Test specimen, method for preparing the test specimen, and method for performing a rolling fatigue test using the test specimen.

Abstract

Obtain a test specimen in which aggregated particles or separated particles are embedded in appropriate positions within the base material. [Solution] The present invention relates to a test specimen used in a thrust-type rolling fatigue test, comprising a base material and particles dispersed in the base material that simulate nonmetallic inclusions. These particles include aggregated particles formed by the aggregation of multiple primary particles, or separated particles formed by the separation of at least a portion of the aggregated particles. The aggregated particles or separated particles can be brought into close contact with the base material of the test specimen. Furthermore, a gap can be formed at the interface between the aggregated particles or separated particles and the base material of the test specimen.

Description

Technical Field

[0001] The present invention relates to a test piece used in a thrust type rolling fatigue test, a method for producing this test piece, and a method for performing a thrust type rolling fatigue test using the test piece.

Background Art

[0002] When parts made of steel such as bearings are used while being repeatedly subjected to fatigue under lubricated conditions, there is a risk of detachment starting from non-metallic inclusions (hereinafter referred to as "inclusions") contained in the steel. Inclusions are inevitably generated during the processes of steel refining, casting, and solidification, and inclusions that cannot be completely removed during these processes are contained in steel parts after subsequent rolling, forging, etc. Further, according to the research of the inventors of the present application, it has been found that gaps formed at the interface between inclusions and the base material affect detachment (for example, Non-Patent Document 1).

[0003] Conventionally, by embedding particles simulating inclusions in steel, detachment starting from inclusions and detachment due to gaps formed at the interface between inclusions and the base material have been evaluated. The form of such inclusions was mainly spherical.

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As forms of inclusions, there are also forms of aggregated particles and separated particles different from spherical ones. Therefore, there has been a demand for a test piece in which aggregated particles or separated particles are embedded at appropriate positions in the base material. [Means for solving the problem]

[0006] (1) A test specimen used in a thrust-type rolling fatigue test, characterized by comprising a base material and particles dispersed in the base material that simulate nonmetallic inclusions, wherein aggregated particles are formed by the aggregation of a plurality of primary particles, or separated particles are formed by the separation of at least a portion of the aggregated particles.

[0007] (2) The test piece according to (1) above, characterized in that the aggregated particles or the separated particles are in close contact with the base material of the test piece.

[0008] (3) The test piece according to (1) above, characterized in that a gap is formed at the interface between the aggregated particles or the separated particles and the base material of the test piece.

[0009] (4) A test specimen according to any one of (1) to (3) above, characterized in that when the sum of the base material and the aggregated particles or the separated particles is 100% by mass, the blending ratio of the aggregated particles or the separated particles is at least 0.05% by mass or more.

[0010] (5) A method for producing a test specimen used in a thrust-type rolling fatigue test, comprising: mixing base material particles constituting the base material of the test specimen with aggregated particles which are formed by the aggregation of multiple primary particles and simulate nonmetallic inclusions to produce a mixture; and subjecting the mixture to hot isostatic pressing so that the test specimen is used and the aggregated particles are in close contact with the base material particles. A method for producing a test specimen, characterized by producing a molded body.

[0011] (6) The method for producing a test specimen according to (5) above, characterized in that a molded body is subjected to tensile processing to produce a molded body that can be used as a test specimen and in which gaps are formed at the interface between the aggregated particles and the base material particles.

[0012] (7) The method for producing a test specimen according to (5) above, characterized in that the molded body is subjected to a stretching process by hot forging to produce a molded body which is used as a test specimen and in which separated particles are formed in which at least a portion of the aggregated particles have been separated, and a gap is formed at the interface between the separated particles and the base material particles.

[0013] (8) The method for producing a test specimen according to (7) above, characterized in that a molded body produced by the stretching process is subjected to hot isostatic pressing to produce a molded body that can be used as a test specimen and in which the separated particles are in close contact with the base material particles.

[0014] (9) A rolling fatigue test method characterized by performing the thrust-type rolling fatigue test using a test specimen described in any one of (1) to (4) above.

[0015] (10) A rolling fatigue test method characterized by performing the thrust-type rolling fatigue test using a test specimen prepared by any one of the preparation methods described in (5) to (8) above. [Effects of the Invention]

[0016] According to the present invention, it is possible to obtain a test specimen having aggregated particles, which are formed by the aggregation of multiple primary particles that simulate one of the states in which nonmetallic inclusions exist in steel, or separated particles, which are formed by the separation of at least a portion of these aggregated particles, within the base material. Since these inclusions are dispersed within the test specimen, it is possible to obtain a test specimen in which the inclusions are embedded in appropriate positions. Thrust-type rolling fatigue testing using this specimen allows for the evaluation of its effects on rolling fatigue and delamination. [Brief explanation of the drawing]

[0017] [Figure 1] This figure shows the configuration of the test specimen according to this embodiment. [Figure 2] This figure shows the configuration of a test specimen in a different embodiment. [Figure 3] This is a photograph showing a dispersed state of aggregated particles. [Figure 4] It is a flowchart showing a method for producing a test piece in a state where agglomerated particles are adhered to a base material. [Figure 5] It is a photograph showing an example of agglomerated particles. [Figure 6] It is a flowchart showing a method for producing a test piece in a state where a gap is formed at the interface between agglomerated particles and a base material. [Figure 7] It is a flowchart showing a method for producing a test piece in a state where separated particles are adhered to a base material. [Figure 8] It is a flowchart showing a method for producing a test piece in a state where a gap is formed at the interface between separated particles and a base material.

Embodiments for Carrying Out the Invention

[0018] (Configuration of Test Piece 1) The configuration of test piece 1 will be described using FIG. 1. FIG. 1 is a diagram showing the configuration of test piece 1. Test piece 1 is a ring-shaped test piece used in a thrust type rolling fatigue test (hereinafter simply referred to as "rolling fatigue test"), and is composed of a base material 10 and particles simulating inclusions. Test piece 1 has a thickness T in a direction orthogonal to the plane of FIG. 1. Note that the shape of test piece 1 may be any shape simulating a rolling component, and is not limited to the shape shown in FIG. 1.

[0019] (Configuration of Base Material 10) Base material 10 has a hole 11 at the central portion in the radial direction and is formed in a ring shape. The outer diameter Ro, inner diameter Ri, and thickness T (size in the direction orthogonal to the plane of FIG. 1) of base material 10 can be appropriately determined according to the conditions of the rolling fatigue test. The steel type of base material 10 can be appropriately selected, and for example, SUJ2 steel can be used. FIG. 2 is a diagram showing the configuration of another test piece 2 having a different shape from FIG. 1. Test piece 2 is a ring-shaped test piece, and the outer edge of test piece 2 has a major diameter: L O , minor diameter: S O and is formed in an elliptical shape, which is different from test piece 1. It is also possible to use test piece 2 instead of test piece 1.

[0020] The hardness (Rockwell hardness) of the base material 10 can be 55 [HRC] or higher, considering the load applied in the rolling fatigue test. Preferably, the hardness of the base material 10 is 58 [HRC] or higher, and more preferably 60 [HRC] or higher.

[0021] Means for adjusting the hardness of the base material 10 include, for example, quenching and tempering, or various surface hardening treatments such as carburizing, nitriding, carbonitriding, and high-frequency induction hardening. Prior to these heat treatments, normalizing and spheroidizing annealing can also be performed. Aging treatment can also be used as a means of adjusting hardness. In that case, normalizing or solution treatment to temporarily dissolve aging precipitate elements can be performed beforehand. Here, the means for adjusting hardness can be appropriately selected depending on the type of steel of the base material 10, and depending on the type of steel of the base material 10, the means for adjusting hardness can be omitted.

[0022] The base material 10 of test specimen 1 contains particles that simulate inclusions, dispersed within it. These particles include aggregated particles, which are formed by the aggregation of multiple primary particles, or separated particles, which are formed when at least a portion of the aggregated particles are separated and dispersed. Primary particles, in this context, are the smallest particles that cannot be divided into smaller particles. Separated particles include primary particles that make up aggregated particles, and aggregated particles (smaller particles than aggregated particles) formed by the aggregation of multiple primary particles. In this case, the base material 10 may contain only primary particles, only aggregated particles, or both primary particles and aggregated particles. The photograph in Figure 3 shows an example of a state where only aggregated particles are dispersed. Since aggregated and separated particles simulate inclusions, this point can be taken into consideration when determining the material of these particles. For example, Al2O3 can be used as the material for aggregated and separated particles.

[0023] The particle sizes of aggregated and separated particles can be determined by considering the diameter of any inclusions that may be present in the steel. For example, according to the example described in Japanese Patent Publication No. 2018-053291, the particle size of 30,000 mm of steel material is determined by extreme value statistics.2 Since the predicted maximum inclusion diameter is observed to be 150 μm or less, the particle size of aggregated particles can be set to 150 μm or less. Furthermore, as the maximum inclusion diameter decreases, separated particles can be used instead of aggregated particles. Inclusions observed in steel are at least 1 μm in size, and in many cases several μm or larger. Therefore, the particle size of primary particles can be determined by considering the smallest inclusions.

[0024] In this embodiment, the test specimen contains dispersed aggregated or separated particles that simulate inclusions, allowing for precise verification of the harmfulness of inclusions without strictly controlling their embedding location. That is, by dispersing the inclusions, a test specimen can be created in which inclusions are embedded at a desired location. It goes without saying that the desired location must be within the stress-loaded volume. As will be explained later in the method for preparing test specimen 1, four types of test specimens listed in Table 1 below can be prepared.

[0025] [Table 1]

[0026] The state in which aggregated particles or separated particles are in close contact with the base material 10, or the state in which gaps are formed at the interface between the aggregated particles or separated particles and the base material 10, can be confirmed using an optical microscope or a scanning electron microscope. The interface between the aggregated particles or separated particles and the base material 10 can be observed, for example, in the state shown in Fig. 1 described in "Materials and Processes (CAMP-ISIJ) Vol.22(2009)-1297". That is, by checking the interface in a state in which the entire aggregated particles or separated particles can be seen within the observation screen, it is possible to understand the state in which the aggregated particles or separated particles are in close contact with the base material 10, or the state in which gaps are formed at the interface between the aggregated particles or separated particles and the base material 10.

[0027] Here, when applying rolling fatigue to particles (aggregated particles or separated particles) that simulate inclusions, it is necessary that the particles (aggregated particles or separated particles) be positioned directly below the trajectory of the rolling element in the rolling fatigue test (corresponding to the "desired position" mentioned above). Since the technical concept of the present invention is to disperse a large number of particles (aggregated particles or separated particles) in the base material 10, the test piece 1 has particles (aggregated particles or separated particles) at the desired position.

[0028] (Method for preparing test specimen 1) The method for preparing the four types of test specimens (the first to fourth test specimens shown in Table 1 above) is described below. The first, third, and fourth test specimens have the shapes shown in Figure 1, and the second test specimen has the shape shown in Figure 2. Therefore, the reference numerals for the first, third, and fourth test specimens are 1, and the reference numeral for the second test specimen is 2.

[0029] (Method for preparing the first test specimen 1) The method for preparing the first test specimen 1, in which aggregated particles adhere closely to the base material, will be explained using the flowchart shown in Figure 4.

[0030] In step S101, the base material particles constituting the base material 10 are prepared. Since the base material 10 is formed from the steel grade described above, the steel grade described above is also used for the base material particles. As the base material particles, for example, gas atomized particles obtained by classification after producing gas atomized particles by the gas atomization method, which have a particle size of a predetermined size or less, can be used. The predetermined particle size can be, for example, 300 μm, taking into consideration the formability for forming into the shape of a test piece. Gas atomized particles with a particle size of a predetermined size or less can be used to suppress the retention of voids during processing by the hot isostatic pressing method described later.

[0031] In step S102, aggregated particles are prepared to simulate inclusions. Specifically, after producing granules by spray freeze granulation, granules with a particle size below a predetermined size obtained by classification can be used as aggregated particles. The particle size of the aggregated particles can be determined considering the diameter of the inclusions; for example, the predetermined particle size can be set to 150 μm. Note that aggregated particles (an example) shown in Figure 5 can be obtained by spray freeze granulation.

[0032] In step S103, the base material particles obtained in step S101 and the agglomerated particles obtained in step S102 are mixed. This allows for the dispersion of a large number of agglomerated particles within the base material particles. The mixing ratio of base material particles and agglomerated particles can be appropriately determined considering the test subject of the rolling fatigue test. For example, the mixing ratio of base material particles can be set to 99.95 to 99.80 mass%, and the mixing ratio of agglomerated particles can be set to 0.05 to 0.20 mass%. While this mixing ratio is a guideline, if the mixing ratio of agglomerated particles exceeds 0.20 mass%, the dispersion density of agglomerated particles becomes too high, which may cause interactions between the agglomerated particles. On the other hand, if it is necessary to consider interactions between agglomerated particles, the mixing ratio of agglomerated particles can be set to 0.20 mass% or higher.

[0033] In step S104, the mixture obtained in step S103 is subjected to processing by hot isostatic pressing (HIP) (hereinafter referred to as "HIP processing"). In HIP processing, a molded body of the mixture can be produced by pressurizing the mixture at a predetermined temperature and pressure. The predetermined temperature can be, for example, 1150°C or higher, and the predetermined pressure can be, for example, 140 MPa or higher. HIP processing compresses the base material particles and aggregated particles, creating a state where there are no gaps at the interface between the base material particles and aggregated particles.

[0034] In step S105, the molded body obtained in step S104 is roughly machined into the shape of the first test piece 1. In step S106, the molded body roughly machined in step S105 is heat-treated to adjust the first test piece 1 to the desired hardness. In step S107, the molded body obtained in step S106 is finished. The finishing process involves surface grinding and mirror polishing. Surface grinding removes the oxide scale generated by the heat treatment. Furthermore, after determining the particle depth by ultrasonic testing described later, the depth from the test piece surface to the particles can be adjusted by surface grinding. Mirror polishing further provides a finish suitable for rolling fatigue testing. In the first test piece 1, numerous aggregated particles simulating inclusions are dispersed in the base material of the first test piece 1, and no gaps are formed at the interface between the base material and the aggregated particles. A rolling fatigue test can then be performed on a test object like the first test piece 1.

[0035] (Method for preparing the second test specimen 2) The method for preparing the second test specimen 2, in which a gap is formed at the interface between the aggregated particles and the base material, will be explained using the flowchart shown in Figure 6. In the flowchart shown in Figure 6, the same reference numerals are used for the same processes as those described in the flowchart shown in Figure 4, and detailed explanations are omitted.

[0036] The processes from step S101 to step S104 result in a molded body that has undergone HIP processing. In this molded body, as described above, there are no gaps at the interface between the base material particles and the aggregated particles.

[0037] In step S108, the molded body obtained in step S104 is subjected to tensile processing. This tensile processing is performed to intentionally create gaps at the interface between the base material particles and aggregated particles inside or on the surface of the molded body. In tensile processing, it is sufficient to apply tensile force to the molded body to create gaps at the interface, and the specific tensile processing can be determined as appropriate.

[0038] For example, in tensile processing, a molded body that has undergone HIP processing can be processed into a disc-shaped ring, and a pair of pins can be used to apply tensile force to the inner diameter of the disc-shaped ring. The pins can be formed using, for example, SUJ2, and their hardness can be adjusted to about 60 HRC by quenching and tempering. By forming a pair of recesses in the inner diameter of the disc-shaped ring and engaging the pins with the inner diameter at a position on the circumference perpendicular to the pair of recesses, the distance between the pair of pins can be increased, thereby applying tensile force to the vicinity of the recesses in the inner diameter of the disc-shaped ring. By understanding the effect of tensile force on the interface between the base material particles and aggregated particles through experiments or simulations beforehand, the tensile force required to create a gap at the interface between the base material particles and aggregated particles can be determined.

[0039] After the tensile processing in step S108, the second test specimen 2 is obtained by performing the processes in steps S106, S105, and S107. The major axis Lo and minor axis So of the second test specimen 2 are determined according to the amount of tensile processing and are not specifically defined. The inner diameter Ri and thickness T (size in the direction perpendicular to the plane of the paper in Figure 2) can be appropriately determined according to the conditions of the rolling fatigue test, as long as the diameter of the raceway is secured. In the second test specimen 2, a large number of aggregated particles simulating inclusions are dispersed in the base material of the second test specimen 2, and a gap is formed at the interface between the base material and the aggregated particles. Then, a rolling fatigue test can be performed on a test object such as the second test specimen 2.

[0040] (Method for preparing the third test specimen 1) The method for preparing the third test specimen 1, in which the separated particles are in close contact with the base material, will be explained using the flowchart shown in Figure 7. In the flowchart shown in Figure 7, the same reference numerals are used for the same processes as those described in the flowchart shown in Figure 4, and detailed explanations are omitted.

[0041] The processes from step S101 to step S104 result in a molded body that has undergone HIP processing. In this molded body, as described above, there are no gaps at the interface between the base material particles and the aggregated particles.

[0042] In step S109, the molded body obtained in step S104 is subjected to stretching by hot forging. This stretching is performed to separate the aggregated particles contained in the molded body into separate particles and to create gaps at the interface between the base material of the molded body and the separate particles. In aggregated particles, multiple primary particles are bonded together in an incomplete state, so by applying external force through hot forging, the aggregated particles can be separated into separate particles (primary particles or aggregated particles).

[0043] By adjusting the amount of stretching during the stretching process, the spacing between separated particles can also be adjusted. The degree of separation of the separated particles can also be adjusted through the amount of stretching, so that not all aggregated particles separate into primary particles. Furthermore, since the external force from the stretching process acts on the base material of the molded body, the base material can be separated from the separated particles, creating a gap at the interface between the base material and the separated particles. It should be noted that forming methods other than stretching may be used as forging methods. Examples of such forming methods include upsetting and part forming.

[0044] In step S110, the molded body obtained in step S109 is subjected to HIP processing. As described above, in the molded body obtained in step S109, a large number of separation particles simulating inclusions are dispersed in the base material of the molded body, and gaps are formed at the interface between the base material and the separation particles. By performing HIP processing on the molded body in this state, the base material can be made to adhere closely to the separation particles, and the gaps formed at the interface between the base material and the separation particles can be eliminated.

[0045] After the HIP processing in step S110, the third test specimen 1 is obtained by performing the processes in steps S106, S105, and S107. In the third test specimen 1, separation particles simulating inclusions are dispersed in the base material of the third test specimen 1, and no gaps are formed at the interface between the base material and the separation particles. Then, a rolling fatigue test can be performed on a test object such as the third test specimen 1.

[0046] (Method for preparing the fourth test specimen 1) The method for preparing the fourth test specimen 1, in which a gap is formed at the interface between the separated particles and the base material, will be explained using the flowchart shown in Figure 8. In the flowchart shown in Figure 8, the same reference numerals are used for the same processes as those described in the flowcharts shown in Figures 4 and 7, and detailed explanations are omitted.

[0047] The processes from step S101 to step S104 yield a molded body that has undergone HIP processing, and the process in step S109 yields a molded body after stretching. In the molded body after stretching, the aggregated particles separate into separated particles and are dispersed in large numbers, and gaps are formed at the interface between the base material particles and the separated particles.

[0048] After the stretching process in step S109, the fourth test specimen 1 is obtained by performing the processes in steps S105, S106, and S107. In the fourth test specimen 1, separation particles simulating inclusions are dispersed in the base material of the fourth test specimen 1, and gaps are formed at the interface between the base material and the separation particles. Then, a rolling fatigue test can be performed on a test object such as the fourth test specimen 1.

[0049] As described above, the first to fourth test specimens 1 are in a state where a large number of aggregated or separated particles, which are simulated as inclusions, are dispersed, so that test specimens with aggregated or separated particles placed at the desired positions can be obtained, and rolling fatigue tests can be performed using such test specimens. Furthermore, by using the first to fourth test specimens 1 described above, rolling fatigue tests can be performed that take into account the presence or absence of gaps at the interface between the inclusions and the base material.

[0050] By using ultrasonic testing to pre-determine the location of particles (inclusions) within specimen 1, and then adjusting the depth of the particles (inclusions) by grinding specimen 1 based on this information, and finishing with mirror polishing, the trajectory of the rolling fatigue test described later is set to pass directly over the particles (inclusions). After performing the rolling fatigue test, the rolling fatigue behavior (cracks, etc.) of the particles (inclusions) and their surroundings can be observed. Furthermore, by causing delamination from the particles (inclusions), the relationship between the size of the particles (inclusions) and the lifespan of specimen 1 can be verified. As an indicator of the lifespan of specimen 1, for example, L 10 Lifespan can be used. 10 Lifetime refers to the lifetime [number of cycles] at which, when peel tests are performed on multiple identical samples (test piece 1) under the same conditions, peeling does not occur in 90% of the samples. On the other hand, in rolling fatigue tests, lifetime can also be determined for each particle (inclusion) size. In rolling fatigue tests, the center of the rolling element's track width can be positioned directly above the particles (inclusions), or the position can be offset from the center of the track width directly above the particles (aggregated particles or separated particles). When rolling fatigue tests are performed when there are a sufficient number of particles (inclusions) in the small test piece 20 (when the proportion of aggregated particles is 0.08% or more), or when rolling fatigue tests are performed on particles (inclusions) that are partially exposed on the surface of the test piece, prior identification of the particle (inclusion) location by ultrasonic testing may not be necessary.

[0051] (Rolling fatigue test) For the rolling fatigue test, a known test method can be employed. Specifically, a race (model number 51305) of a SUJ2 single thrust bearing can be used as the upper plate, and test piece 1 can be used as the lower plate. Multiple rolling elements can be arranged at equal intervals on a circular track between the upper and lower plates. A load can then be applied from the rolling elements to test piece 1 so that a predetermined maximum Hertz contact stress (e.g., 5.26 [GPa]) is applied. [Explanation of Symbols]

[0052] 1,2: Test specimens (1st to 4th test specimens), 10: Base material, 11: Hole

Claims

1. A test specimen used in thrust-type rolling fatigue tests, Base material and The particles dispersed in the matrix material simulate nonmetallic inclusions, and consist of aggregated particles formed by the aggregation of multiple primary particles, or separated particles formed by the separation of at least a portion of the aggregated particles. A test specimen characterized by having the following features.

2. The test piece according to claim 1, characterized in that the aggregated particles or the separated particles are in close contact with the base material of the test piece.

3. The test piece according to claim 1, characterized in that a gap is formed at the interface between the aggregated particles or the separated particles and the base material of the test piece.

4. When the sum of the base material and the aggregated particles or separated particles is 100% by mass, the proportion of the aggregated particles or separated particles is at least 0.05% by mass. A test specimen according to any one of features 1 to 3.

5. A method for preparing test specimens used in thrust-type rolling fatigue tests, A mixture is produced by mixing the base material particles that constitute the base material of the test specimen with aggregated particles that simulate nonmetallic inclusions, which are formed by the aggregation of multiple primary particles. By subjecting the mixture to hot isostatic pressure processing, a molded body is produced in which the aggregated particles are in close contact with the base material particles and can be used as a test piece. A method for preparing a test specimen characterized by the following.

6. The method for producing a test specimen according to claim 5, characterized in that a molded body is subjected to tensile processing to produce a molded body that can be used as a test specimen and in which gaps are formed at the interface between the aggregated particles and the base material particles.

7. The method for producing a test specimen according to claim 5, characterized in that the molded body is subjected to a stretching process by hot forging to produce a molded body that can be used as a test specimen, in which separated particles are formed from at least a portion of the aggregated particles, and a gap is formed at the interface between the separated particles and the base material particles.

8. The method for producing a test specimen according to claim 7, characterized in that a molded body used as a test specimen is produced by hot isostatic pressing on the molded body produced by the stretching process, wherein the separated particles are in close contact with the base material particles.

9. A rolling fatigue test method characterized by performing the thrust-type rolling fatigue test using a test piece described in any one of claims 1 to 3.

10. A rolling fatigue test method characterized by performing the thrust-type rolling fatigue test using a test piece prepared by the manufacturing method described in any one of claims 5 to 8.

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