Induction hardening method and jig used in induction hardening process

The present invention provides a method for induction hardening of a steel member having an outer ring portion and, more specifically, for induction hardening of an outer circumferential surface of the outer ring portion. The method uses two pressing members, each of which includes a flat basal surface and a projecting portion projecting therefrom. Each of the projecting portions has a cross-section in the shape of a perfect circle. Also, the projecting portions have, on the outer circumferences thereof, respective pressing surfaces. While gaps are maintained between axial end faces of the outer ring portion and the basal surfaces of the pressing members, the pressing members are pressed against the outer ring portion so that the pressing surfaces each abut against the inner circumference side of the outer ring portion. While a biasing force is applied to the pressing members, the induction hardening process is applied to the outer circumferential surface. During the induction hardening process, the pressing members are moved toward each other until the basal surfaces abut against respective axial end faces of the outer ring portion.

TECHNICAL FIELD

The present invention relates to a method for improving the dimensional precision in an induction hardening process applied to a ring-shaped steel member.

BACKGROUND ART

Ring-shaped steel members such as gears having teeth on their outer circumferential surface are popularly used as parts in various types of mechanical apparatus. For example, a large number of gears including differential ring gears and counter gears are used in automatic transmissions in automobiles.

Generally, such ring-shaped steel members are required to have high-strength characteristics. An example of a method for imparting high-strength characteristics is an induction hardening process applied to the outer circumferential surfaces of the steel members.

The induction hardening process works extremely well as a method for enhancing the strength of the steel members, but also has a possibility of lowering the degree of precision in the dimensions. For example, after an induction hardening process is performed, the roundness of a ring-shaped part may be inferior to the roundness before the process is performed. If the part is of such a type that a loss of roundness does not lower its performance level, there is no problem. However, if the part is of such a type that a loss of roundness lowers its level of performance, it is necessary to add a correction process, after the induction hardening process is performed, in order to improve the roundness of the part.

For this reason, development of a method for preventing loss of roundness during an induction hardening process has been in demand. However, a satisfactory solution had not yet been found.

For example, Japanese Kokai 11-131133 discloses a jig for improving roundness of a circular-tube-shaped member after an induction hardening process has been performed on the inner circumferential surface of the circular-tube-shaped member. It is, however, not possible to use the disclosed jig to improve the roundness when an induction hardening process is performed on the outer circumferential surface of a part.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

In view of the problems described above with the related art, an objective of the present invention is to provide an induction hardening method and a jig for use therein that are able to prevent a loss of roundness in a ring-shaped steel member.

Means for Solving the Problems

A first aspect of the present invention provides an induction hardening method for treating a steel member having an outer ring portion substantially in the shape of a circular tube, to harden an outer circumferential surface of the outer ring portion. The induction hardening method of the invention uses a pressing member that includes a substrate having a first basal surface and a projecting portion projecting from a central portion of the basal surface, the projecting portion having a cross-section in the shape of a perfect circle and having a pressing surface on its outer circumferential edge. The pressing member is pushed against an axial end face of the outer ring portion with at least a part of the pressing surface abutting against the inner circumferential surface of the outer ring portion while a gap is initially maintained between the axial end face and the basal surface. A force is applied to the outer circumferential surface of the outer ring portion by application of a biasing force to the pressing member in such a direction that the pressing member approaches the steel member, causing the pressing member to move forward relative to the steel member until the basal surface abuts against the axial end face during the induction hardening.

Thus, the induction hardening method according to the present invention uses a jig that includes the pressing member specially configured to include the pressing surface and the basal surface.

During the induction hardening process, the outer circumferential surface of the outer ring portion is first inductively heated and therefore thermally expands. Consequently, the inner circumferential surface of the outer ring portion and the pressing surface naturally move away from each other. However, because a biasing force is applied to the pressing member in such a direction that the pressing member approaches the steel member, i.e., the pressing member moves forward relative to the steel member, at least in an initial stage where the degree of the thermal expansion of the outer ring portion is small, the pressing surface remains abutted against the inner circumferential side of the outer ring portion, whereby a force that enhances the degree of roundness is transferred from the pressing surface to the inner circumferential surface of the outer ring portion. Consequently, it is possible to maintain the roundness during the heating process.

However, as the outer ring portion continues to expand during the heating process, with the pressing member moving forward relative to the steel member undergoing induction hardening, the basal surface around the pressing surface comes into contact with the axial end face of the outer ring portion, thus limiting that forward movement. At this point in time, when the outer ring portion expands further, the inner side of the outer ring portion and the pressing surface are separate from each other. Of course, it is also acceptable to maintain the abutting contact between these elements by adjusting their dimensional relationship.

Subsequently, in the induction hardening process, the steel member is rapidly cooled using, for example, water and the outer ring portion contracts due to the cooling. Consequently, even in the case where the pressing surface and the inner circumferential surface of the outer ring portion separate from each other during the heating process, at the end of the process they return to the state in which they abut against each other. Thus, these two elements press against each other with a high force when the steel member contracts due to the cooling. Because the inner circumferential surface of the outer ring portion is strongly pressed against the pressing surface, the degree of roundness is maintained.

It is preferable that the basal surface of the pressing member extends around the entire outer circumference of the projecting portion, i.e., around the pressing surface. However, it is also acceptable to have the basal surface provided at only one or more portions of the periphery, instead of around the entire periphery.

Further, in a second aspect the present invention provides a jig to be used in a hardening process by attachment to a steel member having an outer ring portion substantially in the shape of a circular tube, while induction hardening is performed on an outer circumferential surface of the outer ring portion. The jig includes a pressing member that includes a substrate having a flat basal surface and a projecting portion projecting from a central portion of the basal surface, the projecting portion having a cross section in the shape of a perfect circle and an outer circumference edge serving as a pressing surface configured so that at least a part abuts against the inner circumferential surface of an outer ring portion of a steel member to be treated, with a gap between an axial end face of the outer ring section and the basal surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferably, the first pressing member is configured so as to abut against the outer ring portion from one axial end face and the second pressing member is configured so as to abut against the outer ring portion from the other axial end face to sandwich the steel member therebetween. The pressing surface of the first pressing member and the pressing surface of the second pressing member respectively abut against axially opposed regions of the inner cylindrical surface of the outer ring portion while the induction hardening process is performed, for application of a biasing force biasing the first and second pressing members together.

The induction hardening process is performed while the steel member is sandwiched between the first pressing member and the second pressing member. As a result, the pressing surfaces of the two pressing members function so as to improve the roundness of their respective contact regions of the outer ring portion. In other words, it is possible to improve the roundness from both axial ends of the outer ring portion. Consequently, it is also possible to avoid taper defects which are caused by diameter variations in the axial direction.

In addition, it is preferable that the pressing surface of the pressing member is a tapered surface having an outside diameter becoming smaller toward the tip thereof. With this arrangement, it is possible to easily bring the pressing surface into contact against the inner circumferential surface of the outer ring portion.

It is also preferable that the pressing surface of the pressing member abuts against a corner (apex angle) portion formed at the intersection between the axial end face and the inner circumferential surface of the outer ring portion. With this arrangement, it is possible to realize more easily contact of the tapered pressing surface against the inner side of the outer ring portion.

The pressing member may be in the shape of a truncated cone or a circular cylinder having a distal end with a flat planar surface, so that the corner where the pressing surface joins that planar surface abuts against the inner circumferential surface of the outer ring portion.

In addition, it is preferred that the steel member is a ring gear having teeth on the outer circumferential surface of the outer ring portion. For example, with ring gears such as differential ring gears used in automatic transmissions of automobiles or the like, it is necessary to harden the surfaces of the teeth provided on the outer circumferential surface, and also it is necessary provide a high level of roundness. In achieving these effects, the induction hardening process according to the present invention is extremely effective.

According to the second aspect of the present invention, the jig used in the hardening process includes first and second pressing members, the first pressing member being configured so as to abut against the outer ring portion from one axial end face and the second pressing member being configured so as to abut against the outer ring portion from the other axial end face. In using the jig, the steel member is sandwiched between the first pressing member and the second pressing member and that the pressing surface of the first pressing member and the pressing surface of the second pressing member respectively abut axially opposing regions of the outer ring portion on the inner circumference side.

When the jig intended for hardening is as described above, it is possible to perform the induction hardening process while the steel member is sandwiched between the two pressing members. Thus, it is possible to maintain the level of roundness and to inhibit taper defects.

Further, it is preferred that the pressing surface of the pressing member is a tapered surface, slanted so that the outside diameter of the pressing member becomes smaller toward the tip thereof. With this design, because of the tapered surface, it is possible to easily achieve abutment of the pressing member against the inner circumference side of the outer ring portion.

The projecting portion having the pressing surface may be in the shape of one of a truncated cone and a circular cylinder. When the projecting portion is in the shape of a truncated cone, the pressing surface will have a tapered surface.

In its first and second aspects, the present invention is applicable to various types of steel members including: a part corresponding to only the outer ring portion; a part that has, inside the outer ring portion, an inner annular portion that is thinner than the outer ring portion; a part in which the inner annular portion is positioned in the vicinity of the center of the inner circumferential surface of the outer ring portion and wherein the outer ring portion has corner (apex angle) portions on both sides; and a part in which the inner annular portion is joined with one of the axial end faces of the outer ring portion, and the outer ring portion has a corner portion only on the axially opposite face.

First Embodiment

An induction hardening method and a jig to be used in a hardening process according to an embodiment of the present invention will be explained with reference toFIG. 1throughFIG. 7.

According to the first embodiment, as shown inFIGS. 7A and 7B, an induction hardening is performed on an outer circumferential surface810of an outer ring portion81of a steel member8. The outer ring is substantially in the shape of a circular tube. To be more specific, the steel member8is a differential ring gear that is a component of an automatic transmission (A/T) in an automobile. The steel member8has, on the outer circumferential surface810of the outer ring portion81, a tooth portion including a large number of teeth811. On the inner circumference side of the outer ring portion81is an inner annular portion83that has a smaller axial thickness than the outer ring portion81. The inner annular portion83extends from the axial middle of the inner circumferential surface816of the outer ring portion81so that the inner circumferential surface816extends axially from both sides of the inner annular portion83. Inner corners821and822are formed at each of the two axial end faces801and802where they intersect the inner circumferential surface816. As shown inFIGS. 4A through 4D, which are explained later, the inner corners821and822are formed by a chamfering process. One of the two inner corners portions serves as the apex angle portion of the present invention.

A jig1used in a hardening process according to the present embodiment includes two pressing members, i.e., first pressing member10and a second pressing member2as shown inFIG. 1. The first pressing member10and the second pressing member2respectively include substrates12and22which respectively provide basal surfaces15and25(flat planes) and projecting portions13and23that are centered on the substrates12and22so as to project from the basal surfaces12and22. Each of the projecting portions13and23has a cross-section in the shape of a perfect circle. Also, the projecting portions13and23have, around their outer circumferential edges, pressing surfaces11and21, respectively. Each of the pressing surfaces11and21is a tapered surface that is slanted with the diameter becoming progressively smaller away from the planar surfaces15,25. In other words, each of the projecting portions13and23is in the shape of a truncated cone of which the lateral face is the tapered surface. The first pressing member11has, on the side opposite the projecting portion13, a back-side projecting portion19for positioning a spring4, described later.

In order to perform the induction hardening process on the steel member8, using the hardening jig1, the first pressing member10and the second pressing member2are pressed against the steel member8from the axially opposite end faces of the steel member8, as shown inFIG. 1.

Subsequently, as shown inFIG. 2, the tapered surface11of the first pressing member10abuts the corner821that is formed between one axial end face801, and the inner circumferential surface816of the outer ring portion81. Also, the tapered surface21of the second pressing member2abuts the corner822formed between the other axial end face802, and the inner circumferential surface816of the outer ring portion81.

Using the jig1, each of the basal surfaces15and25of the first pressing member10and the second pressing member2are maintained axially spaced from the axial end faces801and802of the outer ring portion81.

A biasing board3is provided spaced from the first pressing member10with the spring4interposed therebetween. The spring4is located at the outer circumference of the back-side projecting portion19on the first pressing member10and at the outer circumference of a projecting portion39on the biasing board3. As shown inFIG. 3, the elements described above are supported on a mounting stage71in a sequence starting with the second pressing member2, and the biasing board3is pressed downward, with a predetermined pressure, by a biasing shaft72that is connected to the biasing board3.

With pressure applied as described above, electric current is supplied to a coil75that is disposed around the outer circumferential surface810of the outer ring portion81so that the outer circumferential surface810is inductively heated.

Due to the induction heating, the outer ring portion81thermally expands gradually. Accordingly, as shown inFIGS. 4A to 4D, the position at which the tapered surface11of the first pressing member10abuts against the corner (apex angle) portion821changes from that shown inFIG. 4Ato that shown inFIG. 4B. Also, the basal surface15moves forward and approaches the steel member8up to a position where the basal surface15abuts against the axial end face801of the outer ring portion81. The position at which the second pressing member2abuts against the corner portion822changes in the same manner (not shown in the drawing).

When the outer ring portion81thermally expands further, as shown inFIG. 4C, the corner portion821on the inner circumference side of the outer ring portion81no longer abuts against the pressing surface12of the pressing member10, and thus, there is a gap between them. The positional relationship between the second pressing member2and the corner portion822changes in the same manner (not shown in the drawing). The roundness of the outer ring portion81is maintained by a force exerted from the pressing surface12, until the outer ring portion81is no longer in contact with the pressing surface12.

Next, as shown inFIG. 5, when the heating of the outer circumferential surface810of the outer ring portion81has been completed, cooling water77is sprayed from the coil75to rapidly cool the outer ring portion81from the outer circumferential surface810inward. As a result, as shown inFIG. 4DandFIG. 6, the outer ring portion81contracts due to the cooling. Consequently, once again, the pressing surface11of the first pressing member10abuts against the corner portion821on the inner circumference side of the outer ring portion81, and the pressing surface21of the second pressing member2abuts against the corner portion821on the inner circumference side of the outer ring portion81. Accordingly, a force that improves the level of roundness of the outer ring portion81is applied to the inner circumference side of the outer ring portion81from the pressing surfaces11and21.

Subsequently, the steel member8is removed from the hardening jig1, and thus a sequence of steps in the induction hardening of the steel member8is completed.

As described above, the induction hardening method of the first embodiment uses the hardening jig1that includes the first pressing member10and the second pressing member2that are specially configured with the tapered surfaces11and21extending from basal surfaces15and25, respectively. The tapered surfaces11and21are arranged so as to abut the corner portions821and822respectively, while a gap is maintained between the axial end face801of the outer ring portion81and the basal surface15and also between the axial end face802of the outer ring portion81and the basal surface25. While the biasing force is applied in a direction biasing the pressing members10and2toward the steel member8, the induction hardening process is performed. With this arrangement, it is possible to prevent the degree of roundness of the outer ring portion81from being reduced because the tapered surfaces11and21remain abutted against the corner portions821and822, respectively, during the heating process and the cooling process.

As explained above, by conducting the induction hardening while using the hardening jig1, it is possible to harden the outer circumferential surface810of the ring-shaped steel member8, without loss in the of roundness of the steel-member8.

Second Embodiment

According to a second embodiment of the present invention, as shown inFIGS. 8A to 8D, the first pressing member10and the second pressing member2(not shown inFIGS. 8A to 8D) are arranged in a setting stage before initiating induction hardening, with the first pressing member10having a corner (angle) portion119on pressing surface11of the first pressing member10abutting the inner circumferential surface816of the outer ring portion81, and the second pressing member being configured in the same manner as the first pressing member10.

The second embodiment is different from the first embodiment only in terms of the positions at which the pressing surfaces11and21of the first and the second pressing members10and2abut against the inner circumferential side of the outer ring portion81. Thus, according to the second embodiment, it is possible to achieve the same effect as in the first embodiment.

Third Embodiment

According to a third embodiment of the present invention, as shown inFIGS. 9A to 9D, the first pressing member10and the second pressing member2are arranged in a setting stage before an induction hardening processing is performed, the first pressing member10being configured so that the pressing surface11of the first pressing member10abuts against the inner circumferential surface816of the outer ring portion81, and the second pressing member being configured in the same manner as the first pressing member10.

Again, the third embodiment is different from the first embodiment only in terms of the positions at which the pressing surfaces11and21of the first and the second pressing members10and2abut against the inner circumferential side of the outer ring portion81. Thus, according to the third embodiment, it is possible to achieve the same effect as in the first embodiment.

Experimental

In order to quantitatively evaluate the advantageous effect of the first embodiment, the level of roundness was measured as an ellipticity value obtained after a hardening process had been performed, using samples including ones representing another embodiment and comparison examples. All of the samples used in the evaluation were prepared by performing a hardening process on steel members having the same shape.

Sample No. E1was obtained by induction hardening according to the first embodiment. In other words, the apex angle portions821and822were pressed in a symmetrical manner on the front and the back of the steel member8by the tapered surfaces11and21of the first pressing member10and the second pressing member2, respectively.

Sample No. E2was obtained by, as shown inFIG. 10, performing the same induction hardening process as the one according to the first embodiment but using a hardening jig having a different configuration. In this hardening jig, the first pressing member10that abuts against one axial end face801, was the same as the one used in the first embodiment, whereas the member that abuts against the other axial end face802, was a member91of which the tapered surface911was not at all in contact with the corner (apex angle) portion822. In other words, this sample was obtained with positions which were asymmetrical as between the opposing axial sides of the steel member8.

Sample No. C1was obtained by, as shown inFIG. 11, performing the same induction hardening process as the one according to the first embodiment but using a hardening jig having a different configuration. In this hardening jig, both of the members92and93that respectively abut against the two axial end faces801and802were configured so that neither of the tapered surfaces921and931of the members92and93was ever in contact with the corner portions821and822. In other words, in this example the corner portions821and822of the outer ring portion81were not restrained and were free.

As shown inFIG. 12, sample No. C2was obtained by performing a carburization hardening process, in which the steel member was rapidly cooled after being carburized, instead of an induction hardening process. This hardening jig included a stage941supporting the inner circumferential surface835of the inner annular portion83, while the steel member8was disposed in such a manner that its axis extended horizontally.

In order to evaluate the level of roundness of each of the samples, the diameter of the outer circumference defined by surface810of the outer ring portion81was measured at a plurality of radially spaced positions and at three axially spaced positions (top, middle, and bottom). The difference between the largest diameter and the smallest diameter was calculated as an ellipticity value (μm).

Also, average values of diameters were calculated at two axially spaced positions, namely at a top position and a bottom position, and the difference (μm) between the average values was calculated as taper.

FIG. 13shows the measurements and the evaluations. InFIG. 13, the measurements of ellipticity values are shown as bars for the different positions. Shown below the bars are the ellipticity values and the evaluations. In addition, the values for taper and the evaluations are shown further below.

As can be understood from the drawing, Sample No. E1according to the first embodiment showed the smallest ellipticity value at all of the measuring positions and exhibited an extremely high level of roundness. In addition, the amount of taper was very small, and also, the diameter varied very little between axially spaced position.

Sample No. E2had a smaller ellipticity value than Samples No. C1and No. C2as explained below. No. E2had a sufficiently high level of roundness. On the other hand, the amount of taper was relatively larger, and also, the diameter varied between the axially spaced positions by a larger amount. It is assumed that these results were caused because the process was performed with asymmetrical pressing. Also, it is understood that the process conditions used for No. E2were sufficient for a product that is satisfactory for its intended use as long as the level of roundness is high.

On the other hand, while each of Samples No. C1and No. C2had a small amount of taper and the diameter varied little in the axial direction, the loss of roundness was greater than Samples No. E1and No. E2.