JP7777967B2 - Rolling members and rolling bearings - Google Patents

Description

The present invention relates to rolling members and rolling bearings. More specifically, the present invention relates to rolling members and rolling bearings for hydrogen-utilizing equipment.

To improve the durability of the surface of rolling contact components, nitriding or carbonitriding has traditionally been performed on the surface layer. For example, Patent Document 1 (Japanese Patent No. 3873741) describes a rolling contact component. The rolling contact component described in Patent Document 1 has its surface subjected to carbonitriding, resulting in precipitates being dispersed in the surface layer.

Patent No. 3873741

However, in the rolling contact member described in Patent Document 1, the particle size of the precipitates dispersed in the surface layer is large. Therefore, when the rolling contact member described in Patent Document 1 is used in hydrogen-utilizing equipment, i.e., in an environment exposed to hydrogen, there is a risk of early peeling occurring at the contact surface due to hydrogen embrittlement.

The present invention was made in consideration of the problems with the prior art described above. More specifically, the present invention provides rolling members and rolling bearings for hydrogen-utilizing equipment that can suppress early peeling of contact surfaces due to hydrogen embrittlement.

A rolling member according to one aspect of the present invention has a contact surface and is made of hardened steel. The rolling member is for use in hydrogen-utilizing equipment. The rolling member has a surface layer portion in a region up to 20 μm deep from the contact surface. The steel contains 0.70 to 1.10 mass percent carbon, 0.15 to 0.35 mass percent silicon, 0.30 to 0.60 mass percent manganese, 1.30 to 1.60 mass percent chromium, 0.01 to 0.50 mass percent molybdenum, and 0.01 to 0.50 mass percent vanadium, with the balance consisting of iron and inevitable impurities. The nitrogen content in the surface layer portion is 0.2 to 0.8 mass percent. In a cross-sectional view perpendicular to the contact surface, there are a total of 60 or more precipitates with a grain size of 0.50 μm or less per 100 μm2 in the surface layer ^portion , and the total area ratio of the precipitates in the surface layer portion is 1% or more and 10% or less.

In the above rolling contact member, the steel may contain 0.90 to 1.10 percent by mass carbon, 0.20 to 0.30 percent by mass silicon, 0.40 to 0.50 percent by mass manganese, 1.40 to 1.60 percent by mass chromium, 0.20 to 0.30 percent by mass molybdenum, and 0.20 to 0.30 percent by mass vanadium, with the remainder consisting of iron and unavoidable impurities.

In the above rolling contact member, the nitrogen content in the surface layer may be 0.3 mass percent or more and 0.5 mass percent or less.

In the above rolling member, in a cross-sectional view perpendicular to the contact surface, the surface layer portion may have a total of 80 or more precipitates ^per 100 μm2 having a particle size of 0.50 μm or less, and the total area ratio of the precipitates in the surface layer portion may be 2% or more and 7% or less.

In the above rolling member, the volume fraction of retained austenite at a depth of 50 μm from the contact surface may be 20 percent or more and 40 percent or less.

In the above rolling member, the volume fraction of retained austenite at a depth of 50 μm from the contact surface may be 25 percent or more and 35 percent or less.

The above rolling member may have a hardness of 653 Hv or more and 800 Hv or less at a depth of 50 μm from the contact surface.

A rolling bearing according to one aspect of the present invention comprises a raceway member and rolling elements arranged in contact with the raceway member. The rolling bearing is for use in hydrogen-utilizing equipment. At least one of the raceway member and the rolling elements is the rolling member described above.

The rolling member and rolling bearing according to one aspect of the present invention make it possible to suppress early flaking of the contact surface due to hydrogen embrittlement.

FIG. 1 is a cross-sectional view of a rolling bearing 100. FIG. 2 is an enlarged cross-sectional view of the ball valve 200. FIG. 3 is a cross-sectional view of a hydrogen circulation pump 300. 3A to 3C are process diagrams showing a manufacturing method of the rolling bearing 100. 1 is a graph showing the distribution of carbon and nitrogen content in the surface layer portions of the inner ring and outer ring of Sample 1. 1 shows representative cross-sectional FE-SEM images of the surface layer portions of the inner ring and outer ring of Sample 1.

Details of embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts will be designated by the same reference symbols, and overlapping descriptions will not be repeated.

(Configuration of rolling bearing according to embodiment)
The configuration of a rolling bearing according to an embodiment (hereinafter referred to as "rolling bearing 100") will be described below. The rolling bearing 100 is, for example, a single-direction flat-face thrust ball bearing. However, the rolling bearing 100 is not limited to this. The rolling bearing 100 may also be, for example, a deep groove ball bearing, an angular contact ball bearing, a cylindrical roller bearing, a tapered roller bearing, or a self-aligning roller bearing.

The rolling bearing 100 is for use in hydrogen-utilizing equipment. The hydrogen-utilizing equipment is, for example, a ball valve or compressor for a hydrogen station. The type of compressor is not particularly limited. For example, the compressor may be of a reciprocating type (reciprocating), rotary type (screw), centrifugal type, or axial type. The hydrogen-utilizing equipment may also be a high-pressure hydrogen pressure reducing valve or hydrogen circulation pump for a fuel cell vehicle. The rolling bearing 100 may be used in any application where it is exposed to hydrogen.

Figure 1 is a cross-sectional view of a rolling bearing 100. As shown in Figure 1, the rolling bearing 100 has a central axis A. Figure 1 shows a cross-sectional view of the rolling bearing 100 taken along the central axis A. The rolling bearing 100 has race members (race rings or races) and rolling elements. In the rolling bearing 100, the race members are an inner ring (shaft race) 10 and an outer ring (housing race) 20, and the rolling elements are balls 30. The rolling bearing 100 further has a cage 40.

The inner ring 10 has an annular (ring-shaped) shape. The inner ring 10 has a first surface 10a, a second surface 10b, an inner peripheral surface 10c, and an outer peripheral surface 10d.

The first surface 10a and the second surface 10b form end surfaces in the direction along the central axis A (hereinafter referred to as the "axial direction"). The second surface 10b is the opposite surface of the first surface 10a in the axial direction. The first surface 10a has a raceway surface 10aa. The first surface 10a is recessed toward the second surface 10b within the raceway surface 10aa. In a cross-sectional view, the raceway surface 10aa has a partial arc shape. The raceway surface 10aa is the surface that contacts the balls 30 and forms the contact surface of the inner ring 10.

The inner peripheral surface 10c faces the central axis A. One axial end of the inner peripheral surface 10c is connected to the first surface 10a, and the other axial end is connected to the second surface 10b.

The outer peripheral surface 10d faces away from the central axis A. In other words, the outer peripheral surface 10d is the opposite surface of the inner peripheral surface 10c in the direction perpendicular to the central axis A (hereinafter referred to as the "radial direction"). One end of the outer peripheral surface 10d in the axial direction is continuous with the first surface 10a, and the other end of the outer peripheral surface 10d is continuous with the second surface 10b.

The outer ring 20 has a ring-like shape. The outer ring 20 has a first surface 20a, a second surface 20b, an inner peripheral surface 20c, and an outer peripheral surface 20d.

The first surface 20a and the second surface 20b form end surfaces in the axial direction. The outer ring 20 is arranged so that the first surface 20a faces the first surface 10a. The second surface 20b is the opposite surface of the first surface 20a in the axial direction. The first surface 20a has a raceway surface 20aa. The first surface 20a is recessed toward the second surface 20b within the raceway surface 20aa. In a cross-sectional view, the raceway surface 20aa has a partial arc shape. The raceway surface 20aa is the surface that contacts the balls 30 and forms the contact surface of the outer ring 20.

The inner peripheral surface 20c faces the central axis A. One axial end of the inner peripheral surface 20c is connected to the first surface 20a, and the other axial end is connected to the second surface 20b.

The outer peripheral surface 20d faces away from the central axis A. In other words, the outer peripheral surface 20d is the opposite surface of the inner peripheral surface 20c in the direction perpendicular to the central axis A (hereinafter referred to as the "radial direction"). One end of the outer peripheral surface 20d in the axial direction is continuous with the first surface 20a, and the other end of the outer peripheral surface 20d is continuous with the second surface 20b.

Ball 30 has a spherical shape. There are multiple balls 30. Ball 30 is disposed between first surface 10a and first surface 20a. More specifically, ball 30 is disposed between raceway surface 10aa and raceway surface 20aa. The surface of ball 30 contacts raceway surface 10aa and raceway surface 20aa. In other words, the surface of ball 30 is the contact surface.

The cage 40 holds the balls 30. The cage 40 holds the balls 30 so that the spacing between two adjacent balls 30 in the direction along the circumference centered on the central axis A (hereinafter referred to as the "circumferential direction") is within a certain range.

<Steel used for raceway components and rolling elements>
The inner ring 10, outer ring 20, and ball 30 are formed from steel having the composition shown in Table 1 (hereinafter referred to as the "first composition"). The inner ring 10, outer ring 20, and ball 30 may also be formed from steel having the composition shown in Table 2 (hereinafter referred to as the "second composition"). The steel constituting the inner ring 10, outer ring 20, and ball 30 has been hardened. It is sufficient that at least one of the inner ring 10, outer ring 20, and ball 30 is formed from steel having the first composition (second composition).

Carbon (C) affects the hardness of the contact surfaces (raceway surface 10aa, raceway surface 20aa, and the surface of ball 30) after quenching. If the carbon content in the steel is less than 0.70 mass percent, it is difficult to ensure sufficient hardness at the contact surfaces. Furthermore, if the carbon content in the steel is less than 0.70 mass percent, it is necessary to supplement the carbon content at the surface through carburizing or other treatments, which reduces production efficiency and increases manufacturing costs. If the carbon content in the steel exceeds 1.10 mass percent, cracks (quench cracks) may occur during quenching. For this reason, the carbon content in the first composition steel is set to be 0.70 mass percent or more and 1.10 mass percent or less.

Silicon is added to ensure workability before deoxidation and nitriding during steel refining. If the silicon content in the steel is less than 0.15 mass percent, temper softening resistance will be insufficient. As a result, the hardness of the contact surface may decrease due to tempering after quenching or due to temperature increases during use of the rolling bearing 100. If the silicon content in the steel exceeds 0.35 mass percent, the steel may become too hard, shortening the tool life of cutting tools used to machine the inner ring 10 (outer ring 20, ball 30). This also increases the material cost of the steel. For this reason, the silicon content in the first composition steel is set to 0.15 mass percent or more and 0.35 mass percent or less.

Manganese is added to ensure the hardenability and hardness of the steel. If the manganese content in the steel is less than 0.30 mass percent, it is difficult to ensure the hardenability and hardness of the steel. If the manganese content in the steel exceeds 0.60 mass percent, the steel becomes too hard, which may shorten the tool life of the cutting tools used to machine the inner ring 10 (outer ring 20, ball 30). This also increases the material cost of the steel. For this reason, the manganese content in the steel of the first composition is set to 0.30 mass percent or more and 0.60 mass percent or less.

Chromium is added to ensure the hardenability of the steel and to ensure hardness by forming fine precipitates during nitriding. If the chromium content in the steel is less than 1.30 mass percent, it is difficult to ensure the hardenability and hardness of the steel. If the chromium content in the steel exceeds 1.60 mass percent, the precipitates become coarse and may become the starting point for fatigue fracture. This also increases the material cost of the steel. For this reason, the chromium content in the first composition steel is set to be 1.30 mass percent or more and 1.60 mass percent or less.

Molybdenum is added to ensure the hardenability of steel and to ensure hardness by forming fine precipitates during nitriding. Furthermore, because molybdenum has a strong affinity for carbon, it precipitates in the steel as undissolved carbides during nitriding. These undissolved molybdenum carbides act as precipitation nuclei during quenching, so molybdenum increases the amount of precipitates after quenching.

If the molybdenum content in the steel is less than 0.01 mass percent, it is difficult to ensure the steel's hardenability and hardness. If the molybdenum content in the steel exceeds 0.50 mass percent, precipitates become coarse and may become the starting point for fatigue fracture. This also increases the material cost of the steel. For this reason, the molybdenum content in the steel of the first composition is set to 0.01 mass percent or more and 0.50 mass percent or less.

Vanadium is added to ensure the hardenability of the steel and to ensure hardness by forming fine precipitates during nitriding. If the vanadium content in the steel is less than 0.01 mass percent, it is difficult to ensure the hardenability and hardness of the steel. If the vanadium content in the steel exceeds 0.50 mass percent, the precipitates become coarse and may become the starting point for fatigue fracture. In this case, the material cost of the steel also increases. For this reason, the vanadium content in the first composition steel is set to be 0.01 mass percent or more and 0.50 mass percent or less.

<Surface layer portion 50>
As shown in Figure 1, the inner ring 10, outer ring 20, and ball 30 have a surface layer 50 on their surfaces. The surface layer 50 is a region extending from the surface of the inner ring 10, outer ring 20, and ball 30 to a depth of 20 µm. It is sufficient for the surface layer 50 to be formed on at least the contact surfaces of the inner ring 10, outer ring 20, and ball 30. It is also sufficient for the surface layer 50 to be formed on at least one of the inner ring 10, outer ring 20, and ball 30. The surface layer 50 is formed by nitriding treatment.

In a cross-sectional view perpendicular to the contact surface, a total of 60 or more precipitates having a grain size of 0.5 ^μm or less are present per 100 μm2 in the surface layer portion 50. In a cross-sectional view perpendicular to the contact surface, a total of 80 or more precipitates having a grain size of 0.5 μm or less are present per 100 μm2 in the surface layer portion ⁵⁰ .

When viewed in a cross section perpendicular to the contact surface, the total area ratio of precipitates in the surface layer 50 is 1 percent or more and 10 percent or less. When viewed in a cross section perpendicular to the contact surface, the total area ratio of precipitates in the surface layer 50 is preferably 2 percent or more and 7 percent or less.

The nitrogen content in the surface layer portion 50 is preferably 0.2 mass percent or more and 0.8 mass percent or less. The nitrogen content in the surface layer portion 50 is more preferably 0.3 mass percent or more and 0.5 mass percent or less. However, the nitrogen content in the surface layer portion 50 does not have to be within the ^above range as long as a total of 60 or more particles per 100 μm2 can be present in the surface layer portion 50 and the total area ratio of precipitates in the surface layer portion 50 can be 1 percent or more and 10 percent or less.

The nitrogen content in the surface layer 50 is measured using an electron probe micro analyzer (EPMA).

Precipitates in the surface layer 50 are, for example, carbonitrides and nitrides. Carbonitrides include iron carbides, iron carbides in which the carbon has been substituted with nitrogen, and iron carbides in which the iron has been substituted with an alloying element other than iron. Nitrides are iron nitrides. Precipitates in the surface layer 50 may be carbides, carbonitrides, or nitrides of alloying elements contained in the steel.

The precipitates in the steel of the surface layer 50 may be nitrides containing chromium or vanadium as the main component, or carbonitrides containing chromium or vanadium as the main component.

A nitride whose main component is chromium (vanadium) is a nitride of chromium (vanadium) or a nitride in which some of the chromium (vanadium) sites have been replaced by an alloying element other than chromium (vanadium).

A carbonitride whose main component is chromium (vanadium) is a chromium (vanadium) carbide in which some of the carbon sites have been replaced with nitrogen. The chromium (vanadium) sites of a carbonitride whose main component is chromium (vanadium) may be replaced with an alloying element other than chromium (vanadium).

The area ratio of precipitates is calculated by acquiring a cross-sectional image of the surface layer portion 50 at a magnification of 5000x using a field emission scanning electron microscope (FE-SEM), binarizing the cross-sectional image, and performing image processing on the binarized cross-sectional image. Cross-sectional images of the surface layer portion 50 are acquired from three or more fields of view, and the area ratio is calculated as the average value of these multiple cross-sectional images.

The particle size of each precipitate is obtained by obtaining the area of each precipitate using the same method as above, and then dividing that area by π and multiplying the square root by 2.

<Amount of retained austenite in steel>
The volume fraction of retained austenite in the steel making up the inner ring 10, outer ring 20, and ball 30 at a depth of 50 μm from the contact surface is preferably 20 percent or more and 40 percent or less. It is even more preferable that the volume fraction of retained austenite in the steel making up the inner ring 10, outer ring 20, and ball 30 at a depth of 50 μm from the contact surface be 25 percent or more and 35 percent or less. This improves the durability of the contact surface in an environment containing foreign matter, and suppresses deterioration over time due to the decomposition of retained austenite.

The amount of retained austenite in the steel at a depth of 50 μm from the contact surface is measured using X-ray diffraction. More specifically, the amount of retained austenite in the steel at a depth of 50 μm from the contact surface is measured using an MSF-3M manufactured by Rigaku Corporation.

<Hardness at a depth of 50 μm from the contact surface>
The hardness at a position 50 μm deep from the contact surfaces of the inner ring 10, outer ring 20, and balls 30 is preferably 653 Hv or more and 800 Hv or less. When suppressing hydrogen embrittlement caused by hydrogen generated with the decomposition of lubricating oil, improving the hardness near the contact surfaces to make it less likely that new metal surfaces will form on the contact surfaces is an effective way to suppress hydrogen generation, but in an environment where hydrogen is present regardless of whether new metal surfaces are formed, the hardness at a position 50 μm deep from the contact surfaces of the inner ring 10, outer ring 20, and balls 30 does not have to be 653 Hv or more and 800 Hv or less.

The hardness at a position 50 μm deep from the contact surfaces of the inner ring 10, outer ring 20, and ball 30 is measured using the Vickers hardness test method specified in the JIS standard (JIS Z 2244:2009). The load used during measurement is 300 gf.

<Martensite Blocks in Surface Layer Portion 50>
The steel of the surface layer portion 50 has martensite block grains. The difference in crystal orientation between two adjacent martensite block grains at the grain boundary is 15° or more. From another perspective, even if there is a location where the crystal orientation is misaligned, if the difference in crystal orientation is less than 15°, the location is not considered to be a grain boundary of the martensite block grains. The grain boundary of the martensite block grains is determined by EBSD (Electron Back Scattered Diffraction) method.

In the steel of the surface layer 50, the average grain size of the martensite block grains at a comparative area ratio of 30 percent is preferably 2.0 μm or less. It is even more preferable that in the steel of the surface layer 50, the average grain size of the martensite block grains at a comparative area ratio of 50 percent is 1.5 μm or less. This increases the toughness of the surface layer 50 and improves the shear resistance of the contact surfaces (more specifically, the surfaces of the raceway surfaces 10aa, 20aa, and ball 30).

The average grain size of martensite block grains at a comparative area ratio of 30 percent (50 percent) is measured using the following method. First, a cross-section of the inner ring 10, including the surface layer 50, is observed. At this time, the martensite block grains included in the observation field are identified using the EBSD method. This observation field is an area of 50 μm x 35 μm. Second, the area of each martensite block grain included in the observation field is analyzed using the crystal orientation data obtained using the EBSD method.

Third, the areas of the martensite block grains included in the observation field are added together in descending order of area. This addition is continued until the area reaches 30 percent (50 percent) of the total area of the martensite block grains included in the observation field. The equivalent circle diameter is calculated for each of the martensite block grains that have been added together. This equivalent circle diameter is the square root of the area of the martensite block grain divided by π/4. The average value of the equivalent circle diameters of the martensite block grains that have been added together is considered to be the average diameter of the martensite block grains when the comparison area ratio is 30 percent (50 percent).

In the above, we have described the case where the rolling member according to the embodiment is a component of the rolling bearing 100, but the rolling member according to the embodiment may also be a sliding bearing.

<Application examples of rolling members according to embodiments>
The rolling member according to the embodiment is used in a ball valve 200. Fig. 2 is an enlarged cross-sectional view of the ball valve 200. As shown in Fig. 2, the ball valve 200 includes a body 210, a seat retainer 220, a ball 230, stems 231 and 232, and a plain bearing 240.

The seat retainer 220 is disposed inside the body 210. The seat retainer 220 has an internal space 220a and flow paths 220b and 220c. Flow paths 220b and 220c are connected to the internal space 220a. The ball 230 is disposed in the internal space 220a. The wall surface of the internal space 220a contacts the surface of the ball 230 at the seal portion 220aa.

Stems 231 and 232 are connected to the upper and lower ends of ball 230, respectively. As stems 231 and 232 rotate around their central axis, flow paths 220b and 220c are connected via a through hole (not shown) formed in ball 230. Stems 231 and 232 are passed through through holes formed in body 210 and seat retainer 220. Hydrogen flows through flow paths 220b, 220c, and the through holes formed in ball 230.

The plain bearing 240 is cylindrical and is attached to the body 210 at its outer circumferential surface. The plain bearing 240 rotatably supports the stem 231 (stem 232). The plain bearing 240 is a rolling member according to the embodiment. That is, the plain bearing 240 is made of steel of the first composition or the second composition, and a surface layer 50 is formed on the contact surface.

<Application examples of rolling bearings according to embodiments>
3 is a cross-sectional view of the hydrogen circulation pump 300. The hydrogen circulation pump 300 includes a motor housing 310, a pump housing 320, rotating shafts 331 and 332, a motor stator 341 and a motor rotor 342, gears 351 and 352, rotors 361 and 362, and rolling bearings 371, 372, 373, 374, 375, and 376.

The motor housing 310 is attached to the pump housing 320. One end of the rotating shaft 331 is disposed within the motor housing 310, and the other end of the rotating shaft 331 is disposed within the pump housing 320. One end and the other end of the rotating shaft 331 are rotatably supported by a rolling bearing 371 disposed within the motor housing 310 and a rolling bearing 372 disposed within the pump housing 320, respectively. Between one end and the other end, the rotating shaft 331 is rotatably supported by a rolling bearing 373 and a rolling bearing 374 disposed within the pump housing 320.

The rotating shaft 332 is disposed within the pump housing 320. One end of the rotating shaft 332 is rotatably supported by a rolling bearing 375 disposed within the pump housing 320. The rotating shaft 332 is rotatably supported at a position away from the one end by a rolling bearing 376 disposed within the pump housing 320.

Motor stator 341 is disposed within motor housing 310. Motor rotor 342 is attached to rotating shaft 331 so as to face motor stator 341. Rotating shaft 331 is rotated by motor stator 341 and motor rotor 342. Gears 351 and 352 are attached to rotating shaft 331 and rotating shaft 332, respectively. The rotation of rotating shaft 331 is transmitted to rotating shaft 332 by gears 351 and 352. Gear 351 is located between rolling bearing 373 and rolling bearing 374, and gear 352 is located between rolling bearing 375 and rolling bearing 376.

A pump chamber 320a is formed within the pump housing 320. Rotors 361 and 362 are disposed within the pump chamber 320a. Rotors 361 and 362 are attached to rotating shafts 331 and 332, respectively. Rotor 361 rotates with the rotation of rotating shaft 331, and rotor 362 rotates with the rotation of rotating shaft 332, causing hydrogen to be drawn into and discharged from pump chamber 320a.

Rolling bearings 371, 372, 373, and 375 are deep groove ball bearings. Rolling bearings 374 and 375 are double-row angular contact ball bearings. Rolling bearings 371, 372, 373, 374, 375, and 376 are rolling bearings according to an embodiment. That is, in rolling bearings 371, 372, 373, 374, 375, and 376, the raceway members and rolling elements are made of steel of the first composition or the second composition, and surface layer 50 is formed on the contact surfaces.

(Method for manufacturing rolling bearing according to embodiment)
A method for manufacturing a rolling bearing will be described below.

Figure 4 is a process diagram showing the manufacturing method of the rolling bearing 100. As shown in Figure 4, the manufacturing method of the rolling bearing 100 includes a preparation step S1, a heat treatment step S2, a finishing step S3, and an assembly step S4. The heat treatment step S2 is performed after the preparation step S1. The finishing step S3 is performed after the heat treatment step S2. The assembly step S4 is performed after the finishing step S3.

In the preparation step S1, a workpiece is prepared for the heat treatment step S2 and finishing step S3. If an inner ring 10 and an outer ring 20 are to be formed, a ring-shaped workpiece is prepared, and if a ball 30 is to be formed, a spherical workpiece is prepared. This workpiece is made of steel of the first composition or the second composition.

The heat treatment process S2 includes a heating process S21, a cooling process S22, and a tempering process S23. In the heating process S21, the workpiece is held at a temperature equal to or higher than the _A1 transformation point for a predetermined period of time. The heating process S21 also includes a nitriding treatment of the workpiece. This nitriding treatment is carried out by holding the workpiece at the above heating temperature in an atmosphere containing a nitrogen source gas (e.g., ammonia gas).

The cooling step S22 is performed after the heating step S21. In the cooling step S22, the workpiece is cooled to a temperature equal to or lower than the Ms transformation point. This cooling is performed by, for example, oil cooling. The tempering step S23 is performed after the cooling step S22. In the tempering step S23, the workpiece is held at a temperature lower than the _A1 transformation point for a predetermined time.

In the finishing process S3, the workpiece is finished (grinded and polished) and cleaned. This prepares the inner ring 10, outer ring 20, and balls 30. In the assembly process S4, the inner ring 10, outer ring 20, and balls 30 are assembled together with the cage 40. This completes the manufacturing of the rolling bearing 100 shown in Figure 1.

(Effects of the rolling bearing according to the embodiment)
The effects of the rolling bearing 100 will be described below.

In the rolling bearing 100, the inner ring 10, outer ring 20, and balls 30 are formed from steel of the first or second composition, so when heat treatment step S2 (nitriding treatment) is performed, fine precipitates are precipitated in the surface layer 50. The areas near the fine precipitates in the surface layer 50 become hydrogen trapping sites, reducing the amount of hydrogen penetration in the surface layer 50. Therefore, the rolling bearing 100 is less likely to suffer from early spalling damage due to hydrogen embrittlement.

An embodiment of the rolling bearing 100 will be described below.
<Sample>
Samples 1 and 2 were prepared as rolling bearing samples. Samples 1 and 2 were single-direction thrust ball bearings conforming to JIS standard 51106 model number, with an inner diameter of 30 mm, an outer diameter of 47 mm, and a width of 11 mm.

The inner and outer rings of Sample 1 were formed from steel with the composition shown in Table 3. The composition shown in Table 3 falls within the range of the first and second compositions. The inner and outer rings of Sample 2 were formed from steel with the composition shown in Table 4. The composition shown in Table 4 falls within the composition range of SUJ2 specified in the JIS standard, but falls outside the range of the first and second compositions. The balls of Samples 1 and 2 were formed from stainless steel (SUS440C).

The inner and outer rings of Sample 1 were subjected to heat treatment step S2. The inner and outer rings of Sample 2 were not subjected to heat treatment step S2. More specifically, the inner and outer rings of Sample 2 were quenched and tempered, but not nitrided.

Figure 5 is a graph showing the distribution of carbon and nitrogen content in the surface layer of the inner and outer rings of Sample 1. In Figure 5, the horizontal axis represents distance from the surface (unit: mm), and the vertical axis represents carbon and nitrogen content (unit: mass percent). As shown in Figure 5 and Table 5, the inner and outer rings of Sample 1 were subjected to heat treatment step S2 (nitriding treatment), and therefore nitrogen was contained in the surface layer of the inner and outer rings of Sample 1. On the other hand, as shown in Table 5, the inner and outer rings of Sample 2 were not subjected to heat treatment step S2 (nitriding treatment), and therefore no nitrogen was contained in the surface layer of the inner and outer rings of Sample 2.

As shown in Table 5, the total area ratio of precipitates in the surface layer portions of the inner and outer rings of Sample 1 was 2.2 percent or more and 7.0 percent or less. The total area ratio of precipitates in the surface layer portions of the inner and outer rings of Sample 2 was 0.07 percent or more and 0.24 percent or less.

As shown in Table 5, the number of precipitates in the surface layer portions of the inner ring and outer ring of Sample 1 was equal to or greater than 66 and equal to or less than 425 per ¹⁰⁰ μm2. The number of precipitates in the surface layer portions of the inner ring and outer ring of Sample ² was equal to or greater than 8 and equal to or less than 50 per 100 μm2.

Figure 6 shows a typical cross-sectional FE-SEM image of the surface layer of the inner and outer rings of Sample 1. As shown in Figure 6, the precipitates in the surface layer of the inner and outer rings of Sample 1 were refined (most of the precipitates had a particle size of 0.5 μm or less). In addition, the precipitates in the surface layer of the inner and outer rings of Sample 2 were not refined (most of the precipitates had a particle size of more than 0.5 μm).

<Hydrogen penetration characteristics>
The hydrogen penetration characteristics of the surface layer portions of the raceway members (inner ring and outer ring) of Sample 1 and Sample 2 were evaluated by the following method. In this evaluation, first, the raceway members of Sample 1 and Sample 2 before use were heated from room temperature to 400°C, and the amount of hydrogen released from the raceway members of Sample 1 and Sample 2 before use was measured. Second, the raceway members of Sample 1 and Sample 2 after use for 50 hours in a hydrogen environment were heated from room temperature to 400°C, and the amount of hydrogen released from the raceway members of Sample 1 and Sample 2 after use for 50 hours in a hydrogen environment was measured.

For Sample 2, the ratio of the amount of hydrogen released before and after use (i.e., the amount of hydrogen released after use divided by the amount of hydrogen released before use) was 3.2. On the other hand, for Sample 1, the ratio of the amount of hydrogen released before and after use was 0.9. This comparison experimentally demonstrated that the formation of surface layer 50 on the contact surface suppresses hydrogen penetration into surface layer 50, thereby suppressing early peeling due to hydrogen embrittlement.

Although the embodiments of the present invention have been described above, the above-described embodiments can be modified in various ways. Furthermore, the scope of the present invention is not limited to the above-described embodiments. The scope of the present invention is defined by the claims, and is intended to include all modifications that are equivalent in meaning to and within the scope of the claims.

The above embodiment is particularly advantageously applicable to raceway members such as inner and outer rings, rolling elements such as balls, and rolling bearings using them.

10 Inner ring, 10a First surface, 10aa Raceway surface, 10b Second surface, 10c Inner peripheral surface, 10d Outer peripheral surface, 20 Outer ring, 20a First surface, 20aa Raceway surface, 20b Second surface, 20c Inner peripheral surface, 20d Outer peripheral surface, 30 Ball, 40 Cage, 50 Surface portion, 100 Rolling bearing, 200 Ball valve, 210 Body, 220 Seat retainer, 220a Internal space, 220aa Seal portion, 220b, 220c Flow path, 230 Ball, 231, 232 Stem, 240 Slide bearing, 300 Hydrogen circulation pump, 310 Motor housing, 320 Pump housing, 320a Pump chamber, 331, 332 Rotating shaft, 341 Motor stator, 342 Motor rotor, 351, 352 gears, 361, 362 rotors, 371, 372, 373, 374, 375, 376 rolling bearings, A central shaft, S1 preparation process, S2 heat treatment process, S3 finishing process, S4 assembly process, S21 heating process, S22 cooling process.

Claims (8)

A rolling element made of hardened steel having a contact surface,
The rolling member is for use in hydrogen-utilizing equipment,
the rolling member has a surface layer portion that is a region having a depth of up to 20 μm from the contact surface,
The steel contains 0.70 mass percent to 1.10 mass percent carbon, 0.15 mass percent to 0.35 mass percent silicon, 0.30 mass percent to 0.60 mass percent manganese, 1.30 mass percent to 1.60 mass percent chromium, 0.01 mass percent to 0.50 mass percent molybdenum, and 0.01 mass percent to 0.50 mass percent vanadium, with the balance being iron and unavoidable impurities;
A rolling member, wherein, in a cross-sectional view perpendicular to the contact surface, there are a total of 60 or more precipitates ^per 100 μm2 in the surface layer portion, each having a particle size of 0.50 μm or less, and the total area ratio of the precipitates in the surface layer portion is 1% or more and 10% or less. The rolling contact member of claim 1, wherein the steel contains 0.90 to 1.10 mass percent carbon, 0.20 to 0.30 mass percent silicon, 0.40 to 0.50 mass percent manganese, 1.40 to 1.60 mass percent chromium, 0.20 to 0.30 mass percent molybdenum, and 0.20 to 0.30 mass percent vanadium, with the remainder consisting of iron and unavoidable impurities. A rolling member according to claim 1 or 2, wherein the nitrogen content in the surface layer is 0.3 mass percent or more and 0.5 mass percent or less. 4. The rolling member according to claim 1, wherein, in a cross-sectional view perpendicular to the contact surface, a total of 80 ^or more precipitates having a particle size of 0.50 μm or less are present per 100 μm2 in the surface layer portion, and a total area ratio of the precipitates in the surface layer portion is 2% or more and 7% or less. A rolling member according to any one of claims 1 to 4, wherein the volume fraction of retained austenite at a depth of 50 μm from the contact surface is 20% or more and 40% or less. A rolling contact member according to any one of claims 1 to 5, wherein the volume fraction of retained austenite at a depth of 50 μm from the contact surface is 25% or more and 35% or less. A rolling member according to any one of claims 1 to 6, wherein the hardness at a position 50 μm deep from the contact surface is 653 Hv or more and 800 Hv or less. A rolling bearing,
A track member;
a rolling element disposed in contact with the raceway member,
The rolling bearing is for use in hydrogen-utilizing equipment,
A rolling bearing, wherein at least one of the raceway member and the rolling element is the rolling member according to any one of claims 1 to 7.