U.S. patent application number 13/472353 was filed with the patent office on 2012-09-06 for rolling/sliding part and production method thereof.
This patent application is currently assigned to JTEKT CORPORATION. Invention is credited to Katsuhiko KIZAWA, Tsuyoshi Mikami.
Application Number | 20120222778 13/472353 |
Document ID | / |
Family ID | 38535384 |
Filed Date | 2012-09-06 |
United States Patent
Application |
20120222778 |
Kind Code |
A1 |
KIZAWA; Katsuhiko ; et
al. |
September 6, 2012 |
ROLLING/SLIDING PART AND PRODUCTION METHOD THEREOF
Abstract
A production method of a rolling/sliding part which includes
applying a spherodizing annealing treatment to a steel to render an
average particle diameter of the carbide of the surface layer
portion in a range of 0.3 to 0.6 .mu.m and a maximum particle
diameter thereof 4 .mu.m, to render the surface hardness 62 or more
by Rockwell C hardness and to render a solid solution carbon amount
in a residual austenite of the surface layer portion in a range of
0.95 to 1.15% by weight.
Inventors: |
KIZAWA; Katsuhiko; (Osaka,
JP) ; Mikami; Tsuyoshi; (Yamatotakada-shi,
JP) |
Assignee: |
JTEKT CORPORATION
Osaka
JP
|
Family ID: |
38535384 |
Appl. No.: |
13/472353 |
Filed: |
May 15, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11812750 |
Jun 21, 2007 |
|
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13472353 |
|
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Current U.S.
Class: |
148/233 |
Current CPC
Class: |
C23C 8/02 20130101; F16C
33/62 20130101; F16C 33/64 20130101; C23C 8/22 20130101; C23C 8/80
20130101; F16C 33/32 20130101; Y10T 29/49679 20150115 |
Class at
Publication: |
148/233 |
International
Class: |
C23C 8/22 20060101
C23C008/22; C21D 6/00 20060101 C21D006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 22, 2006 |
JP |
P2006-172181 |
Claims
1. A production method of a rolling/sliding part comprising:
applying a spherodizing annealing treatment to a steel that
contains C: 0.7 to 0.9% by weight, Si: 0.05 to 0.7% by weight, Mn:
0.05 to 0.7% by weight, Cr: 3.2 to 5.0% by weight, Al: 0.04% by
weight or less, P: 0.03% by weight or less, S: 0.03% by weight or
less, Ti: 0.005% by weight or less, O: 0.0015% by weight or less,
at least one kind of Mo: less than 1.0% by weight and V: less than
0.50% by weight and a balance made of Fe and unavoidable impurities
to make a lot of carbide precipitate; processing into a
predetermined shape to produce a processed part raw material;
heating in a carburizing atmosphere of which carbon potential is in
a range of 1.0 to 1.5% at a temperature in a range of 870 to
950.degree. C. to apply a first carburizing treatment followed by
quenching; subsequently heating in a carburizing atmosphere of
which carbon potential is in the range of 1.0 to 1.5% at a
temperature in a range of 870 to 910.degree. C. to apply a second
carburizing treatment followed by quenching; and applying a
tempering treatment to render an area ratio of a total precipitated
carbide of the surface layer portion in a range of 15 to 25%, to
render, 50% or more, by the area ratio, of the total precipitated
carbide present in a surface layer portion a M.sub.7C.sub.3 type
and/or M.sub.23C.sub.6 type, to render an average particle diameter
of the carbide of the surface layer portion in a range of 0.3 to
0.6 .mu.m and a maximum particle diameter thereof 4 .mu.m, to
render the surface hardness 62 or more by Rockwell C hardness and
to render a solid solution carbon amount in a residual austenite of
the surface layer portion in a range of 0.95 to 1.15% by weight.
Description
RELATED APPLICATIONS
[0001] The present Application is a Divisional Application of U.S.
patent application Ser. No. 11/812,750 which was filed on Jun. 21,
2007, the disclosure of which is incorporated herein by
reference.
[0002] The present disclosure relates to the subject matter
contained in Japanese Patent Application No. P2006-172181 filed on
Jun. 22, 2006, which is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The invention relates to a rolling/sliding part and a
production method thereof, in more detail, a rolling/sliding part
that is suitably used as a rolling part that is used as a bearing
ring and a rolling element of a rolling bearing where a lubricant
oil in which a foreign matter is mingled is used or as a sliding
bearing part and a producing method thereof.
[0005] In the range of the specification and claims of the
invention, a rolling/sliding part means apart that comes into a
pure rolling contact, a pure sliding contact and a contact where a
rolling contact and a sliding contact are mingled.
[0006] 2. Relater Art
[0007] As a rolling bearing that is used with a lubricant oil in
which a foreign matter is mingled or a bearing ring and a rolling
element of a rolling bearing that is miniaturized and saved in
weight to improve the fuel cost of, for instance, an automobile, a
rolling/sliding part that is produced in such a manner that a
processed part raw material formed into a predetermined shape from
a bearing steel (high carbon chromium bearing steel) such as JIS
SUJ 2 is heated in a carburizing atmosphere of which carbon
potential is 1.2% or more at a temperature in the range of 840 to
870.degree. C. for 3 hr or more, followed by quenching, further
followed by tempering is known, in which a total carbon amount of a
surface layer portion in the range from a surface up to a depth to
which the maximum shearing force works is rendered in the range of
1.0 to 1.6% by weight, a solid solution carbon amount in a matrix
of the surface layer portion is rendered in the range of 0.6 to
1.0% by weight, carbide is precipitated in the surface layer
portion, an amount of the carbide is rendered in the range of 5 to
20% by area ratio and a grain diameter thereof is rendered 3 .mu.m
or less (JP-A No. 2004-52101).
[0008] However, in applications where more miniaturization of parts
and longer lifetime thereof under severe service conditions are
demanded, performance higher than that of the rolling/sliding part
described in JP-A No. 2004-52101 is in demand. In particular, it
has been known that, when a rolling/sliding part is used in
lubricant oil in which a foreign matter is mingled, in comparison
with a case different therefrom, the lifetime is largely
deteriorated. Accordingly, a countermeasure for that is in strong
demand. As a countermeasure for realizing a longer lifetime in
lubricant oil contaminated with a foreign matter, so far, on a
surface layer portion of a carburized rolling/sliding part, soft
residual austenite is formed by substantially 20%. That is, since
the residual austenite has an effect of mitigating a stress
concentration in the surrounding of an indentation generated when a
foreign matter is bitten in, cracks caused by the generation of
indentation can be retarded in generation and progress.
Accordingly, in order to further improve the performance of an
existing rolling/sliding part, it is considered to further increase
an amount of the residual austenite of a surface layer portion to
such a huge amount as exceeding 30%.
[0009] However, when an amount of the residual austenite of a
surface layer portion is increased in the JIS SUJ2, there is a
large problem described below. That is, since the residual
austenite is a soft structure, when an amount thereof is increased
to 40% or more, the Rockwell C hardness (hereinafter, referred to
as HRC) of 62 or more is difficult to obtain and the static load
capacity becomes insufficient; accordingly, local permanent
deformation caused by overload and impact load during rest or
low-speed rotation is caused to result in disturbing smooth
rotation of a bearing. In order to overcome the problem, it is
considered to apply high concentration carburizing (a carbon amount
of a surface layer portion is made 1.6% by weight or more) to a
processed part raw material made of JIS SUJ2. However, when the JIS
SUJ2 is carburized, carbide precipitated in a surface layer portion
becomes rich with M.sub.3C type carbide {.dbd.Fe.sub.3C, (Fe,
Cr).sub.3C} having the Vickers hardness in the range of 1150 to
1760 such as cementite. The M.sub.3C type carbide is speedier in
comparison with other alloy carbides (M.sub.7C.sub.3, MC) in the
growth speed of carbide at the carburizing (reference literature:
Takayama, Journal of the Japan Institute of Metals, 11, 45(1981),
1195) and low in the hardness (M.sub.7C.sub.3 and MC are 1800 or
more in the Vickers hardness). Accordingly, there is a problem in
that, since an average grain diameter of the carbide becomes
larger, a reinforcement amount due to the dispersed precipitation
is deteriorated and thereby an improvement in the hardness due to
the carbide precipitation becomes smaller. Furthermore, the carbide
is caused to be coarser, resulting in a problem in that an
improvement in the rolling lifetime cannot be sufficiently
obtained.
[0010] However, an approach in which an amount of Cr is increased
to make use of the M.sub.7C.sub.3 type carbide to improve the
rolling life under foreign matter contamination is not a novel one
and an invention described in for instance JP-B No. 06-11899 has
been proposed. However, in the invention, although kinds and area
ratios of carbides after carburization are described, the invention
does not particularly describe a distribution state such as number
and magnitude and only describes that the coarsening has to be
inhibited. Accordingly, since the carbide cannot be sufficiently
miniaturized, there is a problem in that, when the residual
austenite amount is such large as substantially 50%, high hardness
cannot be maintained and a sufficient improvement in the lifetime
cannot be obtained.
[0011] Furthermore, in JP-A No. 01-55423, a method where, with an
existing case hardened steel, total carburization or carburization
nitriding is applied to secure a certain degree of surface hardness
and a residual austenite amount is heightened to improve the
rolling life under a lubrication environment where foreign matter
is mingled is proposed. However, there is a problem in that a
problem same as that in the case of the JIS SUJ2 is caused and
excellent rolling life cannot be obtained.
[0012] The invention intends to resolve the foregoing problems and
to provide a rolling/sliding part that can more elongate the life
when it is used as a bearing ring and a rolling element of a
rolling bearing and as a sliding bearing part, in which lubricant
oil contaminated with foreign matter is used, and can increase
static load capacity to reduce a local permanent deformation amount
that is caused owing to overload and impact load during rest and
low-speed rotation, that is, can increase the indentation
resistance and a production method thereof.
[0013] According to a first aspect of the invention, a
rolling/sliding part is formed of a steel having a surface layer
portion a surface of which is carburized, wherein an area ratio of
a total precipitated carbide of the surface layer portion is in the
range of 15 to 25%, of the total precipitated carbide present of
the surface layer portion 50% or more of the carbide by the area
ratio is made of a M.sub.7C.sub.3 type and/or M.sub.23C.sub.6 type,
an average grain diameter of the carbide of the surface layer
portion is in the range of 0.3 to 0.6 .mu.m, a maximum grain
diameter thereof is 4 .mu.m, the surface hardness is 62 or more by
Rockwell C hardness and a solid solution carbon amount in a
residual austenite of the surface layer portion is in the range of
0.95 to 1.15% by weight.
[0014] Here, the surface layer portion means a portion that is
carburized and clearly contains much C relative to an inner portion
and a depth portion that affects on the life of a rolling and
sliding surface, for instance, in a rolling and sliding part that
is used in oil contaminated with foreign matters, a range from 0 to
50 .mu.m from the outermost surface. In what follows, the surface
layer portion is identical.
[0015] In the rolling/sliding part of the first aspect, reasons why
for limiting the respective numerical values are as follows.
Area Ratio of Total Precipitated Carbide of Surface Layer
Portion
[0016] From results of evaluation tests of examples and comparative
examples described below, it was found that rolling/sliding parts
of which service life ratio (L.sub.10 service life ratio when
L.sub.10 service life of quenched and tempered JIS SUJ2 is assigned
to 1) is 10 or more and of which indentation depth ratio (depth
ratio when the indentation depth of quenched and tempered JIS SUJ2
is assigned to 1) is 0.9 or less exist in a region A hatched with a
chained line in FIG. 1 that expresses relationship between the
service life ratios and the indentation depth ratios and was found
as well that the area ratios of total precipitated carbides of the
rolling/sliding parts and solid solution carbon amounts in residual
austenite exist in a region B hatched with a chained line in FIG. 2
that expresses the relationship therebetween. Based on the results,
the lower limit of the area ratio of carbide in the region B shown
in FIG. 2, that is, 15% is taken as a lower limit value of the area
ratio. Furthermore, an upper limit value of the area ratio, that
is, 25%, is determined because, when the area ratio exceeds the
upper limit value, coarse carbide is generated to be a starting
point of the rolling fatigue crack and thereby short life of the
rolling/sliding part is caused.
Amount of M.sub.7C.sub.3 Type and/or M.sub.23C.sub.6 Type in Total
Precipitated Carbide
[0017] Non-solid solution carbide precipitated in a carburized
layer of high carbon concentration of a surface and remaining as
precipitated after the carburizing is M.sub.3C type carbide in
general steel for machine structural use. However, as mentioned
above in the first aspect of the invention, when, among the total
precipitated carbide present in the surface layer portion, 50% or
more by area ratio of the carbide is formed into M.sub.7C.sub.3
type and/or M.sub.23C.sub.6 type that is high in the hardness and
difficult in coarsening, a grain diameter of carbide of a
carburized surface layer portion can be made finer than that of
steel mainly made of M.sub.3C type carbide and thereby the service
life under the presence of foreign matter can be largely improved.
Furthermore, the M.sub.7C.sub.3 type and M.sub.23C.sub.6 type
carbides are high in the hardness as mentioned above; accordingly,
even under presence of much residual austenite, high hardness can
be maintained and the indentation resistance can be improved. The
M.sub.7C.sub.3 type and M.sub.23C.sub.6 type carbides that are
higher in the hardness and more difficult to be coarsened than the
M.sub.3C type carbide can be predominantly precipitated when, as
well be mentioned below, mainly Cr that is carbide forming element
is contained more abundantly (3.2 to 5.0% by weight) than an
existing steel or when Mo is appropriately added.
Average Grain Diameter and Maximum Grain Diameter of Carbide of
Surface Layer Portion
[0018] When an average grain diameter of carbide of a surface layer
portion is too small, an area ratio of the total precipitated
carbide of a surface layer portion cannot be made such a high area
ratio as 15 to 25%. Moreover, an average grain diameter is
practically difficult to make less than 0.3 .mu.m. Accordingly, the
lower limit value of the average grain diameter is set at 0.3
.mu.m. Furthermore, when the average grain diameter is too large, a
dispersion reinforcement amount is lowered, and, when a solid
solution carbon amount in a residual austenite amount of a surface
layer portion is set at a predetermined amount, predetermined
hardness cannot be obtained; accordingly, the upper limit value is
set at 0.6 .mu.m.
[0019] When the maximum grain diameter of carbide of a surface
layer portion is too large, it works as an internal defect to
deteriorate the service life in clean oil; accordingly, the maximum
limit value is set at 4 .mu.m.
Surface Hardness
[0020] When the surface hardness is too small, the static load
capacity becomes insufficient to result in local permanent
deformation due to overload and impact load during rest or
low-speed rotation. Accordingly, the surface hardness is set at
HRC62 or more.
Solid Solution Carbon Amount in Residual Austenite of Surface Layer
Portion
[0021] From results of evaluation tests of examples and comparative
examples, which are described below, it was found that
rolling/sliding parts of which service life ratio (L.sub.lo service
life ratio when L.sub.10 service life of quenched and tempered JIS
SUJ2 is assigned to 1) is 10 or more and of which indentation depth
ratio (depth ratio when the indentation depth of quenched and
tempered JIS SUJ2 is assigned to 1) is 0.9 or less exist in a
region A shown in FIG. 1 that expresses relationship between the
service life ratios and the indentation depth ratios and found as
well that solid solution carbon amounts in residual austenite of
the rolling/sliding parts exist in a region B shown in FIG. 2 that
expresses the relationship therebetween. Based on the results, the
lower limit of the solid solution carbon amount in the region B
shown in FIG. 2, that is, 0.95% is set as a lower limit value of
the solid solution carbon amount in the residual austenite,
similarly, the upper limit, that is, 1.15% is set as an upper limit
value of solid solution carbon amount in residual austenite.
[0022] It is preferable that the rolling/sliding part of the first
aspect is formed of a steel containing C: 0.7 to 0.9% by weight and
Cr: 3.2 to 5.0% by weight.
[0023] In the invention, a grain diameter of carburized carbide is
made fine to improve the service life under foreign matter
contaminated condition. In order to realize this, the
M.sub.7C.sub.3 type and/or M.sub.23C.sub.6 type carbide that is
slow in the grain growth due to the carburizing heating has to be
predominantly precipitated than the M.sub.3C type carbide. For the
purpose intended, amounts of C and Cr are clearly stated in the
invention.
[0024] Reasons why for limiting the respective numerical values in
the rolling/sliding part of the first aspect of the invention are
as follows.
C Content
[0025] An element C is necessary to increase the hardness after the
quenching to obtain the internal hardness to secure the strength.
Furthermore, C is an indispensable element to make nonsolid
solution carbide remain much before the carburizing and to make
these remain fine and much even after the carburizing to enable to
obtain excellent service life. Accordingly, since a sufficient
amount of C necessary for generating nonsolid solution carbide has
to be added, the lower limit value thereof is set at 0.7% by
weight. However, when C is excessively added, the hardness after
the spherodizing annealing is increased, the machinability before
the carburizing is deteriorated and, as will be described below,
since Cr is added more than in an existing bearing steel, coarse
eutectic carbide that is likely to be a starting point of the
fatigue failure during steel production tends to be generated;
accordingly, the upper limit value is set at 0.9% by weight.
Cr Content
[0026] In the next place, Cr is an important carbide forming
element in the invention. The Cr is an indispensable element in
that it generates a lot of nonsolid solution carbide in a stage
before the carburizing, the nonsolid solution carbide works as
precipitating nucleuses to precipitate fine carbides (in
particular, the M.sub.7C.sub.3 type and/or M.sub.23C.sub.6 type
carbide having high hardness) in a surface carburized layer after
the carburizing, and thereby the longer service life is enabled to
obtain. Accordingly, in order to sufficiently obtain the advantage,
Cr has to be contained much more than the existing bearing steel;
accordingly, the lower limit value is set at 3.2% by weight.
However, when it is excessively contained, it becomes difficult to
inhibit coarse eutectic carbide that becomes a starting point of
the fatigue failure from generating and the cost becomes higher;
accordingly, the upper limit value is set at 5.0% by weight.
[0027] In existing relationship between the average grain diameters
and the area ratios, in general, as the area ratio becomes larger,
the average grain diameter becomes larger. In the first aspect of
the invention, a novel point is in that, even when the area ratio
is large, the average grain diameter is inhibited as far as
possible from becoming coarser. A point that enables to realize
this is in that a steel made of material components that are
optimized in the C content and Cr content in the invention is
subjected to the spherodizing annealing to precipitate a lot of
fine carbide, followed by carburizing at a high concentration to
precipitate alloyed carbide.
[0028] It is preferable that, in the rolling/sliding part of the
first aspect of the invention, a total carbon amount of a surface
layer portion is set in the range of 1.4 to 1.8% by weight.
[0029] Reasons why for limiting the total carbon amount of a
surface layer portion in the rolling/sliding part of the first
aspect of the invention in the range of 1.4 to 1.8% by weight are
as follows.
[0030] That is, when the total carbon amount of the surface layer
portion is too small, a solid solution carbon amount in the
residual austenite cannot be set at a predetermined amount and,
with the area ratio of the carbide maintained such high as 15 to
25%, the surface hardness cannot be set at HRC62 or more;
accordingly, the longer service life and indentation resistance
cannot be simultaneously realized. Accordingly, the lower limit
value of the total carbon amount is set at 1.4% by weight.
Furthermore, when the total carbon amount of the surface layer
portion is too much, coarse carbide (free carbide) is inevitably
generated, thereby the rolling life, in particular, the rolling
life in clean oil is adversely affected. Accordingly, the upper
limit value of the total carbon amount is set at 1.8% by
weight.
[0031] It is preferable that, in the rolling/sliding part of the
first aspect of the invention, the steel is made of C: 0.7 to 0.9%
by weight, Si: 0.05 to 0.7% by weight, Mn: 0.05 to 0.7% by weight,
Cr: 3.2 to 5.0% by weight, Al: 0.04% by weight or less, P: 0.03% by
weight or less, S: 0.03% by weight or less, Ti: 0.005% by weight or
less, O: 0.0015% by weight or less, at least one kind of Mo: less
than 1.0% by weight and V: less than 0.50% by weight and a balance
made of Fe and unavoidable impurities.
[0032] Reasons why for limiting components of C and Cr are same as
mentioned above. Reasons why for limiting components other than C
and Cr are as follows.
[0033] It is preferable that, in the rolling/sliding part of the
first aspect of the invention, inner and outer rings and a rolling
element are provided and at least one of the inner and outer rings
and the rolling element is made of the foregoing parts.
[0034] According to a second aspect of the invention, a production
method of a rolling/sliding part includes:
[0035] applying a spherodizing annealing treatment to a steel that
contains C: 0.7 to 0.9% by weight, Si: 0.05 to 0.7% by weight, Mn:
0.05 to 0.7% by weight, Cr: 3.2 to 5.0% by weight, Al: 0.04% by
weight or less, P: 0.03% by weight or less, S: 0.03% by weight or
less, Ti: 0.005% by weight or less, 0: 0.0015% by weight or less,
at least one kind of Mo: less than 1.0% by weight and V: less than
0.50% by weight and a balance made of Fe and unavoidable impurities
to make a lot of carbide precipitate; after that, processing into a
predetermined shape to produce a processed part raw material; after
that, heating in a carburizing atmosphere of which carbon potential
is 1.0 to 1.5% at a temperature in the range of 870 to 950.degree.
C. to apply a first carburizing process followed by quenching;
subsequently, heating in a carburizing atmosphere of which carbon
potential is 1.0 to 1.5% at a temperature in the range of 870 to
910.degree. C. to apply a second carburizing process followed by
quenching; and applying a tempering treatment to render an area
ratio of a total precipitated carbide of the surface layer portion
in the range of 15 to 25%, to render, of the total precipitated
carbide present in a surface portion, 50% or more of the carbide by
the area ratio a M.sub.7C.sub.3 type and/or M.sub.23C.sub.6 type,
to render an average grain diameter of the carbide of the surface
layer portion in the range of 0.3 to 0.6 .mu.m and a maximum grain
diameter thereof 4 .mu.m, to render the surface hardness 62 or more
by Rockwell C hardness and to render a solid solution carbon amount
in a residual austenite of the surface layer portion in the range
of 0.95 to 1.15% by weight.
[0036] In the first and second aspects of the invention, reasons
why for limiting contents of the respective elements other than C
and Cr of steel being used and for limiting the respective
numerical values in the carburizing areas follows. Reasons why for
limiting the contents of C and Cr in the steel being used are same
as mentioned above. Furthermore, reasons why for limiting an area
ratio of total precipitated carbide of a surface layer portion
after the carburizing, amounts of M.sub.7C.sub.3 type and/or
M.sub.23C.sub.6 type in the total precipitated carbide of the
surface layer portion after the carburizing, an average grain
diameter and the maximum grain diameter of the carbide of the
surface layer portion after the carburizing, the surface hardness
and a solid solution carbon amount in the residual austenite of the
surface layer portion after the carburizing are same as mentioned
above.
Si Content
[0037] At first, Si is an element necessary for deoxidizing when
steel is refined and difficult to dissolve in carbide. Accordingly,
when Si is contained, it disturbs carbide from becoming coarser.
That is, Si is an element that can advantageously suppress the
carbide from growing large. Accordingly, in order to obtain the
advantage, a slight amount thereof has to be contained and the
lower limit value thereof is set at 0.05% by weight. However, when
Si is excessively contained, owing to reinforcement of ferrite, the
hardness after the spherodizing annealing is increased to
deteriorate the machinability before the carburizing; accordingly,
the upper limit value is set at 0.70% by weight.
Mn Content
[0038] In the next place, Mn is an element that stabilizes
austenite. When a content of Mn is increased, an amount of residual
austenite can be readily increased; accordingly, the lower limit
value thereof is set at 0.05% by weight . However, when Mn is
increased, the solid solution temperature of the carbide at the
carburizing heating is lowered. Accordingly, when it is excessively
contained, there is a problem in that an amount of nonsolid
solution carbide is decreased to decrease a hardness improvement
effect due to precipitation of carbide in a carburized layer and
excellent life can be secured only with difficulty. Furthermore,
when Mn is increased, there is another problem in that the hot
workability and the machinability are deteriorated. In this
connection, in the invention, a necessary residual austenite amount
is obtained mainly by increasing C and, in order to secure the
minimum required hardenability, the upper limit of Mn is set at
0.7% by weight. The upper limit value of the Mn content is
preferably set at 0.50% by weight.
Al Content
[0039] Then, Al is an element necessary for deoxidizing during the
refining of steel. However, when the content thereof is increased,
alumina based nonmetallic inclusions are increased, and thereby,
not only cracks and surface flaws tend to be generated during
production of steel material but also peeling start points during
the rolling fatigue tend to be generated. Accordingly, Al is
desirably suppressed to a minimum amount required for deoxidizing;
accordingly, the upper limit value is set at 0.04% by weight.
P Content
[0040] In the next place, P is segregated at an austenite grain
boundary to deteriorate the toughness of steel; accordingly, the
upper limit value thereof is set at 0.03% by weight.
S Content
[0041] Then, S is known to combine with Mn to form MnS to improve
the machinability. However, when S is excessively contained, MnS
becomes coarse to be peeling start point during the rolling
fatigue; accordingly, the upper limit value thereof is set at 0.03%
by weight.
Ti Content
[0042] In the next place, Ti is known to combine with N to form TiN
that is a nonmetallic inclusion to deteriorate the rolling fatigue
life. The TiN inclusion increases with an increase of a content of
Ti and becomes coarser; accordingly, the upper limit value thereof
is set at 0.005% by weight . From the above reasons, the content of
Ti is preferably smaller even within the above range.
O Content
[0043] Then, much of O combines with Al in the steel and Ca
slightly present as an impurity therein to exist in the steel as
oxide based inclusions. The oxide based inclusion is known to be a
peeling start point during rolling fatigue to deteriorate the
rolling fatigue life. Accordingly, steel makers have developed
technologies that enable to reduce an O amount as far as possible
in the steel. From such a background, the upper limit value thereof
is set at 0.0015% by weight. Furthermore, from the same reasons,
the O content is desirably as small as possible even within the
upper limit value.
Mo Content
[0044] In the next place, Mo is a carbide forming element stronger
in the affinity with C than with Cr and raises a solid solution
temperature of carbide during the carburizing to increase an amount
of nonsolid solution carbide. Accordingly, Mo is important element
in the invention in that an amount of fine carbide of a surface
carburized layer after the carburizing is increased to increase the
hardness. Furthermore, Mo is an element that improves the
hardenability of steel, contributes to increase an amount of the
residual austenite and has an effect of effectively precipitating
M.sub.23C.sub.6 type carbide. Accordingly, at least one of two
kinds of Mo and V described below is added to increase the surface
hardness. However, when Mo is excessively contained, not only the
cost is increased but also coarse eutectic carbide that becomes a
start point of the fatigue failure during the production of steel
material; accordingly, the content thereof is set at less than 1.0%
by weight. The lower limit value is not particularly restricted;
however, in order to obtain the advantage, Mo is preferably
contained by 0.10% by weight or more.
V Content
[0045] Next, V is a carbide forming element very strong in the
affinity with C and, since VC that is a generated V carbide is very
high in the solid solution temperature in comparison with that of
carbide of Mo, in a carburizing temperature region of a production
method of the invention described below, much of VC present before
the carburizing is not solid-solved. Accordingly, the nonsolid
solution carbide becomes a precipitating nucleus of carbide in a
carburizing layer during the carburizing to contributes to make the
carbide fine; accordingly, at least one of two kinds including Mo
mentioned above is added to improve the hardness and the service
life. In particular, since much of VC is not solid-solved by
heating during the carburizing, the surface hardness can be
improved more largely than Mo; accordingly, of Mo and V, a single
addition of V can impart higher hardness than a single addition of
Mo. However, when V is contained excessively, since C is disturbed
from diffusing, a carbon concentration of a carburized surface is
increased with difficulty and an amount of solid solution carbon is
decreased owing to the generation of VC to be difficult to secure
an amount of necessary residual austenite; accordingly, the content
thereof is set less than 0.50% by weight. Although the lower limit
value is not particularly restricted, in order to obtain the
advantage, V is preferably contained by 0.05% by weight or
more.
First Carburizing Temperature
[0046] When the carburizing temperature is less than 870.degree.
C., carbon is diffused at a slower speed to necessitate a huge time
and cost to obtain required heat-treatment quality and a huge
amount of soot is generated. Furthermore, when it exceeds
950.degree. C., a solid solution amount of nonsolid solution
carbide before the carburizing increases, a decrease in an amount
of and coarsening of precipitated carbide after the carburizing are
caused and, as austenite grains are made coarser, coarse carbides
precipitate at a grain boundary thereof to deteriorate a function
as a rolling/sliding part. Still furthermore, depending on the
component, when a high temperature is used to process, in some
cases, a ratio of the M.sub.7C.sub.3 type and M.sub.23C.sub.6 type
carbides decreases to increase the M.sub.3C type carbide that is
larger in the growth speed to further promote the coarsening of the
carbide. Accordingly, the carburizing temperature should be
selected within the range of 870 to 950.degree. C. When the first
carburizing is applied at an appropriate temperature like this,
nonsolid solution carbide that is generated before the carburizing
can be suppressed from solid-solving and the carbide is
precipitated much and fine after the carburizing, resulting in
largely improving the service life. Usually, a little before
(before substantially 30 min) applying the quenching at the last of
the carburizing, in order to control the quenching temperature,
frequently, a temperature is a little altered. The carburizing
temperature is a temperature before alteration.
Carbon Potential of First Carburizing Atmosphere
[0047] When the carburizing temperature is set in the range of 870
to 950.degree. C. to suppress the carbide from solid-solving and
growing during the processing and the carbon potential is set at
1.0% or more, fine carbide of which an average grain diameter is in
the range of 0.3 to 0.6 .mu.m and the maximum grain diameter is 4
.mu.m or less can be dispersed and precipitated much in a surface
layer portion so as to be 15 to 25% by the area ratio and, as the
result, the service life in lubricant oil contaminated with foreign
matters can be largely improved. The upper limit value of the
carbon potential is set at 1.5% to inhibit the soot from generating
much.
Second Carburizing Temperature
[0048] When the second carburizing temperature is less than
870.degree. C., the soot is generated much to increase the
maintenance cost of the device and the diffusion speed of the
carbon is deteriorated to lengthen the carburizing time to increase
the production cost. Furthermore, when the second carburizing
temperature exceeds 910.degree. C., the carbide is promoted in the
coarsening to result in incapability of obtaining the maximum grain
diameter of 4 .mu.m or less. Accordingly, the second carburizing
temperature should be set in the range of 870 to 910.degree. C.
Carbon Potential of Second Carburizing Atmosphere
[0049] Reasons why for limiting the carbon potential are same as
that of the first carburizing atmosphere.
[0050] According to the rolling/sliding part of the first aspect of
the invention, not only in the case where clean lubricant oil is
used but also in the case where lubricant oil contaminated with
foreign matter is used, longer service life can be realized, the
static load capacity can be increased to improve the indentation
resistance and a local permanent deformation amount due to overload
and impact load during rest and low-speed rotation can be
lessened.
[0051] It is preferred that, in the rolling/sliding part of the
first aspect of the invention, a C content and a Cr content are
further optimized. Thereby, since fine and high hardness
M.sub.7C.sub.3 type and/or M.sub.23C.sub.6 type carbides can be
precipitated much in a surface layer portion, the carbide can be
assuredly inhibited from growing during the carburizing.
[0052] It is preferred that, in the rolling/sliding part according
to the first aspect of the invention, a total carbon amount of a
surface layer portion is set in an appropriate range. Thereby,
since an area ratio of the carbide of the surface layer portion
after the carburizing can be controlled in an appropriate range, a
rolling bearing and a sliding bearing that use the rolling/sliding
part can be further lengthened in the service life.
[0053] It is preferred that, in the rolling/sliding part according
to the first aspect of the invention, ranges of all addition
elements are optimized. Thereby, similarly to the above, components
are advantageously set so as to precipitate much fine carbides of
M.sub.7C.sub.3 type and/or M.sub.23C.sub.6 type and inclusions of
Ti and Al, which become a start point of the fatigue failure, can
be suppressed low; accordingly, a rolling/sliding part having
longer service life can be obtained.
[0054] It is preferred that, in the rolling/sliding part according
to the first aspect of the invention, the service life in lubricant
oil contaminated with foreign matters and in clean lubricant oil
can be lengthened. Furthermore, the indentation resistance can be
improved and a local permanent deformation amount due to overload
and impact load during rest and low-speed rotation can be
lessened.
[0055] According to a second aspect of the invention, a production
method of the invention of a rolling/sliding part allows assuredly
obtaining a part in which fine M.sub.7C.sub.3 type, M.sub.23C.sub.6
type carbides are precipitated much, resulting in largely
contributing to improve the service life of the rolling/sliding
part and the indentation resistance due to an increase in the
static load capacity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] FIG. 1 is a graph showing relationship between L.sub.10 life
ratios and indentation depth ratios, which are obtained from
results of evaluation tests of rolling/sliding parts of examples
and comparative examples;
[0057] FIG. 2 is a graph showing relationship between area ratios
of total precipitated carbides and solid solution carbon amounts in
residual austenite of rolling/sliding parts of examples and
comparative examples;
[0058] FIG. 3 includes diagrams showing a first carburizing
treatment and a second carburizing treatment of a heat treatment
condition 1;
[0059] FIG. 4 includes diagrams showing a first carburizing
treatment and a second carburizing treatment of a heat treatment
condition 5;
[0060] FIG. 5 is a diagram showing a quenching treatment of a heat
treatment condition 7;
[0061] FIG. 6 is a diagram showing a carburizing treatment of a
heat treatment condition 9;
[0062] FIG. 7 is a diagram showing a carburizing treatment of a
heat treatment condition 10;
[0063] FIG. 8 is a diagram showing a carburizing treatment of a
heat treatment condition 11;
[0064] FIG. 9 is a diagram showing a carburizing treatment of a
heat treatment condition 12;
[0065] FIG. 10 includes diagrams showing a carburizing treatment
and a quenching treatment of a heat treatment condition 17;
[0066] FIG. 11 includes diagrams showing a first carburizing
treatment and a second carburizing treatment of a heat treatment
condition 18;
[0067] FIG. 12 includes a diagram showing a carburizing treatment
and a quenching treatment of a heat treatment condition 21;
[0068] FIG. 13 includes a diagram showing a first carburizing
treatment and a second carburizing treatment of a heat treatment
condition 22; and
[0069] FIG. 14 is a schematic diagram showing a method of forming
an indentation on a test piece in an indentation resistance
test.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0070] In what follows, specific examples of the invention will be
described together with comparative examples thereof.
EXAMPLES 1 THROUGH 6 AND COMPARATIVE EXAMPLES 1 THROUGH 16
[0071] Four kinds of steels having compositions shown in Table 1
are prepared. As mentioned above, in order to obtain excellent
workability and high hardness after the carburizing, from the
necessity of precipitating carbide much before the carburizing, the
steels are spherodizing annealed, followed by forming into 22 kinds
of inner ring materials that are used in a type 6206 rolling
bearing, further followed by heating the inner ring materials under
heat treatment conditions 1 through 22, thereby inner rings
(examples 1 through 6 and comparative examples 1 through 16) are
produced.
TABLE-US-00001 TABLE 1 Kind of Composition (% by weight) Steel C Si
Mn P S Cr Mo V Al Ti O A (SUJ2) 1.03 0.25 0.40 0.015 0.002 1.46 --
-- 0.032 0.0012 0.0005 B 0.2 0.29 0.81 0.02 0.015 1.1 -- -- 0.035
0.0036 0.0014 C 0.79 0.5 0.4 0.019 0.004 3.49 0.15 0.2 0.018 0.002
0.001 D 0.80 0.5 0.4 0.002 0.003 3.48 0.43 0.30 0.02 0.0022
0.0009
[0072] In the Table 1, balances of all steels are Fe and
unavoidable impurities.
[0073] A heat treatment condition 1 includes a first carburizing
treatment, a second carburizing treatment and a tempering
treatment. As shown in FIG. 3, the first carburizing treatment
includes keeping at 930.degree. C. for 6 hr in a carburizing
atmosphere having the carbon potential of 1.3%, subsequently
keeping at 900.degree. C. for 0.5 hr in an appropriate atmosphere,
followed by oil cooling to 80.degree. C. Similarly as shown in FIG.
3, the second carburizing treatment includes keeping at 900.degree.
C. for 6 hr in a carburizing atmosphere having the carbon potential
of 1.3%, followed by keeping at 900.degree. C. for 0.5 hr in an
appropriate atmosphere, further followed by oil cooling to
80.degree. C. The tempering treatment includes air cooling after
keeping at 160.degree. C. for 2 hr.
[0074] In a heat treatment condition 2, the tempering temperature
of the heat treatment condition 1 is set at 200.degree. C.
[0075] A heat treatment condition 3 is same as the heat treatment
condition 1.
[0076] In a heat treatment condition 4, the tempering temperature
of the heat treatment condition 1 is set at 200.degree. C.
[0077] A heat treatment condition 5 includes a first carburizing
treatment, a second carburizing treatment and a tempering
treatment. As shown in FIG. 4, the first carburizing treatment
includes keeping at 900.degree. C. for 5 hr in a carburizing
atmosphere having the carbon potential of 1.3%, followed by keeping
at 900.degree. C. for 0.5 hr in an appropriate atmosphere, further
followed by oil cooling to 80.degree. C. Similarly as shown in FIG.
4, the second carburizing treatment includes keeping at 880.degree.
C. for 3 hr in a carburizing atmosphere having the carbon potential
of 1.3%, followed by keeping at 880.degree. C. for 0.5 hr in an
appropriate atmosphere, further followed by oil cooling to
80.degree. C. The tempering treatment includes air cooling after
keeping at 160.degree. C. for 2 hr.
[0078] In a heat treatment condition 6, the tempering temperature
of the heat treatment condition 5 is set at 200.degree. C.
[0079] A heat treatment condition 7, as shown in FIG. 5, includes
keeping at 830.degree. C. for 40 min followed by oil cooling to
80.degree. C. to quench, further followed by keeping at 180.degree.
C. for 2 hr, followed by air cooling to temper.
[0080] In a heat treatment condition 8, the heating temperature at
the quenching treatment of the heat treatment condition 7 is set at
900.degree. C.
[0081] A heat treatment condition 9, as shown in FIG. 6, includes
keeping at 850.degree. C. for 3.5 hr in a carburizing atmosphere of
which carbon potential is 1.3% to carburize, followed by oil
cooling to 80.degree. C., further followed by keeping at
160.degree. C. for 2 hr, followed by air cooling to temper.
[0082] A heat treatment condition 10, as shown in FIG. 7, includes
keeping at 930.degree. C. for 3 hr in a carburizing atmosphere
having the carbon potential of 1.25%, followed by keeping at
930.degree. C. for 2 hr in a carburizing atmosphere having the
carbon potential of 1.1%, further followed by keeping at
850.degree. C. for 0.5 hr in an appropriate atmosphere to
carburize, followed by oil cooling to 80.degree. C., still further
followed by keeping at 180.degree. C. for 2 hr, followed by air
cooling to temper.
[0083] A heat treatment condition 11, as shown in FIG. 8, includes
keeping at 900.degree. C. for 5.5 hr in a carburizing atmosphere of
which carbon potential is 1.3%, followed by keeping at 870.degree.
C. for 0.5 hr in an appropriate atmosphere to carburize, further
followed by oil cooling to 80.degree. C., still further followed by
keeping at 160.degree. C. for 2 hr, followed by air cooling to
temper.
[0084] A heat treatment condition 12, as shown in FIG. 9, includes
keeping at 900.degree. C. for 5.5 hr in a carburizing atmosphere of
which carbon potential is 1.3%, followed by keeping at 900.degree.
C. for 0.5 hr in an appropriate atmosphere to carburize, followed
by oil cooling to 80.degree. C., still further followed by keeping
at 160.degree. C. for 2 hr, followed by air cooling to temper.
[0085] In a heat treatment condition 13, the heating temperature
and time in the carburizing atmosphere in the carburizing treatment
in the heat treatment condition 11, respectively, are set at
930.degree. C. and 6 hr.
[0086] In a heat treatment condition 14, the heating temperature
and time in the carburizing atmosphere in the carburizing treatment
in the heat treatment condition 11, respectively, are set at
930.degree. C. and 6 hr, and the heating temperature in air where
the carburizing gas is not present is set at 900.degree. C.
[0087] In a heat treatment condition 15, the tempering temperature
of the heat treatment condition 14 is set at 180.degree. C.
[0088] In a heat treatment condition 16, the tempering temperature
of the heat treatment condition 14 is set at 200.degree. C.
[0089] A heat treatment condition 17 includes, as shown in FIG. 10,
keeping at 930.degree. C. for 6 hr in a carburizing atmosphere of
which carbon potential is 1.3%, followed by keeping at 900.degree.
C. for 0.5 hr in an appropriate atmosphere to carburize, further
followed by oil cooling to 80.degree. C., still further followed by
keeping at 900.degree. C. for 1 hr, followed by oil cooling to
80.degree. C. to quench, further followed by keeping at 160.degree.
C. for 2 hr, followed by air cooling to temper.
[0090] A heat treatment condition 18 includes a first carburizing
treatment, a second carburizing treatment and a tempering
treatment. As shown in FIG. 11, the first carburizing treatment
includes keeping at 930.degree. C. for 6 hr in a carburizing
atmosphere of which carbon potential is 1.3%, followed by keeping
at 900.degree. C. for 0.5 hr in an appropriate atmosphere, further
followed by oil cooling to 80.degree. C. Similarly as shown in FIG.
11, the second carburizing treatment includes keeping at
900.degree. C. for 6 hr in a carburizing atmosphere of which carbon
potential is 1.3%, followed by keeping at 850.degree. C. for 0.5 hr
in an appropriate atmosphere, further followed by oil cooling to
80.degree. C. The tempering treatment includes air cooling after
keeping at 160.degree. C. for 2 hr.
[0091] In a heat treatment condition 19, the heating temperature of
the quenching of the heat treatment condition 17 is set at
950.degree. C.
[0092] In a heat treatment condition 20, the heating temperature in
an appropriate atmosphere in the second carburizing treatment of
the heat treatment condition 18 is set at 950.degree. C.
[0093] A heat treatment condition 21 includes, as shown in FIG. 12,
keeping at 850.degree. C. for 8 hr in a carburizing atmosphere of
which carbon potential is 1.29% to carburize, followed by oil
cooling to 80.degree. C., further followed by keeping at
160.degree. C. for 2 hr, followed by air cooling to temper.
[0094] A heat treatment condition 22 includes, as shown in FIG. 13,
keeping at 850.degree. C. for 8 hr in a carburizing atmosphere of
which carbon potential is 1.29%, followed by keeping at 950.degree.
C. for 0.5 hr in an appropriate atmosphere to carburize, further
followed by oil cooling to 80.degree. C., still further followed by
keeping at 160.degree. C. for 2 hr, followed by air cooling to
temper.
[0095] Of thus produced examples 1 through 6 and comparative
examples 1 through 16, kinds of steels of inner rings, heat
treatment conditions, surface hardness of bearing surfaces of inner
rings after heat treatment (HRC), total carbon amounts of surface
layer portions of bearing surfaces, area ratios of carbides
precipitated in the surface layer portion of the bearing surfaces,
maximum grain diameters and average grain diameters of carbides
precipitated in the surface layer portions of the bearing surfaces,
solid solution carbon amounts in residual austenite (.gamma.R) and
total area ratios of M.sub.7C.sub.3 type and M.sub.23C.sub.6 type
carbides are shown in Tables 2A and 2B.
TABLE-US-00002 TABLE 2A Maximum Average Solid Grain Grain Solution
Area Total Area Diameter Diameter Carbon Ratio of Kind Heat Surface
Carbon Ratio of of of Amount M.sub.7C.sub.3/M.sub.23C.sub.6
L.sub.10 of treatment Hardness Amount Carbide Carbide Carbide in
.gamma.R (% Carbide (% Life Indentation Steel Condition (HRC) (%)
(%) (.mu.m) (.mu.m) by weight) by weight) Ratio Depth Ratio Example
1 C 1 65 1.5 15.7 2.5 0.47 0.98 65 11.5 0.875 2 2 63 1.55 18.7 2
0.49 0.95 60 10.2 0.686 3 3 65.3 1.6 16.5 2.7 0.53 1.04 55 14.5
0.75 4 4 63.5 1.6 23 2.9 0.57 1.01 58 12 0.644 5 D 5 64.5 1.45 18
2.2 0.38 1.03 75 16 0.8 6 6 63.5 1.45 21.5 2.9 0.43 0.99 70 12.5
0.5 Comparative 1 A 7 62.6 1.0 6 1.3 0.37 0.68 0 1 1 Example 2 8 64
1.0 3.5 1.1 0.25 0.87 0 1.1 1.2 3 9 65.3 1.3 11 2.2 0.45 0.86 0 3
0.77 4 B 10 61 0.8 1.1 0.3 0.22 0.80 0 1.1 1.155 5 D 11 64.7 1.4 13
1.7 0.31 0.89 70 7.8 0.931
TABLE-US-00003 TABLE 2B Maximum Average Solid Grain Grain Solution
Area Total Area Diameter Diameter Carbon Ratio of Kind Heat Surface
Carbon Ratio of of of Amount M7C3/M23C6 L10 Of treatment Hardness
Amount Carbide Carbide Carbide in .gamma.R (% Carbide (% Life
Indentation Steel Condition (HRC) (%) (%) (.mu.m) (.mu.m) by
weight) by weight) Ratio Depth Ratio Comparative 6 D 12 64 1.4 11.3
2.5 0.33 0.97 70 14.4 1.365 Example 7 C 13 64.4 1.25 12.5 1 0.32
0.96 65 6.7 0.966 8 14 64.4 1.25 11.6 2.4 0.37 1.02 65 12.5 1.442 9
15 63 1.3 9.5 2.2 0.31 0.98 60 5.5 1.25 10 16 62.7 1.25 10.1 1.9
0.31 0.98 60 4.6 1.1 11 17 64.9 1.25 9.8 2.3 0.29 0.84 65 5.8 0.903
12 18 65.5 1.55 23 4.5 0.58 0.85 55 3.5 0.665 13 19 62.5 1.25 5.4
3.3 0.3 1.21 70 11 1.61 14 20 63 1.55 17 4.5 0.65 1.15 60 14 1.46
15 A 21 65.5 1.6 16.5 6.5 0.75 0.88 0 2.8 0.65 16 22 61.5 1.6 11.1
7 0.8 1.04 0 3 1.25
Life Test
[0096] Each of inner rings of examples 1 through 6 and comparative
examples 1 through 16 is combined with an outer ring that is made
of JIS SUJ2 and exposed to an ordinary carburizing treatment and
balls to assemble a type No. 6206C3 ball bearing. The ball bearing
is subjected to the life test with lubricant oil contaminated with
foreign matters. Test conditions are shown in Table 3.
TABLE-US-00004 TABLE 3 Load Fr = 9000 N/set Number of Rotations
2500 rpm Foreign Matter High-speed steel powder, 0.06 mass percent
(hardness: 730 HV, grain diameter: 100 to 150 .mu.m) Lubricant
Turbine oil #68 oil bath (air agitation) Oil Temperature Natural
temperature-up (substantially 100.degree. C.) Calculated Life (Lh)
67.8 h Test Method 2 set .times. 5 times sudden death test Test
Sample Type No. 6206
[0097] A test machine shown in Table 3 can test simultaneously two
ball bearings and radial load in Table 3 means radial load per one
ball bearing.
Indentation Resistance Test
[0098] In order to investigate the indentation resistance of each
of inner rings of examples 1 through 6 and comparative examples 1
through 16, similarly to the cases of inner rings of examples 1
through 6 and comparative examples 1 through 16, after four kinds
of steels having compositions shown in Table 1 are subjected to
spherodizing annealing, 22 kinds of planar test pieces for use in
indentation resistance test are prepared, and the test pieces are
heat treated under the heat treatment conditions 1 through 22.
Then, as shown in FIG. 14, a steel ball (2) is placed on each (1)
of test pieces, under condition shown in Table 4, the steel ball
(2) is pressed through a tool (3) against the test piece (1) by use
of an Amsler head (4) to indent and the indentation depth is
measured by use of a three-dimensional profilometer. It goes
without saying that kinds of steels of 22 kinds of test pieces (1),
heat treatment conditions, surface hardness of bearing surfaces of
inner rings after heat treatment (HRC), total carbon amounts of
surface layer portions of bearing surfaces, area ratios of carbides
precipitated in the surface layer portion of the bearing surfaces,
maximum grain diameters and average grain diameters of carbides
precipitated in the surface layer portions of the bearing surfaces,
solid solution carbon amounts in residual austenite (.gamma.R) and
total area ratios of M.sub.7C.sub.3 type and M.sub.23C.sub.6 type
carbides are same as that shown in Tables 2A and 2B.
TABLE-US-00005 TABLE 4 Test Machine Amsler (10t) Diameter of Steel
Ball D 15/16 inch Load Maximum Contact Surface Four kinds of loads
of Condition Pressure (P max) between 4000, 4500, 5000 and Test
Piece and Steel Ball 5500 Loading Time 1 min
[0099] The L.sub.10 life ratios and indentation depths as well are
shown in Tables 2A and 2B.
[0100] The L.sub.10 life ratio in Tables 2A and 2B is obtained in
such a manner that two ball bearings provided with the same inner
rings are set to a test machine, a test where a time until any one
of the inner rings of the ball bearings is destroyed is measured is
repeated 5 times, an average time up to the failure is taken as the
L.sub.10 life, and the L.sub.10 life is compared with a L.sub.10
life of comparative example 1 (quenched and tempered JIS SUJ2)
assigned to 1 to obtain the L.sub.10 life ratio. Furthermore, the
indentation depth ratio is similarly obtained with an indentation
depth of comparative example 1 (quenched and tempered JIS SUJ2)
assigned to 1.
[0101] As obvious from results shown in Table 2A, in examples 1
through 6, the life can be lengthened and the static load capacity
can be increased to improve the indentation resistance to the
indentation due to the plastic deformation during rest or low-speed
rotation. On the other hand, in comparative examples 1 through 16,
at least one of the life and the indentation resistance is poor in
the performance.
* * * * *