Residual stress prediction method

Abstract

To provide a residual stress prediction method capable of nondestructively predicting residual stress imparted to a hard turning surface by using parameters (flank wear and cutting resistance) confirmable during machining.SOLUTION: The residual stress σ of the hard turning surface is predicted by using the flank wear amount VB of the cutting tool subjected to hard turning as a parameter.SELECTED DRAWING: Figure 11

Description

[Technical Field]

[0001] The present invention relates to a method for predicting residual stress, and more particularly to a method for predicting residual stress on a surface that has been subjected to hard turning. [Background technology]

[0002] In large bearings, rollers can move axially during rotation and come into contact with the rib. When the rollers repeatedly come into contact with the rib, repeated bending stress is generated in the relief groove located at the boundary between the rib and the raceway surface, which can lead to fatigue failure originating at the bottom of the relief groove. As the operating conditions for bearings become increasingly severe in the future, the frequency and contact load between the rollers and rib will increase, and the risk of fatigue failure will also increase, so there is a need to improve the strength of the relief groove.

[0003] Because relief grooves have a complex shape, they are more likely to be incompletely hardened than raceway surfaces or outer diameter surfaces, potentially resulting in abnormal heat treatment structures (grain boundary oxidation layers, decarburized layers, quench cracks, incomplete hardening, etc.). Removing these abnormal heat treatment structures is effective in improving the strength of relief grooves. Grinding is a common method for processing heat-treated parts with high hardness. However, grinding has difficulty processing large removal amounts and precisely processing complex shapes such as relief grooves.

[0004] Hard turning is known as an alternative to grinding for machining high-hardness parts. Hard turning is a lathe-turning method for high-hardness materials with hardnesses of HRC 50 to 65 due to heat treatment or other factors. By using hard turning to machine relief grooves, it is possible to remove any abnormal heat treatment structure in the relief groove, and it is also easy to machine complex shapes. Furthermore, it is possible to impart a bright surface to the surface and compressive residual stress to the interior of the material, which contributes significantly to improving the fatigue strength of the relief groove.

[0005] Patent Document 1 describes a method for applying a 70 kgf / mm force to the grinding relief groove by using a mechanical processing method such as a barrel method or shot peening, a heat treatment method, or a combination of a heat treatment method and a mechanical processing method. 2This publication discloses a solid needle roller bearing in which the above compressive residual stress is formed to improve the fatigue strength of the thin-walled portion in the relief groove.

[0006] As shown in Figure 12, a conventional solid needle roller bearing 37 has a raceway 39 formed on the inner periphery of an outer ring 38, with flanges formed on both axial ends of the raceway 39, and a plurality of needle rollers 41 that roll freely along the raceway 39, held by a cage 42. The outer ring 38 is formed by machining, and ground relief grooves 43 are formed on both axial ends of the raceway 39.

[0007] In the outer ring 38 of this solid needle roller bearing 37, ground relief grooves 43 are formed on both axial ends of the raceway surface 39, so the wall thickness in those areas is thin and the strength is weak. For this reason, it is desirable to improve the strength, particularly of the area of ​​the ground relief grooves 43, to withstand the repeated action of induced thrust forces on the ribs 40 of the outer ring 38 due to skew of the needle rollers 41. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-161367 Summary of the Invention [Problem to be solved by the invention]

[0009] Although it is known that compressive residual stress improves fatigue strength in machined surfaces such as grinding relief grooves, there is concern that tensile residual stress may occur on the material surface depending on tool wear and machining conditions, reducing fatigue strength. Therefore, it is necessary to confirm whether the residual stress is tensile or compressive. Destructive testing, which involves destroying the sample, is a common method for determining the applied residual stress. However, it is difficult to measure residual stress in shaped structures, and the sample is destroyed, making it unusable as a product. Therefore, a simpler method, preferably a nondestructive testing method, for predicting residual stress is needed.

[0010] The present invention has been made in view of the above-mentioned problems, and its object is to provide a residual stress prediction method that can non-destructively predict the residual stress imparted to a machined surface after hard turning, using parameters that can be confirmed during machining (flank wear, cutting resistance). [Means for solving the problem]

[0011] Therefore, the above object of the present invention is achieved by the following configuration [1] relating to a residual stress prediction method. [1] A residual stress prediction method for predicting residual stress on a processed surface obtained by hard turning a hardened metal surface, comprising: Predicting the residual stress on the machined surface using the amount of flank wear of the cutting tool that has undergone hard turning as a parameter. Residual stress prediction method. [Effects of the Invention]

[0012] According to the method for predicting residual stress in a rolling bearing of the present invention, it is possible to non-destructively predict the residual stress imparted to a machined surface that has been hard turned, using parameters that can be confirmed during processing (flank wear, cutting resistance). [Brief explanation of the drawings]

[0013] [Figure 1]FIG. 1 is an enlarged view showing the state of hard turning using a new tool. [Figure 2] FIG. 2 is an enlarged view showing the state of hard turning using a worn tool. [Figure 3] FIG. 3 is a graph showing the relationship between the machining distance and the amount of flank wear. [Figure 4] FIG. 4 is a graph showing the relationship between flank wear and cutting resistance. [Figure 5] FIG. 5(a) is a graph showing the relationship between the flank wear amount and the circumferential residual stress on the surface, and FIG. 5(b) is a graph showing the relationship between the flank wear amount and the axial residual stress on the surface. [Figure 6] FIG. 6(a) is a graph showing the relationship between cutting resistance and circumferential residual stress on the surface, and FIG. 6(b) is a graph showing the relationship between cutting resistance and axial residual stress on the surface. [Figure 7] Figure 7(a) is a graph showing the relationship between flank wear amount and internal minimum circumferential residual stress, and Figure 7(b) is a graph showing the relationship between flank wear amount and internal minimum axial residual stress. [Figure 8] FIG. 8(a) is a graph showing the relationship between the amount of flank wear and the depth at which the circumferential compressive stress is at its maximum, and FIG. 8(b) is a graph showing the relationship between the amount of flank wear and the depth at which the axial compressive stress is at its maximum. [Figure 9] FIG. 9(a) is a graph showing the relationship between cutting resistance and internal minimum circumferential residual stress, and FIG. 9(b) is a graph showing the relationship between cutting resistance and internal minimum axial residual stress. [Figure 10] FIG. 10(a) is a graph showing the relationship between cutting resistance and the depth at which circumferential compressive stress is at its maximum, and FIG. 10(b) is a graph showing the relationship between cutting resistance and the depth at which axial compressive stress is at its maximum. [Figure 11] FIG. 11 is a schematic diagram showing the relationship between the depth of compressive stress and residual stress, with the peripheral speed during machining and the amount of flank wear as parameters. [Figure 12] FIG. 12 is a cross-sectional view showing a part of a conventional needle roller bearing. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of a residual stress prediction method according to the present invention will be described in detail with reference to the drawings. From many years of experience in cutting processing, the applicant has learned that in general cutting processing, the cutting resistance and the amount of flank wear of the cutting tool have a significant effect on the residual stress on the machined surface of the workpiece. On the other hand, hard turning is effective for machining the relief grooves of bearing rings with complex shapes with high precision, but hard turning leaves residual stress on the machined surface. Furthermore, since the residual stress remaining on the machined surface affects the strength of the machined part, predicting the residual stress remaining on the machined surface by hard turning is important for understanding the strength of the machined part. Below, we will explain the relief groove 43 formed on a solid needle roller bearing 37 as an example of a relief groove on a bearing ring that has been hard turned, based on Figure 12.

[0015] As shown in Figure 12, solid needle roller bearing 37 has a raceway surface 39 on the inner periphery of outer ring 38, with flanges formed on both axial ends of raceway surface 39, and a plurality of needle rollers 41 that can roll freely along raceway surface 39 held by cage 42. Outer ring 38 is formed by machining, and raceway surface 39 has relief grooves 43 on both axial ends.

[0016] Therefore, hard turning was performed on a workpiece with a Vickers hardness of Hv=700 by heat treatment, and the cutting resistance F N The relationship between the flank wear amount VB of the cutting tool and the residual stress on the machined surface was also investigated.

[0017] Figure 1 is an enlarged view showing hard turning using a new cutting tool. As shown in Figure 1, when workpiece 1 is cut with new cutting tool 2, cutting resistance A, which is the composite value of principal force B, thrust force C, and feed force D, acts on cutting tool 2. While the cutting resistance A of a new cutting tool 2 is relatively small, in hard turning, thrust force C is larger than in general cutting. In other words, contact stress during cutting increases, generating high compressive residual stress inside. Note that each force component is a three-dimensional vector, but for convenience, it is represented in two dimensions in Figure 1.

[0018] As shown in Figure 2, as the machining distance of the workpiece 1 increases, the flank 2a (the side where the workpiece surface escapes) of the cutting tool 2 wears due to friction between the cutting tool 2 and the workpiece 1, causing flank wear. Flank wear is an important parameter that determines the tool life.

[0019] The flank wear volume VB increases in proportion to the machining distance, almost regardless of the machining peripheral speed, as shown in Figure 3. Note that the flank wear volume VB was measured by measuring the wear width using a digital microscope with a magnification of 150 to 200 times.

[0020] In addition, as shown in Figure 4, when cutting is performed under the same conditions as when a new part was used, the cutting resistance F N increases in proportion to the flank wear amount VB, which increases the amount of heat generated and has a negative effect on the roughness of the machined surface. Note that the cutting resistance of a new cutting tool with a flank wear amount VB of 0 increases in direct proportion to the feed rate.

[0021] The test results of hard turning are shown in Figs. 5 to 10, with peripheral speed as a parameter. Figure 5 shows the relationship between the flank wear amount VB and the residual stress σ (circumferential and axial directions) on the machined surface. N and residual stress σ (circumferential and axial directions) on the machined surface.

[0022] As shown in Fig. 5, the surface residual stress σ increases in the tensile direction in both the circumferential and axial directions until the flank wear volume VB reaches approximately 100 μm, after which it remains at a roughly constant value. In particular, when the flank wear volume VB is 100 μm or more and machining is performed at a peripheral speed of 300 m / min, the thermal stress generated by heat generation during machining causes the surface residual stress to become tensile residual stress. In other words, it is possible to roughly predict whether the residual stress σ on the surface will be tensile or compressive depending on the flank wear amount VB and the peripheral speed. N There is no correlation between the surface residual stress σ and the surface residual stress σ.

[0023] Figure 7 shows the relationship between the flank wear volume VB and the minimum internal residual stress σ (circumferential and axial directions). Figure 8 shows the relationship between the flank wear volume VB and the maximum internal compressive stress depth d (circumferential and axial directions).

[0024] As shown in Fig. 7, the internal residual stress σ is a compressive residual stress, and is a constant value of approximately 1000 to 1250 MPa, regardless of the peripheral speed and flank wear volume VB. This value roughly corresponds to the yield stress of the material. As shown in Figure 8(a), the depth d at which the circumferential compressive stress is at its maximum increases with an increase in the flank wear amount VB, while the depth d at which the axial compressive stress is at its maximum remains almost constant regardless of the circumferential speed and the flank wear amount VB, as shown in Figure 8(b).

[0025] Figure 9 shows the cutting force F N Fig. 10 shows the relationship between the cutting force F and the minimum internal residual stress σ (circumferential and axial directions). N and the depth d (circumferential and axial directions) of maximum internal compressive stress.

[0026] As shown in Figure 9, the cutting force F N In the range of 400N or less, the internal residual stress σ is a compressive residual stress of a substantially constant value, and the cutting resistance F N The compressive residual stress σ can be predicted from As shown in Fig. 10(a), the depth d of the maximum circumferential compressive stress is N On the other hand, as shown in Fig. 10(b), the depth d of maximum axial compressive stress increases with increasing cutting resistance F N No correlation is seen with From this, referring to Fig. 8(a), the depth d of maximum circumferential compressive stress can be calculated by multiplying the flank wear amount VB and cutting resistance F N can be predicted from

[0027] FIG. 11 qualitatively shows the results of FIGS. 5 to 10, grouped into three groups according to the processing conditions. In the figure, process 1 is when the flank wear amount VB and peripheral speed are small, process 2 is when the flank wear amount VB and peripheral speed are medium, and process 3 is when the flank wear amount VB and peripheral speed are large.

[0028] The residual stress σ on the surface gradually increases from σ1 (compressive residual stress) in process 1 to σ2 in process 2 and then to σ3 in process 3. In particular, in machining 3, where the flank wear volume VB and peripheral speed are large, the residual stress on the surface becomes tensile residual stress (σ3) due to thermal stress caused by heat generation during machining. In other words, it is possible to predict whether the residual stress σ on the surface will become compressive residual stress or tensile residual stress from the values ​​of the flank wear volume VB and peripheral speed, which are measurable parameters.

[0029] In addition, the depth d at which the compressive stress is maximum gradually increases from d1 in Process 1 to d2 in Process 2 and then to d3 in Process 3 according to the magnitude of the contact stress and frictional force due to the plastic processing. In other words, the depth d at which the compressive stress is maximum is determined by the cutting resistance F, which is a measurable parameter. N (which is also the flank wear amount VB). The maximum value of the compressive stress is approximately equal to the compressive yield stress.

[0030] As explained above, the flank wear volume VB shows a good correspondence with the occurrence of tensile residual stress on the surface and the depth of compressive residual stress, so by measuring the flank wear volume VB, it is possible to non-destructively predict the occurrence of tensile residual stress on the surface and the depth of compressive residual stress. Furthermore, it is possible to predict residual stress in areas where the shape is complex and it is difficult to measure the residual stress.

[0031] The present invention is not limited to the above-described embodiment, and can be modified, improved, etc. as appropriate. For example, in the present embodiment, the relief groove of a rolling bearing that is subjected to hard turning has been described as an example, but the present invention is not limited to relief grooves of rolling bearings, and can be applied to any part that is subjected to hard turning, and similar effects can be achieved.

[0032] As described above, the present specification discloses the following: (1) A residual stress prediction method for predicting residual stress on a machined surface obtained by hard turning a hardened metal surface, comprising: Predicting the residual stress on the machined surface using the amount of flank wear of the cutting tool that has undergone hard turning as a parameter. Residual stress prediction method. According to this configuration, the residual stress imparted to the hard turned surface can be predicted non-destructively using the amount of flank wear, which is a parameter that can be confirmed during processing.

[0033] (2) Predicting the residual stress on the machined surface by further taking into account the cutting resistance value during the hard turning process as the parameter. (1) A residual stress prediction method according to (1). According to this configuration, by predicting the residual stress by further taking into account the cutting resistance value, it is possible to more accurately and non-destructively predict the residual stress imparted to the hard turned surface.

[0034] (3) The prediction of the residual stress includes determining whether the residual stress is a tensile residual stress or a compressive residual stress. The residual stress prediction method according to (1) or (2). According to this configuration, the strength of the processed portion can be estimated by predicting whether the residual stress on the processed surface is tensile residual stress or compressive residual stress.

[0035] (4) The processed surface is a relief groove formed at a corner between a raceway surface of a raceway ring in a rolling bearing and an inner surface of a flange formed at least at one axial end of the raceway surface. The residual stress prediction method according to any one of (1) to (3). According to this configuration, the residual stress in the relief groove of the raceway of the rolling bearing can be predicted in a non-destructive manner. [Explanation of symbols]

[0036] 1 Workpiece (relief groove of bearing ring) 2 cutting tools F N Cutting resistance (cutting resistance value) VB Flank wear σ residual stress

Claims

1. A residual stress prediction method for predicting residual stress on a processed surface obtained by hard turning a hardened metal surface, comprising: Predicting the residual stress on the machined surface using the amount of flank wear of the cutting tool that has undergone hard turning as a parameter. Residual stress prediction method.

2. The residual stress on the machined surface is predicted by further taking into account the cutting resistance value during the hard turning as the parameter. The residual stress prediction method according to claim 1 .

3. The prediction of the residual stress includes determining and predicting whether the residual stress is a tensile residual stress or a compressive residual stress. The residual stress prediction method according to claim 1 .

4. the processed surface is a relief groove formed at a corner between a raceway surface of a raceway ring in the rolling bearing and an inner surface of a flange formed at least at one axial end of the raceway surface; The residual stress prediction method according to any one of claims 1 to 3.

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