U.S. patent application number 14/785453 was filed with the patent office on 2016-03-17 for surface modification apparatus for alloy steel component, surface modification method for alloy steel component, and method for manufacturing alloy steel component.
The applicant listed for this patent is KABUSHIKI KAISHA F.C.C.. Invention is credited to Satoshi KAWAGASHIRA, Keisuke SUZUKI, Noriyuki UEMATSU.
Application Number | 20160076130 14/785453 |
Document ID | / |
Family ID | 51791533 |
Filed Date | 2016-03-17 |
United States Patent
Application |
20160076130 |
Kind Code |
A1 |
UEMATSU; Noriyuki ; et
al. |
March 17, 2016 |
SURFACE MODIFICATION APPARATUS FOR ALLOY STEEL COMPONENT, SURFACE
MODIFICATION METHOD FOR ALLOY STEEL COMPONENT, AND METHOD FOR
MANUFACTURING ALLOY STEEL COMPONENT
Abstract
Provided are a surface modification apparatus for an alloy steel
component, a surface modification method for an alloy steel
component, and a method for manufacturing an alloy steel component,
which can provide a deep and homogeneous hard layer for a steel
component with a wide shape or a large amount of such steel
components. A surface modification apparatus 100 includes a process
furnace 101, and the process furnace 101 performs a surface
modification process on an alloy steel component 90 including an
alloy steel material to which at least one kind of nitride
formation element such as chromium, molybdenum, or aluminum is
added. The surface modification apparatus 100 forms a compound
layer on a surface of the alloy steel component 90 by exposing the
alloy steel component 90 for at least 180 minutes to an atmosphere
in the process furnace 101 maintained to have an ammonia gas
concentration of 80% or more and a temperature of 620.degree.
C.
Inventors: |
UEMATSU; Noriyuki;
(Shizuoka, JP) ; SUZUKI; Keisuke; (Shizuoka,
JP) ; KAWAGASHIRA; Satoshi; (Shizuoka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA F.C.C. |
Hamamatsu-shi, Shizuoka |
|
JP |
|
|
Family ID: |
51791533 |
Appl. No.: |
14/785453 |
Filed: |
March 20, 2014 |
PCT Filed: |
March 20, 2014 |
PCT NO: |
PCT/JP2014/057717 |
371 Date: |
October 19, 2015 |
Current U.S.
Class: |
148/230 ;
266/252 |
Current CPC
Class: |
C21D 6/00 20130101; C21D
1/76 20130101; C21D 1/06 20130101; C21D 1/74 20130101; C23C 8/26
20130101 |
International
Class: |
C23C 8/26 20060101
C23C008/26; C21D 1/74 20060101 C21D001/74; C21D 6/00 20060101
C21D006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2013 |
JP |
2013-092633 |
Claims
1. A surface modification apparatus for an alloy steel component,
which performs surface modification on an alloy steel component
including an alloy steel material containing a nitride formation
element, the apparatus comprising a surface modification processing
unit for modifying a surface of the alloy steel component by
exposing the alloy steel component for at least 180 minutes to an
atmosphere with an ammonia gas concentration of 80% or more and a
temperature of 610.degree. C. or more and 630.degree. C. or
less.
2. The surface modification apparatus for an alloy steel component
according to claim 1, wherein the temperature is set to 620.degree.
C. by the surface modification processing unit.
3. The surface modification apparatus for an alloy steel component
according to claim 1, wherein the alloy steel component includes at
least one of a corner portion with an acute shape and a hole
portion with a diameter of 8 mm or less.
4. A surface modification method for an alloy steel component
including an alloy steel material containing a nitride formation
element, the method comprising a surface modification processing
step of modifying a surface of an alloy steel component by exposing
the alloy steel component for at least 180 minutes to an atmosphere
with an ammonia gas concentration of 80% or more and a temperature
of 610.degree. C. or more and 630.degree. C. or less.
5. The surface modification method for an alloy steel component
according to claim 4, wherein the temperature is set to 620.degree.
C. in the surface modification processing step.
6. The surface modification method for an alloy steel component
according to claim 4, wherein the alloy steel component includes at
least one of a corner portion with an acute shape and a hole
portion with a diameter of 8 mm or less.
7. A method for manufacturing an alloy steel component including an
alloy steel material containing a nitride formation element, the
method comprising a surface modification processing step of
modifying a surface of an alloy steel component by exposing the
alloy steel component for at least 180 minutes to the atmosphere
with an ammonia gas concentration of 80% or more and a temperature
of 610.degree. C. or more and 630.degree. C. or less.
8. The surface modification apparatus for an alloy steel component
according to claim 2, wherein the alloy steel component includes at
least one of a corner portion with an acute shape and a hole
portion with a diameter of 8 mm or less.
9. The surface modification method for an alloy steel component
according to claim 5, wherein the alloy steel component includes at
least one of a corner portion with an acute shape and a hole
portion with a diameter of 8 mm or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a surface modification
apparatus for an alloy steel component including an alloy steel
material containing a nitride formation element, a surface
modification method for an alloy steel component, and a method for
manufacturing an alloy steel component.
BACKGROUND ART
[0002] Conventionally, various mechanical components made of alloy
steel included in vehicles such as four-wheeled vehicles and
bicycles are subjected to a surface modification process for
enabling the portion of the component that mechanically slides to
have higher wear and abrasion resistance. For example, Patent
Literature 1 has disclosed the thermal processing method for
improving the wear and abrasion resistance and the shock resistance
by modifying the surface of the steel component through a so-called
ion nitration process (also referred to as "plasma nitration
process").
CITATION LIST
Patent Literature
[0003] PATENT LITERATURE 1: JP-A-09-125225
[0004] In the surface modification method according to Patent
Literature 1, the surface of the steel component is modified by the
ion nitration process. This has led to a problem that it is
difficult to form a surface modification layer (hard layer)
homogeneously and the surface-modifiable component is limited due
to the hollow-cathode effect or the edge effect. Here, the
hollow-cathode effect refers to the phenomenon that, if the alloy
steel component has a hole portion with small diameter, discharge
within this hole is so difficult that the surface modification
layer becomes inhomogeneous. The edge effect refers to the
phenomenon that, if the alloy steel component has a corner portion
with an acute, right, or obtuse angle, discharge concentrates on
this corner portion to make the surface modification layer
inhomogeneous. In the ion nitration process, the steel components
to be processed need to be disposed apart from each other in a
process furnace. This has led to another problem that the ion
nitration process is not suitable for the surface modification
process for a large amount of steel components.
[0005] The present invention has been made in view of the above
problems. An object of the present invention is to provide a
surface modification apparatus for an alloy steel component, a
surface modification method for an alloy steel component, and a
method for manufacturing an alloy steel component, which can
provide a deep and homogeneous hard layer for a steel component
with a wide shape or a large amount of such steel components.
SUMMARY OF INVENTION
[0006] An aspect of the present invention for achieving the above
object is a surface modification apparatus for an alloy steel
component, which performs the surface modification on an alloy
steel component including an alloy steel material containing a
nitride formation element. The apparatus includes a surface
modification processing unit for modifying a surface of the alloy
steel component by exposing the alloy steel component for at least
180 minutes to the atmosphere with an ammonia gas concentration of
80% or more and a temperature of 610.degree. C. or more and
630.degree. C. or less. In this case, the nitride formation element
refers to an element which forms a hard nitride due to the
permeation and diffusion of nitrogen, and specifically corresponds
to at least one of chromium, molybdenum, and aluminum. The alloy
steel material containing a nitride formation element is obtained
by adding at least the minimum amount, which is defined by JIS
(Japanese Industrial Standard), of the nitride formation element to
the carbon steel, and specifically 0.3 wt % or more of chromium,
0.08 wt % of molybdenum, and 0.1 wt % of aluminum.
[0007] According to the aspect of the present invention described
above, the surface modification apparatus for an alloy steel
component is structured to include the surface modification
processing unit for exposing the alloy steel component for at least
180 minutes to the atmosphere with an ammonia gas concentration of
80% or more and a temperature of 610.degree. C. or more and
630.degree. C. or less. According to the experiments by the present
inventors, it has been confirmed that the compound layer with a
thickness of 25 .mu.m or more can be formed stably on the surface
of the alloy steel component. In other words, by a surface
modification method for an alloy steel component according to the
present invention, the surface modification process is performed in
an atmosphere where nitrogen generated by the thermal decomposition
of ammonia gas floats. This can form a deep and homogenous hard
layer for the alloy steel component with a wide shape. In the
surface modification apparatus for an alloy steel component
according to the present invention, the alloy steel components to
be processed are disposed in the process furnace with a space
therebetween large enough for the ammonia gas to spread. This
enables the efficient surface modification process to be performed
on a large amount of alloy steel components as compared to the ion
nitration process. According to the experiments by the present
inventors, the ammonia gas concentration is preferably constant
during the surface modification process for the alloy steel
component.
[0008] Another aspect of the present invention is the surface
modification apparatus for an alloy steel component wherein the
temperature is set to 620.degree. C. by the surface modification
processing unit.
[0009] In the other aspect of the surface modification apparatus
for an alloy steel component according to the present invention as
described above, the temperature of the atmosphere to which the
alloy steel component is exposed is set to 620.degree. C.
Therefore, according to the experiments by the present inventors,
the hard layer can be formed more stably. According to the
experiments by the present inventors, the temperature of the
atmosphere to which the alloy steel component is exposed is
preferably 610.degree. C. or more and 630.degree. C. or less, more
preferably 615.degree. C. or more and 625.degree. C. or less, and
the most preferably 620.degree. C. In any of these cases, the
temperature of the atmosphere to which the alloy steel component is
exposed is desirably maintained constant during the process.
[0010] In another aspect of the surface modification apparatus for
an alloy steel component according to the present invention, the
alloy steel component includes at least one of a corner portion
with an acute shape and a hole portion with a diameter of 8 mm or
less. In this case, the corner portion may include, for example, a
portion with an acute angle, a right angle, or an obtuse angle or
an acute shape like a spindle.
[0011] According to the other aspect of the surface modification
method for an alloy steel component according to the present
invention as described above, the homogeneous hard layer can be
formed even though the alloy steel component is formed to have a
corner portion with an acute shape or a hole portion with a
diameter of 8 mm or less.
[0012] The present invention can be implemented not just as the
invention of the surface modification apparatus for an alloy steel
component but also as the invention of the surface modification
method for an alloy steel component and the invention of a method
for manufacturing an alloy steel component.
[0013] Specifically, the surface modification method for an alloy
steel component may be a surface modification apparatus for an
alloy steel component for performing the surface modification on an
alloy steel component including an alloy steel material containing
a nitride formation element, and the method may include a surface
modification processing step of modifying a surface of the alloy
steel component by exposing the alloy steel component for at least
180 minutes to the atmosphere with an ammonia gas concentration of
80% or more and a temperature of 610.degree. C. or more and
630.degree. C. or less.
[0014] In this case, in the surface modification processing step,
the temperature is more preferably 615.degree. C. or more and
625.degree. C. or less, and the most preferably 620.degree. C.
[0015] In this case, the alloy steel component preferably includes
at least one of the corner portion with an acute shape and the hole
portion with a diameter of 8 mm or less.
[0016] The method for manufacturing an alloy steel component may be
a method for manufacturing an alloy steel component including an
alloy steel material containing a nitride formation element, and
the method may include a surface modification processing step of
modifying a surface of the alloy steel component by exposing the
alloy steel component for at least 180 minutes to the atmosphere
with an ammonia gas concentration of 80% or more and a temperature
of 610*C or more and 630.degree. C. or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic view illustrating the outline of an
apparatus structure of a surface modification apparatus for an
alloy steel component used in a surface modification method for an
alloy steel component according to the present invention.
[0018] FIG. 2 is a sectional view illustrating the outline of a
structure of an alloy steel component to which a surface
modification method for an alloy steel component according to the
present invention is applied.
[0019] FIG. 3 is a flowchart illustrating a manufacturing process
for an alloy steel component by a surface modification method for
an alloy steel component according to an embodiment of the present
invention.
[0020] FIG. 4 is a graph expressing the relation between the
hardness and the depth of a hard layer formed on a surface layer of
an alloy steel component by the surface modification method for an
alloy steel component according to the present invention.
[0021] FIG. 5 is a table showing the chemical composition values of
Samples A to E which have been subjected to the surface
modification process by the surface modification method for an
alloy steel component according to the present invention.
[0022] FIG. 6 is a graph expressing the hardness of a compound
layer formed on a surface layer of each of Samples A to E by the
surface modification method for an alloy steel component according
to the present invention.
[0023] FIG. 7 is a graph relatively expressing the thickness of the
compound layer formed on the surface layer of Sample B for each of
different process conditions including the surface modification
method for an alloy steel component according to the present
invention.
[0024] FIG. 8 is a graph relatively expressing the thickness of the
compound layer formed on the surface layer of Sample C for each of
different process conditions including the surface modification
method for an alloy steel component according to the present
invention.
[0025] FIG. 9 is a graph relatively expressing the thickness of the
compound layer formed on the surface layer of Sample D for each of
different process conditions including the surface modification
method for an alloy steel component according to the present
invention.
[0026] FIG. 10 is a graph relatively expressing the thickness of
the compound layer formed on the surface layer of Sample E for each
of different process conditions including the surface modification
method for an alloy steel component according to the present
invention.
DESCRIPTION OF EMBODIMENTS
[0027] An embodiment of a surface modification apparatus for an
alloy steel component, a surface modification method for an alloy
steel component, and a method for manufacturing an alloy steel
component according to the present invention will be described
below with reference to drawings. FIG. 1 is a schematic view
illustrating the outline of an apparatus structure of a surface
modification apparatus 100 for an alloy steel component, which is
used in a surface modification method for an alloy steel component
according to the present invention. In each drawing in the present
specification, some elements are illustrated schematically, for
example, drawn exaggerated in order to help the understanding of
the present invention. Therefore, the size, ratio, etc. among the
elements may be different. This surface modification apparatus 100
corresponds to a thermal processing apparatus for performing a
surface modification process to improve the wear and abrasion
resistance by forming a hard layer on a surface layer of an alloy
steel component 90 such as a cylindrical boss component included in
an alloy steel mechanical component, for example, a power
transmission device such as a clutch in a vehicle like a
four-wheeled vehicle or a bicycle.
[0028] First, description is made of the alloy steel component 90
molded by the surface modification apparatus 100 according to the
present invention. This alloy steel component 90 is a component
included in a power transmission device such as a clutch in a
vehicle, and is formed of an alloy steel material corresponding to
carbon steel to which a nitride formation element has been added.
In this case, the nitride formation element added to the carbon
steel is at least one kind of element that combines with nitrogen
to form a nitride, and corresponds to, for example, chromium,
molybdenum, or aluminum. The amount of the nitride formation
element to be added is more than or equal to the minimum amount
defined by JIS (Japanese Industrial Standard), and specifically, if
the element is chromium, the amount is 0.3 wt % or more, if the
element is molybdenum, the amount is 0.08 wt % or more, and if the
element is aluminum, the amount is 0.1 wt % or more.
[0029] As specifically illustrated in FIG. 2, the alloy steel
component 90 includes a main body portion 91 formed to have an
approximately cylindrical shape. The main body portion 91 includes
a cylindrical sliding portion 92 on which another member of the
power transmission device slides, and a flange portion 93 that
extends in a circular-plate shape outward in the radial direction
from one end (right side in the figure) of the sliding portion 92.
The sliding portion 92 of the main body portion 91 is provided with
a penetration hole 94 in a state of penetrating in the radial
direction of the main body portion 91. The penetration hole 94 is
formed to have a diameter of 8 mm.
[0030] The surface modification apparatus 100 includes a process
furnace 101. The process furnace 101 is a vessel with an
approximately cylindrical shape formed to be airtight so that the
surface modification process is performed on the alloy steel
component 90. The process furnace 101 is formed of a material that
can resist 620.degree. C. corresponding to the process temperature
of the alloy steel component 90, for example, a ceramic material.
This process furnace 101 is mainly formed of a main chamber 101a
and a stand-by chamber 101b.
[0031] The main chamber 101a is a space where the surface
modification process is performed on the alloy steel component 90.
The stand-by chamber 101b is a space where the alloy steel
component 90 is set before or after the alloy steel component 90 is
taken in or out of the main chamber 101a, and is formed to have a
release door 101c that is opened and closed toward the outside.
Between the main chamber 101a and the stand-by chamber 101b is
provided a partition wall 101d that partitions between both
chambers in a manner that the partition wall 101d can freely
connect or disconnect the chambers. Moreover, a conveyance
mechanism 101e is provided between the main chamber 101a and the
stand-by chamber 101b. The conveyance mechanism 101e conveys the
alloy steel component 90 between the stand-by chamber 101b and the
main chamber 101a under the operation control by a control device
111 to be described below. In FIG. 1, the partition wall 101c is
illustrated by a dashed line, and the conveyance mechanism 101e and
the alloy steel component 90 in the stand-by chamber 101b are each
illustrated by a two-dot chain line.
[0032] A heater 102 is provided outside the outer peripheral
surface of the process furnace 101. The heater 102 is an electric
heater used for heating the main chamber 101a in the process
furnace 101 up to 620.degree. C. corresponding to the process
temperature and moreover for maintaining this temperature. The
operation of the hater 102 is controlled by the control device
111.
[0033] The process furnace 101 is connected to a main gas supply
pipe 103, a sub-gas supply pipe 106, and an exhaust pipe 109. The
main gas supply pipe 103 is a pipe for guiding ammonia gas (not
shown) into the process furnace 101. On the upstream side of the
main gas supply pipe 103, a main gas cylinder 105 is connected
through a flow rate adjuster 104. The flow rate adjuster 104 is a
device for adjusting the flow rate of the ammonia gas to be
introduced into the process furnace 101. The flow rate adjuster 104
includes, for example, a gasifier (not shown) for gasifying liquid
ammonia and a flow rate adjustment valve (not shown) for adjusting
the flow rate of the gasified ammonia gas. The operation of the
flow rate adjuster 104 is controlled by the control device 111. The
main gas cylinder 105 is a vessel for storing the liquid
ammonia.
[0034] The sub-gas supply pipe 106 is a pipe for guiding the
nitrogen gas (not shown) into the process furnace 101. On the
upstream side of the sub-gas supply pipe 106, a sub-gas cylinder
108 is connected through a flow rate adjuster 107. The flow rate
adjuster 107 is similar to the flow rate adjuster 104 and is a
device for adjusting the flow rate of the nitrogen gas to be
introduced into the process furnace 101. The flow rate adjuster 107
includes, for example, a gasifier (not shown) for gasifying liquid
nitrogen and a flow rate adjustment valve (not shown) for adjusting
the flow rate of the gasified nitrogen gas. The operation of the
flow rate adjuster 107 is controlled by the control device 111. The
sub-gas cylinder 108 is a vessel for storing the liquid
nitrogen.
[0035] The exhaust pipe 109 is a pipe for guiding the gas in the
process furnace 101 to the outside of the process chamber 101. On
the downstream side of the exhaust pipe 109, an exhaust gas process
device 110 for deodorizing and burning the exhaust gas led out of
the process furnace 101 is connected.
[0036] The control device 111 is configured by a microcomputer
including a CPU, a ROM, a RAM, and the like. The control device 111
further includes an input device (not shown) for an operator to
input an instruction, and a display device (not shown) to show the
operator the operation status of the surface modification apparatus
100. The control device 111 executes the programs stored in advance
in the storage device such as the ROM in accordance with the
operator's instruction, thereby controlling the operation of the
surface modification apparatus 100. Specifically, the control
device 100 controls the operation of each of the heater 102, the
flow rate adjusting devices 104 and 107, and the exhaust gas
process device 110 in accordance with the operator's instruction.
The surface modification apparatus 100 is further provided with a
temperature sensor 112a and a concentration sensor 112b. These
sensors 112a and 112b measure the temperature of the furnace 101
and the ammonia gas concentration, respectively and output the
results to the control device 111.
(Operation of Surface Modification Apparatus 100)
[0037] Next, the surface modification process performed on the
alloy steel component 90 by the surface modification apparatus 100
with the above structure is described with reference to the
flowchart of FIG. 3. FIG. 3 is the flowchart illustrating a process
for manufacturing the alloy steel component 90.
[0038] The operator who manufactures the alloy steel component 90
molds the alloy steel component 90 in a first step. Specifically,
the operator molds the main body portion 91, the sliding portion
92, the flange portion 93, and the penetration hole 94 through
cutting, welding, and grinding of the alloy steel material using
the machining equipment including a machine tool, which is not
shown. In this case, in this embodiment, the alloy steel component
90 is not subjected to a heat treatment including a quenching
process. However, the heat treatment including quenching may be
performed before or after the surface modification process
according to the present invention.
[0039] Next, the operator performs the surface modification process
on the alloy steel component 90 in a second step. In this case, the
surface modification process corresponds to a process of forming a
hard layer made of a nitride on a surface layer of the alloy steel
component 90. Specifically, the operator manipulates the control
device 111 to close the partition wall 101d in the process furnace
101 so as to make the main chamber 101a airtight. After that, the
operator disposes the alloy steel component 90 molded by the
machining in the stand-by chamber 101b through the opening and
closing door 101c. If the operator disposes a plurality of alloy
steel components 90, the operator disposes the alloy steel
components 90 with a space therebetween so that the ammonia gas can
spread in the space.
[0040] Next, the operator manipulates the control device 111 to
introduce the ammonia gas and the nitrogen gas into the main
chamber 101a in the process furnace 101 and heats the process
furnace 101. In this case, the operator introduces the ammonia gas
and the nitrogen gas into the main chamber 101a so that 80% or more
of the gas in the main chamber 101a of the process furnace 101 is
constituted by ammonia gas while the remainder is constituted by
the nitrogen gas. In addition, the operator heats the main chamber
101a of the process furnace 101 so that the chamber has a
temperature of 620.degree. C.
[0041] Next, if the main chamber 101a of the process furnace 101
has an atmosphere with an ammonia gas concentration of 80% and with
a temperature of 620.degree. C. or more, the operator manipulates
the control device 111 to move the alloy steel component 90 from
the stand-by chamber 101b to the main chamber 101a. After that, the
operator manipulates the control device 111 to expose the alloy
steel component 90 for at least 180 minutes to the maintained
atmosphere of the main chamber 101a of the process furnace 101,
specifically the atmosphere of the main chamber 101a maintained to
have an ammonia gas concentration of 80% and a temperature of
620.degree. C. In other words, this process furnace 101 corresponds
to the surface modification processing unit according to the
present invention.
[0042] This causes the nitrogen, which is generated by the thermal
decomposition of the ammonia gas, to permeate into the surface
layer of the alloy steel component 90 in the main chamber 101a of
the process chamber 101, thereby forming the hard layer.
Specifically, on the surface layer of the alloy steel component 90,
the hard layer formed of a compound layer and a diffusion layer in
the order from the outermost surface is generated. In this case,
the surface modification process for the alloy steel component 90
is performed by having the nitrogen floating in the main chamber
101a in contact with the alloy steel component 90. Thus, the hard
layer is generated homogeneously without unevenness for the inside
of the penetration hole 94 and the corner portion of the flange
portion 92.
[0043] After the alloy steel component 90 is exposed to the
atmosphere in the main chamber 101a of the process furnace 101 for
at least 180 minutes, the operator manipulates the control device
111 to stop the supply of the ammonia gas and the nitrogen gas to
the process furnace 101 and stop the heating of the process furnace
101. The operator extracts the alloy steel component 90 out of the
process furnace 101 after the temperature in the process furnace
101 has decreased to be less than a predetermined temperature (for
example, 150.degree. C.).
[0044] Here, description is made of the hardness of the surface
layer of the alloy steel component 90 extracted from the process
furnace 101. FIG. 4 is a graph expressing an example of how the
hardness changes relative to the distance from the surface layer of
the alloy steel component 90 containing the so-called nitride
formation element. According to FIG. 4, it has been confirmed that
the compound layer with a hardness of 780 HmV through 660 HmV to
880 HmV is formed in the range of thicknesses from the surface up
to approximately 25 .mu.m in the alloy steel component 90 and the
diffusion layer is formed to a depth of 0.2 mm from this compound
layer toward the lower layer. This means that the compound layer
with a hardness (750 HmV or more) more than or equal to the
hardness of chromium plating defined in JIS (Japanese Industrial
Standard) is formed on the surface of the alloy steel component
90.
[0045] Next, the operator performs a finishing process in a third
step. Specifically, the operator performs a grinding process on the
outer surface of the alloy steel component 90 extracted from the
process furnace 101 so that the alloy steel component 90 has a
predetermined size or a predetermined surface roughness. In this
case, since the compound layer with a thickness of 25 .mu.m or more
is formed on the surface of the alloy steel component 90, the
operator can easily achieve the predetermined size or the
predetermined surface roughness by a sufficient process margin.
[0046] Here, the results of experiments by the present inventors
are described. FIG. 5 shows the values of the chemical compositions
of Samples A to E used in the present experiments. FIG. 6 shows the
results of measuring the surface hardness of Samples A to E in FIG.
5 that have been subjected to the surface modification process
similar to the process in the above embodiment. In FIG. 6, the
hardness of Sample B satisfying the hardness required in the
specification of the alloy steel component 90 is regarded as 100%,
based on which the hardness of the other Samples A and C to E are
shown. According to the experiment results in FIG. 6, it has been
confirmed that the alloy steel component 09 needs to be formed of
the alloy steel material containing a nitride formation element in
order for the compound layer to have the required hardness.
[0047] Next, FIG. 7 to FIG. 10 show the results of measuring the
thickness of the compound layer of each of Samples B to E, among
Samples A to E in FIG. 5, which have been subjected to the surface
modification process according to the conventional condition, the
temperature changed condition, the time changed condition, the
concentration changed condition, and the condition of the present
invention. In FIG. 7 to FIG. 10, the thicknesses of the compound
layers of Samples C to E are expressed assuming the thickness of
the compound layer that has been increased by 60% of the thickness
of the compound layer of Sample B subjected to the conventional
surface modification process is the target value (achievement rate
100%).
[0048] In this experiment, the conventional technique corresponds
to the conventional nitration process, specifically the gas
soft-nitriding process performed on the alloy steel component 90 in
the atmosphere with a temperature of 530 to 580.degree. C., a
process time of 60 to 180 minutes, and an ammonia gas concentration
of 30 to 50% in the furnace. The conventional technique provides
the compound layer with a hardness of 350 HmV or more in general
and with a thickness of approximately 8 .mu.m to 15 .mu.m.
[0049] In regard to the temperature changed condition, the
conventional condition is changed into a process condition where
just the temperature condition is changed to 620.degree. C.
corresponding to the temperature condition according to the present
invention; under this process condition, the alloy steel component
90 is processed. In regard to the time changed condition, the
conventional condition is changed into a process condition where
just the time condition is changed to 180 minutes or more
corresponding to the time condition according to the present
invention; under this condition, the alloy steel component 90 is
processed. In regard to the concentration changed condition, the
conventional condition is changed into a process condition where
just the concentration condition of the gas confined in the main
chamber 101a is changed to 80% or more of ammonia gas corresponding
to the gas concentration condition according to the present
invention; under this condition, the alloy steel component 90 is
processed. In regard to the condition according to the present
invention, the alloy steel component 90 is processed under the
process condition in the above embodiment, i.e., the temperature is
set to 620.degree. C., the process time is 180 minutes or more, and
the ammonia concentration is 80% or more.
[0050] According to the results of the experiments shown in FIG. 7
to FIG. 10, it has been confirmed that the thickness of the
compound layer formed based on the process condition of the present
invention is clearly larger than that of the compound layer formed
by any process condition that has employed part of the temperature
condition, the time condition, and the ammonia concentration
condition in the process condition in the present invention. In
other words, the thickness of the compound layer formed based on
the present invention cannot be achieved by the process condition
that has employed part of the process condition in the present
invention, and can be achieved only by executing the process
condition including all the temperature condition, the time
condition, and the ammonia concentration condition in the present
invention.
[0051] In regard to how much the thickness of the hard layer formed
by the surface modification process according to the present
invention has increased relative to the conventional thickness, the
experiment results from Samples B to E indicate that the thickness
of the compound layer has increased 1.9 times to 2.5 times and the
thickness of the diffusion layer has increased 1.2 times to 1.6
times. In this case, along with the increase of the compound layer,
the diffusion layer is increased but in regard to the increase
rate, the increase rate of the diffusion layer is smaller than that
of the compound layer. In other words, through the surface
modification process according to the present invention, the
thickness of the compound layer formed on the surface of the alloy
steel component 90 can be increased while the modification of the
mother material inside the alloy steel component 90, more
specifically the tough part thereof is suppressed. Therefore, the
surface modification process according to the present invention is
particularly effective in mechanical parts required to have the
toughness on the inside and have the wear and abrasion resistance
on the surface.
[0052] The thickness of the compound layer formed on the surface of
each of Samples B to E through the surface modification process
according to the present invention is 25 .mu.m to 33 .mu.m. This
thickness is as large as the depth of each hard layer in the
thermally processed product obtained by carburizing and quenching a
conventional chromium molybdenum steel material and plating the
material with chromium and the thermally processed product obtained
by hardening a medium carbon steel material with high frequency and
plating the material with chromium as compared to the thickness of
the compound layer obtained from the conventional gas nitration
process.
[0053] As can be understood from the above description about the
operation, the surface modification apparatus 100 for the alloy
steel component in the above embodiment is structured to have the
process furnace 101 where the alloy steel component 90 is exposed
for at least 180 minutes to the atmosphere with an ammonia gas
concentration of 80% or more and a temperature of 620.degree. C.
According to the experiments by the present inventors, it has been
confirmed that the compound layer with a thickness of 25 .mu.m or
more can be formed stably on the surface of the alloy steel
component 90. In other words, the surface modification apparatus
100 for the alloy steel component 90 according to the present
invention performs the surface modification process in the
atmosphere containing nitrogen generated by the thermal
decomposition of the ammonia gas. This can form the hard layer that
is deep and homogeneous in the alloy steel component 90 with a wide
shape. In the surface modification apparatus 90 for an alloy steel
component according to the present invention, the alloy steel
components 90 to be processed are disposed in the process furnace
101 with a space between the components 90 so that the ammonia gas
spreads in the space. This enables the efficient surface
modification process on a large amount of alloy steel components 90
as compared to the ion nitration process.
[0054] In the implementation of the present invention, various
changes can be made without departing from the purpose of the
present invention and without being limited to the above
embodiment.
[0055] For example, in regard to the concentration of the gas in
the main chamber 101a of the process furnace 101 in the above
embodiment, the ammonia gas constitutes 80% or more with the
remainder constituted by the nitrogen gas. However, the
concentration of the gas in the main chamber 101a is not limited to
that in the above embodiment as long as the ammonia gas constitutes
at least 80%. Therefore, the main chamber 101a may be formed to
contain the ammonia gas only. Alternatively, 80% or more of the gas
concentration in the main chamber 101a may be constituted by the
ammonia gas with the remainder constituted by other gas than the
nitrogen gas, such as carbon gas or hydrogen gas. In these cases,
according to the experiments by the present inventors, the ammonia
gas concentration is preferably maintained constant during the
surface modification process on the alloy steel component 90.
[0056] In the above embodiment, the alloy steel component 90 may be
subjected to the surface modification process while the inside of
the main chamber 101a of the process furnace 101 is maintained to
be heated at 620.degree. C. According to the experiments by the
present inventors, however, it has been confirmed that the effect
of the present invention is achieved if the temperature condition
in the surface modification process in the present invention is in
the range of .+-.10.degree. C. from 620.degree. C., i.e., from
610.degree. C. to 630.degree. C. and that the effect of the present
invention cannot be fully achieved if the temperature is out of
that range. Moreover, according to the experiments by the present
inventors, it has been confirmed that the temperature of the
atmosphere to which the alloy steel component is exposed is more
appropriate to be 615.degree. C. or more and 625.degree. C. or
less, and the most appropriate to be 620.degree. C. It has also
been confirmed that in those cases, the temperature of the
atmosphere to which the alloy steel component is exposed is
desirably maintained to be constant during the process.
[0057] In the above embodiment, the alloy steel component 90 is one
of parts constituting the power transmission device such as a
clutch in a self-propelled vehicle. However, the surface
modification apparatus 100 for the alloy steel component 90
according to the present invention is widely applicable to the
mechanical parts formed of alloy steel. In this case, the surface
modification apparatus 100 for the alloy steel component 90
according to the present invention is appropriate because the thick
and homogeneous compound layer can be formed even for the alloy
steel component 90 including at least one of a corner portion with
the shape having a narrowing end, such as a conical shape or an
acute angle, a right angle, or an obtuse angle and a hole portion
with a diameter of 8 mm or less, preferably 5 mm or less, and more
preferably 4 mm or less. In regard to the alloy steel component 90
including a mechanically slidable portion partly or entirely in the
alloy steel component 90, the hard layer (compound layer) formed by
the surface modification apparatus 100 for the alloy steel
component 90 according to the present invention is effective
because the hard layer can be formed more easily and in a shorter
time than the hard layer formed by the conventional process for
forming the hard layer, specifically the hard layer formed by the
hard chromium plating process after the high-frequency hardening or
the carburizing and quenching.
[0058] In the above embodiment, the surface modification apparatus
100 exposes the alloy steel component 90 for at least 180 minutes
to the atmosphere maintained to have an ammonia gas concentration
of 80% and a temperature of 620.degree. C. in the main chamber
101a. This is based on the experiments by the present inventors,
which have confirmed that the effect of the present invention
cannot fully be achieved if the surface modification process on the
alloy steel component 90 is less than 180 minutes. Thus, the
surface modification apparatus 100 needs to expose the alloy steel
component 90 for at least 180 minutes to the atmosphere maintained
to have an ammonia gas concentration of 80% and a temperature of
610.degree. C. or more and 630.degree. C. or less.
[0059] The surface modification method for an alloy steel component
according to the present invention is widely applicable to the
alloy steel material containing the nitride formation element. In
this case, examples of the alloy steel material containing the
nitride formation element include steel materials containing more
than or equal to the minimum amount, which is defined by JIS
(Japanese Industrial Standard), of at least one element of
chromium, molybdenum, and aluminum, such as chromium alloy steel,
chromium molybdenum steel, nitride steel, and carbon steel,
chromium alloy steel, or chromium molybdenum steel containing 0.1
to 0.3 wt % of aluminum.
DESCRIPTION OF REFERENCE SIGNS
[0060] 90 Alloy steel component [0061] 91 Main body portion [0062]
92 Sliding portion [0063] 93 Flange portion [0064] 94 Penetration
hole [0065] 100 Surface modification apparatus [0066] 101 Process
furnace [0067] 101a Main chamber [0068] 101b Stand-by chamber
[0069] 101c Opening and closing door [0070] 101d Partition wall
[0071] 101e Conveyance mechanism [0072] 102 Heater [0073] 103 Main
gas supply pipe [0074] 104 Flow rate adjuster [0075] 105 Main gas
cylinder [0076] 106 Sub-gas supply pipe [0077] 107 Flow rate
adjuster [0078] 108 Sub-gas cylinder [0079] 109 Exhaust pipe [0080]
110 Exhaust gas process device [0081] 111 Control device [0082]
112a Temperature sensor [0083] 112b Concentration sensor
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