MX2013015262A

MX2013015262A - Manufacturing method for a drive belt ring component.

Info

Publication number: MX2013015262A
Application number: MX2013015262A
Authority: MX
Inventors: Wilhelmus Petrus Maria Janssen; Arjen Bransman
Original assignee: Bosch Gmbh Robert
Priority date: 2011-06-30
Filing date: 2012-06-26
Publication date: 2014-02-27
Also published as: MX340810B; JP2014520957A; WO2013002633A1; CN103732782A

Abstract

The invention concerns a method for manufacturing an endless metal ring (32) for a drive belt suitable for power transmission in a continuously variable transmission including at least a process step (IX) of nitriding or case hardening the ring (32) in an oven chamber (50) containing a process atmosphere that is supplied with both ammonia gas and with hydrogen gas. Preferably, at least the relative volume content of the hydrogen gas present in the oven chamber (50) is actively controlled to a desired value.

Description

MANUFACTURING METHOD FOR A BAND RING COMPONENT OF TRANSMISSION The present invention relates to a method of manufacturing an endless thin flexible metal band, the band of which is typically incorporated into a transmission belt for power transmission between two adjustable pulleys of the well-known continuous variable transmission or applied CVT in motor vehicles.

At least in relation to this application in the transmission band such a band is also referred to as the ring component of the transmission band. Such a transmission belt and its ring component are generally known, for example, from EP-A-1 403 551. In this type of known transmission belt, which is usually referred to as the thrust band, a number of components of ring in at least one, but typically two laminates, that is, assemblies mutually arranged concentrically thereof. The known push band further comprises a number of transverse metal elements which are slidably mounted in the set or sets of rings. In the application of the thrust band thereof, the ring components are typically produced from martensitic steel and in manufacturing are they are, at least, subject to heat treatments of aging and nitriding to provide them with an extraordinary resistance to fatigue and wear resistance properties. More particularly, a surface layer of the ring components is strengthened by soft gas nitriding, whereby the nitrogen atoms (interstitial) are introduced into the outer layers of the atomic network of the martensitic steel by diffusion. The gentle gas nitriding process involves keeping the ring components at a high temperature in a furnace chamber containing an ammonia gas. In the nitriding of soft gas, the ammonia gas dissociates at the surface of the ring component into hydrogen gas and nitrogen atoms that enter the metallic network of the rings by diffusion.

The process steps of the general manufacturing method for such transmission bands and the ring component thereof in particular, as applied for several decades now, become meanwhile well known in the art and are, for example, described in the European patent application EP-A-1 055 738.

A general desire in the well-known soft gas nitriding process is to reduce the intensity thereof in terms of, at least, the concentration of ammonia in the process atmosphere, as much as possible. Obviously, at to do so, the use (and cost) of ammonia gas, as part of the process gas mixture with nitrogen, can be favorably reduced, but also the formation of residual iron-nitride compounds, such as Fe4N, in the surface layer of The ring components can be prevented more effectively in this way. This so-called composite layer usually makes the ring components too fragile to drive the intended transmission belt application thereof. On the other hand, decreasing the intensity of soft gas nitriding substantially increases the risk that substantial fluctuations will appear in the thickness of the nitrided surface layers formed in the present, due to the comparatively low concentration of ammonia in the general atmosphere of the process , which fluctuates easier and more substantially. Moreover, decreasing the intensity will typically disadvantageously increase the processing time required to recognize a given thickness of nitrided surface layer.

It is an object of the present invention to improve the smooth gas nitriding process known as it is applied in the manufacture of the ring components of the transmission belt. More particularly, it is directed to improve the homogeneity and / or consistency of the surface nitrated at a comparatively low concentration of ammonia. It is considered that underlying the present invention is the desire to control in a precise and consistent manner the composition of the process atmosphere in the nitriding of smooth gas as best as possible.

According to the invention, the above objective can be realized by actively controlling the chemical reaction occurring in the process atmosphere of the soft gas nitriding process, ie: 2NH3 < = > N2 + 3H2 in the gas phase (1) or 2NH3 2N + 3¾ on the surface of the ring component where NH3 is the chemical formula of ammonia N is the chemical symbol of nitrogen H is the chemical symbol of hydrogen This active control of these chemical reactions is carried out by supplying not only ammonia gas to the process atmosphere in the furnace chamber, but also hydrogen gas. It was found that, as a result, the nitriding reaction and therefore, also the concentration of ammonia on the surface of the ring component remains favorably much more stable, is say it fluctuates much less through the whole process and / or the process atmosphere. Moreover, by actively controlling the nitriding reaction, it can be controlled accurately and rapidly towards a different equilibrium state thereof, as may be desired when different compositions of material have to be treated, when different thicknesses of nitride layer are required , etc. In other words, the process of nitriding becomes much more flexible, since it has become much better adaptable to vary the (external) circumstances.

In particular, the active control is based on the equilibrium constant KN of the nitriding reaction mentioned above in the gas phase (1): KN = (? [?? 3]) / (? [? 2]? 1 · 5) (2) where p [X] represents the partial pressure in the process atmosphere of compound X and it effectively entails maintaining said equilibrium constant KN at a desired prescribed value selected from within the range between 0.5 and 50 bar "½ depending on the temperature of the process in which the soft gas nitriding is performed. observed that, when the temperature of the process increases, the residual layer of the compound is formed more and more easily, that is, an equilibrium constant KN more Low is required to prevent such a layer of compound from forming.

Because a rapid rate of diffusion of the nitrogen atoms in the ring component and therefore, an economical process of soft gas nitriding is performed in this way, preferably a high process temperature is generally considered, in relation with the present invention, the temperature of the process between 465 and 515 degrees Celsius combined with the equilibrium constant KN is set between 1 and 11 to 465 degrees Celsius and between 1 and 3 to 515 degrees Celsius is considered the optimal process adjusted process of soft gas nitriding according to the present invention, at least for a specific martensitic steel base material (see Figure 7 for suitable adjustments of the equilibrium constant KN in-between these two extreme values). More specifically, in combination with the preferred value of about 10 volume-% hydrogen gas, the process atmosphere of preference is further composed of about 5 to 25 volume-% ammonia gas with a remainder of nitrogen gas .

Finally, it is a preferred feature of the soft gas nitriding process according to the invention that the hydrogen gas supplied to the process atmosphere is obtained from ammonia, in particular by dissociating or cracking the ammonia gas at an elevated temperature in an ammonia cracker to produce hydrogen gas and nitrogen gas in accordance with the nitriding reaction (1) above. Obviously, such a feature allows the soft gas nitriding process to be conducted with only a simple source for the process gas, i.e., pure ammonia gas or, more practically, a mixture thereof with nitrogen gas.

The basic principle of the invention will now be elucidated by way of example, together with a drawing in which: Figure 1 provides a schematic perspective view of a continuously variable transmission with a transmission belt running on two pulleys.

Figure 2 is a schematic illustration of a portion of the known transmission belt shown in perspective, which in part includes two ring assemblies, each including a number of metal ring components arranged concentrically, as well as a plurality of transverse members .

Figure 3 presents in diagram mode an overview of a part of the manufacturing method of the known transmission band, including a step of the smooth gas nitriding process of the ring component of the transmission belt.

Figure 4 shows in more detail the known process step of soft gas nitriding.

Figure 5 diagrammatically represents a novel modified stage of the gentle gas nitriding process illustrating the basic concept of the invention.

Figure 6 diagrammatically represents a further elaboration of the stage of the soft gas nitriding process according to the invention.

Figure 7 is a graph of the possible formation of iron-nitride depending on the nitriding process of soft gas.

Figure 1 shows schematically the central parts of a continuously variable transmission or CVT that is commonly applied in the drive line of the motor vehicles between the engine and the traction wheels thereof. The transmission comprises two pulleys 1, 2, each provided with two conical pulley discs 4, 5, where between a predominantly V-shaped pulley groove is defined and of which a disc 4 is axially movable along a pulley shaft 6, 7 on which it is placed. A transmission belt 3 is wound around the pulleys 1, 2, while being held by friction, that is to say, clamped between the pulley discs 4, 5 thereof, to transmit a torsion T and a rotary movement? companion from a pulley 1, 2 to the other 2, 1. At the same time, the operating radius R of the transmission belt 3 between the discs 4, 5 of the respective pulleys 1, 2 determines the (speed) ratio "i "of the CVT, that is, the relationship between the rotational speeds of the respective pulleys 1, 2. This CVT and its main operation are known per se.

An example of a known transmission belt 3 is shown in more detail in Figure 2 in a section thereof, whose strip 3 is shown to incorporate two endless carriers 31, or ring assemblies 31, which are each composed of a concentrically arranged number, i.e. mutually nested individual ring components 32. The transmission belt 3 further comprises a plurality of plate-like transverse members 30 which are in contact with and held together by the ring assemblies 31. The transverse members 30 assimilate the clamping force exerted between the discs 4, 5 of each pulley, 1,2 by the contact surfaces of the pulley 33 that are provided on each lateral side thereof. These pulley contact surfaces 33 diverge mutually radially outward to essentially match the defined V-angle between the conically shaped pulley discs 4, 5, ie, the V-shaped pulley notch of the pulleys 1, 2. A so-called oscillating edge 34 of each transverse member 30 represents the transition between a radially outer part of the constant thickness and a conical radial inner part thereof. This oscillating edge 34 and the conical shape of the transverse members 30 is what allows the transmission belt 3 to follow a smoothly curved path.

During the operation in the CVT, the transmission belt 3 and in particular its ring components 32 are subjected to cyclically varying traction and bending stresses, i.e., a fatigue load. Typically, the fatigue resistance or fatigue resistance of the ring component 32 therefore determines the functional lifetime of the transmission belt 3 at a given torque T to be transmitted in this way. Therefore, it has long been the general objective in the development of the manufacturing method of the transmission belt to perform a required ring of fatigue resistance in a combined material and minimal processing cost.

Figure 3 illustrates a relevant part of the known manufacturing method for the ring assembly 31 of the transmission belt 3, and it is practiced from the first years of production of the transmission belt, in particular, of automotive application. In Figure 3, the steps of the separate process are indicated as Roman numerals.

In a first stage of the process I a thin sheet or plate 11 of a martensitic steel base material having a thickness of about 0.4 mm is flexed into a cylindrical shape and the ends of the meeting plate 12 are welded together in a second stage II of the process for forming a hollow cylinder or tube 13. In a third stage III of the process, the tube 13 is annealed. After that, in a fourth stage IV of the process, the tube 13 is cut into a number of annular rings 14, which are subsequently - the fifth stage V of the process - rolled to reduce the thickness thereof, typically 0.2. mm, while it lengthens. After rolling, the rings 14 are usually referred to as a ring component of the transmission band 32.

The ring component 32 is subjected to one more, that is, an annealing process step of the VI ring to eliminate the hardening effect by deformation of the pre-rolling process step by recovery and recrystallization of the ring material at a temperature considerably above 600 degrees Celsius, example, around 800 degrees Celsius. After that, in a seventh process step VII, the ring component 32 is calibrated by mounting it around two rotating rollers and extending it to a predefined circumference length by forcing the rollers separately. In this seventh stage of process VII, internal stresses are also imposed on the ring component 32.

After that, the ring component 32 is heat treated in two separate process steps, i.e., an eighth step of process VIII of aging or hardening by higher precipitation and a ninth stage of process IX of nitriding or cementation. More particularly, thermal treatments involve heating the ring component 32 in a furnace chamber 50 containing a controlled gas atmosphere, i.e., the respective process atmosphere. In case of aging, such a process atmosphere is typically composed of nitrogen and some, for example up to 5 volume-% hydrogen. In the case of nitriding, such a process atmosphere also contains ammonia gas which decomposes on the surface of the ring component in the hydrogen gas and nitrogen atoms. These nitrogen atoms enter, that is, they diffuse into the metallic network of the ring component that provides such with wear resistance and a layer of hardened nitrided surface.

Both thermal treatments typically occur within the temperature range of 400 degrees Celsius to 500 degrees Celsius and each can lose from about 45 up to 120 minutes in dependence on the material basis (martensitic steel alloy composition) for the component of ring 32, as well as on the desired mechanical properties thereof. In this latter respect it is pointed out that, typically, it is directed to a ring core hardness value of 520 HV1.0 or more, a ring surface hardness value of 875 HVO .1 or more and a layer thickness of nitrided surface, alternatively denoted as nitrogen diffusion zone, in the range of 25 to 35 microns.

It is noted that it is known in the art that both heat treatments are carried out simultaneously.

Finally, the ring set 31 is therefore formed by stacking radially, that is, a concentrically nested number of ring components 32 formed and processed, as further indicated in Figure 3 in the tenth stage of the X process and final depicted at the moment. Since only a small positive or negative elimination is allowed between the neighboring ring components 32, each ring component 32 of the ring assembly 31 is required to be sized appropriately in relation to the other ring components 32 thereof. For this purpose, however, the individual ring components 32 of the ring assembly 31 are selected from an existence of precisely known ring components 32 of different circumferential lengths.

In Fig. 4 the ninth stage of the smooth gas nitriding process IX is illustrated somewhat in greater detail. The furnace chamber 50 is shown to be supplied with the process gas composed of a mixture of nitrogen and ammonia gas via the supply line 51 and valve (regulator) 52. This supply of the process gas results in a corresponding discharge of the process atmosphere in the furnace chamber 50 by a discharge line 53. Typically, any process atmosphere that is expelled from the furnace chamber 50 is burned off. It will be clear that due to said "fresh" process gas supply and such "used" process atmosphere discharge, the composition of the process atmosphere will not be homogeneous through the furnace chamber 50. Moreover, the opening and Closing the doors 54 for loading and unloading the ring components 32 alter the process atmosphere as well.

Therefore, because the process atmosphere is not typically homogeneous, the resulting nitrided surface layer of the ring component 32 can not be uniformly formed, which deficiency of the known smooth gas nitriding process is aggravated when the concentration of global ammonia in the process atmosphere becomes smaller. Still, a low concentration of ammonia should be favored from a process efficiency perspective.

According to the invention, the homogeneity and / or consistency of the nitrided surface layer of the ring component can favorably and substantially improve, especially at a relatively low concentration of ammonia in the process atmosphere, if the furnace chamber 50 It is not only supplied with ammonia gas, but also with hydrogen gas. This novel configuration of the ninth step of the smooth gas nitriding process IX in the manufacturing method of the ring assembly 31 is illustrated in Figure 5 and includes an additional supply line 55 and associated valve (regulator) 56 for delivery controlled hydrogen gas, that is, in addition to the controlled supply of ammonia gas through the supply line 51 and the associated valve (regulator) 52.

In addition, it is observed that it is also favorable to maintain the overall process atmosphere at a small overpressure relative to atmospheric pressure, to avoid, or at least minimize, the interference of the surrounding atmosphere with the composition of the process atmosphere.

Preferably, the amount of hydrogen gas that is supplied to the process atmosphere is controlled based on the hydrogen content measured in the process atmosphere at a desired value that is selected within the range between 5 and 15 volume-%, by example, 10 volume-%.

According to the invention, the effectiveness of the novel smooth gas nitriding process can be improved by obtaining the hydrogen gas from the process of dissociating the ammonia gas from the process, as schematically illustrated in Figure 6. In Figure 6, the The ammonia-containing process is supplied not only directly to the furnace chamber 50, but also to an ammonia cracker 57 to produce hydrogen gas.

According to the invention, the effectiveness of the novel soft gas nitriding process can be improved by applying a high temperature in the furnace chamber 50. However, it has been found that iron-nitrides are more easily formed, i.e. an increasingly smaller value of the equilibrium constant KN of the nitriding reaction (1), so that it increases the process temperature. These iron-nitrides are per udicial when they inhibit the progress of the nitriding process and can also form a brittle surface layer of the ring component 32 which decreases the fatigue strength thereof.

This last aspect of the soft gas nitriding process, that is, whether or not the iron-nitride formation occurs in the present, is illustrated in the graph of Figure 7 depending on the temperature of the process and the value of the constant equilibrium KN. from this figure 7 it seems that the high temperature of the process is adjusted, the smallest equilibrium constant KN of the nitriding reaction (1) which must be adjusted to avoid iron-nitride formation is reduced. Based on this figure 7, the currently preferred process configurations of a process temperature of about 500 degrees C and an equilibrium constant KN of about 4 are indicated by broken lines.

It is observed that, even if the previous configurations of the soft gas nitriding process are adjusted in such a way that iron-nitrides are formed, the intensity or velocity of such iron-nitride formation still depends on the current magnitude thereof. Such an iron-nitride formation intensity is schematically represented in Figure 7 by the color gradient white to black. Accordingly, although it is preferable to apply the process configurations on the underside of the solid line in Figure 7, a smaller transgression to the upper side thereof may be acceptable.

The invention will now be further defined throughout a set of claims and, apart from the preceding description, also relates to all the details herein, and to all the details and aspects in the discussed drawings that are direct and unambiguous derivable therefrom by the person skilled in the art.

Claims

NOVELTY OF THE INVENTION Having described the present invention as above, it is considered a novelty and therefore the property described in the following is claimed as property: CLAIMS

1. A method for the nitriding of a metal ring for a thrust band for a continuously variable transmission, wherein the ring is placed in a furnace chamber that is supplied with ammonia gas, characterized in that, the furnace chamber is further supplied with hydrogen gas.

2. The method according to claim 1 is characterized in that an amount of hydrogen gas present in the furnace chamber is measured and controlled to a desired amount of hydrogen gas by regulating the hydrogen gas supply to the oven chamber.

3. The method according to claim 1 or 2 is characterized in that the desired amount of the hydrogen gas has a value between 5 and 15 volume-% and preferably is around 10 volume-%.

4. The method according to claim 1, 2 or 3, is characterized in that, by regulating the supply of the hydrogen gas and the supply of the ammonia gas to the furnace chamber, the value of the equilibrium constant KN of the nitriding reaction in the gas phase in the furnace chamber, ie: 2NH3 O N2 + 3H2 Where NH3 is the chemical formula of ammonia N is the chemical symbol of nitrogen H is the chemical symbol of hydrogen whose equilibrium constant KN satisfies the equation: KN = (p [NH3]) / (p [NH2])? 1 · 5) where: p [NH3] represents the partial pressure of the ammonia gas p [H2] represents the partial pressure of hydrogen gas which is controlled to a desired value of the equilibrium constant KN.

5. The method according to claim 4 is characterized in that the desired value of the equilibrium constant KN is between 4 and 11.

6. The method according to one of the preceding claims, characterized in that, the metal ring is nitride at a temperature between 465 and 515 degrees Centigrade.

7. The method of compliance with one of the The preceding claims are characterized in that the desired value of the equilibrium constant KN is about 4 and that the metal ring is nitride at a temperature of about 500 degrees Centigrade.

8. The method according to one of the preceding claims is characterized in that an amount of the ammonia gas present in the furnace chamber has a value between 5 and 25 vol%.

9. The method according to one of the preceding claims is characterized in that the total gas pressure in the furnace chamber is below ambient atmospheric pressure.

10. The method according to any of the preceding claims is characterized in that the hydrogen gas supplied to the furnace chamber is obtained from the ammonia gas by cracking.