WO2025225292A1 - Quenching method - Google Patents

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

The present invention relates to a method for hardening annular workpieces made of steel.

For example, for annular mechanical parts that require high mechanical strength and hardness, such as the raceways of rolling bearings, annular workpieces formed into a specified shape from steel with a carbon content of 0.8% by 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 treatments such as quenching and tempering.

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. Annular workpieces that will ultimately become the raceways are usually hardened full. When hardening full workpieces, they can be heated using either atmospheric heating (furnace heating) or induction heating, but induction heating is increasingly being used, as it has advantages over atmospheric heating, such as significantly less energy required for heating and significantly shorter heating times.

In the heating process for hardening the entire workpiece, the workpiece is induction heated under heating conditions that allow a predetermined amount of carbon to be dissolved into 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 part of the workpiece is heated to approximately 900°C, the target temperature during induction heating, there may be parts of the workpiece that are only heated to approximately 800°C. If the workpiece is cooled and hardened when there are such large temperature differences within the workpiece, or more specifically when the carbide area ratio within the workpiece varies greatly, the metal structure of the workpiece after hardening will be uneven, making it impossible to obtain machine parts with the desired mechanical strength, hardness, etc.

A method believed to be effective in resolving this problem is described in Patent Document 1 below. Specifically, in the heating process of induction heating the workpiece, a high-frequency current is supplied to the heating coil in multiple increments, thereby alternately repeating a temperature-raising step that continuously raises the temperature of the workpiece and a thermal diffusion step that promotes thermal diffusion within the workpiece. In the thermal diffusion step, heat transfer occurs from high-temperature parts to low-temperature parts within the workpiece, reducing temperature differences within the workpiece and making it possible to uniform the carbide area ratio within the workpiece.

Japanese Patent Application Laid-Open No. 2007-262461

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 above-mentioned temperature increase step and thermal diffusion step to be performed a significant number of times. Furthermore, there is a concern that the total time for the heating step will increase accordingly if the number of times (performance time) the thermal diffusion step is performed increases. Furthermore, because heat is dissipated outside the workpiece at the same time as thermal diffusion occurs within the workpiece during the thermal diffusion step, there is also a concern that the total amount of energy required to heat the workpiece will increase as the number of times it is performed increases.

In light of this situation, the present invention aims to reduce the time and energy required to heat a ring-shaped workpiece made of steel, followed by rapid cooling to harden the entire workpiece, while making it possible to homogenize the amount of residual carbide (carbide area ratio) within the workpiece, thereby enabling the production of high-quality, hardened products with the desired mechanical strength, etc., at low cost.

The quenching method according to the present invention, which has been devised to achieve the above object, comprises a heating step of induction-heating an annular workpiece made of steel by passing current through a heating coil, and a cooling step of quenching the entire workpiece by rapidly cooling the workpiece after heating is completed,
The heating process is characterized in that the temperature-raising step, in which the temperature of the workpiece is continuously raised, comprises 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 hardening temperature, and in that 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 reducing the output of the heating coil compared to when the first temperature-raising step was carried out.

In the first heating step of the hardening method of the present invention, the workpiece is heated to the Curie temperature of the steel (approximately 780°C), 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 parts of the workpiece can be kept small. In the thermal diffusion step that follows the first heating step, the output of the heating coil is reduced compared to when the first heating step was performed, preventing the entire workpiece from heating up. Meanwhile, the temperature difference that occurred within the workpiece in the first heating step can be reduced by thermal diffusion (heat transfer from the high-temperature to the low-temperature parts 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 carried out following the thermal diffusion step, the workpiece is heated to a predetermined temperature above the hardening temperature, which causes 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 dissolved into the metal structure of the workpiece can be made uniform throughout the workpiece.

Furthermore, in the present invention, the heating step of continuously raising the temperature of the workpiece is performed only twice, the first and second heating steps described above, so the thermal diffusion step between heating steps to promote thermal diffusion within the workpiece only needs to be performed once. This reduces the total time and total amount of energy required to induction heat the workpiece until it reaches a predetermined temperature in the heating process, while also making it possible to homogenize the amount of residual carbide (carbide area ratio) within the workpiece. Furthermore, by rapidly cooling the workpiece in the cooling process after heating is complete and the carbide area ratio has been homogenized within the workpiece, it is possible to obtain high-quality mechanical parts with no differences in mechanical strength or hardness within the part.

In the heating process, after the secondary heating step, it is preferable to carry out a temperature holding step in which the workpiece is held at the above-mentioned predetermined temperature. In this way, carbon can be dissolved into the metal structure of the workpiece during this temperature holding step as well, which is advantageous for uniforming the carbide area ratio within the workpiece and producing high-quality mechanical parts.

The temperature rise rate of the workpiece during the first temperature rise step can be made faster than the temperature rise rate of the workpiece during the second temperature rise step. This shortens the total heating process time and contributes to reducing the manufacturing costs of mechanical parts that use the workpiece as a base material.

In the heating process, the workpiece can be induction heated by passing current through the outer diameter coil and inner diameter coil, which are positioned radially outside and inside the workpiece and electrically connected in series. By electrically connecting the outer diameter coil and inner diameter coil in series in this way, the amount of current flowing through each of the outer diameter coil and inner diameter coil can be made the same, so the entire workpiece can be heated efficiently without creating a large temperature difference between the outer diameter region and inner diameter region of the workpiece.

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 used in the manufacturing process of a raceway ring of a rolling bearing.

As described above, according to the present invention, when fully hardening an annular workpiece made of steel, it is possible to reduce the total time and total energy required to heat the workpiece while homogenizing the amount of residual carbide (carbide area ratio) within the workpiece. This makes it possible to obtain high-quality hardened products with the desired mechanical strength, etc., and ultimately machine parts, at low cost.

1 is a schematic cross-sectional view of an induction heating device used when performing a heating step of a 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. 10 is a graph showing the temperature transition at two points in an induction-heated workpiece. FIG. 10 is a graph showing the change in carbide area ratio at two points in an induction-heated workpiece. FIG. 2 is a schematic diagram showing a modified example of the induction heating device shown in FIG. 1 .

The following describes an embodiment of the present invention with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of an induction heating device 1 used in a heating step of induction heating an annular workpiece W, which is the object to be hardened, in a hardening method according to one embodiment of the present invention, and FIG. 2 is a block diagram schematically illustrating the electrical circuit of the heating device 1. This induction heating device 1 comprises a workpiece support member 3 that supports from below the workpiece W, which is placed flat with its central axis aligned vertically; a coil unit 2 that induction heats the workpiece W (workpiece W supported by the workpiece support member 3; the same applies hereinafter when describing the heating step); a high-frequency power supply 4 that is electrically connected to the coil unit 2 (its heating coil) and outputs a high-frequency current 8; and a control device 5 that is electrically connected to the high-frequency power supply 4 and controls the output of the high-frequency power supply 4 (heating coil). The heating coil of the coil unit 2 is electrically connected to the high-frequency power supply 4 via an electrode member 6 and power distribution members 7a and 7b.

The annular workpiece W, for example, will eventually become the raceway (outer 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.

The coil unit 2 comprises an outer diameter side coil 2A and an inner diameter side coil 2B as heating coils arranged respectively on the radial outside and inside 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.

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 coil 2A via a power distribution member 7a, and the exit electrode 6b is electrically connected to (one longitudinal end of) the inner diameter coil 2B via a power distribution member 7b. Furthermore, (the other longitudinal end of) the outer diameter coil 2A and (the other longitudinal end of) the inner diameter coil 2B are electrically connected via a power distribution member 7c.

As a result, 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 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, causing the entire workpiece W to heat up.

The heating process performed using induction heating device 1 having the above configuration consists of a primary heating step S1, a thermal diffusion step S2, a secondary heating step S3, and a temperature holding step S4, as shown in Figures 3 and 4. In this heating process, steps S1 to S4 are performed in order, and the workpiece W (as a whole) is induction heated until it reaches a predetermined temperature (approximately 900°C) above the hardening temperature.

A specific example of an embodiment of the heating process, including steps S1 to S4, is described below with reference to Figures 3 and 4. Note that the object to be heated here is an annular workpiece W made of SUJ2 with an outer diameter of 100 to 250 mm and a radial thickness (maximum thickness) of 10 mm or less.

First, in the primary heating step S1, a room-temperature annular workpiece W is placed between the two coils 2A and 2B. Then, electricity is passed through both coils 2A and 2B (high-frequency current 8 is passed through both coils 2A and 2B), continuously heating the workpiece W until the part of the workpiece W that is most susceptible to heating (for example, point A shown in Figure 1) reaches the Curie temperature of the steel (approximately 780°C). At this time, the amount of high-frequency current flowing through both coils 2A and 2B (the output of coils 2A and 2B) is set so that the heating rate (V1) at point A of the workpiece W is 140 to 170°C/second.

Next, in the thermal diffusion step S2, thermal diffusion is performed within the workpiece W by lowering the output of coils 2A and 2B compared to when the primary heating step S1 was performed. In other words, heat is transferred from high-temperature areas within the workpiece W (e.g., point A mentioned above) to low-temperature areas (e.g., point B shown in Figure 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 primary heating step S1. When this temperature difference converges to 20°C or less, the thermal diffusion step S2 is terminated and the process moves to the subsequent secondary heating step S3. Note that the output of 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 in the carbide area ratio within the workpiece W. If the temperature of the high-temperature area within the workpiece W exceeds 800°C, conditions will be adjusted while checking the results of carbide dissolution in the final product.

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

In temperature holding step S4, the output of coils 2A and 2B is reduced from that during secondary heating step S3, and the workpiece W is held 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., 8-10%).

Once temperature holding step S4 is complete and the entire workpiece W has been heated to or above the hardening temperature, the output of high-frequency current 8 from high-frequency power supply 4 (current flow to coils 2A and 2B) is stopped to stop heating of the workpiece W, and the workpiece W is removed from 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), resulting in a hardened product with increased mechanical strength and hardness compared to before hardening.

In the heating process described above, during the first stage of the primary heating step S1, 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 parts of the workpiece W can be kept small. In the thermal diffusion step S2, which follows the primary heating step S1, the output of coils 2A and 2B is reduced compared to when the primary heating step S1 was performed. This prevents the entire workpiece W from heating up, while also reducing the temperature difference that occurred within the workpiece W during the primary heating step S1 through thermal diffusion (heat transfer from the high-temperature parts to the low-temperature parts 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.

In the secondary heating step S3, which follows the thermal diffusion step S2, the workpiece W is heated until it reaches a predetermined temperature above the hardening temperature, causing the dissolution of carbon into the metal structure to proceed (restart) throughout the workpiece W. However, because the temperature of the workpiece W is made approximately uniform in the previous thermal diffusion step S2, the amount of carbon dissolving into the metal structure of the workpiece W can be made uniform throughout the workpiece.

In this embodiment, the heating step of continuously raising the temperature of the workpiece W is performed only twice, the first heating step S1 and the second heating step S3, and therefore the thermal diffusion step S2, which is provided between the heating 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 during the heating process, while also making it possible to homogenize the carbide area ratio within the workpiece W. Furthermore, by rapidly cooling the workpiece W in the cooling process after heating is complete and the carbide area ratio has been made homogenous within the workpiece W, it is possible to obtain a high-quality, hardened product (mechanical part) with no differences in mechanical strength or hardness within the part.

In this embodiment, after the secondary heating 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 during the temperature holding step S4, which is advantageous for uniforming the carbide area ratio within the workpiece W and producing high-quality mechanical parts.

Furthermore, in this embodiment, the temperature rise rate (V1) of the workpiece W during the first temperature rise step S1 is set to be faster than the temperature rise rate (V2) of the workpiece W during the second temperature rise step S2. This shortens the total time of the heating process, contributing to reduced manufacturing costs for mechanical components.

The output of coils 2A and 2B in the heating process described above may be controlled based on tests conducted in advance, or, as shown in Figure 5, 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). Figure 5 shows an example in which two thermometers 10 are installed, but the number of thermometers 10 installed is arbitrary, and one or three or more thermometers may be installed.

The above describes a 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, but the present invention is not limited to this embodiment.

For example, although not shown, the workpiece W may be induction heated using a heating coil (outer diameter side coil 2A) placed only on its outer diameter side, or may be induction heated using a heating coil (inner diameter side coil 2B) placed only on its inner diameter side. In both of these cases, the method for hardening the workpiece W according to the present invention can be applied without any problems.

The above example illustrates the application of the present invention to the full 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 full hardening other workpieces, such as annular workpieces that will become plain bearings, as well as annular workpieces W that will become cages incorporated into rolling bearings or constant velocity universal joints.

The present invention is not limited to the above-described embodiments, and can be embodied in a variety of forms without departing from the spirit of the present invention.

REFERENCE SIGNS LIST 1 induction heating device 2 coil unit 2A outer diameter side coil 2B inner diameter side coil 3 workpiece 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 workpiece

Claims (5)

The method includes a heating step of induction-heating an annular workpiece made of steel by passing current through a heating coil, and a cooling step of quenching the entire workpiece by rapidly cooling the workpiece after heating is completed,
the heating step includes a temperature-raising step of continuously raising the temperature of the workpiece, the temperature-raising step including a first temperature-raising step of raising the temperature of the workpiece until the temperature reaches the Curie temperature of steel, and a second temperature-raising step of raising the temperature of the workpiece until the temperature reaches a predetermined temperature equal to or higher than the quenching temperature;
A hardening method characterized in that a thermal diffusion step is carried out between the first heating step and the second heating step, 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. The quenching method described in claim 1, wherein, in the heating process, after the secondary heating step, a temperature holding step is carried out to hold the workpiece at the predetermined temperature. The hardening method described in claim 1, wherein the temperature rise rate of the workpiece during the first temperature rise step is faster than the temperature rise rate of the workpiece during the second temperature rise step. The hardening method described in claim 1, wherein the heating step involves induction heating the workpiece by passing current through an outer diameter coil and an inner diameter coil, which are positioned radially outside and inside the workpiece, respectively, and electrically connected in series as the heating coils. The hardening method according to claim 1, wherein the workpiece is a raceway of a rolling bearing.