Note: Descriptions are shown in the official language in which they were submitted.
CA 02316183 2006-10-11
Antifriction Bearing Part for High Temperature
Technical Field
The present invention relates to an antifriction bearing part
employed for a power transmission device or an engine part of an
automobile, an aircraft, a ship or an industrial machine, and more
speci5cally, it relates to a low-priced antifriction bearing part for a high
temperature having an exceIlent rolling contact fatigue life under
environment contaminated with foreign matter such as dust or refuse and
under such environment that the temperature of the atmosphere is room
temperature to 300 C.
Background Art
An antifriction bearing employed for a power transmission part or an
engine part of an automobile, an aircraft, a ship or an industrial machine,
which is used under severe environment, is required to have an excellent
rolling contact fatigue life and reliability under such environment. In
particular, the antifriction bearing employed for the above may be
contaminated with foreign matter such as dust, refuse or iron powder and, it
is known that the rolling contact fatigue life remarkably lowers under such
environment as compared with employment under clean environment. As a
countermeasure therefor, a working method of performing carbonitriding on
high-carbon chromium bearing steel such as SUJ2 or case-hardening steel
such as SCM420, SNCM420 or SNCM815 and generating a proper amount of
retained austenite immediately under a rolling surface has been recently
applied and improvement of life under contamination with foreign matter has
been attempted.
However, general carbonitriding is a treatment of a longer time as
compared with quench-and-temper treatment applied to SUJ2 steel or the
like. In such a carbonitrided antifriction bearing, therefore, there is a
problem
that the manufacturing cost remarkably increases as compared
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CA 02316183 2006-10-11
with an antifriction bearing manufactured in a general quench-and-temper
step.
Further, the antifriction bearing employed for an automobile or an
aircraft is used under high-temperature environment and hence required to
have an excellent rolling contact fatigue life characteristic under extremely
severe working environment that is contaminated with foreign matter and is
a high-temperature environment. In general, tempering is executed on an
antifriction bearing used under a high temperature at a high temperature of
at least 300 C for attaining dimensional stability after performing quench
hardening on a high-carbon chromium bearing of SUJ2 or the like or after
performing cementation/quench hardening on case-hardening steel such as
SCM420 or SNCM815.
However, hardness remarkably lowers when tempering these
materials at a high temperature and hence prescribed hardness required for
the antifriction bearing cannot be attained and the rolling contact fatigue
life
and wear resistance are lower. Therefore, a precipitation-hardening type
steel product such as M50 is used for bearing steel used in a high-
temperature range, while it has been impossible for such a steel product to
satisfy the aforementioned needs since the manufacturing cost and the
material cost are high and the working temperature range is limited.
In the antifriction bearing subjected to carbonitriding, retained
austenite is generated immediately under a rolling surface after heat
treatment while nitrogen infiltrates into the steel. Stress concentration
resulting from contamination with foreign matter is relaxed due to the
function of this retained austenite while temper softening resistance is
improved due to the function of nitrogen entering into the steel and change
of the structure caused in the process of rolling contact fatigue is
suppressed, whereby improvement of the rolling contact fatigue life is
attained.
In application to an antifriction bearing for a high temperature,
however, high-temperature tempering must be performed for ensuring
stability of the dimensions as described above. When performing this
high-temperature tempering, the effect cannot be expected since the
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CA 02316183 2006-10-11
retained austenite decomposes and the quantity decreases while temper
softening prevention by nitrogen is also limited and hence sufficient
performance cannot be attained under high-temperature environment
contaminated with foreign matter.
With the recent rapid progress of high output-miniaturization of an
engine in the field of an automobile or the like, a case used under severer
conditions increases at the same time as to the working environment for the
antifriction bearing. It is expected that the working temperature range,
which is about 130 C at room temperature, of an antifriction bearing
employed for an engine part rises to 160 C temperature instantaneously. It
is predicted nowadays that the working temperature range of the antifriction
bearing rises to about 160 C at room temperature and further
instantaneously exceeds 200 C following increase of the output of the engine.
Therefore, it is expected that, when increase of the output or reduction of
the
weight of the engine is hereafter prompted, improvement of the rolling
contact fatigue life under an environment contaminated with foreign matter
and under a high-temperature environment is required.
However, the current high-carbon chromium bearing steel or an
antifriction bearing subjected to cementation or carbonitriding does not have
sufficient heat resistance and cannot maintain a sufficient rolling contact
fatigue life under the expected environment contaminated with foreign
matter and under high-temperature environment. Further, the
precipitation-hardening type bearing steel such as M50 has a problem that
the cost is high, and the situation is such that an antifriction bearing which
is low-priced and excellent in rolling contact fatigue life characteristic
cannot
be provided.
Disclosure of the Invention
The present invention has been proposed in order to solve the
aforementioned problems, and aims at providing an antifriction bearing part
for a high temperature having an excellent rolling contact fatigue life under
an environment contaminated with foreign matter and under a high-
temperature environment and low-priced as compared with the prior art.
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CA 02316183 2007-08-22
The inventors have made deep study to find out a combination of
composition elements capable of obtaining a low-priced antifriction bearing
part of a high temperature having an excellent rolling contact fatigue life
under environment contaminated with foreign matter and under a high-
temperature environment and each content thereof.
Accordingly, an antifriction bearing part for a high temperature
according to the present invention is a part of an antifriction bearing for a
high temperature having an inner ring, an outer ring and a rolling element,
which consists of a steel product containing C (carbon) by at least 0.6 % and
not more than 1.3 %, Si (silicon) by at least 0.3 % and not more than 3.0 %,
Mn (manganese) by at least 0.2 % and not more than 1.5 %, P (phosphorus)
by not more than 0.03 %, S (sulfur) by not more than 0.03 %, Cr (chromium)
by at least 0.3 % and not more than 5.0 %, Ni (nickel) by at least 0.1 % and
not more than 3.0 %, Al (aluminum) by not more than 0.050 %, Ti (titanium)
by not more than 0.003 %, 0 (oxygen) by not more tan 0.0015 %, N (nitrogen)
by not more than 0.015 %, Mo by at least 0.05% and less than 0.25% and
optionally, at least 0.05% and not more than 1.0% of V in mass % as the
contents of alloying elements with the rest consisting of Fe (iron) and
inevitable impurities and has a structure subjected to tempering after
quench hardening or carbonitriding, while the hardness after the tempering
is at least HRC 58 and the maximum carbide size is not more than 8 pm.
In the antifriction bearing art according to the present invention, an
excellent rolling contact fatigue life is attained under environment
contaminated with foreign matter when subjected to quench-and-temper
treatment and also when not subjected to carbonitriding since the same has
the aforementioned structure. Therefore, carbonitriding can be omitted and
hence the manufacturing cost can be lowered.
While it is preferable to omit carbonitriding in view of reduction of the
manufacturing cost as described above, an excellent rolling contact fatigue
life can be attained under environment contaminated with foreign matter
also when performing carbonitriding in place of quench hardening.
Further, it has the aforementioned composition and hence high
hardness of at least HRC 58 can be attained also when subjected to
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CA 02316183 2000-06-23
tempering at a high temperature (e.g., 350 C). The quantity of retained
austenite can be reduced by thus performing tempering at a high
temperature, whereby dimensional stability under high-temperature
environment can be attained while high hardness of at least HRC 58 can be
attained. Therefore, the rolling contact fatigue life and wear resistance
under high-temperature environment can be improved beyond the prior art.
Further, the steel of the aforementioned composition is more low-
priced than the precipitation-hardening type bearing steel such as M50.
Thus, it is possible to obtain an antifriction bearing part for a high
temperature, which has an excellent rolling contact fatigue life under
environment contaminated with foreign matter and under high-
temperature environment and is at a low cost.
The tempering temperature is preferably at least 180 C and not
more than 350 C. The antifriction bearing is generally used at a
temperature of about 100 C, and hence the tempering temperature must be
at least 180 C.
The reasons for limiting the chemical compositions of the antifriction
bearing part for a high temperature according to the present invention are
now described.
(1) As to the content (at least 0.6 % and not more than 1.3 %) of C
C is an element essential for ensuring strength as the antifriction
bearing and must be contained by at least 0.6 % in order to maintain
prescribed hardness after heat treatment, and hence the lower limit of the
C content has been limited to 0.6 %. While a carbide provides an
important role for the rolling contact fatigue life in the present invention
as
described later, it has been proved that a large-sized carbide forms and
causes reduction of the rolling contact fatigue life when C is contained in
excess of 1.3 % in content, and hence the upper limit of the C content has
been limited to 1.3 %.
(2) As to the content (at least 0.3 % and not more than 3.0 %) of Si
Si has a function of suppressing softening in a high temperature
range and improving heat resistance of the antifriction bearing and is hence
preferably added. However, these effects cannot be attained if the Si
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CA 02316183 2000-06-23
content is less than 0.3 %, and hence the lower limit of the Si content has
been limited to 0.3 %. While the heat resistance improves following
increase of the Si content, the effect is saturated and reduction of hot
workability and machinability takes place when Si is contained in a large
quantity exceeding 3.0 % and hence the upper limit of the Si content has
been limited to 3.0 %.
(3) As to the content (at least 0.2 % and not more than 1.5 %) of Mn
Mn is an element employed for deoxidization when preparing steel
and an element improving quench-hardenability and must be added by at
least 0.2 % for attaining this effect, and hence the lower limit of the Mn
content has been limited to 0.2 %. However, the machinability remarkably
lowers when Mn is contained in a large quantity exceeding 1.5 %, and
hence the upper limit of the Mn content has been limited to 1.5 %.
(4) As to the content (not more than 0.03 %) of P
P segregates on austenite grain boundaries of the steel and causes
reduction of toughness and the rolling contact fatigue life, and hence 0.03 %
has been set as the limit of the content.
(5) As to the content (not more than 0.03 %) of S
S harms hot workability of the steel, forms a non-metallic inclusion
in the steel and lowers the toughness and the rolling contact fatigue life,
and hence 0.03 % has been set as the upper limit of the S content. S has
an effect of improving machinability while having the aforementioned
harmful side, and hence a content of up to 0.005 % is allowed although the
content is desirably reduced to the smallest possible level.
(6) As to the content (at least 0.3 % and not more than 5.0 %) of Cr
Cr is an element accomplishing an important function in the present
invention, and added for improvement of quench-hardenability, assurance
of hardness by a carbide and life improvement. Addition of at least 0.3 %
is necessary for obtaining a prescribed carbide, and hence the lower limit of
the Cr content has been limited to 0.3 %. However, a large-sized carbide
forms to result in reduction of the rolling contact fatigue life when Cr is
contained in a large quantity exceeding 5.0 %, and hence the upper limit of
the Cr content has been limited to 5.0 %.
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CA 02316183 2000-06-23
(7) As to the content (not more than 0.050 %) of Al
While Al is used as a deoxidant when preparing steel, it is desirable
to reduce the content since Al forms a hard oxide inclusion and lowers the
rolling contact fatigue life. Remarkable reduction of the rolling contact
fatigue life was recognized when Al was contained in a large quantity
exceeding 0.050 %, and hence the upper limit of the Cr content has been
limited to 0.050 %.
Increase of the preparation cost for steel takes place for rendering the
Al content less than 0.005 %, and hence it is preferable to limit the lower
limit of the Al content to 0.005 %.
(8) As to the content (not more than 0.003 %) of Ti, the content (not
more than 0.0015%) of 0 and the content (not more than 0.015 %) of N
Ti, 0 and N form oxides and nitrides in the steel, become starting
points of fatigue failure as non-metallic inclusions and lower the rolling
contact fatigue life, and hence Ti: 0.003 %, 0: 0.0015 % and N: 0.015 % have
been set as the upper limits of the respective elements.
(9) As to the content (at least 0.1 % and not more than 3.0 %) of Ni
Ni is an element accomplishing an important function in the present
invention, suppresses change of the structure in a rolling contact fatigue
process particularly when used under high-temperature environment, and
has an effect of suppressing reduction of the hardness in a high
temperature range and improving the rolling contact fatigue life. In
addition, Ni improves the toughness for improving the life under foreign
matter environment and has an effect also for improvement of corrosion
resistance. Therefore, Ni must be contained by at least 0.1 %, and hence
the lower limit of the Ni content has been limited to 0.1 %. When
containing Ni in a large quantity exceeding 3.0 %, however, a large
quantity of retained austenite is formed in quench hardening and
prescribed hardness cannot be attained while the steel product cost rises,
and hence the upper limit of the Ni content has been limited to 3.0 %.
Temper hardness of the inventive antifriction bearing part for a high
temperature and the carbides are now mentioned.
(10) Temper Hardness
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CA 02316183 2006-10-11
It is common that a bearing used in a high temperature range is
subjected to tempering at a temperature exceeding the environmental
temperature in order to stabilize the dimensions under the working
environment. The inventors have made detailed investigations related to the
temper hardness and the rolling contact fatigue life under temperature
environment of 200 C, to confirm that correlation is recognized between the
temper hardness and the rolling contact fatigue life and the rolling contact
fatigue life tends to exhibit a longer life as the temper hardness is high. It
has been found that, particularly when the temper hardness is identical, a
bearing for which tempering is executed at a high temperature has a longer
life and a bearing whose temper hardness is high has a longer life also when
performing tempering at a high temperature. Further, it has been proved
that the life tends to abruptly lower and life dispersion increases when the
hardness after tempering is less than HRC 58. In order to improve the life at
a high temperature and reduce dispersion, it is necessary to maintain
hardness of at least HRC 58, and the tempering temperature at this time is
preferably as high as possible.
(11) Carbides
It has been proved that the carbides make the hardness in tempering
maintained while suppressing structural change during rolling contact
fatigue, and has an effect for improvement of the roIling contact fatigue
life.
As a result of investigating the maximum size of the carbides in a depth of
0.1 mm from the surface layer of the bearing and the rolling contact fatigue
life at this time, such a tendency that the life lowers when a large-sized
carbide is present has been recognized and it has been clarified that life
reduction abruptly takes place when a large carbide whose maximum size
exceeds 8 m is present, and hence the maximum size of the carbides has
been defined as 8 m.
Preferably in the aforementioned antifriction bearing part, the steel
product further contains at least one of at least 0.05 % and less than 0.25 %
of Mo and at least 0.05 % and not more than 1.0 % of V in mass %.
Thus, the rolling contact fatigue life under an environment
contaminated with foreign matter and under a high-temperature
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CA 02316183 2000-06-23
environment can be further improved, and the hardness after tempering
can be improved.
The reasons for limiting the aforementioned chemical compositions
are now described.
(12) As to the content (at least 0.05 % and less than 0.25 %) of Mo
Mo improves quench-hardenability of the steel, and has an effect of
preventing softening in tempering by being solidly dissolved in the carbides.
In particular, Mo is added since a function of improving the rolling contact
fatigue life in a high temperature range has been found out. However, the
steel product cost rises while hardness does not lower but machinability
remarkably deteriorates in softening for simplifying cutting when Mo is
contained in a large quantity of at least 0.25 %, and hence the Mo content
has been limited to less than 0.25 %. No effect is attained for carbide
formation if the content of Mo is less than 0.05 %, and hence the lower limit
of the Mo content has been limited to 0.05 %.
(13) As to the content (at least 0.05 % and not more than 1.0 %) of V
V has an effect of bonding with carbon and precipitating a fine
carbide, prompting refinement of crystal grains and improving
strength=toughness while exhibiting a function of improving heat resistance
of the steel product by containing of V, suppressing softening after high-
temperature tempering, improving the rolling contact fatigue life and
reducing dispersion of the life. The content of V at which this effect is
attained is at least 0.05 %, and hence the lower limit of the V content has
been limited to 0.05 %. However, machinability and hot workability lower
if V is contained in a large quantity exceeding 1.0 %, and hence the upper
limit of the V content has been limited to 1.0 %.
Best Mode for Carrying Out the Invention
Examples of the present invention are now described.
Steel products having chemical compositions shown in Table 1 were
dissolved by a vacuum induction furnace, cast into steel ingots of 150 kg in
weight and thereafter heated/held at a temperature of 1200 C for three
hours for executing hot forging, for manufacturing round bars of 50 mm in
-9-
CA 02316183 2000-06-23
diameter. The round bar materials were subjected to treatment of holding
the same at 850 C for one hour as normalizing and thereafter air-cooling
the same, and further subjected to softening of holding the same at 790 C
for six hours, thereafter cooling the same at a cooling rate of 10 C/hour to
650 C and air cooling the same to the room temperature as softening for
simplifying cutting, for making materials for various investigations.
-10-
CA 02316183 2000-06-23
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<Hardness Investigation>
In order to measure temper hardness after quench hardening and
temper hardness after carbonitriding, columnar test pieces of 20 mm in
diameter and 100 mm in length were prepared from materials of 50 mm in
diameter by machining.
Quench hardening was performed by performing heating with a salt
furnace, performing soaking to 850 C for 30 minutes, and thereafter
quench-hardening the test pieces into oil of 80 C. Thereafter tempering of
performing heating identically with a salt furnace, holding the test pieces
at 350 C for two hours and thereafter air-cooling the same was performed
as tempering.
In carbonitriding, a gas atmosphere furnace used in general
production steps was employed for setting a carbon potential to 1.0 to 1.2 %
and the amount of addition of NH3 to 5 to 10 % in an RX gas atmosphere for
holding the test pieces at 850 C for 60 minutes, and thereafter the test
pieces were quench-hardened into oil. Thereafter tempering of 120
minutes was performed at 350 C.
Discoidal test pieces of 10 mm in thickness were cut from central
portions of the test pieces subjected to this quench-and-temper treatment or
the test pieces subjected to tempering after carbonitriding and both
surfaces were polished by wet polishing, for preparing test pieces for
hardness measurement.
As to the hardness, a Rockwell hardness meter was used for
performing hardness measurement on positions 2 mm deep from the
surfaces in sections of the test pieces, and mean values of seven points were
obtained as the temper hardness.
<Rolling Contact Fatigue Life Test>
In order to confirm the performance as the antifriction part, a fatigue
test was made with a thrust rolling contact fatigue life tester and life
evaluation of the respective materials was executed.
As to the test pieces employed for life evaluation, ring-shaped thrust
rolling contact fatigue life test pieces of 47 mm in outer diameter, 29 mm in
inner diameter and 7 mm in thickness were roughly worked from round bar
-12-
CA 02316183 2000-06-23
materials of 50 mm in diameter by machining.
Quench-and-temper treatment and carbonitriding were performed as
heat treatment of the test pieces for which rough working was completed.
As to the treatment, an experimental furnace used in general production
steps was employed.
As to the quench-and-temper treatment, a gas atmosphere furnace
was employed for holding the test pieces at 850 C for 30 minutes while
controlling the carbon potential in an RX gas atmosphere so that neither
decarbonization or cementation took place on the basis of the carbon
content of each steel, and the test pieces were thereafter quench-hardened
into oil. Thereafter tempering of 120 minutes was performed at 350 C.
For carbonitriding, heat treatment was performed under the same
conditions as those for the aforementioned hardness test pieces.
After completion of the heat treatment, both surfaces of the test
pieces were polished and finished into mirror surface states. In the test
pieces subjected to carbonitriding, working margins in polishing were set to
0.1 mm on both surfaces.
The rolling contact fatigue life test was executed with a thrust rolling
contact fatigue life tester. Table 2 shows the conditions of the test. The
test was executed under room temperature environment and under 200 C
environment, and the test was also performed under environment
reproducing environment contaminated with foreign matter.
Table 2
Rolling Contact Fatigue Life Test Conditions
Tester Thrust RoIlin Contact Fatigue Life Tester
Contact Pressure 5.OGPa
Rotational Speed 2000 m
Test Temperature Room Temperature, 200 C
Lubrication Turbo Oil
Quantity of Foreign Matter 0.4g/1000cc
For the fatigue test, 15 repetitive tests were made under the same
-13-
CA 02316183 2000-06-23
conditions for determining such a life that a cumulative damage probability
in a Weibull probability reaches 10 % as the life of each material.
Comparative example No. 13 in Table 2 is general-purpose SUJ2, and the
life value of each material was described with a probability on the
assumption that the life of this quench-and-temper treated material was
1Ø
<Carbides>
Thrust rolling contact fatigue life test pieces were used for
measurement of the carbides present in the steel. In test pieces worked
into the thrust rolling contact fatigue life test pieces by executing various
types of heat treatment, ring cross sections were cut for preparing micro
test pieces for structure observation. The test pieces were mirror-finished,
and further corroded with a picral corrosion solution for performing
observation of carbides. In the micro samples, observation of carbides in
0.1 mm depths from surface layers of rolling surfaces was executed with an
optical microscope and maximum carbides in a visual area of 50 mm2 were
measured.
Results of the aforementioned 350 C temper hardness, rolling contact
fatigue lives at room temperature and 200 C, rolling contact fatigue lives
under foreign matter contamination conditions and maximum carbide sizes
are shown in Table 3 as to inventive Examples and in Table 4 as to
comparative examples.
-14-
CA 02316183 2000-06-23
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CA 02316183 2006-10-11
From the aforementioned results in Table 3 and Table 4, it has been
proved that hardness becomes at least HRC 58 in inventive Examples
having the composition range of the present invention also when
performing tempering of 350 C. In inventive Examples, further, it has
been proved that the rolling contact fatigue lives at room temperature and
200 C and rolling contact fatigue lives under foreign matter conditions rise
as compared with comparative examples also when performing mere
quench-and-temper treatment (HT). It has also been proved that excellent
rolling contact fatigue lives are attained also when performing
carbonitriding in place of quench hardening. In inventive Examples,
further, it has been proved that the maximum sizes of the carbides in the
0.1 mm depths from the surface layers of the rolling surfaces become not
more than 8.0 m.
Examples disclosed this time are to be considered as illustrative and
not restrictive in all points. The range of the present invention is shown
not by the above description but by the scope of claim for patent, and it is
intended that all modifications in the meaning and the range equivalent to
the scope of claim for patent are included.
As hereinabove described, the inventors have found out the optimum
composition elements and the contents thereof, whereby an excellent rolling
contact fatigue life has been attained under foreign matter contamination
conditions without performing carbonitriding while it has been possible to
obtain a low-priced antifriction bearing part for a high temperature capable
of attaining high hardness also when performing tempering at a high
temperature (e.g., 350 C).
Industrial Availability
The present invention is advantageously applicable to an antifriction
bearing part for a high temperature used under an environment contaminated
with foreign matter and under a high-temperature environment.
-17-
