U.S. patent application number 10/559531 was filed with the patent office on 2006-06-15 for nitriding method and device.
Invention is credited to Hitoshi Karasawa, Hideo Kojima, Yutaka Takeuchi.
Application Number | 20060124202 10/559531 |
Document ID | / |
Family ID | 33549350 |
Filed Date | 2006-06-15 |
United States Patent
Application |
20060124202 |
Kind Code |
A1 |
Takeuchi; Yutaka ; et
al. |
June 15, 2006 |
Nitriding method and device
Abstract
A pulse voltage having a frequency of 15 kHz is applied from a
discharging power supply unit (48) to between a crankshaft (12) and
an electrode plate (45) at a current density of 2.5 mA/cm2 to
generate a glow discharge and an electric heater (34) is driven at
a 40% output (64 kW/kg) to heat the crankshaft (12) to up to
400.degree. C., and then heating is continued with the current
density of a glow discharge set at 0.5 mA/cm2 and the output of the
electric heater (34) set at 90% (144 kW/kg), thereby effecting
nitriding at a desired nitriding temperature.
Inventors: |
Takeuchi; Yutaka; (Saitama,
JP) ; Karasawa; Hitoshi; (Saitama, JP) ;
Kojima; Hideo; (Saitama, JP) |
Correspondence
Address: |
ARENT FOX PLLC
1050 CONNECTICUT AVENUE, N.W.
SUITE 400
WASHINGTON
DC
20036
US
|
Family ID: |
33549350 |
Appl. No.: |
10/559531 |
Filed: |
June 10, 2004 |
PCT Filed: |
June 10, 2004 |
PCT NO: |
PCT/JP04/08133 |
371 Date: |
December 2, 2005 |
Current U.S.
Class: |
148/222 |
Current CPC
Class: |
C23C 8/36 20130101 |
Class at
Publication: |
148/222 |
International
Class: |
C23C 8/36 20060101
C23C008/36; C23C 8/24 20060101 C23C008/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 13, 2003 |
JP |
2003-169012 |
Claims
1-12. (canceled)
13. A nitriding treatment method for performing a nitriding
treatment for a workpiece in a heat treatment furnace, said
nitriding treatment method comprising: a first step of applying a
pulse voltage having a predetermined current density at a frequency
of not less than 1 kHz between said heat treatment furnace and said
workpiece to heat said workpiece by means of generated glow
discharge; and a second step of decreasing said current density of
said pulse voltage after a temperature of said workpiece arrives at
350.degree. C., and then heating said workpiece up to a desired
nitriding treatment temperature by using a heating element arranged
around said workpiece, wherein said nitriding treatment is
performed by means of nitrogen ion or nitrogen radical generated by
said glow discharge.
14. The nitriding treatment method according to claim 13, wherein
said workpiece is heated by heat generated by said glow discharge
and said heating element in said first step; and heating is
effected in said second step such that an amount of heat generated
by said heating element is higher than that in said first step.
15. The nitriding treatment method according to claim 13, wherein
said current density of said pulse voltage is gradually decreased
in said second step, while said workpiece is gradually heated up to
said nitriding treatment temperature by using said heating element
arranged around said workpiece.
16. The nitriding treatment method according to claim 13, wherein
said nitriding treatment temperature is maintained to execute said
nitriding treatment after said workpiece arrives at said desired
nitriding treatment temperature in said second step.
17. The nitriding treatment method according to claim 13, wherein
said current density of said pulse voltage is 0.05 to 7
mA/cm.sup.2.
18. The nitriding treatment method according to claim 13, wherein
said current density of said pulse voltage is 0.1 to 4
mA/cm.sup.2.
19. The nitriding treatment method according to claim 13, wherein
said temperature of said workpiece is determined by detecting a
temperature difference between a radiation temperature and a
contact temperature of a dummy workpiece arranged in said heat
treatment furnace, detecting a radiation temperature of said
workpiece, and correcting said radiation temperature of said
workpiece with said temperature difference.
20. A nitriding treatment apparatus for performing a nitriding
treatment for a workpiece in a heat treatment furnace, said
nitriding treatment apparatus comprising: a glow
discharge-generating means which generates glow discharge by
applying a pulse voltage having a predetermined current density at
a frequency of not less than 1 kHz between said heat treatment
furnace and said workpiece; a heating means which heats said
workpiece by using a heating element arranged in said heat
treatment furnace; a temperature-detecting means which detects a
temperature of said workpiece; and a control means which controls
said current density of said glow discharge effected by said glow
discharge-generating means on the basis of said temperature of said
workpiece detected by said temperature-detecting means and which
controls said heating means, wherein said temperature-detecting
means includes: a dummy workpiece radiation thermometer which
detects a radiation temperature of a dummy workpiece arranged in
said heat treatment furnace; a dummy workpiece contact thermometer
which detects a contact temperature of said dummy workpiece; a
workpiece radiation thermometer which detects a radiation
temperature of said workpiece; and a workpiece
temperature-calculating means which calculates said temperature of
said workpiece by calculating a temperature difference between said
radiation temperature and said contact temperature of said dummy
workpiece and correcting said radiation temperature of said
workpiece with said temperature difference.
21. The nitriding treatment apparatus according to claim 20,
wherein said heat treatment furnace includes: a nitriding treatment
chamber which accommodates said workpiece and which is surrounded
by an electrode plate for generating said glow discharge in
cooperation with said workpiece; a heating chamber which involves
said heating element arranged around an outer circumference of said
electrode plate and which is surrounded by a partition wall; and a
cooling means which is arranged around an outer circumference of
said partition wall and to which a cooling liquid for cooling said
partition wall is supplied.
22. The nitriding treatment apparatus according to claim 20,
wherein said heat treatment furnace is a lateral type heat
treatment furnace.
23. The nitriding treatment apparatus according to claim 20,
wherein said workpiece is a crank shaft.
Description
TECHNICAL FIELD
[0001] The present invention relates to a nitriding treatment
method and a nitriding treatment apparatus for performing a
nitriding treatment for a workpiece in a heat treatment
furnace.
BACKGROUND ART
[0002] The nitriding treatment resides in a process in which a
surface of a workpiece composed of a steel material is nitrided to
form an iron nitride layer, and thus the surface of the workpiece
is hardened. The nitriding treatment is widely carried out. A
method for the nitriding treatment includes a plasma nitriding
treatment in which the plasma heating is performed by means of the
glow discharge.
[0003] In the case of such a treatment, when the workpiece is cold,
the glow discharge becomes unstable. Depending on certain
situations, any inconvenience arises such that the arc discharge is
caused, and a part of the workpiece surface is melted, and/or it is
impossible to apply any uniform nitriding treatment. In particular,
when the temperature distribution of the workpiece is nonuniform,
it is impossible to obtain satisfactory products, because the
degree of advance of nitriding differs depending on the positions
of the product.
[0004] In view of the above, in order to heat the workpiece quickly
and uniformly up to a desired temperature at which the nitriding
treatment can be performed stably, a conventional technique is
known, which is constructed such that a heating means such as an
infrared heater or a graphite cloth heating element is arranged
around the workpiece, and the workpiece is heated by using the
heating means together with the glow discharge (see, for example,
Japanese Laid-Open Patent Publication Nos. 52-82641 and
53-23836).
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0005] However, for example, when a plurality of workpieces having
complicated shapes are simultaneously heated in a heat treatment
furnace, it is impossible to obtain any uniform temperature
distribution for the workpieces when the glow discharge and the
heating means are simply used together as in the conventional
technique as described above.
[0006] That is, the heat, which is brought about by the glow
discharge, is basically generated such that the nitrogen ions,
which are produced by the glow discharge, collide with the
workpiece surface. However, when the workpiece is heated to be not
less than a certain temperature, the surrounding workpieces are
heated by the radiation heat of the workpiece itself. Therefore,
the difference in temperature appears between the region which is
disposed close to the heating means and the region which is
disposed separately from the heating means. If a large number of
heating means are arranged among the workpieces, this problem can
be dissolved. However, it is almost impossible to construct the
heating means arranged at appropriate positions among the
workpieces in relation to a production line such as those used in
the various kinds and small quantity production (job shop type
production or flexible manufacturing) in which it is required to
perform the nitriding treatment for the workpieces having different
shapes.
[0007] A general object of the present invention is to provide a
nitriding treatment method and a nitriding treatment apparatus in
which a workpiece is heated quickly and uniformly up to a
temperature at which a stable nitriding treatment can be performed,
making it possible to apply the nitriding treatment in a state in
which the workpiece temperature dispersion is scarcely caused.
[0008] A principal object of the present invention is to provide a
nitriding treatment method and a nitriding treatment apparatus in
which an extremely satisfactory nitriding treatment can be applied
to a large number of workpieces having complicated shapes.
[0009] Another object of the present invention is to provide a
nitriding treatment method and a nitriding treatment apparatus
which can be easily applied to different workpieces of many
types.
[0010] Still another object of the present invention is to provide
a nitriding treatment method and a nitriding treatment apparatus in
which a satisfactory nitriding treatment can be applied when the
present invention is applied to a lateral type heat treatment
furnace.
[0011] Still another object of the present invention is to provide
a nitriding treatment method and a nitriding treatment apparatus in
which an extremely satisfactory nitriding treatment can be applied
by managing the workpiece temperature highly accurately.
MEANS FOR SOLVING THE PROBLEMS
[0012] According to the present invention, the pulse voltage, which
is used to generate the glow discharge, is allowed to have a
frequency of not less than 1 kHz. Accordingly, the period of time,
in which the current is continuously applied, is not more than 1
ms, and the continuation of the arc discharge is avoided with a
short detecting time. Thus, the current is cut off before arrival
at a voltage at which any arc mark appears. Therefore, the stable
glow discharge state can be maintained by avoiding the appearance
of any arc mark, and it is possible to effect the heating without
damaging the workpiece surface. It is preferable that the detecting
time is shorter. Therefore, it is preferable that the power source
to be used for generating the pulse voltage is, for example, those
which operate at about 15 kHz. The workpiece is directly heated by
the glow discharge. Therefore, the present invention does not
depend on, for example, the shape of the workpiece and the
arrangement of the workpiece. In particular, it is possible to
uniformly increase the workpiece temperature in the heat treatment
furnace even in the case of a structure such as those of the
lateral type heat treatment furnace in which any dispersion tends
to appear depending on, for example, the convection current, the
distance from the heating element, and the number of workpieces.
The workpiece can be quickly heated in a state in which the uniform
temperature distribution is maintained by using the glow discharge
and the heating effected by the heating element together. As a
result, the period of time, which is required for all workpieces to
arrive at a certain temperature, is short. Further, it is possible
to shorten the period of time which is required to retain the
workpieces at the certain temperature as well.
[0013] The workpiece temperature is detected during the heating,
and the current density of the glow discharge is decreased after
the temperature arrives at 350.degree. C. Accordingly, it is
possible to avoid the generation of any excessive radiation heat
from the workpiece itself heated by the glow discharge, it is
possible to uniformly maintain the temperature distribution in the
heat treatment furnace, and it is possible to avoid any sudden
increase in temperature. Subsequently, when the workpiece is
further heated from this temperature by using the heating element
arranged around the workpiece, it is possible to set the workpiece
at a desired nitriding treatment temperature highly accurately.
[0014] In this case, if the workpiece temperature exceeds the
desired nitriding treatment temperature, it is feared that any
abnormal microstructure is formed in the workpiece. However, in the
present invention, the heat generation, which is brought about by
the glow discharge, can be adjusted highly accurately. Therefore,
the workpiece can be set at a high temperature at which no
overshoot occurs, and it is possible to shorten the period of time
required for the nitriding treatment.
[0015] It is preferable that the current density of the glow
discharge is 0.05 to 7 mA/cm.sup.2. If the current density is less
than 0.05 mA/cm.sup.2, then the glow discharge is unstable
especially at low temperatures, and it is impossible to perform the
uniform heating. On the other hand, if the current density is
larger than 7 mA/cm.sup.2, then the transition to the arc discharge
is caused, and any damage arises in the workpiece. If the current
density is less than 0.1 mA/cm.sup.2, a long period of time is
required to raise the temperature. Further, if the current density
exceeds 4 mA/cm.sup.2, then ion collision tends to occur, for
example, at projections when the workpiece has the projections or
the like, and it is feared that the heating is advanced abnormally.
Therefore, it is preferable that the current density is within a
range of 0.1 to 4 mA/cm.sup.2.
[0016] Further, in the present invention, in order to detect the
workpiece temperature highly accurately without using any contact
type thermometer, a dummy workpiece is arranged in the heat
treatment furnace. The contact temperature and the radiation
temperature of the dummy workpiece are detected, and the radiation
temperature of the workpiece is detected. The radiation temperature
of the workpiece is corrected on the basis of the difference in
temperature between the contact temperature and the radiation
temperature of the dummy workpiece to estimate the temperature of
the workpiece itself. When the heating, which is effected by the
glow discharge and the heating element, is controlled on the basis
of the workpiece temperature estimated as described above, then the
workpiece temperature can be managed highly accurately, and it is
possible to perform the extremely satisfactory nitriding
treatment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a schematic arrangement of a nitriding
treatment system including a nitriding treatment apparatus
according to an embodiment of the present invention;
[0018] FIG. 2 illustrates workpieces as objectives of the nitriding
treatment and a magazine for accommodating the workpieces;
[0019] FIG. 3 illustrates an arrangement of a lateral type heat
treatment furnace and a control circuit thereof according to the
embodiment of the present invention;
[0020] FIG. 4 shows a flow chart illustrating a nitriding treatment
method according to the embodiment of the present invention;
and
[0021] FIG. 5 illustrates the relationship among the workpiece
temperature, the current density, and the heater output in the
nitriding treatment method according to the embodiment of the
present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022] FIG. 1 shows a schematic arrangement of a nitriding
treatment system 10 including a nitriding treatment apparatus
according to an embodiment of the present invention. As shown in
FIG. 2, a plurality of crank shafts 12 as workpieces are supplied
to the nitriding treatment system 10 in a form of a magazine 16 in
which the plurality of crank shafts 12 are positioned with a jig
14.
[0023] The nitriding treatment system 10 comprises a washing
machine 20 which removes, for example, dust and oil adhered to the
crank shafts 12 transported by a conveyer 18, a conveyer 22 which
receives the washed crank shafts 12 from the conveyer 18 to
transport them to respective work stations, lateral type heat
treatment furnaces 24 which are arranged at the plurality of work
stations along the conveyer 22, and a cooling tank 25 which cools
the crank shafts 12 transported by a conveyer 27 connected to the
terminal end of the conveyer 22.
[0024] FIG. 3 illustrates an arrangement of the lateral type heat
treatment furnace 24 and its control circuit arranged for each of
the work stations.
[0025] The lateral type heat treatment furnace 24 is a heat
treatment furnace for applying the plasma nitriding treatment to
the crank shafts 12. The lateral type heat treatment furnace 24 is
constructed as the lateral type. The magazine 16 with the
positioned crank shafts 12 is placed into/out of the lateral type
heat treatment furnace 24 in the horizontal direction. The lateral
type heat treatment furnace 24 has an inner partition wall 28 and
an outer partition wall 30 which are arranged on a base pedestal
26.
[0026] The inner partition wall 28 forms a nitriding treatment
chamber 32 which involves a nitriding atmosphere for accommodating
the crank shafts 12 together with the jig 14. The space, which is
disposed between the inner partition wall 28 and the outer
partition wall 30, forms a cooling liquid passage 33 for cooling
the inner partition wall 28 to eliminate any influence of the heat
radiation and facilitate the control when the temperature of the
nitriding treatment chamber 32 is raised.
[0027] A plurality of electric heaters 34 (heating elements) are
arranged along the inner partition wall 28 and the base pedestal 26
in the nitriding treatment chamber 32. A dummy workpiece 36, which
has physical properties equivalent to those of the crank shaft 12,
is arranged in the nitriding treatment chamber 32. The dummy
workpiece 36 may be the crank shaft 12 itself.
[0028] A negative electrode 38, which is electrically connected to
the magazine 16, is arranged for the base pedestal 26 by the aid of
an insulator 40. Further, a negative electrode 42, which is
electrically connected to the dummy workpiece 36, is arranged for
the base pedestal 26 by the aid of an insulator 44. An electrode
plate 45 is arranged at the inside of the electric heaters 34 of
the nitriding treatment chamber 32. A positive electrode 46 is
connected to the electrode plate 45. An electric discharge power
source unit 48 (glow discharge-generating means), which applies a
pulse voltage having a frequency of not less than 1 kHz, is
connected between the negative electrodes 38, 42 and the positive
electrode 46. A heater power source unit 50 (heating means) is
connected to the electric heaters 34.
[0029] The nitriding treatment chamber 32 is arranged with a
workpiece radiation thermometer 52 for detecting the radiation
temperature of the crank shaft 12, a dummy workpiece radiation
thermometer 54 for detecting the radiation temperature of the dummy
workpiece 36, and a dummy workpiece contact thermometer 56 composed
of a thermocouple fixed to the dummy workpiece 36 for detecting the
contact temperature of the dummy workpiece 36. The workpiece
radiation thermometer 52, the dummy workpiece radiation thermometer
54, and the dummy workpiece contact thermometer 56 are connected to
a temperature-detecting unit 58 (temperature-detecting means,
temperature-calculating means). The temperature-detecting unit 58
postulates or estimates the temperature of the crank shaft 12 from
the respective detected temperature data.
[0030] A vacuum suction pump 60, which sucks the gas contained in
the nitriding treatment chamber 32 to obtain a desired degree of
vacuum, is connected to the nitriding treatment chamber 32 via a
valve 62. A nitriding treatment gas supply unit 64, which supplies
a gas for performing the plasma nitriding treatment, for example, a
nitriding treatment gas as a mixed gas of nitrogen gas, hydrogen
gas, ammonia gas, and argon gas into the nitriding treatment
chamber 32, is connected to the nitriding treatment chamber 32 via
a valve 66. A cooling liquid supply unit 68 for supplying the
cooling liquid is connected to the cooling liquid passage 33 via a
valve 70. The system is constructed such that the cooling liquid,
which is supplied to the cooling liquid passage 33, is
dischargeable to the outside via a valve 72.
[0031] The electric discharge power source unit 48, the heater
power source unit 50, the temperature-detecting unit 58, the vacuum
suction pump 60, the nitriding treatment gas supply unit 64, and
the cooling liquid supply unit 68 are controlled by a control unit
74 (control means).
[0032] The nitriding treatment system 10 according to the
embodiment of the present invention is basically constructed as
described above. Next, its operation, function, and effect will be
explained in accordance with a flow chart shown in FIG. 4.
[0033] At first, as shown in FIG. 2, the magazine 16, in which the
plurality of crank shafts 12 are positioned in the jig 14, is
prepared. The magazine 16 is placed into the washing machine 20 by
using the conveyer 18 to remove dust and oil adhered to the
respective crank shafts (Step S1).
[0034] Subsequently, the magazine 16, in which the crank shafts 12
have been washed, is transported by using the conveyer 22 to
introduce the magazine 16 into the nitriding treatment chamber 32
of each of the lateral type heat treatment furnaces 24 (Step S2).
In this arrangement, the lateral type heat treatment furnace 24 is
of the horizontal type. Therefore, the following advantages are
obtained as compared with the vertical type. That is, the plurality
of magazines 16 retained with the crank shafts 12 can be easily
placed into/out of the nitriding treatment chambers 32. Further,
the layout of the nitriding treatment system 10 can be simplified.
Furthermore, the number of the crank shafts 12 to be treated at
once can be increased with ease. In contrast, in the case of the
vertical type, it is necessary that the heat treatment chamber has
a large size in the upward direction.
[0035] When the magazine 16 is introduced into the nitriding
treatment chamber 32, the bottom of the jig 14 is connected to the
negative electrode 38 arranged for the base pedestal 26. The dummy
workpiece 36, which has the physical properties equivalent to those
of the crank shaft 12, is previously arranged in the nitriding
treatment chamber 32.
[0036] The nitriding treatment chamber 32 is set to have a
nitriding atmosphere after introducing the magazine 16 into the
nitriding treatment chamber 32 and tightly closing the lateral type
heat treatment furnace 24 (Step S3). In this procedure, the control
unit 74 is operated so that the vacuum suction pump 60 is driven to
suck the air contained in the nitriding treatment chamber 32 until
arrival at a predetermined degree of vacuum, and then the nitriding
treatment gas supply unit 64 is driven to introduce, into the
nitriding treatment chamber 32, the nitriding treatment gas
composed of the mixed gas of, for example, nitrogen gas, hydrogen
gas, ammonia gas, and argon gas.
[0037] Subsequently, the cooling liquid supply unit 68 is driven to
introduce the cooling liquid into the cooling liquid passage 33
disposed between the inner partition wall 28 and the outer
partition wall 30 and start the cooling of the wall surface of the
inner partition wall 28 (Step S4). The control unit 74 sets the
pulse voltage having a frequency of 15 kHz and a current density
.rho. of 2.5 mA/cm.sup.2 for the electric discharge power source
unit 48 (Step S5). Further, the control unit 74 sets the output H
of the heater power source unit 50 to 40% (Step S6). The heater
power source unit 50 is operated at the maximum output (100%) when
the amount of electrical energy (thermal energy) of the crank shaft
12 introduced into the nitriding treatment chamber 32 is 160W per 1
kg. Thus, 40% of the output H is 64W/kg.
[0038] After setting the treatment conditions as described above,
the heating of the nitriding atmosphere in the nitriding treatment
chamber 32 and the nitriding treatment are started (Step S7). In
this procedure, as shown in FIG. 5, when the crank shafts 12 are
introduced, and the setting of the respective treatment conditions
is completed to drive the electric discharge power source unit 48
and the heater power source unit 50 (treatment time tO), then the
glow discharge is generated by the pulse voltage applied between
the negative electrode 38 and the positive electrode 46. The
nitriding treatment gas in the nitriding treatment chamber 32 is
ionized by the glow discharge to make the collision against the
surfaces of the crank shafts 12. Accordingly, the crank shafts 12
are heated. Further, the electric heaters 34, which are arranged
around the outer circumference of the magazine 16 for retaining the
crank shafts 12, generate the heat, and the nitriding atmosphere of
the nitriding treatment chamber 32 is heated by the radiation heat
brought about by the heat generation. The glow discharge is also
caused on the surface of the dummy workpiece 36. Therefore, the
dummy workpiece 36 is also heated in the same manner as described
above.
[0039] In this situation, the voltage having the pulse frequency of
15 kHz is applied between the negative electrodes 38, 42 and the
positive electrode 46. Therefore, a stable glow discharge state, in
which no arc discharge is caused, is maintained even in a state in
which the nitriding atmosphere is not sufficiently subjected to the
increase in temperature up to a temperature at which the stable
nitriding treatment can be started. That is, the period of time, in
which the current is continuously applied, is extremely short, and
the continuation of the arc discharge is stopped with a short
detecting time. Accordingly, the current is cut off before arrival
at a voltage at which any arc mark appears. It is possible to avoid
the appearance of any arc mark and maintain the stable glow
discharge state. Therefore, any situation, in which the surface of
the crank shaft 12 is damaged by the arc discharge, does not occur.
The heating brought about by the glow discharge is not caused by
the radiation heat but caused by the collision of nitrogen ions or
nitrogen radicals produced by the glow discharge against the crank
shafts 12. Therefore, when the temperature is less than 350.degree.
C., then any influence is scarcely exerted by the radiation heat of
the crank shafts 12, and no influence is exerted by the shapes of
the crank shafts 12 and the positional relationship of the crank
shafts 12 arranged adjacently to one another. Therefore, the crank
shafts 12 are uniformly heated by the glow discharge. Further, the
nitriding atmosphere of the nitriding treatment chamber 32 is
heated by the radiation heat from the electric heaters 34 which are
arranged along the inner partition wall 28 and which are set to
provide the low output. Therefore, the crank shafts 12 are quickly
heated up to the desired stable nitriding treatment temperature in
the state in which the temperature distribution is uniformly
maintained.
[0040] During the temperature-raising process, the cooling liquid
is supplied to the cooling liquid passage 33. For example, when the
inner partition wall 28 is cooled and maintained at a temperature
of not more than 100.degree. C. with the cooling liquid, it is
possible to avoid such a situation that the radiation heat from the
electric heaters 34 is reflected by the inner partition wall 28 to
excessively heat the crank shafts 12 arranged inside. Therefore, it
is possible to raise the temperature of the crank shaft 12 more
uniformly.
[0041] On the other hand, the temperature of the crank shaft 12 is
estimated highly accurately by using the temperature of the dummy
workpiece 36 arranged in the nitriding treatment chamber 32 and the
radiation temperature of the crank shaft 12 (Steps S8, S9).
[0042] That is, the workpiece radiation thermometer 52, which is
arranged close to the crank shaft 12, detects the radiation
temperature Trw as the temperature of the radiation heat radiated
from the crank shaft 12. The dummy workpiece radiation thermometer
54, which is arranged close to the dummy workpiece 36, detects the
radiation temperature Trd as the temperature of the radiation heat
radiated from the dummy workpiece 36. Further, the dummy workpiece
contact thermometer 56, which is fixed to the dummy workpiece 36,
detects the correct contact temperature Tcd of the dummy workpiece
36.
[0043] Accordingly, the temperature-detecting unit 58 determines,
as the temperature correction value .DELTA.T, the difference
between the radiation temperature Trd of the dummy workpiece 36
detected by the dummy workpiece radiation thermometer 54 and the
contact temperature Tcd of the dummy workpiece 36 detected by the
dummy workpiece contact thermometer 56 to calculate the temperature
Tw of the crank shaft 12 by using the temperature correction value
.DELTA.T as follows: Tw=Trw+.DELTA.T When the temperature of the
crank shaft 12 is calculated as described above, it is possible to
highly accurately estimate the temperature Tw of the crank shaft 12
in a non-contact manner without fixing any thermometer such as a
thermocouple to the crank shaft 12.
[0044] Subsequently, the control unit 74 executes the process for
judging whether or not the temperature Tw of the crank shaft 12
estimated by the temperature-detecting unit 58 is not less than
400.degree. C. during the heating of the nitriding atmosphere of
the nitriding treatment chamber 32 (Step S10). In this procedure,
if the temperature Tw of the crank shaft 12 is not less than
400.degree. C., then it is impossible to ignore the influence of
the heat radiation of the crank shaft 12 itself, and it is feared
that any temperature unevenness appears on the crank shaft 12 if
the heating by the glow discharge is continued as it is.
[0045] Accordingly, if it is judged that the temperature Tw of the
crank shaft 12 arrives at 400.degree. C. (treatment time t1), then
the control unit 74 controls the electric discharge power source
unit 48 to lower the current density p of the glow discharge to be
applied between the negative electrode 38 and the positive
electrode 46 down to 0.5 mA/cm.sup.2 (Step S11), and the output of
the heater power source unit 50 is raised up to 90%, i.e., 144
kW\kg (Step S12).
[0046] In this procedure, when the current density p is lowered
down to 0.5 mA/cm.sup.2, then it is possible to suppress such a
situation that the radiation heat is radiated from the crank shaft
12 heated by the glow discharge, and it is possible to suppress any
sudden heating which would be otherwise caused by the glow
discharge of the crank shaft 12. When the output of the heater
power source unit 50 is raised up to 90%, it is possible to heat
the crank shaft 12 up to the desired nitriding treatment
temperature at which no overshoot is caused by the radiation heat
from the electric heaters 34, in a state in which the temperature
distribution of the crank shaft 12 is uniformly maintained. When
the temperature of the crank shaft 12 is controlled by using the
electric discharge power source unit 48 and the heater power source
unit 50, for example, it is preferable to perform the following
control. That is, the current density .rho. is gradually lowered
from the treatment time t1 to the treatment time t2 in accordance
with the temperature Tw of the crank shaft 12 estimated by the
temperature-detecting unit 58, while the output of the heater power
source unit 50 is gradually increased from the treatment time t1
until arrival at the desired nitriding treatment temperature of the
crank shaft 12.
[0047] After the temperature Tw of the crank shaft 12 arrives at
the desired nitriding treatment temperature of 570.degree. C. (Step
S13), the control unit 74 controls the heater power source unit 50
to maintain the temperature Tw of the crank shaft 12 at the
nitriding treatment temperature of 570.degree. C. (Step S14).
During this process, the nitriding treatment is advanced on the
surface of the crank shaft 12 in accordance with the reaction of
nitrogen ion and iron ion.
[0048] When the predetermined period of time elapses, and the
nitriding treatment is completed (Step S15, treatment time t3),
then the crank shafts 12 are discharged from the lateral type heat
treatment furnace 24 together with the magazine 16 (Step S16). The
magazine 16, which has been discharged from the lateral type heat
treatment furnace 24, is transported by the conveyer 22, and the
magazine 16 is placed into the cooling tank 25 to perform the
cooling treatment (Step S17). After that, the magazine 16 is
discharged to the outside by the aid of the conveyer 27.
* * * * *