Abstract:
There is provided a bearing for alternators and a bearing for pulleys capable of reducing hydrogen embrittlement or cracking despite that the bearing is used in an environment under such a severe condition that hydrogen is generated from grease or the like and the bearing experiences a load of a condition facilitating hydrogen embrittlement or cracking. The bearing includes a rolling element, an inner ring and an outer ring, and at least one of the rolling element and the inner and outer rings has a nitrogen enriched layer and contains austenite crystal grains having a grain size number exceeding 10.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention The present invention relates generally to bearings for alternators and bearings for pulleys, and particularly to such bearings less prone to flake-accompanied, hydrogen embrittlement or cracking in an ambient of hydrogen.  
           [0002]    2. Description of the Background Art  
           [0003]    In bearings used for alternators or pulleys there is often observed a unique fracture accompanied with flakes at its damage starting point. The flake is dissimilar to a white etching constituent (WEC) introduced by rolling fatigue, a so called butterfly introduced in the vicinity of a non-metal inclusion and the like. The present flake is characterized by failing to have directivity relative to a direction of rolling. A bearing having a damage accompanied with flakes at a starting point apparently shows an increased amount of hydrogen in the steel, and in the flake there is a crack, which extends along a grain boundary deep inside. As such, such a damage as described above has been considered to be attributed to hydrogen. Hereinafter, this flake-accompanied, unique fracture will be referred to as hydrogen embrittlement or cracking.  
           [0004]    Hydrogen embrittlement or cracking is caused possibly because as the bearding rotates, a chemically active fresh metal surface is produced and its catalysis effect decomposes a lubricant and generates hydrogen, which enters into the steel. Accordingly, as an approach against hydrogen embrittlement or cracking, (a 1 ) using a chemically less decomposable lubricant and (a 2 ) blackening to minimize the exposure of fresh metal surface have been considered effective.  
           [0005]    For item (a 1 ), a bearing is proposed, as follows: alkyldiphenylether oil and poly α olefin oil are blended at a ratio in weight of 20:80 to 80:20 to prepare a base oil, which is blended with 5-40% by weight of an aromatic diurea compound or an aromatic urea-urethane compound as a thickener, and furthermore a grease composite with a passivation oxidizer and organic sulfonate added thereto is enclosed in a roller bearing to provide a grease enclosed bearing for an alternator, as disclosed in Japanese Patent Laying-open No. 5-263091.  
           [0006]    For item ( 2   b ), a grease enclosed bearing is proposed that has a raceway having a rolling contact surface covered with an oxide film of 0.1 to 2.5 μm in thickness, as disclosed in Japanese Patent Laying-open No. 2-190615.  
           [0007]    The above proposals, as confirmed in the field of the art, provide effectively prolonged lives.  
           [0008]    In recent years, however, bearings for alternators and those for pulleys are used in increasingly severe environments, and simply taking the above approach is no longer sufficient. There is a demand for an approach that can provide an increased life against hydrogen embrittlement or cracking.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention contemplates a bearing for alternators and a bearing for pulleys capable of reducing hydrogen embrittlement or cracking despite that the bearing is used in an environment under such a severe condition that hydrogen is generated from grease or the like and the bearing experiences a load of a condition facilitating hydrogen embrittlement or cracking.  
           [0010]    The present invention provides a bearing for an alternator that supports a rotor shaft having one end with a pulley attached thereto and having a belt engaged therearound to transmit a driving force to the pulley to allow the rotor shaft to rotate a rotor. The bearing includes a rolling element, an inner ring and an outer ring, and at least one of the rolling element and the inner and outer rings has a nitrogen enriched layer and contains austenite crystal grains having a grain size number exceeding 10. Here, the austenite grains refer to crystal grains of austenite which is phase-transformed during the heating process, and the traces of grains remain after the austenite is transformed into martensite through quenching cooling as described above.  
           [0011]    At least one of the rolling element and inner and outer rings containing austenite grains reduced in size to have a grain size number exceeding 10 that is combined with a nitrogen enriched layer significantly effectively reduces hydrogen embrittlement or cracking. This is possibly attributed to the following reason: in a single crystal grain a certain crystal plane provides hydrogen embrittlement or cracking and the crack does not exactly extend to adjacent crystal grains. To crack adjacent crystal grains it is necessary to store additional energy to do so. Storing energy for example in a hydrogen atom at a specific portion requires a period of time. This should contribute to a reduced rate of hydrogen embrittlement or cracking.  
           [0012]    The austenite grain size number may be determined by the usual method defined by JIS, or determined according to the intercept method for example with the average grain size corresponding to the above grain size number. A smaller austenite grain size is desirable and an austenite grain size number of 11 or greater is further desirable. Alternatively, the average grain size may be 6 μm or less. The austenite grain size number may be achieved in the nitrogen enriched layer. In general, however, the condition of the fineness of austenite is satisfied in the body of steel located inside the nitrogen enriched layer.  
           [0013]    The above-described austenite crystal grain may be any austenite crystal grain having a grain boundary that can be observed after being subjected to a process manifesting a grain boundary such as an etching process for a metal phase sample of the member of interest. The austenite crystal grain is sometimes referred to as a prior austenite grain as described earlier when it has a grain boundary just heated and ready to be subjected to low-temperature quenching. For measurement of the grain size, the average of JIS-defined grain size numbers may be converted to obtain an average grain diameter, as has been described above, or the intercept method or the like may be used in which a straight line is placed on a metal phase structure in an arbitrary direction to obtain an average length between points at which the straight line meets grain boundaries and the average length may be multiplied by a correction coefficient to represent a two- or three-dimensional length between the points.  
           [0014]    The nitrogen enriched layer may be formed by a carbonitriding process, as will be described hereinafter. The nitrogen enriched layer may or may not be enriched with carbon.  
           [0015]    A fine austenite grain as described above is obtained by carbonitriding steel for a bearing component at a temperature higher than an A 1  transformation temperature, then cooling the steel to a temperature of less than the A 1  transformation temperature, and then reheating the steel to a range in temperature of no less than the A 1  transformation temperature and less than an A 3  transformation temperature and also lower than the temperature applied to the carbonitriding process, and quenching the steel. This thermal treatment is referred to as a low-temperature secondary quenching. This production method will be described more specifically in an embodiment.  
           [0016]    In the same application the present invention provides still another bearing for an alternator with at least one of the rolling element and the inner and outer rings having a nitrogen enriched layer and providing a fracture stress value of no less than 2,650 MPa.  
           [0017]    The present inventors have found that the above thermal treatment allows steel having a carbonitrided layer to provide a conventionally unachievable stress value of no less than 2,650 MPa. A rolling bearing of large strength can be obtained exhibiting excellent durability against a radial load and a moment load superimposed on each other when the bearing is applied for an alternator.  
           [0018]    In the same application the present invention provides still another bearing for an alternator with at least one of the rolling element and the inner and outer rings having a nitrogen enriched layer and a hydrogen content of no more than 0.5 ppm.  
           [0019]    The above described thermal treatment (the low-temperature secondary quenching) can provide a reduced ratio of hydrogen contained in a member of steel before it is used for the alternator. This can increase the period of time required for hydrogen entering the steel to reach a critical value at which the steel cracks, and together with other, still unclarified reason can render the steel less prone to hydrogen embrittlement or cracking.  
           [0020]    Smaller amounts of hydrogen are desirable. However, reducing an amount of hydrogen to less than 0.3 ppm requires heating for a long period of time, and austenite grains would have a coarse and large diameter and toughness would rather be impaired. Accordingly, an amount of hydrogen falling within a range of 0.3 to 0.5 ppm is desirably contained. More desirably, an amount of hydrogen falling within a range of 0.35 to 0.45 ppm is contained.  
           [0021]    In measuring the above hydrogen content, diffusible hydrogen is not measured and only the non-diffusible hydrogen released from the steel at a predetermined temperature or higher is measured. Diffusible hydrogen in a sample of small size is released from the sample to be scattered even at room temperature, and therefore the diffusible hydrogen is not measured. Non-diffusible hydrogen is trapped in any defect in the steel and only released from the sample at a predetermined heating temperature or higher. Even if only the non-diffusible hydrogen is measured, the hydrogen content considerably varies depending on the method of measurement. The above mentioned range of hydrogen content is determined by thermal conductimetry. In addition, as detailed later, the measurement may be taken by means of a LECO DH-103 hydrogen determinator or like measuring device.  
           [0022]    In the present bearing for the alternator at least one of the rolling element and the inner and outer rings having the nitrogen enriched layer may have a surface layer containing. globular carbide having an area ratio of no less than 10%.  
           [0023]    The surface layer containing globular carbide having an area ratio of no less than 10% allows the bearing for the alternator to be furthermore resistant to hydrogen embrittlement or cracking.  
           [0024]    The present invention provides a bearing for a pulley that is posed between a pulley having a belt engaged therearound and a rotative shaft passing through the pulley. The bearing includes a rolling element, an inner ring and an outer ring, and at least one of the rolling element and the inner and outer rings has a nitrogen enriched layer and contains austenite crystal grains having a grain size number exceeding 10.  
           [0025]    Pulleys used in automobiles are increasingly miniaturized and subjected to large loads. As such, a rotary shaft increasingly tends to rotate rapidly, and the bearing is used in a condition in which hydrogen is generated for example from grease. A pulley with the above described bearing for a pulley arranged therewith can be driven with a belt arranged such that its center as seen in width is offset relative to a center of the bearing. As such, the above described bearing experiences a radial load and in addition a moment load and the bearing for the pulley has a component periodically experiencing a tensile stress component inducing generated hydrogen to enter into the steel.  
           [0026]    At least one of the rolling element and inner and outer rings containing austenite grains reduced in size to achieve a grain size number exceeding 10 that is combined with a nitrogen enriched layer can significantly reduce hydrogen embrittlement or cracking not only in a bearing for an alternator but also a bearing for a pulley. Furthermore, the effect of providing at least one of the rolling element and inner and outer rings with a fracture stress value of 2,650 MPa and the effect of providing it with a hydrogen content of no more than 0.5 ppm can be considered similarly as considered for the bearing for the alternator.  
           [0027]    In the present bearing for the pulley at least one of the rolling element and the inner and outer rings having the nitrogen enriched layer may have a surface layer containing globular carbide having an area ratio of no less than 10%.  
           [0028]    The bearing for the pulley that has a surface layer containing globular carbide having an area ratio of no less than 10% can further be enhanced in resistance to hydrogen embrittlement or cracking.  
           [0029]    In the above described bearings for the alternator and pulley, respectively, at least one of the rolling element, the inner ring and the outer ring can be formed of material of steel of SUJ2 as defined by JIS. By subjecting the bearing&#39;s member of material of steel SUJ2 to the above-described, low-temperature secondary quenching, fine austenite grains, a high fracture stress value and a low hydrogen content, as described above, can be readily obtained.  
           [0030]    The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    In the drawings:  
         [0032]    [0032]FIG. 1 shows an alternator including a bearing for an alternator in accordance with the present invention in an embodiment;  
         [0033]    [0033]FIG. 2 is an enlarged view of the FIG. 1 bearing;  
         [0034]    [0034]FIG. 3 shows a bearing for a pulley in accordance with the present invention;  
         [0035]    [0035]FIG. 4 represents an exemplary thermal treatment applied to at least one of a rolling elements, an inner ring and an outer ring of bearings for an alternator and a pulley, respectively, in accordance with the present invention in an embodiment;  
         [0036]    [0036]FIG. 5 represents another exemplary thermal treatment applied to at least one of a rolling elements, an inner ring and an outer ring of bearings for an alternator and a pulley, respectively;  
         [0037]    [0037]FIGS. 6A and 6B show a microstructure, particularly austenite grains, of a bearing component, FIG. 6A showing a bearing component of the present invention and FIG. 6B showing a conventional bearing component;  
         [0038]    [0038]FIGS. 7A and 7B diagrammatically show austenite grain corresponding respectively to FIGS. 6A and 6B;  
         [0039]    [0039]FIG. 8 shows a test piece for testing static pressure fracture-strength (measuring fracture stress);  
         [0040]    [0040]FIG. 9A is a front view of a rolling fatigue life tester and FIG. 9B is a side view thereof;  
         [0041]    [0041]FIG. 10 shows a test piece for testing static fracture toughness;  
         [0042]    [0042]FIGS. 11A-11C represent processes in accordance with the present invention in examples V 1  and V 2  and a comparative example C 1 , respectively;  
         [0043]    [0043]FIG. 12 shows a tester reproducing hydrogen embrittlement or cracking; and  
         [0044]    [0044]FIGS. 13A and 13B show hydrogen embrittlement or cracking in a photograph and a schematic representation, respectively. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]    Reference will now be made to the drawings to describe the present invention in embodiment. FIG. 1 shows an alternator  10 . A bearing  1 ,  15  for the alternator receives a shaft  11  inserted therethrough. The shaft has an end protruding outer than the bearing and having a pulley  13  attached thereto. Pulley  13  has a groove  16  along which a transmission belt (not shown) is wound and thus engaged with.  
         [0046]    Bearing  1  is located between a rotor  12  fixed to shaft  12  at a center and pulley  13  and is referred to as a front bearing. The other alternator  15  is located between rotor  12  and the other end and referred to as a rear bearing. The front and rear bearings are both attached to a housing. The present invention may be directed to either the front or rear bearing, although it should be noted that front bearing  1  is more liable to hydrogen embrittlement or cracking as it is located between pulley  13  and rotor  12  and experiences a large bending moment or other similar stress.  
         [0047]    [0047]FIG. 2 is an enlarged view of the FIG. 1 front bearing  1 . Front bearing  1  is configured of an inner ring  5 , an outer ring  7 , a rolling element  3  and a seal  8 . Inner and outer rings  5  and  7  have rolling contact surfaces  5   a  and  7   a , respectively, together holding rolling element  3 . The rolling element may be formed of steel or ceramic. Inner ring  5  is press-fitted on shaft  11 , and shaft  11  is rotatably supported. Furthermore, rolling element  3  is held by a cage (not shown) and between inner and outer rings  5  and  7  at opposite ends seal  8  is arranged to seal the bearing&#39;s interior. In the sealed interior there is a space in which grease is enclosed.  
         [0048]    [0048]FIG. 3 shows a pulley  20  including a bearing  21  for the pulley in accordance with the present invention in an embodiment. Pulley  20  includes a main body  19  fitted on the bearing&#39;s outer ring  7  externally, and a portion  18  receiving a belt wound therearound. Bearing  21  is configured of inner ring  5 , outer ring  7 , rolling element  3  and seal  8 . Inner and outer rings  5  and  7  have rolling contact surfaces  5   a  and  7   a , respectively, together holding rolling element  3  therebetween. Main body  19  externally fitted on outer ring  7  is rotatably supported. Furthermore, rolling element  3  is held by a cage (not shown) and between inner and outer rings  5  and  7  at opposite ends seal  8  is arranged to seal the bearing&#39;s interior. In the sealed interior there is a space in which grease is enclosed.  
         [0049]    Furthermore in FIG. 3 inner ring  5  may be integrated with a shaft used to attach pulley  20  to a counterpart member. More specifically, it may include such a type that a shaft has an outer diameter with a groove.  
         [0050]    In the above described bearing, at least one of outer ring  7 , inner ring  5  and rolling element  3  contains steel having a nitrogen enriched layer and contains austenite grains having a grain size number exceeding  10 . Alternatively, at least one of outer ring  7 , inner ring  5  and rolling element  3  contains steel having a nitrogen enriched layer and also achieve a fracture stress value of no less than 2650 MPa. Alternatively, at least one of outer ring  7  inner ring  5  and rolling element  3  contains steel having a nitrogen enriched layer and a hydrogen content of no more than 0.5 ppm.  
         [0051]    The above described, at least one of the inner and outer rings and the rolling element used at a high velocity and under a high surface pressure which are enclosed by grease, hardly provides hydrogen embrittlement or cracking and can be increased in longevity.  
         [0052]    These rolling bearings have at least one of their outer and inner rings and rolling elements undergoing a thermal treatment including carbonitriding.  
         [0053]    [0053]FIG. 4 shows a thermal treatment pattern according to which primary quenching and secondary quenching are carried out, and FIG. 5 shows a thermal treatment pattern according to which a material is cooled to a temperature lower than the A 1  transformation temperature in a quenching process and thereafter heated again to be finally quenched. Both are embodiments of the present invention. Referring to these figures, in process T 1 , carbon and nitrogen are diffused through a steel matrix while the carbon is sufficiently dissolved therein, and thereafter cooling is done to a temperature below the A 1  transformation temperature. Then, in process T 2  shown in the drawings, heating is done again to a temperature lower than that in process T 1  and then oil-quenching is performed.  
         [0054]    The above described thermal treatments both allow their carbonitriding processes to produce a nitrogen-enriched layer that is a “carbonitrided layer.” Since the material for the carbonitriding process, the steel, has a high concentration of carbon, carbon in the atmosphere of the normal carbonitriding process might not enter the surface of the steel easily. For example, with steel having a high concentration of carbon (approximately 1% by weight), a carburized layer may have a higher concentration of carbon than this value, or a carburized layer may be formed without having a higher concentration of carbon than this value. A concentration of nitrogen in normal steel, however, is typically as low as approximately no more than 0.025 wt % at the maximum although it depends on a concentration of Cr or the like. Therefore, a nitrogen enriched layer can apparently be formed regardless of the concentration of carbon in source steel. It will be appreciated that the above described nitrogen enriched layer may also be enriched with carbon.  
         [0055]    Compared with normal quenching, by which carbonitriding is done and immediately thereafter quenching is done once, the above discussed thermal treatment can provide enhanced crack strength and reduced secular dimensional variation rate while carbonitriding the surface layer. This thermal treatment can also produce a microstructure having austenite crystal grains of a grain size smaller than conventional by one half or more. A bearing component subjected to the above described thermal treatment can have a long rolling fatigue life, an increased anti-crack strength, and a reduced secular dimensional variation rate, and also exhibit significant resistance to hydrogen embrittlement or cracking.  
         [0056]    [0056]FIG. 6A shows a grain of austenite of a bearing steel having been heat-treated as shown in FIG. 4. For comparison, FIG. 6B shows a grain of austenite of a bearing steel which has undergone the conventional thermal treatment. FIGS. 7A and 7B diagrammatically show the grains of austenite that are shown in FIGS. 6A and 6B. In the structures with the crystal grains of austenite, the grain diameter of the conventional austenite is 10 which is a grain size number defined by JIS while that of the present invention through the thermal treatment thereof is for example 12 and thus fine grains are seen. Further, the average grain diameter in FIG. 6A is 5.6 μm measured by the intercept method.  
         [0057]    The present invention in examples will now be described.  
       EXAMPLE 1  
       [0058]    JIS-SUJ2 (1.0 wt % of C-0.25 wt % of Si-0.4 wt % of Mn-1.5 wt % of Cr) was used for Example 1 of the present invention. Samples shown in Table 1 were each produced through the procedure described below.  
         [0059]    JIS G4805 SUJ2 specifies the range of chemistry as follows:  
         [0060]    C:0.95-1.10 wt %, Si:0.15-0.35 wt %, Mn:≦0.50 wt %,  
         [0061]    P:≦0.025 wt %, S:≦0.025 wt %, Cr:1.30-1.60 wt %  
                                                                                                   TABLE 1                                       sample                                        conventional   normal                                   carbonitrided   quenched           A   B   C   D   E   F   sample   sample                        secondary   780 1)     800   815   830   850   870   —   —       quenching       temperature (° C.)       hydrogen amount   —   0.37   0.40   0.38   0.42   0.40   0.72   0.38       (ppm)       crystal grain size   —   12   11.5   11   10   10   10   10       (JIS)       Charpy impact   —   6.65   6.40   6.30   6.20   6.30   5.33   6.70       value (J/cm 2 )       fracture stress   —   2840   2780   2650   2650   2700   2330   2770       (MPa)       relative rolling   —   5.4   4.2   3.5   2.9   2.8   3.1   1       fatigue life (L 10 )                          
 
         [0062]    Samples A-D: Examples of the Present Invention  
         [0063]    Carbonitriding was performed at 850° C. held for 150 minutes in an atmosphere of a mixture of RX gas and ammonia gas. Following the thermal treatment pattern shown in FIG. 4, primary quenching was done from a carbonitriding temperature of 850° C., and secondary quenching was subsequently done by heating to a temperature in a temperature range from 780° C. to 830° C. lower than the carbonitriding temperature. Sample A with a secondary quenching temperature of 780° C. was not tested since quenching of sample A was insufficient.  
         [0064]    Samples E and F: Comparative Examples  
         [0065]    These samples were carbonitrided through the same procedure as that of samples A-D of the present invention, and then secondary quenched at a temperature from 850° C. to 870° C. equal to or higher than the carbonitriding temperature of 850° C.  
         [0066]    Conventional Carbonitrided Sample: Comparative Example  
         [0067]    Carbonitriding was performed at 850° C. held for 150 minutes in an atmosphere of a mixture of RX gas and ammonia gas. Quenching was successively done from the carbonitriding temperature and no secondary quenching was done.  
         [0068]    Normal Quenched Sample: Comparative Example  
         [0069]    Without carbonitriding, quenching was done by increasing the temperature to 850° C. and no secondary quenching was done.  
         [0070]    For the samples above, tests were conducted for (1) measuring the amount of hydrogen, (2) measuring crystal grain size, (3) Charpy impact test, (4) measuring fracture stress and (5) rolling fatigue test, by the methods described below.  
         [0071]    I. Test Methods for Example 1  
         [0072]    (1) Measurement of Hydrogen Amount  
         [0073]    The amount of hydrogen was determined by means of a DH-103 hydrogen determinator manufactured by LECO Corporation to analyze the amount of non-diffusible hydrogen in a steel. The amount of diffusible hydrogen was not measured. Specifications of the LECO DH-103 hydrogen determinator are as follows:  
         [0074]    Analysis range: 0.01-50.00 ppm  
         [0075]    Analysis precision:±0.1 ppm or ±3% H (higher one)  
         [0076]    Analysis sensitivity:0.01 ppm  
         [0077]    Detection method: thermal conductimetry  
         [0078]    Sample weight size: 10 mg-35 g (max:12 mm (diameter)×100 mm (length))  
         [0079]    Furnace temperature range:50° C.-1100° C.  
         [0080]    Reagent: anhydron Mg(ClO 4 ) 2 , Ascarite and NaOH  
         [0081]    Carrier gas: nitrogen gas  
         [0082]    Dosing gas: hydrogen gas  
         [0083]    (Both gases have a purity of at least 99.99% and a pressure of 40 PSI (2.8 kgf/cm 2 ).)  
         [0084]    The procedure of the analysis is roughly described here. A sample was taken by a dedicated sampler and the sample together with the sampler was put into the hydrogen determiner. Diffusible hydrogen therein was directed by the nitrogen carrier gas to a thermal conductimetry detector. The diffusible hydrogen was not measured in this example. Then, the sample was taken out of the sampler to be heated in a resistance heater and non-diffusible hydrogen was directed by the nitrogen carrier gas to the thermal conductimetry detector. The thermal conductivity was measured by the thermal conductimetry detector to determine the amount of non-diffusible hydrogen.  
         [0085]    (2) Measurement of Crystal Grain Size  
         [0086]    The crystal grain size was measured according to the method of testing the crystal grain size of austenite in a steel defined by JIS G 0551.  
         [0087]    (3) Charpy Impact Test  
         [0088]    A Charpy impact test was conducted according to the Charpy impact test method for a metal material defined by JIS Z 2242. A test piece used here was a U-notch test piece (JIS No. 3 test piece) defined by JIS Z 2202.  
         [0089]    (4) Measurement of Fracture Stress  
         [0090]    [0090]FIG. 8 shows a test piece for a static-pressure fracture-strength test (for measuring fracture stress). A load was exerted in direction P in FIG. 8 and the load when the test piece was fractured was measured. Then, the measured load which was a fracture load was converted into a stress by the following stress calculation formula for a curved beam. It is noted that the test piece to be used is not limited to the one shown in FIG. 13 and may be any test piece having a different shape.  
         [0091]    Suppose that a fiber stress on the convex surface of the test piece shown in FIG. 8 is σ 1  and a fiber stress on the concave surface is σ 2 , then, σ 1  and σ 2  are determined by the following formula (JSME Mechanical Engineer&#39;s Handbook, A4-strength of materials, A4-40). Here, N indicates an axial force of a cross section including the axis of the annular test piece, A indicates a cross-sectional area, e 1  indicates an outer radius, e 2  indicates an inner radius, and κ is a section modulus of the curbed beam. 
         σ 1 =( N/A )+{ M /( A ρ o )}[1 +e   1 /{κ(ρ o   +e   1 )}] 
         σ 2 =( N/A )+{ M /( A ρ o )}[1 −e   2 /{κ(ρ o   −e   2 )}] 
         κ=−(1 /A )∫ A {η/(ρ o +η)} dA   
         [0092]    (5) Rolling Fatigue Test  
         [0093]    Test conditions for a rolling fatigue life test are shown in Table 2. FIGS. 9A and 9B schematically show a rolling fatigue life tester, FIG. 9A being a cross-sectional view and FIG. 9B being a side view thereof. Referring to FIGS. 9A and 9B, a test piece  61  undergoing the rolling fatigue life test was driven by a driving roll  51  to rotate while being in contact with balls  53 . Balls  53  were (¾)″ balls guided by a guiding roll  52  to roll. Balls  53  exerted a high surface pressure on test piece  61  while test piece  61  also exerted a high surface pressure on balls  53 .  
         [0094]    II. Results of Tests for Example 1  
         [0095]    (1) Amount of Hydrogen  
         [0096]    As shown in table 1, the conventional carbonitrided sample without being additionally processed has a considerably large hydrogen amount of 0.72 ppm. A reason therefor is considered that ammonia (NH 3 ) contained in the atmosphere in the carbonitriding process is decomposed and then hydrogen enters the steel. On the other hand, the hydrogen amount of samples B-D is reduced to 0.37-0.40 ppm and thus almost a half of that of the conventional sample. This amount of hydrogen is substantially equal to that of the normal quenched sample.  
         [0097]    The above-mentioned reduction of the hydrogen amount can lessen the degree of embrittlement of the steel that is due to hydrogen in the solid solution. In other words, by the reduction of the hydrogen amount, the Charpy impact value of samples B-D of the present invention is remarkably improved.  
         [0098]    (2) Crystal Grain Size  
         [0099]    Regarding the crystal grain size, samples that are secondary quenched at a temperature lower than the quenching temperature in the carbonitriding process (primary quenching), namely samples B-D have austenite grains which are remarkably made fine, i.e., crystal grain size number is 11-12. Samples E and F as well as the conventional carbonitrided sample and the normal quenched sample have austenite grains with the crystal grain size number of 10, which means that the crystal grain size of samples E and F is greater than that of samples B-D of the present invention.  
         [0100]    (3) Charpy Impact Test  
         [0101]    Table 9 shows that the Charpy impact value of the conventional carbonitrided sample is 5.33 J/cm 2  while that of samples B-D of the present invention is higher, ranging from 6.30 to 6.65 J/cm 2 . It is also seen from this that a lower secondary quenching temperature leads to a higher Charpy impact value. The normal quenched sample has a high Charpy impact value of 6.70 J/cm 2 .  
         [0102]    (4) Measurement of Fracture Stress  
         [0103]    The fracture stress corresponds to anti-crack strength. It is seen from Table 1 that the fracture stress of the conventional carbonitrided sample is 2330 MPa. On the other hand, the fracture stress of samples B-D is improved to 2650-2840 MPa. The normal quenched sample has a fracture stress of 2770 MPa which is in the range of the. fracture stress of samples B-F. It is considered that the reduction in hydrogen content greatly contributes to the improved anti-crack strength of samples B-D as well as the reduction in size of austenite crystal grains.  
         [0104]    (5) Rolling Fatigue Test  
         [0105]    According to Table 1, the normal quenched sample has the shortest rolling fatigue life (L10) due to the absence of carbonitrided layer in the surface layer. In contrast, the rolling fatigue life of the conventional carbonitrided sample is 3.1 times as long as that of the normal quenched sample. The rolling fatigue life of samples B-D is remarkably improved as compared with the conventional carbonitrided sample. Samples E and F of the present invention have the rolling fatigue life almost equal to that of the conventional carbonitrided sample.  
         [0106]    In summary, samples B-D of the present invention have the lower hydrogen content, finer austenite crystal grains with the crystal grain size number of at least 11, and improved Charpy impact value, anti-crack strength and rolling fatigue life.  
       EXAMPLE 2  
       [0107]    Example 2 of the present invention is now described. On the following samples X, Y and Z, a series of tests was conducted. A material to be heat-treated that was employed commonly to samples X-Z was JIS-SUJ2 (1.0 wt % of C-0.25 wt % of Si-0.4 wt % of Mn-1.5 wt % of Cr). Samples X-Z were each processed through the following procedure.  
         [0108]    Sample X—comparative example: normal quenching only (without carbonitriding)  
         [0109]    Sample Y—comparative example: quenching directly after carbonitriding (conventional carbonitriding and quenching) Carbonitriding was conducted at 845° C. held for 150 minutes. The atmosphere in the carbonitriding process was a mixture of RX gas and ammonia gas.  
         [0110]    Sample Z—example of the present invention: A bearing material was processed following the thermal treatment pattern shown in FIG. 4. Carbonitriding was conducted at 845° C. held for 150 minutes. The atmosphere in the carbonitriding process was a mixture of RX gas and ammonia gas. Final quenching temperature was 800° C.  
         [0111]    (1) Rolling Fatigue Life  
         [0112]    Test conditions and the test device for the rolling fatigue life test are as shown in Table 2 and FIGS. 9A and 9B. Results of the rolling fatigue life test are shown in Table 3.  
                   TABLE 2                           test piece   φ12 × L22 cylindrical test piece       number of tested pieces   10       counterpart steel ball   ¾″ (19.05 mm)       contact surface pressure   5.88 GPa       load speed   46240 cpm       lubricating oil   turbine VG68 - forced circulation lubrication                  
 
         [0113]    [0113]                                                               TABLE 3                                       life (load count)                sample   L 10  (×10 4  times)   L 10  (×10 4  times)   relative L 10                      X   8017   18648   1.0       Y   24656   33974   3.1       Z   43244   69031   5.4                    
         [0114]    According to Table 3, sample Y which is a comparative example has a rolling fatigue life (L10 life: one out of ten test pieces being damaged) that is 3.1 times as long as that of sample X which is also a comparative example and undergoes normal quenching only, and thus it is seen that the effect of extending the life is obtained through the carbonitriding process. In contrast, sample Z of the present invention has a longer life which is 1.74 times as long as that of sample Y and 5.4 times as long as that of sample X. It is considered that this improvement is obtained mainly from the fine microstructure.  
         [0115]    (2) Charpy Impact Test  
         [0116]    A Charpy impact test was conducted by using a U-notch test piece defined by JIS Z 2242 mentioned above. Test results are shown in Table 4.  
                       TABLE 4                           Charpy impact   relative impact       sample   value (J/cm 2 )   value                   X   6.7   1.0       Y   5.3   0.8       Z   6.7   1.0                  
 
         [0117]    Sample Y (comparative example) having undergone carbonitriding has a Charpy impact value which is not larger than that of sample X (comparative example) having undergone normal quenching, while sample Z has a Charpy impact value equivalent to that of sample X.  
         [0118]    (3) Static Fracture Toughness Test  
         [0119]    [0119]FIG. 10 shows a test piece for a static fracture toughness test. In the notch of the test piece, a pre-crack of approximately 1 mm was made, thereafter a static load by three-point bending was added, and then a fracture load P was determined. Using the following formula (I), a fracture toughness value (KIc value) was calculated. Results of the test are shown in Table 5. 
           KIc =( PL{square root}a/BW   2 ){5.8−9.2( a/W )+43.6( a/W ) 2 −75.3( a/W ) 3 +77.5( a/W ) 4 }  (I) 
         [0120]    [0120]                                   TABLE 5                                   sample   number tested   K 1 C (MPa{square root}m)   relative K 1 C                           X   3   16.3   1.0           Y   3   16.1   1.0           Z   3   18.9   1.2                        
         [0121]    As the pre-crack has a depth greater than the depth of the carbonitrided layer and thus the same results are obtained for samples X and Y (comparative examples), while the result for sample Z (example of the present invention) is approximately 1.2 times as high as that of the comparative examples.  
         [0122]    (4) Static-pressure Fracture-strength Test (Measurement of Fracture Stress)  
         [0123]    A static-pressure fracture-strength test piece as shown in FIG. 8 described above was used. A load was exerted in direction P in FIG. 13 to conduct a static-pressure fracture-strength test. Test results are shown in Table 6.  
                                                         TABLE 6                                       number   static fracture   relative static           sample   tested   strength (kgf)   fracture strength                                        X   3   4200   1.00           Y   3   3500   0.84           Z   3   4300   1.03                      
 
         [0124]    Sample Y having been carbonitrided has a strength somewhat smaller than that of sample X having undergone normal quenching, while sample Z of the present invention has an improved static-pressure fracture-strength compared with sample Y and is thus equivalent to that of sample X.  
         [0125]    (5) Rate of Secular Dimensional Change  
         [0126]    Table 7 shows the rate of secular dimensional change measured under the conditions of 130° C. (holding temperature) and 500 hours (holding time), together with the surface hardness and the amount of retained austenite (0.1 mm depth).  
                                   TABLE 7                                       rate of                   surface       dimensional   relative rate of           number   hardness   retained   change   dimensional       sample   tested   (HRC)   Y (%)     (×10 −5 )   change* )                     X   3   62.5    9.0   18   1.0       Y   3   63.6   28.0   35   1.9       Z   3   60.0   11.3   22   1.2                          
 
         [0127]    As compared with the rate of dimensional change of sample Y having a large amount of retained austenite, sample Z of the present invention has the smaller rate of dimensional change due to the lower amount of retained austenite which is one-half or less.  
         [0128]    (6) Life Test under Contaminated Lubricant Condition  
         [0129]    Ball bearing 6206 was used to evaluate the rolling fatigue life under a contaminated lubricant condition having a predetermined amount of normal contaminants mixed therein. Test conditions are shown in Table 8 and test results are shown in Table 9.  
                   TABLE 8                           load   Fr = 6.86 kN       contact surface pressure   Pmax = 3.2 Gpa       rotational speed   2000 rpm       lubricant   turbine 56 - oil bath lubrication       amount of contaminant   0.4 g/1000 cc       contaminant   grain size: 100-180 μm, hardness: Hv800                  
 
         [0130]    [0130]                                     TABLE 9                       sample   L 10  life_(h)   relative L 10                                  X   20.0   1.0       Y   50.2   2.5       Z   74   3.7                    
         [0131]    Sample Y having undergone conventional carbonitriding has a lifetime which is approximately 2.5 times as long as that of sample X, and sample Z of the present invention has a lifetime which is approximately 3.7 times as long as that of sample X. While sample Z of the present invention has a smaller amount of retained austenite than that of sample Y of the comparative example, sample Z has a long lifetime substantially equivalent to that of sample Y because of influences of entering nitrogen and the fine microstructure.  
         [0132]    It is accordingly seen from the above-discussed results that, sample Z of the present invention, namely a bearing component produced by the thermal treatment method of the present invention can simultaneously achieve three goals: extension of the rolling fatigue life that has been difficult to achieve by the conventional carbonitriding, improvement in crack strength and reduction of the rate of secular dimensional change.  
       EXAMPLE 3  
       [0133]    SUJ2 having undergone two types of processes as shown in FIGS. 11A and 11B was prepared as a material of steel as an example of the present invention minimizing hydrogen embrittlement or cracking, as described above.  
         [0134]    [0134]FIG. 11A represents the present invention in an example V 1  providing a process. More specifically, steel is carbonitrided at a high carbon potential (CP) value of 1.3-1.6 and then from that temperature the steel is oil-quenched. The steel carbonitrided in an ambient with such a high CP value can have a surface layer containing globular or spherodized carbide having an area ratio of no less than 10%. The steel is then tempered at 180° C.  
         [0135]    [0135]FIG. 11B represents the present invention in an example V 2  providing a process. More specifically, steel is initially carbonitrided and quenched, as shown in comparative example C 1  (see FIG. 11C), and then heated to T 1 ° C. (845° C.)-α° C., a temperature lower than an initial quenching temperature by α° C., and oil-quenched. The temperature (T 1 -α)° C. is no lower than the A 1  transformation temperature and lower than the temperature of the preceding, carbonitriding and quenching step. As shown in the figure, quenching the steel starts at the temperature of the carbonitriding step, and the quenching temperature and the carbonitriding temperature are the same. In the present description, a process performed to heat a medium to (T 1 -α)° C. and quench it will be referred to as low-temperature quenching. After the medium is low-temperature quenched, it is tempered at 180° C.  
         [0136]    Furthermore, as a material of steel for a comparative example, four materials of steel, i.e., SUJ2 processed as shown in FIG. 11C, SUJ2 normally quenched, SUJ2 blackened, and 13% Cr steel are used for evaluation for comparison. Table 10 is a list of materials of steel evaluated.  
                                 TABLE 10                           List of Subjects to be Evaluated                Type                   of Steel   Process   Note               Invention in   JIS-SUJ2   See FIG. 1   Large amount of       Ex. V1           globular carbide       Invention in   JIS-SUJ2   See FIG. 1   Fine crystal grains +       Ex. V2           large amount                   of globular                   carbide       Comparative   JIS-SUJ2   See FIG. 1   Nitrided layer       Ex. C1       Comparative   JIS-SUJ2   Normally quenched   —       Ex. C2       Comparative   JIS-SUJ2   Normally quenched +   Oxide film       Ex. C3       blackened       Comparative   13% Cr   Normally quenched   Passive thin       Ex. C4   steel       coating                   (oxide film)                  
 
         [0137]    They were evaluated for hydrogen embrittlement or cracking in a test using a radio load tester as shown in FIG. 12. A bearing  71   a ,  71   b  to be tested was incorporated into two, opposite sides of a main shaft supported by a bearing  73  attached to a center and rotated via a pulley as it is driven by a servo motor. The bearing to be tested receives a radial, biased load and thus flexes as bearing  73  receives a radial load. Bearing  71   a ,  71   b  to be tested is controlled in temperature by a cartridge heater  72   a ,  72   b  attached to a bearing housing. This evaluation test was conducted with bearing  71   a ,  71   b  in the form of a radial needle bearing having an outer diameter of 32 mm, an inner diameter of 24 mm, and a length of 19.8 mm. The test was conducted under conditions as shown in Table 11.  
                         TABLE 11                       Test Conditions                                Contact surface pressure (GPa)   2.61/2.26       Inner ring-side/outer ring-side       Rate of rotation (rpm)   Rapid acceleration and deceleration of           500           → 3,000       Testing temperature (° C.)   100       Lubricant   Water-glycol based, incombustible,           working oil       Condition of lubrication   Oil bath                  
 
         [0138]    The Table 11 test conditions with rapid acceleration and deceleration allow a sufficient reproduction of hydrogen embrittlement or cracking. Table 12 shows a result of the test conducted under these conditions.  
                                                                   TABLE 12                           Test Result                            L 10     L 10                     No.   Longevity   Longevity           Type of Steel   Process   of N   (h)   ratio                        Invention in Ex. V1   JIS-SUJ2   See FIG. 1   10   54.3   4.6       Invention in Ex. V2   JIS-SUJ2   See FIG. 1   10   &gt;122.2* 1     &gt;10.3       Comparative Ex. C1   JIS-SUJ2   See FIG. 1   10   32.6   2.7       Comparative Ex. C2   JIS-SUJ2   Normally quenched   10   11.9   1.0       Comparative Ex. C3   JIS-SUJ2   Normally quenched +   6   62.5* 2     5.3               blackened       Comparative Ex. C4   13% Cr steel   Normally quenched   6   70.6* 2     5.9                                  
 
         [0139]    [0139]FIGS. 13A and 13B show a cross section including hydrogen embrittlement or cracking introduced in comparative example C 1 . FIGS. 13A and 13B show a fracture in a photograph and a schematic representation, respectively. It can be understood that under the above test conditions, hydrogen embrittlement or cracking is induced.  
         [0140]    It can be understood from the Table 12 result that the examples of the present invention are longer in longevity than the comparative examples corresponding to conventional products. In particular, the present invention in example V 2  is further longer in longevity than either comparative example C 2 , a blackened material, and comparative example C 3 , 13% Cr steel blackened, both enhanced in resistance to hydrogen embrittlement or cracking.  
         [0141]    To examine why the present invention in example V 2  provides a particularly long life, the present invention in examples V 1  and V 2  and comparative examples C 1  and C 2  corresponding to conventional products had their respective materials of steel measured at a surface layer to obtain their globular carbide area ratios and austenite grain numbers as defined by JIS. Table 13 shows a result thereof.  
                                                           TABLE 13                           Globular Carbide Area Ratio and Average Austenite Grain Size                Type       Area ratio   JIS defined           of Steel   Process   (%)   grain size                    Invention in Ex. V1   JIS-SUJ2   See FIG. 1   12.9   10.1       Invention in Ex. V2   JIS-SUJ2   See FIG. 1   13.2   12.7       Comparative Ex. C1   JIS-SUJ2   See FIG. 1   7.6   10       Comparative Ex. C2   JIS-SUJ2   Normally   7.9   10.2               quenched                  
 
         [0142]    The present invention in example V 1  contains globular carbide having a larger area ratio than comparative example C 1  and a JIS defined grain size which does not have a significant difference than the comparative examples. The present invention in example V 2  contains globular carbide having a larger area ratio than the comparative examples and a JIS defined grain size smaller than the comparative examples.  
         [0143]    Comparative example C 2  provides a longer life than comparative example C 1 , and the present invention in example V 1  provides a further longer life than comparative example C 2  and the present invention in example V 2  provides a further longer life than the present invention in example V 1 . This reveals that hydrogen embrittlement or cracking can effectively be reduced by the following three factors:  
         [0144]    (1) Presence of a nitrogen enriched layer;  
         [0145]    (2) Globular carbide having an area ratio of no less than 10%; and  
         [0146]    (3) Austenite grain having a JIS defined grain size number exceeding 10.  
         [0147]    The present invention in example V 2 , satisfying all of the above three factors, achieves a significantly long life. Simply satisfying item (1) alone also effectively reduces hydrogen embrittlement or cracking, although it does not provide prolonged life effectively, and accordingly, satisfying at least items (1) and (2), as in the present invention in example V 1 , can provide a rolling bearing less prone to hydrogen embrittlement or cracking in an ambient of hydrogen.  
         [0148]    Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.