Patent Application
20090020194
The present invention relates to an induction hardening method, more particularly to an induction hardening method for a long work of an Fe-based alloy extending along the axis line.
Induction hardening treatment methods for a rod-shaped long work, such as a drive shaft for an automobile engine, include single-shot hardening methods of hardening the entire long work at once, and transfer hardening methods containing placing a high-frequency heating coil around a part of the long work, and transporting the long work in the axial direction to sequentially heat the parts of the long work. In the hardening methods, the long work is heated by a high-frequency heating coil, and then cooled by a cooling liquid.
In the single-shot hardening methods, the long work is clamped at both ends, and forced to be rotated (for example, see Patent Document 1). The long work is heated in this state, then cooled while being rotated, and thereby hardened. A correction roller is placed in the vicinity of the long work. When the long work is distorted in the cooling, the distorted portion of the long work rotated is brought into contact with the correction roller to correct the distortion.
However, the distortion cannot be sufficiently eliminated only by the correction roller, and generally a cold distortion eliminating process is carried out after the hardening treatment. The long work is often cracked by the distortion eliminating process, and a magnetic crack detection is carried out to detect the cracking. Thus, the number of processes is increased, so that it is difficult to improve the production efficiency of the long work obviously.
A general object of the present invention is to provide an induction hardening method capable of preventing the distortion of a long work.
A principal object of the present invention is to provide an induction hardening method not requiring distortion elimination and magnetic crack detection processes.
Another object of the present invention is to provide an induction hardening method capable of producing a long work having an approximately uniform metal structure over the entire work.
A further object of the present invention is to provide such a simple induction hardening method that the distortion of a long work can be corrected even when a correction roller is held for some reasons.
A still further object of the present invention is to provide an induction hardening method having an improved production efficiency of a long work.
According to an aspect of the present invention, there is provided an induction hardening method for hardening a long work containing an Fe-based alloy, wherein the long work is heated by high-frequency induction heating while being rotated, and when the long work is distorted in the hardening, the long work is brought into contact with a correction roller to correct the distortion.
The induction hardening method comprises:
a heating step of heating the long work by the high-frequency induction heating to a first temperature range, within which an austenite is formed, while rotating the long work at a first rotational speed;
a first cooling step of cooling the long work to a second temperature range of higher than the martensite start temperature and at most the pearlite finish temperature while rotating the long work at a second rotational speed higher than the first rotational speed; and
a second cooling step of further cooling the long work having a temperature within the second temperature range while rotating the long work at a third rotational speed lower than the second rotational speed.
The Fe-based alloy used as a material of the long work has a structure, from which the distortion can be easily eliminated, at a temperature lower than the pearlite finish temperature (the Pf temperature). Thus, when the long work is distorted in the cooling step at a maximum rotational speed after the heating step, the distortion can be efficiently eliminated by bringing the long work into contact with the correction roller. This is because the correction roller and the long work can be brought into contact at a high contact frequency.
Thus, in the present invention, the distortion is hardly present in the resultant long work after the second cooling step. Therefore, the cold distortion eliminating process can be omitted, and naturally also the magnetic crack detection for detecting cracks in the long work due to the cold distortion eliminating process can be omitted. The long work production efficiency can be improved by omitting the processes.
In addition, the resultant long work has an approximately uniform metal structure and properties over the entire long work.
The first cooling step is carried out at the maximum rotational speed until the long work is cooled to the predetermined temperature range (the second temperature range). Specifically, the second temperature range is a range of higher than the martensite start temperature (the Ms temperature) and at most the Pf temperature. It is particularly preferred that the long work is cooled to a temperature immediately above the Ms temperature in the first cooling step.
When the rotational speed of the correction roller is changed, practically the speed is gradually increased or decreased due to the inertia. Thus, the rotational speed is in the middle of the increase immediately after the start of the first cooling step, and the rotational speed is in the middle of the decrease immediately after the start of the second cooling step.
It is preferred that at least one correction roller can be freely rotated independently from other correction rollers. In this case, the distortion can be corrected with simple structure, even when the other correction rollers are fixed for some reasons.
In the first cooling step, the cooling time is selected depending on the size, weight, and hardness of the long work. For example, in a case where the long work has a cylindrical shape, as the diameter is increased, the cooling time is lengthened.
The rotational speeds in the heating step and the second cooling step (i.e. the first rotational speed and the third rotational speed) may be equal.
A preferred embodiment of the induction hardening method of the present invention will be described in detail below with reference to accompanying drawings.
The induction hardening method of this embodiment contains a heating step, a first cooling step, and a second cooling step. Thus, a long work is heated in the heating step, and is cooled in the first cooling step and the subsequent second cooling step.
The correction mechanism 12 has a base 18 and first to fourth bearings 20a to 20d formed thereon. A first rotation shaft 22a is supported by the first and second bearings 20a and 20b, while a second rotation shaft 22b is supported by the third and fourth bearings 20c and 20d. Of course the first and second rotation shafts 22a and 22b are rotatable independently.
As shown in
The rotatable chucks 14a, 14b of the clamp mechanism can be moved close to and away from the ends of the long work LW, and in other words can be opened and closed. When the rotatable chucks 14a, 14b are closed, the ends of the long work LW are pressed by the rotatable chucks 14a, 14b, whereby the long work LW is clamped.
The rotatable chucks 14a, 14b can be rotated at a controlled rotational speed under the action of a rotation controlling motor (not shown). The rotational speed can be controlled by changing a rotational force from the rotation controlling motor.
The high-frequency heating coil 16 comprises arches 26a, 26b which are located near the ends of the long work LW, and are curved along the upper half of the long work LW, and straight portions 28a, 28b which are formed to link the ends of the arches 26a, 26b. Further, arms 30a, 30b are disposed on the arches 26a, 26b respectively, and one end of each arm 30a, 30b is supported by an elevating mechanism (not shown). When the arms 30a, 30b are moved downward or upward by the elevating mechanism, the high-frequency heating coil 16 is moved close to the long work LW to surround the upper half or moved away from the upper half.
The long work LW is not particularly limited as long as the height (the length in the axial direction) is equal to or more than the bottom diameter, width, and depth. Preferred examples of the long works LW include drive shafts.
The induction hardening method of this embodiment is carried out as follows.
First, the rotatable chucks 14a, 14b are closed to clamp the ends of the long work LW such as a drive shaft. Then, the arms 30a, 30b of the high-frequency heating coil 16 are moved downward by the elevating mechanism, and finally the upper half of the long work LW is surrounded by the high-frequency heating coil 16 as shown in
The rotatable chucks 14a, 14b are rotated by the rotation controlling motor, and thus the long work LW is rotated. For example, the rotational speed may be 100 to 200 rpm.
In this state, the high-frequency heating coil 16 is energized to start the heating step, so that the long work LW is heated to about 900° C. to 950° C. by electromagnetic induction heating. Thus, the heating step of the induction hardening treatment is started. In the electromagnetic induction heating, austenite transformation is caused in the metal structure of the long work LW composed of an Fe-based alloy.
After a predetermined time, the energization of the high-frequency heating coil 16 is stopped, the heating jacket is moved away from the long work LW, and the rotational speed of the rotatable chucks 14a, 14b are increased. For example, the rotational speed of the rotatable chucks 14a, 14b may be finally increased to 240 to 300 rpm.
Immediately after the heating jacket is moved away from the long work LW, the long work LW is surrounded by the movable cooling jacket.
The movable cooling jacket has a semi-cylindrical shape, and is moved in the longitudinal direction of the long work LW while surrounding a part of the upper half of the long work LW. An injector for spraying a cooling liquid onto the long work LW is disposed on the inner periphery wall of the movable cooling jacket.
Thus, the long work LW is cooled by the cooling liquid emitted from the inner periphery wall of the movable cooling jacket, so that the first cooling step is started. The movable cooling jacket is moved in the longitudinal direction of the long work LW, whereby the entire long work LW is cooled.
In this cooling step, a ferrite or a pearlite is formed in the metal structure of the long work LW (the Fe-based alloy). The metal structure of the long work LW is changed by the ferrite and pearlite formation, and a part of the long work LW may be swelled to generate a distortion before the long work LW is cooled to the pearlite finish temperature (the Pf temperature), at which the pearlite formation is finished. In this case, the distorted part is brought into contact with one of the first to fourth correction rollers 24a to 24d at a rate of 0.83 to 5 times/minute, so that the distortion of the long work LW is corrected. Of course the cooling liquid is continuously emitted from the movable cooling jacket during this correction.
In the temperature range between the austenite forming temperature and the Pf temperature, the long work LW has a structure from which the distortion can be easily eliminated. Therefore, in the first cooling step, the distortion of the long work LW can be efficiently eliminated by bringing the long work LW into contact with the first to fourth correction rollers 24a to 24d while rotating the long work LW at the maximum rotational speed.
In this embodiment, the rotational speed of the long work LW in the first cooling step is higher than that in the high-frequency heating step, so that the first to fourth correction rollers 24a to 24d and the long work LW can be brought into contact at a high contact frequency. As a result, the effect of correcting the distortion of the long work LW is improved.
Further, in this embodiment, the resultant long work LW has an approximately uniform metal structure, and thereby has uniform properties, over the entire work.
The above rotational speed of the rotatable chucks 14a, 14b is maintained (i.e., the first cooling step is continued) until the long work LW is cooled to a predetermined temperature range, within which a large distortion is hardly generated in the long work LW, specifically to the Pf temperature or less. When the long work LW is cooled to a temperature lower than the martensite start temperature (the Ms temperature) in the first cooling step, a so-called hardening crack may be caused by a martensite formed. In view of this problem, in the first cooling step, the long work LW is cooled to a temperature higher than the Ms temperature.
In short, at the end of the first cooling step, the temperature of the long work LW is equal to or lower than the Pf temperature and higher than the Ms temperature. It is preferred that the long work LW is cooled to a temperature immediately above the Ms temperature in the first cooling step. In this case, the dimensional accuracy of the long work LW is improved.
The Pf and Ms temperatures are obtained from a continuous cooling transformation curve (CCT curve) before the first cooling step. For example, in the case of using an S40CM as a material of the long work LW, the Pf and Ms temperatures thereof can be obtained from a CCT curve shown in
In the first cooling step, when the cooling time is excessively short, the long work LW still has a high temperature and is returned to the heated state, resulting in a lowered hardness. On the other hand, when the cooling time is excessively long, the treatment efficiency is deteriorated. Thus, in the first cooling step, the cooling time is controlled such that the hardness of the long work LW is not lowered, and the treatment efficiency is not deteriorated.
The cooling time is selected depending on the diameter, weight, and hardness of the long work LW. Thus, the cooling time is not determined depending on one factor. For example, when the long work LW is composed of an S40CM and has a cylindrical shape with a diameter of about 20 cm, the cooling time may be 10 to 20 seconds.
After the first cooling step, the long work LW is further cooled by spraying the cooling liquid from the movable cooling jacket while reducing the rotational speed of the rotatable chucks 14a, 14b (and the long work LW) in the second cooling step. In the second cooling step, the cooling liquid spraying may be stopped, and the temperature of the cooling liquid may be lower than that in the first cooling step.
For example, the rotational speed in the second cooling step may be equal to that in the heating step. Thus, for example, the rotational speed may be 100 or 150 rpm in both the heating step and the second cooling step. It is preferred that the rotational speed is 180 rpm in the heating step and the second cooling step. In this case, the long work LW can be prevented from being deformed due to the martensitic transformation.
After a predetermined cooling time, the entire induction hardening treatment is completed. The relation between the time and the rotational speed in the above steps is shown in
The resultant long work LW has little or no distortion, and does not have to be subjected to a cold distortion eliminating process for removing the distortion. Also a magnetic crack detection process for detecting cracks in the long work LW due to the cold distortion eliminating process is not required naturally. Thus, the long work LW treatment efficiency, and the long work LW production efficiency are improved.
Though the S40CM material is used in the above embodiment, the material of the long work LW is not particularly limited as long as it is an Fe-based alloy. The Pf and Ms temperatures, etc. of the material other than the S40CM can be obtained using the corresponding CCT curve.
Further, the shape of the long work LW is not limited to the above cylindrical shape with the spherical bottom surfaces. The long work LW may have a polygonal column shape with polygonal bottom surfaces. The bottom surfaces may have different shapes.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-024942 | Feb 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/051425 | 1/30/2007 | WO | 00 | 7/23/2008 |
