatment
gas containing nitrogen which is inexpensive and readily available
as a nitrogen supply source. Consequently, cost for heat treatment
can be reduced.
[0042] The heat treatment gas adopted in the step (S30) may contain
nitrogen gas and may have an oxygen partial pressure less than or
equal to 10.sup.-16 Pa. The heat treatment gas may have an oxygen
partial pressure less than or equal to 10.sup.-16 Pa by containing
reducing gas. For example, hydrogen gas can be adopted as the
reducing gas. Thus, heat treatment gas containing nitrogen which is
inexpensive and readily available as a nitrogen supply source and
having oxidizing capability suppressed to a low level can be
employed. Consequently, cost for heat treatment can be reduced.
[0043] One example of a specific procedure for performing the steps
(S20) to (S40) will now be described with reference to FIG. 2.
Referring to FIG. 2, a batch furnace 1 includes a heat treatment
chamber 11, a carrier portion 12 installed on a bottom wall of heat
treatment chamber 11, and an inlet port 13 and an exhaust port 14
disposed in a wall surface of the heat treatment chamber. Inlet
port 13 can be connected to a gas supply source (not shown) and an
atmosphere gas can be supplied into heat treatment chamber 11
through inlet port 13 as the inlet port is connected to a desired
gas supply source. Exhaust port 14 can be connected to an exhaust
apparatus (not shown) and the atmosphere gas in a heat treatment
furnace can be exhausted through exhaust port 14. The steps (S20)
to (S40) can be performed as below, with the use of this batch
furnace 1.
[0044] Initially, at the step (S20), a steel member 90 prepared at
the step (S10) is arranged on carrier portion 12 in heat treatment
chamber 11. Then, the interior in heat treatment chamber 11 is
adjusted to an oxidative atmosphere. Here, gas in heat treatment
chamber 11 may be discharged through exhaust port 14 and then
oxidative gas may be supplied through inlet port 13, so that the
interior in heat treatment chamber 11 is adjusted to an oxidative
atmosphere, or the interior in heat treatment chamber 11 may be
adjusted to an oxidative atmosphere as inlet port 13 and exhaust
port 14 are opened into the air. Then, in heat treatment chamber 11
adjusted to the oxidative atmosphere, steel member 90 is heated to
and oxidized in a temperature range not lower than 500.degree. C.
and lower than the A.sub.1 transformation point of steel making up
steel member 90. Thus, a film containing vanadium is formed in a
region including a surface of steel member 90.
[0045] As the step (S20) is completed, the step (S30) is performed
in succession. At the step (S30), initially, the atmosphere in heat
treatment chamber 11 is replaced with heat treatment gas.
Specifically, the atmosphere gas in heat treatment chamber 11 is
exhausted through exhaust port 14 and heat treatment gas (for
example, a gas mixture of nitrogen gas and reducing gas) is
supplied through inlet port 13, so that the interior in heat
treatment chamber 11 is replaced with the heat treatment gas. Then,
steel member 90 is heated in heat treatment chamber 11, for
example, to a temperature range not lower than 750.degree. C. and
not higher than 1000.degree. C. and preferably to a temperature
range not lower than 850.degree. C. and not higher than 950.degree.
C., which are temperature ranges not lower than the A.sub.1
transformation point, so that a nitrogen-enriched layer is formed
at the surface layer of steel member 90. Here, after the step (S20)
is completed and before the step (S30) is performed, steel member
90 may be cooled to a room temperature. After the step (S20) is
completed, however, the step (S30) is performed successively
without cooling steel member 90 to a room temperature, so that
energy required for heat treatment can be lowered and a time period
for heat treatment can be shortened.
[0046] When the step (S30) is completed, the step (S40) is
performed in succession. At the step (S40), steel member 90 having
the nitrogen-enriched layer formed is taken out of batch furnace 1
and quench-hardened, for example, by being immersed in an oil bath.
Through the procedure above, the steps (S20) to (S40) can
efficiently be performed with the use of batch furnace 1.
[0047] Alternatively, the steps (S20) to (S40) above may be
performed with the use of a continuous furnace as below. Referring
to FIG. 3, a continuous furnace 2 includes an oxidation furnace 21
serving as an oxidation apparatus, a nitriding furnace 22 serving
as a nitrogen-enriched layer formation apparatus connected to
oxidation furnace 21 with conveyors 24 and 25 serving as a
conveyance apparatus being interposed, and a quenching oil bath 23
which serves as a quenching apparatus connected to nitriding
furnace 22 and holds a quenching oil. In quenching oil bath 23, a
conveyor 26 carrying out a workpiece in quenching oil bath 23 is
disposed. The steps (S20) to (S40) can be performed as below, with
the use of this continuous furnace 2.
[0048] Initially, at the step (S20), steel member 90 prepared at
the step (S10) is placed on conveyor 24. Thus, steel member 90 is
conveyed by conveyor 24 and enters oxidation furnace 21. Since the
interior in oxidation furnace 21 opens, for example, into the air,
it is set to an air atmosphere. In oxidation furnace 21, steel
member 90 is heated to and oxidized in a temperature range not
lower than 500.degree. C. and lower than the A.sub.1 transformation
point of steel making up steel member 90. Thus, a film containing
vanadium is formed in a region including a surface of steel member
90.
[0049] Then, at the step (S30), steel member 90 is conveyed along
an arrow .alpha. on conveyors 24 and 25 and enters nitriding
furnace 22. Here, steel member 90 may enter nitriding furnace 22
without being cooled to a room temperature. The interior in
nitriding furnace 22 is adjusted to an atmosphere of a gas mixture
of nitrogen gas and reducing gas, such as an atmosphere of nitrogen
gas and hydrogen gas as mixed. Then, steel member 90 is heated in
nitriding furnace 22 to a temperature range not lower than the
A.sub.1 transformation point. Thus, a nitrogen-enriched layer is
formed at the surface layer of steel member 90.
[0050] Then, steel member 90 having the nitrogen-enriched layer
formed is conveyed on conveyor 25, so that it falls into quenching
oil bath 23 along an arrow .beta.. Thus, steel member 90 is rapidly
cooled and quench-hardened. Then, quench-hardened steel member 90
is carried out of quenching oil bath 23 on conveyor 26. Through the
procedure above, the steps (S20) to (S40) with the use of
continuous furnace 2 are completed. By thus using continuous
furnace 2, the steps (S20) to (S40) can efficiently be performed
and efficiency in production of machine components can be
improved.
Second Embodiment
[0051] A second embodiment that is another embodiment of the
present invention will now be described r with reference to FIG. 4.
The method of manufacturing a machine component according to the
second embodiment is carried out in a manner basically similar to
that of the first embodiment. However, the method of manufacturing
a machine component of the second embodiment differs from that of
the first embodiment in including a hot forging step.
[0052] In the method of manufacturing a machine component according
to the second embodiment, steel containing vanadium greater than or
equal to 0.1 mass % is prepared at the step (S10), likewise with
the first embodiment. A steel member is produced by forming to a
shape that allows hot forging in a step (S21) that will be
described below.
[0053] Next, a hot forging step is performed as a step (S21). At
this step (S21), the steel member is hot forged. Specifically, the
steel member is shaped by hot forging in the air, for example. At
this stage, the surface layer of the steel member is oxidized by
the oxygen in the air. As a result, the reaction of vanadium in the
steel with the carbon in the steel and nitrogen in the atmosphere
causes formation of a film containing vanadium at the surface of
the steel member, specifically a V--N film, a V--C film, a V--C--N
film, or the like.
[0054] Then, the step (S20) is skipped, and steps (S30) to (S60)
are performed, likewise with the first embodiment, to complete a
machine component.
[0055] In the method of manufacturing a machine component of the
present embodiment, oxidation of the steel member is performed
taking advantage of the hot forging step in the manufacturing
process. Therefore, the method of manufacturing a machine component
of the present invention can be carried out while suppressing
increase in the manufacturing steps.
EXAMPLES
Example 1
[0056] Experiments were carried out to confirm that, by forming a
film containing vanadium through oxidation in a temperature range
lower than A.sub.1 transformation point, formation of a
nitrogen-enriched layer is allowed by subsequent heating in a heat
treatment gas atmosphere containing nitrogen gas and absent of
ammonia gas. A procedure in the experiments is as follows.
[0057] Steel composed of 1.00 mass % of carbon, 0.31 mass % of
silicon, 0.46 mass % of manganese, 1.51 mass % of chromium, and
1.02 mass % of vanadium as well as remainder iron and impurities
(such steel that 1.02 mass % of vanadium had been added to SUJ2
complying with JIS) was prepared and worked to a prescribed shape.
The obtained test piece was subjected to oxidation for 10 hours as
being heated to 700.degree. C. in the air, which was a temperature
lower than the A.sub.1 transformation point. For comparison, a
similar test piece was subjected to oxidation for 1.5 hour as being
heated to 950.degree. C. in the air, which was a temperature not
lower than the A.sub.1 transformation point. These test pieces were
heated to 950.degree. C. in a gas mixture containing 50 volume % of
nitrogen gas and 50 volume % of hydrogen gas and held for 12 hours.
Nitrogen concentration distribution at the surface layer of the
obtained test pieces was analyzed with an electron probe micro
analyser (EPMA). FIG. 5 shows results of analysis. In FIG. 5, the
abscissa represents a depth (a distance) from a surface and the
ordinate represents nitrogen concentration. In FIG. 5, a thin line
corresponds to a sample subjected to oxidation at 950.degree. C.
and a bold line corresponds to a sample subjected to oxidation at
700.degree. C.
[0058] Referring to FIG. 5, even when oxidation is performed at
700.degree. C. which is a temperature lower than the A.sub.1
transformation point, sufficient nitrogen concentration
distribution comparable to that at the time when oxidation is
performed at 950.degree. C. which is a temperature not lower than
the A.sub.1 transformation point is obtained. Thus, by performing
oxidation at a temperature lower than the A.sub.1 transformation
point, change in dimension and deformation due to heat treatment of
a machine component as well as occurrence of decarburization can be
suppressed while a nitrogen-enriched layer having appropriate
nitrogen concentration distribution is formed.
Example 2
[0059] An experiment for confirming whether or not cooling to a
room temperature is necessary after a film containing vanadium was
formed through oxidation and before nitriding was performed was
conducted.
[0060] Initially, a test piece was produced from a steel material
(such a steel material that 1.02 mass % of vanadium had been added
to SUJ2 complying with JIS) similar to that in Example 1. This test
piece was subjected to oxidation as being heated to 700.degree. C.
in the air, which was a temperature lower than the A.sub.1
transformation point, and thereafter, successively without cooling,
the test piece was heated to 950.degree. C. and held for 6 hours in
a gas mixture atmosphere containing 50 volume % of nitrogen gas and
50 volume % of hydrogen gas. Thereafter, nitrogen concentration
distribution at the surface layer of the test piece was examined
with the EPMA as in Example 1. FIG. 6 shows results of examination.
In FIG. 6, the abscissa represents a depth (a distance) from a
surface and the ordinate represents nitrogen concentration.
[0061] Referring to FIG. 6, even when nitriding is successively
performed with a cooling step of cooling steel after oxidation
being intentionally omitted, a nitrogen-enriched layer having
sufficient nitrogen concentration distribution is obtained. Thus,
by performing a nitriding step without performing the cooling step
after oxidation, energy required for heat treatment can be lowered
and a time period for heat treatment can be shortened. Such a heat
treatment process can be performed, for example, with the use of
the batch furnace or the continuous furnace described in the
embodiments above.
[0062] It should be understood that the embodiments and examples
disclosed herein are illustrative and non-restrictive in every
respect. The scope of the present invention is defined by the terms
of the claims, rather than the description above, and is intended
to include any modification within the scope and meaning equivalent
to the terms of the claims.
INDUSTRIAL APPLICABILITY
[0063] The method of manufacturing a machine component of the
present invention can particularly be applied advantageously to the
method of manufacturing a machine component having a
nitrogen-enriched layer at the surface layer.
REFERENCE SIGNS LIST
[0064] 1 batch furnace; 2 continuous furnace; 11 heat treatment
chamber; 12 carrier portion; 13 inlet port; 14 exhaust port; 21
oxidation furnace; 22 nitriding furnace; 23 quenching oil bath; 24,
25, 26 conveyor; and 90 steel member.
* * * * *