WO2018122397A1

WO2018122397A1 - Metal ring component of a drive belt for a continuously variable transmission and its manufacutring method

Info

Publication number: WO2018122397A1
Application number: PCT/EP2018/025000
Authority: WO
Inventors: Bert Pennings
Original assignee: Robert Bosch Gmbh
Priority date: 2016-12-30
Filing date: 2018-01-02
Publication date: 2018-07-05
Also published as: NL1042208B1; JP2020504784A

Abstract

The invention relates to a metal ring (44) for use in a drive belt (3) for a continuously variable transmission, which metal ring (44) is made from a maraging steel alloy including 15 to 20 mass-% nickel, 4 to 18 mass-% cobalt, at least 4 mass-% molybdenum and at least 7 mass-% in total of molybdenum, chromium and/or aluminium and which ring (44) is provided with a nitrided surface layer having a surface hardness of more than 1050 HV0.1.

Description

METAL RING COMPONENT OF A DRIVE BELT FOR A CONTINUOUSLY VARIABLE TRANSMISSION AND ITS MANUFACUTRING METHOD

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 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 used as the basic 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 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 applicant of the rings, it is typically resorted to maraging steel having a basic composition of:

- 15 to 20 mass-% nickel (Ni)

- 4 to 18 mass-% cobalt (Co)

- 4 to 6 mass-% molybdenum (Mo)

- balance iron (Fe)

In the drive belt application not only the yield strength of the rings, but also their surface hardness value and surface compressive stress level are important product characteristics that determine the load carrying capability and longevity of the drive belt. By the said heat treatments of precipitation hardening, i.e. aging, and nitriding, these product characteristics are finally determined. In particular, a surface hardness of up to 1050 HV0.1 and a compressive stress of up to 1400 MPa can be realised with the standing heat treatments. In practice, the surface hardness and surface compressive stress are limited not only by the alloy composition of the maraging steel, in particular by the abundancy of precipitate forming alloying elements therein, but also by factors related to the nitriding heat treatment.

A first known limiting factor related to the nitriding heat treatment is the unwanted phenomenon of over-aging. Over-aging is the process of the fine dispersion intermetallic coherent phases, i.e. the primary, metastable precipitates of Ni3(Co, Mo), becoming coarser and/or reducing in number over time due to their dissolution and replacement with semi-coherent phases such as Fe2Ni/Fe2Mo. Over- aging disadvantageously causes the yield strength of the rings to reduce, effectively limiting the processing time and processing temperature of the gas-soft nitriding heat treatment. A second known limiting factor related to the nitriding heat treatment is the unwanted phenomenon of compound layer formation, as is for example discussed in WO2015/097292. Compound layer formation is the occurrence and growth of an iron-nitride layer on the ring surface in dependence on processing intensity in terms of the processing temperature and the ammonia concentration in the processing gas. Compound layer formation disadvantageously causes the fatigue strength of the rings to reduce, effectively limiting the processing intensity of the gas-soft nitriding heat treatment.

Because of the above-mentioned two limiting factors of the nitriding heat treatment, the nitriding as such, i.e. the surface absorption and subsequent inward diffusion of nitrogen (N), is limited as well, as are the surface hardness and/or the surface compressive stress that can be realised therein.

The present disclosure aims to mitigate at least one of these two limiting factors of the nitriding heat treatment and thus to provide for (the manufacturing of) drive belt ring components with a surface hardness of more than 1050 HV0.1 , in particular of more than 1100 HV0.1 .

The present disclosure is based on the discovery that the said compound layer formation can be avoided or, at least, can be slowed down in relation to the processing intensity of the nitriding heat treatment, at least for a specific range of alloy compositions of the basic material for the rings. In particular, the present disclosure relates to the range of maraging steels having a composition including:

- 15 to 20 mass-% nickel (Ni)

- 4 to 18 mass-% cobalt (Co)

- at least 4 mass-% molybdenum (Mo), and

- at least 7 mass-% in total of molybdenum, chromium and/or aluminium.

During the nitriding the treatment molybdenum, chromium and/or aluminium combine with nitrogen to form metallic nitride precipitates. In particular these latter elements react more easily with nitrogen than iron, such that the forming of iron nitrides on the outer surface of the ring is considerably reduced and, depending on the processing intensity, can possibly be even completely avoided. As a consequence, the nitriding processing intensity could be increased and an increased surface hardness of the (end-product) rings could be realized.

Further according to the present disclosure, the maraging steel composition should preferably satisfy the requirement that the sum of the Molybdenum and Cobalt content thereof amounts to at least 22 mass-% and preferably amounts to between 24 and 28 mass-%. By imposing these additional composition requirements on the maraging steel basic material the ultimately manufactured rings are provided with increased yield strength that is, moreover, favourably balanced with the fatigue strength thereof, in particular in relation to the presently considered application thereof as basic material for the ring component of the drive belt for passenger car transmission application. If the combined Molybdenum and Cobalt content is less than 22 mass-%, the fatigue strength of the rings cannot be optimally exploited in the drive belt in view of the relatively low yield strength thereof, however, if the combined Molybdenum and Cobalt exceeds 28 mass-%, the hardness and brittleness of the ring after precipitation hardening (ageing) result in a reduced fatigue strength thereof.

Yet further according to the present disclosure, the maraging steel composition should preferably include at least two and more preferably all three elements of molybdenum, chromium and/or aluminium. It is considered that by offering different elements to bond with nitrogen, the overall reactivity of the nitride precipitate formation can be improved. In this case, however, the amount of chromium should preferably not exceed 3 mass-% individually. Otherwise, relatively coarse nitride precipitates can be formed during the nitriding heat treatment that would decrease the fatigue strength of the rings.

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. Thus, preferably, Iron is the only other, i.e. balancing element in the alloy composition apart from small amounts of inevitable contaminations such as Oxygen, Nitrogen, Phosphorous, Silicon, etc., e.g. less than 1 mass-% in total.

In relation to the nitriding heat treatment itself and relative to the currently practiced processing intensity, such processing intensity may be favourably increased according to the present disclosure. In particular according to the present disclosure, a processing temperature of more than 500 °C is applied therein, preferably such processing temperature is set at a value in the range between 505 °C and 550 °C, more preferably in the range between 510 °C and 525 °C. Also the ammonia concentration in the processing gas can be chosen relatively high, in particular in the range between 8 vol.-% and 18 vol.-%. 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 drive belt and of a transmission incorporating such known belt;

figure 2 is a schematic cross section oriented in the circumference direction of the known drive belt that includes two sets of a number of maraging steel rings, as well as a plurality of transverse members mounted on such ring sets;

figure 3 represents a diagram of the known manufacturing method of the drive belt ring component that includes the heat treatment of precipitation hardening and gas-soft nitriding and whereof:

figure 4 illustrates the novel process settings of the heat treatment of precipitation hardening and gas-soft nitriding according to the present disclosure.

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 CVT is well-known per se.

An example of a known drive belt 3 is shown in somewhat more detail in figure 2, in a cross-section thereof facing in its circumference direction. In this example, the drive belt 3 incorporates two ring sets 31 , each in the form of a number of mutually nested, flat and thin, i.e. of ribbon-like, flexible metal rings 44. The drive belt 3 further comprises a row of transverse segments 32, whereof one is depicted in front elevation in figure 2. The ring sets 31 are accommodated in a respective one of two axially extending recesses defined by the transverse segments 32. On either side thereof, the transverse segments 32 are provided with contact faces 34 for arriving in friction contact with the pulley discs 4, 5. The contact faces 34 of each transverse segment 32 are mutually oriented at an angle φ that essentially matches an angle of the V-shaped pulley grooves.

It is well-known that during operation in the CVT the rings 44 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 44, also the fatigue strength is an important property and design parameter thereof. Accordingly, maraging steel is used as the base material for the rings 44, which steel can be hardened by precipitation formation (ageing) to improve the overall strength thereof and additionally be surface hardened by nitriding to improve wear resistance and fatigue strength in particular. The maraging steel that is commonly applied as basic material for the rings 44 has a basic composition including 15 to 20 mass-% nickel, 4 to 18 mass-% cobalt, and 4 to 6 mass-% molybdenum with balance iron.

Figure 3 illustrates a relevant part of the known manufacturing method for the drive belt ring component 44, 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 annular rings 44, 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 rings 44 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 44 drops again to room temperature, such microstructure transforms back to martensite, as desired.

After annealing VI, the rings 44 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 rings 44. Thereafter, the rings 44 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 44 in an oven chamber 50 containing a process atmosphere composed of ammonia, nitrogen and hydrogen, for example at a temperature of 475 °C and with 8 vol.-% the ammonia gas present. In the oven chamber, the ammonia molecules decompose at the surface of the rings 44 into hydrogen gas and nitrogen atoms that can enter into the crystal structure of the rings 44. 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 44 is between 25 and 35 micrometre thick, which rings 44 themselves are between 150 and 200 micrometre thick.

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 rings 44 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 44 to realize a minimal radial play or clearance between each pair of adjoining rings 44. 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 the said eighth process step VIII the formation of a compound layer on a part, parts or the whole of the outer surface of the rings 44 is unwanted and is in practice avoided by limiting the intensity of the gas-soft nitriding process as quantified by the processing temperature T and the ammonia concentration [NH3] in the processing atmosphere. In relation to such process settings many different values have been mentioned in the art as particularly appropriate for the nitriding of the maraging steel drive belt ring component. Based on such known sources an upper nitriding intensity for avoiding compound layer formation can be derived as follows:

T = 575 - 12.5 * [NH3] (1 ), with T expressed in °C and [NH3] expressed in vol.-%.

Thus, for example, if a relatively high ammonia concentration of 12% is desired to be applied in ring nitriding, then equation (1 ) that summarizes the prior art prescribes a processing temperature of 425 °C or less. If, on the other hand, a relatively high processing temperature of 500 °C is desired, then equation (1 ) prescribes an ammonia concentration of 6 vol.-% or less.

However, according to the present disclosure, by adding at least 7 mass-% in total of one or more of the alloying elements of molybdenum, chromium and/or aluminium to the said commonly applied basic ring material, more nitrides are formed in a surface layer of the rings, rather than at the surface thereof. In other words, by these alloying elements, the forming of iron nitrides on the outer surface of the ring is considerably reduced and can possibly even be completely avoided in relation to the processing intensity of the nitriding process. Thus, as illustrated in figure 4, the nitriding of such novel basic ring material can be carried out at a relatively high processing temperature of, for example, 520 °C in combination with a relatively high ammonia concentration of, for example, 12%, whereby the rings 44 can be favourably provided with an unprecedented surface hardness value of more than 1050 HV0.1 and/or compressive stress level of more than 1400 MPa that contribute to their wear resistance and fatigue strength in the drive belt application thereof. In particular, a surface hardness value of more than 1100 HV0.1 and/or compressive stress level of more than 1500 MPa can be achieved within the context of the present disclosure.

The present disclosure, in addition to the entirety of the preceding description and all details of the accompanying figures, also concerns and includes all of 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 can also be applied simultaneously therein in 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.

Claims

1 . A metal ring (44) for use in a drive belt (3) for a continuously variable transmission with two pulleys (1 , 2) and the drive belt (3), characterised in that the ring (44) is made from a maraging steel alloy including:

- 15 to 20 mass-% nickel

- 4 to 18 mass-% cobalt

- at least 4 mass-% molybdenum and

- at least 7 mass-% in total of molybdenum, chromium and/or aluminium,

in that the ring (44) is provided with a nitrided surface layer and in that the ring (44) has a surface hardness value of more than 1050 HV0.1 , preferably of more than 1 100 HV0.1 .

2. The metal ring (44) according to claim 1 , characterised in that the said maraging steel alloy further satisfies the requirement that at least two and preferably all three alloying elements of molybdenum, chromium and aluminium are included therein.

3. The metal ring (44) according to claim 1 or 2, characterised in that the said maraging steel alloy contains not more than 8 mass-% molybdenum, not more than 3 mass-% chromium and not more than 3 mass-% aluminium.

4. The metal ring (44) according to claim 1 , 2 or 3, characterised in that the said maraging steel alloy further satisfies the requirement that the combined molybdenum and cobalt content thereof amounts to at least 22 mass-% and preferably having a value in the range from 24 to 28 mass-%.

5. The metal ring (44) according to a preceding claim, characterised in that the said maraging steel alloy further includes iron and less than 1 mass-% of other alloying elements and contaminations.

6. The metal ring (44) according to a preceding claim, characterised in that the ring (44) has a surface compressive (residual) stress of more than minus 1400 MPa, preferably of more than minus 1500 MPa.

7. A method for manufacturing the metal ring (44) according to a preceding claim, wherein the ring (44) is subjected to a process step of nitriding wherein the ring (44) is heat treated in an process atmosphere containing ammonia gas, characterised in that the ring (44) is nitrided at a temperature of more than 500 °C with the process atmosphere containing more than 8 vol.-% ammonia.

8. The ring (44) manufacturing method according to claim 7, characterised in that the ring (44) is nitrided at a temperature in the range between 505 °C and 550 °C, more preferably in the range between 510 °C and 525 °C, with the process atmosphere containing between 8 vol.-% and 18 vol.-% ammonia.