U.S. patent application number 10/137440 was filed with the patent office on 2003-04-24 for bearing pressure-resistant member and process for making the same.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. Invention is credited to Kimura, Toshimitsu, Kurebayashi, Yutaka, Otani, Keizo, Uchiyama, Noriko, Yamaguchi, Takuro.
Application Number | 20030075244 10/137440 |
Document ID | / |
Family ID | 26615300 |
Filed Date | 2003-04-24 |
United States Patent
Application |
20030075244 |
Kind Code |
A1 |
Kurebayashi, Yutaka ; et
al. |
April 24, 2003 |
Bearing pressure-resistant member and process for making the
same
Abstract
A bearing pressure-resistant member, including a matrix and a
carbide dispersed in the matrix, the carbide having an average
particle size of not more than 0.3 .mu.m. A carbon (C) content in
an outer surface of the bearing pressure-resistant member is in a
range of 0.6 to 1.5 mass percent. A process for making the bearing
pressure-resistant member including subjecting a workpiece
containing C to either of gas carburizing and gas carbonitriding to
enhance the C content in the outer surface of the workpiece to 0.6
to 1.5 mass percent, holding the workpiece at a first temperature
not more than an Ac.sub.1 transformation point under reduced
pressure, heating the workpiece to a second temperature not less
than the Ac.sub.1 transformation point under reduced pressure,
followed by holding the workpiece at the second temperature, and
subjecting the workpiece to quenching.
Inventors: |
Kurebayashi, Yutaka;
(Nagoya, JP) ; Kimura, Toshimitsu; (Nagoya,
JP) ; Yamaguchi, Takuro; (Kanagawa, JP) ;
Otani, Keizo; (Kanagawa, JP) ; Uchiyama, Noriko;
(Kanagawa, JP) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
NISSAN MOTOR CO., LTD.
|
Family ID: |
26615300 |
Appl. No.: |
10/137440 |
Filed: |
May 3, 2002 |
Current U.S.
Class: |
148/218 ;
148/233; 148/319 |
Current CPC
Class: |
C23C 8/22 20130101; F16C
33/62 20130101; C23C 8/80 20130101 |
Class at
Publication: |
148/218 ;
148/233; 148/319 |
International
Class: |
C23C 008/22 |
Foreign Application Data
Date |
Code |
Application Number |
May 17, 2001 |
JP |
2001-148517 |
May 29, 2001 |
JP |
2001-160694 |
Claims
What is claimed is:
1. A bearing pressure-resistant member, comprising: a matrix and a
carbide dispersed in the matrix, said carbide having an average
particle size of not more than 0.3 .mu.m; and a C content ranging
from 0.6 to 1.5 mass percent in an outer surface thereof.
2. The bearing pressure-resistant member as claimed in claim 1,
wherein said bearing pressure-resistant member is made of a
mechanical structural steel containing C in an amount ranging from
0.6 to 1.5 mass percent.
3. The bearing pressure-resistant member as claimed in claim 1,
wherein said bearing pressure-resistant member is made of a
mechanical structural steel containing C in an amount ranging from
0.6 to 1.5 mass percent, Cr in an amount ranging from 1.2 to 3.2
mass percent, and Mo in an amount ranging from 0.25 to 2.0 mass
percent, said carbide comprising M.sub.23C.sub.6 carbide containing
at least Cr.
4. The bearing pressure-resistant member as claimed in claim 1,
wherein said bearing pressure-resistant member is made of a
mechanical structural steel which contains Cr in an amount ranging
from 1.2 to 3.2 mass percent and Mo in an amount ranging from 0.25
to 2.0 mass percent, and treated by either of carburizing and
carbonitriding, said carbide comprising M.sub.23C.sub.6 carbide
containing at least Cr.
5. The bearing pressure-resistant member as claimed in claim 1,
wherein an amount of C contained in the bearing pressure-resistant
member is enhanced to said C content by either of carburizing and
carbonitriding.
6. The bearing pressure-resistant member as claimed in claim 1,
wherein an amount of C contained in the bearing pressure-resistant
member is enhanced to said C content by either of gas carburizing
and gas carbonitriding, and a whole amount of H.sub.2 released over
a temperature range of 100.degree. C. to 900.degree. C. is limited
to not more than 0.2 ppm.
7. The bearing pressure-resistant member as claimed in claim 6,
wherein said bearing pressure-resistant member is made of a
mechanical structural steel containing Cr in an amount ranging from
1.2 to 3.2 mass percent and Mo in an amount ranging from 0.25 to
2.0 mass percent, said carbide comprising M.sub.23C.sub.6 carbide
containing at least Cr.
8. A process for making a bearing pressure-resistant member,
comprising: subjecting a workpiece containing C to either of gas
carburizing and gas carbonitriding to enhance a C content in an
outer surface of the workpiece to 0.6 to 1.5 mass percent; holding
the workpiece at a first temperature not more than an Ac.sub.1
transformation point under reduced pressure; heating the workpiece
to a second temperature not less than the Ac.sub.1 transformation
point under reduced pressure, followed by holding the workpiece at
the second temperature; and subjecting the workpiece to
quenching.
9. The process as claimed in claim 8, wherein the workpiece is made
of a mechanical structural steel containing Cr in an amount ranging
from 1.2 to 3.2 mass percent and Mo in an amount ranging from 0.25
to 2.0 mass percent, the first temperature is in a range of
600.degree. C. to 750.degree. C. which is reached by heating the
workpiece at a rate of 0.2.degree. C. to 30.degree. C. per minute
in a temperature range of 500.degree. C. to 650.degree. C., and the
second temperature is in a range not less than the Ac.sub.1
transformation point but not more than a predetermined temperature
T represented by the following formula: T(.degree.
C.)=675+120.Si(%)-27.Ni (%)+30.Cr(%)+215.Mo(%)-400V(%).
10. The process as claimed in claim 9, wherein the rate of heating
the workpiece in the temperature range of 500.degree. C. to
650.degree. C. is in a range of 0.2.degree. C. to 5.degree. C. per
minute.
11. A process for making a bearing pressure-resistant member,
comprising: heating a workpiece made of a mechanical structural
steel containing C in an amount ranging from 0.6 to 1.5 mass
percent, to a first temperature of 600.degree. C. to 750.degree. C.
wherein the workpiece is heated at a rate of 0.2.degree. C. to
30.degree. C. per minute in a temperature range of 500.degree. C.
to 650.degree. C.; holding the workpiece at the first temperature;
heating the workpiece to a second temperature not less than an
Ac.sub.1 transformation point and not more than an Acm
transformation point, followed by holding the workpiece at the
second temperature; and subjecting the workpiece to quenching.
12. The process as claimed in claim 11, further comprising
subjecting the workpiece to either of carburizing and
carbonitriding to enhance the C content in an outer surface of the
workpiece to 0.6 to 1.5 mass percent before heating the workpiece
to the first temperature.
13. The process as claimed in claim 11, wherein the rate of heating
the workpiece in the temperature range of 500.degree. C. to
650.degree. C. is in a range of 0.2.degree. C. to 5.degree. C. per
minute.
14. A process for making a bearing pressure-resistant member,
comprising: heating a workpiece made of a mechanical structural
steel containing C in an amount ranging from 0.6 to 1.5 mass
percent, Cr in an amount ranging from 1.2 to 3.2 mass percent, and
Mo in an amount ranging from 0.25 to 2.0 mass percent, to a first
temperature of 600.degree. C. to 750.degree. C. under reduced
pressure wherein the workpiece is heated at a rate of 0.2.degree.
C. to 30.degree. C. per minute in a temperature range of
500.degree. C. to 650.degree. C. under reduced pressure; holding
the workpiece at the first temperature; heating the workpiece to a
second temperature not less than an Ac.sub.1 transformation point
and not more than a predetermined temperature T represented by the
following formula: T(.degree.
C.)=675+120.Si(%)-27.Ni(%)+30.Cr(%)+215.Mo(%)-400V(%); holding the
workpiece at the second temperature; and subjecting the workpiece
to quenching.
15. The process as claimed in claim 14, further comprising
subjecting the workpiece to either of carburizing and
carbonitriding to enhance the C content in an outer surface of the
workpiece to 0.6 to 1.5 mass percent before heating the workpiece
to the first temperature.
16. The process as claimed in claim 14, wherein the rate of heating
the workpiece in the temperature range of 500.degree. C. to
650.degree. C. is in a range of 0.2.degree. C. to 5.degree. C. per
minute.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an element useable as a
power transmission part such as gears and rolling-contact bearings,
which necessitates a high surface fatigue strength. More
specifically, this invention relates to a bearing
pressure-resistant member suitable for use under a relatively high
bearing pressure in a relatively high temperature range of about
100.degree. C. to about 300.degree. C., and relates to a process
for making the bearing pressure-resistant member.
[0002] There have been proposed methods for enhancing bearing
pressure strength of the above-described power transmission part.
Among the methods, there are hyper-eutectoid carburizing and high
density carburizing. In these carburizing methods, M.sub.3C carbide
such as Fe.sub.3C which is not readily decomposed in the
above-described temperature range, is precipitated so that hardness
of the part and resistance to tempering softening can be enhanced.
European Patent Application Publication No. 1070760 A2
(corresponding to Japanese Patent Application First Publication No.
2001-98343) discloses a bearing pressure-resistant member including
M.sub.23C.sub.6 carbide particles which are precipitated in the
matrix in a dispersed state. M.sub.23C.sub.6 carbide is formed by
carbon (C) and metal combined with the C, which includes chromium
(Cr), molybdenum (Mo) and the like. The precipitation of
M.sub.23C.sub.6 carbide is performed by isothermal heat treatment
carried out between carburizing and quenching, in which a workpiece
is held at a temperature of not more than an Ac.sub.1
transformation point. The bearing pressure-resistant member of this
related technology has excellent surface fatigue strength as
compared with steels in which M.sub.3C carbide is precipitated in
the matrix. M.sub.23C.sub.6 carbide is also disclosed in U.S. Pat.
No. 6,342,109 (issued Jan. 29, 2002, to Takemura et al).
[0003] Such carbides are precipitated in an outer surface of the
element by isothermal heat treatment after subjecting the element
to carburizing to increase a C content in the outer surface. In the
isothermal heat treatment, the element carburized is held at a
temperature at which C produces a solid solution with the matrix.
This precipitation tends to start at a region where Cr forming
carbides is locally present, or at grain boundaries of austenite
particles at which nucleus of the precipitated carbide is produced.
The precipitated carbide is grown in a homogeneously dispersed
state in the outer surface of the element by the isothermal heat
treatment. Thereafter, the element is subjected to quenching to
provide the finished product.
[0004] Further, a heat treatment under controlled atmosphere (gas
carburizing) is generally used as the carburizing method, which
utilizes denatured gas made from propane (C.sub.3H.sub.8) gas. In
addition, U.S. Pat. No. 6,258,179 B1 describes vacuum carburizing
in which hydrogen (H.sub.2) generated upon gas carburizing hardly
infiltrates into a material steel.
SUMMARY OF THE INVENTION
[0005] However, it is difficult to homogeneously disperse
M.sub.23C.sub.6 carbide in the entire structure of the element or
the outer surface layer thereof which has an increased C content by
carburizing, only by the above-described isothermal heat treatment.
Therefore, the element will be provided with a region in which
M.sub.23C.sub.6 carbide is not precipitated, so that the region
will fail to be sufficiently hardened. Further, at the quenching
step following the above-described holding step, dense texture will
not be obtained at the region having no carbide precipitated and
dispersed, causing deterioration in rolling fatigue strength. This
is because carbide can prevent martensite produced upon quenching
from coarsely growing up. In European Patent Application
Publication No. 1070760 A2 as discussed above, a region having no
M.sub.23C.sub.6 carbide precipitated will be generated or an
excessively long treatment time will be required at the holding
step. This causes increase in the production cost.
[0006] Further, residual H.sub.2 infiltrating into the metal of the
element during the gas carburizing as described above, can be
remarkably reduced by the subsequent tempering. However, if the
residual H.sub.2 is insufficiently reduced, the element will suffer
from delayed fracture or deterioration in bending fatigue strength
and toughness. Recently, it has been recognized that rolling
fatigue lives of rolling elements which undergo high bearing
pressure upon coming into rolling contact with counterparts, will
be considerably deteriorated due to the residual H.sub.2.
Furthermore, since H.sub.2 is readily absorbed into the
above-described carbide as well known, the residual H.sub.2 must be
reduced in the gas carburizing treatment. In order to reduce the
residual H.sub.2, there has been proposed baking in which an
element is held at tempering temperature or less for several ten
hours. This leads to decrease in production efficiency and increase
in production cost. On the contrary, if the holding temperature is
raised beyond a certain level, the element will be softened while
the treatment time can be reduced.
[0007] In U.S. Pat. No. 6,258,179 B1 as discussed above, coarse
carbide tends to be produced at grain boundaries of austenite
grains, so that rolling fatigue strength or bending fatigue
strength will be deteriorated. In addition, apparatus for use in
the vacuum carburizing treatment is relatively expensive.
[0008] An object of the present invention is to provide a bearing
pressure-resistant member for use in power transmission, which has
excellent surface fatigue strength with provision of
M.sub.23C.sub.6 carbide homogeneously and finely dispersed in the
matrix. Also, the object of the present invention is to provide a
process for making the bearing pressure-resistant member by holding
the element at a predetermined temperature to precipitate
M.sub.23C.sub.6 carbide in the matrix in homogeneously and finely
dispersed state, the process being capable of improving
significantly surface fatigue strength. A further object of the
present invention is to provide a bearing pressure-resistant member
for use in power transmission, which is reduced in residual H.sub.2
even by gas carburizing or gas carbonitriding, and enhanced bending
fatigue strength or rolling fatigue strength, and a process for
making the bearing pressure-resistant member, capable of
suppressing deterioration in bending fatigue strength or rolling
fatigue strength which will be caused due to delayed fracture or
hydrogen embrittlement.
[0009] According to one aspect of the present invention, there is
provided a bearing pressure-resistant member, comprising:
[0010] a matrix and a carbide dispersed in the matrix, said carbide
having an average particle size of not more than 0.3 .mu.m; and
[0011] a C content ranging from 0.6 to 1.5 mass percent in an outer
surface thereof.
[0012] According to a further aspect of the present invention,
there is provided a process for making a bearing pressure-resistant
member, comprising:
[0013] subjecting a workpiece containing C to either of gas
carburizing and gas carbonitriding to enhance a C content in an
outer surface of the workpiece to 0.6 to 1.5 mass percent;
[0014] holding the workpiece at a first temperature not more than
an Ac.sub.1 transformation point under reduced pressure;
[0015] heating the workpiece to a second temperature not less than
the Ac.sub.1 transformation point under reduced pressure, followed
by holding the workpiece at the second temperature; and
[0016] subjecting the workpiece to quenching.
[0017] According to a still further aspect of the present
invention, there is provided a process for making a bearing
pressure-resistant member, comprising:
[0018] heating a workpiece made of a mechanical structural steel
containing C in an amount ranging from 0.6 to 1.5 mass percent, to
a first temperature of 600.degree. C. to 750.degree. C. wherein the
workpiece is heated at a rate of 0.2.degree. C. to 30.degree. C.
per minute in a temperature range of 500.degree. C. to 650.degree.
C.;
[0019] holding the workpiece at the first temperature;
[0020] heating the workpiece to a second temperature not less than
an Ac.sub.1 transformation point and not more than an Acm
transformation point, followed by holding the workpiece at the
second temperature; and
[0021] subjecting the workpiece to quenching.
[0022] According to a still further aspect of the present
invention, there is provided a process for making a bearing
pressure-resistant member, comprising:
[0023] heating a workpiece made of a mechanical structural steel
containing C in an amount ranging from 0.6 to 1.5 mass percent, Cr
in an amount ranging from 1.2 to 3.2 mass percent, and Mo in an
amount ranging from 0.25 to 2.0 mass percent, to a first
temperature of 600.degree. C. to 750.degree. C. under reduced
pressure wherein the workpiece is heated at a rate of 0.2.degree.
C. to 30.degree. C. per minute in a temperature range of
500.degree. C. to 650.degree. C. under reduced pressure;
[0024] holding the workpiece at the first temperature;
[0025] heating the workpiece to a second temperature not less than
an Ac.sub.1 transformation point and not more than a predetermined
temperature T represented by the following formula:
T(.degree.
C.)=675+120.Si(%)-27.Ni(%)+30.Cr(%)+215.Mo(%)-400V(%);
[0026] holding the workpiece at the second temperature; and
[0027] subjecting the workpiece to quenching.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is an explanatory diagram illustrating first heat
pattern of heat treatment carried out in examples of the present
invention;
[0029] FIG. 2 is an explanatory diagram illustrating second heat
pattern of heat treatment carried out in the examples of the
present invention;
[0030] FIG. 3 is an explanatory diagram illustrating a condition of
gas carburizing carried out in the examples of the present
invention;
[0031] FIG. 4 is an explanatory diagram illustrating third heat
pattern of heat treatment carried out in the examples of the
present invention after the gas carburizing of FIG. 3;
[0032] FIG. 5 is a scanning electron microphotograph of a
cross-section of an outer surface of a specimen of Comparative
Example 1;
[0033] FIG. 6 is an explanatory diagram illustrating fourth heat
pattern of heat treatment carried out in the examples of the
present invention;
[0034] FIGS. 7A and 7B are respectively a plan view and a side view
of specimens used in the examples of the present invention; and
[0035] FIGS. 8A and 8B show a schematic diagram of a thrust rolling
fatigue tester used in examples of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0036] A bearing pressure-resistant member of the present invention
includes a matrix and a carbide dispersed in the matrix. The
carbide has an average particle size of not more than 0.3 .mu.m.
The bearing pressure-resistant member has a carbon (C) content
ranging from 0.6 to 1.5 mass percent in at least an outer surface
thereof. The carbide is precipitated in the matrix in uniformly and
finely dispersed state. Owing to the above-described carbide, the
bearing pressure-resistant member of the invention can be improved
in bearing fatigue strength. The bearing pressure-resistant member
of the invention can be suitably used as parts such as gears and
rolling-contact bearings which are used under high bearing
pressure.
[0037] The bearing pressure-resistant member may be made of a
mechanical structural steel containing C in an amount ranging from
0.6 to 1.5 mass percent. More preferably, the bearing
pressure-resistant member may be made of a mechanical structural
steel containing C in an amount ranging from 0.6 to 1.5 mass
percent, Cr in an amount ranging from 1.2 to 3.2 mass percent, and
Mo in an amount ranging from 0.25 to 2.0 mass percent. The carbide
is preferably M.sub.23C.sub.6 carbide containing at least Cr.
Further, more preferably, the bearing pressure-resistant member may
be made of a mechanical structural steel which contains Cr in an
amount ranging from 1.2 to 3.2 mass percent and Mo in an amount
ranging from 0.25 to 2.0 mass percent and is treated by either of
carburizing and carbonitriding such that the C content is in the
range of 0.6 to 1.5 mass percent.
[0038] Specifically, if the C content is below 0.6 mass percent, an
amount of the carbide which is required for obtaining appropriate
hardness cannot be precipitated. The carbide amount can be
indicated as a ratio of an area of the carbide to a reference area
of the bearing pressure-resistant member. If the C content is more
than 1.5 mass percent, network of M.sub.3C carbide will be produced
to thereby deteriorate mechanical properties of the bearing
pressure-resistant member. Further, if the average particle size of
the carbide is more than 0.3 .mu.m, rolling fatigue life of the
bearing pressure-resistant member will be reduced. Furthermore, if
the Cr content in the material steel is less than 1.2 mass percent,
the amount of carbide will be decreased so that the bearing
pressure-resistant member having excellent rolling fatigue life
cannot be obtained. On the contrary, if the Cr content is more than
3.2 mass percent, machinability of the bearing pressure-resistant
member will be deteriorated. Further, if the Mo content in the
material steel is less than 0.25 mass percent, the M.sub.23C.sub.6
carbide will not be stably precipitated. If the Mo content in the
material steel is less than 2.0 mass percent, the machinability of
the bearing pressure-resistant member will be decreased. Owing to
the Cr content and the Mo content which are in the above-described
ranges, respectively, precipitation of the M.sub.23C.sub.6 carbide
can be facilitated. This serves for assuring good rolling fatigue
strength of the bearing pressure-resistant member even at the
relatively high temperature range of about 100.degree. C. to about
300.degree. C. Further, the M.sub.23C.sub.6 carbide is finely
dispersible in the matrix as compared to other carbides. Therefore,
the bearing pressure-resistant member containing the
M.sub.23C.sub.6 carbide precipitated at the finely dispersed state
in the matrix, can be significantly improved in rolling fatigue
strength.
[0039] The C content in the outer surface of the bearing
pressure-resistant member may be controlled to 0.6 to 1.5 mass
percent by either of carburizing and carbonitriding. Namely, the
amount of C contained in the outer surface of the bearing
pressure-resistant member may be enhanced to 0.6 to 1.5 mass
percent by either of carburizing and carbonitriding. Alternatively,
the amount of C contained in the outer surface of the bearing
pressure-resistant member may be enhanced to 0.6 to 1.5 mass
percent in by either of gas carburizing and gas carbonitriding. Gas
carburizing and gas carbonitriding can be conducted with a
cost-saving facility and at well-controlled carburizing
concentration. This contributes to reduction of the production cost
of the bearing pressure-resistant member and to suppression of
coarse growth of carbide in the matrix. In addition, the bearing
pressure-resistant member of the invention can exhibit high
hardness and, therefore, excellent bearing fatigue strength.
Further, upon the gas carburizing or gas carbonitriding, a whole
amount of hydrogen (H.sub.2) released over a temperature range of
100.degree. C. to 900.degree. C., may be limited to not more than
0.2 ppm. This can prevent hydrogen embrittlement in high-hardness
material steel which will occur due to adsorption of H.sub.2 to the
M.sub.23C.sub.6 carbide. The bearing pressure-resistant member of
the invention, therefore, can exhibit increased bearing fatigue
strength and excellent rolling fatigue life.
[0040] A process for making the bearing pressure-resistant member,
according to the invention, is now explained. A workpiece
containing C is subjected to either of gas carburizing and gas
carbonitriding to enhance the amount of C contained in an outer
surface of the workpiece to 0.6 to 1.5 mass percent. Next, the
workpiece is subjected to a first isothermal heat treatment under
reduced pressure. In the first isothermal heat treatment, the
workpiece is held at a first temperature not more than an Ac.sub.1
transformation point for a first period of time. Subsequently, the
workpiece is subjected to a second isothermal heat treatment under
reduced pressure. In the second isothermal heat treatment, the
workpiece is heated to a second temperature not less than the
Ac.sub.1 transformation point, and then held at the second
temperature for a second period of time. Subsequent to the holding
at the second temperature, the workpiece is subjected to quenching
to provide the bearing pressure-resistant member.
[0041] In the first isothermal heat treatment, an amount of H.sub.2
contained in the workpiece is effectively reduced. In the second
isothermal heat treatment and the quenching subsequent thereto, a
tough structure of martensite or bainite is produced. The first
isothermal heat treatment is conducted in advance of the second
isothermal heat treatment. By the first isothermal heat treatment,
the H.sub.2 content in the workpiece is effectively reduced without
deteriorating the hardness of the workpiece in spite of the first
temperature higher than the temperature of the conventional baking.
Further, since the first temperature is not more than the Ac.sub.1
transformation point, there is shown no increase in grain size of
austenite and no coarse growth of carbide present at grain
boundaries of austenite grains. In addition, the process of the
invention which employs gas carburizing or gas carbonitriding, can
serve for saving the facility cost and readily controlling the
carburizing concentration as explained above. According to the
process of the invention, the bearing pressure-resistant member
having excellent bearing fatigue strength and rolling fatigue life
can be produced with an inexpensive production cost.
[0042] Specifically, the workpiece may be made of a mechanical
structural steel containing Cr in an amount ranging from 1.2 to 3.2
mass percent and Mo in an amount ranging from 0.25 to 2.0 mass
percent. The first temperature may be in a range of 600.degree. C.
to 750.degree. C. which is reached by heating the workpiece at a
rate of 0.2.degree. C. to 30.degree. C. per minute in a temperature
range of 500.degree. C. to 650.degree. C. The second temperature
may be in the range not less than the Ac.sub.1 transformation point
but not more than a predetermined temperature T represented by the
following formula:
T(.degree.
C.)=675+120.Si(%)-27.Ni(%)+30.Cr(%)+215.Mo(%)-400V(%).
[0043] In the first isothermal heat treatment, the H.sub.2 content
in the workpiece is reduced, and at the same time, M.sub.23C.sub.6
carbide is homogeneously precipitated in the matrix. Upon heating
the workpiece to the first temperature, the temperature rise rate
is 0.2.degree. C. to 30.degree. C. per minute in the range of
500.degree. C. to 650.degree. C. wherein nucleus of the
M.sub.23C.sub.6 carbide is produced. This facilitates producing the
nucleus of the M.sub.23C.sub.6 carbide, so that the M.sub.23C.sub.6
carbide can be precipitated at more finely and homogeneously
dispersed state in the matrix. By the second isothermal heat
treatment subsequent to the first isothermal heat treatment, and
the quenching, the precipitated M.sub.23C.sub.6 carbide can be kept
in the homogeneously and finely dispersed state without formation
of excessive solid solution. Then, the workpiece has martensite or
bainite structure in the matrix. As a result, the bearing
pressure-resistant member of the invention can be significantly
increased in rolling fatigue strength.
[0044] A second process for making the bearing pressure-resistant
member, according to the present invention, is explained. A
workpiece made of a mechanical structural steel containing C in an
amount ranging from 0.6 to 1.5 mass percent is heated to a first
temperature of 600.degree. C. to 750.degree. C. wherein the
workpiece is heated at a rate of 0.2.degree. C. to 30.degree. C.
per minute in a temperature range of 500.degree. C. to 650.degree.
C. The workpiece is then held at the first temperature for a first
period of time. Next, the workpiece is heated to a second
temperature not less than an Ac.sub.1 transformation point but not
more than an Acm transformation point, and then held at the second
temperature for a second period of time. Subsequent to the holding
at the second temperature, the workpiece is subjected to
quenching.
[0045] The workpiece may be subjected to either of carburizing and
carbonitriding to enhance a C content in an outer surface of the
workpiece to 0.6 to 1.5 mass percent before heating the workpiece
to the first temperature. Specifically, in a case where the C
content in the outer surface of the bearing pressure-resistant
member is less than 0.6 mass percent, the amount of carbide
required for appropriate hardness cannot be precipitated. In other
words, a required ratio of an area of carbide precipitated in a
local region of the outer surface of the bearing pressure-resistant
member, to the whole area of the local region thereof cannot be
obtained. If the C content in the outer surface of the bearing
pressure-resistant member exceeds 1.5 mass percent, M.sub.3C
carbide will be produced to thereby deteriorate mechanical
properties of the bearing pressure-resistant member.
[0046] Further, if the first temperature is below 600.degree. C., a
rate of diffusion of C will decrease, whereby the growth of
M.sub.23C.sub.6 carbide will be significantly lowered. This causes
increase in the production cost. If the first temperature is more
than 750.degree. C., the C content in the matrix will be consumed
when the M.sub.3C carbide is produced. M.sub.23C.sub.6 carbide,
therefore, cannot grow up so that the hardness of the bearing
pressure-resistant member will not be assured. Furthermore, if the
temperature rise rate is less than 0.2.degree. C. per minute in the
range of 500.degree. C. to 650.degree. C., the treatment time is
excessively prolonged, causing increase in the production cost. If
the temperature rise rate is more than 30.degree. C. per minute in
the range of 500.degree. C. to 650.degree. C., a time required for
producing the nucleus of M.sub.23C.sub.6 carbide will be too short.
This will cause lack of the amount of carbide precipitated.
Preferably, the temperature rise rate is in a range of 0.2.degree.
C. to 5.degree. C. per minute in the range of 500.degree. C. to
650.degree. C.
[0047] Further, if the second temperature is less than the Ac.sub.1
transformation point, the matrix obtained after quenching will not
have martensite or bainite structure. If the second temperature is
more than the Acm transformation point or temperature T as given by
the above-described formula, the M.sub.23C.sub.6 carbide
precipitated will form the solid solution again.
[0048] A third process for making the bearing pressure-resistant
member, according to the present invention, is explained. A
workpiece made of a mechanical structural steel containing C in an
amount ranging from 0.6 to 1.5 mass percent, Cr in an amount
ranging from 1.2 to 3.2 mass percent, and Mo in an amount ranging
from 0.25 to 2.0 mass percent, is heated to a first temperature of
600.degree. C. to 750.degree. C. under reduced pressure. In the
heat treatment, the workpiece is heated at a rate of 0.2.degree. C.
to 30.degree. C. per minute in a temperature range of 500.degree.
C. to 650.degree. C., and held at the first temperature for a first
period of time. Next, the workpiece is heated to a second
temperature not less than an Ac.sub.1 transformation point and not
more than a predetermined temperature T represented by the
following formula:
T(.degree.
C.)=675+120.Si(%)-27.Ni(%)+30.Cr(%)+215.Mo(%)-400V(%)
[0049] The workpiece is then held at the second temperature for a
second period of time. Subsequently, the workpiece is subjected to
quenching.
[0050] The workpiece may be subjected to either of carburizing and
carbonitriding to enhance a C content in an outer surface of the
workpiece to 0.6 to 1.5 mass percent before heating the workpiece
to the first temperature of 600.degree. C. to 750.degree. C.
[0051] The reasons for limitation of the temperature ranges and
component values as described above are summarized as follows.
Here, the contents of the respective components are represented by
mass percent.
[0052] Outer surface C content: 0.6% to 1.5% If the C content is
less than 0.6%, appropriate hardness of the outer surface of the
bearing pressure-resistant member cannot be assured. If the C
content is more than 1.5%, M.sub.3C carbide will be coarsely
precipitated at the grain boundary of austenite grains. In this
case, good bearing fatigue strength cannot be obtained.
[0053] Whole amount of H.sub.2:0.2 ppm or less
[0054] If a whole amount of H.sub.2 is more than 0.2 ppm, hydrogen
embrittlement will occur, causing deterioration of rolling fatigue
life of a steel having a high C content and high hardness.
[0055] First isothermal heat treatment (H.sub.2 reduction
treatment) subsequent to either of gas carburizing, gas
carbonitriding, carburizing and carbonitriding: temperature not
more than Ac.sub.1 transformation point under reduced pressure
[0056] If the temperature is more than the Ac.sub.1 transformation
point, undesired coarse carbide will be precipitated at grain
boundary of austenite grains. The reduced pressure facilitates
reduction of H.sub.2 infiltrated into the steel and prevents
occurrence of decarburization therein.
[0057] Quenching temperature: Ac.sub.1 transformation point or
more
[0058] If the temperature is less than the Ac.sub.1 transformation
point, a steel will not have martensite or bainite structure in the
matrix. Therefore, required hardness of the bearing
pressure-resistant member cannot be obtained.
[0059] Cr content: 1.2% to 3.2%
[0060] Since Cr is an essential for producing M.sub.23C.sub.6
carbide, it is preferable that a material steel contains Cr in an
amount of 1.2% to 3.2%. If the Cr content is less than 1.2%, an
amount of the M.sub.23C.sub.6 carbide precipitated will be
decreased or stable precipitation of the M.sub.23C.sub.6 carbide
cannot be attained. If the Cr content is more than 3.2%, production
cost of the bearing pressure-resistant member will be increased and
machinability thereof will be deteriorated.
[0061] Mo content: 0.25% to 2.0%
[0062] Mo is an element for producing M.sub.23C.sub.6 carbide. It
is preferable that a material steel contains Mo in an amount of
0.25% to 2.0%. If the Mo content is less than 0.25%, an amount of
the M.sub.23C.sub.6 carbide precipitated will be decreased or
stable precipitation of the M.sub.23C.sub.6 carbide cannot be
attained. If the Mo content exceeds 2.0%, there will occur increase
in production cost of the bearing pressure-resistant member and
deterioration of machinability thereof.
[0063] Average particle size of M.sub.23C.sub.6 carbide: 0.3 .mu.m
or less
[0064] If the average particle size of M.sub.23C.sub.6 carbide is
more than 0.3 .mu.m, too long time will be required for obtaining a
uniform structure of the bearing pressure-resistant member in which
the precipitated M.sub.23C.sub.6 carbide is finely and uniformly
dispersed in the matrix. This will cause increase in production
cost. If the average particle size of M.sub.23C.sub.6 carbide is
excessively larger than 0.3 .mu.m, rolling fatigue life of the
bearing pressure-resistant member will be deteriorated.
[0065] Temperature of first isothermal heat treatment subsequent to
either of gas carburizing, gas carbonitriding, carburizing and
carbonitriding: 600.degree. C. to 750.degree. C.
[0066] If the temperature is less than 600.degree. C., a rate of
diffusion of C into the matrix will be small so that the growth of
M.sub.23C.sub.6 carbide will be considerably slow. This will cause
increase in production cost of the bearing pressure-resistant
member. If the temperature is more than 750.degree. C., C will be
consumed for production of M.sub.3C carbide so that the growth of
M.sub.23C.sub.6 carbide cannot be caused. As a result, required
hardness of the bearing pressure-resistant member cannot be
obtained.
[0067] Temperature rise rate upon heating up to the isothermal heat
treatment temperature of 600.degree. C. to 750.degree. C.:
0.2.degree. C. to 30.degree. C. per minute
[0068] If the temperature rise rate is less than 0.2.degree.
C./min, the treatment time will be remarkably prolonged, causing
increase in production cost of the bearing pressure-resistant
member. If the temperature rise rate is more than 30.degree.
C./min, nucleus of M.sub.23C.sub.6 carbide will not be sufficiently
formed so that the M.sub.23C.sub.6 carbide will not be uniformly
and finely precipitated in the matrix.
[0069] Quenching temperature: not less than Ac.sub.1 transformation
point and not more than temperature T represented by the following
formula: T(.degree.
C.)=675+120.Si(%)-27.Ni(%)+30.Cr(%)+215.Mo(%)-400V(%) If the
quenching temperature is less than the Ac.sub.1 transformation
point, the matrix of the bearing pressure-resistant member will not
have martensite or bainite structure. If the quenching temperature
is more than the above-described temperature T, M.sub.23C.sub.6
carbide precipitated in the matrix will form the solid solution
again.
[0070] The production process of the invention can provide a
bearing pressure-resistant member having fine M.sub.23C.sub.6
carbide at least the outer surface, which is precipitated in the
matrix in the homogeneously dispersed state as explained above.
Further, in the process of the invention, reduction of H.sub.2 can
be effectively performed to thereby prevent hydrogen embrittlement
which will be caused by hydrogen infiltration into the matrix.
Accordingly, the bearing pressure-resistant member made by the
process of the invention can exhibit excellent bearing fatigue
strength and enhanced rolling fatigue life because of the
M.sub.23C.sub.6 carbide finely and homogeneously dispersed.
EXAMPLES
[0071] The present invention is described in more detail by way of
examples and comparative examples by referring to the accompanying
drawings. However, these examples are only illustrative and not
intended to limit a scope of the present invention thereto.
Examples 1-6 and Comparative Examples 1-6
[0072] Specimens were prepared from six steel materials A-F for
machine structural use, each having a chemical composition as shown
in Table 1, in the following manner. The specimens made of
materials A, B and C were subjected to carburizing so as to have a
C content of 0.7 to 1.4 mass percent in an outer surface thereof.
Next, the specimens were subjected to heat treatment including
first and second isothermal heat treatments and then quenching. In
the heat treatment, either of heat patterns shown in FIGS. 1 and 2
was used. In FIGS. 1 and 2, t1 represents a first temperature for
the first isothermal heat treatment, and t2 represents a second
temperature for the second isothermal heat treatment. Also, dT
represents a temperature rise rate per minute in a temperature
range of 500.degree. C. to 650.degree. C. Subsequently, the
specimens heat-treated were subjected to tempering at a temperature
of 170.degree. C. for two hours, and then to grinding to finish the
outer surface. The specimens made of materials D, E and F were not
subjected to carburizing but subjected to the same heat treatment
as described above. The specimens heat-treated were then subjected
to tempering and grinding in the same manner as described above.
Meanwhile, in Example 3, the specimen was cooled down to a
temperature of 600.degree. C. after carburizing, and then was
subjected to the same heat treatment as described above.
1TABLE 1 Steel Chemical Composition (mass %) T* Material C Si Mn Ni
Cr Mo V (.degree. C.) A 0.18 1.41 1.11 1.48 3.15 0.44 0.33 860 B
0.20 1.03 0.30 2.10 2.18 1.21 -- 1067 C 0.33 0.82 0.24 2.32 1.62
1.88 0.13 1112 D 0.65 1.55 0.34 1.52 2.65 0.99 0.21 1028 E 0.87
0.82 0.18 2.00 2.02 0.78 -- 948 F 1.18 1.10 0.28 1.04 1.81 1.44
0.18 1071 T* (.degree. C.) = 675 + 120 .times. Si (%) - 27 .times.
Ni (%) + 30 .times. Cr (%) + 215 .times. Mo (%) - 400 .times. V
(%)
[0073] Each of the thus-prepared specimens was cut into a disk
shape shown in FIGS. 7A and 7B, which had a diameter of 60 mm and a
thickness of 5 mm. The cut specimen was subjected to rolling
fatigue test under conditions shown in Table 2, to evaluate the
rolling fatigue life, i.e., the life up to flaking. The rolling
fatigue life was obtained as L50 at a cumulative fracture
probability of 50% based on Weibull distribution. The rolling
fatigue test was conduced using a thrust rolling fatigue tester
shown in FIGS. 8A and 8B. As illustrated in FIG. 8B, specimen 3 was
set in contact with three steel bearing balls 5 in traction 5 oil
4. In FIG. 8B, only two steel bearing balls 5 were shown. Specimen
3 was brought into rolling contact with steel bearing balls 5 when
a shaft of the tester rotates about the rotation axis as indicated
by curved arrow of FIG. 8B. The results were shown in Table 3.
2TABLE 2 Testing Machine Thrust rolling fatigue tester Bearing
Pressure 5.23 GPa Maximum Shearing Stress Depth* 0.1 mm from
outer-most surface Revolution Number 2000 rpm Lubricating Oil
Transmission oil Oil Temperature 150.degree. C. Counterpart Steel
Ball Three balls made of JIS SUJ2 steel, having 3/8-inch diameter
*Depth at which maximum shearing stress was caused.
[0074] Further, each of the thus-prepared specimens was subjected
to evaluation of properties of the outer surface in the following
manner. The specimen was cut vertically, and the vertical cross
section was subjected to etching with a nital etchant composed of
3% nitric acid alcohol solution. Microphotograph at the
magnification of 10,000 of an outer portion of the vertical cross
section was obtained using a scanning electron microscope (SEM).
The outer portion had a depth of 0.1 mm from the outer-most surface
of the specimen. The microphotograph was subjected to image
analysis to measure an average particle size of precipitated
carbide. Further, a local region of the vertical cross section was
observed with an optical microscope to obtain an area ratio of
non-carbide-precipitated portion where no carbide was precipitated,
to the whole local region. The local region was located at a depth
of 0.1 mm +/- 0.05 mm from the outer-most surface of the specimen.
The results were shown in Table 3.
3 TABLE 3 Steel Heat Heat Treatment Material Pattern dT (.degree.
C./min) t1 (.degree. C.) t2 (.degree. C.) Example 1 A 5 630 850
Example 2 B 5 650 870 Example 3 C 10 690 880 Example 4 D 0.3 610
830 Example 5 E 10 640 900 Example 6 F 15 730 850 Comparative A 5
630 880 Example 1 Comparative B 5 770 870 Example 2 Comparative C
10 580 880 Example 3 Comparative D 40 610 830 Example 4 Comparative
E 10 640 950 Example 5 Comparative F 15 760 850 Example 6 Area
Ratio of Average Particle Non-Carbide- Rolling Fatigue Size of
Precipitated Life (L50) .times. M.sub.23C.sub.6 Carbide (.mu.m)
Region (%) 10000 Example 1 0.18 0 880 Example 2 0.17 0 1000 Example
3 0.22 0 1000 Example 4 0.16 0 960 Example 5 0.21 0 1000 Example 6
0.27 0 1000 Comparative 0.12 33 300 Example 1 Comparative 0.31 18
290 Example 2 Comparative 0.21 45 320 Example 3 Comparative 0.09 21
210 Example 4 Comparative 0.14 62 180 Example 5 Comparative 0.29 14
260 Example 6
[0075] It was recognized from Table 3 that carbides 5 were
precipitated in the outer surfaces of the specimens obtained in
Examples 1-6. The precipitated carbides were finely and
homogeneously dispersed in the matrix so that the specimens
obtained in Examples 1-6 exhibited the excellent rolling fatigue
lives. On the contrary, it was found that the specimens obtained in
Comparative Examples 1 and 5 had the regions of the outer surfaces
which had no precipitated carbide because the precipitated carbide
formed a solid solution with the matrix again due to the relatively
high temperature t2. It was found that the specimens obtained in
Comparative Examples 2 and 6 had too small amount of
M.sub.23C.sub.6 carbide precipitated in the outer surfaces due to
the relatively high temperature t1. It was found that the specimen
obtained in Comparative Example 3 had the region of the outer
surface which had no precipitated carbide because the growth rate
of carbide was small due to the relatively low temperature t1.
Further, it was confirmed that the specimens obtained in
Comparative Example 4 had inhomogeneously precipitated carbide in
the outer surface which was caused by less production of carbide
nucleus due to the large temperature rise rate dT. As a result, the
specimens obtained in Comparative Examples 1-6 exhibited the
shortened rolling fatigue lives as shown in Table 3.
Examples 7-10 and Comparative Examples 7-16
[0076] Specimens were prepared from five steel materials G-K for
machine structural use, each having a chemical composition as shown
in Table 4, in the following manner. The specimens were subjected
to normalizing and then gas carburizing under conditions shown in
FIG. 3. A carburizing gas composition (C potential) was adjusted so
as to provide a C content in an outer surface of each specimen in a
range of 0.7 to 0.8 mass percent. Next, the specimens were
subjected to heat treatment including first and second isothermal
treatments and then quenching. In the heat treatment, a heat
pattern shown in FIG. 4 was used under temperature conditions TC1
to TC5 shown in Table 5. In FIG. 4, t1 and t2 and dT represent the
same temperatures and temperature rise rate as described in
Examples 1-6 and Comparative Examples 1-6.
4TABLE 4 Steel Chemical Composition (mass %) T* Material C Si Mn Ni
Cr Mo V (.degree. C.) G 0.20 0.93 0.26 2.32 1.50 0.96 0.19 899 H
0.18 1.03 0.30 2.10 2.18 1.21 -- 1067 I 0.21 0.25 0.72 -- 0.93 --
-- 733 J 0.21 0.22 0.69 -- 1.06 0.21 -- 778 K 0.19 0.18 0.56 1.79
0.57 0.28 -- 726 T* (.degree. C.) = 675 + 120 .times. Si (%) - 27
.times. Ni (%) + 30 .times. Cr (%) + 215 .times. Mo (%) - 400
.times. V (%)
[0077]
5 TABLE 5 Temperature Condition dT (.degree. C./min) t1 (.degree.
C.) t2 (.degree. C.) TC1 0.8 610 820 TC2 5 700 880 TC3 5 540 820
TC4 10 760 880 TC5 40 640 820
[0078] Each of the thus-prepared specimens was subjected to
evaluation of properties of the outer surface in the same manner as
described in Examples 1-6 and Comparative Examples 1-6. The results
were shown in Table 7.
[0079] On the other hand, each of the thus-prepared specimens was
cut into the same disk shape as described in Examples 1-6 and
Comparative Examples 1-6. The cut specimen was subjected to
grinding to finish the outer surface. Thereafter, the specimen was
subjected to the rolling fatigue test to evaluate the rolling
fatigue life L10 at a cumulative fracture probability of 10% based
on Weibull distribution. The rolling fatigue test was conducted
using the same testing machine as described in Examples 1-6 and
Comparative Examples 1-6 except for using the test conditions shown
in Table 6. The results were shown in Table 7.
6 TABLE 6 Testing Machine Thrust rolling fatigue tester Bearing
Pressure (Load) 5.2 GPa (10 kg) Revolution Number 2000 rpm
Lubricating Oil Nissan Traction Fluid KTF-1 Oil Temperature
150.degree. C. Counterpart Steel Ball Three balls made of JIS SUJ2
steel, having 3/8-inch diameter
[0080]
7 TABLE 7 Area Ratio Average Rolling of Non- Particle Fatigue
Temper- Carbide- Size of Life Steel ature Precipitated Carbide
(L10) .times. Material Condition Region (%) (.mu.m) 10000 Example 7
G TC1 0 0.19 79 Example 8 G TC2 0 0.21 118 Example 9 H TC1 0 0.21
75 Example 10 H TC2 0 0.23 101 Comparative G TC3 38 0.17 11 Example
7 Comparative G TC5 31 0.23 14 Example 8 Comparative H TC3 27 0.20
31 Example 9 Comparative H TC4 10 0.08 18 Example 10 Comparative I
TC4 --* --* 8 Example 11 Comparative I TC5 16 0.21 15 Example 12
Comparative J TC3 41 0.22 9 Example 13 Comparative J TC4 5 0.12 19
Example 14 Comparative K TC3 84 0.13 17 Example 15 Comparative K
TC4 --* --* 12 Example 16 *No carbide precipitation
[0081] It was confirmed from Table 7 that carbides were
precipitated in the outer surface of the specimens obtained in
Examples 7-10. The precipitated carbides were finely and uniformly
dispersed in the matrix so that the specimens obtained in Examples
7-10 exhibited the excellent rolling fatigue lives. On the other
hand, it was found that the specimens obtained in Comparative
Examples 7, 9, 13 and 15 had the regions of the outer surface which
had no precipitated carbide because carbide was grown at a small
rate due to the remarkably low temperature t1. Therefore, the
specimens obtained in Comparative Examples 7, 9, 13 and 15
exhibited the rolling fatigue lives less than those in Examples
7-10. It was found that the specimens obtained in Comparative
Examples 10, 11, 14 and 16 had substantially no precipitated
carbide in the outer surface due to the excessively high
temperatures t1 and t2, and therefore had the shortened rolling
fatigue lives. Further, it was found that the specimens obtained in
Comparative Examples 8 and 12 had the regions of the outer surface
which had no precipitated carbide because less production of
carbide nucleus was caused due to the larger temperature rise rate
dT. As a result, the specimens obtained in Comparative Examples 8
and 12 exhibited the shortened rolling fatigue lives as shown in
Table 7.
[0082] FIG. 5 shows a microphotograph of the cross-section, taken
with SEM, of the outer surface of the specimen obtained in
Comparative Example 1. In FIG. 5, a black part depicts the region
having precipitated carbide, and a blank part depicts the region
having no precipitated carbide.
Examples 11-16 and Comparative Examples 17-25
[0083] Specimens were prepared in the following manner. The
specimens were made of four steel materials L-O for machine
structural use, each having a chemical composition as shown in
Table 8. The specimens were subjected to normalizing and then gas
carburizing under conditions shown in FIG. 3. A carburizing gas
composition (C potential) was adjusted so as to have such a C
content in an outer surface of each specimen as shown in Table 10.
Subsequently, the specimens were subjected to heat treatment in
which a heat pattern shown in FIG. 6 was used under temperature
conditions TC6 to TC10 shown in Table 9, under reduced ambient
pressure of 100 Pa. In FIG. 6, t1, t2 and dT represent the same
temperatures and temperature rise rate as described in Examples 1-6
and Comparative Examples 1-6. In Examples 11-16 and Comparative
Examples 17, 18 and 23-25, the specimens were subjected to the same
tempering as described in Examples 7-10 and Comparative Examples
7-16 after the heat treatment, but not held at temperature t3 for
five hours. In Comparative Examples 19-22, the specimens were not
subjected to the first isothermal heat treatment at temperature t1,
but subjected to the second isothermal heat treatment at
temperature t2 and the same tempering and then held at temperature
t3 for five hours.
8TABLE 8 Steel Chemical Composition (mass %) T* Material C Si Mn Ni
Cr Mo V (.degree. C.) L 0.20 1.00 0.30 2.00 1.50 1.50 0.19 1032.5 M
0.18 1.03 0.39 2.10 2.10 0.70 -- 955 N 0.22 0.20 0.65 -- 1.50 0.20
-- 787 O 0.20 1.00 0.30 1.90 -- 0.70 -- 894 T* (.degree. C.) = 675
+ 120 .times. Si (%) - 27 .times. Ni (%) + 30 .times. Cr (%) + 215
.times. Mo (%) - 400 .times. V (%)
[0084]
9TABLE 9 Temperature Condition dT (.degree. C./min) t1 (.degree.
C.) t2 (.degree. C.) t3 (.degree. C.) TC6 5 650 820 -- TC7 5 800
820 -- TC8 35 650 880 -- TC9 -- -- 880 300 TC10 -- -- 820 120
[0085] Each of the thus-prepared specimens was cut into test pieces
which were subjected to evaluation of properties. Hardness of each
test piece was measured using a Vickers hardness tester. The C
content in the test piece was measured by emission spectrochemical
analysis. The microstructure of the precipitated carbide in the
test piece was identified based on an electron beam diffraction
image obtained by replica method. Average particle size and area
ratio of the precipitated carbide were obtained by image analysis
of a SEM microphotograph of the test piece. Whole amount of H.sub.2
was obtained from a characteristic curve showing a H.sub.2 amount
released upon heating the specimens from 100.degree. C. to
900.degree. C. The results were shown in Table 7.
[0086] On the other hand, each of the thus-prepared specimens was
cut into the same disk shape as described in Examples 1-6 and
Comparative Examples 1-6. The specimen was subjected to grinding to
finish the outer surface. Next, the specimen was subjected to the
rolling fatigue test to evaluate the same rolling fatigue life L50
as described in Examples 1-6 and Comparative Examples 1-6. The
rolling fatigue test was conducted using the same testing machine
under the same test conditions as described in Examples 7-10 and
Comparative Examples 7-16. In Comparative Examples 23-25, H.sub.2
was charged into the rolling fatigue tester to examine the
influence of H.sub.2 on the rolling fatigue lives of the specimens.
The results were shown in Table 10.
10TABLE 10 Surface Temper- Surface Steel C Content ature Hardness
H.sub.2 Amount Material (mass %) Condition (Hv) (ppm) Example 11 L
1.23 TC6 803 -- Example 12 L 0.91 TC6 762 -- Example 13 M 1.32 TC6
794 -- Example 14 M 0.82 TC6 755 -- Example 15 L 0.92 TC7 766 --
Example 16 L 0.88 TC8 754 -- Comparative L 1.65 TC6 798 -- Example
17 Comparative L 0.51 TC6 470 -- Example 18 Comparative L 0.89 TC9
620 0.4 Example 19 Comparative L 0.89 TC10 768 0.7 Example 20
Comparative N 0.96 TC10 753 0.7 Example 21 Comparative O 0.82 TC10
741 0.6 Example 22 Comparative L 1.23 TC6 803 0.7 Example 23
Comparative L 1.23 TC6 803 1.2 Example 24 Comparative L 1.23 TC6
803 1.9 Example 25 Precipitated Carbide Average Micro- Particle
Area Rolling Fatigue structure size (.mu.m) Ratio (%) Life (L50)
.times. 10.sup.7 Example 11 M.sub.23C.sub.6 0.22 13 11.5 Example 12
M.sub.23C.sub.6 0.17 9 10.2 Example 13 M.sub.23C.sub.6 0.20 16 10.9
Example 14 M.sub.23C.sub.6 0.19 8 9.8 Example 15 M.sub.3C 7.25 1
2.1 Example 16 M.sub.23C.sub.6 0.35 4 5.1 Comparative M.sub.3C +
M.sub.23C.sub.6 3.46 22 0.1 Example 17 Comparative --* --* --* 0.4
Example 18 Comparative --* --* --* 0.9 Example 19 Comparative --*
0.19 1 1.8 Example 20 Comparative M.sub.3C 0.15 1 0.4 Example 21
Comparative --* --* --* 1.1 Example 22 Comparative M.sub.23C.sub.6
0.22 13 1.3 Example 23 Comparative M.sub.23C.sub.6 0.22 13 0.4
Example 24 Comparative M.sub.23C.sub.6 0.22 13 0.3 Example 25 --*
No data detected.
[0087] It was found from Table 10 that no H.sub.2 released from the
specimens obtained in Examples 11-16 was detected and the specimens
exhibited the excellent rolling fatigue lives. On the contrary, it
was found that the specimen obtained in Comparative Example 17
exhibited the deteriorated rolling fatigue life because network of
M.sub.3C carbide was precipitated at grain boundaries of austenite
particles due to too large C content in the outer surface of the
specimen. It was found that the specimen obtained in Comparative
Example 18 exhibited the deteriorated rolling fatigue life because
sufficient hardness was not obtained due to too small C content in
the outer surface of the specimen. It was found that the specimen
obtained in Example 15 had no M.sub.23C.sub.6 carbide precipitated
but coarse M.sub.3C carbide precipitated at grain boundaries of
austenite particles because the first temperature t1 was more than
Ac.sub.1 transformation point. As a result, the specimen obtained
in Example 15 exhibited the rolling fatigue life shorter than that
of the specimen obtained in each of Examples 11-14. The specimen
obtained in Example 15 is suitably applicable to low-power
automobiles. It was found that the specimen obtained in Example 16
exhibited non-uniformity in precipitation of carbide because
production of nucleus of M.sub.23C.sub.6 carbide was insufficient
due to the large temperature rise rate dT. The specimen obtained in
Example 16, therefore, had the shorter rolling fatigue life than
that of the specimen obtained in each of Examples 11-14. The
specimen obtained in Example 16 also is suitably applicable to
low-power automobiles. Further, it was found that the specimen
obtained in Comparative Example 19 had no M.sub.23C.sub.6 carbide
precipitated in the outer surface and the lower surface hardness
because the first isothermal heat treatment was omitted. The
specimen obtained in Comparative Example 19, therefore, exhibited
the deteriorated rolling fatigue life. It was found that the
specimen obtained in each of Comparative Examples 20-22 had no
M.sub.23C.sub.6 carbide precipitated in the outer surface and the
H.sub.2 amount insufficiently reduced, because the first isothermal
heat treatment was omitted and the temperature t3 after tempering
at 170.degree. C. was lower. As a result, the specimens obtained in
Comparative Examples 20-22 exhibited the deteriorated rolling
fatigue lives. Further, it was confirmed that the specimens
obtained in Comparative Examples 23-25 exhibited the rolling
fatigue lives which were deteriorated as the residual H.sub.2
amount increased.
[0088] This application is based on prior Japanese Patent
Applications No. 2001-148517 filed on May 17, 2001 and No.
2001-160694 filed on May 29, 2001, the entire contents of which are
hereby incorporated by reference.
[0089] Although the invention has been described above by reference
to certain examples of the invention, the invention is not limited
to the examples described above. Modifications and variations of
the examples described above will occur to those skilled in the art
in light of the above teachings. The scope of the invention is
defined with reference to the following claims.
* * * * *