Process for hardening a rack using a gas flow

Abstract

Hardening method for a rack of a power steering system of a vehicle, the method comprising: A heating step in which at least one area to be treated of an outer surface of the rack is heated by means of an electric current induced in said area; Characterized in that the method comprises: A cooling step in which the at least one area is cooled by means of at least a first gas flow projected onto said area. Figure 1

Description

Title of the invention: Method for hardening a rack using a gas flow Technical field

[0001] The invention relates to the field of methods for manufacturing a power steering rack, and more particularly to a method for hardening said rack and a hardening device. State of the prior art

[0002] A vehicle steering system is intended to enable a driver to control a trajectory of the vehicle by modifying an orientation angle of the vehicle's wheels by means of a steering wheel.

[0003] There are steering systems in which a variation in rotation of the vehicle wheels is effected by a mechanical assembly composed of a steering pinion which meshes with a rack. The rack is mounted to slide in a longitudinal direction in a steering casing. The two ends of the rack, external to the casing, are coupled respectively to two steering rods, which are themselves associated respectively with the left and right steering wheels of the vehicle.

[0004] The rack comprises on the one hand a toothing formed of teeth and, on the other hand, a toothing back opposite the toothing. The toothing extends in a longitudinal direction of the rack.

[0005] During the manufacture of the rack, a quenching operation is carried out in order to harden the surface of said rack. More precisely, the quenching operation is a heat treatment which aims to transform the metal of the rack over a quenching thickness from an austenitic crystalline structure obtained during a heating step, into a martensitic crystalline structure.

[0006] There is a known method for carrying out this quenching consisting of generating electric currents in the rack, by induction or by conduction, so as to increase the surface temperature, then putting the heated part in contact with water so as to quickly cool the rack.

[0007] The disadvantage of such a method is that it requires a large quantity of water to be decontaminated and cooled, generates water vapor, uses additives in the water such as polymers, and causes distortion of the rack which may require the addition of a step of straightening said rack.

[0008] There is therefore a need for a tempering process that is particularly more ecological. Statement of the invention

[0009] One embodiment relates to a hardening method for a rack of a power steering system of a vehicle, the method comprising: - A heating step in which at least one area to be treated of an outer surface of the rack is heated by means of an electric current induced in said area;

[0010] Characterized in that the method comprises: - A cooling step in which the at least one zone is cooled by means of at least a first gas flow projected onto said zone.

[0011] The quenching process consists of carrying out a heat treatment which aims to transform the metal of the rack over a quenching thickness. The heating step makes it possible to obtain an austenitic crystalline structure and then the cooling step makes it possible to transform the austenitic crystalline structure into a martensitic crystalline structure.

[0012] The rack comprises an outer surface to be treated. In other words, the rack comprises an outer surface whose crystalline structure must be modified over a hardening thickness which is preferably less than a total thickness of the rack.

[0013] Thus, an internal part of the rack, that is to say the part between the quenching thickness and a center of the rack, is not quenched, in other words, its crystalline structure is not modified. The internal part compresses the treated zone so as to create compressive stresses improving fatigue resistance of the rack.

[0014] Furthermore, a transformation into martensite by quenching of the outer surface to be treated makes it possible to improve the surface hardness of the outer surface and thus increase the wear resistance of the rack.

[0015] The quenching process comprises a heating step followed by a cooling step.

[0016] The heating step is carried out by means of an electric current induced in the area to be treated. A temperature reached by the area to be treated depends on the material of the area to be treated. With certain grades of steel, the temperature of the area to be treated is higher than 700°C, for example higher than 850°C, or 900°C.

[0017] The cooling step consists of reducing the temperature of the zone so as to transform the austenitic crystalline structure obtained during the heating step into the martensitic crystalline structure. For this, a cooling rate of the zone must be at least equal to a martensitic critical rate. The rate The critical martensitic strength is determined in the laboratory depending on the material and more particularly depending on the grade of the rack steel.

[0018] The method is innovative in that the cooling is carried out by means of a first gas flow projected onto the area. In other words, the gas flow is voluntarily directed at a determined flow rate, temperature and pressure onto the area. The first gas flow is an artificially created flow to cool the area.

[0019] Indeed, the applicant has found that it is possible to obtain, contrary to what is commonly accepted, a martensitic structure by cooling with a gas, that is to say with a fluid having a drasticity lower than that of water, for the quenching thickness commonly accepted for racks, for example a few millimeters.

[0020] Drasticity corresponds to the cooling power of the fluid, that is to say an ability of the fluid to evacuate calories from a metal. The drasticity for a fluid is calculated by the formula:

[0021] [Math.l]

[0022] With:

[0023] H: the drasticity in mm-1

[0024] a: the exchange coefficient in W / m2K

[0025] X: the thermal conductivity coefficient of the material to be cooled in W / mK

[0026] Using a gas during the cooling step allows for slower cooling than using water. Stresses exerted in the rack during the cooling step are therefore reduced, as are the resulting deformations and cracks.

[0027] The method according to the invention is therefore less expensive than that of the state of the art in that it reduces the number of racks that are non-compliant due to the presence of cracks or excessive deformation.

[0028] Furthermore, the method according to the invention does not require the use of water, which will be partially evaporated, and which must be decontaminated and cooled, and additives which can be partially burned when brought into contact with the hot rack and thus create potentially toxic fumes.

[0029] Finally, the use of a gas makes it possible to avoid pollution of the heating elements, such as inductors, leading to their degradation and replacement.

[0030] The subject matter of the present disclosure may also have one or more of the following characteristics taken alone or in combination.

[0031] In some embodiments, at least a second gas stream is projected onto one end of the rack.

[0032] Thus, the second gas flow allows constant cooling of the rack, and in particular of the center of the rack. The cooling step is improved.

[0033] In certain embodiments, an outlet pressure of a nozzle for spraying the first and / or second gas flow is between 0.5 bar and 30 bar, for example between 1 bar and 20 bar, or greater than or equal to 1 bar, for example greater than 5 bar or 20 bar.

[0034] Thus the gas flow is projected under pressure so as to improve the drasticity of the gas and therefore the efficiency of the cooling step.

[0035] In some embodiments, the gas of the first or second gas stream is air, and / or at least one neutral gas.

[0036] The first or second gas flow can therefore also be a mixture of air and neutral gases, or a mixture of neutral gases.

[0037] Using air allows for a particularly inexpensive and environmentally friendly cooling step in that the air does not need to be cooled, recycled or depolluted.

[0038] Using a neutral gas allows for an improvement in the drasticity of the gas.

[0039] For example, the neutral gas can be chosen from: nitrogen, helium, argon.

[0040] In some embodiments, during the heating step, the at least one area is heated by induction of electric current or by conduction of electric current.

[0041] When the area is heated by induction, the electric current is induced on the surface of the area to be treated of the rack.

[0042] When the area is heated by conduction, the electric current is transmitted to the surface of the area to be treated of the rack by two electric electrodes.

[0043] In certain embodiments, the heating step and the cooling step are carried out simultaneously on two different zones.

[0044] In other words, a first zone of the rack is heated while a second zone is cooled. The first zone will then be cooled while a third zone will be heated.

[0045] This is a continuous quenching process.

[0046] In certain embodiments, the area to be treated corresponds to an entire exterior surface to be treated.

[0047] In other words, the entire surface to be treated is heated and then cooled. The quenching process is discontinuous.

[0048] In some embodiments, the rack is made of steel.

[0049] In some embodiments, the rack is a solid bar at least in cylindrical part.

[0050] Thus the center of the rack is full of material which is not heated and which cools the area to be treated by conduction. The cooling step is therefore improved.

[0051] Another aspect of the invention relates to a quenching device configured to implement the method according to the invention, the device comprising an inductor configured to induce an electric current in the zone, and a cooler provided with at least one projection nozzle configured to project the first gas flow onto said zone.

[0052] In certain embodiments, the device also comprises a device for recovering and recycling the projected gases.

[0053] In certain embodiments, the device also comprises a device for cooling the projected gases. Brief description of the drawings

[0054] The invention will be better understood, thanks to the following description, which relates to one or more embodiments according to the present invention, given as non-limiting examples and explained with reference to the appended schematic drawings, in which:

[0055] [Fig-1] is a theoretical representation of the method according to the invention;

[0056] [Fig.2] is a schematic representation of the process when a heating step and a cooling step is carried out simultaneously;

[0057] [Fig.3] is a schematic representation of the process when the heating step and the cooling step are carried out one after the other according to a first embodiment;

[0058] [Fig.4] is a schematic representation of the process when the heating step and the cooling step are carried out one after the other according to a second embodiment; Description of the embodiments

[0059] Only the elements necessary for understanding the invention have been shown. To facilitate reading of the drawings, the same elements bear the same references from one figure to another.

[0060] The method 100 according to the invention is a hardening method for a rack 1 of a power steering system of a vehicle.

[0061] In certain embodiments, the rack 1 is made of a steel, for example a steel of a grade which allows the formation of non-equilibrium constituents (martensite or bainite) without necessarily carrying out the cooling step, i.e. under so-called “natural” cooling conditions.

[0062] In certain embodiments, the rack 1 is a solid bar that is at least partly cylindrical.

[0063] The rack 1 comprises an outer surface to be treated. In other words, an outer surface whose crystalline structure must be modified over a hardening thickness e, which is preferably less than a total thickness of the rack 1.

[0064] The quenching method 100 consists of carrying out a heat treatment which aims to transform the metal of the rack 1 over the quenching thickness e from an austenitic crystalline structure A obtained during a heating step into a martensitic crystalline structure M.

[0065] As shown in [Fig.l], the method 100 comprises the heating step E1 in which a zone to be treated Zt of the rack 1 is heated by means of an electric current induced in said zone to be treated Zt. Under the effect of the heat, the zone to be treated Zt takes on an austenitic crystalline structure.

[0066] A temperature reached by the zone to be treated Zt depends on the material of the zone to be treated. With certain steels, the temperature of the zone to be treated Zt is higher than 700°C, for example higher than 850°C or 900°C.

[0067] In some embodiments, the at least one zone Zt is heated by induction of the electric current as illustrated in Figures 2 and 3, or by conduction of the electric current as illustrated in [Fig.4].

[0068] When the area is heated by induction, the electric current is induced on the surface of the area to be treated Zt of the rack 1 by an inductor 2.

[0069] When the area is heated by conduction, the electric current is transmitted to the surface of the area to be treated Zt of the rack 1 by two electric electrodes 2'.

[0070] During the heating step, an internal part of the rack 1, that is to say the part between the quenching thickness e and a center of the rack 1, is not quenched, in other words, its crystalline structure is not modified. The internal part compresses the treated zone Zt so as to create compressive stresses which improve the fatigue resistance of the rack 1.

[0071] The method 100 comprises, following the heating step E1, a cooling step E2. The cooling step E2 consists of reducing the temperature of the zone Zt so as to transform the austenitic crystalline structure obtained during the heating step E1 into a martensitic structure M. For this, a cooling rate of the zone Zt must be at least equal to a critical martensitic rate. The critical martensitic rate is determined in the laboratory depending on the material and more particularly depending on the grade of the steel of the rack.

[0072] In the cooling step E2, the at least one zone Zt is cooled by means of at least a first gas flow Fl projected onto said zone Zt.

[0073] The method 100 is innovative in that the cooling is carried out by means of a first gas flow Fl projected onto the zone Zt. In other words, the gas flow Fl is voluntarily directed according to a determined flow rate, temperature and pressure onto the zone Zt. The first gas flow Fl is an artificially created flow to cool the zone Zt.

[0074] Indeed, the applicant has found that it is possible to obtain, contrary to what is commonly accepted, a martensitic structure M by cooling with a gas, that is to say with a fluid having a drasticity lower than that of water, for the quenching thickness commonly accepted for racks, for example a few millimeters.

[0075] Drasticity corresponds to the cooling power of the fluid, that is to say an ability of the fluid to evacuate calories from a metal.

[0076] In certain embodiments, at least a second gas flow F2 is projected onto one end of the rack 1.

[0077] The second gas flow F2 allows constant cooling of the rack 1 as illustrated in [Fig.2], and in particular from the center of the rack. The cooling step E2 is improved. In addition, the center of the rack 1 being full of material which is not heated, the center cools the area to be treated Zt by conduction C.

[0078] In certain embodiments, an outlet pressure of a projection nozzle 3 of the first F1 and / or of the second F2 gas flow is greater than or equal to 1 bar, for example greater than 5 bar or 20 bar.

[0079] Thus the gas flow F1, F2 is projected under pressure so as to improve the drasticity of the gas and therefore the efficiency of the cooling step.

[0080] In some embodiments, the gas of the first F1 or second F2 gas stream is air, and / or at least one neutral gas.

[0081] The first F1 or the second F2 gas flow can therefore also be a mixture of air and neutral gases, or a mixture of neutral gases.

[0082] The use of air allows a particularly inexpensive and ecological E2 cooling step in that the air does not need to be cooled, recycled or depolluted.

[0083] Using a neutral gas also allows for an improvement in the drasticity of the gas.

[0084] For example, the neutral gas can be chosen from: nitrogen, helium, argon.

[0085] Of course, the use of neutral gas requires the use of an enclosure of containment, not shown, in order not to pollute and make toxic the environment in which the quenching process is carried out, and in order to recycle said neutral gas.

[0086] Using a gas during the cooling step E2 allows for slower cooling than with water. Stresses acting in the rack 1 during the cooling step E2 are therefore reduced as well as the resulting deformations and cracks.

[0087] The method 100 according to the invention is therefore less expensive than that of the state of the art in that it reduces the racks 1 which are non-compliant due to the presence of cracks or excessive deformations.

[0088] Furthermore, the method 100 according to the invention does not require the use of water, which will be partially evaporated, and which must be decontaminated and cooled, and of additives which can be partially burned when brought into contact with the hot rack and thus create potentially toxic fumes.

[0089] Finally, the use of a gas makes it possible to avoid pollution of the heating elements, such as inductors 2, leading to their degradation and replacement.

[0090] Another aspect of the invention relates to a quenching device configured to implement the method 100, the device comprises an inductor 2 configured to induce the electric current in the zone Zt, and a cooler provided with at least one projection nozzle 3 configured to project the first gas flow F1 onto said zone Zt.

[0091] In certain embodiments, as illustrated in [Fig.2], the device is configured so that the heating step E1 is carried out simultaneously with the cooling step E2 on two different zones of the rack 1.

[0092] In other words, a first zone of the rack 1 is heated while a second zone is cooled. The first zone will then be cooled while a third zone will be heated.

[0093] This is a continuous, or in-line, quenching process.

[0094] In certain embodiments, as illustrated in Figures 3 and 4, the area to be treated Zt corresponds to the entire exterior surface to be treated. Thus, the area to be treated Zt is heated initially and then in a second step the area to be treated Zt is cooled. The quenching process is then discontinuous.

[0095] In certain embodiments, the device also comprises a device for recovering and recycling the projected gases.

[0096] In certain embodiments, the device also comprises a device for cooling the projected gases.

[0097] Although the present invention has been described with reference to specific embodiments, it is obvious that modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various illustrated / mentioned embodiments may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

[0098] It is also obvious that all the characteristics described with reference to a method are transposable, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device are transposable, alone or in combination, to a method.

Claims

Claims

1. A hardening method for a rack of a power steering system of a vehicle, the method comprising: - A heating step in which at least one area to be treated of an outer surface of the rack is heated by means of an electric current induced in said area; Characterized in that the method comprises: - A cooling step in which the at least one area is cooled by means of at least a first gas flow projected onto said area.

2. A quenching method according to claim 1, wherein at least a second gas stream is projected onto one end of the rack.

3. A quenching method according to any one of the preceding claims, wherein an outlet pressure of a nozzle for spraying the first and / or second gas flow is between 0.5 bar and 30 bar, for example between 1 bar and 20 bar, or greater than or equal to 1 bar.

4. A quenching method according to any preceding claim, wherein the gas of the first or second gas stream is air, and / or at least one neutral gas.

5. A quenching method according to any preceding claim, wherein during the heating step, the at least one zone is heated by induction of the electric current or by conduction of the electric current.

6. A quenching method according to any preceding claim, wherein the heating step and the cooling step are carried out simultaneously on two different zones.

7. A hardening method according to any one of claims 1 to 5, wherein the area to be treated corresponds to an entire exterior surface to be treated.

8. A hardening method according to any preceding claim, wherein the rack is made of steel.

9. A hardening method according to any preceding claim, wherein the rack is a solid bar which is at least partly cylindrical.

10. A quenching device configured to implement the method according to any one of the preceding claims, the device comprising an inductor configured to induce an electric current in the zone, and a cooler provided with at least one projection nozzle configured to project the first gas flow onto said zone.

Images