Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16G—BELTS, CABLES, OR ROPES, PREDOMINANTLY USED FOR DRIVING PURPOSES; CHAINS; FITTINGS PREDOMINANTLY USED THEREFOR
  • F16G5/00—V-belts, i.e. belts of tapered cross-section
  • F16G5/16—V-belts, i.e. belts of tapered cross-section consisting of several parts
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/24—Nitriding
  • C23C8/26—Nitriding of ferrous surfaces

Definitions

• the present disclosure relates to a heat treatment process in a manufacturing method of a ring set for a drive belt, as defined by the preamble of the claim 1 hereinafter.
• the drive belt is mainly used as the means for power transmission between two adjustable pulleys of the well-known continuously variable transmission that is mainly applied in motor vehicles.
• the present type of drive belt is generally known and is composed of a multitude of transverse elements or segments that are slidably incorporated on one or more ring sets that are each composed of a number of mutually nested, flexible belt rings.
• the transverse elements are not connected to the ring sets, but rather slide along the circumference thereof during operation in the CVT. Also the individual belt rings of the ring sets slide relative to one another during operation.
• the belt rings of the drive belt which are alternatively denoted hoops, loops or endless bands, are produced from steel, in particular a maraging steel, that combines -amongst others- the mechanical characteristics of great tensile and (bending) fatigue strength with a relatively favourable possibility to process the steel from sheet-shaped base material towards the desired shape and material properties of the end-product belt rings.
• These desired material properties comprise a fair hardness of the ring core material, for combining the characteristics of a great tensile strength together with a sufficient elasticity and ductility to allow longitudinal bending of the belt ring, and a much harder outer or surface layer, for providing wear resistance to the belt ring.
• the overall manufacturing method for such drive belts has become well known in the art.
• An important part of such overall manufacturing method is the heat treatment of the belt rings, which heat treatment includes the process steps of precipitation hardening and of nitriding.
• Precipitation hardening is also known as aging and is realised through heating the belt rings to a temperature exceeding 400 degrees Celsius fC), at which temperature microscopic metallic precipitates incubate and grow at random locations throughout the ring material. As the precipitates grow, the hardness of the ring material increases until, generally speaking, a maximum hardness value is reached, after which the hardness of the ring material typically starts to decrease again (so- called "over-aging").
• precipitation hardening is normally performed in an inert or a reducing process atmosphere, such as nitrogen gas or nitrogen gas with some hydrogen gas mixed-in.
• Nitriding provides the belt rings with an additionally hardened and, moreover, compressively (pre-)stressed surface layer.
• the belt rings are kept in an ammonia gas (NH3) containing process atmosphere at a temperature of, likewise, more than 40CTC. At such temperature, the ammonia molecules dissociate at the surface of the belt rings, forming hydrogen gas and nitrogen atoms, which latter nitrogen atoms enter into the crystal lattice of the ring material.
• NH3 ammonia gas
• the nitrogen atoms move away from the surface into the ring material by diffusion at a rate that is dictated by the process temperature, thus providing the belt ring with a nitrided surface layer of increasing thickness.
• the thickness of the nitride layer, as well as closely related material properties, such as a hardness and a compressive residual stress at the ring surface, that are obtained in/by the nitriding process thus depend on the composition of the nitriding process atmosphere, in particular the ammonia and hydrogen concentrations therein, and on the temperature and duration of the nitriding process. Together, these three parameters of process atmosphere composition, process temperature and process time/duration thus represent the main process settings of the nitriding process.
• the nitride layer thickness of the belt rings is of predominant importance in determining the longevity of the belt.
• the wear and fatigue properties of the belt ring will be insufficient, or, if the nitride layer is too thick, the ring material will be too brittle and ring stress levels will exceed the elastic limit during operation. In either case, the belt ring, and hence the drive belt as a whole, will fail prematurely. Therefore, once a desired or target value has been specified for the nitride layer thickness, it is essential that such target value is accurately and consistently realised in the (mass) manufacture of the drive belt.
• the present disclosure aims to improve the existing manufacturing method of the drive belt ring set, in particular in terms of the process step of ring set nitriding therein. According to the present disclosure such improvement is found in the manufacturing method according to the claim 1 hereinafter.
• an active control of the process step of ring set nitriding is provided that affects the thickness of the nitride layer both at the ring surfaces facing one another inside of the ring set and at the ring surfaces facing towards the outside of the ring set in a largely corresponding manner.
• the process step of ring set nitriding is prolonged, if an actual (i.e. measured) nitride layer thickness is smaller than the target value for such thickness, and it is shortened, if that actual (i.e. measured) nitride layer thickness is larger than that target value.
• the supply of the process gas, in particular of the ammonia gas, and the process temperature are preferably left unchanged, at least in response to such thickness deviation.
• figure 1 provides a schematically depicted example of the well-known continuously variable transmission provided with a drive belt
• figure 2 is a section of the drive belt shown in perspective
• figure 3 schematically illustrates the presently relevant part of the known manufacturing method of the ring set component of the drive belt
• figure 4 provides a figurative representation of the process step of gas-soft nitriding in the manufacturing method according to figure 3,
• 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 drive wheels thereof.
• the transmission comprises two pulleys 1 , 2, each provided with two conical pulley discs 4, 5, where between a predominantly V- shaped groove is defined and whereof one disc 4 is axially moveable along a respective pulley shaft 6, 7 over which it is placed.
• a drive belt 3 is wrapped around the pulleys 1 , 2 for transmitting a rotational movement ⁇ and an accompanying torque T from the one pulley 1 , 2 to the other 2, 1 .
• the transmission generally also comprises activation means that impose on the said at least one disc 4 an axially oriented clamping force Fax directed towards the respective other pulley disc 5 such that the belt 3 is clamped there between. Also, a (speed) ratio of the transmission between the rotational speed of the driven pulley 2 and the rotational speed of the driving pulley 1 is determined thereby.
• FIG. 2 An example of a known drive belt 3 is shown in detail in figure 2 in a section thereof, which belt 3 incorporates two ring sets 31 that are each composed of a set of -in this example- six thin and flat, i.e. band-like, flexible belt rings 32.
• the belt 3 further comprises a multitude of plate-like metal transverse elements 33 that are held together by the ring sets 31 that are each located in a respective recess of the transverse elements 33.
• the transverse elements 33 take-up the said clamping force Fax, such when an input torque Tin is exerted on the so-called driving pulley 1 , friction between the discs 4, 5 and the belt 3, causes a rotation of the driving pulley 1 to be transferred to the so-called driven pulley 2 via the likewise rotating drive belt 3.
• the drive belt 3 and in particular the belt rings 32 thereof are subjected to a cyclically varying tensile and bending stresses, i.e. a fatigue load.
• a fatigue load i.e. the resistance against metal fatigue, i.e. the fatigue strength of the rings 32 thus determines the functional life span of the drive belt 3 at a given torque T to be transmitted thereby. Therefore, it has been a long standing general aim in the development of the ring set manufacturing method to realise the required ring fatigue strength at a minimum combined material and processing cost.
• Figure 3 illustrates the presently relevant part of the known overall drive belt 3 manufacturing method, i.e. of the manufacturing of the ring set(s) 31 thereof, wherein separate process steps are indicated by way of Roman numerals.
• a thin sheet or plate 1 1 of base material that typically has a thickness in the range between 0.4 mm and 0.5 mm is bend into a cylindrical shape and the meeting plate ends 12 are welded together in a second process step II to form an open, hollow cylinder or tube 13.
• the tube 13 is annealed.
• the tube 13 is cut into a number of annular hoops 14, which are subsequently -process step five V- rolled to reduce the thickness thereof to a value between 0.150 and 0.200 mm, typically to about 185 microns, while being elongated.
• the hoops 14 are usually referred to as belt rings 32.
• the belt rings 32 are then subjected to a further, i.e. ring annealing process step VI for removing the work hardening effect of the previous rolling process (i.e. step five V) by recovery and re-crystallisation of the ring material at a temperature considerably above 600 degree Celsius (' E C"), e.g. about 800 °C.
• a seventh process step VII the belt rings 32 are calibrated, i.e. they are mounted around two rotating rollers and stretched to a predefined circumference length by forcing the said rollers apart.
• an internal stress distribution is imposed on the belt rings 32.
• ring sets 31 are assembled, each from a number of the belt rings 32 of suitable, mutually adapted circumference length, by stacking these belt rings 32 one around the other.
• the ring sets 31 are heat-treated in a ninth process step IX of precipitation hardening or aging IX-A and of gas-soft nitriding IX-N. More in particular, aging and nitriding involve heating the ring sets 31 to a temperature of between 400 and 500 °C, typically 485 °C, in a furnace containing a controlled gas atmosphere that is typically composed of nitrogen, hydrogen and ammonia gas, typically around 10 vol.-% ammonia gas.
• the exact process settings of the heat treatment are selected in dependence on the base material of the belt rings 32 (i.e. the alloy composition of the maraging steel), as well as on the mechanical properties that are desired for the belt rings 32. In this latter respect it is remarked that, typically, it is aimed at a core hardness value of at least 500 HV1.0, at a surface hardness value of at least 800 HV0.1 and at a thickness of the nitrided surface layer, alternatively denoted nitrogen diffusion zone, of 25 to 35 microns.
• the duration of the above heat treatment is then obtained as a consequence of these mechanical properties, the process temperature and the process atmosphere composition, which in practice will normally have a value in the range from 30 to 90 minutes.
• FIG 4 the nitriding part of the ninth process step IX, i.e. the heat treatment of ring set 31 nitriding is schematically illustrated.
• a gap 34 between these two rings 32 is highly exaggerated (i.e. in reality the width of such gap 34 is far less than the thickness of the belt rings 32 and may amount to only a couple of microns or so).
• the ring set 31 is immersed in a process atmosphere containing gaseous ammonia molecules that are schematically represented in figure 4 by four circles each: a large circle representing a nitrogen atom and three smaller circles representing the hydrogen atoms of the ammonia molecules. At least some of the ammonia molecules will dissociate at the surfaces of the belt rings 32, whereby three hydrogen atoms are released to allow the one nitrogen atom to enter into the crystal lattice of the belt ring 32, which ammonia dissociation reaction is schematically represented in figure 4 inside the dashed ellipses. As part of the ammonia dissociation reaction, the released hydrogen atoms combine to form hydrogen gas.
• the ammonia dissociation reaction can thus be represented in a formula, as follows: 2NH 3 2[N] + 3H 2 (1 )
• the rate at which the ammonia dissociation reaction (1 ) occurs at the surface of the belt rings 32 is affected differently between the inside and the outside of the gap 34.
• the supply of ammonia molecules is critical, such that the rate of the ammonia dissociation reaction varies less in relation to the process temperature then outside the gap 34.
• the ammonia molecules are relatively abundant, such that the ammonia dissociation is predominantly determined by the process temperature, in particular if the ammonia concentration in the process atmosphere is sufficiently high and the process temperature is sufficiently low.
• a change in either one of the parameters of the ammonia concentration in the process atmosphere and of the process temperature of the ninth process step IX of combined aging and nitriding is found to affect the thickness of the nitride layer differently between, on the one hand, the surfaces of the belt rings 31 that face the gap(s) 34 and, on the other hand, those ring surfaces that constitute the external surfaces of the ring set 31 as a whole.
• the active control of the ninth process step IX of combined aging and nitriding is opted to realise the active control of the ninth process step IX of combined aging and nitriding, more in particular the active control of the thickness of the nitride layer provided to the belt rings 32 treated therein, through the parameter of the duration thereof. That is to say that the composition of the process gas supplied to the process atmosphere and the process temperature applied in this ninth process step IX are predetermined for providing the belt rings 32 with desired material properties, such as a desired nitride layer thickness, and that a drifting of the process in terms of the resulting, i.e. the actual nitride layer thickness deviating from the said desired thickness thereof, is counteracted by adapting the process duration, in particular in proportion to such deviation.
• the process temperature applied therein is set to the -at least in comparison with what is presently commonly applied in practice- relatively low value of less than 475 °C, preferably of about 470 °C.
• the ammonia concentration in the process gas supplied to the process atmosphere applied therein is set to the -at least in comparison with what is presently commonly applied in practice- relatively high value of more than 12 vol.-%, preferably of about 15 vol.-%.

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• Engineering & Computer Science (AREA)
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• Mechanical Engineering (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
• Heat Treatment Of Articles (AREA)

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