U.S. patent application number 10/394804 was filed with the patent office on 2004-01-15 for method and apparatus for dynamic nitriding.
Invention is credited to Fujino, Ryoji, Fujita, Hideto, Fujita, Yoshiyuki, Ogawa, Yasutaka.
Application Number | 20040007292 10/394804 |
Document ID | / |
Family ID | 29774578 |
Filed Date | 2004-01-15 |
United States Patent
Application |
20040007292 |
Kind Code |
A1 |
Fujita, Hideto ; et
al. |
January 15, 2004 |
Method and apparatus for dynamic nitriding
Abstract
A nitriding method includes the steps of nitriding an object
held under a nitriding gas atmosphere in a sealed furnace and
applying vibration energy to one or both of the nitriding gas and
the object W to facilitate nitriding. A nitriding apparatus
includes a nitriding furnace for holding an object W to be nitrided
in a sealed manner, means 30 for supplying a nitriding gas to the
furnace, and means 2 for applying vibration to the atmosphere gas
in the furnace 1 to faciliate nitriding. According to the method or
apparatus, nitriding-resistant or complex-shaped materials can be
nitrided at high efficiency and a nitrided layer can be formed at a
lower temperature for a shorter time as compared with conventional
processes.
Inventors: |
Fujita, Hideto; (Kyoto,
JP) ; Fujita, Yoshiyuki; (Kyoto, JP) ; Fujino,
Ryoji; (Kyoto, JP) ; Ogawa, Yasutaka; (Kyoto,
JP) |
Correspondence
Address: |
Martin G. Linihan
Hodgson Russ LLP
Suite 2000
One M&T Plaza
Buffalo
NY
14203-2391
US
|
Family ID: |
29774578 |
Appl. No.: |
10/394804 |
Filed: |
March 21, 2003 |
Current U.S.
Class: |
148/238 ;
266/251 |
Current CPC
Class: |
C23C 8/26 20130101; B01J
4/04 20130101; C23C 8/24 20130101; B06B 1/10 20130101; B01J 19/185
20130101 |
Class at
Publication: |
148/238 ;
266/251 |
International
Class: |
C23C 008/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 15, 2002 |
JP |
2002-204961 |
Claims
1. A method for dynamically nitriding an object, comprising the
steps of: nitriding said object held under a nitriding gas
atmosphere in a sealed furnace; and applying vibration energy to
one or both of said nitriding gas and said object to facilitate
nitriding.
2. The method according to claims 1, wherein said vibration energy
is so selected as to generate vibration at a vibration frequency of
400 to 5000 vibrations per minute.
3. A gas vibration type nitriding apparatus, comprising: a
nitriding furnace for holding an object to be nitrided in a sealed
manner; means for supplying a nitriding gas to said nitriding
furnace; and means for applying vibration to an atmosphere gas in
said nitriding furnace, wherein nitriding will be facilitated by
allowing said atmosphere gas to vibrate.
4. A gas vibration type nitriding apparatus, comprising: a
nitriding furnace for holding an object to be nitrided in a sealed
manner; gas supply lines for supplying a nitriding gas and/or one
or more atmosphere gases other than said nitriding gas to said
nitriding furnace; and means for applying vibration to one or more
gases in some or all of said gas supply lines, wherein nitriding
will be facilitated by atmosphere gas vibration.
5. The gas vibration type nitriding apparatus according to claim 3
or 4, wherein said means for applying vibration is a fast vibrating
diaphragm, and one or more gases being supplied to said furnace are
allowed to collide against said vibrating diaphragm to apply
vibration to atmosphere gas in said furnace.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a nitriding method that can
provide dramatically enhanced nitriding efficiency in gas nitriding
processes of such materials as steel members and
nitriding-resistant materials, and an apparatus for use
therein.
DESCRIPTION OF THE RELATED ART
[0002] Conventionally, nitriding processes are used to harden the
surface of steel members. The nitriding processes are carried out
at a relatively low temperature as compared with cementation
processes and therefore can provide the nitrided materials with
less deformation or distortion. The nitriding processes can also
form a very hard nitrided layer in the surface of steels and
therefore are widely used as a surface treatment process for
providing a good wear or corrosion resistance. Known examples of
the nitriding process include a gas nitriding process, a salt bath
nitriding process, and an plasma nitriding process.
[0003] Generally, the salt bath nitriding process uses cyanide
salts, which can provide a harmful working environment, and
requires a high cost for waste disposal. The ion nitriding process,
which uses a discharge process under reduced pressure, is suited
for the treatment of simple-shaped objects. In the ion nitriding
process, however, it would be difficult to evenly nitride objects
that have complicated shapes, small holes, or deep holes.
[0004] In the gas nitriding process, the object such as a steel
product is heated in a nitrogen-containing nitriding gas such as
ammonia gas. The heated nitriding gas is decomposed into nitrogen
atoms, which chemically react with the iron components in the steel
surface to form a nitrided layer for hardening the steel surface.
The gas nitriding process can be carried out in a good working
environment and applied to complex-shaped objects, so that it can
be free from the problem with the salt bath or ion process.
[0005] The gas nitriding process, however, requires a step of
removing a passive state film from the surface of
nitriding-resistant materials such as austenitic stainless steels
before nitriding. It can also provide complex-shaped materials with
uneven thickness of the nitrided layer or form insufficiently
nitrided portions at small or deep holes.
[0006] In general, the nitriding gas is superficially brought into
contact with the object in the nitriding process so that the
chemical reactions involved in the nitriding are slow and the
process for a thick nitrided layer needs a long time (at least 40
hours) or a high temperature treatment (550 to 580.degree. C. or
above). Such a long time or high temperature process can reduce the
hardness of the nitrided layer, increase the embrittled layer
(white layer), or increase the dimensional change, and adversely
affect the metallurgical properties of the object. The long time
process can also increase the usage of the nitriding gas, decrease
the productivity of the nitrided product, or provide the product
with low cost-performance.
SUMMARY OF THE INVENTION
[0007] In light of the above-mentioned problems, the present
invention is directed to a new epoch-making nitriding method that
can nitride nitriding-resistant materials or complex-shaped
materials with good efficiency and form a excellent nitrided layer
at a low temperature for a short time period in contrast to
conventional gas nitriding processes, and an apparatus for use
therein.
[0008] According to the present invention, it provides a nitriding
method in which a nitriding reaction of an object is allowed to
proceed under vibrating conditions. Such a method includes the
steps of nitriding the object held under an atmosphere gas in a
sealed nitriding furnace and applying vibration energy to one or
both of a nitriding gas and the object to facilitate nitriding.
[0009] The present invention is also directed to an apparatus
including means for applying vibration to one or both of gas and an
object to be nitrided.
[0010] In a general gas nitriding process, the surface of steel is
heated and brought into contact with a nitriding gas such as
ammonia gas. By the catalysis of the steel surface, the ammonia gas
is decomposed into active atomic nitrogen, which reacts with the
iron components in the steel to form a nitrided layer.
[0011] For example, by the nitriding reaction of iron (Fe) with
ammonia (NH.sub.3), a nitrided layer ([Fe]N layer) is formed in the
surface of steel according to the following formula:
NH.sub.3+[Fe].fwdarw.H.sub.2+[Fe]N (1)
[0012] As a nitrided iron, there are Fe.sub.2N and Fe.sub.4N.
[0013] On the steel containing an alloy such as aluminum (Al),
chromium (Cr), titanium (Ti), and vanadium (V) alloys, the
activated nitrogen atoms react with the alloy element to form
aluminum nitride (AlN), chromium nitride (Cr.sub.2N.sub.3), or the
like according to the following formulas:
NH.sub.3+Al.fwdarw.H.sub.2+AlN (2)
NH.sub.3+Cr.fwdarw.H.sub.2+Cr.sub.2N.sub.3 (3)
[0014] Such materials as iron nitrides (Fe.sub.2N and Fe.sub.4N),
aluminum nitride (AlN), and chromium nitride (Cr.sub.2N.sub.3) are
insoluble in iron, hard and stable, and have the function of
hardening the steel surface.
[0015] The inventor has made active investigations on the gas
nitriding process and found that the activity of the atomic
nitrogen generated by the contact of the nitriding gas with the
steel surface significantly affects the progress of the nitriding
reaction. Thus, the inventor has made various experiments to find
out a method for facilitating the activation of the nitriding gas.
In order to facilitate the activation, the nitriding reaction
temperature could be raised, but in such a case, embrittled layers
can be formed, or distortion in shape can occur as mentioned
above.
[0016] As a result of the experiments and the consideration, the
inventor has found that by the contact of the steel surface with a
nitriding gas being provided with vibration, the activation of the
nitrogen can be dramatically enhanced in the catalytic reaction
between the nitriding gas and the steel, and the efficiency in
nitriding steels can drastically be improved. In the description,
such an improved process is called a dynamic nitriding method.
[0017] The nitrided material obtained by this method is hard and
stable and contains fine precipitates dispersed in the alpha iron
lattice. As a result, it has been confirmed that the alpha iron
lattice is provided with high distortion and the steel is
significantly hardened.
[0018] Preferred means for applying vibration to the nitriding gas
includes a technique of allowing the nitriding gas to collide
against a diaphragm in a sealed nitriding furnace in supplying the
nitriding gas to the furnace so that a shock wave is generated in
the nitriding gas-containing atmosphere gas in the furnace. A
principal part of the technique is a step of producing a dynamic
nitriding reaction, in other words, producing a catalytic reaction
between the nitriding gas and the steel in such a state that
molecular vibration or mechanical vibration is involved, in
addition to the conventional fluid catalytic reaction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a side view showing the mechanism of a gas
vibration type nitriding apparatus;
[0020] FIGS. 2A and 2B are schematic views showing a first example
of the vibration-applying means;
[0021] FIG. 3 is a schematic view showing a second example of the
vibration-applying means;
[0022] FIG. 4 is a diagram showing a gas flow;
[0023] FIG. 5 shows hardness of an object (an austenitic stainless
steel) with respect to distances from the surface of the
object;
[0024] FIG. 6 shows hardness of an object (a martensitic stainless
steel) with respect to distances from the surface of the
object;
[0025] FIG. 7 shows hardness of an object (a hot work tool steel)
with respect to distances from the surface of the object;
[0026] FIG. 8 is a micrograph showing a surface side section of a
martensitic stainless steel object nitrided according to the
present invention; and
[0027] FIG. 9 is a micrograph showing a surface side section of a
martensitic stainless steel object nitrided by a conventional
process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] Referring to the attached drawings, the present invention is
described in detail in the following example.
[0029] FIG. 1 shows an example of the dynamic nitriding method
according to the present invention. In this example using a gas
vibration type nitriding apparatus, vibration is applied to a
nitriding gas being introduced into a nitriding furnace in the step
of supplying the nitriding gas to the nitriding furnace, so that
shock and vibration is being generated in the atmosphere gas (a
mixture of the nitriding gas and an inert gas) in the furnace while
the nitriding is allowed to proceed. In the drawing, numeral 1
represents a sealed nitriding furnace in which an object W to be
nitrided such as a steel product is placed. On the sides of the
furnace 1 are arranged a gas control unit 3 for controlling the
feed rate, pressure and the like of the nitriding gas being
supplied to the nitriding furnace 1 and a temperature control unit
4 for controlling the temperature of the nitriding furnace 1.
Numeral 2 represents vibration-applying means provided according to
the present invention, which will be described below in detail.
[0030] The nitriding furnace 1 has a furnace body 10, a retort 11
for holding the object W in the furnace body 10, and a heater 12
for heating the retort 11. The upper portion of the furnace 1 is
provided with an opening 13 for pulling out the retort 11. The
upper portion of the nitriding furnace 1 is also provided with a
nitriding gas supply line 30 for supplying the nitriding gas into
the furnace body 10, and an inert gas supply line 31 for supplying
an inert gas (e.g. N.sub.2 gas) excluding the nitriding gas into
the furnace body 10. Hereinafter, the nitriding gas supply line 30
and the inert gas supply line 31 are simply called the gas supply
lines 30 and 31. The lower portion of the nitriding furnace 1 is
provided with a gas exhaust line 32 for discharging air or the
atmosphere gas from the furnace body 10.
[0031] An upper portion of the retort 11 has an opening flange 14,
in which a cover 15 is detachably provided. The temperature control
unit 4 controls the heater 12 in response to the temperature
detected by a temperature sensor 40 placed in the furnace body 10.
The gas control unit 3 controls the introduction and discharge of
the gas, the combination and exchange of the gases, the flow rate
and pressure of the gas, and the like.
[0032] Examples of the object W to be nitrided include an
austenitic stainless steel (SUS304), a martensitic stainless steel
(SUS420J2), and a hot work tool steel (SKD61). Before nitriding,
the object W is placed on a mount 17 provided in the retort 11 and
then the cover 15 is attached to the retort 11 to seal it.
[0033] FIGS. 2 and 3 are schematic diagrams showing first and
second examples of the vibration-applying means 2,
respectively.
[0034] Referring to FIG. 2A, the first example of the
vibration-applying means 2 is described. A passage pipe 24 is
attached to the upper portion of the nitriding furnace 1. A
nitriding gas introducing pipe 30, which serves as the nitriding
gas supply line 30, is connected to an upper side 24a of the
passage pipe 24. A cylindrical agitator 20 is inserted and freely
slidably provided in a lower portion of the passage pipe 24. The
agitator 20 is suspended by a supporting rod 23 from a vibration
transmitting plate 27c outside the furnace 1. The agitator 20 has
an outlet to the furnace 1 space, and a diaphragm 21 is attached to
an outlet portion of the agitator 20. Numeral 25 represents a motor
for applying vibration to the agitator 20. The motor 25 is fixed
onto a casing 16, and as shown in FIG. 2B, its rotation is
converted into up-and-down movements of the supporting rod 23
through a cam 27a and a cam-follower 27b, so that vibration is
transmitted to the agitator 20.
[0035] The nitriding gas supplied from the nitriding gas
introducing pipe 30 flows from the upper side 24a to the lower side
in the passage pipe 24 and collides against the agitator 20 which
is vibrating at a high speed in up-and-down directions. By such a
mechanism, vibration energy is applied to the nitriding gas, which
involves a vibration-shock wave and is discharged into the furnace
1 through an outlet hole 22 formed in the agitator 20. At the
moment, the nitriding gas also collides against the diaphragm 21
vibrating at a high speed to absorb additional vibration energy.
The nitriding gas with the additional vibration energy is
discharged into the nitriding furnace 1 to chemically react with
the object W. In addition, a flexible sealing mechanism (such as a
bellows mechanism) is provided at the upper portion of the passage
pipe 24 to ensure the sealing between the passage pipe 24 and the
supporting rod 23, and sleeve bearings are provided at upper and
central portions of the supporting rod 23 to improve the stability
and durability under the vibration.
[0036] Referring to FIG. 2B, the structure of the
vibration-applying means 2 is described. In the drawing, numeral 26
represents an output shaft of the motor 25. Upon rotation of the
shaft 26, the cam 27a attached to it rotates in the direction
indicated with arrow a and comes into contact with the cam-follower
27b having a corrugated shape and placed on the plate 27c, so that
the cam 27a allows the cam-follower 27b to vibrate in the
up-and-down directions indicated with arrow b together with the
plate 27c. The plate 27c holding the cam-follower 27b is movable in
the up-and-down directions and supported by a plurality of
supporting columns 28b, which are firmly planted on the top of the
body of the nitriding furnace 1, through coiled springs 28a. To the
plate 27c is attached the upper end of the supporting rod 23
suspending the agitator 20. By this mechanism, the running torque
of the motor shaft 26 is transmitted through the cam 27a and the
cam-follower 27b to the plate 27c in the form of vertical vibration
of the plate 27c, which results in vertical vibration of the
agitator 20 and the diaphragm 21 through the supporting rod 23.
[0037] Referring to FIG. 3, the second example of the
vibration-applying means 2 is described. This example of the
vibration-applying means 2 also has a motor 25, an output shaft 26,
a cam 27a, a cam-follower 27b, a plate 27c, a coiled spring 28a,
and a supporting column 28b in the same configuration as those of
the first example, and the description thereof is omitted.
[0038] The difference between this example and the first example is
that a passage pipe 24 is directly attached to the plate 27c, and a
diaphragm 21 is attached to a lower end of the passage pipe 24 so
that the passage pipe 24 itself, through which the nitriding gas
passes, is allowed to vertically vibrate for vibration of the
diaphragm 21. This example is free of the agitator 20. A flexible
sealing mechanism is provided at an upper portion of the passage
pipe 24 to ensure the sealing between the passage pipe 24 and the
top side of the nitriding furnace 1. A sleeve bearing is also
provided at an upper portion of the supporting rod 23 to improve
the stability and durability under the vibration. In this example,
the passage pipe 24 is allowed to vibrate at an amplitude raging
from about 1 to 10 mm and at a vibration frequency ranging from
about 400 to 5,000 vibrations per minute (vpm) depending on the
type of the object to be nitrided.
[0039] In the first example, the agitator 20 has the outlet hole
22. Alternatively, in the second example, a lower end portion of
the passage pipe 24 has a plurality of outlet holes 24b from which
the nitriding gas is discharged through the passage pipe 24. The
discharged nitriding gas is allowed to collide against the
diaphragm 21 vibrating at a high speed so that the vibration energy
is applied to the nitriding gas, which involves a vibration-shock
wave together with the atmosphere gas in the retort 11 and reacts
with the object W.
[0040] FIG. 4 is a diagram showing the gas flow. Referring to FIG.
4, the gas flow in the nitriding process is described in
detail.
[0041] First, a supply valve V1 for supplying the gas into the
retort 11 and an exhaust valve V2 for discharging the gas from the
retort 11 are shut. An exhaust valve V3 is then opened, and a
vacuum pump VP is run to evacuate air from the retort 11. After the
evacuation, the supply valve V1 is opened to introduce the
nitriding gas (NH.sub.3) into the retort 11. If necessary, an inert
gas (such as N.sub.2) may be introduced before or together with the
introduction of the nitriding gas.
[0042] After the retort 11 is filled up with an atmosphere gas
containing the nitriding gas, an exhaust valve V4 is opened and the
pressure of the atmosphere gas in the retort 11 is controlled with
a mass flow controller MC or a pressure regulator PR. The pressure
of the atmosphere gas is determined depending on the shape or
material of the object W, or the hardness requirement for the
nitrided layer.
[0043] After the pressure of the atmosphere gas becomes stable in
the retort 11, the temperature inside the nitriding furnace 1 is
elevated with the heater 12 (see FIG. 1). The temperature inside
the nitriding furnace 1 is from 300 to 600.degree. C. (depending on
the shape or material of the object W, or the hardness requirement
for the nitrided layer). In this example, a sufficient effect was
obtained at a lower temperature (about 350.degree. C.) than that of
a conventional nitriding process (about 550.degree. C.).
[0044] Vibration is then applied to the nitriding gas with the
vibration-applying means 2, while the nitriding gas is introduced
into the retort 11 for a certain time period. After the conclusion
of the nitriding, the heater 12 is turned off and the object W is
allowed to cool before taken out.
[0045] In this example, the vibration-applying means 2 is placed on
the upper side of the nitriding furnace 1. Alternatively, such
means may be placed on the lower side. If the nitriding furnace 1
holds a number of small objects W, the vibration-applying means 2
may be arranged at each of the upper and lower sides of the
nitriding furnace 1 so that all the objects W can effectively be
nitrided.
[0046] In addition, the object W may be rotated, while vibration is
applied to the nitriding gas-containing atmosphere gas being
supplied to the nitriding furnace 1. Such a process can reduce
unevenness in nitriding the object W and provide uniform nitriding
over the entire surface.
[0047] Nitriding processes were carried out according to the
present invention and a conventional technique under the same
conditions of temperature and time. The advantage of the present
invention is examined from the resulting data.
[0048] FIGS. 5 to 7 show hardness of the object with respect to
distances from the surface of the object. Parts A and B are a table
and a graph each showing, in comparison, data obtained by the
process according the present invention and the conventional
process.
[0049] Referring to FIG. 5, the experimental data were obtained by
nitriding austenitic stainless steel (SUS304) objects. The drawing
shows that a rigid nitrided layer about 10 .mu.m in thickness is
formed in the surface of the object according to the present
invention, but only a trace of nitrided layer is formed by the
conventional process.
[0050] Referring to FIG. 6, the experimental data were obtained by
nitriding martensitic stainless steel (SUS420J2) objects. The
drawing shows that a rigid nitrided layer 15 to 20 .mu.m in
thickness is formed in the surface of the object according to the
present invention, but a rigid nitrided layer only less than 5
.mu.m in thickness is formed by the conventional process.
[0051] Referring to FIG. 7, the experimental data were obtained by
nitriding hot work tool steel (SKD61) objects. The drawing shows
that a rigid nitrided layer 10 to 15 .mu.m in thickness is formed
in the surface of the object according to the present invention,
but only a trace of nitrided layer is formed by the conventional
process.
[0052] FIG. 8 is a micrograph (.times.400) showing a surface side
section of the martensitic stainless steel (SUS420J2) object, which
was nitrided according to the present invention. FIG. 9 is a
micrograph (.times.400) showing a surface side section of the
martensitic stainless steel (SUS420J2) object, which was nitrided
by the conventional process. The drawings show that a nitrided
layer about 17 .mu.m in thickness is formed in the surface of the
object according to the present invention, but a nitrided layer
only about 4 .mu.m in thickness is formed by the conventional
process.
[0053] From the resulting data, it has been confirmed that
according to the present invention, the nitriding time can
significantly be reduced under the same temperature conditions as
those of the conventional gas nitriding method. According to the
present invention, a sufficient nitriding effect was also obtained
at a low temperature on objects each having a complicated shape, a
small hole, a deep hole, or an unpierced hole.
[0054] In the above experimental examples, the diaphragm 21 was
allowed to vibrate at a vibration frequency of about 1500 to 3500
vibrations per minute. At such a vibration frequency, a sufficient
effect was confirmed. According to the results obtained by
additional experiments, the nitriding effect has also been
confirmed at a vibration frequency of the diaphragm 21 ranging from
400 to 5000 vibrations per minute.
[0055] In the above description, the dynamic nitriding method and
apparatus according to the present invention are exemplified by the
gas vibration type nitriding method and apparatus. In such
examples, the vibration-applying means 2 is placed in the nitriding
gas supply line 30, and the nitriding gas being provided with
vibration is introduced into the nitriding furnace 1 so that the
atmosphere gas in the furnace 1 is allowed to vibrate while the
fluid catalytic reaction with the object surface is allowed to
proceed. The experimental data and the photographs of the
structures obtained by the examples are also shown. However, such
examples are not intended to be limiting upon the scope of the
invention.
[0056] For example, the vibration-applying means 2 may be provided
in the inert gas supply line 31 so as to allow the nitriding gas in
the furnace 1 to vibrate. This method may be combined with the
above-described method so that the nitriding effect can further be
improved.
[0057] In place of the vibration-applying means 2 provided in the
gas supply lines 30 and 31, the diaphragm 21 or the like may be
placed in the vicinity of the work (object to be nitrided) W in the
retort 11. The diaphragm 21 may be allowed to vibrate through
electromagnetic type or mechanical type vibration-applying means so
as to provide the atmosphere gas with a vibration-shock wave in the
vicinity of the object W. Such a mechanism can produce a similar
effect and, of course, may be combined with the above-described
mechanism.
[0058] Generally, atomic nitrogen from the nitriding gas greatly
contributes to the nitriding reaction and therefore the vibration
directly applied to the nitriding gas should be most effective.
[0059] In the dynamic nitriding method according to the present
invention, the step of applying vibration to the nitriding gas or
the atmosphere gas may be replaced by the step of applying
vibration energy to the object W itself. By such a step, the
activation of the nitriding reaction of the object W with the
nitriding gas can be facilitated so that a similar effect can be
produced. Specifically, a small object W may be placed on a
vibrating mount and allowed to vibrate under the nitriding gas
atmosphere in the retort 11. The vibration of the object W itself
may also be combined with the vibration of the atmosphere gas. In
such a case, the nitriding would further be facilitated, and the
nitriding time can be reduced.
[0060] Besides the mechanical vibration, the vibration-applying
means 2 may comprise means for applying ultrasonic vibration to the
nitriding gas-containing atmosphere gas or means for generating low
frequency molecular vibration. Such means can complementarily be
combined with the mechanical vibration means so that the effect can
further be enhanced.
[0061] The above-described methods, including the method comprising
the step of applying vibration or a shock wave to the gas in
nitriding, the method comprising the step of allowing the object to
vibrate in nitriding, and the method in which both of the
above-mentioned steps are combined, are collectively called the
dynamic nitriding method in contrast to the conventional static
nitriding method. The dynamic nitriding method is characterized in
that high rate nitriding can be achieved at a low temperature.
INDUSTRIAL APPLICABILITY
[0062] In the present invention, the nitriding reaction is allowed
to occur under the environmental conditions including the
artificial application of vibration. In such conditions, the
catalytic effect of the steel surface can be enhanced and the
thermal decomposition reaction of the nitriding gas can be
accelerated. In the present method, the dissociation of nitrogen
atoms from ammonia gas is so facilitated that much nascent hydrogen
can be generated and such hydrogen atoms can provide a stronger
reducing action. As a result, a more stable nitrided layer can be
obtained together with an etching effect on the steel surface.
[0063] As described above, according to the present invention,
effective nitriding can be performed on nitriding-resistant
materials such as austenitic stainless steels and martensitic
stainless steels, and complex-shaped objects having edges, small
holes, or deep holes. The present invention is also advantageously
applied to frequently used materials such as hot work tool steels,
because nitriding can be performed at a lower temperature for a
shorter time period with the embrittled layer (white layer)
significantly reduced and with less harmful effect on the internal
structure of the object than the conventional nitriding method.
[0064] According to the present invention, the process at a lower
temperature for a shorter time period can reduce the usage of the
nitriding gas and nitriding furnace heating energy, improve the
working environment, and therefore be economically and
environmentally advantageous.
* * * * *