WO2018019435A1

WO2018019435A1 - Flexible steel ring made from maraging steel and provided with a nitrided surface layer

Info

Publication number: WO2018019435A1
Application number: PCT/EP2017/025223
Authority: WO
Inventors: Bert Pennings
Original assignee: Robert Bosch Gmbh
Priority date: 2016-07-27
Filing date: 2017-07-27
Publication date: 2018-02-01
Also published as: CN109563907A; JP2019528409A; CN109563907B; NL1041998B1; JP6934934B2

Abstract

The invention relates to a flexible ring (44) for use in a drive belt (3) for a continuously variable transmission, which ring (44) is made from maraging steel and is provided with a nitrided surface layer, whereof the microstructure includes a volume fraction austenite phase of between 2 and 10 vol.%.

Description

FLEXIBLE STEEL RING MADE FROM MARAGING STEEL AND PROVIDED WITH A NITRIDED SURFACE LAYER

The present invention relates to a flexible steel ring according to the preamble of claim 1 hereinafter. This type of ring is used as a component of a drive belt for a continuously variable transmission for, in particular, automotive use such as in passenger motor cars. The drive belt is typically composed of two sets of mutually concentrically arranged rings that are inserted in a recess of transverse members of the drive belt. The drive belt comprises a plurality of these transverse members that are arranged in mutual succession along the circumference of such ring sets. In such drive belt application thereof, an individual ring normally has a thickness of only 0.2 mm or less, typically of about 0.18 mm.

In the transmission the drive belt is used for transmitting a driving power between two shafts, whereto the drive belt is passed around two rotatable pulleys, respectively associated with one such transmission shaft and provided with two conical discs defining a circumferential V-groove of the pulley wherein the drive belt is accommodated. By varying an axial separation between the respective discs of the two pulleys in a coordinated manner, the drive belt's radius at each pulley -and hence the rotational speed ratio between the transmission shafts- can be varied, while maintaining the drive belt in a tensioned state. This transmission and drive belt are generally known in the art and are, for example, described in the European patent publication EP- A-1243812.

It is further generally known in art that the performance of the drive belt is directly linked to not only the combined tensile strength of the ring sets, but to a large extent also the fatigue strength of the individual rings thereof. This is because, during rotation of the drive belt in the transmission, the tension and bending stress in the rings oscillate. In practice it is universally resorted to special steel compositions, in particular so-called maraging steels, as the base material for the rings in order to realize the desired performance of the drive belt. Additionally to this end, the rings are precipitation hardened, i.e. aged, and nitrided in one combined or two subsequent heat treatment (s) that is/are part of the ring production process.

A common, longstanding desire and general aim in the further development and/or improvement of the drive belt ring component has been to increase the fatigue strength thereof. One approach aimed at such increase is provided by the European patent application No. EP-A-2832870. This document teaches to include a process step of melting and solidifying surface layers of the drive belt ring components, in particular of the annular member or tube wherefrom these rings are cut and (post- ) processed . According to EP-A-2832870, segregation of molybdenum in the melted/solidified surface layers results in molybdenum rich and molybdenum poor areas in those layers. Further according to EP-A-2832870, molybdenum is an austenite stabilizing element, such that in the segregated, molybdenum rich areas in the melted/solidified surface layers, a certain austenite phase fraction is retained within the martensic phase matrix. In particular according to EP-A-2832870, in case of a maraging steel base material composed of 18% by weight nickel, 9% by weight cobalt, 5% by weight molybdenum, 0.45% by weight titanium, 0.1% by weight aluminum and less than 0.03% by weight carbon, such fraction amounts to about 2 to 3% by volume and provides a remarkable improvement of fatigue strength.

Another approach is provided by the Japanese patent applications No. JP-A-2002-3946 and JP-A-2004-315875 that teach that austenite phase austenite phase can also be reverse formed by heat treatment, i.e. without actually melting the rings. In particular according to these latter two documents, the rings are heated to and held at a temperature within a specific temperature range (i.e. from 550 to 670 deg.C.) that is lower than an austenitizing start temperature (of about 750 deg.C.) of the base material. In this way 15 up to 35% by volume of such reverse-transformed austenite is formed.

These known approaches thus rely on a dedicated, i.e. additional process step in the overall ring manufacturing process for forming the reverse-transformed austenite fraction, which disadvantageously adds to the cost and complexity of such overall process. Furthermore, when using the 1^st approach, the said retained austenite phase is obtained only in the said molybdenum rich areas of the melted/solidified layers of the tube and/or ring. Such an in omogeneous microstructure of the nitrided surface layer provides the ring with a lesser fatigue strength than a comparable, but homogeneous microstructure of that surface layer. On the other hand, when using the 2^nd approach, the reverse-transformed austenite phase is obtained not only in the said nitrided surface layer but throughout the entire (cross-section of the) ring. However, such a homogeneous microstructure throughout the ring provides it with a lesser fatigue strength than a ring with a lesser or no reverse-transformed austenite phase fraction in its core than in its nitrided surface layer. Additionally, these known approaches diverge in terms of the supposedly optimum volume fraction of the reverse-transformed austenite phase.

The present disclosure aims to improve upon these known approaches for increasing the fatigue strength of the rings by including a certain reverse-transformed austenite phase fraction therein. In particular the present disclosure aims to provide a further approach for increasing the fatigue strength of the drive belt ring component by forming a certain reverse-transformed austenite phase fraction therein, which further approach: i) does not require a dedicated process step that is specifically and/or exclusively aimed at forming the said reverse-transformed austenite phase fraction; and

ii) forms the said reverse-transformed austenite phase fraction predominantly in and essentially equally distributed throughout, the nitrided surface layer of the ring. According to the present disclosure, a desired austenite phase fraction of between 2 and 10 volume-%, preferably of between 4 and 8 volume-%, is formed in the nitrided surface layer of the ring advantageously during the nitriding heat treatment, when the following conditions are met:

- the maraging steel base material includes less than 7% by weight cobalt and preferably includes 5% by weight or less cobalt; and

- the nitriding heat treatment is carried out at 490 to 525 deg.C. and preferably at 500 to 515 deg . C.

Other process parameters of the nitriding heat treatment, such as the ammonia content of the process gas supplied to the nitriding process atmosphere and the nitriding processing time, are selected such that the ring is provided with the nitrided surface layer of a desired, i.e. 15 to 30 micron, thickness, substantially without a so-called compound layer being formed on the outer surface of the ring, as discussed in WO2013/002633 and WO2015/097292 a/o.

According to the present disclosure it has been experimentally determined that the conventionally applied, relatively high cobalt content of the base material inhibits the formation of reverse-transformed austenite phase. This can explain why, according to JP-A-2002-3946 and JP-A-2004-315875, such a high temperature is required for the formation of reverse-transformed austenite phase in these conventionally applied base materials, which high temperature cannot be achieved in the nitriding heat treatment .

Furthermore, according to the present disclosure, the nitrogen that is introduced in the surface layer of the ring during the nitriding heat treatment was found to have a catalytic effect on the said formation of reverse- transformed austenite phase. This can explain why, according to JP-A-2002-3946 and JP-A-2004-315875, such a long processing time is required for the formation of reverse-transformed austenite phase in the known, dedicated reverse-transformed austenite phase forming heat treatment. Also due to the said catalytic effect of nitrogen, no or less reverse-transformed austenite phase is formed in the core of the ring than in the nitrided surface layer encasing it. Hereby, a decline of the toughness of the ring is favorably minimized.

It is noted that the cobalt content of the maraging steel is conventionally set at 7% by weight or more, because cobalt is an element that catalyzes precipitate formation for age hardening the ring. Therefore, i.e. in order to compensate for the relatively low cobalt content according to the present disclosure, some small amount of between 0.5 and 1.5% by weight of aluminum and/or of chromium is preferably added to the base material that is otherwise composed of 18% by weight nickel, less than 7% by weight, preferably 5% by weight or less cobalt, from 5 up to 7% by weight of molybdenum, with balance iron and other elements in trace amounts of less than 0.1% by weight only.

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 is a schematic illustration of a known drive belt and of a transmission incorporating such known belt.

Figure 2 is a schematic illustration of a part of the known drive belt, which includes two sets of a number of flexible steel rings, as well as a plurality of transverse members.

Figure 3 figuratively represents a known manufacturing method of the drive belt ring component that includes the heat treatments of precipitation hardening and nitriding.

Figure 4 is a photographic representation of a cross- section of the drive belt ring component revealing the microstructure thereof.

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 elements 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 elements 32. On either side thereof, the transverse elements 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 element 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 tensile 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.

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 11 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 circum^¬ ference 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 so-called gas-soft nitriding, i.e. case hardening. More in particular, such combined heat treatment involves keeping the rings 44 in an oven chamber 50 containing a controlled gas atmosphere that comprises ammonia, nitrogen and hydrogen gas. In the oven chamber, i.e. in the process atmosphere, 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 ring 44. By these interstitial nitrogen atoms the resistance against wear as well as against fatigue fracture is known to be increased remarkably. Inter alia it is noted that such combined heat treatment can alternatively be carried out in the separate and subsequent stages of ageing and nitriding, which alternative process setup is known in the art. 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 ring 44 is between 25 and 35 micron thick.

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 above, known manufacturing method, the heat treatment (s) of aging and nitriding (process step VIII) are arranged such that the fully martensitic micro- structure of the rings 44, which has been obtained after cooling down from the annealing temperature applied in the sixth VI process step, is retained therein. In particular, the formation of an austenite phase fraction is securely avoided in the known manufacturing method by limiting the temperature and duration of the said heat treatment ( s ) , in particular in relation to the specific composition of the maraging steel applied in the manufacture of the rings 44.

The former, conventional approach is based on the prevailing technical insight that any residual or reverted austenite crystals within the predominant martensitic microstructure of a maraging steel end-product will reduce the toughness and/or strength thereof, which is indeed undesired for the presently considered drive belt ring component 44. However, in accordance with the present disclosure, a small amount of austenite phase in the nitrided surface layer could instead be experimentally correlated to an improvement the fatigue strength of the ring 44. Specifically in the nitrided outer surface layer of the rings 44 where a compressive residual stress prevails, the known detrimental effect of the austenite phase fraction on toughness of the ring material is apparently more than offset by a beneficial effect of a higher ductility thereof.

Figure 4 illustrates the microstructure of an actual ring 44 in the nitrided surface layer thereof. In figure 4 the austenite phase AP has been made visible in a cross- section thereof by chemical etching and light microscopy (LM) , such that crystals/grains of austenite appear as whitish specs in between the martensitic phase crystals that are much darker in figure 4. In this figure 4 the austenite phase AP represents about 6% of the overall surface area of the cross-section of the ring 44. The volume fraction of the austenite phase AP essentially corresponds to such surface area fraction thereof, since the austenite crystals are randomly placed and oriented relative to the plane of the cross-section of figure 4, and thus lies well within the presently claimed range therefor, namely of 2 to 10 volume-%. As mentioned hereinabove, the austenite phase AP fraction in the nitrided surface layer can be controlled either by the temperature applied in the aging and nitriding heat treatment in the eight process step VIII or, at least above a certain, critical process temperature, by increasing the duration thereof. More in particular, a higher temperature and a longer duration increase the amount of austenite that is reverted in the said heat treatment and vice versa.

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 in the appended set of claims. Bracketed references in the claims do not limit the scope thereof, but are merely provided as non-limiting example of a respective feature. Separately 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 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 encompass ( es ) amendments, modifications and practical applications thereof, in particular those that lie within reach of the person skilled in the relevant art.

Claims

1. Flexible ring (44) destined for use as or in a drive belt (3) for a continuously variable transmission with two pulleys (1, 2) and the drive belt (3), which ring (44) is made from a maraging steel containing less than 7% by weight, preferably 5% by weight or less of cobalt and which ring (44) is provided with a nitrided surface layer having a microstructure containing at least 2% by volume and at most 10% by volume of austenite phase in a predominantly martensic phase matrix, characterized in that the said austenite phase is distributed essentially homogeneously throughout the nitrided surface layer of the ring (44) and in that a core of the ring (44) inside the said nitrided surface layer contains less austenite phase than the said nitrided surface layer.

2. The flexible ring (44) according to the claim 1, characterized in that the nitrided surface layer thereof contains at least 4% by volume and at most 8% by volume of austenite phase.

3. The flexible ring (44) according to the claim 1 or 2, characterized in that the said core of the ring (44) contains at most 6% by volume, preferably at most 4% by volume of austenite phase and most preferably contains no austenite phase or only in a residual amount.

4. The flexible ring (44) according to the claim 1, 2 or 3, characterized in that the maraging steel contains between 0.5 and 1.5% by weight of aluminum and/or of chromium.

5. A method for manufacturing a flexible steel ring (44) from material steel containing less than 7% by weight, preferably 5% by weight or less of cobalt, wherein the ring (44) is subjected to a nitriding heat treatment that is carried out in an ammonia containing process gas at a temperature in the range from 490 to 525 degrees Centigrade, preferably in the range from 500 to 515 degrees Centigrade for nitriding a surface layer of the ring (44) and for providing such nitrided surface layer with an essentially homogeneously distributed austenite phase fraction in the range from 2 to 10% by volume in a predominantly martensic phase matrix.

6. The method for manufacturing a flexible steel ring (44) according to claim 5, wherein the nitrided surface layer is provided with an austenite phase fraction in the range from 4 to 8% by volume in nitriding heat treatment.

7. The method for manufacturing a flexible steel ring (44) according to claim 5 or 6, wherein a core of the ring

(44) inside the said nitrided surface layer is provided with an austenite phase fraction of 6% by volume at most, preferably 4% by volume and most preferably is not provided with austenite phase or only in a residual amount in nitriding heat treatment.

8. The method for manufacturing a flexible steel ring (44) according to claim 5, 6 or 7, wherein the ring (44) is provided with a nitrided surface layer of at least 15 micron and at most 30 micron thickness in the nitriding heat treatment .