The present disclosure relates to an endless and flexible metal band that is used as a ring component in a drive belt for power transmission between two adjustable pulleys of the well-known continuously variable transmission or CVT applied in motor vehicles. In the drive belt a number of such metal rings are incorporated in at least one, but often two laminated, i.e. mutually radially nested sets thereof. The known drive belt further comprises a number of transverse segments that are slidably mounted on such ring set(s) and that are typically made from metal as well.

Maraging steel is typically used as the basic material for the metal rings, because this material provides a great resistance against wear as well as against bending and/or tensile stress fatigue, at least after the appropriate heat treatments thereof including precipitation hardening by aging and surface hardening by nitriding, in particular so-called gas-soft nitriding. The basic alloying elements of maraging steel are iron, nickel, cobalt and molybdenum with possible smaller amounts of titanium, chromium, aluminium, etc. and with balance iron. Although presently not yet broadly applied, other precipitation hardening steel alloys are known as a potential alternative to maraging steel.

In the drive belt application of the metal rings, not only their yield strength, but also surface hardness value and surface compressive stress level are important product characteristics, which determine the load carrying capability and longevity of the drive belt. By the said heat treatments of aging and nitriding, these product characteristics are finally determined. In particular, in nitriding, a surface layer of the metal rings is enriched with nitrogen to realise a surface hardness of 1000 HV0.1 and beyond and a compressive stress of 1000 MPa and beyond. By such nitrided surface layer, the metal rings are provided with an exceptional resistance against wear caused by, for example, the impact and/or the sliding contact of the transverse segments and/or of the transmission pulleys with the ring set during operation of the drive belt in the transmission.

In drive belt manufacturing, it is the current, standard practice to set a required value for the thickness of the nitrided surface layer that a level of wear resistance of the metal rings required to guarantee the service life of the drive belt. The gas-soft nitriding process is then controlled to obtain such nitride layer thickness, in particular by controlling the partial pressures of ammonia and hydrogen in the process gas, the temperature of the process gas and the duration of the nitriding process.

The above conventional setup of the belt manufacturing process, in particular of the metal ring nitriding process part thereof, has proven satisfactory over many years now. However, according to the present disclosure, with the new generation of maraging steel compositions that, over the past decade or so, have been developed for drive belt application and that have only recently been introduced in drive belt series production, this conventional setup is no longer optimal. Such a basic composition includes for example:

- 17 to 19 mass-% nickel (Ni),

- 15 to 17 mass-% cobalt (Co),

- at least 5 mass-%, preferably between 6.5 and 8 mass-% molybdenum (Mo), and with possible smaller amounts of titanium, chromium, aluminium, etc. and balance iron.

In particular, it is an insight underlying the present disclosure that due to the downsizing of the drive belt made possible a/o by these new maraging steel composition, the tensile stress loading of the metal rings became an increasingly relevant parameter in determining the operating limits of the drive belt. According to a further insight underlying the present disclosure, such tensile or yield strength of the metal rings can, in principle, be increased by decreasing the nitride layer thickness of the nitrided surface layer that is relatively brittle.

Based on the above insights, the present disclosure proposes to quantify and control the nitriding process in manufacturing, not in relation to the nitride layer thickness, i.e. the depth of the nitrogen diffusion penetration, but in relation to the nitrogen concentration over depth, i.e. perpendicular distance inward of the ring surface. According to the present disclosure, a relatively high concentration of nitrogen -in comparison with current practice- is favoured close to the ring surface to at least maintain its customary wear resistance, while improving the yield strength thereof. At the same time, the nitrogen concentration deeper in the metal ring, i.e. further away from its surface, is preferably lowered relative to the current practice. The above mentioned, relatively high molybdenum content of the maraging steel is able to realize such high nitrogen content, by the molybdenum atoms having a high affinity towards bonding with the nitrogen atoms close to the ring surface.

In particular according to the present disclosure, the depth range of 5 to 20 micron is considered. Closer to and further away from the ring surface the nitrogen concentration does not seem to vary much anyway, at least not in relation to the composition of the maraging steel. According to the present disclosure, the nitrided surface layer is in this respect quantified by:

- a nitrogen content by weight at 5 micron depth of 0.80 mass-% or more, preferably more than 1.0 mass-%; and/or

- a nitrogen content by weight at 20 micron depth of 0.15 mass-% or less, preferably less than 0.10 mass-%.

Further according to the present disclosure, the nitrided surface layer is additionally or alternatively quantified by an absolute value of the local gradient of the nitrogen content versus depth profile at 10 to 15 micron depth of more than 0.050 mass-%/micron, preferably more than 0.075 mass-%/micron.

According to another aspect of the present disclosure and based on the above, same insights, the nitrided surface layer, i.e. its thickness, can also be optimized along the cross-sectional circumference of the metal rings. In particular, the nitride layer thickness along such circumference is adapted to the surface hardness and/or compressive stress that is locally required from the metal rings, or from an individual metal ring even. For example, it can be advantageous to take into account that the axial side faces of the metal rings are more severely impacted by the said contact with the transverse segments or the pulleys, than the radially inner and outer main faces thereof. On the other hand, the material characteristics at the radially oriented, main faces to a large extend determine the bending fatigue strength of the metal rings, which benefits from a more ductile, less brittle microstructure. Furthermore, as a secondary effect, it can be taken into account that only the radial inner main face of the innermost metal ring of the ring set and the radial outer main face of the outermost metal ring of the ring set arrive in contact with the transverse segments.

Based on these latter insights, the present disclosure proposes to quantify and control the nitriding process in manufacturing such that the nitride layer thickness of the metal ring is largest at its axial side faces and decreases towards the axial middle of both of the radially oriented, main faces of the metal ring for in-between rings of the ring set. Possibly, such decrease in nitride layer thickness can also be applied at only the radially outwardly oriented main face of the innermost metal ring of the ring set and only the radially inwardly oriented main face of the outermost metal ring of the ring set, with the other radially oriented main faces thereof that arrive in contact with the transverse segment during operation, being provided with a nitride surface layer of essentially constant thickness. According to the present disclosure, the nitrided surface layer is in this respect quantified by a thickness at the axial middle of the metal ring of at most 50% of its thickness at the axial side faces of the metal ring. In particular, the nitride layer thickness is 20 micron or more at the axial side faces of the metal ring and 10 micron or less at the axial middle of the radially oriented, main faces of the metal ring. Preferably in this respect, the nitride surface layer is absent in the axial middle of the metal ring.

This particular embodiment of the metal ring is particularly favourable for the drive belt provided with only one ring set, in which case the metal rings applied therein are relatively wide, in particular have a width of 15 mm or more.

It is noted that in manufacturing, i.e. in gas-soft nitriding, the above defined differentiation of the nitride layer thickness according to the present disclosure, can be realised by increasingly shielding the metal ring from the process gas atmosphere from its side faces towards the axial middle thereof. For example, the said middle section of the radially oriented main faces of the metal ring may be covered during nitriding by an impermeable or semi-permeable covering layer. Such covering layer can be applied only temporarily, to be removed after nitriding.

The above aspects of the present disclosure are ideally applied in combination. In particular, even though the nitrided surface layer is relatively thin at the axial middle of (a radially oriented main face of) the metal ring, a sufficient surface hardness is in this case still provided by the high concentration of nitrogen close to the ring surface.

The above-described drive belt, the ring component thereof and its manufacturing method will now be explained further with reference to the drawing figures, whereof:

figure 1 is a schematic illustration of a known transmission incorporating two variable pulleys and a drive belt;

figure 2 illustrates two known drive belt types in cross-section, each provided with a set of metal rings and with a multitude of transverse segments that are slidably mounted on such ring set along its circumference;

figure 3 provides a diagrammatic representation of the presently relevant part of the known manufacturing method of the metal ring component of the drive belt; figure 4 is a graph of the nitrogen content in a nitrided surface layer of the metal ring component;

figure 5 is a schematic cross-section of the metal ring component illustrating a thickness of the nitrided surface layer

Figure 1 shows the central parts of a known continuously variable transmission or CVT that is commonly applied in the drive-line of motor vehicles between the engine and the driven wheels thereof. The transmission comprises two pulleys 1 , 2 that are each provided with a pair of conical pulley discs 4, 5 mounted on a pulley shaft 6 or 7, between which pulley discs 4, 5 a predominantly V-shaped circumferential pulley groove is defined. At least one pulley disc 4 of each pair of pulley discs 4, 5, i.e. of each pulley 1 , 2, is axially moveable along the pulley shaft 6, 7 of the respective pulley 1 , 2. A drive belt 3 is wrapped around the pulleys 1 , 2, located in the pulley grooves thereof, for transmitting a rotational movement and an accompanying torque between the pulley shafts 6, 7.

The transmission typically also comprises activation means (not shown) that -at least during operation- impose on the said axially moveable pulley disc 4 of each pulley 1 , 2 an axially oriented clamping force that is directed towards the respective other pulley disc 5 of that pulley 1 , 2, such that the drive belt 3 is clamped between each such disc pair 4, 5. These clamping forces not only determine a friction force that can maximally be exerted between the drive belt 3 and a respective pulley 1 , 2 to transmit the said torque, but also radial positions R of the drive belt 3 in the pulley grooves. These radial position(s) R determine a speed ratio of the transmission. This type of transmission and its operation are well-known per se.

In figure 2, two known examples of the drive belt 3 are schematically illustrated in a cross-section thereof facing in the circumference direction thereof. In both examples, the drive belt 3 comprises transverse segments 32 that are arranged in a row along the circumference of an annular carrier in the form of one or two sets 31 of metal rings 41. In either example of the drive belt 3, the ring set 31 is laminated, i.e. is composed of a number of mutually nested, flat and thin, i.e. of ribbon-like individual metal rings 41 . In practice, mostly 6, 9, 10 or 12 metal rings 41 of 185 micrometer thickness are applied in the ring set 31.

On the left-side of figure 2 an embodiment of the drive belt 3 is illustrated including two such ring sets 31 , each accommodated in a respective slot of the transverse segment 32 that opens towards a respective axial side thereof. These slots are defined between a base part 33 and a head part 35 of the transverse segment 32 on either side of a relatively narrow web part 34 interconnecting the base part 33 to the head part 35.

On the right-side of figure 2 an embodiment of the drive belt 3 is illustrated incorporating only a single ring set 31. In this case, the ring set 31 is accommodated in a central recess of the transverse segment 32 that opens towards the radial outside of the drive belt 3. This central recess is defined between the base part 33 and two pillar parts 36 of the transverse segment 32 that respectively extend from either axial side of the base part 33 in radial outward direction. In such radial outward direction, the central recess is partly closed-off by respective, axially extending hook parts 37 of the pillar parts 36.

On either side thereof, the transverse segments 32 of both of the drive belts 3 are provided with contact faces 38 for arriving in friction contact with the pulley discs 4, 5. The contact faces 38 of each transverse segment 32 are mutually oriented at an angle f that essentially matches an angle of the V-shaped pulley grooves. The transverse segments 32 are typically made from metal as well.

It is well-known that, during operation in the transmission, the individual metal rings 41 of the drive belt 3 are tensioned by a/o a radially oriented reaction force to the said clamping forces. A resulting ring tension force is, however, not constant and varies not only in dependence on a torque to be transmitted by the transmission, but also in dependence on the rotation of the drive belt 3 in the transmission. Therefore, in addition to the yield strength and wear resistance of the metal rings 41 , also the fatigue strength is an important property and design parameter thereof. Accordingly, maraging steel is used as the base material for the metal rings 41 , which steel can be hardened by precipitation formation (ageing) to improve the overall strength thereof and additionally be surface hardened by nitriding (gas-soft nitriding) to improve wear resistance and fatigue strength in particular.

Figure 3 illustrates a relevant part of the known manufacturing method for the metal ring, as it is typically applied in the art for the production of metal drive belts 3 for automotive application. The separate process steps of the known manufacturing method 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 in an oven chamber 50. Thereafter, in a fourth process step IV, the tube 13 is cut into a number of metal rings 41 , which are subsequently -process step five V- rolled to reduce the thickness thereof to, typically, around 0.2 mm, while being elongated. The thus elongated metal rings 41 are 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 degrees Celsius, e.g. about 800°C, in an oven chamber 50. At such high temperature, the microstructure of the ring material is completely composed of austenite-type crystals. However, when the temperature of metal rings 41 drops again to room temperature, such microstructure transforms back to martensite, as desired.

After annealing VI, the metal rings 41 are calibrated in a seventh process step VII by being mounted around two rotating rollers and stretched to a predefined circumference length by forcing the said rollers apart. In this seventh process step VII of ring calibration, also internal stresses are imposed on the metal rings 41. Thereafter, the metal rings 41 are heat-treated in an eighth process step VIII of combined ageing, i.e. bulk precipitation hardening, and nitriding, i.e. case hardening. More in particular, such combined heat treatment involves keeping the metal rings 41 in an oven chamber 50 containing a process atmosphere composed of ammonia, nitrogen and hydrogen. In the oven chamber the ammonia molecules decompose at the surface of the metal rings 41 into hydrogen gas and nitrogen atoms that can enter into the microstructure of the metal rings 41 . These nitrogen atoms partly remain as interstitial atoms in the microstructure and partly bond with some of the alloying elements of the maraging steel, such as molybdenum in particular, to form intermetallic precipitates (e.g. Mo ₂ N). These interstitials and precipitates are known to remarkably increase the resistance of the metal rings 41 against wear as well as against fatigue fracture. Inter alia it is noted that such combined heat treatment can alternatively be followed or preceded by an aging treatment (without simultaneous nitriding), i.e. in a processing gas that is free from ammonia. Such separate aging treatment is applied when the duration of the nitriding treatment is too short to simultaneously complete the precipitation hardening process.

A number of the thus processed metal rings 41 are assembled in a ninth process step IX to form the ring set 31 by the radially stacking, i.e. concentrically nesting of selected metal rings 41 to realize a minimal radial play or clearance between each pair of adjoining metal rings 41. It is noted that it is also known in the art to instead assemble the ring set 31 immediately following the seventh process step VII of ring calibration, i.e. in advance of the eighth process step VIII of ring ageing and ring nitriding.

Many different ranges and values are mentioned in the art as being particularly appropriate for the process settings applied in the said eighth process step VIII or process steps of ring ageing and ring nitriding, also in relation to the specific composition of the maraging steel base material for the metal rings 41 . In practice, the nitriding process settings are defined so as to provide the metal rings 41 with a nitrogen diffusion zone, i.e. nitride surface layer of between 25 and 35 micrometer thickness. In figure 4, the dashed line represents the measured nitrogen content [N] in mass-% as a function of a measurement depth D from the outer surface of the metal ring 41 that is typically found in the currently mass produced drive belt 3. At around 30 micron depth, the said measured nitrogen content drops to zero, defining the extent, i.e. thickness of the nitrided surface layer.

The present disclosure considers that the thickness of the nitrided surface layer is, in fact, only of secondary relevance and that the mechanical properties of the metal ring 41 are, instead, mainly determined by the nitrogen concentration in the nitride surface layer. According to the present disclosure, it is thus preferable to control the nitriding process in the eighth process step VIII in relation to the nitrogen concentration over depth. In particular, according to the present disclosure and as indicated by the solid line in figure 4, close to the surface of the metal ring 41 , a relatively high concentration of nitrogen is set for providing wear resistance, while the nitrogen concentration deeper in the metal ring 41 is set relatively low to improve the ductility and/or yield strength of the metal ring 41. As a consequence, the nitrogen content versus depth profile of the nitrided surface layer according to the present disclosure, also shows a relatively high local gradient halfway its total thickness, i.e. of around (minus) 0.075 mass-%/micron at around 13 micron depth in figure 4, which compares with around (minus) 0.035 mass-%/micron at around 15 micron depth according to the state of the art.

The present disclosure also considers that, at least as a secondary optimisation, the thickness of the nitrided surface layer can be optimized along the cross-sectional circumference of the metal rings 41. In particular, according to the present disclosure and as illustrated in figure 5 in a schematic cross section of the metal ring 41 , the thickness TNSL of the nitrided surface layer is largest at its axial side faces TNSL-af and decreases towards the axial middle of the radially oriented, main faces TNSL-rf of the metal ring 41. According to the present disclosure, by such measure the ductility and fatigue strength of the metal ring 41 is improved, in particular in the sliding contact between adjacent metal rings 41 of the ring set 31 , without detriment to its resistance against wear. The present disclosure, in addition to the entirety of the preceding description and all details of the accompanying figures, also concerns and includes all the features of the appended set of claims. Bracketed references in the claims do not limit the scope thereof, but are merely provided as non-binding examples of the respective features. The claimed features can be applied separately in a given product or a given process, as the case may be, but it is also possible to apply any combination of two or more of such features therein.

The invention(s) represented by the present disclosure is (are) not limited to the embodiments and/or the examples that are explicitly mentioned herein, but also encompasses amendments, modifications and practical applications thereof, in particular those that lie within reach of the person skilled in the relevant art.