NL2029820B1 - Metal ring for a drive belt - Google Patents

Abstract

The invention relates to a metal ring (41) for use in a drive belt (3) for a continuously variable transmission, which metal ring (41) is made from a maraging steel alloy including 5 15 to 20 mass-% nickel, 4 to 18 mass-% cobalt, 4 to 8 mass-% molybdenum, 0.5 to 2.5 mass-% aluminium and up to 3.5% chromium, with balance iron, and which metal ring (41) is provided with a nitrided surface layer. According to the invention, aluminium nitride inclusions are present in the nitrided surface layer that have a largest size, as quantified by the diameter of virtual circle encompassing it in a 2D cross-section of the ring (41), of 6 10 micron or less.

Description

METAL RING FOR A DRIVE BELT

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 metal 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 treatment thereof including the combined precipitation hardening (i.e. aging) and nitriding, in particular so-called gas-soft nitriding, thereof. 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 following range of nominal maraging steel alloy compositions is considered: - 15 to 20 mass-% nickel (Ni), - 4 to 18 mass-% cobalt (Co), - 4 to 8 mass-% molybdenum (Mo), - 0.510 2.5 mass-% aluminium (Al), - up to 3.5 mass-% chromium (Cr), and -apart from small amounts of inevitable contaminations such as oxygen (O), nitrogen (N), carbon (C), phosphorous (P) and/or silicon (Si)- with - 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 nitriding, nitrogen atoms originating from ammonia gas that decomposes at the ring surface, are introduced into the metal lattice of the ring by diffusion. In a surface layer of the ring these nitrogen atoms react with the available molybdenum, aluminium and/or chromium to form nanoscale Mo, Al and/or Cr-type nitride precipitates, the formation whereof is enhanced by the presence of cobalt that decreases the solubility of the other alloying elements in the iron-nickel matrix. By these nanoscale nitride precipitates in the nitrided surface layer the said surface residual compressive stress is realised, which surface compressive stress favourably reduces the occurrence of (fatigue) crack initiation on surface imperfections. At the same time, aluminium nitrides are present in the material on microscale as well, which larger nitrides are referred to as (non-metallic) inclusions.

These larger nitride inclusions are present not only in the said nitrided surface layer, but also to the inside of such layer, i.e. in the core of the ring. Nevertheless, these are typically most detrimental to the fatigue strength nearest the surface, where the highest tensile stress occurs during operation. Thus, the maximum size and abundance of these nitrides inclusions are carefully controlled during the preparation of the base material and in ring manufacturing.

For example the Japanese patent JP3690774-B2 teaches an upper size limit for the nitride inclusions of 8 micron, as quantified by an effective diameter determined by the smallest (virtual) circle circumscribing, i.e. encompassing the inclusion in a 2D cross- section of the ring, albeit in relation to titanium nitride inclusions in a titanium-containing maraging steel alloy composition. In fact, measurements by applicant in relation to aluminium containing maraging steel corroborate such known technical teaching for aluminium nitrides. Namely, an ultimate fatigue fracture of the ring was found to initiate just as likely on an aluminium nitride inclusion with an effective diameter of 8 micron, as on such an inclusion with a smaller effective diameter of 7 or even 6 micron. JP3690774-

B2 even considered titanium nitride inclusions with an effective diameter of 2 micron.

Therefore, i.e. since a (further) reduction of the upper size limit of the nitride inclusions to below 8 micron does not (further) increase the fatigue strength of the rings, no effort has been spend to (further) reduce the nitrogen contamination and/or to (further) limit nitride inclusion growth in nitriding.

Departing from the above technical teachings and insights according to the state of the art, the present invention aims to (further) improve the fatigue strength of the drive belt ring component. At odds with the state of art for titanium nitride inclusions, the present invention found a different fatigue behaviour for aluminium nitride inclusions.

Namely, even though the ring fatigue strength improvement, as a function of decreasing aluminium nitride inclusion size, likewise satiates, i.e. diminishes to zero when the upper size limit for the effective diameter of the aluminium nitride inclusions is around 8 micron, applicant surprisingly did not observe any fatigue fracture whatsoever originating from an aluminium nitride inclusion with an effective diameter of 6 micron or less. This led applicant to the technical insight that by setting the upper size limit of the aluminium nitride inclusions, as quantified by the effective diameter thereof, to 8 micron, the ultimate fatigue fracture of the rings is determined by the said surface imperfections thereof rather than by the aluminium nitride inclusions. As a result, the ring fatigue strength can be remarkably improved. It is hypothesised that such small aluminium nitride inclusions are tightly embedded in the iron-nickel matrix, such that their relatively high brittleness can be accommodated, i.e. compensated for by the relatively elastic matrix, at least compared to titanium nitride inclusions.

The upper size limit for aluminium nitride inclusions of 6 micron in terms of the effective diameter thereof can be conveniently realised by limiting the aluminium content in the nominal maraging steel alloy composition to between 0.5 and 1.5 mass-% and preferably to between 0.5 and 1 mass-%. In these cases, preferably, a substantial amount of at least 1 mass-%, more preferably between 1.5 and 2.5 mass-% of chromium is included therein, to compensate for the relative small aluminium content. Moreover, the nitriding heat treatment is preferably preceded by a separate precipitation hardening heat treatment in an inert atmosphere to a/o form Ni3Al intermetallic precipitates, such that less aluminium is available for reacting with the nitrogen atoms in the (subsequent) nitriding heat treatment to avoid the said microscale aluminium nitride inclusions growing undesirably large (i.e. >6 micron effective diameter).

The above-described drive belt, its ring component and the technical insight behind the present invention 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 continuously variable 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 is a SEM photograph of an enlarged cross-section of the drive belt ring component with an aluminium nitride inclusion having a size that is effectively defined by the diameter D of a virtual circle circumscribing it;

Figure 4 is a plot that schematically represents the known relationship between a largest size of the nitride inclusions present in the ring (in terms of its effective diameter D) and the resulting fatigue strength thereof (in terms of the number of stress cycles until failure); and

Figure 5 plots the measured fatigue strength of several rings in terms of a number of stress cycles until ring fatigue fracture on the X-axis versus the measured effective diameter of the aluminium nitride inclusion that initiated such fracture on the Y-axis.

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 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 thickness of 0.19 millimetre.

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 5 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.

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 surface hardened by nitriding (gas-soft nitriding) to improve wear resistance and fatigue strength in particular. Both such processes of precipitation hardening and of surface nitriding are carried out at an elevated temperature of, typically, between 450 and 500 degrees centigrade and nitriding is additionally carried out in an ammonia gas-containing atmosphere. In practice, both such processes are carried out simultaneously in a single, i.e. combined heat treatment, to minimise process equipment and maximise process throughput.

Presently, the following range of nominal maraging steel alloy compositions is considered: - 15 to 20 mass-% nickel (Ni), - 4 to 18 mass-% cobalt (Co), - 4 to 8 mass-% molybdenum (Mo), - 0.5 to 2.5 mass-% aluminium (Al), - up to 3.5 mass-% chromium (Cr),

and -apart from small amounts of inevitable contaminations such as oxygen (O), nitrogen (N), carbon (C), phosphorous (P) and/or silicon (Si)- with - balance iron (Fe).

In particular, the aluminium and/or chromium are added

It is well-known that in the preparation of the base material for the rings 41, relatively large, i.e. micron-scale non-metallic aluminium nitride inclusions form, the maximum size whereof can be controlled, however at the expense of additional and/or slower processing, such as a (vacuum) remelting. Close to the surface of the rings 41, where bending stress is highest, these inclusions are most detrimental to the fatigue strength thereof. A real example of such a non-metallic nitride inclusion in the iron-nickel matrix, in this case an aluminium-nitride (AIN) inclusion, is illustrated in figure 3 that depicts the same SEM photograph of a cross-section of the ring 41 at two enlargements.

The size of the inclusion can be conveniently quantified by an effective diameter D thereof, as illustrated an the right in figure 3, that is the diameter of the smallest (virtual) circle that encompasses the inclusion in such cross-section (of course taking into account the enlargement in question).

Because nitride inclusions, such as aluminium and titanium nitride, locally raise the stress level during operation of the drive belt 3, they typically serve as crack initiation locations and can thus cause the ultimate fatigue fracture of the ring 41. In this respect, the figure 4 illustrates the known general behaviour of the fatigue strength of maraging steel in terms of the number of stress cycles until failure and in relation to the largest nitride inclusions therein. This figure 4 illustrates that the said fatigue strength increases when the nitride inclusion in the maraging steel are regulated to a smaller size, however with a diminishing return. At an effective (nitride) inclusion diameter of 10 micron, such fatigue strength increase is known to satiate.

Figure 5 is a plot of the effective inclusion diameter versus fatigue strength, similar to the plot of figure 4, however presenting the outcome of multiple fatigue tests performed by applicant in relation to the aluminium containing maraging steel having a nominal composition of 19 mass-% nickel, 13 mass-% cobalt, 5 mass-% molybdenum, 1 mass-% aluminium and 1 mass-% chromium. These fatigue tests were performed at various test settings, in terms of a cyclically applied stress amplitude due to bending that is superimposed on a constant tensile stress, which test settings reflect the actual loads experienced by the rings 41 during (normal) operation of the drive belt 3 in the transmission. In particular in figure 5, the said effective diameter D of the nitride inclusion that actually initiated the ultimate fatigue fracture is plotted as a function of the number of stress (amplitude) cycles until such ultimate fatigue fracture, i.e. until failure.

Figure 5 confirms once again that at an effective inclusion diameter D around 8 micron of the in this case aluminium nitride causing the fatigue fracture, e.g. 8 micron plus or minus 2 micron, the fatigue strength of the maraging steel rings 41 is more or less independent of such diameter. Nevertheless, as an additional and new insight, figure 5 reveals that none of the observed fatigue fractures of the rings 41 initiates from an aluminium nitride inclusion having an effective diameter of less than ~6.5 micron. Based on this insight, it is presently proposed to limit the size of the individual aluminium nitride inclusions, i.e. to control the largest occurring effective inclusion diameter D to 6 micron at most, as indicated by the greyed area in figure 5.

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 that lie within reach of the person skilled in the relevant art.

Claims (5)

CONCLUSIONS 1. A metal ring (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 mass nickel (Ni), - 4 to 18 mass % cobalt (Co), - 4 to 8 mass % molybdenum (Mo), - 0.5 to 2.5 mass % aluminum (Al), - up to 3, 5 mass-% chromium (Cr) and remainder iron (Fe), which ring (41) is provided with a nitrated surface layer and which ring (41) comprises microscopic aluminum nitride inclusions, characterized in that, at least in said nitrated- surface layer of the ring (41) in its 2D section, the aluminum nitride inclusions each fit within an imaginary circle of 6 microns in diameter. The metal ring (41) of claim 1, characterized in that all aluminum nitride inclusions therein each fit within an imaginary circle having a diameter of 6 microns The metal ring {41) according to claim 1 or 2, characterized in that the steel alloy more specifically contains between 0.5 and 1.5 mass % aluminum and preferably between 0.5 and 1 mass % aluminum . The metal ring (41) according to claim 1, 2 or 3, characterized in that the steel alloy more specifically contains at least 1% by mass of chromium and preferably contains between 1.5 and 2.5% by mass of chromium. The metal ring (41) according to claim 1 or 2, characterized in that the steel alloy has a nominal composition of 19 mass % nickel, 13 mass % cobalt, 5 mass % molybdenum, 1 mass % aluminum and 1 mass % chromium.