NL1041998B1

NL1041998B1 - Flexible steel ring made from maraging steel and provided with a nitrided surface layer

Info

Publication number: NL1041998B1
Application number: NL1041998A
Authority: NL
Inventors: Pennings Bert
Original assignee: Bosch Gmbh Robert
Priority date: 2016-07-27
Filing date: 2016-07-27
Publication date: 2018-02-01
Also published as: JP6934934B2; CN109563907B; WO2018019435A1; JP2019528409A; CN109563907A

Description

Octrooicentrum

Nederland

(21) Aanvraagnummer: 1041998 © Aanvraag ingediend: 27/07/2016

Θ 1041998

BI OCTROOI @ Int. Cl.:

F16G 1/20 (2017.01) C21D 9/40 (2017.01) F16G 5/16 (2017.01)

Aanvraag ingeschreven:	(73) Octrooihouder(s):
01/02/2018	Robert Bosch G.m.b.H.
	te Stuttgart, Germany, DE.
(43) Aanvraag gepubliceerd:
	(72) Uitvinder(s):
(47) Octrooi verleend:	Bert Pennings te Goirle.
01/02/2018
(45) Octrooischrift uitgegeven:	(74) Gemachtigde:
13/02/2018	ir. G.A.J.M. Plevier te Tilburg.

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

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.%.

NL BI 1041998

I* 34

Dit octrooi is verleend ongeacht het bijgevoegde resultaat van het onderzoek naar de stand van de techniek en schriftelijke opinie. Het octrooischrift komt overeen met de oorspronkelijk ingediende stukken.

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 EPA-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.

According to the present disclosure, the fatigue strength of the rings can be improved upon by fine-tuning the process settings of the heat treatments of ageing and nitriding, in particular in terms of the processing temperature and duration, such that a certain fraction of austenite phase is formed in the nitrided surface layer of the rings that otherwise has a martensitic microstructure. In particular according to the present disclosure, the austenite phase fraction should amount to between 2 and 10 volume-% of the nitrided surface layer of the ring, more preferably 4 to 8 volume-%. According to the present disclosure, such austenite phase fraction in the predominantly martensite phase of the microstructure in the nitrided surface layer was unexpectedly found to improve the fatigue strength of the rings.

It is noted that up to now, a fully martensitic microstructure of the maraging steel is typically aimed for in industry and in drive belt ring manufacture in particular. In fact, it is known in the art that any residual or reverted austenite crystals in a predominant martensitic microstructure can detrimentally affect the toughness and therefore likely also reduce the fatigue strength of the steel. However, in accordance with the present disclosure, it was discovered that by allowing some austenite phase to form in the nitrided surface layer, the fatigue strength of the ring could in fact be improved. In this respect, it is hypothesized that the austenite phase fraction favorably reduces the brittleness of the nitrided surface layer and that, in terms of the fatigue strength of the ring, the detrimental effect of the austenite phase fraction of reducing toughness is more than offset by the beneficial effect thereof of improved ductility.

Preferably, the said austenite phase fraction is present in the nitrided surface layer, but not, or at least to a lesser extent, in the core material of the ring that is encased within the nitrided surface layer thereof. Hereby, a decline of the toughness of the ring is favorably minimized. Therefore, in a more detailed embodiment of the ring according to the present disclosure, the austenite phase fraction of the core material of the ring should be controlled to less than 6 volume-%, preferably to less than 4 volume-%, more preferably down to zero.

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 gas-soft nitriding.

Figure 4 is a photographic representation of a crosssection 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 temperatue 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 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 microstructure 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 crosssection 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

- 8 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

CONCLUSIES

1. Flexibele ring (44) bestemd voor gebruik als, of althans in, een drijfriem (3) voor een continu variabele transmissie met twee poelies (1, 2) en de drijfriem (3), welke ring (44) is gemaakt van maraging staal en is voorzien van een genitreerde oppervlaktelaag, met het kenmerk, dat een microstructuur van de genitreerde oppervlaktelaag van de ring (44) tenminste 2 en ten hoogste 10 volume-% austenietkristallen omvat.
2. De flexibele ring (44) volgens de conclusie 1 met het kenmerk, dat de microstructuur van de genitreerde oppervlaktelaag daarvan tenminste 4 en ten hoogste 8 volume-% austenietkristallen omvat.
3. De flexibele ring (44) volgens de conclusie 1 of 2 met het kenmerk, dat de microstructuur van de genitreerde oppervlaktelaag daarvan een overwegend martensitische microstructuur bezit.
4. De flexibele ring (44) volgens de conclusie 1, 2 of 3 met het kenmerk, dat een microstructuur van het binnen de genitreerde oppervlaktelaag van de ring (44) gelegen kernmateriaal van de ring (44) ten hoogste 6 volume-% austenietkristallen omvat.
5. De flexibele ring (44) volgens de conclusie 4 met het kenmerk, dat een microstructuur van het binnen de genitreerde oppervlaktelaag van de ring (44) gelegen kernmateriaal van de ring (44) ten hoogste 4 volume-% austenietkristallen omvat.
6. Een werkwijze voor het uit maraging staal vervaardigen van een flexibele ring (44), waarin de ring wordt onderworpen aan één of meer warmtebehandelingen voor het precipitatieharden en het gasnitreren van een oppervlaktelaag daarvan, met het kenmerk, dat in de genoemde één of meer warmtebehandelingen austenietkristallen in de genoemde oppervlaktelaag van de ring (44) worden gevormd.

5
7. De werkwijze volgens de conclusie 6 met het kenmerk dat na afloop van de genoemde één of meer warmtebehandelingen de daarin gevormde austenietkristallen tenminste 2 en ten hoogste 10 volume-% van de genoemde oppervlaktelaag van de flexibele ring (44) uitmaken.
8. De werkwijze volgens de conclusie 6 of 7 met het kenmerk dat tenminste de warmtebehandeling van het gasnitreren van de oppervlaktelaag daarvan wordt uitgevoerd bij een temperatuur waarbij althans een deel

15 van de martensietkristallen in de genoemde oppervlaktelaag in austenietkristallen worden omgezet.

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