Source: http://www.google.com/patents/US8002907?dq=6760745
Timestamp: 2014-03-14 13:28:17
Document Index: 26746789

Matched Legal Cases: ['Application No. 200410008248', 'Application No. 200710146996', 'Application No. 05703409', 'Application No. 2002', 'Application No. 2002', 'Application No. 2002', 'Application No. 2002', 'Application No. 2002', 'Application No. 2002', 'Application No. 2002', 'Application No. 2003', 'Application No. 2004']

Patent US8002907 - Bearing's component, heat treatment method thereof, heat treatment apparatus ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsObtain a bearing's component, a heat treatment method thereof, and a rolling bearing having a long life against rolling contact fatigue and high anti-crack strength, and suppressed long-term dimensional variation rate. The bearing's component is carbonitrided at a carbonitriding temperature higher than...http://www.google.com/patents/US8002907?utm_source=gb-gplus-sharePatent US8002907 - Bearing's component, heat treatment method thereof, heat treatment apparatus, and rolling bearingAdvanced Patent SearchPublication numberUS8002907 B2Publication typeGrantApplication numberUS 10/897,016Publication dateAug 23, 2011Filing dateJul 23, 2004Priority dateAug 29, 2003Also published asCN1605636A, CN100453661C, EP1510589A1, US20050045247Publication number10897016, 897016, US 8002907 B2, US 8002907B2, US-B2-8002907, US8002907 B2, US8002907B2InventorsChikara OhkiOriginal AssigneeNtn CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (101), Non-Patent Citations (35), Classifications (28), Legal Events (1) External Links: USPTO, USPTO Assignment, EspacenetBearing's component, heat treatment method thereof, heat treatment apparatus, and rolling bearingUS 8002907 B2Abstract Obtain a bearing's component, a heat treatment method thereof, and a rolling bearing having a long life against rolling contact fatigue and high anti-crack strength, and suppressed long-term dimensional variation rate. The bearing's component is carbonitrided at a carbonitriding temperature higher than an A1 transformation point of steel for the bearing's component and then cooled to a temperature lower than the A1 transformation point, and subsequently, using a heat treatment apparatus that successively moves and heats each individual bearing's component, reheated to a range of quenching temperature of no less than the A1 transformation point and less than the carbonitriding temperature to be quenched.
the temperature raising rate in heating to temperature T2 (quenching temperature) is set to be at least 3� C./min at a depth of 2 mm from the surface of the bearing's component.
5. The bearing's component according to claim 4, wherein said range of quenching temperature is 790� C. to 830� C.
SUMMARY OF THE INVENTION It is an object of the present invention to provide a bearing's component, a heat treatment method thereof, a heat treatment apparatus and a rolling bearing having a longer life under rolling contact fatigue, an increased anti-crack strength, and a reduced long-term dimensional variation rate.
In the heat treatment method of a bearing's component according to the present invention, the range of quenching temperature in reheating may be a temperature range of 790� C. to 830� C.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for illustrating a heat treatment method in accordance with an embodiment of the present invention.
FIGS. 7A to 7D each show a microstructure of a bearing's component, more specifically, an austenite grain, wherein FIG. 7A is of temperature raising rate of 1� C./min, FIG. 7B is of temperature raising rate of 2.5� C./min, FIG. 7C is of temperature raising rate of 7.5� C./min, and FIG. 7D is of temperature raising rate of 25� C./min.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following, an embodiment of the present invention will be described with reference to the drawings. FIGS. 1 and 2 show heat treatment method according to the embodiment of the present invention. FIG. 1 shows a heat treatment pattern according to which primary quenching and secondary quenching are carried out, and FIG. 2 shows a heat treatment pattern according to which a material is cooled to a temperature lower than the A1 transformation point in a quenching process and thereafter heated again to be finally quenched. In these figures, at temperature T1, carbon, nitrogen and the like are diffused in a steel matrix of a bearing's component and carbon is sufficiently included in the steel. Thereafter, at temperature T2 in the figures, the bearing's component is reheated to a temperature lower than temperature T1, from which the steel is subjected to oil quenching.
The following two points should be noted in FIGS. 1 and 2: (1) the temperature raising rate in heating to temperature T2 is set to be at least 3� C./min at a depth of 2 mm from the surface of the bearing's component, and (2) in quenching from temperature T2, the average cooling rate for lowering the temperature by 400� C. from the heating temperature at a depth of 2 mm from the surface of a bearing's component is set to be at least 20� C./sec, or quench severity (cooling power) of the cooling medium in quenching is set to be at least 0.1 cm−1. By (1), fine austenite grains without mixed grains can be obtained, and by (2), hardness that is sufficient to ensure durability and the like can be obtained. Additionally, it is also important for obtaining appropriate amount of retained austenite.
FIG. 3 illustrates the above-mentioned average cooling rate. FIG. 3 shows the case where heating temperature T2 is 800� C. The temperature raising rate is an average temperature raising rate from A1 transformation point to temperature T2. Additionally, given that the average cooling rate from heating temperature T2 to the temperature lower than that by 400� C. is at least 20� C./sec, point A in FIG. 3 should only be at most 20 seconds on the axis of abscissas.
EXAMPLE FIG. 5A shows austenite crystal grain size of a bearing steel applied with the heat treatment pattern shown in FIG. 1. For comparison, FIG. 5B shows austenite crystal grain size of a bearing steel which has undergone the conventional heat treatment. JIS-SUJ2 (1.0 wt % of C�0.25 wt % of Si�0.4 wt % of Mn�1.5 wt % of Cr) of steel material was used in both of the methods. FIGS. 6A and 6B diagrammatically show the crystal grain sizes of austenite that are shown in FIGS. 5A and 5B. In the structures with the crystal grain sizes of austenite, the grain size of the conventional austenite is 10 which is a grain size number defined by JIS while that of the present invention through the heat treatment method thereof is 12 and thus fine grains are seen. Further, the average grain size in FIG. 5A is 5.6 μm measured by the intercept method.
Next, the effect of the temperature raising rate to heating temperature T2 exerted to generation of mixed grains of austenite crystal grains is described. JIS-SUJ2 as sample was heated to 800� C. in accordance with the heat pattern shown in FIG. 1 and with varied temperature raising rate to temperature T2. Thereafter, oil quenching was performed and the austenite grains were examined. The results are shown in FIGS. 7A-7D. FIGS. 8A-8D diagrammatically represents FIGS. 7A-7D.
FIGS. 8A and 8B show austenite grains when heated at temperature raising rates of 1� C./min and 2.5� C./min, respectively. It can be seen that coarse austenite grains are grown among fine austenite grains. The coarse austenite grains are grown by merging with fine austenite grains, and while they are coarse grains, their grain boundaries show curved continuous portions with small curvatures, as if grain boundaries of fine grains are remained.
In FIGS. 8C and 8D, the difference between larger grain sizes and smaller grain sizes among the mixed grains becomes less significant, and the structure can be regarded as uniform grains. The term mixed grain structure is defined by JIS G0551. As described above, very fine austenite grains can be obtained when the above-discussed temperature raising rate is set to at least 3� C./min. When the temperature raising rate is lower than 3� C./min, very large coarse grains are grown. As a result, mechanical properties such as durability are degraded. Such limitation in the temperature raising rate is significantly effective in avoiding the mixed grain structure.
Next, using the same steel material, the relationship between quench severity (cooling power) of oil and quench hardness was examined. The samples of steel material are JIS-SUJ2, each of which are ring-shaped and have an outer diameter of 60 mm, and a length of 10 mm, while their inner diameter, and hence, the thickness is different. Their thickness is varied in a range of 2 mm-8 mm. As to the heat treatment pattern, the pattern of FIG. 1 was employed, and quench severity was changed by using different cooling oils in quenching from temperature T2. As oil with low quench severity, a hot oil was employed, and as oil with high quench severity, a cold oil was employed. A semi-hot oil was employed as an intermediate oil between them. Quench severity was varied in a range of 0.1-0.14 cm−1. After the steel was quenched, it was tempered at 180� C. and the hardness thereof was measured. The hardness is average Vickers hardness (HV) at a depth of 0.2 mm from the surface of circumferential length central portion of each ring sample. The number tested was 3. The results are shown in Table 1.
Ring thickness of samples
Quench severity of oil (1/cm)
HV780
HV790
HV760
Next, On the following samples A, B and C, a series of tests was conducted. A material to be heat-treated that was employed commonly to samples A-C was JIS-SUJ2 (1.0 wt % of C�0.25 wt % of Si�0.4 wt % of Mn�1.5 wt % of Cr).
Sample B�comparative example: quenching directly after carbonitriding (conventional carbonitriding and quenching)
Sample C�example of the present invention: a bearing steel processed following the heat treatment pattern shown in FIG. 1.
K1C (MPa{square root over (m)})
Relative K1C A
Table 7 shows the rate of long-term dimensional variation measured under the conditions of 130� C. of holding temperature and 500 hours of holding time, together with the surface hardness and the amount of retained austenite (at 0.1 mm depth).
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