METHOD FOR MANUFACTURING ROLLING COMPONENT

Abstract

A method for selecting a steel material for a rolling component to be used in an environment in which hydrogen enters steel. The method includes determining a maximum contact surface pressure Pmax acting on a rolling surface of the rolling component, and using, as a material for the rolling component, a material whose inclusion has a radius not greater than a radius d determined by the following formula (1) and not less than 1.0 m,

d=64729(Pmax/4).sup.1.441 (1).

Claims

1. A method for selecting a steel material for a rolling component to be used in an environment in which hydrogen enters steel, the method comprising: determining a maximum contact surface pressure Pmax acting on a rolling surface of the rolling component; and using, as a material for the rolling component, a material whose inclusion has a radius not greater than a radius d determined by the following formula (1) and not less than 1.0 m,
d=64729(Pmax/4).sup.1.441 (1).

2. A method for manufacturing a rolling component made of a steel material to be used in an environment in which hydrogen enters steel, the method comprising selecting the steel material by using the method for selecting the rolling component material as claimed in claim 1.

3. A method for manufacturing a bearing ring of a rolling bearing to be used in an environment in which hydrogen enters steel, the method comprising selecting a steel material for the bearing ring by the method for selecting the rolling component material as claimed in claim 1, wherein the maximum contact surface pressure Pmax is 1.0 GPa.

4. A method for manufacturing a rolling element of a rolling bearing to be used in an environment in which hydrogen enters steel, the method comprising selecting a steel material for the rolling element by the method for selecting the rolling component material as claimed in claim 1, wherein the maximum contact surface pressure Pmax is 1.0 GPa.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In any event, the present invention will become more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings, However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views, and:

[0029] FIG. 1 includes diagrams (A) to (D) in which the diagram (A) is a chart showing a relationship between a shear stress amplitude and the number of load application at occurrence of a shear crack obtained in a hydrogen-charged ultrasonic torsional fatigue test, the diagram (B) is a picture of a shear crack in a test piece broken from the surface origin, the diagram (C) is an optical microscope photograph of a shear crack occurrence site, and the diagram (D) is a partially enlarged electron microscope photograph of the photograph of the diagram (C);

[0030] FIG. 2 shows a chart showing a relationship between the circumferential position and the axial position of a crack occurrence position examined in the test, and an electron microscope photograph of an inclusion;

[0031] FIG. 3 is a chart showing a relationship between stress around an origin inclusion and an inclusion radius d;

[0032] FIG. 4 is an explanatory diagram of an ultrasonic torsional fatigue tester used in the test;

[0033] FIG. 5 is an explanatory diagram showing a hydrogen charging method used in the test;

[0034] FIG. 6 is a schematic diagram of a test piece for the test;

[0035] FIG. 7 is a chart showing a relationship between a maximum contact surface pressure Pmax and a maximum shear stress T in each contact state;

[0036] FIG. 8 is an explanatory diagram showing a concept of a relationship between a rolling shear fatigue crack and an inclusion; and

[0037] FIG. 9 is an explanatory diagram of a Mode II crack.

DESCRIPTION OF EMBODIMENTS

[0038] A rolling component material, a rolling component, a method for selecting the rolling component material, and a method for manufacturing the rolling component according to an embodiment of the present invention will be described with reference to the drawings. The rolling component material is a steel material for a rolling component to be used in an environment in which hydrogen enters steel, and the radius of a non-metallic inclusion of the steel material is not greater than a radius d determined by the following formula (1) and is not less than 1.0 m,

d=64729(Pmax/4).sup.1.441 (1)

[0039] The enviromnent in which hydrogen enters steel means an environment in which water enters or an environment in which sliding easily occurs. When a fresh surface or a newly formed surface of a metal occurs due to sliding, the activity of such a fresh surface is high, and therefore, a lubricant is decomposed to generate hydrogen. In addition, the rolling component is a generic term for mechanical element components each of which makes rolling contact or makes rolling contact with sliding as described above, and examples thereof include a bearing ring and a rolling element of a rolling bearing, a joint component having a raceway groove of a constant velocity joint, a torque transmission ball, and a screw shaft, a nut and a ball of a ball screw. Moreover, the inclusion may include holes. in the case where holes are present therein, a possibility of breakage is the highest.

[0040] Examples of the rolling component to be used in an environment in which hydrogen enters steel include electrical auxiliary equipment (lighting, an air-conditioner, a wiper, a power window, etc.), a construction machine (in particular, a revolving seat and a revolving component thereof), a ball screw of a CVT (continuously variable transmission), a machine tool, a windmill, and an acceleration and deceleration machine.

[0041] The reason why the radius d of the inclusion is not less than 1.0 m is that carbide does not become a starting point and the size of carbide is less than 1.0 m.

[0042] The radius d and the stress r in a hydrogen entry environment were obtained by a hydrogen-charged ultrasonic torsional fatigue test as described below. A method of the test will be described in detail later.

[0043] Diagram (A) of FIG. 1 shows the results of the number of load application (number of cycles) and a shear stress amplitude (MPa) when a shear crack occurred as shown in diagram (B) of FIG. 1 by the test, In the diagram (A) of FIG. 1, each plot of a white triangle shows a case where a shear crack, of a subsurface origin, occurred with hydrogen charging, and each plot of a black triangle shows a case where a shear crack, of a surface origin, occurs with hydrogen charging. For comparison, each plot of a black circle shows a case where a shear crack occurred without hydrogen charging.

[0044] In an ultrasonic torsional fatigue test in an ordinary environment, a crack occurs from the surface of a test piece. This is because the surface receives greatest stress. However, in the case of the test piece subjected to hydrogen charging, as shown in diagram (A) of FIG. 2, it was found that a crack occurred from an internal portion of the test piece. In diagram (A) of FIG. 2, the horizontal axis and the vertical axis indicate a circumferential position and an axial position, respectively, in the test piece shown in diagram (B) of FIG. 2, and each plotted point indicates the starting position or origin position of a breakage.

[0045] A stress at the plotted origin position of each breakage, that is, a stress acting on an origin inclusion is determined by torsional moment applied to the test piece and the distance of the origin position from the center of the test piece. In addition, the size (radius) d of the inclusion that was the origin of each breakage was obtained from an electron microscope photograph (SEM image) as shown in diagram (C) of FIG. 2. FIG. 3 shows a relationship between the stress T at each breakage origin position and the size (radius) d of the inclusion obtained thus. It was found that the lower limit at which a fracture does not occur is obtained from the following formula (1):

d=64729(Pmax/4).sup.1.441 (1).

[0046] In the case where the radius of the non-metallic inclusion of the steel material is not greater than the radius d determined by the above formula (1), when the steel material is used for a rolling component, even if such a rolling component is used in a hydrogen entry environment, the rolling component is not fractured due to rolling fatigue.

[0047] The maximum contact surface pressure Pmax acting on a rolling surface of the rolling component is determined on the basis of the use of the rolling component, and is set, for example, to the following value for each use. When the maximum contact surface pressure Pmax is determined, the maximum value of the radius d of the inclusion is also determined as follows. [0048] (1) In a bearing ring and a rolling element of a rolling bearing used in electrical auxiliary equipments (lighting, an air-conditioner, a wiper, a power window etc); [0049] maximum contact surface pressure Pmax is 2.0 GPa, and maximum value of the radius d of the inclusion is 8.4 m. [0050] (2) In a bearing ring and a rolling element of a turning seat bearing in construction machines; [0051] maximum contact surface pressure Pmax is 3.5 GPa, and maximum value of the radius d of the inclusion is 3.7 m. [0052] (3) In a screw shaft, a nut, and a ball of a ball screw in CVTs (continuously variable transmission); [0053] maximum contact surface pressure Pmax is 2.7 GPa, and maximum value of the radius d of the inclusion is 5.4 m. [0054] (4) In a bearing ring and a rolling element of a main shaft bearing in machine tools; [0055] maximum contact surface pressure Pmax is 2.0 GPa, and maximum value of the radius d of the inclusion is 8.4 m. [0056] (5) In a bearing ring and a rolling element of a rolling bearing used as a main shaft bearing of windmills; [0057] maximum contact surface pressure Pmax is 2.5 GPa, and maximum value of the radius d of the inclusion is 6.1 m. [0058] (6) In a bearing ring and a rolling element of a rolling bearing used in acceleration and deceleration machines in windmill generators; [0059] maximum contact surface pressure Pmax is 2.5 GPa, and maximum value of the radius d of the inclusion is 6.1 m.

[0060] The method of the hydrogen-charged ultrasonic torsional fatigue test used in the above test will be described.

[0061] In this test, as shown in FIG. 4, hydrogen charging unit 2 for charging hydrogen to a test piece 1 made of the rolling component material is included, and with the use of a testing apparatus, completely-alternating ultrasonic torsional vibration is applied to the test piece 1 after the hydrogen charging, and data of the rolling component material is collected in a hydrogen entry environment.

[0062] The hydrogen charging is performed on the test piece 1 by cathode electrolytic charging as described below. The cathode electrolytic hydrogen charging is performed as shown in FIG. 5, by immersing a platinum electrode 24 and a test piece 23 into an electrolyte 22 within a container 21 and applying a voltage with the test piece 23 as a negative side and the electrode 24 as a positive side.

[0063] FIG. 4 shows a shear fatigue characteristic evaluation apparatus that applies completely-alternating ultrasonic torsional vibration to the test piece 1. The apparatus includes: a test device body 10 including a torsional vibration converter 7 and an amplitude-increasing horn 8; an oscillator 4; an amplifier 5; and control/data collector 3.

[0064] In the test device body 10, the amplitude-increasing horn 8 is mounted to the torsional vibration converter 7 installed on an upper portion of a frame 6, such that the amplitude-increasing horn 8 projects downward. The test piece 1 is detachably attached to the distal end of the amplitude-increasing horn 8. Ultrasonic vibration generated by the torsional vibration converter 7 is expanded as vibration in forward and reverse rotation directions about an axis O of the amplitude-increasing horn 8, and then, is transmitted to the test piece 1. The test device body 10 includes a test piece cooling unit 9 that forcedly cools the test piece 1. The test piece cooling unit 9 is composed of, for example, a nozzle that is connected to a compressed air generating source (not shown) of a blower via a pipe and through which air is blown to the test piece 1. Switching between air blowing and cessation of blowing can be performed by an electronic valve (not shown) or by turning on/off the compressed air generating source.

[0065] The torsional vibration converter 7 is operable to generate torsional vibration that causes forward and reverse rotation about the rotation axis O at the frequency of the AC power when two-phase AC power is applied thereto. The AC power applied to the torsional vibration converter 7 is AC power in which a voltage has positive/negative symmetry as in a sine wave, and the generated torsional vibration is completely-alternating vibration, that is, vibration that is symmetrical in the forward rotation direction and in the reverse rotation direction.

[0066] The amplitude-increasing horn 8 is formed in a tapered shape, and has, at a distal end surface thereof, a mount portion composed of a female screw hole to which a test piece is concentrically attached. The amplitude-increasing horn 8 is fixed at a proximal end thereof to the torsional vibration converter 7. The amplitude-increasing horn 8 changes the amplitude of the torsional vibration applied from the torsional vibration converter 7 to generate the proximal end thereof, to increased amplitude at the distal end thereof The material of the amplitude-increasing horn 8 is, for example, a titanium alloy.

[0067] The oscillator 4 includes an electronic device generating a voltage signal of a frequency in the ultrasonic region that is to be a frequency at which the amplitude-increasing horn 8 is vibrated. The oscillatory frequency of the oscillator 4 is fixed or is adjustable, for example, within the range of 20000+500 Hz.

[0068] The amplifier 5 is composed of an electronic device that amplifies output of the oscillator 4 and then applies AC power having a frequency in the ultrasonic region to the torsional vibration converter 7. The magnitude of output of the AC power and ON/OFF of the amplifier 5 are controllable by an external input.

[0069] The control/data collector 3 provides an input for control of the magnitude of the output, ON/OFF, or the like, to the amplifier 5, and also collects, from the amplifier 5, data including the vibration frequency, a state of the output of the amplifier 5 or the like, and the number of times of load application (the number of cycles) during the test. The control/data collector 3 further has a function to control the test piece cooling unit 9. The control/data collector 3 includes a computer such as a personal computer and a program (not shown) to be executed by the computer. An input device 11 such as a keyboard and a mouse and a screen display device 12 such as a liquid crystal display device which displays an image are connected to the control/data collector 3. Each of the input device 11 and the screen display device 12 may be provided as a part of the computer.

[0070] According to the test method described above, the ultrasonic torsional fatigue test is carried out in which ultrasonic torsional vibration having a vibration frequency in the ultrasonic region is applied to a test piece. Therefore, a torsional fatigue test can be carried out in which a load is repeatedly applied at a very high speed. Thus, before charged hydrogen is scattered, shear fatigue can be applied to a test piece made of a metallic material to be evaluated, and therefore, a shear fatigue characteristic in a hydrogen entry environment can be rationally and rapidly evaluated. For example, when vibration is continuously applied at 20000 Hz, the number of times of load application reaches 10.sup.7 times only in 8.3 min. Since the test piece is resonated, shear fatigue fracture can be efficiently caused to occur by input of small energy.

[0071] FIG. 6 shows a schematic diagram of the test piece 1. Although not shown in FIG. 6, a male screw portion for fixing to the distal end of the amplitude-increasing horn 8 is provided at one end of the actual test piece 1. The test piece 1 has a dumbbell shape including cylindrical shoulder portions 1a, 1a at both ends and an intermediate thin portion 1b that is connected to the shoulder portions 1a, 1a at both sides. The thin portion 1b has a cross-sectional shape along an axial direction including a circular art curve 1ba. However, the shape of the test piece 1 is not limited thereto.

[0072] The present invention is not limited to the above-described embodiment, and various additions, changes, or deletions can be made without departing from the gist of the present invention. Therefore, these are construed as included within the scope of the present invention.

REFERENCE NUMERALS

[0073] 1 . . . test piece

[0074] 2 . . . hydrogen charging unit

[0075] 4 . . . oscillator

[0076] 6 . . . came

[0077] 7 . . . torsional vibration converter

[0078] 8 . . . amplitude-increasing horn