Quenching method

Abstract

In the present invention, in a heating process for induction heating an annular steel workpiece W so that the workpiece W undergoes full-body quenching, a temperature raising step for continuously raising the temperature of the workpiece W includes: a primary temperature-raising step S1 for raising the temperature of the workpiece W until reaching the Curie temperature of the steel; and a secondary temperature-raising step S3 for raising the temperature of the workpiece W until reaching a predetermined temperature higher than or equal to the quenching temperature. Between the temperature raising steps S1 and S3, a thermal diffusion step S2 is carried out for performing thermal diffusion in the workpiece W by lowering the output of heating coils 2A, 2B more than when the primary temperature-raising step S1 is carried out.

Description

Quenching method

[0001] The present invention relates to a method for hardening an annular workpiece made of steel.

[0002] For example, for annular mechanical parts that are required to have high mechanical strength and hardness, such as the raceways of rolling bearings, annular workpieces formed into a predetermined shape from steel with a carbon content of 0.8 mass % or more (for example, SUJ2, a type of high-carbon chromium bearing steel specified in JIS G4805) are used, which have been subjected to heat treatment such as quenching and tempering.

[0003] There are two types of hardening: full hardening, in which the entire workpiece is hardened, and surface hardening, in which only the surface layer of the workpiece is hardened. The annular workpiece that will ultimately become the raceway is usually hardened full. When hardening the full workpiece, the workpiece can be heated by either atmospheric heating (furnace heating) or induction heating. However, induction heating is increasingly being adopted, as it has the advantages of significantly reducing the amount of energy required for heating and significantly shortening the heating time compared to atmospheric heating.

[0004] In the heating process for hardening a workpiece as a whole, the workpiece is induction-heated under heating conditions that allow a predetermined amount of carbon to be dissolved in the workpiece's metal structure (resulting in a heated product with a predetermined carbide area ratio). However, if the output of the heating coil is increased to rapidly heat the workpiece in order to improve hardening efficiency, large temperature differences may occur within the workpiece depending on the workpiece's shape and thickness. For example, even if a portion of the workpiece reaches the target temperature of approximately 900°C during induction heating, there may be portions of the workpiece that only reach approximately 800°C. If the workpiece is cooled and hardened under such conditions of large temperature differences within the workpiece, or more specifically, under conditions of large variations in the carbide area ratio within the workpiece, the metal structure of the hardened workpiece will be nonuniform, making it impossible to obtain machine parts with the desired mechanical strength, hardness, etc.

[0005] A method that is thought to be effective in solving this problem is described in the following Patent Document 1. Specifically, in a heating process in which a workpiece is induction-heated, a high-frequency current is supplied to a heating coil in multiple steps, thereby alternately repeating a temperature-raising step in which the temperature of the workpiece is continuously raised, and a thermal diffusion step in which thermal diffusion within the workpiece is promoted. In the thermal diffusion step, heat transfer occurs from high-temperature parts to low-temperature parts within the workpiece, thereby reducing the temperature difference within the workpiece and making it possible to uniform the carbide area ratio within the workpiece.

[0006] Japanese Patent Application Laid-Open No. 2007-262461

[0007] The heating method described in Patent Document 1 is proposed as a method for tempering a workpiece. Because the quenching temperature is typically set significantly higher than the tempering temperature, applying the heating method described in Patent Document 1 to the heating process during quenching would likely require the temperature-raising step and the thermal diffusion step to be performed numerous times. Furthermore, there is a concern that the total time of the thermal diffusion step will increase as the number of times it is performed (i.e., the duration of the thermal diffusion step) increases. Furthermore, because the thermal diffusion step involves both thermal diffusion within the workpiece and heat radiation to the outside of the workpiece, there is also a concern that the total amount of energy required to heat the workpiece increases as the number of times it is performed increases.

[0008] In view of the above circumstances, the present invention aims to make it possible to uniformize the amount of residual carbide (carbide area ratio) within a workpiece while reducing the time and amount of energy required to heat the workpiece when induction heating an annular workpiece made of steel and then rapidly cooling the workpiece to harden the entire workpiece, thereby making it possible to obtain a high-quality hardened product with the desired mechanical strength, etc. at low cost.

[0009] The quenching method of the present invention, which has been devised to achieve the above-mentioned object, comprises a heating step in which an annular workpiece made of steel is induction-heated by passing current through a heating coil, and a cooling step in which the workpiece is rapidly cooled after heating is complete to quench the entire workpiece, and is characterized in that the heating step, in which the temperature of the workpiece is continuously raised, comprises a first heating step in which the temperature of the workpiece is raised until it reaches the Curie temperature of the steel, and a second heating step in which the temperature of the workpiece is raised until it reaches a predetermined temperature equal to or higher than the quenching temperature, and that between the first heating step and the second heating step, a thermal diffusion step is carried out in which thermal diffusion is carried out within the workpiece by lowering the output of the heating coil compared to when the first heating step was carried out.

[0010] In the first heating step of the hardening method according to the present invention, the workpiece is heated to the Curie temperature (approximately 780°C) of the steel, which has little effect on carbon dissolution. Therefore, even if a large temperature difference occurs within the workpiece, the difference in the amount of remaining carbide (carbide area ratio) between the high-temperature and low-temperature regions of the workpiece can be minimized. In the thermal diffusion step following the first heating step, the output of the heating coil is reduced compared to the first heating step. This prevents the entire workpiece from heating up, while also reducing the temperature difference within the workpiece caused by the first heating step through thermal diffusion (heat transfer from the high-temperature region to the low-temperature region of the workpiece). Reducing the temperature difference within the workpiece in this way simultaneously reduces the difference in carbide area ratio within the workpiece. In the secondary heating step that follows the thermal diffusion step, the workpiece is heated to a predetermined temperature that is equal to or higher than the hardening temperature, causing the dissolution of carbon into the metal structure to proceed (restart) throughout the workpiece. However, because the temperature of the workpiece is made approximately uniform in the previous thermal diffusion step, the amount of carbon that dissolves into the metal structure of the workpiece can be made uniform within the workpiece.

[0011] In the present invention, the temperature-raising step for continuously raising the temperature of the workpiece is performed only twice, the first and second temperature-raising steps, so that the thermal diffusion step between the temperature-raising steps to promote thermal diffusion within the workpiece only needs to be performed once. This reduces the total time and energy required to induction-heat the workpiece to a predetermined temperature in the heating step, while homogenizing the amount of residual carbide (carbide area ratio) within the workpiece. Furthermore, by rapidly cooling the workpiece after heating with a homogenized carbide area ratio within the workpiece in the cooling step, a high-quality mechanical part with no differences in mechanical strength or hardness within the part can be obtained.

[0012] In the heating process, it is preferable to carry out a temperature holding step in which the workpiece is held at the above-mentioned predetermined temperature after the second temperature raising step is carried out. In this way, the carbon can be dissolved into the metal structure of the workpiece in the temperature holding step as well, which is advantageous in making the carbide area ratio in the workpiece uniform and manufacturing high-quality mechanical parts.

[0013] The temperature rise rate of the workpiece during the first temperature rise step can be faster than the temperature rise rate of the workpiece during the second temperature rise step, thereby shortening the total time required for the heating process and contributing to reducing the manufacturing costs of mechanical parts using the workpiece as a base material.

[0014] In the heating process, the workpiece can be induction heated by passing current through an outer diameter side coil and an inner diameter side coil, which are arranged radially outside and inside the workpiece and electrically connected in series. By electrically connecting the outer diameter side coil and the inner diameter side coil in series, the amount of current flowing through the outer diameter side coil and the inner diameter side coil can be made the same, so that the entire workpiece can be heated efficiently without causing a large temperature difference between the outer diameter side region and the inner diameter side region of the workpiece.

[0015] In the hardening method according to the present invention described above, the workpiece to be hardened can be a raceway ring of a rolling bearing. In other words, the hardening method according to the present invention can be preferably employed in the manufacturing process of a raceway ring of a rolling bearing.

[0016] As described above, according to the present invention, when quenching an annular workpiece made of steel, the amount of residual carbide (carbide area ratio) in the workpiece can be made uniform while suppressing the total time and total amount of energy required to heat the workpiece, thereby making it possible to obtain high-quality quenched products with desired mechanical strength, etc., and ultimately machine parts, at low cost.

[0017] Fig. 1 is a schematic cross-sectional view of an induction heating device used when carrying out the heating step of the heat treatment method according to the present invention. Fig. 2 is a block diagram schematically showing an electric circuit of the induction heating device shown in Fig. 1. Fig. 3 is a graph showing the transition of temperature at two points in an induction-heated workpiece. Fig. 4 is a graph showing the transition of carbide area ratio at two points in an induction-heated workpiece. Fig. 5 is a schematic view showing a modified example of the induction heating device shown in Fig. 1.

[0018] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0019] Fig. 1 is a schematic cross-sectional view of an induction heating apparatus 1 used in a heating step of induction heating an annular workpiece W, which is a hardening target, in a hardening method according to one embodiment of the present invention. Fig. 2 is a block diagram showing the electrical circuit of the induction heating apparatus 1. The induction heating apparatus 1 includes a workpiece support member 3 that supports from below the workpiece W placed flat with its central axis aligned vertically; a coil unit 2 that induction heats the workpiece W (supported by the workpiece support member 3; the same applies hereinafter to the heating step); a high-frequency power source 4 electrically connected to the coil unit 2 (its heating coil) and outputting a high-frequency current 8; and a control device 5 electrically connected to the high-frequency power source 4 and controlling the output of the high-frequency power source 4 (heating coil). The heating coil of the coil unit 2 is electrically connected to the high-frequency power source 4 via an electrode member 6 and power distribution members 7a and 7b.

[0020] The annular workpiece W will eventually become, for example, a raceway (outer ring or inner ring) of a rolling bearing, and is made of steel with a carbon content of 0.8% by mass or more (for example, SUJ2, which is classified as a bearing steel specified in JIS G4805). In addition to SUJ2, other steels with a carbon content of 0.8% by mass include SUJ3, which is classified as a bearing steel like SUJ2, and SKD11, SKD12, SKD3, and SKD31, which are classified as tool steels specified in JIS G4404.

[0021] The coil unit 2 comprises an outer diameter side coil 2A and an inner diameter side coil 2B as heating coils respectively arranged on the radially outer and inner sides of the workpiece W, and a coil support member (not shown) made of an insulating material such as ceramics that supports both coils 2A and 2B. Both the outer diameter side coil 2A and the inner diameter side coil 2B are formed into a predetermined shape by bending a tubular body made of a conductive metal such as a copper pipe, and a coolant flows through their internal space while current is flowing (while the workpiece W is being heated). This prevents the coils themselves from overheating.

[0022] The electrode member 6 has an entrance electrode 6a and an exit electrode 6b fixed via an insulating layer. The entrance electrode 6a is electrically connected to (one longitudinal end of) the outer diameter side coil 2A via a power distribution member 7a, and the exit electrode 6b is electrically connected to (one longitudinal end of) the inner diameter side coil 2B via a power distribution member 7b. In addition, (the other longitudinal end of) the outer diameter side coil 2A and (the other longitudinal end of) the inner diameter side coil 2B are electrically connected via a power distribution member 7c.

[0023] As described above, the outer diameter side coil 2A and the inner diameter side coil 2B are electrically connected in series. Therefore, when the control device 6 issues an output command for the high frequency current 8 to the high frequency power supply 4, the high frequency current 8 flows along the following path: high frequency power supply 4 → entrance electrode 6a → power distribution member 7a → outer diameter side coil 2A → power distribution member 7c → inner diameter side coil 2B → power distribution member 7b → exit electrode 6b → high frequency power supply 4. Because the outer diameter side coil 2A and the inner diameter side coil 2B are electrically connected in series, the amount of high frequency current flowing through both coils 2A and 2B is the same. As a result, the outer diameter surface and inner diameter surface of the workpiece W are first induction-heated simultaneously, and the heat from the heated outer diameter surface and inner diameter surface is transferred toward the core of the workpiece W, thereby raising the temperature of the entire workpiece W.

[0024] The heating process carried out using the induction heating device 1 having the above configuration consists of a primary temperature-raising step S1, a thermal diffusion step S2, a secondary temperature-raising step S3, and a temperature-holding step S4, as shown in Figures 3 and 4. In this heating process, the above steps S1 to S4 are carried out in order, and the workpiece W (as a whole) is induction-heated until it reaches a predetermined temperature (about 900°C) that is equal to or higher than the hardening temperature.

[0025] A specific example of the heating process including steps S1 to S4 will be described below with reference to Figures 3 and 4. Here, the heating target is an annular workpiece W made of SUJ2 and having an outer diameter of 100 to 250 mm and a radial thickness (maximum thickness) of 10 mm or less.

[0026] First, in the primary heating step S1, an annular workpiece W at room temperature is placed between the coils 2A and 2B, and then electricity is passed through the coils 2A and 2B (high-frequency current 8 is passed through the coils 2A and 2B), thereby continuously heating the workpiece W until a portion of the workpiece W that is easy to heat up (for example, point A shown in FIG. 1) reaches the Curie temperature of steel (approximately 780°C). At this time, the amount of high-frequency current flowing through the coils 2A and 2B (the output of the coils 2A and 2B) is set so that the heating rate (V1) of point A of the workpiece W is 140 to 170°C / sec.

[0027] Next, in the thermal diffusion step S2, the output of the coils 2A and 2B is reduced compared to when the first heating step S1 was performed, thereby diffusing heat within the workpiece W. In other words, heat is transferred from high-temperature areas (e.g., point A) within the workpiece W to low-temperature areas (e.g., point B shown in FIG. 1 ) to reduce the temperature difference within the workpiece W. Here, the temperature difference between points A and B is approximately 100°C at the end of the first heating step S1. Once this temperature difference converges to 20°C or less, the thermal diffusion step S2 is terminated and the subsequent second heating step S3 begins. Note that the output of the coils 2A and 2B during the thermal diffusion step S2 is controlled so that the temperature of the high-temperature areas within the workpiece W is between 770 and 800°C. This is because, if the temperature exceeds 800°C, carbon dissolution into the metal structure of the high-temperature areas rapidly progresses, promoting non-uniformity of the carbide area ratio within the workpiece W. If the temperature of the high-temperature areas within the workpiece W exceeds 800°C, adjustments are made to the conditions while checking the carbide dissolution results in the final product.

[0028] In the second heating step S3, the output of the coils 2A and 2B is increased to a level higher than that during the thermal diffusion step S2, thereby continuously raising the temperature of the workpiece W (more specifically, for example, the point A of the workpiece W) until it reaches a predetermined temperature (approximately 900°C) that is equal to or higher than the quenching temperature. The output of the coils 2A and 2B during the second heating step S3 is set to be lower than that during the first heating step S1, so that the temperature rise rate (V2) of the workpiece W is 20 to 35°C / second.

[0029] In the temperature holding step S4, the output of the coils 2A and 2B is lowered to a level lower than that during the secondary temperature increase step S3, thereby holding the workpiece W at the above-mentioned predetermined temperature (approximately 900°C) for a predetermined time. This causes carbon to dissolve into the metal structure of the workpiece W, reducing the carbide area ratio of the workpiece W. This temperature holding step S4 is carried out for a predetermined time until the carbide area ratio of the workpiece W falls within a predetermined numerical range (e.g., 8 to 10%).

[0030] Once the temperature holding step S4 is completed and the entire workpiece W is heated to or above the hardening temperature, the output of the high-frequency current 8 from the high-frequency power supply 4 (the supply of current to the coils 2A and 2B) is stopped to stop heating of the workpiece W, and the workpiece W is removed from the induction heating device 1. The removed workpiece W is transported to a cooling process (not shown) and then rapidly cooled, for example, by immersion in a coolant. As a result, the entire workpiece W is hardened (its metal structure becomes martensite), and becomes a hardened product with increased mechanical strength and hardness compared to before hardening.

[0031] In the first heating step S1 performed in the heating process described above, the workpiece W is heated until it reaches the Curie temperature of steel (approximately 780°C), which has little effect on carbon dissolution. Therefore, even if a large temperature difference occurs within the workpiece W, the difference in carbide area ratio between the high-temperature and low-temperature portions of the workpiece W can be kept small. In the thermal diffusion step S2 performed following the first heating step S1, the output of the coils 2A and 2B is reduced compared to when the first heating step S1 was performed. This prevents the temperature of the entire workpiece W from rising, while reducing the temperature difference that occurred within the workpiece W in the first heating step S1 through thermal diffusion (heat transfer from the high-temperature portion to the low-temperature portion of the workpiece W). Reducing the temperature difference within the workpiece W in this way simultaneously reduces the difference in carbide area ratio within the workpiece W.

[0032] In the secondary heating step S3 that follows the thermal diffusion step S2, the workpiece W is heated until it reaches a predetermined temperature that is higher than the hardening temperature, so that the dissolution of carbon into the metal structure of the workpiece W progresses (resumes) throughout the entire workpiece W. However, since the temperature of the workpiece W is made approximately uniform in the previous thermal diffusion step S2, the amount of carbon that dissolves into the metal structure of the workpiece W can be made uniform within the workpiece.

[0033] In this embodiment, the temperature-raising step for continuously raising the temperature of the workpiece W is performed only twice, the first temperature-raising step S1 and the second temperature-raising step S3, and therefore the thermal diffusion step S2, which is provided between the temperature-raising steps to promote thermal diffusion within the workpiece W, only needs to be performed once. This reduces the total time and total energy required to induction-heat the workpiece W until it reaches a predetermined temperature in the heating step, while also making it possible to homogenize the carbide area ratio within the workpiece W. Furthermore, by rapidly cooling the workpiece W after heating is complete and the carbide area ratio has been made homogenous within the workpiece W in the cooling step, a high-quality, hardened product (machine part) can be obtained that has no differences in mechanical strength or hardness within the part.

[0034] In this embodiment, after the secondary temperature increase step S3, a temperature holding step S4 is performed in which the workpiece W is held at the above-mentioned predetermined temperature (approximately 900° C.). In this way, carbon can be dissolved into the metal structure of the workpiece W even in the temperature holding step S4, which is advantageous in uniforming the carbide area ratio in the workpiece W and manufacturing high-quality mechanical components.

[0035] In this embodiment, the temperature increase rate (V1) of the workpiece W during the first temperature increase step S1 is set to be faster than the temperature increase rate (V2) of the workpiece W during the second temperature increase step S2, thereby shortening the total time of the heating process and contributing to reducing the manufacturing costs of the mechanical parts.

[0036] The outputs of the coils 2A and 2B in the heating process described above may be controlled based on tests conducted in advance, or may be controlled while monitoring the temperature of the workpiece W with a thermometer 10 (based on the temperature of the workpiece W measured by the thermometer 10), as illustrated in Fig. 5. Fig. 5 illustrates an example in which two thermometers 10 are installed, but the number of thermometers 10 to be installed is arbitrary, and one or three or more thermometers may be installed.

[0037] The method for hardening a workpiece W according to one embodiment of the present invention and the induction heating device 1 used to carry out this hardening method have been described above, but the embodiment of the present invention is not limited to this.

[0038] For example, although not shown, the workpiece W may be induction heated by a heating coil (outer diameter side coil 2A) arranged only on its outer diameter side, or by a heating coil (inner diameter side coil 2B) arranged only on its inner diameter side. In such cases, the quenching method for the workpiece W according to the present invention can be applied without any problems.

[0039] The above has exemplified the application of the present invention to the entire hardening of an annular workpiece W that will become the raceway (outer or inner ring) of a rolling bearing. However, the present invention can also be preferably used when the entire hardening of other workpieces, such as an annular workpiece W that will become a plain bearing, or an annular workpiece W that will become a retainer incorporated into a rolling bearing or a constant velocity universal joint.

[0040] The present invention is not limited to the above-described embodiment, and can be embodied in various other forms without departing from the spirit of the present invention.

[0041] REFERENCE SIGNS LIST 1 induction heating device 2 coil unit 2A outer diameter side coil 2B inner diameter side coil 3 work support member 4 high frequency power supply 5 control device 10 thermometer S1 primary temperature rise step S2 thermal diffusion step S3 secondary temperature rise step S4 temperature holding step W work

Claims

1. A quenching method comprising a heating step in which an annular workpiece made of steel is induction-heated by passing current through a heating coil, and a cooling step in which the workpiece is rapidly cooled after heating is complete to quench the entire workpiece, wherein the heating step comprises a temperature-raising step in which the temperature of the workpiece is continuously raised, the temperature-raising step comprising a first temperature-raising step in which the temperature of the workpiece is raised until it reaches the Curie temperature of the steel, and a second temperature-raising step in which the temperature of the workpiece is raised until it reaches a predetermined temperature equal to or higher than the quenching temperature, and wherein a thermal diffusion step is carried out between the first temperature-raising step and the second temperature-raising step in which heat is diffused within the workpiece by lowering the output of the heating coil compared to when the first temperature-raising step was carried out.

2. A quenching method according to claim 1, wherein in the heating step, a temperature holding step is carried out after the second temperature raising step, in which the workpiece is held at the predetermined temperature.

3. A quenching method according to claim 1, wherein the temperature rise rate of the workpiece during the first temperature rise step is set to be faster than the temperature rise rate of the workpiece during the second temperature rise step.

4. A quenching method as described in claim 1, wherein the heating step involves induction heating the workpiece by passing current through an outer diameter side coil and an inner diameter side coil, which are arranged radially outside and inside the workpiece, respectively, and electrically connected in series as the heating coils.

5. The hardening method according to claim 1, wherein the workpiece is a raceway of a rolling bearing.

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