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 endless bands or hoops. Such rings are typically 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, possibly with small amounts of other alloying elements such as less than 1 mass-% titanium and/or of impurities and with balance iron.

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 European patent publication EP-A-1753889. 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 in accordance with EP-A-1753889, the rings are subjected to a heat treatment process that includes both, i.e. that combines, the heat treatments of precipitation hardening, i.e. ageing or core hardening, and of gas soft-nitriding, i.e. of case hardening by insertion of Nitrogen atoms, in an ammonia-containing process gas atmosphere. This known combined process lasts for 45 up to 65 minutes at a temperature of 440 up to 480 degrees Celcius and in an atmosphere containing 10 or more volume-% or more ammonia gas.

The known combined heat treatment process is very cost effective in relation to the more conventional, sequential heat treatment processes that are, for instance, described in EP-A-105573.8, _^ but it also includes the disadvantage that it is more- difficult to control. In particular, it has been found more difficult with the combined heat treatment process to reach the material properties desired for the rings in the drive belt application thereof, at least reliably and/or in mass-production. More in particular, it was found that with the combined heat treatment process the microstructure of the rings at and/or near the outer surface thereof is often sub- optimal in terms of the fatigue strength of the rings.

The present invention aims to improve the standing combined heat treatment process in the sense that the drive belt rings produced thereby have a fatigue strength that at least approximates and preferably even exceeds that of drive belt rings produced according to the sequential heat treatment processes of aging and nitriding.

According to the invention, the above aim can be realised with the improved combined heat treatment process according to claim 1 hereinafter. According to the invention, the temperature in the combined heat treatment process gradually decreases as the combined process proceeds.

By this measure it is realised that in the early stages of the combined heat treatment process, thus when the process temperature is still comparatively high, the ring ageing is comparatively intensive, i.e. occurs at a comparatively high rate, whereas the ring nitriding is hardly influenced at all, i.e. occurs at a more or less steady rate. As the temperature decreases as the combined process proceeds, the ring ageing process slows down and over-ageing is avoided. Moreover, the nitriding of the ring mainly occurs only after already a substantial precipitation hardening of the ring has occurred. The invention relies on the experimental observation by applicant that the thus obtained microstructure provides the ring with significantly higher fatigue strength than what results from the standing combined heat treatment process.

According to the invention, the said aim can also be realised with the improved combined heat treatment process according to claim 7 hereinafter. According to the invention, the ammonia content of the process gas gradually increases as the combined process proceeds.

By this measure it is additionally realised that in the early stages of the combined heat treatment process, thus when the ammonia concentration is still comparatively low, the ring nitriding is comparatively mild, i.e. occurs at a comparatively low rate, whereas the ring ageing is hardly influenced at all, i.e. occurs at a mo_re_ or less constant rate. As the ammonia concentration increases as ^" the combined process proceeds, the nitriding of the ring intensifies, however, at this time already a substantial precipitation hardening of the ring has occurred. The invention again relies on the experimental observation by applicant that the thus obtained microstructure provides the ring with significantly higher fatigue strength than what results from the standing combined heat treatment process. It is hypothesized that in the standing combined heat treatment process, the nitrogen that enters the lattice of the maraging steel alloy can react not only with the alloy elements to form precipitates, but also forms nitride-compounds near the outer surface of the ring that disadvantageously serve as fatigue crack initiators.

Preferably, both the above measures are applied simultaneously to further enhance the effect of the invention of improving the fatigue strength of the drive belt ring component when manufactured with the combined process of ageing and nitriding. In this latter case, during the combined process the temperature of the process gas atmosphere is preferably decreased from 480-520 °C to 400-440 °C and the ammonia gas concentration in the process gas supplied to the ring is preferably increased from 0-6 vol.-% to 4-12 vol.-%, preferably mixed-in with nitrogen gas only.

Preferably also, the decrease in process temperature and/or the increase in ammonia concentration in accordance with the invention (are controlled to) continue(s) for as long as the process lasts, i.e. from start till end thereof. Preferably also, said decrease and/or increase (are controlled to) occur in an essentially linear fashion. Particularly effective process settings in this respect are a linear decrease of the said temperature from 500°C to 420 °C and a linear increase of the said ammonia concentration from 2 vol.-% to 5 vol.-% from the start till the end of the combined heat treatment process.

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 that includes the process step of combined ageing and nitriding.

Figure 4 illustrates the process settings of the temperature T in °C of and the ammonia NH3 concentration in vohjme-% in the process gas- supplied-to-the ring- during the combined ageing and nitriding thereof in accordance with the invention.

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 4 and 5, 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 (see figure 2), 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 11 is bent into a cylindrical shape, whereby the sheet ends

12 that meet each other are welded together in a second process step II to form a tube 13. In a third step III of the process the tube 13 is annealed at a temperature of more than 800 °C in an inert surroundings, e.g. a vacuum or an atmosphere that is largely composed of nitrogen gas N ₂ . 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 VI 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 the rings 14 areJneat treated forJ30 minutes-and-at 460 °C in. a combined-heat treatment-process step of ring core hardening or ageing and of ring case hardening or gas soft-nitriding nitriding. In this example, the process gas atmosphere is predominantly composed of inert gas, i.e. nitrogen gas "N ₂ ", but also comprises 10 volume-% ammonia gas "NH ₃ ".

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 manufacturing method, at least the combined heat treatment process step thereof may be significantly improved by, as illustrated in the figure 4, applying either one or both of the measures of:

- allowing or controlling the temperature of the process gas atmosphere during the combined process to gradually decrease (in the illustrated example starting from 500 °C at the beginning of the combined process and decreasing to 420 °C at the end thereof),

or of:

- allowing or controlling the ammonia concentration of the process gas, which is supplied to the ring 14 in the combined process and is preferably additionally composed of nitrogen gas only, to gradually increase (in the illustrated example starting from 2 vol.-% at the beginning of the combined process and increasing to 5 vol.-% at the end thereof).

According to the invention, this new set-up of the combined ageing and nitriding process provides significantly improved fatigue strength of the thus processed ring, as well as of the ultimately manufactured drive belt. According to the invention, such favourable fatigue strength increase results from the effect that in the early stages of the combined heat treatment process, when the nitride atoms have not yet penetrated the ring material very far, a comparatively rapid, initial precipitation growth is realised. Whereas, in the later stage of the combined heat treatment process exactly the opposite effect is realised to prevent over-aging, while still ensuring a sufficient penetration of the nitrogen atoms into the ring (surface) material.

Preferably, e.g. to prevent over-ageing and/or so-called compound layer formation, the combined heat treatment process lasts between 10 and 60 minutes from start till end.

The invention, apart from the pre^edjng_descrip_tion _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.