Machine parts

Abstract

To provide a machine component capable of suppressing aging of a dimension thereof, and capable of improving rolling motion fatigue life of an impression origin type on a surface thereof, the machine component being used for a hydrogen utilization appliance.SOLUTION: A machine component is formed of a steel and has a surface. The steel has been subjected to hardening and tempering. The machine component has a nitriding layer on a surface thereof, and a core part which is separated from the surface by a distance larger than that by which the nitriding layer is separated from the surface. The machine component is used for a hydrogen utilization appliance. Nitrogen concentration in the steel on the surface is equal to or greater than 0.3 mass%. Hardness of the steel on the surface is 850 Hv or greater. A residual austenite amount in the steel in the core part is 9 volume% or less. Dislocation concentration of the residual austenite in the steel in the core part, is equal to or greater than 4.0×1014 m-2, and the steel is a bearing steel.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a machine part, and more particularly to a machine part for hydrogen-utilizing equipment, which is made of hardened and tempered steel. [Background technology]

[0002] For example, bearings are used in hydrogen-utilizing equipment for hydrogen stations. As the operating rate of hydrogen stations increases or bearings become smaller, the temperature of rolling bearings increases during use.

[0003] In hardened and tempered rolling bearings, the retained austenite decomposes as the operating temperature rises. As a result, the volume expansion associated with the decomposition of the retained austenite causes dimensional changes in the components of the rolling bearing. Furthermore, if the dimensional change rate of the rolling bearing's raceways increases, problems such as creep may occur, the clearance between the raceway surface and the rolling elements may decrease, resulting in increased contact pressure and premature damage, and abnormal noise and vibration may increase due to reduced dimensional accuracy.

[0004] Japanese Patent Laid-Open Publication No. 2001-99163 (Patent Document 1) describes a raceway ring for a rolling bearing. The raceway ring described in Patent Document 1 is made of steel that has been quenched and tempered. In the raceway ring described in Patent Document 1, the amount of retained austenite in the steel is substantially zero. The raceway ring described in Patent Document 1 is suppressed from changing in dimension over time during use. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-99163 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the raceway ring described in Patent Document 1, the hardness of the steel on the surface is less than 752 Hv. When the hardness of the steel on the surface of the raceway ring is reduced, the indentation-initiated rolling contact fatigue life decreases. Therefore, there is room for improvement in the indentation-initiated rolling contact fatigue life of the raceway ring described in Patent Document 1.

[0007] The present invention has been made in view of the above-mentioned problems of the prior art. More specifically, the present invention provides a mechanical component for hydrogen-utilizing equipment that can suppress dimensional changes over time and can improve the rolling contact fatigue life of the surface indentation-initiated type. [Means for solving the problem]

[0008] A mechanical component according to one aspect of the present invention is made of steel and has a surface. The mechanical component is for use in hydrogen-utilizing equipment. The steel is quenched and tempered. The mechanical component has a nitriding layer on the surface and a core that is further from the surface than the nitriding layer. The nitrogen concentration in the steel at the surface is 0.3 mass percent or more. The hardness of the steel at the surface is 850 Hv or more. The amount of retained austenite in the steel at the core is 9 volume percent or less. The dislocation density of the retained austenite in the steel at the core is 4.0 × 10 14 m -2 That's all. The steel is bearing steel.

[0009] In the above machine part, the nitrogen concentration in the steel at the surface is X (unit: mass percent), and the dislocation density of martensite in the steel at the surface is Y (unit: m -2 ) then 934893.48+379.96×X-330.96×Y 2 -5.41×10 4 ×logY+783.83×logX 2 ≧0 may be satisfied.

[0010] In the mechanical component, the hardness of the steel on the surface after holding at 160°C for 2500 hours may be 850 Hv or more.

[0011] In the mechanical component, the steel may contain 0.95 mass percent or more and 1.10 mass percent carbon, 1.40 mass percent or more and 1.60 mass percent or less chromium, less than 0.30 mass percent silicon, less than 0.50 mass percent manganese, and less than 0.0080 mass percent sulfur. [Effects of the Invention]

[0012] According to the machine component of the present invention, it is possible to suppress dimensional changes over time and to improve the rolling fatigue life of the surface indentation-initiated type. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a cross-sectional view of the inner ring 10. [Figure 2] FIG. 2 is an enlarged cross-sectional view of the ball valve 200. [Figure 3] FIG. 3 is a cross-sectional view of a hydrogen circulation pump 300. [Figure 4] 3A to 3C are process diagrams showing a manufacturing method of the inner ring 10. [Figure 5] 2 is a cross-sectional view of a workpiece 20. FIG. [Figure 6] 4 is a schematic graph showing the shape of the surface of a bearing ring on which an indentation is formed. [Figure 7] 1 is a graph showing the relationship between the hardness of the steel on the surface of the inner ring 10 and the swelling around the indentation. DETAILED DESCRIPTION OF THE INVENTION

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and redundant description will not be repeated.

[0015] (Configuration of mechanical component according to embodiment) The configuration of the mechanical component according to the embodiment will be described below.

[0016] The mechanical component according to the embodiment 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 any of a reciprocating type (reciprocating), rotary type (screw type), centrifugal type, and axial type. The hydrogen-utilizing equipment may be a high-pressure hydrogen pressure reducing valve or a hydrogen circulation pump for a fuel cell vehicle. The mechanical component according to the embodiment may be used in an application where it is exposed to hydrogen.

[0017] The mechanical component according to the embodiment is, for example, a raceway ring of a rolling bearing. The mechanical component according to the embodiment may also be a rolling element of a rolling bearing. The rolling bearing is, 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 mechanical component according to the embodiment may also be a sliding member such as a shaft, a ball screw, or a plain bearing, or a structural member such as a housing. Here, an inner ring 10 of a rolling bearing will be described as an example of the mechanical component according to the embodiment.

[0018] The configuration of the inner ring 10 will be described below. Fig. 1 is a cross-sectional view of inner ring 10. As shown in Fig. 1, inner ring 10 has a first end face 10a, a second end face 10b, an inner circumferential surface 10c, and an outer circumferential surface 10d. The first end face 10a, the second end face 10b, the inner circumferential surface 10c, and the outer circumferential surface 10d form the surface of inner ring 10. Inner ring 10 is ring-shaped.

[0019] The central axis of the inner ring 10 is defined as the central axis A. The direction along the central axis A is defined as the axial direction. The direction perpendicular to the central axis A and passing through the central axis A is defined as the radial direction. The direction along the circumference of a circle centered on the central axis A is defined as the circumferential direction.

[0020] The first end face 10a and the second end face 10b are axial end faces of the inner ring 10. The second end face 10b is the surface opposite the first end face 10a in the axial direction.

[0021] The inner peripheral surface 10c extends in the circumferential direction. The inner peripheral surface 10c faces the central axis A. One end and the other end in the axial direction of the inner peripheral surface 10c are connected to the first end face 10a and the second end face 10b, respectively. Although not shown, the inner ring 10 is fitted onto a shaft at the inner peripheral surface 10c.

[0022] The outer peripheral surface 10d extends in the circumferential direction. 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 radial direction. One end and the other end of the outer peripheral surface 10d in the axial direction are connected to the first end face 10a and the second end face 10b, respectively. The outer peripheral surface 10d has a raceway surface 10da. The raceway surface 10da is the portion of the outer peripheral surface 10d that contacts the rolling elements (not shown). The raceway surface 10da is located, for example, at the center of the outer peripheral surface 10d in the axial direction. The raceway surface 10da extends in the circumferential direction. In a cross-sectional view, the raceway surface 10da is partially arc-shaped.

[0023] The inner ring 10 is made of steel. The steel that constitutes the inner ring 10 is hardened and tempered. The steel that constitutes the inner ring 10 may be bearing steel. Bearing steel refers to high-carbon chromium steel with a carbon concentration of 0.90 mass percent or more and 1.1 mass percent or less and a chromium concentration of 0.3 mass percent or more and 2.1 mass percent or less.

[0024] Specific examples of bearing steel include SUJ2, SUJ3, SUJ4, and SUJ5 specified in JIS standards, 50100, 51100, 52100, and A485 Grade 1 specified in ASTM standards, and 100Cr6, 100CrMnSi4-4, 100CrMnSi6-4, 100CrMo7, 100CrMo7-3, and 100CrMnMoSi8-4-6 specified in ISO standards.

[0025] The carbon concentration in the steel constituting the inner ring 10 may be, for example, 0.95 mass percent or more and 1.10 mass percent or less. The steel constituting the inner ring 10 may contain 1.40 mass percent or more and 1.60 mass percent or less chromium, less than 0.30 mass percent silicon, less than 0.50 mass percent manganese, and less than 0.0080 mass percent sulfur. In this case, the steel constituting the inner ring 10 does not need to contain silicon, manganese, or sulfur.

[0026] The surface of the inner ring 10 has been subjected to nitriding or carbonitriding treatment. That is, the inner ring 10 has a nitrided layer 11 on its surface (the surface of the inner ring 10 is formed as the nitrided layer 11). Nitrogen is dissolved in the steel in the nitrided layer 11. The portion of the inner ring 10 that is farther from the surface than the nitrided layer 11 is referred to as the core portion 12. From another perspective, the core portion 12 is the interior portion of the inner ring 10 other than the nitrided layer 11. Nitrogen is not dissolved in the steel in the core portion 12. The nitrogen concentration in the steel at the surface of the inner ring 10 is 0.3 mass percent or more. It is preferable that the nitrogen concentration in the steel at the surface of the inner ring 10 be 0.4 mass percent or more. The nitrogen concentration at the surface of the inner ring 10 is measured using, for example, an EPMA (Electron Probe Micro Analyzer).

[0027] The hardness of the steel on the surface of the inner ring 10 is 850 Hv or more. After holding at 160°C for 2500 hours, the hardness of the steel on the surface of the inner ring 10 is preferably 850 Hv or more. The hardness of the steel on the surface of the inner ring 10 is measured using the Vickers hardness test method specified in the JIS standard (JIS Z 2244:2009). The load used when measuring the hardness of the steel on the surface of the inner ring 10 is 300 g. The hardness of the steel on the surface of the inner ring 10 is measured at at least three locations and obtained by averaging the measured values.

[0028] The amount of retained austenite in the steel in the core portion 12 is 9 volume percent or less. The amount of retained austenite in the steel in the core portion 12 is, for example, 0.1 volume percent or more.

[0029] The amount of retained austenite in the steel in the core portion 12 is measured using X-ray diffraction. When measuring the amount of retained austenite in the steel in the core portion 12 by X-ray diffraction, the X-ray tube is preferably a chrome tube. When measuring the amount of retained austenite in the steel in the core portion 12, the inner ring 10 is preferably electropolished to prevent work-induced transformation of the retained austenite.

[0030] The dislocation density of the retained austenite in the steel in the core 12 is 4.0 × 10 14 m -2 The dislocation density of the retained austenite in the steel in the core portion 12 is preferably 6.0×10 14 m -2 That's all.

[0031] The nitrogen concentration in the steel at the surface of the inner ring 10 is X (unit: mass percent), and the dislocation density of martensite in the steel at the surface of the inner ring 10 is Y (unit: m -2 ) and the hardness of the steel on the surface of the inner ring 10 is Z (unit: Hv). In this case, Z is calculated as follows: 935743.48 + 379.96 × X - 330.96 × Y 2 -5.41×10 4 ×logY+783.83×logX 2 It can be calculated using (Equation 1). Equation 1 can be obtained by performing multiple regression analysis after determining the value of Z when X and Y are changed through experiments. From Equation 1, in order to make the hardness of the steel on the surface of the inner ring 10 850 Hv or more, it is necessary to obtain the following: 934893.48 + 379.96 × X - 330.96 × Y 2 -5.41×10 4 ×logY+783.83×logX 2 ≧0 (Equation 2) is satisfied.

[0032] The dislocation density of the retained austenite in the steel in the core portion 12 and the dislocation density of the martensite in the steel on the surface of the inner ring 10 are measured by X-ray diffraction. More specifically, first, an X-ray profile is obtained by performing X-ray diffraction using a cobalt tube-type X-ray diffractometer. Second, the half-width obtained from the X-ray profile is separated into crystallite size and strain, and the Williamson-Hall equation is applied to calculate the dislocation density of the retained austenite in the steel in the core portion 12 and the dislocation density of the martensite in the steel on the surface of the inner ring 10. The dislocation density of martensite is calculated using strain obtained by Rietveld analysis of the peaks in the X-ray profile corresponding to the (110), (200), (211), and (220) planes, while the dislocation density of retained austenite is calculated using strain obtained by Rietveld analysis of the peaks in the X-ray profile corresponding to the (111), (200), (220), (311), and (222) planes. Note that using Rietveld analysis improves the accuracy of dislocation density measurement, and the instrument constants are taken into account when performing the Rietveld analysis.

[0033] The dimensional change rate of the inner ring 10 after holding at 160°C for 2500 hours is 40×10 -5 Preferably, the dimensional change rate of the inner ring 10 after being held at 160°C for 2500 hours is 15×10 or less. -5 The dimensional change rate of the inner ring 10 is calculated by subtracting the dimension of the inner ring 10 before holding from the dimension of the inner ring 10 after holding, and dividing the result by the dimension of the inner ring 10 before holding.

[0034] <Application examples of rolling members according to embodiments> The mechanical component 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.

[0035] 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 formed therein. The 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.

[0036] Stems 231 and 232 are connected to the upper and lower ends of ball 230, respectively. When stems 231 and 232 rotate around the 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. Note that hydrogen flows through flow paths 220b, flow paths 220c, and the through holes formed in ball 230.

[0037] The sliding bearing 240 is cylindrical and is attached to the body 210 at its outer peripheral surface. The sliding bearing 240 rotatably supports the stem 231 (stem 232). The sliding bearing 240 is a mechanical component according to an embodiment. That is, the sliding bearing 240 has a nitrided layer on its surface, and also has a core that is more distant from the surface than the nitrided layer. The nitrided layer and core in the sliding bearing 240 have the same configurations as the nitrided layer 11 and core 12, respectively.

[0038] <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.

[0039] Motor housing 310 is attached to pump housing 320. One end of rotating shaft 331 is disposed within motor housing 310, and the other end of rotating shaft 331 is disposed within pump housing 320. One end and the other end of rotating shaft 331 are rotatably supported by rolling bearing 371 disposed within motor housing 310 and rolling bearing 372 disposed within pump housing 320, respectively. Rotating shaft 331 is rotatably supported between its one and other ends by rolling bearing 373 and rolling bearing 374 disposed within pump housing 320.

[0040] Rotating shaft 332 is disposed within pump housing 320. One end of rotating shaft 332 is rotatably supported by rolling bearing 375 disposed within pump housing 320. Rotating shaft 332 is rotatably supported at a position away from the one end by rolling bearing 376 disposed within pump housing 320.

[0041] 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 shafts 331 and 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.

[0042] A pump chamber 320a is formed within pump housing 320. A rotor 361 and a rotor 362 are disposed within pump chamber 320a. Rotors 361 and 362 are attached to rotary shafts 331 and 332, respectively. Rotor 361 rotates with the rotation of rotary shaft 331, and rotor 362 rotates with the rotation of rotary shaft 332, causing hydrogen to be drawn into pump chamber 320a and discharged from pump chamber 320a.

[0043] Rolling bearings 371, 372, 373, and 375 are deep groove ball bearings. Rolling bearings 374 and 375 are double-row angular contact ball bearings. The components (races and rolling elements) of rolling bearings 371, 372, 373, 374, 375, and 376 are mechanical components according to the embodiment. That is, in rolling bearings 371, 372, 373, 374, 375, and 376, the raceways and rolling elements have nitrided layers on their surfaces and cores that are further from the surfaces than the nitrided layers. The nitrided layers and cores of the raceways and rolling elements have the same structures as nitrided layer 11 and core 12, respectively.

[0044] (Mechanical component manufacturing method according to the embodiment) A method for manufacturing the inner ring 10 will be described below.

[0045] Fig. 4 is a process diagram showing a method for manufacturing inner ring 10. As shown in Fig. 4, the method for manufacturing inner ring 10 includes a preparation step S1, a nitriding step S2, a quenching step S3, a cooling step S4, a tempering step S5, and a post-treatment step S6.

[0046] In the preparation step S1, a workpiece 20 is prepared. FIG. 5 is a cross-sectional view of the workpiece 20. As shown in FIG. 5, the workpiece 20 is ring-shaped and has a first end face 20a, a second end face 20b, an inner circumferential surface 20c, and an outer circumferential surface 20d. The first end face 20a, the second end face 20b, the inner circumferential surface 20c, and the outer circumferential surface 20d are surfaces that will become the first end face 10a, the second end face 10b, the inner circumferential surface 10c, and the outer circumferential surface 10d, respectively, after the post-processing step S6 is completed. The workpiece 20 is formed from the same steel as the inner ring 10.

[0047] In the nitriding step S2, the workpiece 20 is subjected to a nitriding treatment. The nitriding treatment of the workpiece 20 is performed by heating and holding the workpiece 20 in an atmospheric gas containing a nitrogen source. The heating temperature and the nitrogen concentration in the atmospheric gas in the nitriding step S2 are set so that a compound layer is not formed on the surface of the workpiece 20. By performing the nitriding step S2, nitrogen penetrates from the surface of the workpiece 20 to the interior, and the nitrogen is dissolved in the workpiece 20. Note that the nitriding step S2 is performed so that the nitrogen reaches a position deeper than the position that will become the surface of the inner ring 10 after the post-treatment step S6 is performed.

[0048] The workpiece 20 may be subjected to a carbo-nitriding process instead of the nitriding process S2. The carbo-nitriding treatment of the workpiece 20 is performed by heating and holding the workpiece in an atmospheric gas containing a nitrogen source and a carbon source. The heating temperature in the carbo-nitriding process and the carbon and nitrogen concentrations in the atmospheric gas are set so that a compound layer is not formed on the surface of the workpiece 20. By performing the carbo-nitriding process, carbon and nitrogen penetrate from the surface of the workpiece 20 to the interior, and the carbon and nitrogen are dissolved in the workpiece 20. The carbo-nitriding process is performed so that the nitrogen and carbon reach a position deeper than the position that will become the surface of the inner ring 10 after the post-treatment process S6 is performed.

[0049] In the quenching step S3, the workpiece 20 is quenched. The workpiece 20 is quenched by heating the workpiece 20 to a temperature equal to or higher than the A1 transformation point of the steel that constitutes the workpiece 20, and then cooling it to a temperature equal to or lower than the Ms transformation point of the steel that constitutes the workpiece 20. By performing the quenching step S3, martensite and retained austenite are generated in the steel that constitutes the workpiece 20.

[0050] In the cooling step S4, sub-zero treatment is performed on the workpiece 20. In the cooling step S4, cryo-treatment (ultra-sub-zero treatment) may be performed on the workpiece 20. In the sub-zero treatment, the workpiece 20 is cooled to a temperature above -100°C and below room temperature. In the cryo-treatment, the workpiece 20 is cooled to a temperature below -100°C. By performing the cooling step S4, part of the retained austenite in the steel that constitutes the workpiece 20 is transformed into martensite.

[0051] In the tempering step S5, the workpiece 20 is tempered. The workpiece 20 is tempered by heating the workpiece 20 to a temperature below the A1 transformation point of the steel that constitutes the workpiece 20. More specifically, the workpiece 20 is tempered by heating the workpiece 20 to a temperature of about 180°C. In the post-treatment step S6, the surface of the workpiece 20 is subjected to machining such as grinding and polishing. In this manner, the inner ring 10 having the structure shown in FIG. 1 is manufactured.

[0052] (Effects of the mechanical component according to the embodiment) In the bearing rings of rolling bearings used in hydrogen-utilizing equipment, creep can occur due to dimensional changes caused by the decomposition of martensite in the steel as temperatures rise during use. One possible way to suppress dimensional changes over time in quenched and tempered steel bearing rings is to temper them at high temperatures to reduce the amount of retained austenite. However, while high-temperature tempering can suppress dimensional changes over time by reducing the amount of retained austenite, it also reduces the hardness of the steel on the surface of the bearing ring.

[0053] When a foreign object becomes caught between the surface of the raceway and the rolling element, an indentation is formed on the surface of the raceway. FIG. 6 is a schematic graph showing the shape of the surface of the raceway on which an indentation is formed. As shown in FIG. 6, the surface of the raceway bulges around the indentation. FIG. 7 is a graph showing the relationship between the hardness of the steel on the surface of the inner ring 10 and the bulge around the indentation. In FIG. 7, the horizontal axis represents hardness (unit: Hv), and the vertical axis represents the amount of bulge around the indentation. Note that the vertical axis in FIG. 7 represents the ratio to the amount of bulge when cooling step S4 is not performed. As shown in FIG. 7, the amount of bulge around the indentation increases as the hardness of the steel on the surface of the inner ring 10 decreases.

[0054] If the amount of swelling around the indentation becomes large, stress will be concentrated on the swelling around the indentation, making fatigue fracture more likely to occur from the indentation. Therefore, if tempering is performed at high temperature to suppress dimensional changes over time, the rolling fatigue life of the bearing ring may be insufficient.

[0055] When the amount of retained austenite in steel is low and the retained austenite is surrounded by martensite with a high dislocation density (i.e., poor deformability), the retained austenite is constrained or stressed by the martensite, reducing its lattice spacing (lattice constant), resulting in a high dislocation density of the retained austenite in the steel. Even when the volume of such retained austenite expands due to decomposition, it is constrained by the surrounding martensite with a high dislocation density, so that even if the retained austenite decomposes during use, the dimensional change associated with the decomposition is small.

[0056] The inner ring 10 has been subjected to sub-zero treatment or cryo-treatment, thereby reducing the amount of retained austenite in the steel in the core portion 12. More specifically, in the inner ring 10, the amount of retained austenite in the steel in the core portion 12 is 9 volume percent or less.

[0057] Furthermore, since the inner ring 10 has undergone sub-zero treatment or cryo-treatment, the dislocation density of martensite in the steel in the core portion 12 is increased, and as a result, the dislocation density of the retained austenite in the steel in the core portion 12 is also increased. More specifically, in the inner ring 10, the dislocation density of the retained austenite in the steel is 4.0 × 10 14 m -2 That's all.

[0058] As described above, since the retained austenite is surrounded by martensite with a high dislocation density in the steel of the core portion 12, even if the retained austenite in the steel of the core portion 12 decomposes due to the rise in temperature that accompanies use of the inner ring 10, the volume expansion that accompanies the decomposition is restrained by the surrounding martensite with a high dislocation density, and dimensional change is therefore unlikely to occur. In this way, dimensional change over time that accompanies use of the inner ring 10 is suppressed.

[0059] Furthermore, because the inner ring 10 has not been tempered at high temperatures, the decomposition of martensite at the surface of the inner ring 10 progresses only slightly. Furthermore, the nitrogen concentration in the steel at the surface of the inner ring 10 is 0.3 mass percent or more, and the steel at the surface of the inner ring 10 is solid-solution strengthened. As a result, the hardness of the steel at the surface of the inner ring 10 is 850 Hv or more. As shown in Figure 7, as the hardness of the steel at the surface of the inner ring 10 increases, the amount of swelling around the indentation decreases. Therefore, the inner ring 10 also improves the rolling fatigue life at the indentation origin.

[0060] (Hardness evaluation test) Samples 1 to 17 were prepared to evaluate the relationship between hardness, martensite dislocation density, and nitrogen concentration on the surface of quenched and tempered steel machine parts. Samples 1 to 17 were ring-shaped, with an inner diameter of 54 mm, an outer diameter of 60 mm, and a width of 15 mm. As shown in Table 1, the steel type, nitrogen concentration in the steel at the sample surface, and dislocation density of martensite in the steel at the sample surface were varied for Samples 1 to 17. "OK" and "NG" in the "Fulfillment of Equation 2" column in Table 1 indicate that the above equation 2 is fulfilled and that the above equation 2 is not fulfilled, respectively.

[0061] [Table 1]

[0062] The nitrogen concentration in the steel at the surface of each sample was adjusted by changing the heating temperature and holding time in the nitriding or carbonitriding treatment, and the dislocation density of martensite in the steel at the surface of each sample was adjusted by changing the cooling temperature and holding time in the sub-zero treatment or cryo-treatment.

[0063] The above formula 2 was satisfied in Samples 1 to 5, Sample 7, and Samples 9 to 12. On the other hand, the above formula 2 was not satisfied in Samples 6, Sample 8, and Samples 13 to 17.

[0064] In Samples 1 to 5, Sample 7, and Samples 9 to 12, the hardness of the steel on the sample surface was 850 Hv or more. On the other hand, in Samples 6, Sample 8, and Samples 13 to 17, the hardness of the steel on the sample surface was less than 850 Hv.

[0065] From this comparison, it became clear that the hardness of the steel on the surface of the mechanical component becomes 850 Hv or more and the rolling fatigue life is improved when the above formula 2 is satisfied. From another perspective, it became clear that the hardness of the steel on the surface of the mechanical component, and therefore the rolling fatigue life of the mechanical component, is improved by increasing both the nitrogen concentration in the steel on the surface of the mechanical component and the dislocation density of martensite in the steel on the surface of the mechanical component.

[0066] (Evaluation test of dimensional changes over time) To evaluate the change in dimensions over time, the above-mentioned Samples 3, 7, and 10 were used. Table 2 shows the amount of retained austenite in the steel at the core 12 of Samples 3, 7, and 10, and the dislocation density in the steel at the core 12 of Samples 3, 7, and 10.

[0067] [Table 2]

[0068] The dislocation density of martensite in the steel in the core 12 of each sample and the amount of retained austenite in the martensite in the steel in the core 12 of each sample were adjusted by changing the cooling temperature and holding time in the sub-zero treatment or cryo-treatment.

[0069] The dislocation density of martensite in the steel at the core 12 of the sample is 4.0 × 10 14 m -2 The condition A is that the amount of retained austenite in the steel in the core portion 12 of the sample is 9 volume percent or less.

[0070] In Sample 3 and Sample 7, Conditions A and B were satisfied. On the other hand, in Sample 10, Conditions A and B were not satisfied. In Sample 3 and Sample 7, the dimensional change rate after holding at 160°C for 2500 hours was 40×10 -5 On the other hand, in Sample 10, the dimensional change rate after holding at 160°C for 2500 hours was 50 × 10 -5 This comparison reveals that satisfying conditions A and B suppresses the dimensional changes of machine parts over time.

[0071] The dislocation density of martensite in the steel at the core 12 of the sample is 6.0 × 10 14 m -2 or more is defined as condition C. Sample 3 satisfies condition C, and the dimensional change rate after holding at 160°C for 2500 hours is 15×10 -5 On the other hand, in sample 7, condition C was not satisfied, and the dimensional change rate after holding at 160°C for 2500 hours was 15 × 10 -5 This comparison reveals that satisfying condition C further suppresses the dimensional changes of mechanical parts over time.

[0072] Condition D is defined as a nitrogen concentration of 0.4 mass percent or more on the surface of the sample. Sample 3 satisfied condition D, and the surface hardness after holding at 160°C for 2500 hours was 850 Hv or more. On the other hand, Sample 7 did not satisfy condition C, and the surface hardness after holding at 160°C for 2500 hours was less than 850 Hv. This comparison reveals that by satisfying condition D, the surface hardness of mechanical parts can be maintained even after use in high-temperature environments.

[0073] Although the embodiments of the present invention have been described above, the above embodiments can be modified in various ways. Furthermore, the scope of the present invention is not limited to the above embodiments. The scope of the present invention is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0074] A central shaft, S1 preparation process, S2 nitriding process, S3 quenching process, S4 cooling process, S5 tempering process, S6 post-treatment process, 10 inner ring, 10a first end face, 10b second end face, 10c inner peripheral surface, 10d outer peripheral surface, 10da raceway surface, 11 nitriding layer, 12 core portion, 20 workpiece, 20a first end face, 20b second end face, 20c inner peripheral surface, 20d outer peripheral surface, 200 ball valve, 210 body, 220 seat retainer, 220a internal space, 220aa seal portion, 220b, 220c flow path, 230 ball, 231 stem, 232 stem, 240 plain bearing, 300 hydrogen circulation pump, 310 motor housing, 320 pump housing, 320a Pump chamber, 331 Rotating shaft, 332 Rotating shaft, 341 Motor stator, 342 Motor rotor, 351, 352 Gears, 361, 362 Rotors, 371, 372, 373, 374, 375, 376 Rolling bearings.

Claims

1. A machine part made of quenched and tempered steel and having a surface, a nitriding layer on the surface in which nitrogen is dissolved, and a core located farther from the surface than the nitriding layer; The mechanical component is for use in hydrogen-utilizing equipment, The nitrogen concentration in the steel at the surface is 0.3 mass percent or more, The hardness of the steel at the surface is 850 Hv or more, The amount of retained austenite in the steel in the core is 9 volume percent or less, The dislocation density of the retained austenite in the steel in the core portion is 4.0 × 10 １４ m －２ That's all, the steel is a bearing steel, The mechanical part, wherein the bearing steel is SUJ2 as specified in the JIS standard.

2. 2. The mechanical component according to claim 1, wherein the hardness of the steel at the surface after holding at 160°C for 2500 hours is 850 Hv or more.

3. 2. The machine component according to claim 1, wherein the steel contains, by weight, 0.95 to 1.10 percent carbon, 1.40 to 1.60 percent chromium, less than 0.30 percent silicon, less than 0.50 percent manganese, and less than 0.0080 percent sulfur.

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