JP4152281B2 - Heat treatment method for bearing parts - Google Patents

Description

  More specifically, the present invention relates to a bearing component, a heat treatment method, a heat treatment apparatus, and a rolling bearing that are used in a part that requires a long life against rolling fatigue and requires high cracking resistance and aging change. Relates to a bearing component used in a reduction gear, a drive pinion and a transmission bearing, a heat treatment method thereof, a heat treatment apparatus, and a rolling bearing.

As a heat treatment method for providing a long life against rolling fatigue of a bearing component, there is 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. (For example, JP-A-8-4774, JP-A-11-101247). By using this carbonitriding treatment, 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 and needs to be kept at a high temperature for a long time, it is difficult to improve the cracking strength. In addition, an increase in the dimensional change rate due to increase in retained austenite is also a problem.

  On the other hand, in order to secure a long life against rolling fatigue, improve the cracking strength, and prevent an increase in the rate of dimensional change over time, it is possible to carry out it by steel alloy design. 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. Therefore, a bearing component having a long life against rolling fatigue, high strength, and high dimensional stability is required.

  The present invention provides a bearing component, a heat treatment method, a heat treatment apparatus, and a rolling bearing that have a long life against rolling fatigue, have high cracking strength, and suppress an increase in the rate of dimensional change over time. With the goal.

The heat treatment method for bearing parts of the present invention is a heat treatment method for bearing parts made of JIS standard SUJ2 material, heat treated at a temperature exceeding the A1 transformation point of the steel of the bearing component, and then cooled to a temperature below the A1 transformation point. Thereafter, the reheating temperature within the quenching temperature range above the A1 transformation point and below the heat treatment temperature is set to 3 ° C./min or more at a position 2 mm deep from the surface of the bearing part. Reheating, quenching is performed, and the quenching is performed using a cooling medium having a quenching intensity of 0.1 cm-1 or more, and the quenching is performed at a position 2 mm from the surface of the bearing component, and from the reheating temperature to 400 ° C. the average cooling rate of decrease is performed under conditions such that a 20 ° C. / sec or more. The heat treatment at a temperature exceeding the A1 transformation point is carbonitriding, and the quenching temperature range is a temperature range of 790 ° C. to 830 ° C. (Claim 1).

  With this configuration, after heating to the temperature of the heat treatment, cooling to a temperature lower than the A1 transformation point, and then heating to a temperature lower than the temperature of the heat treatment for final quenching, the austenite grain size is made fine. be able to. It should be particularly noted that the interior of the bearing component can be made as fine as the surface layer by the above method. Here, the austenite grains refer to austenite grains obtained from the traces of the grain boundaries of the austenite grains before quenching, which remain after quenching.

Further, by using a cooling medium having a quenching intensity of 0.1 cm ⁻¹ or more for the second low-temperature quenching, a hardness sufficient to ensure durability and the like can be obtained.

In order to obtain a sufficient hardness, it is particularly important to use the quenching cooling medium described above. When the quenching intensity of the cooling medium is less than 0.1 cm ⁻¹ , at a position of 2 mm from the surface of the bearing component, It becomes difficult to make the average cooling rate from the heating temperature to 400 ° C lower than 20 ° C / second, and sufficient hardness cannot be obtained. For this reason, the quenching intensity of the cooling medium is set to 0.1 cm ⁻¹ or more. More preferably at 0.13 cm ^-1 or more.

  Usually, when trying to obtain very fine austenite grains, a mixture of fine grains and coarse grains tends to be formed. The phenomenon that tends to be mixed when trying to obtain fine austenite grains is explained thermodynamically because the interfacial energy at the austenite grain boundaries increases. When austenite grains are mixed, the mechanical properties are determined by coarse grains, and even if fine grains are obtained in the most corners, improvement in mechanical properties cannot be expected so much.

  By using the above production method and avoiding mixed grains to obtain fine austenite grains, the Charpy impact value, fracture toughness value, crack strength, rolling fatigue life, and the like can be improved. In addition, the structure | tissue which consists of a substantially uniform grain instead of a mixed grain is called a sized structure or a sized structure. The distinction between sized and mixed particles will be explained later.

  According to this method, in addition to obtaining fine grained austenite grains, the austenite grain boundaries in the carbonitriding process and the austenite grains in the final quenching are further cooled by cooling to a temperature at which the austenite transforms, for example. You can make the world unrelated. In the austenite grain boundary in the carbonitriding process, there is a region where carbides and nitrides are precipitated along the grain boundary. For this reason, when it is handed over to the austenite grain boundary at the time of quenching, carbide and nitride remain along the austenite grain boundary in that region. Such carbides and nitrides along the austenite grain boundaries tend to be edged at the edges, increasing the stress concentration and tending to become crack initiation points in rolling fatigue. According to the heat treatment method of the present invention, during the carbonitriding process, carbides and nitrides are precipitated at the austenite grain boundaries at that time. However, the austenite grain boundary formed at the time of austenite transformation after austenite transformation is usually irrelevant to the austenite grain boundary during carbonitriding. For this reason, the carbides and nitrides generated during the carbonitriding process tend to be rounded at the edges to reduce the (carbide / matrix) interface. As a result, carbides and nitrides are less likely to act as crack starting points while contributing to wear resistance and improved deformation resistance at high temperatures.

  In addition, the steel for bearing parts is steel normally used for bearing parts, Comprising: It is steel used by applying heat processing, such as normal hardening.

Further , since the austenite crystal grains are reheated and quenched to a temperature at which growth of the austenite crystal grains is difficult to occur, the austenite grain size can be reduced.

  Next, embodiments of the present invention will be described with reference to the drawings. 1 and 2 show a heat treatment method according to an embodiment of the present invention. FIG. 1 is a heat treatment pattern showing a method of performing primary quenching and secondary quenching, and FIG. 2 shows a method of cooling the material to below the A1 transformation point temperature during quenching, and then reheating and finally quenching. It is the heat processing pattern shown. In these drawings, in the treatment T1, carbon and nitrogen are diffused in the steel base and the carbon is sufficiently dissolved, and then cooled to less than the A1 transformation point. Next, in process T2 in the figure, reheating is performed at a lower temperature than process T1, and oil quenching is performed therefrom.

The points to be noted in FIGS. 1 and 2 are that (1) the quenching intensity (cooling ability: JIS standard) of the cooling medium in quenching from the temperature T2 is 0.1 cm ⁻¹ or more, or from the temperature T2. When quenching, two points of an average cooling rate of 20 ° C./second or more at a position 2 mm from the surface of the bearing part until the temperature drops from the heating temperature to 400 ° C., and (2) temperature rise at the time of heating to T2 The temperature speed is set to 3 ° C./min or more at a position 2 mm from the surface of the bearing part. (1) makes it possible to obtain sufficient hardness to ensure durability and the like, and (2) makes it possible to obtain fine austenite grains free of mixed grains. It is also important to obtain an appropriate amount of retained austenite.

  FIG. 3 is a diagram for explaining the average cooling rate. FIG. 3 shows a case where the heating temperature T2 is 800 ° C. The temperature increase rate is an average temperature increase rate from A1 point to T2. Further, the average cooling rate from the heating temperature T2 to the temperature lower by 400 ° C. is 20 ° C./second or more as long as the point A shown in FIG. 3 is 20 seconds or less on the horizontal axis.

  According to the above heat treatment, the structure is refined due to cooling after carbonitriding and then reheating to low temperature and quenching. Therefore, carbonitriding the surface layer and improving crack strength And the aged dimensional change rate can be reduced. According to the heat treatment method of the present invention, 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 component subjected to the above heat treatment has a long life against rolling fatigue, can improve the cracking strength, and can also reduce the rate of dimensional change over time.

  FIG. 4 is a view showing a continuous heat treatment apparatus (continuous furnace) 30 that heats the bearing parts after the carbonitriding process to a temperature T2. The heat treatment apparatus 30 is provided with an oil tank 31 storing oil 33 for oil quenching on the rear side, and includes a quenching guide 27 for immersing the bearing component 21 in the oil tank 31. The bearing component 21 is placed from the front side on the conveying member 25 that continuously rotates in an endless track shape, and is conveyed to the main body portion 26 through the preheating portion 28 and the like. Heating means such as a radiant tube 23 is disposed in the main body 26 to heat the bearing component 21. The heating means may be a heater or a gas burner. The atmosphere in the furnace is preferably a non-oxidizing atmosphere, but may not be a non-oxidizing atmosphere.

The capacity | capacitance of the furnace main-body part 26, etc. are set so that said temperature increase rate may be ensured. When the bearing part 21 is heated uniformly, the bearing part 21 is made to reach the rear side of the furnace body 26. The bearing parts 21 homogeneously heated to T2 are individually and continuously dropped into the oil tank 31 through the quenching guide 27. The bearing component 21 is accommodated in a gutter 35 disposed in the oil tank 31 and taken out together. For quenching, it is not necessary to use oil, and various cooling media can be used. Any cooling medium, such as molten salt, water containing a surfactant, or mere water, may be used as long as the quenching intensity can be 0.1 cm ⁻¹ or more.

  Even with a batch system, the above temperature increase rate can be secured, but if the heat treatment of countless bearing parts is accommodated in a bucket and performed together, a sufficient cooling rate may not be obtained depending on the product position. . The continuous heat treatment apparatus described above can arrange a large number of products with the same quality. In the oil tank or the cooling medium storage tank, the temperature of the cooling medium is kept within a predetermined range so that the temperature of the cooling medium does not exceed a predetermined upper limit according to the situation where the bearing parts are continuously immersed and quenched. It is desirable to provide a cooling device or the like. Further, when the cooling device is not provided, the target bearing component has a predetermined capacity so that the cooling medium does not exceed a predetermined upper limit temperature while the continuous reheating and quenching are completed. It is desirable.

  FIG. 5A shows the austenite grain size of the bearing steel to which the heat treatment pattern shown in FIG. 1 is applied. For comparison, FIG. 5B shows the austenite grain size of the bearing steel obtained by the conventional heat treatment method. The steel materials used are all JIS standard SUJ2 materials (1.0 wt% C-0.25 wt% Si-0.4 wt% Mn-1.5 wt% Cr). FIGS. 6A and 6B show the austenite grain sizes illustrated in FIGS. 5A and 5B. 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.5 (a) was 5.6 micrometers as a result of measuring by the intercept method.

  Next, the influence of the heating rate on the heating temperature T2 on the mixed grain formation of austenite crystal grains will be described. JIS standard SUJ2 material was used as a test body, and the temperature was increased to 800 ° C. while changing the temperature rising rate to temperature T2 in accordance with the heat pattern shown in FIG. After that, oil quenching was performed and austenite grains were investigated. The results are shown in FIGS. FIGS. 8A to 8D are diagrams schematically showing FIGS. 7A to 7D.

  FIGS. 8 (a) and (b) are the ones heated at a heating rate of 1 ° C./min and 2.5 ° C./min, respectively, but coarse austenite grains are generated in the fine austenite grains. . Coarse austenite grains grow while merging fine austenite grains. Despite being coarse, the grain boundary has a continuous portion with a small curvature such that the grain boundary of fine austenite remains as it is.

  8 (c) and 8 (d), the degree of difference between the particle size of the large category and the particle size of the small category in the mixed grains is reduced, resulting in a structure that can be said to be sized. The mixed grain structure follows the definition of JISG0551. As described above, very fine austenite grains are obtained when the heating rate is 3 ° C./min or more, and when the heating rate is less than 3 ° C./min, very large coarse grains are obtained. grow up. As a result, mechanical properties such as durability deteriorate. In order to avoid such a mixed grain structure, the above-mentioned limitation of the temperature rising rate is very effective.

Next, using the same steel material, the relationship between oil quenching intensity (cooling ability) and quenching hardness was investigated. The steel material of the test body was a JIS standard SUJ2 material, the shape of the test body was a ring shape, the outer diameter was 60 mm, the length was 10 mm, and the inner diameter, and hence the wall thickness, was changed. The wall thickness was changed in the range of 2 mm to 8 mm. As the heat treatment pattern, the pattern shown in FIG. 1 was adopted, and the quenching intensity was changed by changing the cooling oil in quenching from the temperature T2. Hot oil was used as the oil with low quenching intensity, cold oil was used as the high oil, and semi-hot oil was used as the intermediate oil. The quenching intensity was changed in the range of 0.1 to 0.14 cm ⁻¹ . After quenching, after tempering at 180 ° C., the hardness was measured. The hardness is an average Vickers hardness (HV) at a depth position of 0.2 mm from the surface of the center portion of the outer peripheral length of each ring specimen. The N number was 3. The results are shown in Table 1.

According to Table 1, if the intensity is 0.1 cm ⁻¹ or more, HV750 can be obtained with a thickness of 4 mm, and HV600 can be obtained with a thickness of 8 mm. Therefore, if the intensity is maintained at 0.1 cm ⁻¹ or more, quenching sufficient to ensure mechanical properties can be performed.

Next, 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 as the material for heat treatment. did.
(A material: comparative example): Only normal quenching (without carbonitriding).
(B material: comparative example): quenching as it is after carbonitriding (conventional carbonitriding quenching).
(C material: Example of the present invention): Bearing steel subjected to the heat treatment pattern of FIG.

(1) Rolling fatigue life Table 2 and FIG. 9 show the test conditions of the rolling fatigue life test and a schematic diagram of the test apparatus. That is, the driving wheel 11 is rotationally driven in a state in which the cylindrical test piece 1 of φ12 × L22 is supported between the hard sphere 13 in contact with the guide wheel 12 and the driving wheel 11, and the life at that time (L10 life) is increased. This was done by measuring. Table 3 shows the rolling fatigue life test results.

  According to Table 3, the B material of the comparative example shows 1.6 times the L10 life of the A material that was subjected only to normal quenching in the comparative example (the life that one of the 10 test pieces breaks), and carburized. The effect of extending the life by nitriding is recognized. On the other hand, the C material of the present invention example has a long life of 2.7 times that of the B material and 5.0 times that of the 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 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 was about 1.5 times as high as the A material. .

(3) Test of static fracture toughness value The test piece shown in FIG. 10 was used as a test piece for static fracture toughness test, and after introducing a precrack about 1 mm, a static load by three-point bending was applied, The breaking load P was determined. The following formula (I) was used to calculate the fracture toughness value (K _I c value). However, B in the formula (I) is the thickness of the test piece. The test results are shown in Table 5.
_{K I c = (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 larger 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 Crush Strength Test A static crush strength test piece having the shape shown in FIG. 11 was used. 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 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 foreign matter lubrication Using a ball bearing 6206, the rolling fatigue life under foreign matter lubrication in which a predetermined amount of standard foreign matter was mixed was evaluated. 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 has less retained austenite than the B material of the comparative example, it has a substantially equivalent long life due to the intrusion of nitrogen and the influence of the refined microstructure.

  From the above results, the material C of the example of the present invention, that is, the bearing part produced by the heat treatment method of the present invention, has a long rolling fatigue life, which is difficult with the conventional carbonitriding process, and an 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 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 explaining the average cooling rate in the heat processing method of embodiment of this invention. It is a figure which shows the example of the heat processing apparatus used with the heat processing method of embodiment of this invention. It is a figure which shows the microstructure of a bearing component, especially an austenite grain, (a) is a bearing component of the example of this invention, (b) is a conventional bearing component. (A) shows the former austenite grain boundary illustrated in FIG. 5 (a), and (b) shows the former austenite grain boundary illustrated in FIG. 5 (b). It is a figure which shows the microstructure of bearing components, especially an austenite grain, (a) is temperature rising rate 1 degree-C / min, (b) is temperature rising rate 2.5 degree-C / min, (c) is temperature rising rate 7 respectively. .5 ° C./min, (d) is a rate of temperature increase of 25 ° C./min. The illustrated austenite grain boundaries are shown, (a) in FIG. 7 (a), (b) in FIG. 7 (b), (c) in FIG. 7 (c), (d) in FIG. 7 (d), respectively. Correspond. 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. It is a figure which shows the test piece of a static crushing strength test.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Rolling fatigue life test piece, 11 Drive wheel, 12 Guide wheel, 13 Hard ball, T1 Carbonitriding temperature, T2 Quenching heating temperature, 21 Bearing parts, 23 Radiant tube, 25 Conveying member, 26 Main part, 27 Quenching guide , 28 Preheating part, 30 Heat processing apparatus, 31 Oil tank, 33 Quenching oil, 35 bucket.

Claims (1)

A heat treatment method for a bearing component made of JIS standard SUJ2 material, after heat treatment at a temperature exceeding the A1 transformation point of the steel of the bearing component, cooling to a temperature below the A1 transformation point, and thereafter at a temperature equal to or higher than the A1 transformation point Reheating is performed at a reheating temperature included in a quenching temperature range lower than the temperature of the heat treatment so that a temperature rising rate at a depth of 2 mm from the surface of the bearing part is 3 ° C./min or more, and quenching is performed. The quenching is performed using a cooling medium having a quenching intensity of 0.1 cm -1 or more, and the quenching is performed at a position 2 mm from the surface of the bearing component, and an average cooling rate until the temperature decreases to 400 ° C. from the reheating temperature. Is performed under the condition of 20 ° C./second or more ,
A heat treatment carbonitriding treatment at a temperature exceeding the transformation point A1, the quenching temperature range is a temperature range of 790 ℃ ~830 ℃, the heat treatment method of the bearing components.

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