NL1043487B1 - Ring component of a drive belt for a continuously variable transmission - Google Patents

Abstract

The invention relates to a flexible metal band (41) for use in a drive belt (3) for a continuously variable transmission, which flexible metal band (41) has a nitrided surface layer and is made from a maraging steel alloy including 15 to 20 mass-% nickel, 10 to 14 mass-% cobalt, 4 to 6 mass-% molybdenum, up to 2.5 mass-% chromium and up to 2.0 mass-% aluminium, whereof the cobalt (Co) content and the aluminium (Al) content satisfy the criterion: 19 mass-% < (Co + 6*AI) < 21 mass-%.

Description

RING COMPONENT OF A DRIVE BELT FOR A CONTINUOUSLY VARIABLE

TRANSMISSION The present invention 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 rings are incorporated in at least one, but typically 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 base material for the 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, i.e. aging and nitriding, in particular so-called gas-soft nitriding. The basic alloying elements of maraging steel are iron, nickel, cobalt and molybdenum and can vary within a broad range, however, specifically for the presently considered drive belt application of the rings, the international patent application published as WO 2018/122397 A1 recently disclosed a range of maraging : steels having a basic alloy composition of: - 15 to 20 mass-% nickel (Ni), - 4 to 18 mass-% cobalt (Co), - at least 4 mass-% molybdenum (Mo), - at least 7 mass-% in total molybdenum (Mo), chromium (Cr) and/or aluminium (Al), and - balance iron (Fe).

In the drive belt application thereof, not only the yield strength of the rings, but also their surface hardness and surface residual compressive stress are important product characteristics that have a significant impact on the load carrying capability and longevity of the drive belt. In particular, these latter product characteristics to a large extent determine the fatigue strength and wear resistance of the rings. In practice, these product characteristics of the ring surface are determined not only by the composition of the maraging steel, in particular by the abundancy of precipitate forming alloying elements therein, but also by the process parameters of the nitriding heat treatment thereof.

In the heat treatment of nitriding, nitrogen atoms are introduced in the microstructure by lattice diffusion from the ring surface. In a surface layer of the ring these nitrogen atoms react with the available molybdenum, aluminium and/or chromium to form (Mo, Al, Cr)-type nitrides, which nitride formation is enhanced by the presence of cobalt that decreases the solubility of the other alloying elements in the iron-nickel matrix. By these nitrides in the nitrided surface layer the said surface residual compressive stress is realised, which surface compressive stress results in and is balanced by a residual tensile stress in the core of the ring. This core residual tensile stress thus effectively limits the (additional) tension that can be tolerated by the ring during operation of the drive belt without exceeding its yield strength. Accordingly, even though an increase in the thickness of the nitrided surface layer thereof typically supports the fatigue strength and wear resistance of the ring, it decreases the yield strength thereof. Therefore, in practice in ring nitriding, a balance is struck between the thickness of the nitrided surface layer, in particular in terms of the remaining thickness of the core of the ring, and the surface hardness and surface residual compressive stress that can be reached.

Based on the afore-mentioned insight, the present invention aims to maximize the surface residual compressive stress in the ring for a given nitrided layer thickness obtained in the said nitriding heat treatment, i.e. to maximize the nitride concentration in the ring close to the ring surface. Within the teaching of WO 2018/122397 A1, such increase in nitride concentration would require the amount of molybdenum, chromium and aluminium in the maraging steel composition to be increased to significantly above 7 mass-% in combination with an amount of cobalt towards the top end of the disclosed cobalt range. However, this known solution not only increases the costing of the maraging steel, but was also found to disadvantageously increase the thickness of the nitrided surface layer. According to the present invention, a more favourable option for maximizing the surface compressive stress is, however, available.

According to the present invention, it was surprisingly found that by mutually correlating the amount of aluminium and the amount of cobalt in the maraging steel alloy, the nitride formation can be significantly enhanced, in particular without substantially increasing the nitrided surface layer thickness simultaneously. As a result, a high surface compressive stress can be realised in the ring at a relatively low cobalt content. In particular according to the present invention such favourable result is realized with a basic composition of the maraging steel including between

9.0 and 14 mass-% cobalt and satisfying the criterion:

19 mass-% < (Co + 6*Al) < 21 mass-% (1) With this particular relationship between the cobalt content and the aluminium content according to criterion (1), a balance is struck between the formation of aluminium-type nitrides in the nitrided surface layer and the formation of aluminium- type intermetallic precipitates (Ni3Al) in the ring core. Optimum results were in this respect achieved with a basic composition of the maraging steel including between

1.0 and 1.5 mass-% aluminium and satisfying the criterion: (Co + 6*Al) = 20 mass-% (2).

Within the range of cobalt-to-aluminium ratios resulting from the above criterion (1) and criterion (2) in particular, significantly more nitrogen is captured in the nitrides formed in the nitriding heat treatment than outside such range, as can be derived from a nitrogen concentration versus depth profile obtained by glow-discharge optical emission spectroscopy for instance. In particular, close to the surface of the ring, the nitride concentration decreases considerably less sharply in relation to a (nitriding) depth below the surface of the ring, as compared to maraging steels with an alloy composition outside the ranges according to criterion (1) or (2). As a result, a high residual compressive stress is realised in the nitrided surface layer of the ring, against a relatively small increase in the residual tensile stress in the core of the ring. In particular, the increase in such core residual tensile stress is smaller than the increase in the surface residual compressive stress, since it is predominantly realised by the increased nitride concentration close to the surface of the ring, rather than by an increased thickness of the nitrided surface layer.

Additionally according the present invention, also the molybdenum and chromium content are preferably set within a relatively narrow range each, with an optimum found at 4.0 to 6.0 mass-% molybdenum and up to 2.5 mass-% chromium.

In the latter case, the nitrided surface layer is favourably characterised by a mixture of both (Al, Mo, Cr)-type and (Al, Mo)-type nitrides near the surface of the ring and a mixture of (Al, Mo, Cr)-type nitrides and Ni3Al intermetallic precipitates at the boundary between the nitrided surface layer and the core of the ring. Hereby, a favourable transition from the compressive residual stress in the nitrided surface layer towards the tensile stress in the ring core is obtained.

Although the presently considered maraging steel compositions may include amounts of other alloying elements, such as titanium, this is not required within the present context. In this case, only trace amounts of other elements are present therein, such as inevitable phosphorous and silicon contamination.

It is noted that within the context of the present invention, the nitriding heat treatment itself can be carried out as is customary in the art. The above-described favourably effects are realised almost entirely by the specific maraging steel composition according to the present invention and do thus not rely on or otherwise require a special or specific setup of the aging and nitriding heat treatments. In fact, the presently considered range of maraging steels is even suitable for the combined heat treatment of simultaneous aging and nitriding that is described in the European patent EP 1753889 B1 in relation to a conventional maraging steel composed of 18 mass-% nickel, 5 mass-% molybdenum, 16.5 mass-% cobalt and balance iron.

The above-described drive belt, the ring component thereof and its manufacturing method will now be explained in more detail by means of a non- limiting, illustrative embodiment thereof and with reference to the drawing, 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 a schematic cross-section, each provided with a set of nested, flexible metal rings and with a plurality of metal transverse segments that are slidably mounted on such ring-set along the circumference thereof, Figure 3 provides a diagrammatic representation of the presently relevant part of the known overall manufacturing process of the drive belt ring component that includes the heat treatments of precipitation hardening and gas-soft nitriding and whereof: Figure 4 is a graph linking an amount of nitrogen present in the material of the ring in relation to a distance from the outer surface of the ring (i.e. a nitrogen abundance vs. depth profile).

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.

5 The transmission generally also comprises activation means that -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 these discs 4, 5 of the pulleys 1, 2. These clamping forces not only determine a friction force that can be exerted between the drive belt 3 and a respective pulley 1, 2, but also radial positions R of the drive belt 3 at the pulleys 1, 2 between the respective pulley discs 4, 5 thereof. These radial position(s) R determine a speed ratio of the transmission. This type of transmission, its activation means and their 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. The known 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. A thickness of the transverse segments 32 is small relative to a circumference length of such ring-set(s) 31, in particular such that several hundred transverse segments 32 are comprised in the said row thereof. In either example of the drive belt 3 in figure 2, the ring-set 31 is laminated, i.e. is composed of a number of mutually nested, flat, thin and flexible individual rings 41. Although in the accompanying figures the ring-set 31 is illustrated to be composed of 5 nested rings 41, in practice, mostly 6, 9, 10 or 12 rings 41 are applied in such ring-set 31, each with a nominal thickness of 185 micrometer.

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 laterally oriented recess of the transverse segment 32 that opens towards a respective, i.e.

left and right, axial sides thereof. Such lateral openings are defined between a body part 33 and a head part 35 of the transverse segment 32 on either side of a relatively narrow neck part 34 that is provided between and interconnects the body part 33 and 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 centrally located recess of the transverse segment 32 that opens towards the radial outside of the drive belt 3. Such central opening is defined between a base part 39 and two pillar parts 36 of the transverse segment 32 that respectively extend from either axial side of the base part 39 in radial outward direction. In such radial outward direction, the central opening 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 ¢ 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 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 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 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 drive belt ring component 41, 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 | a thin sheet or plate 20 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 21 are welded together in a second process step Il to form a hollow cylinder or tube 22. In a third step lll of the process, the tube 22 is annealed in an oven chamber 50. Thereafter, in a fourth process step IV, the tube 22 is cut into a number of annular rings 41, which are subsequently -process step five V- rolled to reduce the thickness thereof by a factor of around 2, while being elongated. The thus elongated 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 rings 41 drops again to room temperature, such microstructure transforms back to martensite, as desired.

After annealing VI, the 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 VI! of ring calibration, also internal stresses are imposed on the rings 41. Thereafter, the 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 rings 41 in an oven chamber 50 containing a controlled process atmosphere of a mixture of ammonia, hydrogen and nitrogen gas at a controlled temperature. It is known in the art to control the ammonia concentration in the process atmosphere to a value between 5 and 25% by volume, to control the hydrogen concentration in the process atmosphere to a value between 5 and 15% by volume and to control the temperature of the process atmosphere to a value between 450 and 525 °C. Practically applied values in this respect are around 10% by volume ammonia gas, around 5% by volume hydrogen gas and 470 °C.

In the oven chamber, the ammonia molecules decompose at the surface of the rings 41 into hydrogen gas and nitrogen atoms that can enter into the crystal structure of the rings 41. By these interstitial nitrogen atoms the resistance against wear as well as against fatigue fracture is known to be increased remarkably. Typically, the eighth process step VIII of combined ring ageing and nitriding is carried out until a nitrided layer or nitrogen diffusion zone formed at the outer surface of the rings 41 reaches a desired thickness of, for example, 25 micrometer.

Inter alia it is noted that such combined heat treatment can alternatively be followed or preceded by an aging heat treatment, i.e. without simultaneous nitriding, in a processing gas that is free from ammonia. Such separate aging heat treatment is applied when the duration of the nitriding heat treatment is too short to simultaneously complete the precipitation hardening process.

A number of the thus processed 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 rings 41 to realize a minimal radial play or clearance between each pair of adjoining rings 41. Inter alia it is noted that it 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.

In figure 4, the two dashed lines DL1, DL2 and the single solid line SL1 each represent the measured (by means of so-called glow-discharge optical emission spectroscopy or GDOES) nitrogen content [N] in mass-% as a function of a measurement depth D below the outer surface of a respective one of three rings 41. The hashed area in figure 4 marks a range wherein the GDOES measurement does not provide accurate results, due to start-up limitations and/or unavoidable test sample surface contamination. The locally measured nitrogen content [N] that is plotted in figure 4 is representative of the amount of nitrides that are locally present in a respective ring 41, i.e. that are locally formed in ring nitriding. In turn such local nitride amount is representative of the local compressive residual stress in the ring

41.

The three rings 41 that generated the figure 4 measurement data DL1, DL2 and SL1 are all produced by means of the said process steps I-VIIl of the above manufacturing method, however, starting from different base materials. The two dashed lines DL1 and DL2 represent two conventional maraging steels that are composed of 18 mass-% nickel, 16.5 mass-% cobalt, 5 mass-% respectively 7 mass- % molybdenum and balance iron. It can be seen in figure 4 that by increasing the molybdenum content from 5 mass-% (line DL1) to 7 mass-% (line DL2), the measured nitrogen content [N] and thus the amount of nitrides increases, at least down to a depth D of approximately 15 micron below the surface of the ring 41. This is to be expected, since molybdenum is a nitride forming alloying element. However, the effect of the additional molybdenum on the nitrogen take-up is limited.

The amount of nitrides in the nitride surface layer can be increased further by adding aluminium and/or chromium to the base material instead of or in addition to increasing the molybdenum content. To this end, the broad and open-ended range of alloy compositions with 15 to 20 mass-% nickel, with 4 to 18 mass-% cobalt, with at least 7 mass-% in total of molybdenum, chromium and/or aluminium and with balance iron is proposed in the art. However, according to the present invention, superior results are achieved in a relatively narrow subrange within such known open-ended range of alloy compositions. In particular according to the present invention, in the presence of aluminium in the alloy composition, a specific ratio between the aluminium content and the cobalt content provides a remarkably effective nitriding heat treatment in terms of the amount of nitrides formed therein close to the outer surface of the ring 41: 19 mass-% < (Co + 6*Al) < 21 mass-% (1) The solid line SL1 in figure 4 represents one specific maraging steel composition within such narrow range of alloy compositions according to the present invention that is composed of 19 mass-% nickel, 13 mass-% cobalt, 5.0 mass-% molybdenum, 1.0 mass-% chromium, 1.1 mass-% aluminium and balance iron. From figure 4 it appears that for such latter maraging steel composition the amount of nitrides, as determined by the measured nitrogen content [N], is dramatically and favourably increased relative to the said two conventional maraging steels over the essentially the whole thickness of the nitrided surface layer, favourably without simultaneously increasing such thickness. For example, in the range of depths D from 5 micron to 10 micron, such increase in the measured nitrogen content [N] exceeds a factor of 2. In principle this also implies that for the presently considered range of maraging steel alloy compositions a reduced thickness of the nitrided surface layer can be applied, resulting in a favourably reduced core residual tensile stress at a comparable surface residual compressive stress. In particular according to the present invention, the thickness of the nitrided surface layer can be reduced from a conventional 22.5 to 27.5 micron to 12.5 to 17.5 micron.

The present invention, 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 is 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.

Claims (5)

CONCLUSIONS A flexible metal belt (41) for use in a drive belt (3) for a continuously variable transmission with two pulleys (1, 2) and the drive belt (3) and made of a steel alloy containing: - 15 to 20% by weight nickel - 4 to 18 percent by weight cobalt, - at least 4 percent by weight molybdenum, - at least 7 percent by weight in total molybdenum, chromium and/or aluminum, - balance iron, which tire (41) is provided with a nitrided surface layer, characterized by: that the steel alloy contains more specifically 9.0 to 14 weight percent cobalt, 4.0 to 6.0 weight percent molybdenum and at most 2.5 weight percent chromium and that the proportions of cobalt and aluminum therein satisfy the condition that: 19 weight percent < (Co + 6*Al) < 21 wt% (1), where Co stands for the proportion in wt.% cobalt and Al for the proportion in wt.% aluminum. The flexible metal strip (41) according to claim 1, characterized in that the steel alloy contains more specifically 1.0 to 1.5 weight percent aluminum and the proportions of cobalt and aluminum therein satisfy the condition that: (Co + 6*Al) = 20 weight percent (2). The flexible metal strip (41) according to claim 1 or 2, characterized in that the steel alloy contains more specifically 13 weight percent cobalt, 1.1 weight percent aluminum, 5.0 weight percent molybdenum and 1.0 weight percent chromium. The flexible metal band (41) according to any preceding claim, characterized in that the nitrided surface layer of the band (41) has a thickness of between 12.5 and 17.5 micrometers. The flexible metal band (41) according to claim 4, characterized in that the band (41) has a nominal thickness of 185 micrometers.