JP4382769B2 - Heat treatment method for bearing parts, bearing parts and rolling bearing - Google Patents

Description

  The present invention relates to a heat treatment method for bearing parts, bearing parts, and rolling bearings used in reduction gears, drive pinions, transmission bearings, etc., and has a long rolling fatigue characteristic, high cracking resistance, and aging resistance. The present invention relates to a heat treatment method for bearing parts, bearing parts, and rolling bearings.

As a heat treatment method that gives a long life against rolling fatigue of a bearing component, a method of performing a carbonitriding process on the surface layer portion of the bearing component by adding ammonia gas to the atmosphere RX gas during quenching heating, etc. (For example, refer to Patent Documents 1 and 2). By using this carbonitriding treatment method, the surface layer portion can be hardened, retained austenite can be generated in the microstructure, and the rolling fatigue life can be improved.
JP-A-8-4774 Japanese Patent Laid-Open No. 11-101247

  However, since the carbonitriding method described above is a diffusion treatment that diffuses carbon and nitrogen, it must be kept at a high temperature for a long time. For this reason, it is difficult to improve the cracking resistance due to the coarsening of the structure. In addition, an increase in the dimensional change rate due to increase in retained austenite is also a problem.

  On the other hand, it is possible to cope with rolling fatigue by adjusting the composition by alloy design of steel in order to ensure a long life, improve crack strength, and prevent an increase in the rate of dimensional change over time. However, the alloy design causes problems such as an increase in raw material costs.

  Future bearing parts are required to have characteristics that can be used under larger load conditions and at higher temperatures than in the past as the usage environment increases in load and temperature. For this reason, a bearing part having high strength, long rolling fatigue characteristics, high cracking resistance and dimensional stability is required.

  An object of the present invention is to provide a bearing component heat treatment method, a bearing component, and a rolling bearing that have high cracking resistance and dimensional stability and are excellent in rolling fatigue life.

Heat treatment method of the bearing components in accordance with the present invention, after the bearing component formed of steel having a composition of JIS standard SUJ2, and carbonitriding in the carbonitriding temperature is a temperature above the A ₁ transformation point, A ₁ transformation point cooled to below the temperature, then there is a ₁ transformation point or higher, then reheated to quenching heating temperature lower than the carbonitriding temperature, performing quenching. And quenching heating temperature is 800 degreeC or more and 815 degrees C or less.

  The bearing component according to the present invention is heat-treated by the heat treatment method for a bearing component according to the present invention. And the grain size number of the austenite crystal grain of the said steel is 11 or more.

  In the rolling bearing according to the present invention, at least one of the outer ring, the inner ring and the rolling element is a bearing component according to the present invention.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing a rolling bearing in an embodiment of the present invention. In FIG. 1, this rolling bearing 10 mainly has an outer ring 1, an inner ring 2, and rolling elements 3. Although the drawings show radial bearings, ball bearings, tapered roller bearings, roller bearings, and needle roller bearings are also subject to the embodiments of the present invention. The rolling element 3 is supported by a cage disposed between the outer ring 1 and the inner ring 2 so as to be able to roll.

Next, heat treatment including carbonitriding performed on at least one bearing part of the outer ring, the inner ring and the rolling element of the rolling bearing will be described. FIG. 2 is a diagram for explaining a heat treatment method according to the embodiment of the present invention. Moreover, FIG. 3 is a figure explaining the modification of the heat processing method in embodiment of this invention. FIG. 2 is a heat treatment pattern showing a method of performing primary quenching and secondary quenching, and FIG. 3 is a method of cooling the material to below the A ₁ transformation point temperature during quenching, and then re-heating and finally quenching. It is the heat processing pattern which shows. Both are exemplary embodiments of the present invention. In these figures, in the treatment T1, carbon and nitrogen are diffused in the steel base and the carbon is sufficiently dissolved, and then cooled to below the A ₁ transformation point. Next, in process T2 in the figure, reheating is performed at a temperature lower than that of process T1, and oil quenching is performed therefrom.

  Rather than performing normal quenching, that is, carbonitriding once after the carbonitriding treatment, the crack strength can be improved and the aging change rate can be reduced while carbonitriding the surface layer portion. As described above, according to the above heat treatment method, it is possible to obtain a microstructure in which the grain size of austenite crystal grains is ½ or less of the conventional one. The bearing parts subjected to the above heat treatment have a long rolling fatigue characteristic, can improve the cracking strength, and can also reduce the rate of dimensional change over time.

  FIG. 4 is a view showing the microstructure of the bearing component, particularly austenite grains. FIG. 4A shows a bearing component of the present invention, and FIG. 4B shows a conventional bearing component. That is, FIG. 4A shows the austenite grain size of the bearing steel to which the heat treatment pattern shown in FIG. 2 is applied. For comparison, FIG. 4B shows the austenite grain size of the bearing steel obtained by the conventional heat treatment method. FIGS. 5A and 5B are diagrams showing the austenite grain boundaries illustrated in FIGS. 4A and 4B. From the structure showing the austenite crystal grain size, the conventional austenite grain size is No. 10 in the JIS standard grain size number, and according to the heat treatment method of the present invention, No. 12 fine grains can be obtained. Moreover, the average particle diameter of Fig.4 (a) was 5.6 micrometers as a result of measuring by the intercept method.

  Example 1 of the present invention was performed using JIS standard SUJ2 material (1.0 wt% C-0.25 wt% Si-0.4 wt% Mn-1.5 wt% Cr). The manufacturing history of each sample shown in Table 1 is shown below.

(Samples A to D; examples of the present invention): carbonitriding 850 ° C., holding time 150 minutes. The atmosphere was a mixed gas of RX gas and ammonia gas. In the heat treatment pattern shown in FIG. 2, primary quenching was performed from a carbonitriding temperature of 850 ° C., and then secondary quenching was performed by heating to a temperature range of 780 ° C. to 830 ° C. lower than the carbonitriding temperature. However, Sample A having a secondary quenching temperature of 780 ° C. was excluded from the test due to insufficient quenching.
(Sample E, F; Comparative Example): carbonitriding is carried out in the same history and the present invention embodiment to D, were subjected to secondary quenching temperature at 850 ° C. 870 ° C. above carbonitriding treatment temperature 850 ° C..
(Conventional carbonitriding product; comparative example): Carbonitriding treatment at 850 ° C., holding time of 150 minutes. The atmosphere was a mixed gas of RX gas and ammonia gas. Quenching was performed as it was from the carbonitriding temperature, and secondary quenching was not performed.
(Normally hardened product; comparative example): without quenching and carbonitriding, it was heated to 850 ° C. and quenched. Secondary quenching was not performed.

  (1) Measurement of hydrogen content, (2) Measurement of crystal grain size, (3) Charpy impact test, (4) Measurement of fracture stress value, (5) Rolling fatigue test Was done. Next, these test methods will be described.

I Test Method of Example 1 (1) Measurement of hydrogen content The amount of hydrogen was analyzed for the amount of non-diffusible hydrogen in the steel using a DH-103 hydrogen analyzer manufactured by LECO. The amount of diffusible hydrogen is not measured. The specification of this LECO DH-103 type hydrogen analyzer is shown below.

Analysis range: 0.01 to 50.00 ppm
Analysis accuracy: ± 0.1 ppm or ± 3% H (whichever is greater)
Analysis sensitivity: 0.01ppm
Detection method: Thermal conductivity method Sample weight size: 10 mg to 35 g (maximum: diameter 12 mm × length 100 mm)
Heating furnace temperature range: 50 ° C to 1100 ° C
Reagents: Anhydrone Mg (ClO ₄ ) ₂ , Ascarite NaOH
Carrier gas: nitrogen gas, gas dosing gas: hydrogen gas, both gases have a purity of 99.99% or more and a pressure of 40 PSI (2.8 kgf / cm ² ).

The outline of the measurement procedure is as follows. A sample collected with a dedicated sampler is inserted into the hydrogen analyzer together with the sampler. Internal diffusible hydrogen is directed to the thermal conductivity detector by a nitrogen carrier gas. The diffusible Hydrogen is not measured in this example. Next, the sample is taken out from the sampler and heated in a resistance heating furnace, and non-diffusible hydrogen is guided to the thermal conductivity detector by nitrogen carrier gas. The amount of non-diffusible hydrogen can be known by measuring the thermal conductivity with a thermal conductivity detector.

(2) Measurement of crystal grain size The crystal grain size was measured based on the JIS G 0551 steel austenite grain size test method.

(3) Charpy impact test The Charpy impact test was performed based on the Charpy impact test method of the metal material of JIS Z2242. As a test piece, a U-notch test piece (JIS No. 3 test piece) shown in JIS Z 2202 was used.

(4) Measurement of Fracture Stress Value FIG. 6 is a diagram showing a test piece for a static crush strength test (measurement of a fracture stress value). The load until it is broken by applying a load in the P direction in the figure is measured. Thereafter, the obtained fracture load is converted into a stress value by the following bending beam stress calculation formula. In addition, a test piece is not restricted to the test piece shown in FIG. 6, You may use the test piece of another shape.

Assuming that the fiber stress on the convex surface of the test piece of FIG. 6 is σ ₁ and the fiber stress on the concave surface is σ ₂ , σ ₁ and σ ₂ are obtained by the following formulas (Mechanical Engineering Handbook A4 Knitting Material Dynamics A4-40) . Here, N is the axial force of the cross section including the axis of the annular test piece, A is the cross-sectional area, e ₁ is the outer radius, and e ₂ is the inner radius. Further, κ is a section modulus of the curved beam.

σ ₁ = (N / A) + {M / (Aρ _o )} [1 + e ₁ / {κ (ρ _o + e ₁ )}]
σ ₂ = (N / A) + {M / (Aρ _o )} [1-e ₂ / {κ (ρ _o −e ₂ )}]
κ = − (1 / A) ∫ _A {η / (ρ _o + η)} dA
(5) Rolling fatigue test Table 2 shows the test conditions for the rolling fatigue life test. FIG. 7 is a schematic view of a rolling fatigue life tester. FIG. 7A is a front view, and FIG. 7B is a side view. 7 (a) and 7 (b), the rolling fatigue life test piece 21 is driven by the drive roll 11 and is in contact with the ball 13 and rotating. The ball 13 is a (3/4) ″ ball and is guided by a guide roll and rolls while exerting a high surface pressure with the rolling fatigue life test piece 21.

II Test Results of Example 1 (1) Hydrogen Content The conventional carbonitrided product that has been carbonitrided has a very high value of 0.72 ppm. This is presumably because ammonia (NH ₃ ) contained in the carbonitriding atmosphere decomposed and hydrogen entered the steel. On the other hand, in Samples B to D, the hydrogen amount is reduced to almost half of 0.37 to 0.40 ppm. This amount of hydrogen is at the same level as that of ordinary quenched products.

  By reducing the amount of hydrogen described above, embrittlement of steel due to hydrogen solid solution can be reduced. That is, the reduction in the amount of hydrogen greatly improves the Charpy impact value of Samples B to D of the present invention example.

(2) Crystal grain size When the secondary quenching temperature is lower than the quenching (primary quenching) temperature during carbonitriding, that is, in the case of Samples B to D, the austenite grains have grain size numbers 11 to 12 and Remarkably miniaturized. The austenite grains of Samples E and F, the conventional carbonitrided product, and the normal quenching product have a crystal grain size number 10 and are coarser than the samples B to D of the examples of the present invention.

(3) Charpy impact test According to Table 1, the Charpy impact value of the samples B to D of the present invention example is 6 compared to the Charpy impact value of the conventional carbonitrided product being 5.33 J / cm ^2. A high value of .30 to 6.65 J / cm ² is obtained. Among these, the lower the secondary quenching temperature, the higher the Charpy impact value tends to be. The normally hardened product has a Charpy impact value as high as 6.70 J / cm ² .

(4) Measurement of fracture stress value The fracture stress value corresponds to the crack resistance strength. According to Table 1, the conventional carbonitrided product has a fracture stress value of 2330 MPa. Compared to this, the fracture stress values of Samples B to D are improved to 2650 to 2840 MPa. The fracture stress value of the normally quenched product is 2770 MPa, which is equivalent to the fracture stress values of Samples B to F. Such improved cracking resistance strengths of Samples B to D are presumed to have a great effect by reducing the hydrogen content, along with the refinement of austenite crystal grains.

(5) According to the rolling contact fatigue test Table 1, usually sintered Irihin is reflecting that does not have a carbonitrided layer in the surface layer portion, the rolling fatigue life L ₁₀ is the lowest. Compared to this, the rolling fatigue life of the conventional carbonitrided product is 3.1 times. The rolling fatigue life of Samples B to D is significantly improved as compared with the conventional carbonitrided product. Samples E and F of the present invention are substantially equivalent to conventional carbonitrided products.

  In summary, Samples B to D of the present invention have a reduced hydrogen content, an austenite grain size of 11 or more, and improved Charpy impact value, crack resistance strength and rolling fatigue life. .

Next, Example 2 will be described. A series of tests were performed on the following A material, B material, and C material. JIS standard SUJ2 material (1.0% by weight C-0.25% by weight Si-0.4% by weight Mn-1.5% by weight Cr) is used for the material for heat treatment, which is common to materials A to C. did. The manufacturing histories of the A material to the C material are as follows.
(A material: comparative example): Only normal quenching (without carbonitriding).
(B material: comparative example): quenching as it is after carbonitriding (conventional carbonitriding quenching). Carbonitriding temperature 845 ° C, holding time 150 minutes. The atmosphere of the carbonitriding process was RX gas + ammonia gas.
(C material: Example of the present invention): Bearing steel subjected to the heat treatment pattern of FIG. Carbonitriding temperature 845 ° C, holding time 150 minutes. The atmosphere of the carbonitriding process was RX gas + ammonia gas. The final quenching temperature was 800 ° C.

(1) Rolling fatigue life Test conditions and test equipment for the rolling fatigue life test are as shown in Table 2 and FIG. 7 as described above. The rolling fatigue life test results are shown in Table 3.

According to Table 3, the B material of the comparative example shows 3.1 times the L ₁₀ life (the life that one of the 10 test pieces breaks) of the A material that was also subjected to only ordinary quenching in the comparative example, The effect of extending the life by carbonitriding is recognized. On the other hand, C material of the example of the present invention has a long life of 5.4 times that of A material. The main reason for this improvement is thought to be the refinement of the microstructure.

(2) Charpy impact test The Charpy impact test was performed by the method according to the above-mentioned JISZ2242 using the U notch test piece. The test results are shown in Table 4.

  The Charpy impact value of the B material (comparative example) subjected to carbonitriding was not higher than that of the normally quenched A material (comparative example), but the C material had the same value as the A material.

(3) Test of Static Fracture Toughness Value FIG. 8 is a diagram showing a test piece of a static fracture toughness test. After introducing a precrack about 1 mm into the notch of this test piece, a static load by three-point bending was applied to determine the fracture load P. The following formula (I) was used for calculation of the fracture toughness value (K _Ic value). The test results are shown in Table 5.
K _Ic = (PL√a / BW ² ) {5.8−9.2 (a / W) +43.6 (a / W) ² −75.3 (a / W) ³ +77.5 (a / W ⁴ }… (I)

  Since the pre-crack depth is greater than the carbonitrided layer depth, there is no difference between the A material and B material of the comparative example. However, the C material of the present invention example was able to obtain a value about 1.2 times that of the comparative example.

(4) Static crushing strength test (measurement of fracture stress value)
As described above, the static crushing strength test piece had the shape shown in FIG. In the figure, a static crushing strength test was performed by applying a load in the P direction. The test results are shown in Table 6.

  The B material subjected to the carbonitriding process has a slightly lower value than the A material subjected to normal quenching. However, the C material of the present invention has a higher static crushing strength than the B material, and a level comparable to that of the A material is obtained.

(5) Aged dimensional change rate The measurement results of the aged dimensional change rate at a holding temperature of 130 ° C. and a holding time of 500 hours are shown in Table 7 together with the surface hardness and the retained austenite amount (0.1 mm depth).

  It can be seen that the C material of the example of the present invention is suppressed to half or less compared to the dimensional change rate of the B material having a large amount of retained austenite.

(6) Life test under lubrication mixed with foreign matter Using a ball bearing 6206, the rolling fatigue life was evaluated under lubrication mixed with foreign matter in which a predetermined amount of standard foreign matter was mixed. The test conditions are shown in Table 8, and the test results are shown in Table 9.

  Compared to the A material, the B material subjected to the conventional carbonitriding treatment was about 2.5 times longer, and the C material of the example of the present invention had a long life of about 2.3 times. Although the C material of the present invention example has less retained austenite than the B material of the comparative example, almost the same long life is obtained due to the intrusion of nitrogen and the influence of the refined microstructure.

  From the above results, the C material of the present invention example, that is, the bearing component manufactured by the heat treatment method of the present invention, has a long rolling fatigue life, which is difficult with conventional carbonitriding treatment, and improved crack strength. It was found that the three items of reduction of the aging dimensional change rate can be satisfied simultaneously.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a schematic sectional drawing which shows the rolling bearing in embodiment of this invention. It is a figure explaining the heat processing method in embodiment of this invention. It is a figure explaining the modification of the heat processing method in embodiment of this invention. It is a figure which shows the microstructure of a bearing component, especially an austenite grain. (a) is a bearing part of the example of the present invention, and (b) is a conventional bearing part. (A) shows the austenite grain boundary illustrated in FIG. 4 (a), and (b) shows the austenite grain boundary illustrated in FIG. 4 (b). It is a figure which shows the test piece of a static crushing strength test (measurement of a fracture stress value). It is the schematic of a rolling fatigue life tester. (a) is a front view, (b) is a side view. It is a figure which shows the test piece of a static fracture toughness test.

Explanation of symbols

  1 outer ring, 2 inner ring, 3 rolling element, 10 rolling bearing, 11 drive roll, 12 guide roll, 13 (3/4) "ball, 21 rolling fatigue life test piece, T1 carbonitriding temperature, T2 quenching heating temperature.

Claims (3)

The bearing component formed of steel having a composition of JIS standard SUJ2, after carbonitriding process in the carbonitriding temperature is a temperature above the A ₁ transformation point, cooled to a temperature below the A ₁ transformation point, then, the A ₁ Re-heated to a quenching heating temperature that is equal to or higher than the transformation point and lower than the carbonitriding temperature, and performs quenching,
The heat treatment method for bearing parts, wherein the quenching heating temperature is 800 ° C. or higher and 815 ° C. or lower. A bearing part that is heat-treated by the heat-treating method for a bearing part according to claim 1 and that has an austenite crystal grain number of 11 or more. A rolling bearing, wherein at least one of the outer ring, the inner ring, and the rolling element is a bearing component according to claim 2 .

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