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Title:
HEAT TREATMENT PROCESS FOR A DRIVE BELT METAL RING COMPONENT
Document Type and Number:
WIPO Patent Application WO/2010/002240
Kind Code:
A1
Abstract:
The invention provides for a heat treatment process in a manufacturing method for a metal ring (14) for use in a drive belt (1) at least comprising a process step (VIII-O) of oxidising the ring (14) in an oxygen containing atmosphere and a subsequent process step (VIII-N) of nitriding the ring (14) in an ammonia containing atmosphere, which process step (VIII-O) of oxidising the ring (14) the oxygen containing atmosphere is performed at a temperature of more than 450 °C and/or for minutes or more.

Inventors:
PENNINGS BERT (NL)
ALEXANDROV OLEG ALEXANDROVICH (NL)
DERKS MICHEL JOSEPH MARIE (NL)
TRAN MINH-DUC (NL)
Application Number:
PCT/NL2008/050432
Publication Date:
January 07, 2010
Filing Date:
June 30, 2008
Export Citation:
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Assignee:
BOSCH GMBH ROBERT (DE)
PENNINGS BERT (NL)
ALEXANDROV OLEG ALEXANDROVICH (NL)
DERKS MICHEL JOSEPH MARIE (NL)
TRAN MINH-DUC (NL)
International Classes:
F16G5/16; C23C8/10; C23C8/26
Domestic Patent References:
WO2007133062A12007-11-22
WO2007145502A12007-12-21
WO2006054885A12006-05-26
Foreign References:
JP2004043962A2004-02-12
JP2005330565A2005-12-02
JP2006328486A2006-12-07
JPS62235463A1987-10-15
Attorney, Agent or Firm:
PLEVIER, Gabriël Anton Johan (AM Tilburg, NL)
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Claims:
CLAIMS

1. Heat treatment process in a manufacturing method for a metal ring (14) for use in a drive belt (1 ) at least comprising a process step (VIII-O) of oxidising the ring (14) in an oxygen containing atmosphere and a subsequent process step (VIII-N) of nitriding the ring (14) in an ammonia containing atmosphere, characterised in that in the process step (VIII-O) of oxidising the ring (14) the oxygen containing atmosphere is maintained at a temperature of more than 450 °C.

2. Heat treatment process according to claim 1 , characterised in that the process step (VIII-O) of oxidising the ring (14) has a duration of a least 5 minutes. 3. Heat treatment process in a manufacturing method for a metal ring (14) for use in a drive belt (1 ) at least comprising a process step (VIII-O) of oxidising the ring (14) in an oxygen containing atmosphere and a subsequent process step (VIII-N) of nitriding the ring (14) in an ammonia containing atmosphere, characterised in that in the process step (VIII-O) of oxidising the ring (14) has a duration of at least 15 minutes.

4. Heat treatment process according to claim 3, characterised in that in the process step (VIII-O) of oxidising the ring (14) the oxygen containing atmosphere is maintained at a temperature of more than 330 °C.

5. Heat treatment process according to claim 4, characterised in that the process step (VIII-O) of oxidising the ring (14) has a duration between 20 minutes and 90 minutes, and in that the oxygen containing atmosphere is maintained at a temperature between 400 <€ and 575 0C.

6. Heat treatment process according to claim 4, characterised in that the process step (VIII-O) of oxidising the ring (14) has a duration between 30 minutes to 60 minutes and in that the oxygen containing atmosphere is maintained at a temperature between 440 °C and 480 0C.

7. Heat treatment process according to any one of the preceding claims, characterised in that the ring (14) is made from a maraging steel alloy, and in that the process step (VIII-O) of oxidising the ring (14) is preceded by a process step (VIII-A) of aging the ring (14) at a temperature between 430 °C and 500 °C and for 30 minutes or less.

8. Heat treatment process according to claim 5 or 6, characterised in that the ring (14) is made from a maraging steel alloy comprising 17-19 mass-% nickel, 4-6 mass- % molybdenum, 8-18 mass-% cobalt, less than 1 mass-% titanium and balance iron, and in that the heat treatment process in the metal ring manufacturing method consists exclusively of the said oxidising and nitriding process steps (VIII-O; VIII-N). 9. Heat treatment process according to any one of the preceding claims, characterised in that the ring (14) has a thickness between 0.150 mm and 0.250 mm.

10. Heat treatment process according to any one of the preceding claims, characterised in that in between the said oxidising and nitriding process steps (VIII-O; VIII-N) the intermediate-product ring (14) has a hardness value between 400 HV1 .0 and 500 HV1.0.

Description:
HEAT TREATMENT PROCESS FOR A DRIVE BELT METAL RING COMPONENT

The present invention relates to a manufacturing method for a metal ring to be used in a drive belt, more in particular a heat treatment process part thereof as defined by the preamble of the following claim 1 . The drive belt is typically used as the means for power transmission between two adjustable pulleys of the well-known continuously variable transmission that is mainly applied in motor vehicles.

One well known type of drive belt is described in detail in EP-A-1 403 551 and is composed of a multitude of relatively thin transverse metal elements that are slideably incorporated on two laminated endless tensile means that are each composed of a set of mutually nested flat metal rings, alternatively denoted bands or hoops. Such rings are produced from a precipitation hardening steel, such as a maraging steel, that combines a/o the properties of great tensile strength and good resistance against tensile stress and bending fatigue with a relatively favourable possibility to process the steel from sheet material towards the desired shape and material characteristics of the end-product rings, which, preferably, should not vary along the circumference of the rings. The present invention in particular relates to the range of maraging steel alloys having a basic composition with 17 to 19 mass-% nickel, 4 to 6 mass-% molybdenum, 8 to 18 mass-% cobalt and with balance iron, possibly with some, i.e. less than 1 mass-%, titanium added.

These desired material characteristics comprise a fair hardness of the ring core material for combining the properties of a great tensile strength together with sufficient elasticity to allow longitudinal bending of the ring and an extremely hard outer surface layer of the ring to provide wear resistance. Additionally, the outer surface layer is provided with a residual compressive stress to provide a high resistance against metal fatigue, which is a significant feature of the rings because of the numerous numbers of load and bending cycles the rings are subjected to during the service lifetime of the belt.

The basics of the known manufacturing method for such rings have become well known in the art and are, for example, described in the Japanese publication JP- A-2004-043962. The rings are formed out of a sheet base material, which is bent and welded into a cylindrical shape, or tube, which is heat treated, i.e. annealed, to restore the original material properties, i.e. to largely remove changes therein that were introduced by the bending and welding. The tube is then cut into a number of hoops, which are subsequently rolled and elongated to a required thickness, which is typically about 0.185 mm in the end product. After rolling the hoops are usually referred to as rings or bands. The rings are subjected to a further annealing step to remove the internal stresses introduced during rolling. Thereafter, the rings are calibrated, i.e. they are mounted around two rotating rollers and stretched to a predefined circumference length.

Finally, the rings are subjected to a heat treatment process that includes three separate steps, wherein in each step a processing gas of different composition is applied. First of all the rings are precipitation hardened, i.e. aged, in an atmosphere predominantly composed of nitrogen (N 2 ), secondly the rings are oxidised in an atmosphere containing a substantial amount of oxygen (O 2 ), e.g. ambient air, and thirdly the rings are nitrided, i.e. case hardened, in an atmosphere containing a substantial amount of Ammonia (NH 3 ).

A common, long standing desire and general aim in the further development and/or advancement of the above ring manufacturing method has been to enhance the effectiveness thereof, not only in terms of improving the material properties realised in the end-product ring in view of the drive belt application thereof, but also in terms of providing such favourable material properties in the most cost effective way possible. According to the present invention, in particular this latter aspect of cost effectiveness can be improved upon in the known ring manufacturing method. To this end the present invention provides for a novel specification of the heat treatment process part of the overall manufacturing method for the metal ring component of a drive belt. According to the invention, the above known heat treatment process may be significantly improved and even simplified by increasing the intensity of the step of ring oxidising, while simultaneously shortening the preceding step of ring aging. Hereby, the ring oxidising intensity is determined by either one or both of the temperature applied therein and the duration thereof.

In the above respect it is noted that, where according to JP-A-2004-043962 the duration and temperature of the ring oxidising is to be strictly limited to less than 15 minutes and 450 degrees Centigrade respectively, such limitations are presently considered to be causally linked to the ring aging as specified by JP-A-2004-043962. According to the present invention, the substantial loss of tensile strength of the end- product ring reported in JP-A-2004-043962 is likely to be caused by the phenomenon of over-aging. Over-aging occurs when the intermetallic precipitates, which are formed in the maraging steel matrix of the rings and which continue to grow for as long as the material is maintained at a sufficiently high temperature, i.e. also during ring nitriding and high-temperature ring oxidising, grow larger than a certain, critical size. Therefore, it is presently considered that, although the overall duration of the ring heat treatment should indeed not exceed a certain limit, at least not in relation to the processing temperature(s) applied therein, it may to a certain extent be shifted between the respective durations of the separate heat treatment process steps of ring aging, ring oxidising and ring nitriding.

The advantage of the novel process specification according to the invention, i.e. the freedom to shift between the said respective durations, is that the processing capacities of the respective heat treatment process steps may be more freely and possibly even equally distributed such that, as a result, a favourably more efficient and cost effective overall ring manufacturing method may be realised. These respective capacities are, for example, determined by the size of the oven or oven chamber that is respectively required for each step in the manufacturing chain or line.

In a preferred embodiment of the invention, the known separate step of ring aging, which preceded the step of ring oxidising in the conventional heat treatment process of the drive belt ring component, is omitted altogether. Obviously, such would substantially improve the cost effectiveness of the overall ring manufacturing method, because the separate aging oven or aging chamber is not required anymore. In this latter respect it is noted that, based on the above considerations underlying the present invention, one would expect that a similar result may also be obtained by increasing the duration of the step of ring nitriding while simultaneously decreasing the ammonia concentration in the processing gas, i.e. by providing that the aging process and the nitriding process may be completed in the same time, i.e. simultaneously in a single step of combined ring aging and ring nitriding. However, applicant has discovered that with such specific set-up of the ring manufacturing method the material properties for the end-product ring will not be optimal. Unexpectedly, the phenomenon of discontinuous precipitation was found to occur in such manufacturing set-up, which phenomenon is likely to reduce the (metal) fatigue strength of the rings.

For reliably avoiding such phenomenon an additional criterion is presently introduced. According to the invention such additional criterion entails that in the heat treatment process after the step of oxidising has been be completed and before the step of ring nitriding is commenced the (core) hardness value of the intermediate- product ring amounts to 400 HV1.0 or more. More preferably, for additionally reliably avoiding the phenomenon of over-aging, such (core) hardness value of the intermediate-product ring amounts to 500 HV1 .0 at most.

The above-described basic features of the invention will now be elucidated by way of example with reference to the accompanying figures. Figure 1 is a schematic illustration of the drive belt the present invention relates to and of the transmission in which such belt is applied.

Figure 2 is an illustration of the manner in which a laminated tensile means and a transverse element are mutually oriented within the drive belt.

Figure 3 figuratively represents the known manufacturing method of a metal ring applied in the endless tensile means of the drive belt.

Figure 4 illustrates the heat treatment process part of the above manufacturing method that is optimised in accordance with the invention.

Figure 5 illustrates a novel setup of the said heat treatment process part according to the invention. Figure 6 is photograph of an enlarged cross section of the ring, which ring incorporates a nitrided surface layer that suffers from the phenomenon of discontinuous precipitation.

In the drawings, the separate process steps of the known and the new manufacturing method are indicated by way of Roman numerals. Figure 1 shows schematically a continuous variable transmission (CVT) with a drive belt 1 wrapped around two pulleys 1 and 2, which belt 1 is made up of a laminated tensile means 2 in the form two sets of mutually nested endless thin and flat metal rings 14, alternatively denoted bands 14 and an essentially continuous array of transverse elements 3, alternatively denoted transverse elements 3, which are mounted along the circumference of the tensile means 2 and which may freely slide there along. Such a continuous variable transmission is known per se.

Figure 2 depicts a front view of a transverse element 3 and a cross section of the laminated tensile means 2. The transverse element 3 laterally shows a side face 6 by which it rests against the conical face of one sheave of either a drive or a driven pulley. The rings 14 of the tensile means 2 are produced of high quality steel, e.g. maraging steel. A typical thickness of the rings 14 ranges from 0.15 to 0.25 mm, a typical width thereof ranges from 8 to 35 mm and a typical circumference length thereof ranges from 500 to 1000 mm.

Figure 3 illustrates the presently relevant part of the known manufacturing method for the above described belt 1 , in particular for the rings 14 thereof, as is practised since the early years of metal push belt production. In a first process step I a sheet of base material 1 1 is bent into a cylindrical shape, whereby the sheet ends 12 that meet each other are welded together in a second process step Il to form a tube 13. In a third step III of the process the tube 13 is annealed. Thereafter, in a fourth process step IV the tube 13 is cut into a number of hoops 14, which are subsequently -process step five V- rolled and elongated to a thickness. After rolling the hoops 14 are usually referred to as rings 14 or bands 14. The rings 14 are subjected to a further annealing process step Vl to remove the internal stresses introduced during rolling. Thereafter, in a seventh process step VII, the rings 14 are calibrated, i.e. they are mounted around two rotating rollers and stretched to a predefined circumference length. In this seventh process step VII, also an internal stress distribution is imposed on the rings 14, which defines the so-called curling radius of the respective ring 14.

Finally, in the eighth step VIII of the known manufacturing method, which is illustrated in more in detail in the following figure 4, the rings 14 are heat treated in three separate steps, wherein in each step a processing gas of different composition is applied. Firstly, the rings 14 are precipitation hardened, i.e. aged, (step VIII-A) in an atmosphere predominantly composed of nitrogen (N 2 ), secondly the rings are oxidised (step VIII-O) in ambient air, i.e. an atmosphere containing a substantial amount of oxygen (O 2 ), and thirdly the rings are nitrided, i.e. case hardened, (step VIII-N) in an atmosphere containing a substantial amount of Ammonia (NH 3 ).

In the known process step of ring ageing VIII-A the rings 14 are heated to a temperature of 430 and 480 0 C for several hours, which process duration is known to depend mainly on the composition of the ring material used. For example, increasing the mass-% content of cobalt (Co) in the basic maraging steel alloy composition is known to significantly speed up the nucleating (i.e. forming) of the iron-molybdenum (Fe x MOy) and nickel-molybdenum (Ni x Mo y ) precipitates. Indeed, it is known that a process duration of less than 2 hours, even down to 45 to 90 minutes can be realised by including at least 8, up to 18 mass-% cobalt in the maraging steel alloy. During the ring aging step VIII-A the hardness of the ring material increases as the intermetallic precipitates continue to form and grow in the steel matrix. It is also known to speed op the ring ageing step VIII-A by applying a process temperature of up to 500 °C.

In the known process step of ring oxidising VIII-O the rings 14 are heated to a temperature of between 330 and 450 °C for 5 to 15 minutes. During the ring oxidising step VIII-O the surface of the ring 14 is cleaned and prepared or "activated" for the nitriding process.

In the known process step of ring nitriding VIII-N the rings 14 are heated to a temperature of between 420 and 500 0 C for 35 to 80 minutes, which process duration is known to depend mainly on the process temperature. During the ring nitriding step VIII-N the rings 14 are provided with a nitrided diffusion zone or surface layer of typically 25 to 35 microns of extreme hardness and provided with a considerable compressive stress.

From a number of thus processed rings 14 the tensile means 2 is formed by nesting a number of purposely selected rings 14, i.e. concentrically placing the rings 14 one around the other, as is also indicated in figure 3, whereby only a small positive or negative play is allowed between the adjacent rings 14 of the tensile means 2.

According to the invention, the above known heat treatment process may be significantly improved by increasing the duration and/or the temperature of the ring oxidising step VIII-O, whereby the duration of the preceding ring aging step VIII-N may be decreased considerably as required by the maraging steel alloy composition of the rings 14. Thus, in accordance with the present invention, the rings 14 are oxidised (process step VIII-O) at a temperature of more than 450 °C and/or for 15 minutes or more. In relation to the above-mentioned maraging steel alloy composition the preceding ring aging step VIII-A is thereby favourably reduced to 30 minutes or less.

In a preferred embodiment of the invention, the rings 14 are oxidised (process step VIII-O) at a temperature of between 400 and 575 0 C for 20 to 90 minutes. According to the invention, specifically in relation to the above-mentioned maraging steel alloy composition, the preceding, separate process step of ring aging VIII-A may in this latter case be completely and favourably omitted from the heat treatment process of the drive belt ring component, as is schematically illustrated in figure 5. In this latter novel, simplified heat treatment process according to the invention, the rings 14 are preferably oxidised (process step VIII-O) at a temperature of between 440 and 480 0 C for 30 to 60 minutes.

Finally, applicant has discovered that the combined intensity of the separate process steps of ring aging step VIII-A (if included in the heat treatment process at all) and of ring oxidising VIII-O should preferably satisfy a minimum requirement. Otherwise, the phenomenon of discontinuous precipitation might occur in the subse- quent process step of ring nitriding VIII-N, which phenomenon is illustrated in figure 6 and which is likely to reduce the (metal) fatigue strength of the end-product rings 14.

Figure 6 is a photograph of an enlarged cross-section of a ring 14 that incorporates the nitrided surface layer NSL. It can be seen therein that near the outer surface of the ring 14 the material has a relatively course structure that was related to the said discontinuous precipitation DP, meaning that locally the alloying elements of the maraging steel, such as the molybdenum, have formed nitrides on the grain boundaries, instead of the desired intermetallic compounds.

In particular, it has been found that the process step of ring nitriding VIII-N should preferably only be performed, i.e. started, after the (core) hardness value of the intermediate-product ring 14 has reached a value of at least 400 HV1.0 in the preceding ring oxidising step VIII-O, in which case the said discontinuous precipitation DP does not occur. On the over hand, the said (core) hardness value of the intermediate-product ring should preferably not exceed a value of 500 HV1.0 to avoid the said over-aging in the subsequent ring nitriding step VIII-N. The invention, apart from the preceding description and all details of the drawing that may not be described, however immediately and unambiguously evident to a person skilled in the art, further relates to all details of the following set of claims.