U.S. patent application number 15/774749 was filed with the patent office on 2018-11-15 for gas quenching method.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. The applicant listed for this patent is NISSAN MOTOR CO., LTD.. Invention is credited to Yukichi OKAYAMA, Tsuyoshi SUGIMOTO.
Application Number | 20180327874 15/774749 |
Document ID | / |
Family ID | 58694773 |
Filed Date | 2018-11-15 |
United States Patent
Application |
20180327874 |
Kind Code |
A1 |
SUGIMOTO; Tsuyoshi ; et
al. |
November 15, 2018 |
GAS QUENCHING METHOD
Abstract
A gas quenching method of the present invention includes a first
stage (t1 to t2) at which a workpiece is subjected to a rapid
cooling by forcibly circulating a cooling gas, a second stage (t2
to t3) at which the circulation of the cooling gas is stopped and
pressure is reduced inside the furnace to conduct heat insulation,
and a third stage (as from t3) at which the workpiece is cooled
again by the cooling gas. At the second stage, the workpiece is
maintained at an intermediate temperature that is higher than
martensite transformation start temperature, and, during this,
temperature throughout the workpiece is made uniform. Therefore, it
is possible to achieve a uniform quenching and suppress distortion
caused by difference of the cooling speed.
Inventors: |
SUGIMOTO; Tsuyoshi;
(Kanagawa, JP) ; OKAYAMA; Yukichi; (Kanagawa,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NISSAN MOTOR CO., LTD. |
Yokohama-shi, Kanagawa |
|
JP |
|
|
Assignee: |
NISSAN MOTOR CO., LTD.
Yokohama-shi, Kanagawa
JP
|
Family ID: |
58694773 |
Appl. No.: |
15/774749 |
Filed: |
November 11, 2015 |
PCT Filed: |
November 11, 2015 |
PCT NO: |
PCT/JP2015/081698 |
371 Date: |
May 9, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 1/767 20130101;
C21D 1/20 20130101; C21D 1/773 20130101; C21D 1/22 20130101; C21D
1/19 20130101; C21D 2211/008 20130101; C21D 9/40 20130101; C21D
1/62 20130101; C21D 1/613 20130101 |
International
Class: |
C21D 1/20 20060101
C21D001/20; C21D 1/62 20060101 C21D001/62 |
Claims
1. A gas quenching method in which a workpiece made of steel is
heated and then cooled for quenching by allowing a cooling gas to
flow around the workpiece in a furnace, the gas quenching method
comprising: stopping supply of the cooling gas in the middle of the
quenching before the workpiece reaches a martensite transformation
start temperature; reducing pressure inside the furnace and making
temperature throughout the workpiece uniform by radiation cooling,
while temperature of the workpiece is maintained at an intermediate
temperature that is higher than the martensite transformation start
temperature; and resuming supply of the cooling gas after the
temperature throughout the workpiece has been made uniform, thereby
conducting the quenching to pass the martensite transformation
start temperature.
2. The gas quenching method as claimed in claim 1, wherein the
temperature throughout the workpiece is made uniform, while the
temperature of the workpiece is maintained at a temperature that is
higher than the martensite transformation start temperature and
lower than a bainite transformation curve.
3. The gas quenching method as claimed in claim 1, wherein the
workpiece has a surface that is previously subjected to a
carburizing treatment.
4. A gas quenching method, comprising: a first step of subjecting a
workpiece made of steel to a rapid cooling in a furnace by a
cooling gas from a heated condition; a second step of stopping
supply of the cooling gas to the workpiece and reducing pressure
inside the furnace, such that, in the middle of a temperature
lowering of the workpiece, the workpiece is maintained at an
intermediate temperature that is higher than a martensite
transformation start temperature; and a third step of conducting a
rapid cooling again by the cooling gas after temperature of the
workpiece has been made uniform.
Description
TECHNICAL FIELD
[0001] This invention relates to a gas quenching method in which a
workpiece is heated and then cooled by using a cooling gas, as a
quenching of steel.
BACKGROUND TECHNOLOGY
[0002] Quenching of steel is a heat treatment technology to obtain
a martensite structure by turning steel into a high-temperature
condition and then rapid cooling. Hitherto, there has been adopted
many times a liquid quenching method in which cooling after heating
is conducted by using, as a cooling agent, a liquid, such as oil,
water or a polymer solution, which is relatively high in cooling
property to conduct quenching of relatively large parts. In this
liquid quenching, however, boiling occurs non-uniformly during
quenching. As a result, the cooling speed becomes non-uniform,
thereby making quality unstable. Furthermore, it is necessary to
have a washing step for removing the cooling agent after quenching,
and a waste water treatment resulting from the washing also becomes
a major problem.
[0003] From such point, in recent years, attention has been
attracted to a gas quenching in which an inert gas, such as
nitrogen gas, is used as the cooling agent, and the cooling gas is
allowed to flow, for example, around workpieces arranged in a
furnace, thereby conducting rapid cooling or quenching of the
workpieces.
[0004] Furthermore, Non-patent Publication 1 discloses, as a type
of the gas quenching method, an isothermal quenching (also called
multi-stage quenching) in which an isothermal maintenance is
conducted for a certain period of time in the middle of the cooling
by using a hot gas of a high temperature of around 300.degree. C.
In this method, the cooling gas is previously heated to around
300.degree. C. by using factory exhaust heat or the like, and this
hot gas is circulated through a gas furnace that accommodates
workpieces heated to around 1000.degree. C., thereby cooling the
workpieces and conducting an isothermal treatment on the workpieces
to a temperature of around 300.degree. C. that is in equilibrium
with the temperature of the hot gas. Then, after the temperature
equilibrium, it is switched to circulation of the cooling gas
having low temperatures by passing through a cooler, thereby
cooling the workpieces to complete quenching.
[0005] It is described in Non-patent Publication 1 that distortion
of the workpiece is reduced by conducting such a multi-stage
quenching, as compared with a normal continuous quenching.
[0006] However, in a conventional method to achieve the multi-stage
quenching by using a plurality of gases having different
temperatures like Non-Patent Publication 1, it becomes necessary to
provide the gas furnace with a heat exchanger for heating gas, a
cooler for cooling gas, a damper for switching the passage, and so
on. This makes the facility complicated.
[0007] Furthermore, it is aimed to obtain an isothermal condition
by an equilibrium between temperature of the hot gas and
temperature of the workpiece. Therefore, it takes time during which
temperature of the workpiece reaches the target isothermal
treatment temperature, and the cycle time of the quenching
treatment as a whole becomes long.
PRIOR ART PUBLICATIONS
Non-Patent Publications
[0008] Non-patent Publication 1: Akihiro HAMABE, "Using Preheated
Inactive Gas for Vacuum Hardening and Isothermal Heat Treatment
after Carburizing", Journal of the Vacuum Society of Japan, 2010,
Vol. 53, No. 1, pages 49-52.
SUMMARY OF THE INVENTION
[0009] According to the present invention, there is provided a gas
quenching method in which a workpiece made of steel is heated and
then cooled for quenching by allowing a cooling gas to flow around
the workpiece in a furnace, the gas quenching method
comprising:
[0010] stopping supply of the cooling gas in the middle of the
quenching before the workpiece reaches a martensite transformation
start temperature;
[0011] reducing pressure inside the furnace and making temperature
throughout the workpiece uniform by radiation cooling, while
temperature of the workpiece is maintained at an intermediate
temperature that is higher than the martensite transformation start
temperature; and
[0012] resuming supply of the cooling gas after the temperature
throughout the workpiece has been made uniform, thereby conducting
the quenching to pass the martensite transformation start
temperature.
[0013] That is, in the quenching method of the present invention,
in the middle of a quenching using a cooling gas, supply of the
cooling gas is stopped, and pressure inside the furnace is reduced
to suppress cooling speed of the workpiece. In particular, the
cooling action by convection is rapidly suppressed by reducing
pressure inside the furnace, resulting in substantially only
radiation cooling. In other words, the furnace turns into a heat
insulated condition by the pressure reduction, such that the
workpiece is temporarily maintained at the intermediate
temperature. At this time, heat transfers in the workpiece from a
relatively high-temperature site to a relatively low-temperature
site, thereby making the temperature throughout the workpiece
uniform. Therefore, at the subsequent cooling by supplying the
cooling gas, temperatures throughout the workpiece pass the
martensite transformation start temperature almost at the same time
and with similar temperature gradients. Thus, the quenching is
conducted more uniformly.
[0014] According to the present invention, it is possible to
achieve a multi-stage quenching without necessity of a plurality of
gases with different temperatures, and distortion of the workpiece
resulting from quenching is reduced by making the temperature
throughout the workpiece uniform. Furthermore, as compared with a
conventional method using a hot gas, it is possible to conduct the
cooling and the isothermal treatment until the intermediate
temperature within a short period of time, thereby shortening the
cycle time of the quenching treatment as a whole.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is an explanatory view of a structure of a gas
quenching furnace used in the gas quenching method of the present
invention;
[0016] FIG. 2 is an explanatory view showing steps of the gas
quenching method of Example;
[0017] FIG. 3 is a perspective view showing one example of the
workpiece;
[0018] FIG. 4 is a perspective view of a lower link as a whole to
become the workpiece; and
[0019] FIG. 5 is a characteristic diagram showing a comparison
between Example and Comparative Example in the amount of distortion
resulting from the quenching.
MODE FOR IMPLEMENTING THE INVENTION
[0020] In the following, an embodiment of the present invention is
explained in detail.
[0021] FIG. 1 shows one example of gas quenching furnace 1 used in
the gas quenching method of the present invention. This gas
quenching furnace 1 is a vertical furnace with an elliptical shape
that is elongated in vertical direction when viewed from the front.
It is formed at its upper part with fan 2 that circulates the
cooling gas in gas quenching furnace 1 and stirs the cooling gas.
At its lower part, there is disposed one-stage or multi-stage tray
3 on which a plurality of the after-mentioned workpieces as the
targets of the quenching treatment are arranged. This tray 3 has a
latticed structure having many openings such that flow of the
cooling gas (shown by arrow G in the drawing) sent by fan 2 is
allowed to pass through the tray 3 and then flow in an upward
direction. This tray 3 is taken into and out of the furnace through
a door not shown in the drawings.
[0022] Gas quenching furnace 1 has a sealed structure that is
resistant against a predetermined depressurized condition, and is
equipped outside with depressurization pump 4 for depressurizing
the furnace. This depressurization pump 4 is connected to the space
inside the furnace through depressurization passage 5, and
depressurization passage 5 is equipped with on-off valve 6 with
solenoid valve, etc.
[0023] Furthermore, gas quenching furnace 1 is equipped with gas
introducing passage 7 for introducing a cooling gas, such as
nitrogen gas, hydrogen gas, helium gas or argon gas, into the
furnace, and gas discharging passage 9 for discharging the cooling
gas from the furnace. Gas introducing passage 7 is equipped with
on-off valve 8 with solenoid valve, etc. Gas discharging passage 9
is similarly equipped with on-off valve 10 with solenoid valve,
etc.
[0024] FIG. 2 shows an embodiment of the gas quenching method of
the present invention using the above-mentioned gas quenching
furnace 1. A workpiece used in this embodiment is one prepared by
machining chromium steel of SCr420 as base material into a
predetermined shape and then previously conducting a carburizing
treatment on the surface by gas carburizing. The target carbon
concentration of the surface in the carburizing treatment is 0.6%.
Therefore, the material on the surface of the workpiece is one
equivalent to SCr460. The carburizing treatment is conducted in
another furnace. After annealing from the carburizing treatment
temperature, it is introduced together with tray 3 into gas
quenching furnace 1 in a condition where it has been subjected to a
reheating until 1050.degree. C. for quenching.
[0025] After closing the door (not shown in the drawings) of gas
quenching furnace 1, the cooling gas is introduced into gas
quenching furnace 1 through gas introducing passage 7. Once filled
with the cooling gas, the inside of gas quenching furnace 1 is
turned into a sealed condition by closing on-off valve 8, etc.
Then, fan 2 is driven to cool the workpiece by forcibly circulating
the cooling gas. As the cooling gas, for example, nitrogen gas
having a temperature adjusted to 40.degree. C. is used.
[0026] FIG. 2(a) shows temperature change of the workpiece, FIG.
2(b) shows an on-off condition of the gas cooling or fan 2, and
FIG. 2(c) shows an on-off condition of depressurization of the
furnace or depressurization pump 4. From time t1, the workpiece is
rapidly cooled by forcibly circulating the cooling gas. With this,
temperature of the workpiece is abruptly lowered. FIG. 2(a) also
shows a bainite transformation curve (B) where transformation into
bainite occurs resulting from the cooling prior to martensite
transformation, but the speed of the temperature lowering by the
cooling gas is set not to pass this nose-shape bainite
transformation curve.
[0027] Following such rapid cooling period, before temperature of
the workpiece reaches the martensite transformation start
temperature, fan 2 is stopped at time t2 to stop circulation and
stirring of the cooling gas. At substantially the same time as
this, depressurization pump 4 is energized to depressurize the
inside of gas quenching furnace 1. By stopping fan 2, cooling by
the cooling gas is suppressed. However, the inside of gas quenching
furnace 1 turns into a thermally insulated condition by
depressurizing the inside of gas quenching furnace 1. That is, the
cooling action by convection is rapidly suppressed, resulting in
slightly only radiation cooling by radiation from the surface of
the workpiece. With this, the cooling speed of the workpiece
becomes very small, and temperature of the workpiece is temporarily
maintained at an intermediate temperature that is higher than
martensite transformation start temperature, as shown in FIG. 2(a).
The target intermediate temperature is, for example, 300.degree.
C., which is slightly higher than martensite transformation start
temperature (Ms).
[0028] During the rapid cooling period between times t1 to t2,
there are some differences in cooling speed throughout the
workpiece. As shown by solid line F in FIG. 2(a), the temperature
lowering progresses early at a site with a rapid cooling speed. In
contrast, as shown by broken line L, the progress of the
temperature lowering becomes slow at a site with a relatively slow
cooling speed. Therefore, at time t2, there occur temperature
differences among the sites. While the workpiece is substantially
in a heat insulated condition by stopping fan 2 and the
depressurization, heat transfers from a relatively high-temperature
site to a relatively low-temperature site, and an isothermal
condition is obtained throughout the workpiece at the target
intermediate temperature (e.g., 300.degree. C.) which is slightly
higher than martensite transformation start temperature. That is,
temperature shown by solid line F and temperature shown broken line
L of FIG. 2(a) converge and are maintained at around 300.degree.
C.
[0029] Herein, to control stopping of fan 2 and turning-on of
depressurization pump 4, it is optional to monitor the actual
temperature of the workpiece by using, for example, an
infrared-type temperature sensor, etc. and to execute stopping of
fan 2 and turning-on of depressurization pump 4 when becoming a
predetermined temperature that is slightly higher than the target
intermediate temperature in an isothermal condition in view of the
delay of temperature change. Alternatively, it is optional to
experimentally determine the necessary time in which the
temperature lowers to a predetermined temperature from time t1 and
then to execute stopping of fan 2 and turning-on of
depressurization pump 4 when the elapsed time from time t1 has
reached the predetermined value. In one embodiment, the initial
rapid cooling period from time t1 to time t2 is, for example, about
45 seconds.
[0030] Once completing an isothermal condition throughout the
workpiece by maintaining the intermediate temperature, at time t3,
depressurization pump 4 is turned off, the cooling gas is
reintroduced into gas quenching furnace 1 through gas introducing
passage 7, and fan 2 is driven to restart rapid cooling of the
workpiece by forcibly circulating the cooling gas. The cooling gas
may be the same one as that of the initial rapid cooling period.
For example, there is used a nitrogen gas of which temperature has
been adjusted to 40.degree. C.
[0031] By the above rapid cooling, temperature of the workpiece
lowers to cross martensite transformation start temperature (Ms)
(that is, pass martensite transformation start temperature (Ms)) to
conduct quenching. At this time, an isothermal condition is
achieved throughout the workpiece. Thus, throughout the workpiece,
timing and temperature gradient (cooling speed) when passing
martensite transformation start temperature become constant.
Therefore, martensite transformation occurs evenly therethroughout
to obtain an even quenching.
[0032] The necessary time from time t2 to time t3 is, for example,
about 30 seconds in one embodiment. To control restarting of the
cooling at time t3, it suffices to experimentally determine the
time necessary for an isothermal condition and to restart cooling
when the elapsed time from time t2 has reached a predetermined
value. Alternatively, it is optional to monitor the actual
temperatures of a plurality of sites of the workpiece by using an
infrared-type temperature sensor, etc. and to restart cooling when
these have converged on generally the same temperature.
[0033] Cooling as from time t3 is conducted, for example, for about
2 to 5 minutes in one embodiment.
[0034] In this way, in the quenching method of the above-mentioned
embodiment, as a gas quenching using a single cooling gas, there is
achieved a multi-stage quenching including the first stage of a
rapid cooling period between time t1 and time t2, the second stage
of an isothermal period between time t2 and time t3, and the third
stage of a rapid cooling period as from time t3. In this way, by
having the second stage as a period for obtaining an isothermal
condition at the intermediate temperature which is slightly higher
than martensite transformation start temperature, it is possible to
conduct a uniform quenching with a small distortion resulting from
the quenching. Furthermore, it is possible in the second stage to
rapidly lower the cooling speed by using heat insulation by the
depressurization. Therefore, the necessary time of the first stage
and the second stage becomes short. Thus, for example, as compared
with a conventional method of using a hot gas, the cycle time
becomes shorter.
[0035] Herein, as shown in FIG. 2(a), the temperature of the second
stage between time t2 and time t3 is set at a temperature that is
higher than martensite transformation start temperature (Ms) and is
lower than the nose-shape bainite transformation curve. That is,
the intermediate temperature and the period of the second stage are
set such that the characteristic of temperature change of the
workpiece does not cross the bainite transformation curve. With
this, transformation into bainite during quenching is
suppressed.
[0036] FIG. 3 shows one example of the workpiece suitable for the
quenching method of the present invention. This workpiece is a
component constituting a part of lower link 11 (see FIG. 4) in a
multi-link type piston crank mechanism of an internal combustion
engine. As described in, for example, Japanese Patent Application
Publication 2015-42849, this type of lower link 11 is one for
connecting an upper link with one end connected to a piston pin and
a crank pin of a crankshaft. As shown in FIG. 4, it is formed at
its center with a cylindrical crank pin bearing portion 12 to be
fitted onto the crank pin. Furthermore, it is provided with a pin
boss portion 13 for an upper pin and a pin boss portion 14 for a
control pin at positions on opposite sides by almost 180 degrees
with an interposal of this crank pin bearing portion 12. This lower
link 11 as a whole forms a parallelogram close to rhombus. On
division surface 15 passing through center of crank pin bearing
portion 12, it is formed of two divided parts of lower link upper
11A containing the pin boss portion 13 for upper pin and lower link
lower 11B containing the pin boss portion 14 for control pin. The
workpiece of the above embodiment is the above-mentioned lower link
upper 11A.
[0037] Pin boss portion 13 for upper pin in this lower link upper
11A has a bifurcated structure to sandwich the upper link at its
center portion in the axial direction. That is, it is formed into a
pair of wall-like ones opposite to each other with an interposal of
a center recess portion 16.
[0038] This workpiece, that is, lower link upper 11A, is disposed
on the above-mentioned tray 3 with a posture shown in FIG. 3. That
is, it is retained to have an upright posture in which one side
surface 17 (see FIG. 4) perpendicular to division surface 15
becomes a bottom surface that is brought into contact with tray 3
and in which division surface 15 stand upright from tray 3. Then,
the cooling gas is guided to be parallel with division surface 15
in gas quenching furnace 1, and the cooling gas is allowed to flow
along the front and back surfaces of a pair of wall-like pin boss
portions 13.
[0039] In quenching against such workpiece, wall-like pin boss
portion 13 has a thinner thickness as compared with a part in the
vicinity of division surface 15 and is widely exposed to the gas
flow. Therefore, in general, wall-like pin boss portion 13 becomes
a portion with a rapid cooling speed, and a thick portion in the
vicinity of division surface 15 becomes a portion with a slow
cooling speed. Furthermore, an outer surface and an inner surface
(the surface on the side of recess portion 16) of wall-like pin
boss portion 13 are different in cooling speed. As a result, as
quenching progresses, it tends to have a distortion in which
wall-like pin boss portion 13 is displaced in the axial direction
of lower link 11.
[0040] According to the multi-stage quenching method of the above
embodiment, it is possible to suppress distortion of such wall-like
pin boss portion 13 in the axial direction.
[0041] FIG. 5 shows results of comparative experiments in the case
of the multi-stage quenching method of Example and in the case of a
simple continuous quenching to continue cooling by the cooling gas
as Comparative Example, in terms of change of the distance between
the pair of pin boss portions 13 (in other words, the width of the
recess portion 16 in the axial direction) due to the above
distortion. Herein, in quenching of Example, as the first stage,
nitrogen gas of 40.degree. C. was introduced under a pressure of
0.6 MPa, and it was circulated by fan 2, thereby conducting a rapid
cooling for 1 minute. Then, as the second stage, it was
depressurized to 1 kPa, followed by maintaining for 30 seconds.
Furthermore, as the third stage, nitrogen gas of 40.degree. C. was
introduced under a pressure of 0.6 MPa, and it was circulated by
fan 2, thereby conducting a cooling for 1 minute. In Comparative
Example, nitrogen gas of 40.degree. C. was introduced under a
pressure of 0.6 MPa, and it was circulated by fan 2, thereby
conducting a cooling for two minutes and thirty seconds.
[0042] As shown in the drawing, according to the multi-stage
quenching of Example, as compared with the continuous quenching,
there was obtained a result that distortion of pin boss portion 13
in the axial direction was reduced by half.
[0043] As above, one embodiment of the present invention was
explained, but the present invention is not limited to the above
embodiment. Various modifications are possible, including the
treatment temperature, time, etc. Furthermore, the present
invention is also suitable for quenching of lower link lower 11B of
lower link 11 shown in FIG. 4 and can be applied to quenching of
other various parts.
* * * * *