The present disclosure relates to a manufacturing method for an endless, thin and flexible metal band, which band is typically incorporated in a drive belt for power transmission between two adjustable pulleys of the well-known continuously variable transmission or CVT applied in motor vehicles. At least in relation to its application in the drive belt such band is also referred to as a metal ring. Such a drive belt and the metal ring applied therein are generally known, e.g. from EP-A-1 403 551 . In this known type of drive belt, which is usually referred to as a pushbelt, a number of such metal rings are incorporated in at least one, but typically two laminated, i.e. mutually concentrically arranged sets thereof. The known pushbelt further comprises a number of transverse metal elements that are slidably mounted on such ring sets.

At least in the said pushbelt application thereof, the metal ring is typically produced from a maraging steel that is a/o subjected to the heat treatments of aging or precipitation hardening and nitriding or case hardening to provide the metal ring with extraordinary fatigue strength and wear resistance properties. In particular, a surface layer of the metal ring is strengthened by gas-soft nitriding, whereby (interstitial) nitrogen atoms are introduced in the outer layers of the atomic matrix of the maraging steel by diffusion. The gas-soft nitriding process entails keeping the metal ring at a temperature of several hundred degrees Centigrade in an ammonia gas containing oven chamber. In the gas-soft nitriding the ammonia gas dissociates at the surface of the metal ring into hydrogen gas and nitrogen atoms that can enter into the atomic matrix of the metal ring by diffusion.

The process steps of the general manufacturing method of such metal rings have become well-known in the art and are, for instance, described in the international patent application publication WO2013/002633. In this publication it is mentioned that, although the efficiency of the gas-soft nitriding process can be increased by increasing the intensity thereof, in particular in terms of the ammonia concentration in and/or the temperature of the oven chamber, it must be avoided that a layer of iron-nitrides compounds, such as Fe ₄ N, is formed on the surface of the metal ring at too high a process intensity. This so-called compound layer is namely known to have a detrimental effect on the fatigue strength of the metal ring. The presence of the compound layer on the surface of the metal ring can be linked to a discoloration of the ring surface after etching. Thus, the process settings of the gas- soft nitriding process can -in principle- be determined by means of a relatively straight forward trial-and-error approach based on the (absence of the) said discoloration after etching.

Figure 7 of WO2013/002633 provides an empirically determined graph of possible iron-nitride compound layer formation in dependence on the process settings/intensity of the gas-soft nitriding process, which graph is presently included as figure 5. This type of graph is known as a Lehrer diagram.

In the Lehrer diagram, the Y-axis represents the equilibrium constant K _N of the chemical reaction occurring between ammonia (NH3), nitrogen (N2) and hydrogen (H2) molecules in the nitriding gas atmosphere, namely:

2NH ₃ N ₂ + 3H ₂ (1 ) which equilibrium constant K _N is thus calculated as follows:

K _N = (p[NH ₃ ])/(p[H ₂ f ⁵ ) (2) wherein p[X] represent the (partial) pressure in the process atmosphere of X. Within the teaching of WO2013/002633, the process settings of the metal ring nitriding process will be determined close to, but on the safe side of (i.e. below) the boundary line of the compound layer formation in the Lehrer diagram. An equilibrium constant K _N of 4 bar ^"½ at a temperature T of 500 degree Centigrade is specifically mentioned in WO2013/002633 in this respect.

However, following experimental investigations underlying the present disclosure, it was unexpectedly discovered that -in contrast with the known teaching- by applying process settings in the gas-soft nitriding process that are increasingly further away from the said boundary line in the Lehrer diagram, the fatigue strength of the metal ring continues to improve. Only when the applied process settings are considerably removed from the said boundary line, the fatigue strength appears to be at an optimum, since no (further) improvement is then observed anymore.

The above observations caused applicant to conduct detailed investigations of several metal rings that had been nitrided with various process settings close to, but below the said boundary line in the Lehrer diagram. As part of these investigations, the nitrided metal rings were analysed by means of X-ray diffractometry (XRD) that demonstrated the presence of iron-nitrides, even though with the etching method no discoloration of the ring surface was observed. In fact, the iron-nitrides appeared to be present not in a layer on the ring surface, but in the form of small sized, in particular nanometre to sub-micrometer scale particles in the atomic matrix of the metal rings. Furthermore, the detected quantity and size of the iron-nitride particles could be (inversely) linked to the fatigue strength of the metal rings.

Based on the above observations and investigations it is presently hypothesized that the (formation of the) iron-nitride compound layer is preceded by the said small-sized iron-nitride particles forming and growing in the atomic matrix and, in particular, coinciding with surface defects of the metal ring. At these surface defects these interstitial iron-nitride particles increase the (local) brittleness of the ring material thus (locally) making the metal ring more susceptible to the initiation and subsequent growth of a fatigue fracture, i.e. thus reducing the resistance against fatigue fracture of the metal ring as a whole. Therefore, according to the present disclosure, the determining criterion for selecting the process settings of the metal ring nitriding process should not be the Lehrer diagram and/or the formation of the compound layer, but rather a maximum quantity and size of the interstitial iron-nitride particles that precede such compound layer formation. For the practical application thereof, this latter criterion has been empirically quantified in applying a minimum difference of 4 bar ^"½ between the said boundary line of the Lehrer diagram in figure 5 and the equilibrium constant K _N of the nitriding reaction that is actually applied in the gas-soft nitriding process at a given process temperature. For example, where according to figure 5, at the preferred nitriding process temperature of 500 °C, no iron-nitrides will be formed if the equilibrium constant K _N of the nitriding reaction is set to less than (approximately) 6 bar ^"½ , according to the present disclosure the optimum ring fatigue strength is realised only at a value of 2 bar ^"½ (i.e. 6 minus 4) or less. Alternatively, in combination with a preferred nominal value for the equilibrium constant K _N of 6 bar ^"½ , optimum ring fatigue strength is realised at a process temperature of (approximately) 480 °C in accordance with the present disclosure.

In a preferred embodiment of the metal ring nitriding process in accordance with the present disclosure, the considerations of providing the rings with an optimum fatigue strength in a nitriding gas atmosphere with a comfortably controllable composition and temperature, the equilibrium constant K _N of the nitriding reaction is controlled to 6 ± 2 bar ^"½ and the temperature T is controlled to 470 ± 5 °C. The (required) process duration of such metal ring nitriding process is given by the time it takes to provide the nitrided surface layer of the rings with a required thickness. Typically, such required thickness amounts to between 0.030 ± 0.005 mm only, which thickness will normally be reached in 60 ± 12 minutes in the above-defined, controlled process gas atmosphere.

The above-described basic features of the present disclosure will now be elucidated by way of example with reference to the accompanying figures.

Figure 1 provides a schematic perspective view of a continuously variable transmission with a drive belt running over two pulleys.

Figure 2 is a schematic illustration of a part of the known drive belt shown in perspective, which part includes two ring sets, each including a number of concentrically arranged metal rings, as well as a plurality of transverse members.

Figure 3 diagrammatically resents an overview of a part of the known drive belt manufacturing method, including a process step of gas-soft nitriding of the metal ring component thereof.

Figure 4 shows the known process step of gas-soft nitriding in more detail.

Figure 5 is a graph indicating the possible iron-nitride compound layer formation in the process step of gas-soft nitriding in dependence on the process settings applied therein.

Figure 6 is a graph illustrating a relationship between a fatigue characteristic of the metal ring and an amount of iron-nitride formed at its surface in the process step of gas-soft nitriding.

Figure 7 is the graph of figure 5, however, additionally indicating the technical insight underlying the present disclosure.

Figure 1 schematically shows the central parts of a continuously variable transmission or CVT that is commonly applied in the drive line of motor vehicles between the engine and the drive wheels thereof. The transmission comprises two pulleys 1 , 2 that are each provided with two conical pulley discs 4, 5, where between a predominantly V-shaped pulley groove is defined and whereof one disc 4 is axially moveable along a respective pulley shaft 6, 7 over which it is placed. A drive belt 3 is wrapped around the pulleys 1 , 2, while being held by friction, i.e. clamped between the pulley discs 4, 5 thereof, for transmitting a torque T and an accompanying rotational movement ω from the one pulley 1 , 2 to the other 2, 1 . At the same time, the running radii R of the drive belt 3 between the discs 4, 5 of the respective pulleys 1 , 2 determine the (speed) ratio of the CVT, i.e. the ratio between the rotational speeds of the respective pulleys 1 , 2. An example of a known drive belt 3 is shown in more detail figure 2 in a section thereof, which belt 3 is shown to incorporate two endless carriers 31 , or ring sets 31 , that are each composed of a number of concentrically arranged, i.e. mutually nested individual metal rings 32. The drive belt 3 further comprises a plurality of plate-like transverse members 30 that are in contact with and held together by the ring sets 31 . The transverse members 30 take-up the clamping force exerted between the discs 4, 5 of each pulley 1 , 2 via pulley contact faces 33 that are provided on either lateral side thereof. These pulley contact faces 33 are mutually diverging in radial outward direction to essentially match the V-angle defined between the conically-shaped pulley discs 4, 5, i.e. of the V-shaped pulley groove of the pulleys 1 , 2. A so-called rocking edge 34 of each transverse member 30 represents the transition between a radially outer part of constant thickness and a tapered radial inner part thereof. This rocking edge 34 and tapered shape of the transverse members 30 is what allows the drive belt 3 to follow a smoothly curved trajectory.

During operation in the CVT the drive belt 3 and in particular the metal rings 32 thereof are subjected to a cyclically varying tensile and bending stresses, i.e. a fatigue load. Typically, the resistance against fatiguing or fatigue strength of the metal ring 32 thus determines the functional life span of the drive belt 3 at a given torque T to be transmitted thereby. Therefore, it has been a long standing general aim in the development of the drive belt manufacturing method to realise a required ring fatigue strength at a minimum combined material and processing cost.

Figure 3 illustrates a relevant part of the known manufacturing method for the ring set 31 of the drive belt 3, as it is practised since the early years of drive belt production for, in particular, automotive application. In figure 3, the separate process steps are indicated by way of Roman numerals.

In a first process step I a thin sheet or plate 1 1 of a maraging steel base material having a thickness of around 0.4 mm is bend into a cylindrical shape and the meeting plate ends 12 are welded together in a second process step II to form a hollow cylinder or 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 annular hoops 14, which are subsequently -process step five V- rolled to reduce the thickness thereof to, typically, 0.2 mm, while being elongated. After rolling, the hoops 14 are usually referred to as a metal ring 32.

The metal ring 32 is subjected to a further, i.e. ring annealing process step VI for removing the work hardening effect of the previous rolling process step by recovery and re-crystallization of the ring material at a temperature considerably above 600 degree Celsius, e.g. about 800 degree Celsius. Thereafter, in a seventh process step VII, the metal ring 32 is calibrated by mounting it around two rotating rollers and stretching it to a predefined circumference length by forcing the said rollers apart. In this seventh process step VII, also internal stresses are imposed on the metal ring 32.

Thereafter, the metal ring 32 is heat-treated in two separate process steps, namely an eighth process step VIII of ageing or bulk precipitation hardening and a ninth process step IX of nitriding or case hardening. More in particular, both such heat-treatments involve heating the metal ring 32 in an oven chamber 50 containing a controlled process gas atmosphere. In case of ageing (process step VIII) such process gas atmosphere is typically composed of nitrogen gas and some, e.g. up to 5 volume-%, hydrogen gas. In case of nitriding (process step IX) such process gas atmosphere contains nitrogen, hydrogen and ammonia gas, which ammonia gas (in part) decomposes at the surface of the metal ring 32 into hydrogen gas and nitrogen atoms. These nitrogen atoms enter, i.e. diffuse into the atomic matrix of the metal ring 32 to provide a wear resistant and hardened nitrided surface layer.

Both these heat-treatments of aging and nitriding typically occur within the temperature range from 400 degrees Celsius to 500 degrees Celsius and can each last for about 45 to over 120 minutes in dependence on the base material (maraging steel alloy composition) of the metal ring 32, as well as on the mechanical properties desired therefor. In this latter respect it is remarked that, typically, it is aimed at a ring core hardness value of 520 HV1 .0 or more, a ring surface hardness value of 875 HV0.1 or more and at a thickness of the nitrided surface layer, alternatively denoted nitrogen diffusion zone, in the range from 25 to 35 micron.

Finally, the ring set 31 is formed by radially stacking, i.e. concentrically nesting, a number of thus formed and processed metal rings 32, as is further indicated in figure 3 in the last depicted, i.e. eleventh process step XI. Obviously, the metal rings 32 of the ring set 31 have to be suitably dimensioned there for, e.g. have to differ slightly in circumference length to allow the metal rings 32 to be fitted one around the other. To this end the subsequent metal rings 32 of the ring set 31 are typically purposively selected in a prior, i.e. in a tenth process step X, from a stock of metal rings 32 of different, but known circumference length.

As a general remark, it is noted that the overall drive belt 3 manufacturing method described hereinabove, serves merely as an example. Several minor and even major modifications thereof are known already. For example, it is known to perform the heat treatments of ageing and nitriding in/as a single, combined process step, i.e. to perform the above eighth and ninth process steps VIII, IX simultaneously.

In figure 4 the ninth process step IX of nitriding is illustrated in somewhat more detail. The oven chamber 50 can be accessed via doors 54 for charging and discharging of metal rings 32 to and from such chamber 50. Furthermore, the process gas atmosphere in the oven chamber 50 is controlled by means of the regulator valves 51 , 52 and 56 that respectively control a gas flow of hydrogen, nitrogen and ammonia to such chamber 50. The temperature inside the oven chamber 50, i.e. the temperature of the process gas atmosphere, is -in this embodiment example- controlled by means of an electric heating coil 55. A discharge line 53 is provided allow any excess process gas to escape from the oven chamber 50.

It is known that during the nitriding of the metal ring 32 (process step IX) a so- called compound layer of iron-nitrides can be formed on the outer surface thereof in dependence on the process temperature T and the process gas atmosphere composition. Figure 5 provides an empirically determined graph of possible iron- nitride compound layer formation on maraging steel in dependence on those process settings of the gas-soft nitriding process, whereof the process gas atmosphere composition is represented by the equilibrium constant K _N of the reversible ammonia decomposition reaction:

2NH ₃ N ₂ + 3H ₂ (1 )

In figure 5 a relatively clearly defined boundary line marks the transition between the said nitriding process settings T, K _N with or without the formation of the compound layer. Since the compound layer is known to be highly detrimental to the fatigue strength of the metal ring 32, the nitriding process settings T, K _N are in practice always selected from just below the said boundary line. For example, an equilibrium constant K _N of 4 bar ^"½ and a temperature T of 500 degree Centigrade is specifically mentioned in WO2013/002633 in this respect, which combination of process settings is marked "x" in the graph of figure 5.

Figure 6 provides a graph of experimental data underlying the present disclosure that links a characteristic SR that is representative to the fatigue strength of the metal rings 32, i.e. that is quantified as the percentage of specimens of the metal rings 32 that survive a particular fatigue test, in relation to a relative amount C of iron-nitride (Fe4N) measured near the surface of the metal ring 32 with the XRD technique. In figure 6, the said relative amount C of iron-nitride (Fe4N) is expressed as a percentage of the absolute amount of iron-nitride (Fe4N) that is measured on a metal ring 32 that has been nitrided (process step IX) with process settings T, K _N on the boundary line of compound layer formation, which latter, absolute amount thus represent the 100% value of the said relative amount C.

Figure 6 reflects the present observation that even if the nitriding process settings T, K _N are set such that no compound layer is formed on the surface of the metal ring 32, iron-nitride molecules can still be detected by XRD somehow. Apparently, in these circumstances, the iron-nitride is not present as a fully or partially formed surface layer, but as small sized particles in the atomic matrix of the metal ring 32 instead. Still, even these iron-nitride particles have a detrimental effect on the fatigue strength of the metal ring 32, as is apparent from the graph of figure 6. As is also apparent from figure 6, when the said relative amount C decreases below 10%, hardly any effect on the said fatigue strength is measured anymore.

Thus, in accordance with the present disclosure, the fatigue strength of the metal ring 32 can be improved by selecting the process settings T, K _N in the nitriding heat treatment (process step IX) at some distance below the said boundary line for compound layer formation in figure 5. More in particular in this latter respect, at a given process temperature T of the nitriding process step IX, the equilibrium constant K _N of the process gas atmosphere thereof should be set at least 4 bar ^"½ below the K _N value defined the said boundary line in the graph of figure 5. This novel requirement for the process settings T, K _N in the nitriding heat treatment (process step IX) is illustrated in the graph of figure 7 by the line L. For comparison, also the said boundary line for compound layer formation of figure 5 and the mark "x" representing the conventionally preferred process settings are indicated in figure 7 as well.

Finally, the cross-hatched area A in figure 7 indicates the preferred range for the nitriding process settings T, K _N in terms of the optimum between the realised ring fatigue strength improvement and duration of the nitriding process step IX. After all the lower the process temperature T and/or the lower the equilibrium constant K _N of the process gas atmosphere are/is, the longer it will take to provide the metal ring 32 with the nitrided surface layer of a required thickness. According to the present disclosure, such preferred range for the nitriding process settings T, K _N is defined by an equilibrium constant K _N of the process gas atmosphere of 6 ± 2 bar ^"½ and a process temperature T of 470 ± 5 °C.