Metal working – Method of mechanical manufacture – Obtaining plural product pieces from unitary workpiece
Reexamination Certificate
2000-05-26
2003-10-14
Vidovich, Gregory (Department: 3726)
Metal working
Method of mechanical manufacture
Obtaining plural product pieces from unitary workpiece
C148S228000, C148S230000
Reexamination Certificate
active
06631542
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a laminated ring for use as a belt in transmitting power in a continuously variable transmission, and a heat treatment apparatus for use in such a method.
2. Description of the Related Art
Continuously variable transmissions have a power transmitting belt trained around a pair of pulleys. The power transmitting belt comprises a laminated ring composed of a plurality of rings and mounted on and held by an element of predetermined shape.
The laminated ring is straight in shape when traveling between the pulleys, and curved in shape when running along the pulleys. The laminated ring undergoes severe bending deformations due to repetitive cycles of the straight and curved states. Therefore, the laminated ring is required to have a mechanical strength large enough to withstand the severe bending deformations.
One known material capable withstanding such severe bending deformations is maraging steel. The maraging steel is a low-carbon steel containing 17 to 19% of Ni, and Co, Mo, Ti, etc. When the maraging steel is heated to a suitable temperature after being subjected to a solution treatment, it causes age hardening in a martensitic state, resulting in an ultra-high strength steel that is highly strong and highly tough. The maraging steel is highly suitable for use as the material of the laminated ring.
The laminated ring has heretofore been manufactured according to the following process: The ends of a thin sheet of maraging steel that is an ultra-high strength steel are welded to each other, producing a cylindrical drum. The cylindrical drum is subjected to a first solution treatment in order to uniformize the hardness that has been partly increased due to the heat applied when the thin sheet of maraging steel was welded. Then, the cylindrical drum is severed into rings of certain width, which are rolled to a predetermined length. The rings are then subjected to a second solution treatment in order to recrystallize the rolled structure for restoring the metal crystal grain configuration that has been deformed by the rolling process. The rings that have been subjected to the second solution treatment are corrected into predetermined circumferential lengths, and aged and nitrided for increased hardness. The rings of slightly different circumferential lengths are fitted together into a laminated ring.
In the above process of manufacturing the laminated ring, the rings are aged to cause the maraging steel to develop aged hardness thereby to impart a high strength to the rings. Specifically, the rings are placed in an aging chamber, and the interior of the aging chamber is heated to a predetermined aging temperature, e.g., 480 to 520° C., and thereafter is held at the aging temperature for a predetermined period of time. The aging is performed in an inactive gas atmosphere such as nitrogen or the like in order to prevent an oxide layer from being formed on the a surface of the rings.
When the rings are aged, an intermetallic compound is separated out in the metal structure of the maraging steel, resulting in aged hardness for high strength and high toughness. The rings are then case-hardened because they need to have wear resistance and fatigue-resistant strength for use as the laminated ring.
The rings are case-hardened by nitriding the rings to form a nitride layer in their surface. The rings are nitrided by a gas nitriding process or a gas soft-nitriding process.
According to the gas nitriding process or the gas soft-nitriding process, nitrogen produced when ammonia is decomposed penetrates the metal structure of the maraging steel for thereby producing a nitride layer in the surface of the rings to harden the same for increased wear resistance and increased fatigue-resistant strength. If the rings are nitrided, then the rings are repeatedly heated to respective temperatures in the aging chamber and the nitriding chamber, kept at the temperatures for respective periods of time, and then cooled. Consequently, periods of time required for the aging and nitriding processes are increased, resulting in an increase in the manufacturing cost.
One solution to the above problem would be to successively age and nitride the rings in one processing chamber. Specifically, the rings are placed in the processing chamber, and the interior of the processing chamber is heated to an aging temperature and kept at the aging chamber for a predetermined period of time to age the rings. Thereafter, the interior of the processing chamber is not cooled, but the atmosphere in the processing chamber is replaced with an ammonia gas atmosphere or a mixed atmosphere of ammonia and RX gases, and the interior of the processing chamber is kept at a predetermined nitriding temperature for a predetermined period of time to effect gas nitriding or gas soft-nitriding on the rings. However, the above proposed process is disadvantageous in that the atmosphere changed for the nitriding process after the aging process tends to be not stabilized easily.
Another solution would be to simultaneously age and nitride the rings in one processing chamber. Specifically, the heating of the rings for the gas nitriding or gas soft-nitriding doubles as the heating of the rings for the aging. However, this process is problematic in that it is difficult to adjust the atmosphere in order to achieve an appropriate aging hardness and a nitride layer of a suitable depth.
It is known in the above process of manufacturing the laminated ring that the aging process causes the rings to shrink their volume thereof to reduce their circumferential lengths. It is also known that when the nitriding process is carried out, the volume of the rings is increased to increase their circumferential lengths because nitrogen penetrates the surface of the rings to form the nitride layer therein.
As a result, the rings which have been aged and nitrided suffer errors on the corrected circumferential lengths due to the reduction in the circumferential lengths in the aging process and the increase in the circumferential lengths in the nitriding process. In view of such errors, the rings are measured for their circumferential lengths after the aging process and the nitriding process, and classified according to the magnitude of errors of the measured circumferential lengths with respect to the corrected circumferential lengths.
Then, those of the classified rings which have such circumferential lengths that they can be fitted together are chosen and combined into a laminated ring. In order to allow easy selection of the classified rings which have such circumferential lengths that they can be fitted together, the errors of the circumferential lengths after the aging process and the nitriding process with respect to the corrected circumferential lengths should preferably be kept in the range of ±80 &mgr;m.
According to the conventional manufacturing process, however, the errors of the circumferential lengths of rings after they have been aged and nitrided with respect to the corrected circumferential lengths occasionally become unduly large. After such rings are classified, they are not used and are left to stand for a long period of time until they encounter rings whose circumferential lengths are suitable for being fitted together.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method of manufacturing a laminated ring by processing rings in reduced periods of time according to an aging process and a nitriding process, the nitriding process being carried out in a stable atmosphere.
Another object of the present invention is to provide a method of manufacturing a laminated ring of excellent dimensional stability by allowing easy selection of rings which have been aged and nitrided and whose circumferential lengths are suitable for being fitted together.
Still another object of the present invention is to provide a heat treatment apparatus that is suitable for use in such a method of manufacturing a laminate
Imai Hitoshi
Karasawa Hitoshi
Nakajima Katsuyuki
Omgba Essama
Vidovich Gregory
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