U.S. patent application number 11/989930 was filed with the patent office on 2009-12-24 for ion nitriding method.
This patent application is currently assigned to Honda Motor Co., Ltd.. Invention is credited to Kazuo Minemura, Takahiro Yamaguchi.
Application Number | 20090314389 11/989930 |
Document ID | / |
Family ID | 37727341 |
Filed Date | 2009-12-24 |
United States Patent
Application |
20090314389 |
Kind Code |
A1 |
Minemura; Kazuo ; et
al. |
December 24, 2009 |
Ion Nitriding Method
Abstract
When a metal material such as an Fe alloy or Ni alloy is heated
in the presence of an amino resin such as a melamine-formalin
resin, a passivation film on the surface of the metal material is
reduced and removed. During the reduction and removal of the
passivation film, hydrogen sputtering may be also performed when
the temperature becomes higher than 400.degree. C., preferably not
less than 450.degree. C. By performing a glow discharge, the metal
material having an exposed active metal surface is nitrided with N
liberated from the amino resin. For example, the amino resin is
applied onto the surface of the metal material directly or by using
a solvent such as water.
Inventors: |
Minemura; Kazuo;
(Tochigi-ken, JP) ; Yamaguchi; Takahiro;
(Tochigi-ken, JP) |
Correspondence
Address: |
SQUIRE, SANDERS & DEMPSEY L.L.P.
8000 TOWERS CRESCENT DRIVE, 14TH FLOOR
VIENNA
VA
22182-6212
US
|
Assignee: |
Honda Motor Co., Ltd.
|
Family ID: |
37727341 |
Appl. No.: |
11/989930 |
Filed: |
August 4, 2006 |
PCT Filed: |
August 4, 2006 |
PCT NO: |
PCT/JP2006/315529 |
371 Date: |
February 4, 2008 |
Current U.S.
Class: |
148/222 |
Current CPC
Class: |
F01L 2301/00 20200501;
C23C 8/36 20130101; C23C 8/02 20130101; F01L 3/04 20130101; F01L
3/02 20130101; C23C 8/38 20130101 |
Class at
Publication: |
148/222 |
International
Class: |
C23C 8/36 20060101
C23C008/36 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 5, 2005 |
JP |
2005-228621 |
Claims
1. An ion nitriding method comprising applying electricity to a
metal material, thereby generating glow discharge to carry out a
nitriding treatment on said metal material, wherein said metal
material is ion-nitrided in the presence of an amino resin in said
nitriding treatment.
2. An ion nitriding method according to claim 1, wherein said amino
resin is applied to a surface of said metal material before said
nitriding treatment.
3. An ion nitriding method according to claim 2, wherein said amino
resin is applied to said surface of said metal material using a
solvent.
4. An ion nitriding method according to claim 1, wherein said amino
resin and said metal material are placed in a heat treatment
furnace before said nitriding treatment.
5. An ion nitriding method according to claim 1, wherein said amino
resin is a melamine resin, a urea resin, an aniline resin, or a
formalin resin.
6. An ion nitriding method according to claim 5, wherein said
melamine resin contains a repeating unit having a composition
formula of C.sub.6H.sub.3N.sub.9.
7. An ion nitriding method according to claim 1, wherein a heat
treatment furnace is used as an anode and said metal material is
used as a cathode to apply said electricity.
Description
TECHNICAL FIELD
[0001] The present invention relates to an ion nitriding method for
introducing and diffusing nitrogen in a surface of a metal material
under glow discharge.
BACKGROUND ART
[0002] A nitriding treatment, which is a surface modification
treatment for introducing and diffusing nitrogen in a surface of a
metal material to form a hard nitride, has been widely used with a
purpose of hardening the surface to improve its abrasion
resistance. This nitriding treatment is generally used for a
material mainly composed of a Group 8 metal of the Periodic Table,
e.g. an iron group alloy such as a stainless steel.
[0003] On the surface of the iron group alloy, a passivation film
of an iron oxide is spontaneously formed due to oxidation by
atmospheric oxygen. The passivation film inhibits the nitriding
process, resulting in decreased nitriding efficiency.
[0004] Thus, generally a treatment for removing the passivation
film is carried out before the nitriding treatment. As the removing
treatment, a wet method of soaking the stainless steel in an
aqueous cyanide solution, etc. has been conventionally used.
However, the wet method is disadvantageous in that the cyanide is
toxic, whereby the workers are burdened and a removing mechanism
has to be installed in the workplace. Further, in this method, an
apparatus for treating a waste liquid is required, thereby
increasing equipment investment.
[0005] In view of solving such problems, a method for removing the
passivation film by sputtering is proposed in Patent Document 1 and
Non-Patent Document 1. It is reported in Patent Document 1 and
Non-Patent Document 1 that a mixed gas of hydrogen with nitrogen or
ammonia is introduced into a treatment furnace, glow discharge is
caused in the treatment furnace to generate hydrogen and ammonia
ions, and the passivation film is reduced and removed by the ions.
A passivation film-reducing mechanism and a nitride-diffusing
mechanism are both described in Non-Patent Document 1.
[0006] Further, a method proposed in Patent Document 2 contains the
steps of introducing only a hydrogen gas into a treatment furnace
at a temperature lower than 350.degree. C., preferably lower than
150.degree. C., and starting hydrogen gas sputtering at a
relatively low temperature. It is described in Patent Document 2
that the passivation film can be reduced from inside such that
H.sup.+ is diffused and introduced inside an iron group alloy base
material, and the material is heated to a temperature equal to or
higher than the reduction temperature of the passivation film, to
transfer H.sup.+ to the surface of the material.
Patent Document 1: Japanese Patent Publication No. 02-002945
Patent Document 2: WO 01/34867
[0007] Non-Patent Document 1: Takao Takase, Ion Nitriding Method,
Appearance of Rival of Soft Nitriding Method, KINZOKU, Separate
print of March issue, 1973, Page 48 to 54
DISCLOSURE OF THE INVENTION
[0008] When a passivation film is reduced and removed from a steel
having a relatively high Cr or Ni content by the method described
in Patent Document 1 or Non-Patent Document 1, the passivation film
cannot be sufficiently removed due to a small diffusion depth of
H.sup.+ or NH.sub.4.sup.+ in some cases. In such cases,
disadvantageously, the thickness of a compound layer or a nitrided
layer formed by the nitriding treatment is not uniform, or the
compound layer or nitrided layer is formed on only a part of the
steel.
[0009] In the method described in the Patent Document 2, only the
diffusion and introduction of hydrogen ion into the iron group
alloy base material is caused, the passivation film being not
reduced. The characteristics of the diffusion and introduction of
hydrogen ion, such as the diffusion depth, amount, and distribution
of hydrogen ion, are greatly changed depending on sputtering
conditions. When the hydrogen ion is diffused and introduced into
an excessively deep position, or when the amount of the introduced
hydrogen ion is too large, the hydrogen ion often causes hydrogen
embrittlement. This tendency is particularly noticeable in a
complicated member of an internal-combustion engine, etc.
[0010] Thus, for example, the hydrogen ion is diffused and
introduced in the desired state by highly precisely controlling the
sputtering condition, to prevent the so-called hydrogen
embrittlement of the iron group alloy base material by remaining
hydrogen ion. However, to perform such highly precise control, it
is necessary to obtain data by repeating many tests
complicatedly.
[0011] A general object of the present invention is to provide an
ion nitriding method that can be carried out under secure
environment easily and readily.
[0012] A principal object of the present invention is to provide an
ion nitriding method that can be carried out with reduced material
procurement costs.
[0013] Another object of the present invention is to provide an ion
nitriding method capable of producing a compound layer and a
nitrided layer that have a large thickness and are substantially
uniform in any portion.
[0014] A further object of the present invention is to provide an
ion nitriding method capable of preventing embrittlement of a
nitriding-treated metal material.
[0015] According to an aspect of the present invention, there is
provided an ion nitriding method comprising applying electricity to
a metal material, thereby generating glow discharge, to ion-nitride
the metal material in the presence of an amino resin.
[0016] When the metal material is heated in the presence of the
amino resin, the amino resin is thermally decomposed and C, N, and
H are liberated therefrom. The liberated C, N, and H interact with
O to produce HCN or NO, which attacks and removes a passivation
film, and eventually the passivation film is vanished. Thus, in the
present invention, the substantially entire passivation film on the
surface of the metal material can be easily removed by the
remarkably simple process of heating the metal material in the
presence of the amino resin.
[0017] Further, the amino resin is nontoxic according to Material
Safety Data Sheet, and therefore the process can be carried out
under secure environment. It should be noted that the above HCN is
produced only in a slight amount of several-thousand ppm and is
rapidly decomposed into nitrogen and carbon dioxide gas in waste
gas combustion, so that an HCN-removing apparatus is not
required.
[0018] The removal of the passivation film proceeds also during the
heating process, and thereby temperature holding processes for the
removal of the passivation film are not required in the present
invention. Thus, due to the removal of the passivation film, the
period for completing the ion nitriding treatment is not made
longer, and the efficiency of the ion nitriding treatment is not
reduced.
[0019] The amino resin is thermally decomposed into a gas phase,
and exists as an ambient gas around the metal material. The gas
phase contains the liberated N and acts as an N source for
nitriding the metal material.
[0020] As compared with metal materials ion-nitrided without the
amino resin, the metal material ion-nitrided in the presence of the
amino resin has a higher hardness, and the hard region thereof
extends more deeply. In the present invention, the ion nitriding
treatment can be carried out without precisely controlling the
types and ratios of gases, reaction temperature, reaction time,
etc. as long as it is carried out in the presence of the amino
resin. Various metal materials can be hardened by the method of the
present invention.
[0021] Thus, as compared with ion nitriding processes without the
amino resin, the simple easy process of nitriding in the presence
of the amino resin according to the present invention can increase
the hardness of the metal material higher. Further, the hardened
region of the metal material extends more deeply in the present
invention.
[0022] The metal material with the increased hardness is excellent
in abrasion resistance and strength. Thus, an abrasion-resistant,
high-strength metal material can be obtained in the present
invention.
[0023] For example, the amino resin may be applied to the surface
of the metal material. In this case, it is preferred that the amino
resin is applied to the surface of the metal material using a
solvent, so that application unevenness is reduced, and the
passivation film removal and the nitriding proceed substantially
uniformly in any portion.
[0024] The amino resin may be placed in a heat treatment furnace
together with the metal material and then heat-treated without the
application.
[0025] The amino resin is a resin produced by polycondensation
between an amino group and formaldehyde. Typical examples of such
resins include melamine resins, urea resins, aniline resins, and
formalin resins.
[0026] Among these resins, the melamine resins are particularly
preferred, and the melamine resin preferably contains a repeating
unit having a composition formula of C.sub.6H.sub.3N.sub.9. In this
case, the residue of the resin is hardly attached to the heat
treatment furnace during the ion nitriding treatment, whereby the
number of maintenance can be significantly reduced, and the
maintenance work can be easily carried out.
[0027] Preferred examples of the metal materials to be ion-nitrided
include alloys mainly composed of Group 8 elements of the Periodic
Table of Elements, such as Fe alloys and Ni alloys.
[0028] It is preferred that the heat treatment furnace is used as
an anode and the metal material is used as a cathode to apply the
electricity. In this case, it is not necessary to place another
electrode in the heat treatment furnace, resulting in a simple
structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic, overall structural view showing an
intake engine valve used in an ion nitriding method according to an
embodiment of the present invention;
[0030] FIG. 2 is a graph showing an example of a temperature
pattern of the ion nitriding method;
[0031] FIG. 3 is a graph showing an example of a temperature
pattern of the ion nitriding method, containing an ion nitriding
treatment with a nitriding gas;
[0032] FIG. 4 is a graph showing an example of a temperature
pattern of the ion nitriding method, containing a hydrogen gas
sputtering treatment and an ion nitriding treatment;
[0033] FIG. 5 is a schematic, overall structural view showing an
exhaust engine valve used in an ion nitriding method according to
another embodiment of the present invention;
[0034] FIG. 6 is a table showing the thickness and Hv of nitrided
layers formed by a plasma ion nitriding treatment; and
[0035] FIG. 7 is a table showing the materials of engine valves,
and the thicknesses and surface hardnesses of compound layers or
nitride layers formed on the valves by a gas soft nitriding
treatment using a melamine resin, in contrast to those obtained by
not using the melamine resin.
BEST MODE FOR CARRYING OUT THE INVENTION
[0036] A preferred embodiment of the ion nitriding method of the
present invention will be described in detail below with reference
to the drawings.
[0037] An ion nitriding method according to an embodiment contains
a first step of applying an amino resin to a surface of an intake
engine valve 10 (a metal member) shown in FIG. 1, and a second step
of ion-nitriding the intake engine valve 10 with the amino resin
applied.
[0038] The intake engine valve 10 is installed in an
internal-combustion engine of an automobile and reciprocally moved.
The intake engine valve 10 has an integral structure of a wide head
12, a long bar 14, and an end 16, and is composed of SUH11M. Thus,
the intake engine valve 10 is a Cr-containing steel material, i.e.
a Cr-containing Fe alloy.
[0039] In the surface of the intake engine valve 10, the Fe alloy
SUH11M is oxidized by atmospheric oxygen, whereby a passivation
film of an oxide is spontaneously formed. Therefore, the amino
resin is applied onto the passivation film in the first step.
[0040] The amino resin is a resin produced by polycondensation
between an amino group (--NH.sub.2) and formaldehyde, and typical
examples thereof include melamine resins represented by the
following structural formula (1), urea resins represented by the
following structural formula (2), aniline resins represented by the
following structural formula (3), and formalin resins. These amino
resins are commercially available in the state of solid or
powder.
##STR00001##
[0041] The amino resin is particularly preferably a
melamine-formalin resin containing a repeating unit having a
composition formula of C.sub.6H.sub.3N.sub.9. C.sub.6H.sub.3N.sub.9
is a substance used specifically for salt bath nitriding. When the
ion nitriding treatment is carried out using C.sub.6H.sub.3N.sub.9,
the residue of the resin is hardly attached to inner walls of a
heat treatment furnace advantageously.
[0042] In the first step, it is preferred that a suspension liquid
prepared by dispersing a powder of the amino resin in a solvent
such as water is applied to the surface of the metal material,
though the amino resin powder may be applied directly to the
surface. In this case, application unevenness can be reduced, so
that the thickness of the formed compound layer or nitrided layer,
etc. can be substantially uniform.
[0043] The application may be achieved by a brush coating method
using a brush. Of course, known application technologies other than
the brush coating method may be used.
[0044] After the amino resin is applied directly or preferably by
using the suspension liquid, the intake engine valve 10 and a heat
treatment furnace are electrically connected to an electric power
source. In this process, they are connected such that a furnace
wall of the heat treatment furnace acts as an anode and the intake
engine valve 10 acts as a cathode.
[0045] Then, the intake engine valve 10 is placed in the heat
treatment furnace, and the heat treatment furnace is sealed to
start the second step. A procedure of vacuuming the inside of the
heat treatment furnace is started, while a procedure of rising the
temperature of the furnace is started. The temperature rising rate
may be 3.degree. C. to 5.degree. C./minute.
[0046] In the procedure of rising the temperature of the heat
treatment furnace, the amino resin begins to be decomposed. As a
result, C, N, and H contained in the amino resin are liberated
therefrom to generate HCN. In a case where O interacts therewith,
NO is also generated. The passivation film is reduced by attack of
HCN or NO, and is removed finally. The active metal surface of the
intake engine valve 10 is exposed in this manner.
[0047] As described above, according to this embodiment, the
substantially entire passivation film on the surface of the intake
engine valve 10 (the metal material) can be easily removed by the
remarkably simple process of heating the intake engine valve 10
coated with the amino resin. Further, in this method, an existing
apparatus such as the heat treatment furnace can be utilized,
whereby particular additional equipment investment is not
required.
[0048] It should be noted that the above HCN is produced only in an
amount of several-thousand ppm, and is burned as the temperature of
the heat treatment furnace rises. Thus, it is not necessary to set
up an HCN-removing apparatus. Further, pretreatments such as
treatments for removing an oxide film by using hydrofluoric acid,
etc. are not required in this embodiment, and the amino resin is
nontoxic, whereby the method can be carried out under secure
environment.
[0049] As shown in a temperature pattern of FIG. 2, after the heat
treatment furnace is heated to a predetermined temperature,
electricity is applied to the intake engine valve 10 and the heat
treatment furnace. The current and voltage may be about 25 A and
about 220 to 250 V, respectively. When the electricity is applied,
glow discharge is generated between the anode of the furnace wall
of the heat treatment furnace and the cathode of the intake engine
valve 10.
[0050] When the glow discharge is generated, the N liberated from
the amino resin attacks the exposed metal surface of the intake
engine valve 10. As a result, the liberated N is introduced and
diffused in the metal surface, to nitride the surface.
[0051] Since the passivation film has been removed from the intake
engine valve 10, the N does not have to penetrate the passivation
film. Therefore, the time required for the nitriding treatment can
be shortened, and the thermal energy can be reduced.
[0052] Further, since the passivation film can be removed in the
temperature rising procedure during the nitriding treatment in this
embodiment, the method can be carried out without a particular heat
treatment process such as a treatment of keeping the metal material
at a constant temperature to remove the passivation film. Thus, the
nitriding treatment efficiency is not reduced due to the removal of
the passivation film by the amino resin.
[0053] When the N is introduced and diffused inside the surface of
the intake engine valve 10, the compound layer or the nitrided
layer is formed. The thickness of the nitrided layer or the
compound layer, i.e. the distance of the nitrogen diffusion in the
intake engine valve 10, is significantly larger as compared with
the case of a gas soft nitriding treatment carried out under the
same conditions except for the absence of the amino resin. In the
case of removing the passivation film under the presence of the
amino resin and then ion-nitriding the metal material, the
thickness of the compound layer or the nitrided layer can be
increased, so that even a deeper region of the intake engine valve
10 can be hardened.
[0054] Specifically, in the case of using the intake engine valve
10 of SUH11M, the compound layer formed by the method using the
melamine-formalin resin according to this embodiment has a
thickness of approximately 81 .mu.m, though the compound layer
formed by the conventional gas soft nitriding treatment has a
thickness of approximately 40 .mu.m. Thus, the resultant intake
engine valve 10 obtained by the treatment of this embodiment has a
high hardness in a deeper region, as compared with the conventional
gas soft nitriding treatment.
[0055] After the ion nitriding treatment of this embodiment, a fine
martensite formed on the intake engine valve 10 can be observed by
an electron beam probe microanalyzer (EPMA).
[0056] In the embodiment, as described above, the amino resin is
applied to the surface of the intake engine valve 10 directly or by
using a solvent, and the intake engine valve 10 is then heated,
whereby substantially all the passivation film on the intake engine
valve 10 can be removed easily and readily. When the intake engine
valve 10 is subjected to the ion nitriding treatment, the intake
engine valve 10 is nitrided substantially uniformly. Thus, the
thickness unevenness of the compound layer or nitrided layer and
the generation of a portion without the compound layer or nitrided
layer can be reduced, and the resultant intake engine valve 10 has
the layer with a large thickness and has a high hardness even in
the deeper portion.
[0057] Further, in this embodiment, the same agent can be used for
removing the passivation film and ion-nitriding the metal material.
Therefore, the material procurement cost can be reduced in the
embodiment.
[0058] The procedure of rising the temperature of the furnace is
continued. For example, when the temperature reaches 520.degree.
C., this temperature is maintained for 60 minutes. Also, the
vacuuming procedure is continued to maintain the inner pressure of
the furnace at approximately 0.7 to 1.5 Torr.
[0059] In the above steps, in the case of applying the resin
containing the repeating unit having the composition formula of
C.sub.6H.sub.3N.sub.9 to the intake engine valve 10, the residue of
the resin is hardly attached to the walls of the heat treatment
furnace. Thus, the number of maintenance of the heat treatment
furnace can be significantly reduced, and the maintenance work can
be simply and easily carried out. Further, the thickness of the
compound layer can be increased.
[0060] After the temperature holding process, the temperature is
lowered to approximately 200.degree. C., the heat treatment furnace
is opened, and the intake engine valve 10 is taken out. The
compound layer or the nitrided layer is formed on the surface of
the intake engine valve 10 by the ion nitriding treatment. The
surface of the intake engine valve 10 taken out is hard due to the
compound layer or the nitrided layer.
[0061] As shown in FIG. 3, a mixed gas of ammonia gas and RX gas,
etc. may be flowed into the furnace after the temperature reaches
approximately 500.degree. C., and the above temperature holding may
be carried out under the flow. In this case, the ammonia gas is
ionized to produce nitrogen ion, and the nitrogen ion collides with
the surface of the intake engine valve 10, which the passivation
film is reduced and removed from. The surface of the intake engine
valve 10 is nitrided also by the gas to form the compound layer or
the nitrided layer.
[0062] As shown in FIG. 4, after the start of the glow discharge, a
procedure of introducing a hydrogen gas may be started at a point A
higher than 400.degree. C. shown in FIG. 4, and thereby the
passivation film may be reduced and removed by the amino resin and
also by hydrogen gas sputtering. For example, the inner pressure of
the heat treatment furnace may be approximately 0.7 to 1.5
Torr.
[0063] The hydrogen gas is converted to the plasma state, H.sup.+
in the plasma collides with the surface of the intake engine valve
10 under an electric field. The H.sup.+ is introduced as a
diffusive hydrogen ion into the passivation film on the surface of
the intake engine valve 10.
[0064] The temperature is sufficient for reducing the passivation
film, and the introduced diffusive hydrogen ion is rapidly reacted
with the passivation film. Thus, the diffusive hydrogen ion attacks
the passivation film to reduce and remove the film. H.sub.2O is
generated by the reduction of the passivation film, and is rapidly
discharged to the outside of the system.
[0065] In the case of starting the hydrogen gas sputtering at a
temperature of 350.degree. C. or lower, the hydrogen ion is
diffused and introduced in the intake engine valve 10, the
passivation film being not reduced. In the case of starting the
hydrogen gas sputtering at a temperature of higher than 350.degree.
C. and not higher than 400.degree. C., the rate of the diffusion
and introduction of the hydrogen ion is too high as compared with
the rate of reducing the passivation film.
[0066] In this case, the characteristics of the diffusion and
introduction of the hydrogen ion, such as the diffusion depth,
amount, and distribution of the hydrogen ion, are greatly changed
depending on sputtering conditions. For example, when the voltage
is excessively high, the amount or depth of the diffused and
introduced hydrogen ion is excessively increased. Thus, the
sputtering conditions have to be highly precisely controlled to
obtain desired diffusion and introduction characteristics of the
hydrogen ion at a temperature of 400.degree. C. or lower.
[0067] In contrast, in the case of starting the hydrogen gas
sputtering at a temperature higher than 400.degree. C., the
diffusive hydrogen ion collides with the intake engine valve 10, is
introduced in the passivation film, rapidly causes the reduction of
the passivation film, and thus is rapidly consumed. Meanwhile the
hydrogen gas is emitted from the inside of the intake engine valve
10, whereby the amount or depth of the diffused and introduced
hydrogen ion is not excessively increased even when the voltage is
excessively high. Thus, the reduction and removal of the
passivation film can be easily controlled.
[0068] The hydrogen gas sputtering is preferably carried out at a
temperature of 450.degree. C. or higher. At the temperature, the
reduction of the passivation film is remarkably accelerated while
the hydrogen ion is emitted at a high rate from the intake engine
valve 10. Thus, the passivation film can be reduced and removed
without highly precisely controlling the sputtering conditions.
[0069] It is preferred that the hydrogen gas is supplied in
addition to the nitriding gas also in the temperature holding
process. In this case, the hydrogen gas sputtering proceeds along
with the ion nitriding, so that reproduction of the passivation
film on the surface of the intake engine valve 10 can be prevented.
The volume ratio between the nitriding gas and the hydrogen gas may
be, for example 2:1.
[0070] A second embodiment, which contains the steps of placing an
amino resin in a container and introducing the container in a heat
treatment furnace together with an exhaust engine valve 20 shown in
FIG. 5, will be described below. In the second embodiment, the
amino resin is not applied to a surface of the exhaust engine valve
20, and is placed in the powder state in the heat treatment
furnace.
[0071] The exhaust engine valve 20 has a wide head 22, a long bar
24, and an end 26. The bar 24 is divided at a portion slightly
closer to the head 22 than its center. Thus, the bar 24 has a first
bar 28 and a second bar 30 that is slightly longer than the first
bar 28.
[0072] The head 22 and the first bar 28 are composed of NCF600, and
the second bar 30 and the end 26 are composed of SUH11M. In the
surface of the exhaust engine valve 20, NCF600 and SUH11M are
spontaneously oxidized, whereby a passivation film of an oxide is
formed.
[0073] The exhaust engine valve 20 having such a structure is
degreased using an organic solvent, etc., and is placed in the heat
treatment furnace together with the above described amino resin.
Then, the exhaust engine valve 20 and the heat treatment furnace
are electrically connected to an electric power source such that
the valve 20 acts as a cathode and a furnace wall of the furnace
acts as an anode, to start electricity application. The current and
voltage may be about 25 .ANG. and about 220 to 250 V,
respectively.
[0074] When the amino resin is placed in the heat treatment
furnace, the amount of the amino resin may be about 1% to 10% of
the weight 1 kg of the metal material. For example, when the weight
of the exhaust engine valve 20 is 10 kg, 0.1 to 1 kg of the amino
resin may be added to the container and placed in the heat
treatment furnace.
[0075] Then, a compound layer and a nitride layer are formed on the
exhaust engine valve 20 by carrying out an ion nitriding treatment
in the manner shown in FIGS. 2 to 4.
[0076] The exhaust engine valve 20 may comprise a so-called
superalloy such as 30Ni15Cr or 75Ni15Cr. For example, in the case
of using 75Ni15Cr in the exhaust engine valve 20, a compound layer
having a thickness of approximately 5 .mu.m can be formed such that
the exhaust engine valve 20 and a melamine resin are placed in the
heat treatment furnace, the weight ratio of the melamine resin to
the exhaust engine valve 20 being 5%, and they are kept at
540.degree. C. for 2 hours under glow discharge to carry out the
ion nitriding treatment.
[0077] The materials of the head 22 and the first bar 28 are not
limited to NCF600, and may be NCF3015, NCF440, or the like. Also
the materials of the second bar 30 and the end 26 are not limited
to SUH11M, and may be SKH51 or the like. The thickness and Hv of
nitrided layers, formed by subjecting NCF3015, NCF440, and SKH51 to
the above hydrogen gas sputtering treatment and ion nitriding
treatment, are shown in FIG. 6. In the case of using NCF600 or
SUH11M, the nitriding treatment was started at 500.degree. C., the
temperature of the heat treatment furnace reached 520.degree. C. 20
minutes after the start, and this temperature was kept for 40
minutes. On the other hand, in the case of using NCF3015 or SKH51,
the nitriding treatment was started at 520.degree. C., the
temperature rise was stopped, and the temperature of 520.degree. C.
was kept for 60 minutes.
[0078] In the embodiment, the passivation film can be easily and
readily removed from the surface of the various metal material to
accelerate the nitriding treatment in this manner. Further, any
material can be ion-nitrided in the presence of the amino resin
without precisely controlling the types and ratios of gases,
reaction temperature, reaction time, etc.
[0079] The temperature, kept in the ion nitriding treatment, is not
limited to the above temperature of approximately 520.degree. C.
When the temperature is excessively high, the rate of the
reproduction of the passivation film is too high as compared with
the rate of the reduction and removal thereof, and the produced
nitride is diffused in the intake engine valve 10 or the exhaust
engine valve 20, so that the nitrided layer is hardly formed. To
prevent the reproduction of the passivation film and the diffusion
of the nitride, the temperature is preferably 590.degree. C. or
lower, more preferably 550.degree. C. or lower, most preferably
520.degree. C. to 540.degree. C.
[0080] Though the iron group alloys and Ni alloys are used as the
metal material in the above described embodiments, the metal
material is not particularly limited thereto. The metal material
may be any member composed of a metal alloy containing, as a main
component, a Group 8 metal element of the Periodic Table, such as a
Cr alloy. The conditions of the plasma nitriding treatment, such as
the kept temperature and the treatment time, may be appropriately
selected depending on the material of the work.
[0081] In the above embodiment, the electricity is applied to the
intake engine valve 10 such that the furnace wall is used as an
anode and the intake engine valve 10 is used as a cathode. For
example, the electricity may be applied such that an electrode is
installed in the heat treatment furnace, the electrode is used as
an anode, and the intake engine valve 10 is used as a cathode.
Example 1
[0082] An engine valve was prepared from each of metal materials
shown in FIG. 7, the engine valve and a melamine resin were placed
in a heat treatment furnace, and the engine valve was ion-nitrided
at 520.degree. C. for 60 minutes. For comparison, the metal
materials were ion-nitrided in the same manner except for not
placing the melamine resin in the heat treatment furnace. The
thickness of the compound layer or nitride layer and the surface
hardness of each engine valve ion-nitrided in the presence of the
melamine resin are shown in FIG. 5 in multiples of those of each
engine valve ion-nitrided without the melamine resin. It is clear
from FIG. 5 that the thickness of the compound layer or nitride
layer and the surface hardness can be increased by using the
melamine resin in the ion nitriding treatment. This means that,
when the ion nitriding treatment is carried out in the presence of
the melamine resin, a passivation film is removed and whereby the
nitride is substantially uniformly diffused deep into the metal
material.
INDUSTRIAL APPLICABILITY
[0083] In the present invention, the metal material is ion-nitrided
in the presence of the amino resin. By the simple procedure, the
passivation film on the surface of various metal material can be
easily removed and the surface can be easily nitrided under secure
work environment. Thus, the compound layer or nitrided layer having
a substantially uniform thickness can be formed on the
approximately entire surface of the metal material.
[0084] Further, in the present invention, the thickness of the
compound layer or nitrided layer can be increased. In other words,
the metal material can show improved hardness, strength, abrasion
resistance, etc. even in a deep inside region.
* * * * *