U.S. patent application number 09/778779 was filed with the patent office on 2001-08-23 for distortion control method and cooling power measuring device.
This patent application is currently assigned to NTN Corporation. Invention is credited to Iihara, Michio, Maruyama, Takeshi, Yamaguchi, Masami, Yoshitomi, Jun.
Application Number | 20010015247 09/778779 |
Document ID | / |
Family ID | 18562886 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015247 |
Kind Code |
A1 |
Iihara, Michio ; et
al. |
August 23, 2001 |
Distortion control method and cooling power measuring device
Abstract
A distortion control method that can suppress distortion of a
member during quenching and a cooling power measuring device that
can precisely measure cooling power are provided. In the distortion
control method, when the member is subjected to quenching using
liquid cooling medium, the cooling power of the cooling medium
being used is maintained within a prescribed range, so that
variation in distortion suffered by the member is restricted.
Inventors: |
Iihara, Michio; (Iwata-shi,
JP) ; Yamaguchi, Masami; (Iwata-shi, JP) ;
Yoshitomi, Jun; (Iwata-shi, JP) ; Maruyama,
Takeshi; (Iwata-shi, JP) |
Correspondence
Address: |
McDERMOTT, WILL & EMERY
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
NTN Corporation
|
Family ID: |
18562886 |
Appl. No.: |
09/778779 |
Filed: |
February 8, 2001 |
Current U.S.
Class: |
148/713 |
Current CPC
Class: |
C21D 1/60 20130101; C21D
1/55 20130101 |
Class at
Publication: |
148/713 |
International
Class: |
C22F 001/16 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 17, 2000 |
JP |
2000-039301(P) |
Claims
What is claimed is:
1. A distortion control method, wherein when a member is subjected
to quenching using liquid cooling medium, cooling power of the
cooling medium being used is maintained in a prescribed range to
suppress variation in distortion suffered by the member.
2. The distortion control method according to claim 1, wherein said
cooling medium is cooling water including coolant, and a change in
the cooling power of the cooling water due to a running change of
said coolant is held in a fixed range.
3. The distortion control method according to claim 1 for measuring
the cooling power of said cooling medium by quenching of a sample
member in a prescribed shape formed of a material that does not
transform in a measured temperature range, wherein the measurement
is conducted by immersing said sample member in said cooling medium
and stopping said sample member at a quenching stop position with
positioning accuracy within a range of .+-.0.03 mm.
4. The distortion control method according to claim 2 for measuring
the cooling power of said cooling medium by quenching of a sample
member in a prescribed shape formed of a material that does not
transform in a measured temperature range, wherein the measurement
is conducted by immersing said sample member in said cooling medium
and stopping said sample member at a quenching stop position with
positioning accuracy within a range of .+-.0.03 mm.
5. The distortion control method according to claim 4, wherein, as
the change in cooling power of said cooling medium is great for a
prescribed time period from a start of use of said coolant and then
becomes smaller and is stabilized, said measurement is conducted
using the cooling water that has entered such stabilized range.
6. The distortion control method according to claim 4, wherein the
cooling power of said cooling water is maintained in the prescribed
range by resupplying new liquid of said coolant.
7. The distortion control method according to claim 4, wherein the
distortion of said member during the quenching is controlled using
a cooling power transition table indicating a running change of the
cooling power of said cooling medium from a start of use of new
liquid of said coolant.
8. The distortion control method according to claim 4, wherein the
distortion of said member during the quenching is controlled using
a cooling power transition table indicating running changes of the
cooling power of said cooling medium and of a concentration of
coolant measured by a saccharimeter from a start of use of new
liquid of said coolant.
9. The distortion control method according to claim 3, wherein the
cooling power of said cooling medium is adjusted using a distortion
table indicating a relation between distortion of said member
during said quenching and the cooling power of said cooling
medium.
10. The distortion control method according to claim 4, wherein the
cooling power of said cooling medium is adjusted using a distortion
table indicating a relation between distortion of said member
during said quenching and the cooling power of said cooling
medium.
11. The distortion control method according to claim 4, wherein the
cooling power of said cooling medium is evaluated by a time
required, when quenching the member, to lower a temperature of the
member from a temperature T.sub.1 to a temperature T.sub.2 that is
lower than temperature T.sub.1.
12. The distortion control method according to claim 4, wherein the
cooling power of said cooling medium is evaluated, assuming that
the quenchings of the same sample are conducted using cooling
medium including only new coolant, by obtaining a concentration of
the new coolant that should be included in the relevant cooling
medium to obtain a cooling time the same as said cooling time.
13. A device for measuring cooling power of cooling medium in a
quenching device, comprising: a heating device for heating a sample
member; a cooling medium bath for storing the cooling medium for
use in cooling said sample member heated; a transfer device for
transferring said sample member from said heating device to said
cooling medium bath, and for partially immersing said sample member
into said cooing medium and holding said sample member at a
prescribed position; and a transfer control device for controlling
an operation of said transfer device; said transfer device and said
transfer control device suppressing variation in the quenching stop
position at which said sample member immersed in said cooling
medium is to be held, within a range of .+-.0.03 mm.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of controlling
distortion due to heat treatment during quenching of automotive
parts or the like, and a device for measuring cooling power of
liquid cooling medium for use in a quenching device.
[0003] 2. Description of the Background Art
[0004] It is often the case that, once quenching of automotive
parts is completed, the parts are put to use without being
subjected to further processing. Thus, distortion due to the
quenching should be limited in a small range. In particular, with
wheel driving parts, such distortion will lead to noise at the time
of engagement of toothed gears, or degradation in durability, and
therefore, restriction of such distortion is a critical issue. A
number of examinations and analyses have been made in an effort to
restrict the distortion. However, such distortion due to heat
treatment results from a variety of factors that affect to one
another in a complicated manner. Further, the testing itself
incorporates variation therein. Thus, detailed analyses have not
been made successfully; a general tendency for each factor would be
found at best.
[0005] In an effort to stabilize distortion, e.g., in over pin
diameter (OPD) of an outer ring of constant velocity universal
joint (CVJ), the inventors have focused on management of the
following factors causing such distortion: (1) an output of
high-frequency coil for heating; (2) concentration of coolant
within liquid cooling medium; and (3) temperature of the cooling
medium, and have succeeded in producing good results.
[0006] Due to increasingly stringent demands for suppressing
distortion of automotive parts, however, it has become no longer
possible to fulfill such demands with the conventional techniques.
Thus, to quantitatively extract the effects leading to the
distortion as described above, the inventors conducted a
measurement of cooling power of liquid cooling medium in a
quenching line in a strict manner allowing no variation to be
incorporated therein, and examined a relation between the cooling
power and the distortion. As a result, the inventors have succeeded
in clarifying the effects of the cooling power of the cooling
medium on the distortion due to heat treatment, which had been
uncertain before conduction of such strict measurement.
SUMMARY OF THE INVENTION
[0007] An object of the present invention is to provide a
distortion control method that allows suppression of distortion due
to heat treatment during quenching of a member, and a cooling power
measuring device that enables strict measurement of cooling power
of cooling medium that plays an important role in such distortion
control.
[0008] According to a distortion control method of the present
invention, when a member is being subjected to quenching using
liquid cooling medium, cooling power of the cooling medium being
used is maintained within a prescribed range so as to suppress
variation in distortion suffered by the member.
[0009] In the course of examination of a relation between
distortion due to the quenching and cooling power of the cooling
medium, the inventors made a measurement of the cooling power with
high precision excluding variation inherent to such cooling power
measurement, and as a result, they succeeded in clarifying a
significant influence of the cooling power on the distortion. The
cooling power is measured as a cooling time that is required for
cooling a sample member of a prescribed form by a prescribed
temperature range. The correlation between this cooling time and
the distortion is not clearly recognized if (a) the material of the
sample member transforms in the prescribed temperature range; (b)
an immersion depth of the sample member in the cooling medium
varies in a range of .+-.1 mm; and (c) a thick oxide film is formed
on the surface of the sample member when heating the sample member.
However, by measuring the cooling power using, as a material of the
sample member, Ni-based alloy such as Inconel that (a') maintains
an austenite phase from room temperature to high temperature and
(b') is excellent in oxidation resistance, thereby forming almost
no oxide film, and (c') by positioning the sample member in the
cooling medium with accuracy within a range of .+-.0.03 mm, it has
become possible to confirm that the cooling power of the cooling
medium in the quenching device significantly affects the
distortion. This phenomenon was made clear for the first time as
the result of the high-precision measurement of the cooling power.
Conventionally, the effect of the cooling power on the distortion
was not recognized exactly, but was known vaguely as a kind of
tendency. As a result of clarification of such phenomenon as
described above, it has become clear that the distortion due to
heat treatment can be controlled by maintaining the cooling power
of the cooling medium in a fixed range. The quenching as mentioned
above may be induction hardening, or the entire member may be
heated in a heating furnace. Throughout the specification, the
cooling power is expressed using a convenient measure. More
specifically, a cooling time in which a sample member is cooled by
a prescribed temperature range, or a concentration of coolant
included in liquid cooling medium that is expressed as an
equivalent new coolant concentration, as will be described later,
is used as the measure to express the cooling power.
[0010] In the distortion control method of the preset invention,
the cooling medium is cooling water including coolant, and a change
of cooling power of the cooling water due to a running change of
the coolant is maintained in the fixed range.
[0011] The coolant is a water-soluble liquid polymer, such as
polyalkylene glycol (PAG). Normally, the coolant is dissolved into
cooling water in concentration of 5-20% for prevention of quenching
crack. Using the coolant, a uniform vapor film is formed on the
surface of the member undergoing the heat treatment, which helps
slow down the cooling, thereby preventing the quenching crack.
Thus, by dissolving the coolant in the cooling medium and
maintaining the cooling power in a fixed range, it becomes possible
to suppress the distortion while preventing the quenching crack.
Since the coolant consists of liquid polymer as described above,
the polymerization degree of such high polymer is lowered as heat
history is accumulated during the quenching. Therefore, as
operating days pass from the first day of use of new liquid of
coolant, the cooling power of the cooling medium increases in a
constant manner. If the cooling power is expressed using the
equivalent new coolant concentration as will be described below,
the value decreases in a constant manner. During this, however, the
concentration of coolant measured by a saccharimeter does not
exhibit a significant change. This means that the cooling power
cannot be estimated by only measuring the concentration using the
saccharimeter. The cooling power ceases to increase after 30 days
have passed since the day on which cooling medium in the quenching
device was renewed and the use of new liquid of coolant was
started. It is said that, when the polymerization degree of polymer
is lowered to a certain extent, the polymer is stabilized against
thermal shock. The stabilization of the cooling power after a lapse
of 30 days as described above is considered because the polymer has
reached such low polymerization degree. The distortion would not be
controlled accurately if only the cooling power of newly applied
cooling medium is considered without paying attention to such
change in cooling medium over time.
[0012] The distortion control method of the present invention is a
method of measuring cooling power of cooling medium employing
quenching of a sample member in a prescribed form made of a
material that does not transform in a temperature range to be
measured. The measurement is conducted by immersing the sample
member into the cooling medium and positioning the member at its
quenching stop position with accuracy within a range of .+-.0.03
mm.
[0013] By performing such high-precision cooling power measurement
with positioning accuracy within a range of .+-.0.03 mm as
described above, it has become clear for the first time that the
cooling power has a significant influence on the distortion. Thus,
by utilizing such high-precision cooling power measurement, the
distortion can be controlled effectively. For more effective
control of the distortion, positioning accuracy within a range of
.+-.0.015 mm will be desirable. As described above, the cooling
power may be expressed as a cooling time that is required for
cooling a sample member of a prescribed shape by a prescribed
temperature range. In this case, the cooling power is higher as the
cooling time is shorter. Alternatively, the cooling power may be
expressed as an equivalent new coolant concentration. More
specifically, a cooling time that is actually required for cooling
a sample member using cooling medium as a target of measurement is
correlated with a cooling time that would be required when the same
sample member is cooled using cooling medium including only new
coolant. It is then calculated what % of new coolant should be
included in the cooling medium to achieve the same cooling time as
with the target cooling medium. This percentage is called the
"equivalent new coolant concentration". For example, it can be said
like: cooling medium initially containing 15% of new coolant has
been used for 20 days, and now the equivalent new coolant
concentration of this cooling medium is decreased to 12%. In this
case, the cooling power is improved as the equivalent new coolant
concentration decreases.
[0014] The change in cooling power of the cooling medium is
significant for a prescribed time period from the start of use of
the new liquid of coolant, and it then becomes smaller and comes to
stabilize. The distortion control method of the present invention
utilizes cooling water that has entered such stabilized stage.
[0015] For example, when the cooling power of cooling medium
including coolant is represented by the equivalent new coolant
concentration, the concentration decreases for about 30 days from
the start of use of the new coolant in an unvaried manner, if
cooling medium is not resupplied or partly removed. After the lapse
of 30 days, however, the change in the equivalent new coolant
concentration becomes small. Thus, by using the cooling water as
described above, the running change of the cooling power becomes
negligible as cooling power of an approximately constant level is
maintained, and therefore, it becomes possible to readily control
the distortion due to heat treatment.
[0016] In the distortion control method of the present invention,
new liquid of coolant is resupplied to keep the cooling power of
the cooling water in a fixed range.
[0017] As the cooling power of new liquid of coolant is known, it
is possible to adjust the cooling power of the cooling water by
increasing/decreasing the ratio occupied by the new liquid of
coolant within the cooling water.
[0018] In the distortion control method of the present invention,
the distortion of the member during the quenching is controlled
using a cooling power transition table that indicates a running
change in cooling power of cooling medium from the start of use of
the new liquid of coolant.
[0019] As the control is done based on this cooling power
transition table, it is unnecessary to measure the cooling power
day by day. It is possible to determine the cooling power of the
quenching device at any time simply by calculating how many
operating days have passed since the start of use of the new liquid
of coolant. As a result, convenient and accurate control of
distortion is enabled. The measurement of cooling power is
desirably conducted using the high-precision measuring method as
described above. The cooling power transition table may be
represented as a graph or table, or even as enumeration of data.
The same applies to any table that will be described below.
[0020] In the distortion control method of the present invention,
the distortion of the member during the quenching is controlled
using a cooling power transition table in which transition in
cooling power of cooling medium from the start of use of new liquid
of coolant as well as transition in concentration of coolant
measured using a saccharimeter are indicated as a function of
time.
[0021] Because of the presence of such table, when the same
quenching is being repeated, it is possible to check the cooling
power simply by measuring the concentration of the coolant by the
saccharimeter and by calculating how long the coolant has been
used. Accordingly, it becomes possible to confirm the cooling power
in a simple and convenient manner.
[0022] In the distortion control method of the present invention,
the cooling power of cooling medium is adjusted using a distortion
table in which a relation between distortion of a member during
quenching and cooling power of cooling medium is indicated.
[0023] With the presence of such table that makes a specific value
of the distortion realized, it becomes possible to control as
described above. The distortion is normally expressed in % as a
ratio of a dimension of a member as a target of measurement after
quenching relative to its dimension before the quenching. However,
it may be expressed as an absolute value of such difference in
dimension.
[0024] In the distortion control method of the present invention,
the cooling power of the cooling medium is evaluated based on a
time required, upon quenching of a member, to lower a temperature
of the member from a temperature T.sub.1 to a temperature T.sub.2
that is lower than T.sub.1.
[0025] To derive the cooling power according to an academic
definition, complicated calculations will be required on its way.
Thus, in the present invention, the cooling power is conveniently
represented by a time required for cooling as described above.
According to the present invention, sample members of an identical
shape formed of an identical material are cooled using
approximately the same cooling medium by the same temperature
range. Thus, the cooling time can be utilized as a highly accurate
and convenient measure of the cooling power. The cooling power
according to the academic definition requires a material constant
of the sample member, for example, such that it can be applied even
if the cooling medium or cooling temperature range, or the material
of the sample member changes. Although such cooling power according
to the academic definition is derived using complicated
calculations, its accuracy is rather low.
[0026] In the distortion control method of the present invention,
the cooling power of the cooling medium is evaluated, assuming that
the quenchings of the same sample are conducted using cooling
medium including only new coolant, by obtaining a concentration of
the new coolant that should be included in the cooling medium to
obtain the same cooling time as with the target cooling medium.
[0027] The cooling power can also be expressed accurately and
conveniently by employing such equivalent new coolant concentration
derived from the cooling time as described above.
[0028] The cooling power measuring device of the present invention
is a device for measuring cooling power of cooling medium in a
quenching device. This measuring device includes: a heating device
for heating a sample member; a cooling medium bath for storing the
cooling medium for use in cooling the heated sample member; a
transfer device for transferring the sample member from the heating
device to the cooling medium bath and for immersing the sample
member into the cooling medium and holding the member at a
prescribed position; and a transfer control device for control of
an operation of the transfer device. The transfer device and the
transfer control device are configured to suppress variation in a
quenching stop position at which the sample member immersed in the
cooling medium is to be held, within a range of .+-.0.03 mm.
[0029] A cooling time that is required for cooling a sample member
by a prescribed temperature range is greatly affected by its stop
position. Therefore, the stop position should be controlled with
positioning accuracy within the range of .+-.0.03 mm; otherwise,
the distortion control as described above cannot be conducted
accurately. To achieve more accurate control of the distortion,
positioning accuracy within the range of .+-.0.015 mm is desirable.
As the cooling medium bath, a temperature-controlled bath is
preferred. As the heating device, a high-frequency coil is
preferable which enables rapid heating so that generation of oxide
film is restricted. For positioning the sample member in a shorter
period of time, it is preferred that the member is partially
immersed into the cooling medium and held at a prescribed position.
Instead, however, the entire sample member may be immersed
therein.
[0030] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a partly diagrammatic sectional view of an outer
ring of constant velocity universal joint being a member that is
subjected to quenching according to an embodiment of the present
invention.
[0032] FIG. 2 is a cross sectional view taken along a line II-II of
FIG. 1.
[0033] FIG. 3 shows a configuration of a cooling power measuring
device that is used for measuring cooling power of cooling water in
a quenching device according to the embodiment of the present
invention.
[0034] FIG. 4 shows a cooling curve that is obtained when cooling a
sample member in the cooling power measuring device shown in FIG.
3.
[0035] FIG. 5 shows variation in cooling power in a measurement of
the cooling power, wherein positioning accuracy of the sample
member at a quenching stop position is within .+-.0.5 mm.
[0036] FIG. 6 shows variation in cooling power in a measurement of
the cooling power according to the present invention, wherein the
positioning accuracy of the sample member at the quenching stop
position is within .+-.0.015 mm.
[0037] FIG. 7 shows cooling curves for the sample members in the
measurement of the cooling power according to the present
invention.
[0038] FIG. 8 illustrates how to obtain an equivalent new coolant
concentration from a cooling time of the sample member according to
the present invention.
[0039] FIG. 9 shows changes over time of the equivalent new coolant
concentration (cooling power) of the present invention and a
concentration of coolant measured by a saccharimeter.
[0040] FIG. 10 shows a change over time of a difference between the
equivalent new coolant concentration (cooling power) of the present
invention and the concentration of coolant measured by the
saccharimeter.
[0041] FIG. 11 shows a relation between an outer diameter changing
rate of a member subjected to quenching and the equivalent new
coolant concentration.
[0042] FIG. 12 shows a relation between the outer diameter changing
rate of the member subjected to quenching and the concentration of
coolant measured by the saccharimeter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] Embodiment of the present invention will now be described
with reference to the drawings. Referring to FIG. 2 showing the
cross section taken along the line II-II of FIG. 1, a dimension of
over pin diameter (OPD) is critical in terms of distortion. The
outer ring of constant velocity universal joint as shown is
subjected to induction hardening to cure its surface for the
purposes of improving its wear resistance and fatigue
characteristics. This quenching is performed at a temperature
exceeding 800.degree. C. at which carbon steel constituting the
outer ring of the constant velocity universal joint is
austenitized. Thus, by the quenching, the carbon steel is
transformed to a hardened structure with the above-described
characteristics being improved. Here, cooling power of cooling
medium for use in the quenching is important. A method of measuring
the cooling power of the cooling medium will now be described.
[0044] Referring to FIG. 3, the cooling medium 15 is extracted from
the cooling water actually used in a cooling medium bath in a
quenching line in a factory. To accurately comprehend the change in
the cooling water over time, it is necessary to extract the cooling
water day by day in the course of measurement. Sample member 11 is
preferably fabricated using Incoloy, which is Ni-based alloy that
maintains an austenite phase and does not transform from room
temperature to high temperature. Incoloy also exhibits good heat
resistance and forms almost no oxide film. Therefore, it will not
cause considerable variation even if it is repeatedly used for the
quenching. Sample member 11 is formed in a cylindrical shape having
a diameter of 10 mm and a thermocouple 12 is embedded in its
center. For measurement of the cooling power, sample member 11 is
heated by a high-frequency coil 13 to 550.degree. C. as measured by
thermocouple 12, and held at the temperature for a prescribed time
period. Thereafter, sample member 11 is immersed into cooling water
15 including coolant as a target of measurement, which is held at
100.degree. C. in a temperature-controlled bath 14, to a prescribed
depth 16 for cooling. According to the present embodiment,
positioning accuracy for positioning sample member 11 at a
prescribed position is within .+-.0.015 mm. The electrical signal
sent from thermocouple 12 undergoes data processing, and is
displayed as a cooling curve on a chart having a time axis as its
horizontal axis, as shown in FIG. 4. From this cooling curve, the
time required for cooling the member from 500.degree. C. to
150.degree. C. is derived, which is used as a measure of the
cooling power. Conventionally, the accuracy for positioning the
sample member at its stop position was low, i.e., on the order of
.+-.0.5 mm. With such poor accuracy, the variation in the cooling
time was as much as 3.2 seconds, as shown in FIG. 5. In the present
invention, however, the accuracy for positioning sample member 11
at its stop position as described above was improved. Specifically,
by achieving the positioning accuracy within .+-.0.015 mm, the
variation in the cooling time was limited within 0.8 seconds, as
shown in FIG. 6. Throughout the measurement of the cooling power as
described above, cooling medium containing only new coolant was
always used. It is noted that, even if the positioning accuracy as
described above is set within .+-.0.03 mm, cooling power utilizable
for the control of the distortion could be obtained.
[0045] FIG. 7 shows cooling curves each obtained when cooling is
conducted utilizing cooling medium including the stated percentage
of new coolant, with positioning accuracy of the sample member
within .+-.0.015 mm. From FIG. 7, it is noticed that, as the
content of the new coolant increases, the cooling becomes slower
and the cooling power decreases. The straight line shown in FIG. 8
represents a relation between the cooling time and the coolant
concentration when a sample member is immersed and cooled in
cooling medium including only new coolant (equivalent new coolant
concentration). From this straight line, it becomes possible to
obtain an equivalent new coolant concentration from the cooling
time actually obtained from the cooling medium used in a quenching
line of a factory. For example, referring to FIG. 8, when the
cooling time obtained from the cooling medium as a target of
measurement is 30.7 seconds, the equivalent new coolant
concentration of this cooling medium can be determined as 9.2%.
Before improvement of the positioning accuracy, with that of at
least .+-.0.5 mm, the cooling time would vary on the order of .+-.2
seconds, leading to variation in equivalent new coolant
concentration on the order of .+-.2%. With such a large variation,
the change of cooling power over time could not be detected, and
therefore, it would be unimaginable to control the distortion by
the cooling power. The above-described method of expressing the
cooling power as the equivalent new coolant concentration derived
from the cooling time is referred to as a cooling faculty (CF)
method. Conventionally, as simple means for measuring the
concentration of coolant within the cooing medium, a saccharimeter
has been used. Hereinafter, for the purposes of comparison, the
concentration measured by the saccharimeter according to the prior
art will also be described.
[0046] Transition in cooling power of cooling medium over time is
shown in FIG. 9, wherein a horizontal axis represents operating
days that have passed from the day on which the entire cooling
medium was renewed and the use of new liquid of coolant started.
Obtained by the CF method is the cooling power, measured using the
method as shown in FIG. 3 with improved positioning accuracy, and
expressed as the equivalent new coolant concentration as described
above. According to FIG. 9, the equivalent new coolant
concentration starts to decrease from the first day of the use of
new liquid of coolant. Such decrease ceases after 25 days have
passed from the start day, and thereafter, the concentration is
held approximately at a fixed level. In FIG. 9, the concentration
measured by the saccharimeter is also shown. This shows a change
similar to that of the equivalent new coolant concentration,
although any specific pattern cannot be observed from the change.
FIG. 10 shows a change over time of the difference between the
equivalent new coolant concentration and the concentration measured
by the saccharimeter. It decreases in an unvaried manner for almost
30 days, and thereafter, there comes a time period in which almost
no change is observed. Utilizing the graph of FIG. 10, it becomes
possible, by simply measuring the concentration using the
saccharimeter, to obtain the equivalent new coolant concentration
from the concentration measured and the number of days passed from
the start day.
[0047] As seen from FIG. 11, it is clear that there is a strong
correlation between the distortion and the equivalent new coolant
concentration. On the contrary, it cannot be said that there is a
certain correlation between the distortion and the concentration
measured by the saccharimeter, as shown in FIG. 12.
[0048] As explained above, the present invention was inspired by
the distinct correlation between cooling power and distortion that
is observable only when cooling power is measured by positioning a
member to be cooled in cooling medium with high positioning
accuracy within a range of .+-.0.03 mm, or even within .+-.0.015
mm. According to the present invention, it is possible to keep
track of cooling power precisely even when the cooling medium
changes over time. Thus, distortion due to heat treatment can be
controlled to a minimum.
[0049] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
* * * * *