Heat Treatment System

Abstract

A nitrogen enriched layer is formed in a primary heat treatment, and requenching is conducted in a secondary heat treatment, while the heat treatment efficiency is improved across the entire system. In a primary heat treatment device 1, a bearing component is heated in a heater 11 at a temperature exceeding the A1 transformation point, and is then cooled in a cooler 12 to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component. The bearing component that has undergone primary heat treatment is then subjected to high frequency heating in a heater 21 of a secondary heat treatment device 2, at a temperature exceeding the A1 transformation point, and is then cooled in a cooler 22 to a temperature less than the A1 transformation point. Following cooling by the cooler 22, the component is tempered using high frequency heating.

Description

TECHNICAL FIELD

The present invention relates to a heat treatment system for conducting a two-stage heat treatment on a steel component.

BACKGROUND ART

An example of a heat treatment method that has been adapted for steel machine components that require a long service life to rolling fatigue, such as the bearing portions of a rolling bearing, is disclosed in Japanese Patent Laid-Open Publication No. 2003-226918. This method comprises the steps of subjecting the steel for the bearing components to carbonitriding treatment at a carbonitriding treatment temperature that exceeds the A1 transformation point, subsequently cooling the steel to a temperature less than the A1 transformation point, and then reheating and quenching the steel within a quenching temperature region (790 to 830° C.) exceeding the A1 transformation point but less than the carbonitriding treatment temperature.

According to this method, the presence of a carbonitrided layer at the surface of the steel increases the hardness of the bearing components, and because the quenching temperature during the reheating is restricted to a temperature at which the growth of austenite crystal grains is suppressed, the average particle size of the austenite crystal grains can be reduced to no more than 8 μm. As a result, the grain boundary strength is increased, which produces effects such as an improvement in the service life relative to rolling fatigue, and an improvement in the cracking resistance.

For example, the bearings used in reduction gears, drive pinions, and transmissions and the like operate under severe conditions, including high loads and contamination of the lubricant with foreign matter (such as abrasion dust from the gears), and with recent demands for even higher speeds and greater miniaturization, the severity of these operating conditions continues to increase. In order to cope with these recent trends, Japanese Patent Laid-Open Publication No. 2003-226918 proposes a heat treatment method capable of producing bearing components with superior basic performance, including improved service life relative to rolling fatigue, greater crack resistance and better resistance to dimensional changes over time. This heat treatment method comprises the steps of subjecting the steel for the bearing components to carbonitriding treatment at a carbonitriding treatment temperature that exceeds the A1 transformation point, subsequently cooling the steel to a temperature less than the A1 transformation point, and then reheating and quenching the steel within a quenching temperature region (790° C. to 830° C.) exceeding the A1 transformation point but less than the carbonitriding treatment temperature. According to this heat treatment method, the austenite crystal grains within the microstructure of the heat treated bearing components can be miniaturized to an average particle size of no more than 8 μm, enabling the production of bearing components with superior basic performance, including improvements in the service life relative to rolling fatigue, the Charpy impact value, the fracture toughness, and the compressive failure strength.

In the invention disclosed in the above publication, a total of two heat treatments, namely a primary treatment and a secondary treatment, are performed, although tempering is necessary following the secondary heat treatment, in order to prevent quenching crack during the quenching process. If the heating time for this tempering process is long, then it cannot be matched to the heating time of the secondary heat treatment, meaning the heat treated product must be halted within the production line, which causes long down times for the machinery used to perform the primary and secondary heat treatments, resulting in reduced heat treatment efficiency and longer heat treatment times. An object of the present invention is to improve the heat treatment efficiency across the entire system, while forming a nitrogen enriched layer during a primary heat treatment, and then conducting requenching within a secondary heat treatment.

Furthermore, another object of the present invention is to provide a heat treatment system for conducting the above heat treatment method on steel components such as bearing components, and in particular, to provide a heat treatment system that is capable of subjecting each steel component to a uniform heat treatment.

In the invention disclosed in the above publication, the heating temperature of the secondary heat treatment must be strictly controlled in order to ensure uniform miniaturization of the crystal grains throughout the whole component. If this secondary heating is conducted in an atmospheric furnace, in a similar manner to a conventional quenching process, and the atmospheric temperature inside the furnace is measured, then differences can develop between the measured temperature and the actual temperature of the bearing component, making strict temperature control difficult. An object of the present invention is to provide a heat treatment system for forming a nitrogen enriched layer during a primary heat treatment, and then conducting requenching within a secondary heat treatment, wherein the heating temperature in the secondary heat treatment is strictly controlled.

Furthermore, in the invention disclosed in the above publication, a total of two heat treatments, namely a primary treatment and a secondary treatment, are performed, but in those cases where the article undergoing heat treatment is a particularly thin-walled member or a member of varying thickness, the occurrence of quenching distortion during the heat treatment is a concern. Amongst the bearing components within a rolling bearing, the outer ring and the inner ring are thin-walled members. In the case of a tapered roller bearing, since the thickness of the outer ring and the inner ring is also non-uniform, there is a real danger of quenching distortion. Moreover, in order to ensure satisfactory bearing performance, since these bearing components require extremely high levels of dimensional precision, quenching distortion must be suppressed as far as possible. Another object of the present invention is to suppress quenching distortion of the steel component, while forming a nitrogen enriched layer during a primary heat treatment, and then conducting requenching within a secondary heat treatment.

Furthermore, in the invention disclosed in the above publication, a total of two heat treatments, namely a primary treatment and a secondary treatment, are performed, but because the heating time required for the primary heat treatment is different from that required for the secondary heat treatment, the heating times for the primary heat treatment and the secondary heat treatment cannot be easily balanced, and there is a danger that the heat treated product must be halted within the production line, causing longer down times for the machinery used to perform the heat treatments, and resulting in reduced heat treatment efficiency. An object of the present invention is to improve the heat treatment efficiency across the entire system, while forming a nitrogen enriched layer during a primary heat treatment, and then conducting requenching within a secondary heat treatment.

DISCLOSURE OF THE INVENTION

A heat treatment system according to the present invention comprises: a primary heat treatment device for heating a steel component at a temperature exceeding the A1 transformation point and then cooling the component to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component; and a secondary heat treatment device for heating the steel component that has undergone primary heat treatment, at a temperature exceeding the A1 transformation point, and then cooling the component to a temperature less than the A1 transformation point, wherein the secondary heat treatment device includes an induction heater, and tempering is performed by induction heating following cooling within the secondary heat treatment device.

According to this heat treatment system, the heat treatment by the primary heat treatment device forms a nitrogen enriched layer in which nitrogen is diffused throughout the component surface, thus increasing the surface hardness of the steel component. On the other hand, the austenite grains within the steel structure following the primary heat treatment are considerably large, but because the secondary heat treatment is subsequently conducted while controlling a heating temperature and a heating time by the subsequent induction heating, the austenite grains can be reduced in size to approximately one half of the size observed in conventional components, enabling a fine grain size with an austenite grain size number exceeding 10 to be achieved. As a result of these characteristics, abrasion resistance and cracking resistance can be improved in comparison with conventional components, enabling a significant increase in the service life relative to rolling fatigue.

In the system of the present invention, both the heating of the secondary heat treatment, and the tempering following the secondary heat treatment are conducted using induction heating. Compared with heating in an atmospheric gas from a combustion furnace or the like, induction heating offers the advantages of better heating efficiency and shorter heating times, and moreover because induction heating uses electrical energy, control of the heating output is also very easy. Accordingly, by conducting both the heating of the secondary heat treatment, and the tempering following the secondary heat treatment using induction heating, the heating times for the two heating steps can be balanced relatively easily.

According to this aspect of the invention, a nitrogen enriched layer is formed in the primary heat treatment, and when requenching is then conducted within the secondary heat treatment, the heating time for the secondary heat treatment, and the heating time for the tempering step following the secondary heat treatment can be balanced easily. Accordingly, the need to halt heat treated products within the production line can be minimized, and the down times required for the various machinery can be reduced, enabling an improvement in the heat treatment efficiency across the entire system.

Furthermore, the present invention also provides a heat treatment system comprising: a primary treatment device for heating a steel component at a primary heating temperature exceeding the A1 transformation point and then cooling the component to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface layer of the component; and a secondary treatment device for heating the steel component that has undergone heat treatment by the primary treatment device, at a secondary heating temperature exceeding the A1 transformation point by induction heating, and then cooling the component to a temperature less than the A1 transformation point.

In the primary treatment device, the steel component is heated at a primary heating temperature that exceeds the A1 transformation point, thus forming a nitrogen enriched layer in which nitrogen is diffused throughout the surface layer of the component, and the component is then cooled to a temperature less than the A1 transformation point. Subsequently, in the secondary treatment device, induction heating at a secondary heating temperature exceeding the A1 transformation point and quenching are conducted. Consequently, by controlling the heating temperature and the heating time, the austenite crystal grains within the micro structure of the heat treated steel component can be reduced in size, enabling a fine grain size to be achieved, with a grain size number, determined in accordance with the austenite grain size test method prescribed in JIS G0551, exceeding 10. As a result, a steel component with excellent service life relative to rolling fatigue, and superior levels of crack resistance and resistance to dimensional changes over time can be obtained.

Furthermore, the quenching of the steel component in the secondary treatment device is performed piece by piece using an induction heating system (such as high frequency quenching), and consequently both non-uniformity in the heat treatment quality within each individual steel component, and variations in the quality of the heat treatment across different steel components can be minimized, enabling uniform steel components of high reliability to be produced. In the secondary treatment device, die quenching may be conducted following heating of the steel component to the secondary heating temperature using the induction heating system. In this description, die quenching refers to a treatment method wherein quenching is conducted with the heated article constrained by a die, and includes press quenching in which the article is constrained by applying pressure to the die.

In the primary treatment device, appropriate methods for dispersing nitrogen within the surface layer of the steel component to form a nitrogen enriched layer include nitridation and carbonitridation, although considering the associated heating temperatures and the need to prevent decarburization, carbonitridation is preferred. Moreover, in terms of cost and quality, gas carbonitriding is preferred.

The primary treatment device and the secondary treatment device each have a basic structure comprising a heater for heating a steel component to a desired temperature (the primary heating temperature or the secondary heating temperature), and a cooler for subsequently cooling the component. For example, in those cases where gas carbonitriding is conducted in the primary treatment device, a heating furnace in which the steel component is heated within an atmospheric gas comprising a carburizing gas containing added ammonia is used as the heater for the primary treatment device. This heating furnace may be either a continuous-type or batch-type furnace. The heater of the secondary treatment device is a heater that uses inductive heating (such as high frequency heating) to heat the steel component, and is constructed from a high frequency heating device. There are no particular restrictions on the cooling system used for the coolers of the primary treatment device and the secondary treatment device, and suitable cooling methods that can be adopted include air cooling, gas cooling using a gas such as N₂, oil cooling, water cooling, and salt bath cooling.

According to this aspect of the present invention, because the quenching of the steel component in the secondary treatment device is performed piece by piece using an induction heating system (such as high frequency quenching), both non-uniformity in the heat treatment quality within each individual steel component, and variations in the quality of the heat treatment across different steel components can be minimized. Thus, steel components with excellent service life relative to rolling fatigue, superior levels of crack resistance and resistance to dimensional changes over time, and uniformly high reliability can be obtained.

Furthermore, a heat treatment system according to the present invention comprises: a primary heat treatment device for heating a steel component at a temperature exceeding the A1 transformation point and then cooling the component to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component, and a secondary heat treatment device for heating the steel component that has undergone primary heat treatment, at a temperature exceeding the A1 transformation point, and then cooling the component to a temperature less than the A1 transformation point, wherein induction heating is conducted in the secondary heat treatment device, the temperature of the steel component undergoing induction heating is detected, and the induction heater is operated under feedback control based on the detected temperature value.

In this aspect of the present invention, induction heating such as high frequency heating is conducted in the secondary heat treatment device, the temperature of the steel component undergoing induction heating is detected, and the induction heater is operated under feedback control based on the detected temperature value, and consequently the secondary heating temperature can be held reliably and precisely within a narrow temperature range based on the actual temperature of the steel component, enabling the production of a high quality steel component in which the crystal grain size has been reduced uniformly throughout the entire component.

In this case, in order to absolutely minimize any temperature errors, the temperature of the steel component undergoing induction heating is preferably detected using a non-contact type temperature sensor.

As described above, according to this aspect of the present invention, a nitrogen enriched layer is formed in the primary heat treatment, and when requenching is then conducted within the secondary heat treatment, the heating temperature within the secondary heat treatment device is able to be controlled with good precision. Accordingly, heating irregularities within the secondary heat treatment can be prevented, enabling the crystal grain size to be reduced uniformly throughout the entire component, thus ensuring a more stable quality level for the steel component.

Furthermore, a heat treatment system according to the present invention comprises: a primary heat treatment device for heating a steel component at a temperature exceeding the A1 transformation point and then cooling the component to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component; and a secondary heat treatment device for heating the steel component that has undergone primary heat treatment, at a temperature exceeding the A1 transformation point, and then cooling the component to a temperature less than the A1 transformation point, wherein the secondary heat treatment device conducts induction heating and die quenching.

In this system of the present invention, as described above, both induction heating and die quenching are conducted within the secondary heat treatment device, and consequently a heat treated product with little distortion and good dimensional precision can be obtained, and even thin-walled components or components with varying thickness can be produced with favorable dimensional precision.

According to this aspect of the present invention, a nitrogen enriched layer is formed in the primary heat treatment, and when requenching is then conducted within the secondary heat treatment, a steel component with little thermal distortion and a high level of dimensional precision can be produced at low cost. In particular, the invention can also be ideally applied to thin-walled components or components with varying thickness.

Furthermore, a heat treatment system according to the present invention comprises: a primary heat treatment device for heating a steel component at a temperature exceeding the A1 transformation point and then cooling the component to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component, and a secondary heat treatment device for heating the steel component that has undergone primary heat treatment, at a temperature exceeding the A1 transformation point, and then cooling the component to a temperature less than the A1 transformation point, wherein a plurality of secondary heat treatment devices are disposed in parallel.

In this system of the present invention, as described above, a plurality of secondary heat treatment devices are disposed in parallel, and consequently the secondary heat treatment can be conducted simultaneously at a plurality of different locations, meaning the heat treatment efficiency of the secondary heat treatment can be improved.

In such cases, induction heating is preferably conducted within each of the secondary heat treatment devices. Compared with heating in an atmospheric furnace such as a combustion furnace, induction heating offers better work efficiency, and heating can be completed within a shorter time, meaning that by conducting such induction heating at a plurality of locations in parallel, the heating efficiency of the secondary heat treatment can be improved dramatically. Accordingly, the heat treatment efficiency within the primary heat treatment and the secondary heat treatment can be balanced, enabling an improvement in the heating efficiency across the entire system.

In this case, if die quenching is conducted within the secondary heat treatment devices, then heat treated products with little distortion and a high level of dimensional precision can be produced, and favorable dimensional precision can be ensured even for thin-walled components or components of varying thickness.

According to this aspect of the present invention, a nitrogen enriched layer is formed in the primary heat treatment, and when requenching is then conducted within the secondary heat treatment, the heating efficiency within the secondary heat treatment can be improved dramatically. Accordingly, the heat treatment efficiency within the primary heat treatment and the secondary heat treatment can be balanced, enabling an improvement in the heating efficiency across the entire system.

In each of the heat treatment systems described above, the method used for forming the nitrogen enriched layer in the primary heat treatment is preferably carbonitridation, and in terms of cost and quality, gas carbonitriding is preferred. Gas carbonitriding can be conducted in an atmospheric furnace using an atmospheric gas comprising a carburizing gas containing added ammonia.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a schematic illustration of a heat treatment system according to the present invention;

FIG. 2 is a cross-sectional view showing a deep groove ball bearing;

FIG. 3 is a heat treatment cycle diagram;

FIG. 4 is a diagram showing a schematic illustration of the construction of a heat treatment system according to the present invention;

FIG. 5 is a heat treatment cycle diagram for a heat treatment system;

FIG. 6 is a cross-sectional view showing a schematic illustration of a heat treatment system according to the present invention;

FIG. 7 is a cross-sectional view showing a schematic illustration of a heat treatment system according to the present invention;

FIG. 8 is a cross-sectional view of a tapered roller bearing; and

FIG. 9 is a cross-sectional view showing a schematic illustration of a heat treatment system according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As follows is a description of a first embodiment of the present invention, applied to a bearing component that represents one example of a steel component.

FIG. 1 shows a schematic illustration of the construction of a heat treatment system according to the present invention. As shown in the figure, this heat treatment system comprises a primary heat treatment device 1, a secondary heat treatment device 2, two washing devices 5 and 6, and a tempering device 7. A bearing component produced by a molding process (not shown in the drawings) comprising, for example, forging, and then turning, is fed sequentially through the primary heat treatment device 1 and the secondary heat treatment device 2, and undergoes heating and cooling within each of the devices, as the primary heat treatment and the secondary heat treatment, respectively.

The term “bearing component” refers to a bearing component of a rolling bearing such as a ball bearing, a tapered roller bearing, a roller bearing or a needle roller bearing. As one example, FIG. 2 shows a deep groove ball bearing 4 comprising, as the main structural elements, an outer ring 41, an inner ring 42, and rolling elements (balls) 43, and of these structural elements, in this description, the term “bearing components” describes those elements that are subjected to rolling contact with an opposing member, namely the outer ring 41, the inner ring 42 and the rolling elements 43. Examples of the materials that can be used for these bearing components include bearing steel such as SUJ2 prescribed in the JIS, as well as high temperature bearing steel comprising C: 0.6 to 1.3 wt %, Si: 0.3 to 3.0 wt %, Mn: 0.2 to 1.5 wt %, Cr: 0.3 to 5.0 wt %, and Ni: 0.1 to 3 wt % (and preferably also comprising Mo: 0.05 to 0.25 wt % and V: 0.05 to 1.0 wt %), and medium carbon steel comprising C: 0.4 to 0.8 wt %, Si: 0.2 to 0.9 wt %, Mn: 0.7 to 1.3 wt % and Cr: no more than 0.7 wt %.

The primary heat treatment device 1 comprises a heater 11 and a cooler 12. In FIG. 1, a continuous system is shown for the heater 11, but a batch-type furnace can also be used. The heater 11 is formed, for example, from an atmospheric furnace that uses an atmospheric gas comprising a carburizing gas containing added ammonia. Inside this heater 11, a bearing component is heated at a temperature T1 (from 800° C. to 900° C., for example, 850° C.) exceeding the A1 transformation point for a predetermined time of 40 minutes for example (primary heating), as shown in FIG. 3. This heating causes activated nitrogen to diffuse into the surface layer of the bearing component, thus hardening that surface layer (gas carbonitriding). The primary heating performed by the heater 11 has a primary object of forming a nitrogen enriched layer at the surface of the component, and at least nitridation must occur, although carburization is not essential. However, depending on the conditions, carburization may be essential as well as nitridation, particularly in cases where decarburization is a concern, or cases where the carbon content of the steel is insufficient and a satisfactory level of hardness cannot be achieved. The heater 11 may also use a vacuum furnace, a salt bath furnace or an induction heater or the like. Following heat treatment, the bearing component is cooled to below the Ms point by the cooler 12 (using oil cooling, for example), and is then transported to a washing device 5 for washing and removal of the cooling liquid.

As shown in FIG. 1, a bearing component that has undergone carbonitriding in the primary heat treatment device 1 is then supplied to the secondary heat treatment device 2. The secondary heat treatment device 2 comprises a heater 21 for performing induction heating such as high frequency heating, and a cooler 22. The bearing component supplied to the heater 21 is positioned with a suitable clearance from an inductor (not shown in the figure), and as shown in FIG. 3, is then subjected to induction heating (secondary heating), at a secondary heating temperature T2 (for example, 880° C. to 900° C.) exceeding the A1 transformation point for a predetermined time (for example, 1.5 to 2 seconds), by electrically conducting the inductor. FIG. 3 shows a temperature exceeding the A1 transformation point but less than the primary heating temperature T1 in the primary heat treatment device 1 as the secondary heating temperature T2; however, the upper limit of the secondary heating temperature T2 may exceed T1. In the induction heating, the heating temperature and the heating time can be precisely controlled and the treatment can be conducted within a short time. Consequently, the austenite crystal grains within the microstructure of the bearing components can be miniaturized. At this time, whether or not the austenite crystal grains are miniaturized can be evaluated by a product of the heating temperature and the heating time. For example, if the maximum heating temperature in an induction heater is low, the austenite crystal grains can be miniaturized by prolonging the heating time.

Following completion of this heating, the bearing component is transported to the cooler 22 and is cooled to below the Ms point (using oil cooling, for example) to effect quenching. Instead of transporting the bearing component to a cooler 22 that is separate from the heater 21 as in the above example, the bearing component may also be subjected to spray cooling while still positioned at the induction heating location inside the heater 21.

Following completion of the above secondary heat treatment, the bearing component is washed in the washing device 6 to remove the cooling liquid, and is then transported to the tempering device 7 where, as shown in FIG. 3, tempering is conducted at a suitable temperature T3 (180° C. for example). This tempering is conducted using induction heating such as high frequency heating.

In the description above, oil cooling was used as the cooling method within the primary heat treatment device 1 and the secondary heat treatment device 2, but other cooling methods such as water cooling, air cooling or gas cooling can also be employed, and different cooling methods may also be used for the primary heat treatment device 1 and the secondary heat treatment device 2. In this embodiment, the washing devices 5 and 6 were provided because the primary heat treatment and the secondary heat treatment both use oil cooling, but if water cooling, air cooling or gas cooling were used, then these washing devices would be unnecessary.

In a bearing component that has undergone heat treatment in the steps described above, since a nitrogen enriched layer (with a nitrogen content of 0.1 to 0.7 wt %) is formed at the surface layer of the component, a high hardness exceeding Hv700 can be achieved, and the austenite grains within the micro structure are reduced in size to yield an austenite grain size number exceeding 10. Furthermore, the breaking stress value for the bearing component is at least 2650 MPa, the hydrogen concentration within the steel is no more than 0.5 ppm, and the residual austenite content within the steel is from 13 to 25%, which represent far superior physical properties to conventional components. As a result of the above properties, the cracking resistance and the abrasion resistance can be improved, and a marked improvement in the service life relative to rolling fatigue can also be achieved.

In the present invention, as described above, both the heater 21 of the secondary heat treatment device 2 and the tempering device 7 that is used following the secondary heat treatment use an induction heating device such as a high frequency heating device, and provided induction heating is used, the heating efficiency is better and the heating time considerably shorter than those cases where atmospheric gas heating is conducted in an atmospheric furnace or the like, and moreover because induction heating uses electrical energy, control of the heating output is also very easy. Accordingly, by suitable control of the heating output, either by varying the power input to the inductor of the heater 21 and/or the tempering device 7, or by varying the heating time, the heating times required for the two heating steps can be easily balanced. As a result, the need to halt heat treated products within the production line can be minimized, and the down times required for the various machinery can be reduced, enabling an improvement in the heat treatment efficiency across the entire system.

Furthermore, induction heating offers a number of advantages including the ability to evenly heat each component on a piece by piece basis, the ability to heat with improved heating efficiency and shorter heating times, the ability to perform localized heating and the freedom to determine the thickness of the hardened layer, and the ability to improve the fatigue strength through surface compressive residual stress by enabling rapid heating and rapid cooling, and consequently by conducting induction heating in both the heater 21 and the tempering device 7, further reductions in the cost of the bearing components, and further improvements in the quality and the service life relative to rolling fatigue can be obtained.

As follows is a description of a second embodiment of the present invention, in which bearing components of a deep groove ball bearing shown in FIG. 2 are used as examples of steel components.

FIG. 4 is a diagram showing a schematic illustration of the construction of a heat treatment system according to this second embodiment. This heat treatment system comprises a primary heat treatment device 1 and a secondary heat treatment device 2. A bearing component produced by a molding process comprising forging and/or turning or the like (not shown in the drawings), is fed sequentially through the primary heat treatment device 1 and the secondary heat treatment device 2, and undergoes a two-stage heat treatment comprising heating and cooling within each of the devices.

The primary heat treatment device 1 comprises a heater 11, a cooler 12 and a washing device 13. The heater 11 is formed, for example, from a heating furnace that heats the bearing component in an atmospheric gas comprising a carburizing gas containing added ammonia. Inside this heater 11, the bearing component is heated at a primary heating temperature T1 (from 800° C. to 950° C., for example, 850° C.) exceeding the A1 transformation point for a predetermined time (of 40 minutes for example) (primary heating), as shown in FIG. 5, and this heating causes activated nitrogen to diffuse into the surface layer of the bearing component, thus forming a nitrogen enriched layer (in this example, a carbonitrided layer). This primary heating has a primary object of forming a nitrogen enriched layer at the surface of the component, and at least nitridation must occur, although carburization is not essential. However, depending on the conditions, carburization may be essential as well as nitridation, particularly in cases where decarburization is a concern, or cases where the carbon content of the steel is insufficient and a satisfactory level of hardness cannot be achieved. Following heat treatment, as shown in FIG. 5, the bearing component is cooled to below the Ms point by the cooler 12 (using oil cooling, for example), and is then transported to the washing device 13 for washing and removal of the cooling liquid (the oil, for example). In the cooler 12, instead of cooling the component to a temperature below the Ms point, the component may also be held at a constant temperature of approximately 500° C. Furthermore, in FIG. 4, a continuous heating furnace is shown as the heater 11 for the primary heat treatment device 1, but as shown by the dotted lines in the same figure, a batch-type heating furnace 11′ may also be employed. Similarly, in FIG. 4, oil cooling is used for the cooler 12 within the primary heat treatment device 1, but as shown by the dotted lines in the same figure, a cooler 12′ that uses air cooling or gas cooling, such as cooling with N₂ gas, may also be employed. In such a case, since adhesion of cooling liquid to the bearing component does not occur within the cooler 12′, the subsequent washing device 13 can be omitted, and the system can be structured so that the bearing components pass directly from the cooler 12′ to the heater 21 of the secondary heat treatment device 2. This not only enables the structure of the primary heat treatment device 1 to be simplified, but also shortens the processing time.

Having undergone heat treatment in the primary heat treatment device 1, the bearing component is transported to the secondary heat treatment device 2 via a transport device such as a conveyor. The secondary heat treatment device 2 comprises a heater 21, a cooler 22, a washing device 23 and a tempering device 24. The heater 21 is a device for heating the bearing component by induction heating (such as high frequency heating), and is formed from a high frequency heating device. In the heater 21, each bearing component is treated on a piece by piece basis, and as shown in FIG. 5, is subjected to induction heating at a secondary heating temperature T2 exceeding the A1 transformation point, for a predetermined time. FIG. 5 shows a case where the secondary heating temperature T2 is lower than the primary heating temperature T1; however, the upper limit of the secondary heating temperature T2 may exceed T1. In the induction heating, the heating temperature and the heating time can be precisely controlled and the treatment can be conducted within a short time. Consequently, the austenite crystal grains within the microstructure of the bearing components can be miniaturized. Furthermore, because heating of each bearing component is conducted on a piece by piece basis, non-uniformity in the heat treatment quality within each individual bearing component, and variations in the quality of the heat treatment across different bearing components can be minimized. As shown in FIG. 5, following heat treatment, the bearing component is cooled to below the Ms point (using oil cooling, for example) in the cooler 22, and is then transported to the washing device 23 for washing and removal of the cooling liquid. Subsequently, the bearing component is transported to the tempering device 24, and is tempered at a suitable temperature T3 (180° C. for example). The tempering device 24 may also be disposed separately from the secondary heat treatment device 2. Furthermore, if the cooler 22 uses air cooling, gas cooling or water cooling, then the washing device 23 can be omitted.

In a bearing component that has undergone heat treatment in the steps described above, a nitrogen enriched layer (with a nitrogen content of 0.1 to 0.7 wt %) is formed at the surface layer of the component, meaning a high hardness exceeding Hv700 can be achieved, and the austenite grains within the micro structure are reduced in size to yield a crystal grain size number exceeding 10. Furthermore, the breaking stress value for the bearing component is at least 2650 MPa, the hydrogen concentration within the steel is no more than 0.5 ppm, and the residual austenite content within the steel is from 13 to 25%, which represent far superior physical properties to conventional components. As a result of the above properties, the cracking resistance and the abrasion resistance can be improved, and a marked improvement in the service life relative to rolling fatigue can also be achieved.

As follows is a description of a third embodiment of the present invention, in which bearing components of a deep groove ball bearing shown in FIG. 2 are used as examples of the steel components.

FIG. 6 shows a schematic illustration of the construction of a heat treatment system according to this third embodiment. As shown in the figure, this heat treatment system comprises a primary heat treatment device 1, a secondary heat treatment device 2, washing devices 3 and 5, and a tempering device 6. A bearing component produced by a molding process (not shown in the drawings) comprising forging, and then turning or the like, is fed sequentially through the primary heat treatment device 1 and the secondary heat treatment device 2, and undergoes heating and cooling within each of the devices, as the primary heat treatment and the secondary heat treatment, respectively.

The primary heat treatment device 1 comprises a heater 11 and a cooler 12. In FIG. 6, a continuous system is shown for the heater 11, but a batch-type furnace can also be used. The heater 11 is formed, for example, from an atmospheric furnace that uses an atmospheric gas comprising a carburizing gas containing added ammonia. Inside the furnace of this heater 11, a bearing component is heated at a temperature T1 (from 800° C. to 900° C., for example, 850° C.) exceeding the A1 transformation point for a predetermined time of 40 minutes for example (primary heating), as shown in FIG. 3. This heating causes activated nitrogen to diffuse into the surface layer of the bearing component, thus hardening that surface layer (gas carbonitriding). The heater 11 has a primary object of forming a nitrogen enriched layer at the surface of the component, and at least nitridation must occur, although carburization is not essential. However, depending on the conditions, carburization may be essential as well as nitridation, particularly in cases where decarburization is a concern, or cases where the carbon content of the steel is insufficient and a satisfactory level of hardness cannot be achieved. The heater 11 may also use a vacuum furnace, a salt bath furnace or an induction heater or the like. Following heat treatment, the bearing component is cooled to below the Ms point by the cooler 12 (using oil cooling, for example), and is then transported to the washing device 3 for washing and removal of the cooling liquid.

As shown in FIG. 6, a bearing component that has undergone carbonitriding in the primary heat treatment device 1 is then supplied to the secondary heat treatment device 2 via a transport device such as a conveyor. The secondary heat treatment device 2 is used for conducting high frequency quenching, and comprises a heater 21 and a cooler 22. As shown in FIG. 3, the bearing component is then subjected to induction heating (secondary heating) in the heater 21, at a temperature T2 exceeding the A1 transformation point, for a predetermined time. Because this secondary heating is conducted within a short time by induction heating, of course in the case where the heating is conducted at a temperature less than the primary heating temperature T1 and also even in the case where the induction heating is conducted at a temperature exceeding the primary heating temperature T1, the austenite grains within the steel can be reduced in size, by adjusting the heating temperature and the heating time. Following completion of this heating, the bearing component is transported to the cooler 22 and is cooled to below the Ms point (using oil cooling, for example) to effect quenching. Instead of conducting the cooling in a cooler 22 that is independent from the heater 21 as in the above example, the bearing component may also be cooled while still positioned at the induction heating location inside the heater 21. In order to ensure satisfactory dimensional precision following the quenching, die quenching can be conducted in the secondary heat treatment device 2 following the high frequency heating. Such die quenching enables an improvement in the precision of thin-walled components such as the outer ring and the inner ring of a roller bearing, as well as components of varying thickness such as the outer ring and the inner ring of a tapered roller bearing, meaning favorable bearing performance can be achieved with good stability. Die quenching refers to a treatment in which quenching is conducted with the heated article constrained by a die, and includes press quenching in which the article is constrained by applying pressure to the die.

Following completion of the secondary heat treatment, the bearing component is removed from the secondary heat treatment device 2, washed in the washing device 5 to remove the cooling liquid, and is then transported to the tempering device 6 where, as shown in FIG. 3, tempering is conducted at a suitable temperature T3 (180° C. for example). In order to improve the treatment efficiency by shortening the heating time, this tempering is preferably conducted using induction heating such as high frequency heating.

A sensor 9 that uses a non-contact method for detecting the temperature (the surface temperature) of the bearing component undergoing induction heating is provided in the heater 21 of the secondary heat treatment device 2. Examples of sensors that can be used as this sensor 9 include infrared temperature sensors. Inside the heater 21, the bearing component is supported with a predetermined clearance relative to an inductor that is not shown in the figure, and the sensor 9 measures the temperature of the supported bearing component using a non-contact method, and transmits the detected value to a control device 8. Using the detected temperature data, the control device 8 determines whether or not the bearing component undergoing heating has reached the predetermined secondary heating temperature T2, and whether or not the component temperature is within a predetermined temperature range, and based on the results of these determinations, conducts feedback control of the induction heater 21. Control of the induction heater 21 is mainly achieved by altering either the power supplied to the inductor or the heating time.

In the description above, oil cooling was used as the cooling method within the primary heat treatment device 1 and the secondary heat treatment device 2, but other cooling methods such as water cooling, air cooling or gas cooling can also be employed, and different cooling methods may also be used for the primary heat treatment device 1 and the secondary heat treatment device 2. In this embodiment, the washing devices 3 and 5 were provided because the primary heat treatment and the secondary heat treatment both use oil cooling, but if water cooling, air cooling or gas cooling were used, then these washing devices would be unnecessary.

In a bearing component that has undergone heat treatment in the steps described above, a nitrogen enriched layer (with a nitrogen content of 0.1 to 0.7 wt %) is formed at the surface layer of the component, meaning a high hardness exceeding Hv700 can be achieved, and the austenite grains within the micro structure are reduced in size to yield an austenite grain size number exceeding 10. Furthermore, the breaking stress value for the bearing component is at least 2650 MPa, the hydrogen concentration within the steel is no more than 0.5 ppm, and the residual austenite content within the steel is from 13 to 25%, which represent far superior physical properties to conventional components. As a result of the above properties, the cracking resistance and the abrasion resistance can be improved, and a marked improvement in the service life relative to rolling fatigue can also be achieved.

The heater 21 of the secondary heat treatment device 2 is an induction heater that uses an electromagnetic induction phenomenon to convert electrical energy directly to thermal energy inside the steel structure, thus heating the steel. Consequently by adjusting the heating conditions such as the power input to the inductor or the heating time, the heating output can be controlled simply and precisely. Accordingly, by operating the heating conditions of the heater 21 under feedback control from the control device 8, based on the detected values from the sensor 9, the secondary heating temperature T2 can be held reliably within a predetermined temperature range. Furthermore, because induction heating is a piece by piece heating method, the types of heating irregularities that can occur with the use of atmospheric furnaces, caused by the charging location within the furnace, do not arise. Accordingly, a steel component in which the crystal grain size has been reduced uniformly throughout the entire component can be obtained, and the characteristic effects of the above type of two-stage heat treatment, namely favorable abrasion resistance and cracking resistance, or an improvement in the service life relative to rolling fatigue, can be achieved with good stability.

Furthermore, induction heating offers additional advantages, such as the ability to perform localized heating and the freedom to determine the thickness of the hardened layer, and the ability to improve the fatigue strength through surface compressive residual stress by enabling rapid heating and rapid cooling, and consequently further reductions in the cost of the bearing components, and further improvements in the quality and the service life relative to rolling fatigue can be obtained.

As follows is a description of a fourth embodiment of the present invention, in which a bearing component is used as an example of the steel component.

FIG. 7 shows a schematic illustration of the construction of a heat treatment system according to this fourth embodiment. As shown in the figure, this heat treatment system comprises a primary heat treatment device 1, a secondary heat treatment device 2, two washing devices 3 and 5, and a tempering device 6. A bearing component produced by a molding process (not shown in the drawings) comprising forging, and then turning or the like, is fed sequentially through the primary heat treatment device 1 and the secondary heat treatment device 2, and undergoes heating and cooling within each of the devices, as the primary heat treatment and the secondary heat treatment respectively.

The term “bearing component” refers to a bearing component of a rolling bearing such as a ball bearing, a tapered roller bearing, a roller bearing or a needle roller bearing. As one example, FIG. 8 shows a tapered roller bearing 4 comprising, as the main structural elements, an outer ring 41, an inner ring 42, and rolling elements (tapered rollers) 43, and of these structural elements, in this description, the term “bearing components” describes those elements that are subjected to rolling contact with an opposing member, namely the outer ring 41, the inner ring 42 and the rolling elements 43. Examples of the materials that can be used for these bearing components include bearing steel such as SUJ2, as well as high temperature bearing steel comprising C: 0.6 to 1.3 wt %, Si: 0.3 to 3.0 wt %, Mn: 0.2 to 1.5 wt %, Cr: 0.3 to 5.0 wt %, and Ni: 0.1 to 3 wt % (and preferably also comprising Mo: 0.05 to 0.25 wt % and V: 0.05 to 1.0 wt %), and medium carbon steel comprising C: 0.4 to 0.8 wt %, Si: 0.2 to 0.9 wt %, Mn: 0.7 to 1.3 wt % and Cr: no more than 0.7 wt %.

The primary heat treatment device 1 comprises a heater 11 and a cooler 12. In FIG. 7, a continuous system is shown for the heater 11, but a batch-type furnace can also be used. The heater 11 is formed, for example, from an atmospheric furnace that uses an atmospheric gas comprising a carburizing gas containing added ammonia. Inside this heater 11, a bearing component is heated at a temperature T1 (from 800° C. to 900° C., for example, 850° C.) exceeding the A1 transformation point for a predetermined time of 40 minutes for example (primary heating), as shown in FIG. 3, and this heating causes activated nitrogen to diffuse into the surface layer of the bearing component, thus hardening that surface layer (gas carbonitriding). The heater 11 has a primary object of forming a nitrogen enriched layer at the surface of the component, and at least nitridation must occur, although carburization is not essential. However, depending on the conditions, carburization may be essential as well as nitridation, particularly in cases where decarburization is a concern, or cases where the carbon content of the steel is insufficient and a satisfactory level of hardness cannot be achieved. The heater 11 may also use a vacuum furnace, a salt bath furnace or an induction heater or the like. Following heat treatment, the bearing component is cooled to below the Ms point by the cooler 12 (using oil cooling, for example), and is then transported to the washing device 3 for washing and removal of the cooling liquid.

As shown in FIG. 7, a bearing component that has undergone carbonitriding in the primary heat treatment device 1 (in the figure, an outer ring 41 of a tapered roller bearing is shown as an example) is then supplied to the secondary heat treatment device 2, which conducts induction heating such as high frequency heating. The bearing component 41 is positioned within the inner periphery of an inductor 21, and as shown in FIG. 3, is then subjected to induction heating (secondary heating) at a temperature T2 exceeding the A1 transformation point. By controlling heating conditions such as a heating temperature and a heating time, the austenite grains within the steel are reduced in size. Following completion of this heating, as shown in FIG. 7, the bearing component 41 is cooled to below the Ms point using a cooling liquid such as oil in the state where the bearing component 41 is fitted into and held by a die 22, thus effect quenching (die quenching). The quenching may be conducted by spraying the cooling liquid such as oil from apertures provided in various locations within the die 22. Alternatively, instead of conducting the cooling at the induction heating location as shown in the figure, the bearing component may also be transported to a separate location from the induction heating location for cooling.

Following completion of the secondary heat treatment, the bearing component is washed in the washing device 5 to remove the cooling liquid, and is then transported to the tempering device 6 and tempered at a suitable temperature T3 (180° C. for example). In order to improve the treatment efficiency by shortening the heating time, this tempering is preferably conducted using induction heating such as high frequency heating.

In the description above, oil cooling was used as the cooling method within the primary heat treatment device 1 and the secondary heat treatment device 2, but other cooling methods such as water cooling, air cooling or gas cooling can also be employed, and different cooling methods may also be used for the primary heat treatment device 1 and the secondary heat treatment device 2. In this embodiment, the washing devices 3 and 5 were provided because the primary heat treatment and the secondary heat treatment both use oil cooling, but if water cooling, air cooling or gas cooling were used, then these washing devices would be unnecessary.

In a bearing component that has undergone heat treatment in the steps described above, a nitrogen enriched layer (with a nitrogen content of 0.1 to 0.7 wt %) is formed at the surface layer of the component, meaning a high hardness exceeding Hv700 can be achieved, and the austenite grains within the micro structure are reduced in size to yield an austenite grain size number exceeding 10. Furthermore, the breaking stress value for the bearing component is at least 2650 MPa, the hydrogen concentration within the steel is no more than 0.5 ppm, and the residual austenite content within the steel is from 13 to 25%, which represent far superior physical properties to conventional components. As a result of the above properties, the cracking resistance and the abrasion resistance can be improved, and a marked improvement in the service life relative to rolling fatigue can also be achieved.

In this embodiment of the present invention, induction heating and die quenching are conducted within the secondary heat treatment device 2, as described above. In this case, theoretically, because the induction heating causes minimal thermal distortion, and the quenching following the heating process is conducted as die quenching, a bearing component with little thermal distortion and a high level of dimensional precision can be produced at low cost, and a favorable level of dimensional precision can be achieved even for thin-walled components such as the outer ring or the inner ring of a ball bearing, or for components with varying thickness such as the outer ring 41 or the inner ring 42 of a tapered roller bearing. Accordingly, the quality of the bearing components can be improved, meaning favorable bearing performance can be achieved with good stability.

Furthermore, induction heating offers additional advantages, such as the ability to heat each individual structural component uniformly on a piece by piece basis, the ability to heat with improved heating efficiency and shorter heating times, the ability to perform localized heating and the freedom to determine the thickness of the hardened layer, and the ability to improve the fatigue strength through surface compressive residual stress by enabling rapid heating and rapid cooling, and consequently further reductions in the cost of the bearing components, and further improvements in the quality and the service life relative to rolling fatigue can be obtained.

As follows is a description of a fifth embodiment of the present invention, in which bearing components of a tapered roller bearing shown in FIG. 8 are used as examples of steel components.

FIG. 9 shows a schematic illustration of the construction of a heat treatment system according to this fifth embodiment. As shown in the figure, this heat treatment system comprises a primary heat treatment device 1, a washing device 3, and a plurality of secondary heat treatment devices 2, washing devices 5, and tempering devices 6, which are arranged in parallel relative to the primary heat treatment device 1 and the washing device 3. Bearing components produced by a molding process (not shown in the drawings) comprising forging, and then turning or the like, are fed sequentially through the primary heat treatment device 1 and the secondary heat treatment devices 2, and undergo heating and cooling within each of the devices, as the primary heat treatment and the secondary heat treatment respectively. A separate washing device 5 and tempering device 6 are positioned as subsequent stages to each of the plurality of secondary heat treatment devices 2.

The primary heat treatment device 1 comprises a heater 11 and a cooler 12. In FIG. 9, a continuous system is shown for the heater 11, but a batch-type furnace can also be used. The heater 11 is formed, for example, from an atmospheric furnace that uses an atmospheric gas comprising a carburizing gas containing added ammonia. Inside this heater 11, a bearing component is heated at a temperature T1 (from 800° C. to 900° C., for example, 850° C.) exceeding the A1 transformation point for a predetermined time of 40 minutes for example (primary heating), as shown in FIG. 3. This heating causes activated nitrogen to diffuse into the surface layer of the bearing component, thus hardening that surface layer (gas carbonitriding). The heater 11 has a primary object of forming a nitrogen enriched layer at the surface of the component, and at least nitridation must occur, although carburization is not essential. However, depending on the conditions, carburization may be essential as well as nitridation, particularly in cases where decarburization is a concern, or cases where the carbon content of the steel is insufficient and a satisfactory level of hardness cannot be achieved. The heater 11 may also use a vacuum furnace, a salt bath furnace or an induction heater or the like. Following heat treatment, the bearing component is cooled to below the Ms point by the cooler 12 (using oil cooling, for example), and is then transported to the washing device 3 for washing and removal of the cooling liquid.

As shown in FIG. 9, bearing components (in the figure, outer rings 41 of a tapered roller bearing are shown as examples) that have undergone carbonitriding in the primary heat treatment device 1 are divided up and supplied, via a transport device such as a conveyor that is not shown in the figure, to one of the secondary heat treatment devices 2 which conduct induction heating such as high frequency heating. Within each of the secondary heat treatment devices 2, a bearing component 41 is held within the inner periphery of an inductor 21, and as shown in FIG. 3, is then subjected to induction heating (secondary heating) at a predetermined temperature T2 exceeding the A1 transformation point. Because this secondary heating is conducted within a short time, the austenite grains within the steel can be reduced in size, irrespective of whether the heating temperature T2 is higher or lower than the primary heating temperature, by adjusting the heating temperature and the heating time. Following completion of this heating, the bearing component 41 is fitted into the die 22, and cooled to below the Ms point using a cooling liquid such as oil, thus effect quenching (die quenching). The quenching may be conducted by spraying the cooling liquid such as oil from apertures provided in various locations within the die 22. Alternatively, instead of conducting the cooling with the component held at the induction heating location as shown in the figure, the bearing component may also be transported to a separate location from the induction heating location for cooling.

Following completion of the secondary heat treatment, the bearing components are removed from each of the secondary heat treatment devices 2, washed in the corresponding washing devices 5 to remove the cooling liquid, and are then transported to the corresponding tempering devices 6 and tempered at a suitable temperature T3 (180° C. for example), as shown in FIG. 3. In order to improve the treatment efficiency by shortening the heating time, this tempering is preferably conducted using induction heating such as high frequency heating.

In the description above, oil cooling was used as the cooling method within the primary heat treatment device 1 and the secondary heat treatment devices 2, but other cooling methods such as water cooling, air cooling or gas cooling can also be employed, and different cooling methods may also be used for the primary heat treatment device 1 and the secondary heat treatment devices 2. In this embodiment, the washing devices 3 and 5 were provided because the primary heat treatment and the secondary heat treatment both use oil cooling, but if water cooling, air cooling or gas cooling were used, then these washing devices would be unnecessary.

In a bearing component that has undergone heat treatment in the steps described above, a nitrogen enriched layer (with a nitrogen content of 0.1 to 0.7 wt %) is formed at the surface layer of the component, meaning a high hardness exceeding Hv700 can be achieved, and the austenite grains within the micro structure are reduced in size to yield an austenite grain size number exceeding 10. Furthermore, the breaking stress value for the bearing component is at least 2650 MPa, the hydrogen concentration within the steel is no more than 0.5 ppm, and the residual austenite content within the steel is from 13 to 25%, which represent far superior physical properties to conventional components. As a result of the above properties, the cracking resistance and the abrasion resistance can be improved, and a marked improvement in the service life relative to rolling fatigue can also be achieved.

In this embodiment of the present invention, as described above, bearing components 41 that have passed through a common primary heat treatment device 1 are divided up and subjected to simultaneous secondary heating within a plurality of secondary heat treatment devices 2, and high frequency heating is performed within each of these secondary heat treatment devices 2, which enables the heating to be completed within a short time period. As a result, the heating efficiency of the secondary heat treatment can be improved dramatically, meaning the heat treatment efficiency within the primary heat treatment and the secondary heat treatment can be balanced, enabling an improvement in the heating efficiency across the entire system. The number of secondary heat treatment devices 2 can be selected in accordance with the difference in heat treatment efficiency between the primary heat treatment and the secondary heat treatment, and can be any number that enables a balance between the two treatments. In the embodiment described above, only a single primary heat treatment device 1 was provided, but a plurality of these devices can also be provided in parallel, and the heat treatment efficiency then balanced with a plurality of secondary heat treatment devices 2 (in such cases, the number of secondary heat treatment devices 2 is greater than the number of primary heat treatment devices 1).

Furthermore, because the high frequency heating conducted in the secondary heat treatment devices 2 theoretically causes minimal thermal distortion, and because the quenching following the heating process is conducted as die quenching, bearing components with little thermal distortion and a high level of dimensional precision can be produced at low cost, and a favorable level of dimensional precision can be achieved even for thin-walled components such as the outer ring or the inner ring of a ball bearing, or for components with varying thickness such as the outer ring 41 or the inner ring 42 of a tapered roller bearing. Accordingly, the quality of the bearing components can be improved, meaning favorable bearing performance can be achieved with good stability.

Furthermore, induction heating offers additional advantages, such as the ability to heat each individual structural component uniformly on a piece by piece basis, the ability to perform localized heating and the freedom to determine the thickness of the hardened layer, and the ability to improve the fatigue strength through surface compressive residual stress by enabling rapid heating and rapid cooling, and consequently further reductions in the cost of the bearing components, and further improvements in the quality and the service life relative to rolling fatigue can be obtained.

In the above descriptions of each of the embodiments, a bearing component was used as an example of the target article undergoing heat treatment, but the present invention is not restricted to bearing components, and can be applied to a wide range of mechanical components that require a favorable service life relative to rolling fatigue or superior cracking resistance, such as the structural components of constant velocity universal joints, or even general steel components.

Furthermore, the values of the primary heating temperature T1, the secondary heating temperature T2 and the tempering temperature T3 detailed above in each of the embodiments represent the values in the case where bearing steel SUJ2 is used as the steel material. These temperatures T1, T2 and T3 may differ from the values listed above depending on the nature of the steel material used.

Claims

1. A heat treatment system comprising: a primary heat treatment device for heating a steel component at a temperature exceeding the A1 transformation point and then cooling the component to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component; and a secondary heat treatment device for heating the steel component that has undergone primary heat treatment, at a temperature exceeding the A1 transformation point, and then cooling the component to a temperature less than the A1 transformation point, wherein the secondary heat treatment device includes an induction heater, and tempering is performed by induction heating following cooling within the secondary heat treatment device.

2. A heat treatment system comprising: a primary treatment device for heating a steel component at a primary heating temperature exceeding the A1 transformation point and then cooling the component to a temperature less than the A transformation point, thus forming a nitrogen enriched layer at the surface layer of the component; and a secondary treatment device for heating the steel component that has undergone heat treatment by the primary treatment device, at a secondary heating temperature exceeding the A1 transformation point, and then cooling the component to a temperature less than the A1 transformation point.

3. A heat treatment system as claimed in claim 2, wherein

the primary treatment device includes a heater for conducting gas carbonitriding.

4. A heat treatment system as claimed in claim 2, wherein

die quenching is conducted within the secondary treatment device.

5. A heat treatment system comprising: a primary heat treatment device for heating a steel component at a temperature exceeding the A1 transformation point and then cooling the component to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component, and a secondary heat treatment device for heating the steel component that has undergone primary heat treatment, at a temperature exceeding the A1 transformation point, and then cooling the component to a temperature less than the A1 transformation point, wherein induction heating is conducted in the secondary heat treatment device, the temperature of the steel component undergoing induction heating is detected, and the induction heater is operated under feedback control based on the detected temperature value.

6. A heat treatment system as claimed in claim 5, wherein

the temperature of the steel component undergoing induction heating is detected using a non-contact type temperature sensor.

7. A heat treatment system comprising: a primary heat treatment device for heating a steel component at a temperature exceeding the A1 transformation point and then cooling the component to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component; and a secondary heat treatment device for heating the steel component that has undergone primary heat treatment, at a temperature exceeding the A1 transformation point, and then cooling the component to a temperature less than the A1 transformation point, wherein the secondary heat treatment device conducts induction heating and die quenching.

8. A heat treatment system comprising: a primary heat treatment device for heating a steel component at a temperature exceeding the A1 transformation point and then cooling the component to a temperature less than the A1 transformation point, thus forming a nitrogen enriched layer at the surface of the component, and a secondary heat treatment device for heating the steel component that has undergone primary heat treatment, at a temperature exceeding the A1 transformation point, and then cooling the component to a temperature less than the A1 transformation point, wherein a plurality of secondary heat treatment devices are disposed in parallel.

9. A heat treatment system as claimed in claim 8, wherein induction heating is conducted within each of the secondary heat treatment devices.

10. A heat treatment system as claimed in claim 8, wherein die quenching is conducted within each of the secondary treatment devices.

11. A heat treatment system as claimed in claim 1, wherein

gas carbonitriding is conducted within the primary heat treatment device.

12. A heat treatment system as claimed in claim 5, wherein

gas carbonitriding is conducted within the primary heat treatment device.

13. A heat treatment system as claimed in claim 7, wherein

gas carbonitriding is conducted within the primary heat treatment device.

14. A heat treatment system as claimed in claim 8, wherein

gas carbonitriding is conducted within the primary heat treatment device.

15. A heat treatment system as claimed in claim 3, wherein

die quenching is conducted within the secondary treatment device.

16. A heat treatment system as claimed in claim 9, wherein

die quenching is conducted within each of the secondary treatment devices.