JPS6127445B2 - - Google Patents

Description

[Detailed description of the invention]

B. Field of Industrial Application This invention is used to temper annular parts such as inner and outer rings of rolling bearings, and the annular parts, which have been previously hardened by high-frequency induction heating, are heated again by high-frequency induction to an appropriate temperature below the transformation point. The present invention relates to an induction tempering method for tempering. B. Prior art In the prior art induction tempering of the inner and outer rings of a rolling bearing, as shown in FIG. Hardened layer 1 of annular part 1
A certain outer diameter (or inner diameter) a is again subjected to high-frequency induction heating using the heating coil 2. However, when an annular part 1 whose outer diameter (or inner diameter) is induction hardened is induction tempered from the outer diameter (or inner diameter) where the hardened layer 1a is located, the compressive residual stress due to induction hardening becomes large tensile residual stress, and the hardened layer often Cracks may occur in 1a. The reason for the generation of tensile residual stress is that when the annular component 1 is induction tempered from the outer diameter (or inner diameter) where the hardened layer 1a is located, the temperature gradient due to high frequency induction heating is higher at the outer diameter (or inner diameter) and If there is such a temperature gradient, the volume shrinkage due to the transformation of hardened martensite to tempered martensite in the microstructure is larger closer to the surface, so the tensile residual stress increases. When it occurs, it becomes. Further, in the case of induction tempering from the side with the hardened layer 1a, there are problems in that it is difficult to control the hardness due to occurrence of hardness unevenness due to induction heating, and variations in dimensions occur.
Therefore, in order to solve such problems, methods are being considered that use a power source with a frequency as low as possible, reduce the supplied power, and lengthen the heating time. However, although this method is effective for products with simple shapes such as shafts, it is effective for products with complex shapes such as the inner and outer rings of ball bearings whose raceway surfaces have curvature. It is difficult to solve problems such as uneven hardness. Moreover, because the tempering process takes a long time, the advantages of induction tempering cannot be exploited, and it is currently quite difficult to maintain the hardness after tempering at HRC58 or higher, which is required for rolling bearings. It is not suitable for tempering the inner and outer rings of rolling bearings. Therefore, in the inner and outer rings of rolling bearings,
Tempering by heating in an electric furnace is often carried out, but equipment such as electric furnaces not only takes up space, but also requires time for the tempering process, which wastes energy and increases manufacturing costs compared to induction tempering. It has become the cause of C. Purpose of the Invention This invention was made to compensate for the drawbacks of conventional induction tempering, which does not generate tensile residual stress but rather increases compressive residual stress, thereby improving rolling fatigue strength, which is the basic performance of bearings. The present invention provides an induction tempering method that contributes to the present invention. D. Structure of the Invention The gist of the present invention is to perform high-frequency induction heating from the opposite side of the hardened layer to perform tempering by heat conduction after induction hardening. E. Example Fig. 2 shows a specific example in which the induction tempering method of the present invention is applied to the inner and outer rings of a ball bearing.
a indicates the inner ring, and b indicates the outer ring. In the drawings, 3 denotes an annular part such as the inner and outer rings of a ball bearing to be tempered by the induction tempering method of the present invention, and 3a denotes a hardened layer previously applied to the outer diameter (or inner diameter) of the annular part 3 by induction hardening. , 4 is a heating coil provided facing the inner diameter (or outer diameter) of the annular component 3 on the side opposite to the hardened layer 3a. In this invention, after the outer diameter (or inner diameter) of the annular component 3 is induction hardened, the inner diameter (or outer diameter) on the opposite side of the hardened layer 3a is heated by high frequency induction using a heating coil 4, and then cooled to room temperature and tempered. completed. As described above, this invention uses heat conduction to temper the annular part 3 from the opposite side of the hardened layer 3a on the outer diameter (or inner diameter), so the temperature gradient is greater on the inside than on the outer diameter (or inner diameter) surface. The closer to the surface, the smaller the volume shrinkage due to the transformation of hardened martensite into tempered martensite during tempering. As a result, compressive residual stress is generated in the surface layer. FIG. 3a shows a comparison of the residual stress distribution of a specimen tempered by induction tempering according to the conventional method with that of a specimen tempered by electric furnace. The specimen shape is a ring specimen (hereinafter referred to as a ring specimen) with an outer diameter of 50φ, a minor curvature of 25R, an inner diameter of approximately 27φ (1/6 taper), and a width of 12mm.The steel type is bearing steel, but after normal quenching. Each piece is tempered by high-frequency induction heating from the outer diameter or by heating the entire piece in an electric furnace. As can be seen from FIG. 3a, in the conventional induction tempering method, the residual stress in the surface layer shifts to the tensile side compared to the electric furnace tempered specimen.
The transfer life test results for this ring specimen are shown in Table 1 below.
Shown below.

[Table] According to Table 1, the rolling life of the induction-tempered specimens prepared by the conventional method is much shorter than that of the electric furnace tempered specimens, and cracks occur on the rolling surface. As described above, the adverse effects of induction tempering by conventional methods can naturally be expected not only for through-hardened materials but also for induction-hardened materials. Therefore, in the conventional induction tempering method, in addition to using a power source with a frequency as low as possible, a method is adopted in which the entire product is heated by long-term heating or intermittent heating with a low power supply. Next, we used the conventional method on a cylindrical transfer fatigue specimen with a diameter of 12 mm and a length of 22 mm (hereinafter referred to as a 12φ cylindrical specimen), and attempted to heat the entire body for a long time (5 seconds or more). The results of the rolling life test of the induction tempered specimens are shown in Table 2 below, and from these results it can be said that there is no difference in rolling life between induction tempered products and electric furnace tempered products when overall heating is used.

[Table] Figure 3b shows an example of the measurement results of residual stress on the surface layer of the specimen shown in Table 2, and residual compressive stress can be left even during induction tempering if the entire specimen is heated. However, induction tempering, which heats the entire product by applying high-frequency heating over a long period of time, only takes about 5 seconds to heat a small specimen with a simple shape, such as the 12φ cylindrical specimen mentioned above, but For larger products such as 6206 (deep groove ball bearing) outer rings, which will be described later, a longer time is required, which completely negates the advantage of high-frequency heating, which has short-term heating as its greatest weapon. On the other hand, in the induction tempering method according to the present invention, heating time of 1 second or less is sufficient for, for example, the above-mentioned ring-shaped specimen and the following 6206 outer ring. Table 3 below shows the bearing life test results when the 6206 outer ring was subjected to induction tempering according to the present invention.The high frequency heating time was 0.8 to 1.0 seconds, and the advantages of high frequency heating were fully utilized.

[Table] As can be seen from Table 3, the transfer life of the induction tempered product of the present invention is twice as long as that of the electric furnace tempered product. As shown in Figure 3c, the reasons for the improved lifespan are that the residual stress of the induction tempered product of this invention shifts to the compression side compared to the electric furnace tempered product (increase in compressive residual stress) and the hardness due to short-time tempering. This may be due to a decrease in the rate of decline. F. Effects of the Invention In this invention, after induction hardening, high-frequency induction heating is performed from the opposite side of the hardened layer to perform tempering by thermal conduction, which does not generate tensile residual stress but rather increases compressive residual stress. In addition to contributing to improving the rolling fatigue strength, which is the basic performance of bearings,
Since it is suitable for uniform heating of the hardened layer, it is possible to eliminate uneven hardness even for products with complex shapes such as inner and outer rings of ball bearings with rolling surfaces with curvature. Hardness can be easily controlled with small variations.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a conventional induction tempering method, where a shows the case of the inner ring and b shows the case of the outer ring. Second
The figure is a schematic view showing the induction tempering method according to the present invention, where a shows the case of the inner ring and b shows the case of the outer ring. FIG. 3 is a graph showing a comparison of residual stress distributions between furnace tempered products and induction tempered products. 3...Annular part, 3a...Hardened layer, 4...Heating coil.

Claims (1)

[Claims] 1. A method for induction tempering an annular workpiece having a rolling axial surface, which is characterized in that the tempering is performed by high-frequency induction heating from the opposite side of a quenched layer that has been previously induction-hardened by heat conduction.