Hairspring for clock movement and method for manufacturing same

Abstract

The present invention relates to a method for manufacturing a spiral spring comprising: a) a step of providing a blank with a core made of Nb-Ti; b) a step of beta-quenching said blank; c) a step of deforming said blank in several sequences; d) a step of winding in order to form the spiral spring; e) a step of final heat treatment on the spiral spring, and being characterised in that: - the blank of step a) comprises a layer of X, with a material X selected from among Cu, Sn, Fe, Pt, Pd, Rh, Al, Au, Ni, Ag, Co and Cr or an alloy of one of these elements around the core made of Nb-Ti; - it comprises a heat treatment step for partially transforming said layer of X into a layer of X, Ti inter-metals around the core made of Nb-Ti, said step being performed between step b) and step c) or between two sequences of the deformation step c); - a step of removing said part of the layer of X, said step being performed between step b) and step c), between two sequences of the deforming step c) or between step c) and step d).

Description

Scope of the invention

[0001] The invention relates to a method for manufacturing a spiral spring intended to equip a balance wheel of a watch movement and the spiral spring resulting from the method. Background of the invention

[0002] The manufacture of balance springs for watchmaking must contend with constraints that are often, at first glance, incompatible: need for high yield strength, ease of processing, particularly wire drawing and rolling, excellent fatigue resistance, stability of performance over time, small cross-sections.

[0003] The design of spiral springs also focuses on thermal compensation to ensure consistent timekeeping performance. This requires achieving a thermoelastic coefficient close to zero. The goal is also to produce spiral springs with limited sensitivity to magnetic fields.

[0004] New spirals have been developed from niobium and titanium alloys. However, these alloys present sticking and binding problems in drawing or wire drawing dies and against rolling rollers, making them virtually impossible to transform into thin wires using standard processes employed, for example, for steel.

[0005] To overcome this drawback, it has been proposed to deposit, before shaping in the dies and rolling mill, a layer of a ductile material, particularly copper, onto the Nb-Ti blank. A manufacturing process for a niobium-titanium alloy containing between 40 and 60% titanium by weight is known from document EP 3 502 288. This manufacturing process includes, before the deformation stage, the deposition of a surface layer of a ductile material.

[0006] This copper layer on the wire presents a disadvantage. It does not allow for precise control of the wire's geometry during calibration and rolling. These dimensional variations in the Nb-Ti core of the wire result in significant variations in the coil torques. Summary of the invention

[0007] To remedy the aforementioned disadvantages, the present invention proposes a method for manufacturing a spiral spring which makes it easier to shape by deformation while avoiding the disadvantages associated with copper.

[0008] To this end, the manufacturing process for the spiral spring according to the invention includes a heat treatment step designed to transform a portion of the copper layer coating the Nb-Ti core into a layer of Cu,Ti intermetallics and to remove the remaining copper layer. This intermetallic layer then forms the outer layer that comes into contact with the dies and rolling rollers. It is chemically inert and ductile, and allows the spiral wire to be easily drawn and rolled. It also has the advantage of facilitating the separation of the spirals after the fixing step following the drawing process.

[0009] The intermetallic layer remains on the spiral after the manufacturing process. It is sufficiently thin, with a thickness between 20 nm and 10 microns, preferably between 300 nm and 1.5 µm, so as not to significantly alter the thermoelastic coefficient (TEC) of the spiral. Furthermore, it is perfectly bonded to the Nb-Ti core.

[0010] The invention is more specifically described for a Cu layer partially transformed into a Cu,Ti intermetallic layer. However, the present invention is applicable to other elements such as Sn, Fe, Pt, Pd, Rh, Al, Au, Ni, Ag, Co, and Cr, which are also capable of forming intermetallics with Ti. It is also applicable to an alloy of one of these elements. Brief description of the figures

[0011] There figure 1represents a microscopy of the blank with a core made of NbTi 47 alloy coated with a layer of Cu partially transformed into intermetallics by the heat treatment process according to the invention. figure 2 represents, according to the prior art, the XRD spectrum of this alloy with the Cu layer in the absence of heat treatment according to the process of the invention. figure 3 represents the XRD spectrum of this same alloy with the Cu layer in the presence of the heat treatment according to the process of the invention. figure 4 is an enlargement of the XRD spectrum of the figure 3 for the peaks relating to intermetallics. Detailed description of the invention

[0012] The invention relates to a method for manufacturing a spiral spring intended for use in the balance wheel of a watch movement. This spiral spring is made of a binary alloy comprising niobium and titanium. The invention also relates to the spiral spring produced by this method.

[0013] The manufacturing process involves the following steps: a) a step of providing a blank with an Nb-Ti core made from an alloy consisting of: niobium: balance at 100% by weight, titanium: between 5 and 95% by weight, traces of one or more elements selected from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu and Al, each of said elements being present in an amount between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being between 0% and 0.3% by weight, b) a beta-type quenching step of said blank, so that the titanium of said alloy is essentially in the form of a solid solution with the niobium in the beta phase, c) a step of deforming the blank in several sequences, d) a step of stretching to form the spiral spring, e) a final heat treatment step on the spiral spring.

[0014] According to one variant, the blank in step a) comprises a layer around the Nb-Ti core of a material X selected from Cu, Sn, Fe, Pt, Pd, Rh, Al, Au, Ni, Ag, Co, and Cr, or an alloy of one of these elements. For example, it could be Cu, Cu-Sn, Cu-Ni, etc. According to another variant, the process includes a step of adding said material X around the Nb-Ti core to form the layer in X, said step being carried out between step a) and the deformation step c).

[0015] The manufacturing process also includes a heat treatment step to partially transform the X layer into an X,Ti intermetallic layer around the Nb-Ti core. The heat treatment is carried out at a temperature between 200 and 900°C for 15 minutes to 100 hours. The blank thus comprises successively the Nb-Ti core, the X,Ti intermetallic layer, and the remaining portion of the X layer, this step being performed between steps b) and c) or between two sequences of the deformation step c).

[0016] The manufacturing process then includes a step of removing the remaining part of the X layer. This step is carried out between step b) and step c), between two sequences of the deformation step c) or between step c) and step d).

[0017] The process is now described in more detail.

[0018] In step a),The core is made of an Nb-Ti alloy containing between 5 and 95% titanium by weight. According to a preferred variant, the alloy used contains between 40 and 60% titanium by weight. Preferably, it contains between 40 and 49% titanium by weight, and more preferably between 46% and 48% titanium by weight. The percentage of titanium is sufficient to obtain a maximum proportion of Ti precipitates in the form of the alpha phase, while being reduced to avoid the formation of martensitic phases that lead to alloy brittleness problems during processing. According to another variant, the titanium content is reduced more significantly to avoid the formation of these hard phases. The titanium content is then less than 40% by weight. It is between 5 and 40% by weight (not including the upper limit). More specifically, it is between 5 and 35%, preferably between 15 and 35% and more preferably between 27 and 33%.

[0019] In a particularly advantageous manner, the Nb-Ti alloy used in the present invention contains no other elements except for possible and unavoidable traces. This prevents the formation of brittle phases.

[0020] More specifically, the oxygen content is less than or equal to 0.10% by weight of the total, or even less than or equal to 0.085% by weight of the total.

[0021] More specifically, the tantalum content is less than or equal to 0.10% by weight of the total.

[0022] More specifically, the carbon content is less than or equal to 0.04% by weight of the total, in particular less than or equal to 0.020% by weight of the total, or even less than or equal to 0.0175% by weight of the total.

[0023] More specifically, the iron content is less than or equal to 0.03% by weight of the total, in particular less than or equal to 0.025% by weight of the total, or even less than or equal to 0.020% by weight of the total.

[0024] More specifically, the nitrogen content is less than or equal to 0.02% by weight of the total, in particular less than or equal to 0.015% by weight of the total, or even less than or equal to 0.0075% by weight of the total.

[0025] More specifically, the hydrogen content is less than or equal to 0.01% by weight of the total, in particular less than or equal to 0.0035% by weight of the total, or even less than or equal to 0.0005% by weight of the total.

[0026] More specifically, the silicon content is less than or equal to 0.01% by weight of the total.

[0027] More specifically, the nickel content is less than or equal to 0.01% by weight of the total, in particular less than or equal to 0.16% by weight of the total.

[0028] More specifically, the content of ductile material, such as copper, in the alloy is less than or equal to 0.01% by weight of the total, in particular less than or equal to 0.005% by weight of the total.

[0029] More specifically, the aluminum content is less than or equal to 0.01% by weight of the total.

[0030] The Nb-Ti core of the blank in step a) is coated with a layer of material X as listed above. The application of the X layer around the core can be achieved galvanically, by PVD, CVD, or mechanically. In the latter case, a tube of material X is fitted onto a bar of the Nb-Ti alloy. The assembly is deformed by hammering, drawing, and / or wire drawing to thin the bar and form the blank provided in step a). The X layer is applied during the manufacturing process of the spiral spring between step a) and the deformation step c). The thickness of the X layer is chosen so that the ratio of the surface area of ​​material X to the surface area of ​​the Nb-Ti core for a given wire cross-section is less than 1, preferably less than 0.5, and more preferably between 0.01 and 0.4. For example, the thickness is preferably between 1 and 500 micrometers for a wire with a total diameter of 0.2 to 1 millimeter.

[0031] Beta-type tempering step b) This is a solution treatment. Preferably, it is carried out for a duration of between 5 minutes and 2 hours at a temperature between 700°C and 1000°C, under vacuum, followed by cooling under gas. More specifically, this beta quenching is a solution treatment at 800°C under vacuum for 5 minutes to 1 hour, followed by cooling under gas.

[0032] Step c)The deformation process is carried out in several stages. Deformation is defined as deformation by wire drawing and / or rolling. Advantageously, the deformation stage comprises at least successively a first wire drawing stage, a second calibration wire drawing stage, and a third rolling stage, preferably with a rectangular profile compatible with the entry cross-section of a drawing spindle. Each stage is performed with a given strain ratio between 1 and 5, this strain ratio corresponding to the classic formula 2In(d0 / d), where d0 is the diameter of the last beta hardening, and where d is the diameter of the work-hardened wire. The overall cumulative strain over this succession of stages results in a total strain ratio between 1 and 14.

[0033] The manufacturing process includes a heat treatment step to partially transform the X layer into an X,Ti intermetallic layer around the Nb-Ti core. This step is carried out for 15 minutes to 100 hours at a temperature between 200 and 900°C. Preferably, it is carried out for 5 to 20 hours at 400 and 500°C. This heat treatment step can be used to precipitate titanium in the alpha phase.

[0034] At the end of this step, according to the invention, the intermetallic layer has a thickness between 20 nm and 10 µm, preferably between 300 nm and 1.5 µm, more preferably between 400 and 800 nm, and even more preferably between 400 and 600 nm. The remaining X layer has a thickness between 1 and 25 µm. In the case of Cu, the intermetallic layer comprises, for example, Cu₄Ti, Cu₂Ti, CuTi, Cu₃Ti₂, and CuTi₂. By way of illustration, microscopy with the figure 1This represents the structure of the blank after heat treatment at 450°C of a niobium-titanium alloy with 47% titanium by weight, coated with a copper layer. The following layers are observed successively: the NbTi 47 core, the Cu,Ti intermetallic layer with a thickness of approximately 700 nm, and the remaining copper layer with a thickness of approximately 5 µm. figure 3 represents the XRD spectrum for the same spiral spring alloy according to the invention after removal of the copper layer and after the stretching and fixing steps. For comparison, the XRD spectrum for the same alloy with the copper layer but without the heat treatment is shown in the figure. figure 2 A series of small peaks are observed next to the Nb peak, which are shown in magnification at the figure 4 There are peaks for Cu4Ti, Cu2Ti, CuTi, Cu3Ti2 and CuTi2.

[0035] This heat treatment aimed at forming intermetallics can be carried out before the deformation step c) or between two deformation sequences during step c). Advantageously, it is carried out in step c) between the first wire drawing sequence and the second calibration wire drawing sequence.

[0036] Next, the remaining X-shaped layer is removed, leaving the intermetallic layer as the outermost layer. This step can be carried out by chemical etching in a cyanide-based or acidic solution, such as nitric acid. It should be noted that the present invention does not preclude the possibility that certain intermetallics may also be dissolved in the acid. This is, for example, the case with Cu₄Ti in a nitric acid solution.

[0037] The X-layer can be removed at different points in the process depending on the desired effect. Preferably, it is removed in step c) before the calibration drawing to precisely control the final dimensions of the spiral wire. The intermetallics in the outer layer then prevent the wire from sticking in the dies, against the rolling rollers, and between the spirals during fixing. More preferably, it is removed between the first drawing sequence and the second calibration drawing sequence. According to a less advantageous variant, it is removed after the calibration drawing before rolling, to prevent the wire from sticking against the rolling rollers and between the spirals during fixing. According to another less advantageous variant, it is removed at the end of the deformation step c) before the winding step.In this case, the outer layer of intermetallics only serves to prevent sticking between the spirals during fixing.

[0038] The step of stretching (d) to form the spiral spring is followed by step e) of final heat treatment on the spiral spring. This final heat treatment is a precipitation treatment of Ti in alpha phase lasting between 1 and 80 hours, preferably between 5 and 30 hours, at a temperature between 350 and 700°C, preferably between 400 and 600°C.

[0039] Finally, it should be noted that the process may include intermediate heat treatments between the deformation sequences within this same time and temperature range.

[0040] The spiral spring produced using this method has a yield strength greater than or equal to 500 MPa, preferably greater than 600 MPa, and more precisely between 500 and 1000 MPa. Advantageously, it has a modulus of elasticity less than or equal to 120 GPa, and preferably less than or equal to 100 GPa.

[0041] The spiral spring comprises an Nb-Ti core coated with a layer of X,Ti intermetallics, where X is selected from Cu, Sn, Fe, Pt, Pd, Rh, Al, Au, Ni, Ag, Co and Cr or an alloy of one of these elements, said intermetallic layer having a thickness between 20 nm and 10 µm, preferably between 300 nm and 1.5 µm, more preferably between 400 nm and 800 nm, or even between 400 nm and 600 nm. Preferably, the intermetallic layer is a Cu,Ti layer.

[0042] The core of the spiral spring has a bi-phase microstructure comprising niobium in the beta phase and titanium in the alpha phase.

[0043] In addition, the spiral spring made according to the invention has a thermoelastic coefficient, also called CTE, which allows it to guarantee the maintenance of chronometric performance despite the variation in operating temperatures of a watch incorporating such a spiral spring.

[0044] The process of the invention enables the production, and more particularly the shaping, of a balance spring made of a niobium-titanium alloy, typically containing 47% titanium by weight (40-60%). This alloy exhibits high mechanical properties, combining a very high yield strength, exceeding 600 MPa, and a very low modulus of elasticity, on the order of 60 GPa to 80 GPa. This combination of properties is well-suited for a balance spring. Furthermore, such an alloy is paramagnetic.

Claims

1. A method for manufacturing a spiral spring intended to equip a balance wheel of a horological movement, comprising: a) a step of providing a blank with an Nb-Ti core made of an alloy consisting of: - niobium: 100% balance by weight, - titanium: between 5 and 95% by weight, - traces of one or more elements selected from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu and Al, each of said elements being present in an amount comprised between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being comprised between 0% and 0.3% by weight, b) a step of beta-quenching said blank, so that the titanium of said alloy is essentially in the form of a solid solution with the beta-phase niobium, c) a step of deforming the blank in several sequences, d) a step of winding in order to form the spiral spring, e) a step of final heat treatment on the spiral spring, - the blank of step a) comprising, around the Nb-Ti core, a layer of X with a material X selected from among Cu, Sn, Fe, Pt, Pd, Rh, Al, Au, Ni, Ag, Co, and Cr or an alloy of one of these elements, or the method comprises a step of supplying said material X around the Nb-Ti core to form the layer of X, said step being carried out between step a) and step c), the method being characterised in that : - it comprises a heat treatment step for 15 minutes to 100 hours at a temperature comprised between 200°C and 900°C to partially transform said layer of X into a layer of X,Ti intermetals around the Nb-Ti core, the blank thus successively comprising the Nb-Ti core, the layer of X,Ti intermetals and part of the layer of X, said step being carried out between step b) and step c) or between two sequences of the deforming step c), - it comprises a step of removing said part of the layer of X, said step being carried out between step b) and step c), between two sequences of the deforming step c) or between step c) and step d). - the layer of intermetals has a thickness comprised between 20 nm and 10 µm.

2. The manufacturing method according to claim 1, characterised in that the deforming step c) includes at least successively a first drawing sequence, a second calibration drawing sequence and a third rolling sequence.

3. The manufacturing method according to claim 1, characterised in that the heat treatment step is carried out between two sequences of the deforming step c).

4. The manufacturing method according to the preceding claim, characterised in that the heat treatment step is carried out between the first sequence and the second sequence.

5. The manufacturing method according to the preceding claim, characterised in that the step of removing said part of the layer of X is carried out between the first sequence and the second sequence.

6. The manufacturing method according to claim 2, characterised in that the step of removing said part of the layer of X is carried out between the second sequence and the third sequence.

7. The manufacturing method according to claim 2 , characterised in that the step of removing said part of the layer of X is carried out between step c) and step d).

8. The manufacturing method according to claim 1, characterised in that the step of removing said part of the layer of X is carried out by chemical attack in a solution based on cyanides or acids.

9. The manufacturing method according to claim 1, characterised in that said step of β-quenching is a dissolution heat treatment, with a duration comprised between 5 minutes and 2 hours at a temperature comprised between 700°C and 1000°C, under vacuum, followed by cooling under gas.

10. The manufacturing method according to claim 1, characterised in that the final heat treatment of step e) is a treatment of precipitating alpha-phase titanium of a duration comprised between 1 hour and 80 hours at a temperature comprised between 350°C and 700°C, preferably between 5 hours and 30 hours between 400°C and 600°C.

11. The manufacturing method according to claim 1, characterised in that it includes between each sequence or between certain sequences of the deforming step c) an intermediate heat treatment of precipitating alpha-phase titanium of a duration comprised between 1 hour and 80 hours at a temperature comprised between 350°C and 700°C, preferably between 5 hours and 30 hours between 400°C and 600°C.

12. The manufacturing method according to claim 1, characterised in that the layer of X has a thickness comprised between 1 and 500 µm.

13. The manufacturing method according to claim 1, characterised in that each sequence is performed with a deformation rate comprised between 1 and 5, the global accumulation of the deformations over all the sequences leading to a total deformation rate comprised between 1 and 13.

14. The manufacturing method according to claim 1, characterised in that the Ti content is comprised between 40 and 55% by weight, preferably between 45 and 49% by weight.

15. The manufacturing method according to claim 1, characterised in that the Ti content is greater than or equal to 5% and less than 40%.

16. A spiral spring intended to equip a balance wheel of a horological movement, comprising an Nb-Ti core made of an alloy consisting of: - niobium: 100% balance by weight, - titanium: between 5 and 95% by weight, - traces of elements selected from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said elements being present in an amount comprised between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being comprised between 0% and 0.3% by weight, characterised in that the Nb-Ti core is coated with a layer of X,Ti intermetals with X selected from among Cu, Sn, Fe, Pt, Pd, Rh, Al, Au, Ni, Ag, Co and Cr or an alloy of one of these elements, said layer of intermetals having a thickness comprised between 20 nm and 10 µm.

17. The spiral spring according to claim 16, characterised in that the layer of intermetals has a thickness comprised between 300 nm and 1.5 µm.

18. The spiral spring according to claim 16, characterised in that the layer of intermetals has a thickness comprised between 400 nm and 800 nm, preferably between 400 and 600 nm.

19. The spiral spring according to claim 16, characterised in that X is Cu and in that the layer of intermetals includes Cu2Ti, CuTi, Cu3Ti2 and CuTi2.

20. The spiral spring according to claim 16, characterised in that the Ti content is comprised between 40 and 55% by weight, preferably between 45 and 49% by weight.

21. The spiral spring according to claim 16, characterised in that the Ti content is greater than or equal to 5% and less than 40%.

22. The spiral spring according to the preceding claim, characterised in that the Ti content is comprised between 5 and 35%, preferably between 15 and 35% and more preferably between 27 and 33%.

23. The spiral spring according to claim 16, characterised in that the Nb-Ti core has a bi-phase microstructure including beta-phase niobium and alpha-phase titanium.

24. The spiral spring according to claim 16, characterised in that it has an elastic limit greater than or equal to 500 MPa, preferably 600 MPa, and a modulus of elasticity less than or equal to 120 GPa, preferably less than or equal to 100 GPa.

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