JP7516295B2 - Test piece for rolling fatigue test, rolling fatigue test method using said test piece, and method for manufacturing test piece - Google Patents

Description

The present invention relates to test pieces and the like used in rolling fatigue tests.

Despite being used under proper lubrication conditions, there have been reports of short-life flaking, in which bearings fail earlier than expected, and this is an obstacle to the realization of smaller, lighter bearing designs. It is known that this type of flaking is caused by non-metallic inclusions in the steel. Non-metallic inclusions are inevitably produced during the refining, casting, and solidification processes of steel, and any that cannot be completely removed during these processes are contained in the steel. Furthermore, non-metallic inclusions remain and are contained in the bearing material after further downstream processes such as rolling and forging. Flaking that originates from these non-metallic inclusions usually begins slightly inside the part, rather than on the surface. This is due to high shear stress occurring slightly inside the bearing race when the bearing race and rolling elements (balls, rollers, etc.) come into rolling contact.

As described above, because flaking due to fatigue progresses inside the component, direct observation of rolling fatigue is difficult. Furthermore, it is rare for the inclusions that caused the flaking to be found on the fracture surface after flaking has occurred. Therefore, although there is no doubt that inclusions affect bearing life, the direct relationship between the size, shape, dispersion state, chemical composition, etc. of the inclusions (hereinafter referred to as "properties of inclusions") and life has not yet been clarified.

The _L10 life is widely used as a life indicator for rolling bearings. The _L10 life refers to the life when 90% of the test pieces do not flake when a life test is performed on multiple samples under the same conditions. In other words, the life of a bearing is usually evaluated probabilistically. In order to realize a steel that does not cause short-life flakes, it is necessary to directly verify the relationship between the properties of inclusions and life or rolling fatigue, rather than relying on probability theory, and to clarify the quantitative relationship between the two.

On the other hand, in Non-Patent Document 1, SUJ2 steel was artificially created with many cavities remaining and dispersed within the steel, the rolling fatigue crack behavior for these cavities was observed, and the results of comparing this behavior with stress simulations for cavities or general inclusions showed that harmfulness is aggravated when there are gaps (voids) between the inclusions and the matrix.

In this regard, Non-Patent Document 2 discloses a technology that significantly improves rolling fatigue life by subjecting steel to hot isostatic pressing (HIP) processing to close the gaps between inclusions and the parent phase.

According to Non-Patent Document 2, it is clear that the state of the inclusion-parent phase interface is a factor in changing the lifespan. However, after spalling has occurred, it is practically difficult to verify whether there was a gap around the inclusion beforehand, and it was not possible to estimate the contribution of the gap to the length of the lifespan.

Therefore, in order to understand the relationship between the properties of inclusions and life or rolling fatigue, it is essential to align the state of the inclusion-parent phase interface, which has a strong influence on life, in a constant state prior to rolling fatigue testing, i.e., to fix the interface conditions. In addition, it is presumed that it is relatively large inclusions that cause short-life spalling in bearings, and it is also necessary to consider that such inclusions are unlikely to exist in the extremely small stress-loaded volume within the limited number of evaluation rolling fatigue test pieces.

In recent years, there has been a demand for bearing steel that not only pursues longer life, but also improves the reliability of parts by suppressing sudden short-life flaking. Therefore, in order to realize such bearing products, it is necessary to clarify the relationship between the properties of inclusions and life, and to understand the properties of inclusions that are the starting point of short-life flaking.

However, to do this, it is necessary to understand in advance the properties of the inclusions, which are factors that affect rolling fatigue, the state of the interface between the parent phase and the inclusion, and their location in the steel, and then to conduct rolling fatigue tests on the inclusions and verify the properties of the inclusions and their lifespan, or, if they have not spalled, the properties of the inclusions and the fatigue state around them, in a one-to-one comparison.

This is because, although the various factors mentioned above are highly likely to affect the life span, it is difficult to isolate and verify their effects after testing. Patent Document 1 is known as a technique for conducting rolling fatigue tests with the properties of the target inclusions known in advance, and verifying their effects on life span and rolling fatigue through one-to-one comparison.

In this patent, a test piece is prepared in which an artificial inclusion is artificially contained therein, and then the position of the inclusion is adjusted and a rolling fatigue test is performed. In the embodiment, a method for preparing a test piece using _{spherical Al2O3} and a rolling fatigue test method are shown, which assumes spherical inclusions among inclusions in actual steel materials (spherical inclusions among D-type inclusions in the classification of JIS standard JIS G 0555).

JP 2018-102759 A

Takeshi Fujimatsu, Kazuhiko Hiraoka, and Atsuyuki Yamamoto, "Crack Initiation Behavior from Internal Defects in Rolling Contact Fatigue of High Carbon Chromium Bearing Steel," Iron and Steel Institute of Japan, Vol. 1.94, No. 1 (2008), pp. 13-20 K. Hashimoto, T. Fujimatsu, N. Tsunekage, K. Hiraoka, K. Kida, and E. C. Santos, "Effect of inclusion/matrix interface cavities on internal-fracture-type rolling contact fatigue life," Materials & Design, Elsevier B.V. (Netherlands), Vol. 32, Issue 10, December 2011, pp. 4980-4985

However, Patent Document 1 discloses findings related to test specimens containing spherical inclusions, which are simple in shape, and does not specifically consider rolling fatigue test methods that take into account the properties and arrangement of inclusions that are not spherical.

The present invention aims to provide a rolling fatigue testing method that takes into account the properties and arrangement of inclusions that are not spherical.

In order to solve the above problems, the test piece used in the rolling fatigue test of the present invention has (1) a test piece main body and at least one inclusion of type A, type B, type C, or type D inclusion as specified in JIS G 0555, embedded to a predetermined depth from the surface of the test piece main body, and having a shape other than spherical.

(2) The test piece described in (1) above, characterized in that a marking member that can be distinguished from the parent phase of the test piece main body is embedded in a position in the test piece main body different from the inclusion.

(3) The test piece described in (2) above, characterized in that the marking member is formed in a cylindrical shape and has a composition different from that of the parent phase of the test piece main body.

(4) A test piece according to any one of (1) to (3) above, characterized in that the hardness of the test piece is 55 HRC or more.

(5) A rolling fatigue test method, comprising performing a thrust rolling fatigue test using a test piece according to any one of (1) to (4) above.

(6) A rolling fatigue test method for performing a thrust type rolling fatigue test using the test piece described in (3) or (4) above, characterized in that the rolling fatigue test is performed by identifying the surface position of the test piece body directly above the inclusion from the position of the marker member, and setting a track so that the rolling element passes through this identified surface position.

(7) The rolling fatigue test method described in (6) above, characterized in that the identified surface position is corrected based on the test results of an ultrasonic flaw detection test.

(8) The method for manufacturing the test piece described in (1) above, characterized in that the inclusion extraction process includes a process of rolling or forging a steel material, a process of taking a test piece for axial fatigue from the rolled material rolled in the rolling process or the forged material forged in the forging process, and a process of fracturing the test piece by subjecting the test piece to a fatigue test using ultrasonic vibration in the axial direction, and a process of obtaining information on the properties of the inclusions manifested on the fracture surface of the test piece and removing the inclusions together with the fracture surface of the test piece.

(9) A method for manufacturing a test piece as described in (8) above, characterized in that a steel embrittlement process is carried out by charging the rolled or forged material with hydrogen before carrying out the fatigue test using ultrasonic vibration.

According to the present invention, it is possible to extract non-spherical inclusions (non-metallic) that are generated during the steel manufacturing process and remain distributed in the steel even after it has undergone subsequent rolling, forging, etc., identify their properties, and then place the extracted inclusions in a very small stress-loaded volume within a rolling fatigue test piece to perform a rolling fatigue test.

FIG. 2 is a structural diagram of a test piece according to an embodiment. 4 is a flowchart showing a manufacturing process of a test piece according to an embodiment. FIG. 2 is a diagram showing an example of the shape of a test piece for inclusion extraction. 1 is an optical microscope image showing the fracture surface of a test piece that broke after ultrasonic fatigue testing. This is an enlarged secondary electron image of a portion of Figure 4, taken by SEM, of the inclusion that emerged as the fracture origin.

Below, the test specimen and rolling fatigue test method according to the embodiment of the present invention will be described in detail with reference to the drawings.

Figure 1 is a structural diagram of the test piece of this embodiment. Figure 1(a) is a front view, and Figure 1(b) is a view of the test piece of Figure 1(a) cut along the X-X cross section as seen from the X arrow. Note that, for ease of explanation, some dimensional relationships are exaggerated in Figure 1.

The test specimen 100 of this embodiment includes a hollow disk-shaped test specimen body 100A having an inner diameter hole 101 at the center. An inclusion 105 is embedded below a position corresponding to a raceway in a thrust type rolling fatigue test described later on a mirror-polished surface of the test specimen body 100A.
The inclusions 105 are nonmetallic inclusions, which are inclusions that are generated in the steels specified in the JIS standard or various steels that deviate from the JIS standard due to the steel manufacturing process. The JIS standard (JIS G 0555: Microscopic test method for nonmetallic inclusions in steel) presents a method in which a cross section parallel to the rolling direction (or forging direction) of a sample taken from a steel bar is mirror-polished, and the nonmetallic inclusions that appear on the polished surface are observed with an optical microscope to evaluate their distribution. These are used as a means of confirming the quality of steel. Nonmetallic inclusions are roughly classified according to their shape and type into type A (sulfide type), type B (alumina type, particle group consisting of single particles aligned in the deformation direction of the steel), type C (silicate type, elongated individual particles with an aspect ratio (length/thickness) generally exceeding 3), and type D (granular oxide type, single particles that are not deformed and have a low aspect ratio of less than 3 in an angular or circular shape). Of these, in the present invention, any type of nonmetallic inclusions except for spherical inclusions can be used. Also, only one type of inclusion among the A to D type inclusions may be embedded, or an inclusion that is a combination of multiple types of inclusions (for example, a combination of D type inclusions and A type inclusions) may be embedded.

The X-X line extends in the radial direction of the test piece 100, and a pair of cylindrical marker members 107 are embedded at a position on the X-X line that is a predetermined distance away from the center of the hole 103, whose bottom surface is machined to be flat. The marker members 107 are arranged to clearly identify the position of the hole 103 relative to the parent phase of the test piece main body 100A. Specifically, by making the marker members 107 and the parent phase of the test piece main body 100A from materials with different compositions, the interface can be clearly identified. Details will be described later, but by making the test piece main body 100A from SUJ2 steel (see JIS G 4805) and the marker members 107 from stainless steel, the interface between the parent phase of the test piece main body 100A and the marker members 107 can be clearly identified.

The mark member 107 can be formed in a cylindrical shape, for example, with a diameter of 0.5 mm and a height of 3 mm.
In Figure 1, for the sake of convenience, cylindrical sample 104 (the preparation method of which will be described later) with inclusion 105 exposed on its bottom surface, hole 103 with a flat bottom surface, marker member 107, and small drill hole 106 are shown with solid lines; however, by performing the hot isostatic pressing process described later, the interfaces between inclusion 105, cylindrical sample 104, and hole 103 with a flat bottom surface come into close contact with each other, and the interface between marker member 107 and small drill hole 106 also come into close contact with each other.

2 is a flow chart showing the manufacturing process of the test piece of this embodiment. The manufacturing process will be described in detail with reference to the specific conditions under which the test piece 100 was manufactured as an example.
(Step S1)
In step S1, the test piece body 100A is prepared. The test piece body 100A is the remaining part of the test piece 100 excluding the cylindrical sample 104, the inclusion 105, and the marker member 107. For the material of the test piece body 100A and the cylindrical sample 104 (wherein the maximum diameter part of the inclusion 105 is exposed at the bottom surface due to fatigue fracture described later), for example, SUJ2 steel (see JIS G 4805), which is widely known as a material for bearing steel, can be used. However, the present invention is not limited to this, and any type of steel for which fatigue characteristics are to be obtained can be appropriately selected. The process for preparing the test piece main body 100A includes a rolling process for rolling SUJ2 steel, a normalizing process for performing normalizing treatment on the rolled material, a spheroidizing annealing process for performing spheroidizing annealing on the normalized rolled material, and a shaping process for processing the material into a test piece shape (i.e., a hollow disk shape with both sides parallel to each other). The rolling process may be substituted by a forging process. In the above, when a hypereutectoid steel such as SUJ2 is used, a spheroidizing annealing process is performed as a pretreatment for obtaining an appropriate quenched structure in the quenching process described later. On the other hand, when a hypoeutectoid steel is used, a carburizing quenching process is performed instead of quenching, so there is no need to perform the spheroidizing annealing process. In the shaping process, it is desirable to polish one side 102 smoothly. This is because, as described later, one side 102 is drilled (hole 103, etc.). However, it is not necessary to perform a buffing finish process.

In the rolling process in the examples, SUJ2 steel was rolled to a diameter of 65 mm. In the normalizing process in the examples, the rolled material was held at a heating temperature of 865°C for 1 hour, followed by air cooling as a normalizing treatment. In the spheroidizing annealing process in the examples, a heat treatment was carried out with a maximum heating temperature of 800°C, and the material was held at that temperature for a specified time, followed by slow cooling as a spheroidizing annealing treatment.

In this embodiment, SUJ2 steel is used as an example, but the present invention is not limited to this, and other steels can be used. In this case, the normalizing treatment and spheroidizing annealing treatment in step S1, and the normalizing treatment and spheroidizing annealing treatment when reprocessing the test piece described below, can be performed under conditions suitable for the selected steel type, or can be omitted depending on the steel type.

Next, in the shape processing step in the embodiment, a hollow disk-shaped test piece body 100A was produced with an outer diameter of 60 mm (corresponding to dimension A in FIG. 1), an inner diameter of 20 mm (corresponding to dimension B in FIG. 1), and a thickness of 8 mm (corresponding to dimension C in FIG. 1). Both sides of the test piece body 100A were processed to be parallel. One side 102 was polished smoothly.
Needless to say, the outer diameter, inner diameter, and thickness of the test piece body 100A are not limited to the above values, and can be changed as appropriate depending on the test conditions.

(Step S2)
In step S2, a hole 103 (which can be machined by end milling, for example) with a flat bottom (i.e., cylindrical with a bottom) and a pair of small drill holes 106 are formed in the test piece body 100A. The hole 103 can be machined at a position corresponding to the orbit on the polished surface 102 of the test piece body 100A. In the embodiment, the dimensions of the hole 103 are 2.6 mm in diameter and 2 mm in depth.
The small drilled holes 106 are formed in a cylindrical shape with a bottom at positions on either side of the hole 103 in the radial direction of the test piece body 100A. In the embodiment, the dimensions of the small drilled holes 106 were 0.6 mm in diameter and 3 mm in depth.
Needless to say, the diameter and depth of the holes 103 and 106 can be appropriately changed depending on the test conditions and the shape of the cylindrical sample 104 and the size of the marker member 107 described later. The hole 103 is formed on the upper surface side in the structural diagram of the test piece of this embodiment illustrated in FIG. 1, but it may be formed after the lower surface side is polished smoothly in the same manner as the above-mentioned one side 102. In this case, the pair of small drill holes 106 are also machined on the upper surface side. In this case, the length of the cylindrical sample 104 including the inclusion 105 and the depth of the hole 103 are set to be within the depth range of the marker member 107 from the surface of the one side 102. By setting the radial distance (the above-mentioned specified distance) from the outer diameter portion of the cylindrical sample 104 to the marker member 107 to, for example, 1 mm or more, it is possible to prevent the chemical components of the marker member 107 from affecting the fatigue characteristics of the cylindrical sample 104 when the marker member 107, which is made of a material with a different composition from the cylindrical sample 104, is tightly attached to the cylindrical sample 104 by hot isostatic pressing processing described below.

(Step S3)
In step S3, a test piece for inclusion extraction is prepared to obtain the inclusion 105 to be introduced into the test piece body 100A. This inclusion 105 is used as a defect to be introduced into the test piece body 100A. For this purpose, a test piece for inclusion extraction is prepared from the steel material to be evaluated, a fatigue test is performed, and an inclusion (extracted inclusion) that appears at the origin of the fracture surface after the test can be used as the inclusion 105 to be introduced into the test piece body 100A. In this case, the inclusion is an inclusion that was accidentally included in a steel test piece randomly collected. The method of the fatigue test is not particularly limited, but if it is desired to shorten the test time, ultrasonic fatigue testing may be selected. In addition, for the steel material to be evaluated, a defect that is expected to be an inclusion is detected by a method such as ultrasonic flaw detection testing, a test piece is prepared to include the defect, and the steel material is fractured by a fatigue test. If an inclusion appears at the origin of the fracture surface after the test, the inclusion may be used.

Figure 3 is a cross-sectional view of the inclusion extraction test piece cut along its longitudinal direction. Here, dimensional information of the inclusion extraction test piece is also shown as an example. However, the dimensions are not limited to these and can be adjusted as appropriate. The inclusion extraction test piece can be manufactured by first preparing a base material test piece that will be the base material of the test piece to be used in the inclusion extraction test (base material test piece preparation process), and then subjecting the base material test piece to a shape processing process (shape processing process). Each process included in step S3 will be described in detail below.

(Regarding the process of preparing the base material test piece in step S3)
The manufacturing process of the base material test piece will be described by taking SUJ2 steel as an example. This process includes a rolling process in which SUJ2 steel is rolled, a normalizing process in which the rolled material is normalized, and a spheroidizing annealing process in which the normalized rolled material is spheroidized. In the rolling process in the example, SUJ2 steel was processed to φ65 mm by hot rolling. In the normalizing process in the example, a normalizing process was performed in which the material was held at a heating temperature of 865°C for 1 h and then air-cooled. In the spheroidizing annealing process in the example, a heat treatment was performed with a maximum heating temperature of 800°C, and a spheroidizing annealing process was performed in which the material was held at that temperature for a predetermined time and then slowly cooled. When using steel other than SUJ2 steel, the base material test piece may be manufactured by a method suitable for each steel type, taking into account the processing method after this step. For example, in the case of hypoeutectoid steel, it is not necessary to perform spheroidizing annealing after normalizing. In addition, a forging process may be adopted instead of the rolling process in this step.

(Regarding the shape processing step S3)
The following explanation will be given based on the above-mentioned classification of inclusions. When D-type inclusions are extracted and used by fatigue testing, it is desirable to perform an axial fatigue test (the direction of repeated stress loading on the test piece is the axial direction, and the fracture surface in this case is perpendicular to the axial direction) in which the evaluation volume is ³⁰⁰ mm3 or more. Furthermore, it is preferable to take the test piece for inclusion extraction so that its long axis direction is approximately parallel to the rolling direction of the rolled material. When it is desired to extract and use inclusions that are elongated in the rolling direction or aligned in the rolling direction, such as A-type, B-type, and C-type inclusions, it is preferable to take the test piece for inclusion extraction so that the long axis direction is the direction in which the cross-sectional area of the inclusion is maximum, that is, the direction perpendicular to the rolling direction. In the case of forged material, the forging direction is used as the reference instead of the rolling direction, and test pieces can be taken in accordance with the above-mentioned method for rolled material. In this case, in the case of a base material in which the extracted inclusions are type D despite multiple tests being performed, it may be easier to detect types A, B, and C by reducing the evaluation volume of the test piece for inclusion extraction. Note that, since the present invention is merely directed to extracting and utilizing inclusions contained in steel material, the type and size of the inclusions will be derived from the history of the steel material to be evaluated (the manufacturing process of the steel material).
In the examples, as an example, aiming at extracting inclusions elongated in the rolling direction (other than the D-type inclusions in the above classification), when processing the base material test piece into the shape of the inclusion extraction test piece shown in Figure 3, rough processing (in other words, rough processing) was performed so that the longitudinal direction of the ultrasonic fatigue test piece was perpendicular to the rolling direction of the rolled material, and this roughly processed inclusion extraction test piece was subjected to quenching and tempering treatment and then finish processing.

(Step S4)
In step S4, a fatigue test is performed on the test piece for inclusion extraction to obtain inclusions. Here, an ultrasonic fatigue test is taken as an example to explain the method.
In order to apply ultrasonic fatigue to the ultrasonic fatigue test piece for inclusion extraction, it is necessary for the ultrasonic fatigue test piece to satisfy a predetermined resonance frequency. Therefore, before carrying out the ultrasonic fatigue test, the resonance frequency of the processed test piece for inclusion extraction is confirmed. The resonance frequency of the ultrasonic fatigue tester used in this example is 20,000 Hz ± 500 Hz, but the resonance frequency of the test piece is preferably within 20000 Hz ± 200 Hz, more preferably within 20,000 Hz ± 30 Hz. If the resonance frequency of the test piece for inclusion extraction does not satisfy these preferred ranges, the resonance frequency can be adjusted to the range by adjusting the length of the test piece for inclusion extraction. The resonance frequency of the test piece for inclusion extraction used in the examples is 19,980 Hz, which satisfies the above-mentioned conditions.
However, if the size of the test piece for inclusion extraction is increased to increase the risk volume within the output range of the ultrasonic fatigue testing machine, the maximum stress that can be applied to the test piece for inclusion extraction decreases with the size of the risk volume. Therefore, it is possible to promote fracture by embrittling the steel material using hydrogen charging as described below.

After confirming the resonant frequency, prior to carrying out the ultrasonic fatigue test, hydrogen charging can be performed on the test piece for inclusion extraction in order to break the test piece for inclusion extraction more quickly. This causes hydrogen embrittlement of the steel material, and the breaking stress of the test piece for inclusion extraction can be reduced. In addition, the brittle fracture at this time makes the characteristics around the inclusion that is the fracture origin flat, and the cylindrical sample 104 containing the inclusion 105 that is the origin of the fracture by hot isostatic pressing processing described later and the hole 103 machined in the test piece main body 100A can be easily attached. The method of hydrogen charging is not particularly limited, but can be, for example, a method of immersing the test piece in various electrolytes, a method of exposing the test piece to high-pressure hydrogen gas, or a method of performing electrolysis in an electrolyte with the test piece as the cathode.

In the examples, hydrogen charging was performed by a cathode charging method in which electrolysis was performed in an electrolyte solution using the test piece for inclusion extraction as the cathode. An electrolyte solution containing sodium chloride and ammonium thiocyanate added to pure water was used. When the electrolyte solution was 100 mass%, the concentration of sodium chloride was 3 mass% and the concentration of ammonium thiocyanate was 0.3 mass%. The hydrogen charging time was 6 continuous hours. The average current density flowing through the test piece for inclusion extraction was 1.0 mA/ ^cm2 , and the test temperature was 50°C.

Since the hydrogen diffusion coefficient into steel is temperature dependent, hydrogen charging can be made more efficient by increasing the temperature of the electrolyte. Therefore, hydrogen charging can be made more efficient by adjusting the temperature of the electrolyte to a temperature higher than room temperature, for example.

The hydrogen charged into the inclusion extraction test specimen is gradually released after the inclusion extraction test specimen is taken out into the atmosphere, so it is desirable to subject the specimen to ultrasonic fatigue testing immediately after hydrogen charging.

The ultrasonic fatigue test is a test in which a test piece is subjected to repeated axial loads of tension and compression by ultrasonic vibration. It is known that when a test piece is continuously vibrated by ultrasonic waves, the test piece heats up due to internal friction caused by repeated tension and compression at high speed. This heat increases the ductility of the test piece, thereby preventing it from breaking from the inclusions. Therefore, in order to perform an appropriate test, it is necessary to suppress the heat generation of the test piece for inclusion extraction, for example, by blowing compressed air cooled by a chiller onto the test piece for inclusion extraction as necessary, or by performing intermittent operation in which ultrasonic oscillation is repeated and stopped. In the examples, as a means of suppressing the heat generation of the test piece for inclusion extraction, both the blowing of compressed air and intermittent operation in which ultrasonic oscillation is repeated for 0.11 sec and stopped for 0.40 sec were performed.

Furthermore, materials with low hardness tend to generate heat more easily due to the internal friction in the test piece under the same load stress compared to steels with high hardness. In other words, if the output of the ultrasonic fatigue tester is simply increased to speed up the test, materials with low hardness will generate significant heat, making it impossible to perform an appropriate fatigue test. Therefore, it is desirable to select the output of the ultrasonic fatigue tester appropriately according to the hardness of the inclusion extraction test piece in the quenched and tempered state or the carburized, quenched and tempered state (which varies depending on the type of steel). In the embodiment, ultrasonic fatigue tests were performed at a test stress of 900 MPa, and it was confirmed that the fracture life was 5 x ¹⁰⁵ cycles, and the test piece could be fractured within a reasonable time.

(Step S5)
In step S5, information regarding the properties of the inclusions appearing on the fracture surface of the inclusion extraction test piece is obtained. As described above, the properties of the inclusions include the size, shape, dispersion state, and chemical composition of the inclusions. This information can be obtained, for example, by observation and analysis using an EDS-equipped scanning electron microscope (SEM-EDS).
For example, an SEM image of the fracture surface of the test piece for inclusion extraction is obtained by an EDS-equipped scanning electron microscope, and the obtained SEM image is analyzed to obtain the size of the inclusion as the square root (√area) of the projected area of the inclusion. Similarly, the size of the inclusion may be obtained as a circle equivalent diameter by image analysis. Furthermore, the chemical composition of the inclusion can be grasped by detecting the characteristic X-rays of the inclusion excited by the electron beam. Information on the shape and dispersion state can also be obtained by observation with a scanning electron microscope. By carrying out this step S5, the classification of the extracted inclusions (classification of A, B, C, and D, or classification of whether they are combined) can be determined.

Figure 4 shows an example of an optical microscope image taken by photographing the fracture surface of a test piece for inclusion extraction after an ultrasonic fatigue test. The area near the origin of the fracture surface exhibits a fish-eye pattern as shown by the dashed area in the figure, and the inclusion (non-metallic inclusion) that was the origin of the fracture has been extracted near the center.

Figure 5 is an enlarged SEM image of the inclusion shown in Figure 4, and in this example, it is confirmed that it is a B-type inclusion. If the required inclusion is not obtained, the above inclusion extraction steps can be repeated until the required inclusion is obtained.

(Step S6)
In step S6, the inclusions on the fracture surface of the inclusion extraction test piece extracted as described above are cut so as to include the inclusions, to prepare a cylindrical sample 104 including the inclusions 105. At this time, since the position of the inclusions 105 on the fracture surface of the inclusion extraction test piece is not necessarily located at the center of the cylindrical test piece 104, it is advisable to obtain the position information and use the information when arranging the inclusions later. In addition, when the major axis and minor axis of the inclusions are clearly different, it is advisable to obtain information on the orientation of the inclusions. Then, the cylindrical sample 104 is placed inside the hole 103 so that the fracture surface side on which the inclusions 105 are present in the cylindrical sample 104 faces the bottom surface of the hole 103 whose bottom surface has been machined to be flat. Here, the arrangement direction of the inclusions 105 on the bottom surface of the hole 103 whose bottom surface has been machined to be flat can be changed according to the purpose of the fatigue test. In the embodiment, the inclusions 105 were disposed so that the extension direction of the inclusions 105 coincided with the X-X direction in Fig. 1 (the radial direction of the test piece 100 passing through the pair of marker members 107). At this time, a part of the interface between the cylindrical test piece 104 and the hole 103 may be spot welded so that the storage position of the test piece does not shift before the hot isostatic pressing process performed thereafter.

In addition, a stainless steel marker member 107 is embedded in the small drill hole 106. After embedding, the parent phase of the test piece main body 100A and the marker member 107 are joined by hot isostatic pressing, and the interface between the parent phase and the marker member 107 can be identified, which can be used to identify the embedded position of the inclusion 105. In other words, when the plane along the polished surface 102 side of the test piece 100 is defined as a coordinate plane consisting of mutually orthogonal X-axis and Y-axis, the position where the inclusion 105 exists can be identified based on the identified coordinate position of the marker member 107. This makes it possible to place the inclusion 105 under the track of the rolling element during the thrust type rolling fatigue test described later.

(Step S7)
In step S7, hot isostatic pressing (HIP) is performed on the test piece 100. As a pretreatment for hot isostatic pressing, the test piece 100 is placed in a separately prepared low carbon steel case (not shown), a core bar (not shown) is inserted into the inner diameter hole 101 of the test piece 100, and the case is then sealed. After sealing the case, the inside of the case is evacuated and hot isostatic pressing is performed, and then the case is cooled. The heating temperature in the hot isostatic pressing is 1150°C or higher, preferably 1160°C or higher. The pressure in the hot isostatic pressing is preferably 110 MPa or higher, more preferably 140 MPa or higher. In the embodiment, the pressure is set to 147 MPa, the heating temperature is set to 1170°C, and the holding time at the heating temperature is 5 hours.

By applying hot isostatic pressing, the cylindrical sample 104 and the inclusion 105 can be brought into close contact with the hole 103, and the marker member 107 can be brought into close contact with the small drill hole 106. This makes it possible to eliminate gaps at the interface between the inclusion 105 and the parent phase, thereby eliminating the effect of gaps at the interface on the rolling fatigue test.

(Step S8)
In step S8, a normalizing process and a spheroidizing process are performed on the test piece 100, and a re-processing process is performed in which the portion made of SUJ2 is again processed into the shape of the test piece (hollow disk shape). In the normalizing process in the example, a normalizing treatment was performed in which the test piece was held at a heating temperature of 865°C for 1 hour and then air-cooled. In the spheroidizing process in the example, a heat treatment was performed with a maximum heating temperature of 800°C, and the test piece was held at that temperature for a predetermined time and then slowly cooled, as a spheroidizing annealing treatment.
In the reprocessing step in the embodiment, the processed test piece 100 was processed into a hollow disk shape having an outer diameter (corresponding to dimension A in FIG. 1) of 58 mm, an inner diameter (corresponding to dimension B in FIG. 1) of 20 mm, and a thickness (corresponding to dimension C in FIG. 1) of 6 mm.

(Step S9)
In step S9, the test piece 100 is quenched and tempered to adjust the hardness of the test piece 100. The hardness (Rockwell hardness) of the test piece 100 after quenching and tempering is preferably 55 HRC or more, and more preferably 58 HRC or more. If the hardness is lower than this, rolling fatigue will progress not only in the defect and its surroundings, but also in the parent phase itself, making it difficult to distinguish and verify the harmfulness of the defect itself.
In the embodiment, the heating temperature for quenching was 835°C, the heating time was 0.5h, and oil quenching was used as the quenching quenching method. The tempering temperature was 180°C, the tempering time was 1.5h, and air cooling was used as the cooling method after the tempering temperature was reached. The hardness of the test piece 100 after quenching and tempering was about 62HRC.

(Step S10)
After removing the oxide scale of the test piece 100 generated by the heat treatment by surface grinding, the depth of the inclusion 105 in the test piece 100 is identified by ultrasonic testing at a frequency of 50 MHz, and the test piece 100 is buffed and polished based on this depth information, so that the inclusion 105 is arranged in the high shear stress depth region under the thrust type rolling fatigue test conditions described below. During the ultrasonic testing, in addition to the depth of the inclusion 105, the coordinate position directly above the embedded portion of the inclusion 105 can also be identified. In the buffing and polishing process, the surface is polished by rotating a buff coated with an abrasive material and bringing it into contact with the surface. Cotton, felt, etc. can be used for the buff. If the position cannot be identified by the 50 MHz ultrasonic testing, the depth of the inclusion 105 may be identified by using ultrasonic testing at a higher frequency.

The test piece 100 used in thrust rolling fatigue testing is completed through the process described above in steps S1 to S10. Then, thrust rolling fatigue testing can be carried out on inclusions with properties other than spherical shapes. This allows precise verification of the harmfulness of the inclusions (their impact on lifespan and rolling fatigue).

The thrust type rolling fatigue test will be described in detail with reference to an example. As a preparation before performing the thrust type rolling fatigue test, the position of the marking member 107, which serves as a precise marking embedded in the small drill hole 106, was identified by visually inspecting the buffed surface. Next, a track was arranged so that the rolling element would pass between a pair of marking members 107. A single thrust bearing race (model number 51305) made of SUJ2 was used as the upper plate to be combined with the test piece 100. A rolling element was arranged between the test piece 100 and the upper plate. SUJ2 steel balls with a diameter of 3/8 inches were used as the rolling element. Three of these rolling elements were arranged equally spaced at 120° intervals. As described above, the position directly above the embedded portion of the inclusion 105 can be identified by utilizing the coordinate position of the marking member 107. At this time, if the coordinate position information of the ultrasonic flaw detection test of the inclusion 105 is also utilized, the track position can be determined more precisely. That is, the position directly above the embedded location of the inclusion 105, which is identified by utilizing the coordinate position of the marker member 107, can be corrected based on the test results of the ultrasonic flaw detection test. Regarding the layout of the track, taking into consideration that the behavior of rolling fatigue may change depending on the layout of the inclusion within the track width, the track may be set so that the center of the track width passes directly above the embedded location, or it may be intentionally shifted from the center of the track width as appropriate, and can be set appropriately according to the purpose of the test.

Next, a load is applied so that a maximum Hertzian contact stress of 4.5 GPa is applied to the contact area between the rolling element and the test piece 100. The load cycle speed is, for example, 1800 cycles/min, and lubrication is by immersion in an ISO VG68 oil bath. By performing a thrust type rolling fatigue test at room temperature, the rolling fatigue behavior of the inclusion 105 can be evaluated. These conditions for the thrust type rolling fatigue test can be adjusted according to the environment to be evaluated.

As described above, by using the rolling fatigue testing method with defect introduction according to this embodiment, an inclusion 105 other than a sphere whose properties are known in advance can be introduced into a target position in the test piece body 100A, a rolling fatigue test can be performed on it, and the rolling fatigue behavior such as cracks around the inclusion embedding point can be reliably observed after the test based on the position information known in advance.

Furthermore, if it is possible to induce spalling from the inclusions 105, it will also be possible to verify the relationship between the size of the inclusions 105 and the lifespan. Furthermore, with this method, tests are performed under conditions in which the composition and shape of the inclusions 105 that impart rolling fatigue and the state of the inclusion-parent phase interface are determined in advance, so it is easy to reflect these conditions in analysis using the finite element method, making it possible to perform more advanced verification than ever before.

As explained above, this embodiment is a method in which non-spherical non-metallic inclusions that are generated and contained in steel during the manufacturing process are extracted, and the size, composition, and shape of the inclusions are determined in advance and then introduced into a rolling fatigue test piece made of bearing steel. The inclusions and the parent phase are then brought into close contact with each other, and a rolling fatigue test is then performed on the inclusions to precisely verify their harmful effects on rolling fatigue (effects on life and fatigue).

In addition, with this method, even if the test is stopped before delamination occurs, the location of the inclusion in the steel is precisely identified in advance, making it possible to carry out cross-sectional observation of the fatigue condition around it.

The rolling fatigue testing method using inclusions, the properties of which have been identified in this embodiment, is performed from a state in which various information about the inclusions that affect life, such as their size, shape, composition, the state of the gap with the parent phase, and their location in the steel, is known in advance, making it possible to precisely verify the harmfulness of the inclusions (their impact on life and rolling fatigue).

When using this test method, the precise location of the inclusions is known in advance, making it easy to observe the fatigue condition around the inclusions. By using the traditional cross-sectional observation method or a non-destructive observation method, it is expected that more precise verification of the fatigue behavior around the inclusions can be achieved than ever before.

Although an embodiment has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. This new embodiment can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the gist of the invention. This embodiment and its variations are included in the scope and gist of the invention, and are included in the scope of the invention and its equivalents described in the claims.

Reference Signs List 100 Test piece 100A Test piece body 101 Inner diameter hole 102 Polished surface 103 Hole with flat bottom 104 Cylindrical sample 105 Inclusion 106 Small drill hole

Claims (9)

A test piece used in a rolling fatigue test,
A test piece main body portion,
at least one of type A inclusions, type B inclusions, type C inclusions and type D inclusions as defined in JIS G 0555, embedded to a predetermined depth from the surface of the test piece body, and having a shape other than a sphere;
having
A test piece , characterized in that a marker member that can be distinguished from the parent phase of the test piece body is embedded in a position in the test piece body different from the inclusions. 2. The test piece according to claim 1 , wherein the marker member is formed in a cylindrical shape and has a composition different from that of the parent phase of the test piece body. 3. The test piece according to claim 1, wherein the hardness of the test piece is 55 HRC or more. 4. A method for testing rolling fatigue, comprising the step of carrying out a thrust type rolling fatigue test using the test piece according to claim 1 . A rolling fatigue testing method for performing a thrust type rolling fatigue test using the test piece according to claim 2 or 3 , comprising the steps of:
A rolling fatigue testing method characterized in that the surface position of the test piece main body directly above the inclusion is identified from the position of the marker member, and a track is set so that a rolling element passes through this identified surface position, thereby performing the thrust type rolling fatigue test. 6. The rolling fatigue testing method according to claim 5 , further comprising the step of correcting the specified surface position based on the results of an ultrasonic flaw detection test. A method for manufacturing a test piece used in a rolling fatigue test , comprising the steps of:
The test piece has a test piece main body and at least one of A-type inclusions, B-type inclusions, C-type inclusions and D-type inclusions as defined in JIS G 0555, embedded to a predetermined depth from the surface of the test piece main body, and having a shape other than a sphere;
As the inclusion extraction step,
A step of rolling or forging a steel material;
A fracture process in which a test piece for axial fatigue is taken from the rolled material obtained in the rolling process or the forged material obtained in the forging process, and a fatigue test is performed on the test piece by ultrasonic vibration in the axial direction, thereby fracture the test piece;
and an inclusion removal process for obtaining information regarding the properties of inclusions appearing on the fracture surface of the test piece and removing a sample including the inclusions from the fracture surface of the test piece ,
a step of placing the specimen in a hole formed in the surface of the test piece body such that a fractured surface side of the specimen having an inclusion faces a bottom surface of the hole;
and a HIP process for performing hot isostatic pressing on the test piece body in which the sample is housed.
A method for manufacturing a test piece comprising the steps of: 8. The method for manufacturing a test piece according to claim 7 , further comprising the step of embrittling the steel by charging the rolled or forged material with hydrogen before carrying out the fatigue test using ultrasonic vibration. The housing step includes a step of embedding a marking member that can be distinguished from the parent phase of the test piece main body at a position different from the inclusions.
The method for producing a test piece according to claim 7 or 8.