Balance spring for a timepiece movement

Abstract

The invention relates to a balance spring intended to equip a balance of a timepiece movement, characterized in that it is made of an alloy comprising, in weight percent: Ti content of 1-80 wt%, H content of 0.17-2 wt%, total content of all other elements less than or equal to 0.3 wt%, the balance of the 100 wt% consisting of Nb, Ti, H and possible trace other elements chosen from O, C, Fe, N, Ni, Si, Cu and Al. The invention further relates to a method for its manufacture, with a step of thermochemical treatment of a blank made of Nb-Ti alloy in an atmosphere comprising hydrogen, so as to enrich the Nb-Ti alloy with hydrogen in interstitial form.

Description

Invention Field

[0001] This invention relates to a balance spring intended for equipping a balance wheel in a watch movement. It further relates to a method for manufacturing such a balance spring. Background of the Invention

[0002] The balance wheel and hairspring used in watchmaking are subject to limitations that, at first glance, are usually irreconcilable:

[0003] - High yield strength is required.

[0004] - Easy to manufacture, especially easy for wire drawing and rolling operations.

[0005] - Excellent fatigue strength,

[0006] - A stable performance level over time,

[0007] - Small cross-section.

[0008] The alloy chosen for the balance spring must also possess the property of maintaining its chronograph performance despite temperature variations in the watch. Therefore, the alloy's thermoelastic coefficient, or CTE, is crucial. To create a chronograph oscillator with a balance wheel made of CuBe or nickel-silver, a CTE of + / - 10 ppm / ℃ must be achieved.

[0009] The following formulas relate the CTE of the alloy, the coefficient of thermal expansion (α) of the balance wheel and hairspring, and the coefficient of thermal expansion (β) of the balance wheel to the thermal coefficient (CT) of the oscillator:

[0010]

[0011] Variables M and T represent the speed in s / d and temperature in °C, respectively, and E is the Young's modulus of the balance spring, where (1 / E · dE / dT) is the CTE of the balance spring alloy, and the coefficient of thermal expansion is expressed in °C. -1 express.

[0012] In practice, CT is calculated as follows:

[0013]

[0014] Its value must be between -0.6 and +0.6 s / d℃.

[0015] In the prior art, it is known that balance springs used in the watchmaking industry are made of binary Nb-Ti alloys, where the weight percentage of Ti is typically 40-60% by weight, and more specifically 47% by weight. Through deformation and appropriate heat treatment, this balance spring possesses a two-phase microstructure, where the β phase is a solid solution of Nb and Ti, and the α phase is Ti in precipitate form. Cold-rolled β-phase Nb and Ti solid solution has a high positive CTE, while the α-phase Ti has a high negative CTE, thus bringing the CTE of the two-phase alloy close to zero, which is particularly advantageous for CT (Chemical Temperature Coefficient).

[0016] However, using binary Nb-Ti alloys for balance springs also has some drawbacks. As mentioned above, binary Nb-Ti alloys are particularly advantageous for low temperature coefficients (CTs). However, their composition is not optimized for mid-temperature error, which is a measure of the rate curvature approximated by a straight line passing through two points (8°C and 38°C). The rate can deviate from this linear behavior between 8°C and 38°C, and the mid-temperature error at 23°C is a measure of this deviation at that temperature. It is calculated according to the following formula:

[0017]

[0018] Typically, for NbTi47 alloys, the mid-temperature error is +4.5 s / d, while it should preferably be -3 to +3 s / d. Invention Overview

[0019] The purpose of this invention is to propose a new manufacturing method and a new chemical composition for balance springs that can reduce mid-temperature error while maintaining a thermal coefficient close to 0.

[0020] For this purpose, the present invention relates to a balance spring for watches made of an alloy of niobium, titanium, and hydrogen. More specifically, the balance spring is made of an alloy having the following weight percentages:

[0021] - Ti content is 1-80% by weight.

[0022] - H content is 0.17-2% by weight.

[0023] - The total content of all other elements is less than or equal to 0.3% by weight.

[0024] - The balance up to 100% by weight consists of Nb.

[0025] It consists of Nb, Ti, H and possible trace amounts of other elements selected from O, C, Fe, N, Ni, Si, Cu and Al.

[0026] The addition of hydrogen can produce a balance spring with a near-zero mid-temperature error and a near-zero thermal coefficient.

[0027] According to the present invention, hydrogen is added to the Nb-Ti alloy by thermochemical treatment under a controlled atmosphere during the manufacturing process.

[0028] More specifically, the manufacturing method includes, in sequence:

[0029] a) The step of producing or supplying billets made of an alloy, said alloy being composed of Nb, Ti and possible trace amounts of other elements selected from O, C, Fe, N, Ni, Si, Cu and Al, wherein the Ti content is 1-80% by weight and the total content of all other elements is less than or equal to 0.3% by weight, with the balance being Nb up to 100% by weight.

[0030] b) The steps of performing β-type solution treatment and quenching on the billet, so that the titanium and niobium of the alloy are essentially in the form of β-phase solid solution.

[0031] c) The step of applying a series of deformation processes to the alloy, wherein optionally at least one heat treatment is performed between two processes and / or after the series of deformation processes.

[0032] d) The winding step used to form the balance spring.

[0033] e) The final so-called fixing heat treatment step,

[0034] The method is characterized by including an additional thermochemical treatment step in an atmosphere containing hydrogen, the thermochemical treatment step being performed during the solution treatment in step b), during the heat treatment in step c), during the final heat treatment in step e), before step b), between step b) and step c), between step c) and step d), between step d) and step e), or after step e).

[0035] Advantageously, the thermochemical treatment is carried out on the recrystallized structure.

[0036] The resulting balance spring contains hydrogen primarily or entirely in an interstitial form. The term "primarily," as opposed to "entirely," must be understood to mean a very localized presence of small amounts of hydrides that cannot be excluded. Regarding its microstructure, it is formed by a single β phase of Nb and Ti in a solid solution.

[0037] In addition to its low temperature error and low thermal coefficient, the balance spring produced using the method according to the invention has an ultimate tensile strength Rm greater than or equal to 500 MPa, and more precisely 800-1000 MPa. Advantageously, it has an elastic modulus greater than or equal to 80 GPa, and preferably greater than or equal to 90 GPa.

[0038] Other features and advantages of the invention will become apparent as you read the following detailed description. Brief description of the attached diagram

[0039] Figure 1 The variation of the mid-temperature error with thermal coefficient for the ternary Nb-Ti-H grade (grade) with 47 wt% Ti according to the present invention is shown.

[0040] Figure 2 The variation of the mid-temperature error with thermal coefficient for a binary Nb-Ti grade with 47 wt% Ti according to the prior art is shown.

[0041] Figure 3 The variation of Young's modulus of the Nb-Ti-H alloy according to the present invention with temperature is shown after thermochemical treatment at 652°C for 15 minutes under 4 bar of hydrogen. In this figure, the Young's modulus is normalized to the Young's modulus at 23°C.

[0042] Figure 4 The X-ray diffraction pattern (XRD pattern) of the same alloy is shown.

[0043] Figure 5 This is an enlarged view of the XRD pattern centered at θ=39°, with the left peak (Inv) having the reference peak (Ref) on the right, without any thermochemical treatment.

[0044] Detailed Explanation

[0045] This invention relates to a watch balance spring made of an alloy of niobium (Nb), titanium (Ti), and hydrogen (H). More specifically, the alloy comprises components having the following weight percentages:

[0046] - Ti content is 1-80% by weight.

[0047] - H content is 0.17-2% by weight.

[0048] - The total content of all other elements present in trace amounts is less than or equal to 0.3% by weight.

[0049] - The balance up to 100% by weight consists of Nb.

[0050] It consists of Nb, Ti, H and possible trace amounts of other elements selected from O, C, Fe, N, Ni, Si, Cu and Al.

[0051] Preferably, the hydrogen content is 0.2-1.5% by weight, more preferably 0.5-1% by weight.

[0052] Preferably, the titanium content is 20-60% by weight, more preferably 40-50% by weight.

[0053] Apart from any potential and unavoidable trace elements, the alloys used in this invention do not contain any elements other than Ti, Nb, and H.

[0054] More specifically, the oxygen content is less than or equal to 0.10% by weight of the total composition, or even less than or equal to 0.085% by weight of the total composition.

[0055] More specifically, the carbon content is less than or equal to 0.04% by weight of the total composition, particularly less than or equal to 0.020% by weight of the total composition, or even less than or equal to 0.0175% by weight of the total composition.

[0056] More specifically, the iron content is less than or equal to 0.03% by weight of the total composition, particularly less than or equal to 0.025% by weight of the total composition, or even less than or equal to 0.020% by weight of the total composition.

[0057] More specifically, the nitrogen content is less than or equal to 0.02% by weight of the total composition, particularly less than or equal to 0.015% by weight of the total composition, or even less than or equal to 0.0075% by weight of the total composition.

[0058] More specifically, the silicon content is less than or equal to 0.01% by weight of the total composition.

[0059] More specifically, the nickel content is less than or equal to 0.01% by weight of the total composition, and particularly less than or equal to 0.16% by weight of the total composition.

[0060] More specifically, the copper content is less than or equal to 0.01% by weight of the total composition, and particularly less than or equal to 0.005% by weight of the total composition.

[0061] More specifically, the aluminum content is less than or equal to 0.01 by weight of the total composition.

[0062] According to the present invention, the alloy is enriched with hydrogen by thermochemical treatment in an atmosphere containing hydrogen as a carrier gas.

[0063] This thermochemical treatment can be performed at different steps in the process of manufacturing the balance wheel and hairspring, as follows:

[0064] a) Producing or supplying billets made of an alloy composed of Nb, Ti, and possibly trace amounts of other elements selected from O, C, Fe, N, Ni, Si, Cu, and Al, wherein the Ti content is 1-80% by weight, and the total content of all other elements is less than or equal to 0.3% by weight, with the balance being Nb up to 100% by weight.

[0065] b) The billet is subjected to a so-called β-type solution treatment and quenching, so that the titanium and niobium are essentially in the form of a β-phase solid solution.

[0066] c) The alloy is subjected to a deformation process, optionally involving one or more heat treatments. The term "deformation" here is to be understood as deformation performed by drawing and / or rolling. Drawing may require the use of one or more drawplates in the same or different processes if necessary. Drawing continues until a wire with a circular cross-section is obtained. Rolling may be performed during the same deformation process as drawing, or in a separate process. Advantageously, the final process applied to the alloy is a rolling operation, preferably having a rectangular profile compatible with the inlet cross-section of the winder spindle.

[0067] d) Winding to form the balance spring,

[0068] e) Perform the final fixation heat treatment.

[0069] According to the invention, the thermochemical treatment can be performed during the solution treatment in step b), during the heat treatment in step c), during the final fixation heat treatment in step e), or between steps a) and b), between steps b) and c), between steps c) and d), between steps d) and e), or after step e). Advantageously, this treatment is performed in step e) at the end of the manufacturing method. Performing the thermochemical treatment at the end of the manufacturing method prevents any possible release of hydrogen into the atmosphere during any subsequent steps (e.g., under vacuum). This also allows the geometry, thermal coefficient, and mid-temperature error of the hairspring to be fixed in a single heat treatment process.

[0070] The thermochemical treatment is carried out in an atmosphere containing hydrogen at a holding temperature of 100-900°C, preferably 500-800°C, more preferably 600-700°C. The thermochemical treatment can also be carried out in an atmosphere containing 100% H2 at an absolute pressure of 5 mbar-10 bar, preferably 0.5-7 bar, more preferably 1-6 bar, and even more preferably 3.5-4.5 bar. The thermochemical treatment can also be carried out in an atmosphere containing a gas mixture (e.g., a mixture of Ar and H2) at a total pressure of 5 mbar-10 bar, preferably 0.5-7 bar, more preferably 1-6 bar, and even more preferably 3.5-4.5 bar, wherein the volume percentage of H2 is 5-90%. Advantageously, the thermochemical treatment is carried out for a duration of 1 minute to 5 hours.

[0071] In step b), the so-called β-type solution treatment and quenching prior to the deformation process is a treatment performed in a vacuum at a temperature of 600°C–1,000°C for a duration of 5 minutes to 2 hours, followed by cooling under gas. More specifically, the treatment is performed in a vacuum at 800°C for 1 hour, followed by cooling under gas.

[0072] In step c), each deformation process is performed at a given deformation ratio of 1-5, which satisfies the conventional formula 2ln(d0 / d), where d0 is the diameter of the final β-quench and d is the diameter of the cold-rolled wire. The total cumulative deformation throughout this series of processes results in a total deformation ratio of 1-14.

[0073] More specifically, the method includes one to five deformation steps.

[0074] More specifically, the first process includes a first deformation in which the cross-section is reduced by at least 30%.

[0075] More specifically, except for the first process, each process includes a deformation in which the cross-section is reduced by 25%.

[0076] Heat treatment can be performed between deformation processes and / or after all deformation processes. This heat treatment can serve several purposes: to perform the β-type solution treatment and quenching described above, to precipitate the α-phase titanium, or to restore / recrystallize the structure. β-type solution treatment and quenching are performed in a vacuum at a temperature of 600°C–1000°C for a duration of 5 minutes to 2 hours, followed by cooling under gas. Precipitation of the α-phase titanium is performed at a temperature of 300–500°C for a duration of 1 hour to 200 hours. Restoration / recrystallization is performed at a temperature of 500–600°C for a duration of 30 minutes to 20 hours.

[0077] In step e), the final heat treatment is carried out at a temperature of 300°C to 700°C for a duration of 1 to 200 hours. More specifically, it is carried out at a holding temperature of 400°C to 600°C for a duration of 5 to 30 hours.

[0078] Furthermore, the method may advantageously include an additional step, performed after step a) of producing or supplying the alloy billet and before the deformation process in step c), namely, adding a surface layer of a ductile material selected from copper, nickel, copper-nickel alloys, copper-manganese alloys, gold, silver, nickel-phosphorus (Ni-P), and nickel-boron (Ni-B), etc., to the billet to facilitate the wire forming operation during the deformation process. Additionally, the ductile material layer is removed from the wire, particularly by etching, between the final deformation processes, after the deformation process, or after the winding step d).

[0079] In one alternative embodiment, a surface layer of ductile material is deposited to form the balance spring, the pitch of which is not a multiple of the strip thickness. In another alternative embodiment, a surface layer of ductile material is deposited to form the spring, the pitch of which is variable.

[0080] In a specific watchmaking application, ductile material is thus added at a given time to facilitate the wire forming operation, resulting in a thickness of 10 to 500 micrometers on a wire with a final diameter of 0.3 to 1 mm. The ductile material layer is removed from the wire, particularly by etching, and then the wire is flattened before actually manufacturing the hairspring itself by winding. Alternatively, the ductile material layer is removed after flattening and before winding.

[0081] The addition of ductile material can be electroplated or mechanical; in this case, it is a sleeve or tube of ductile material, which is fitted onto an alloy bar with a large diameter and then thinned during the deformation step of the composite bar.

[0082] This layer can be removed, in particular, by using cyanide-based or acid-based solutions, such as nitric acid, for etching.

[0083] Returning to the additional thermochemical treatment step, the addition of hydrogen was intended to reduce the intermediate temperature error. The experiment was conducted on a binary Nb-Ti alloy with 47 wt% Ti and 53 wt% Nb. The thermochemical treatment was carried out during the final fixation heat treatment in step e), in an atmosphere containing 100% H2, under the conditions given in Table 1 below. The thermochemical treatment was performed on recrystallized structures (R) that had undergone a deformation process ending with recrystallization heat treatment, or on cold-rolled structures (E) that had undergone a deformation process without subsequent recrystallization heat treatment. The intermediate temperature error (ES) was measured at 23°C using the following formula:

[0084]

[0085] This is the rate change at 23°C compared to the rate at 8°C and the rate at 38°C, forming a linear curve. For example, the rates at 8°C, 23°C, and 38°C can be measured using a Witschi timer. The coefficient of thermal expansion (CT) is measured using the same equipment and the following formula:

[0086] .

[0087] The measurement results are provided in Table 1.

[0088] Table 1

[0089]

[0090] R = recrystallization, E = cold rolling.

[0091] Samples 01 to 04 have a hydrogen content of 0.3–1 wt%. For samples treated at a hydrogen pressure of 4 bar, all samples exhibited a mid-temperature error of -3 to +3 s / d, as expected, close to 0. The temperature coefficient (CT) also remained as expected in the range of -0.6 to +0.6 s / d °C. Sample 01 yielded the best values; for this sample, thermochemical treatment was performed on the recrystallized structure, with thermal coefficients and mid-temperature errors close to 0, expressed in s / d / °C and s / d respectively. This sample has a hydrogen content of approximately 0.6 wt%.

[0092] The results for samples 01 to 04 were plotted using the variation of intermediate temperature error (ES) with thermal coefficient (CT). Figure 1 In general, a direct correlation between CT and ES is observed when the balance spring alloy contains hydrogen. This is contrary to what was observed in previous tests on binary alloys with 47 wt% titanium and 53 wt% niobium. In the latter case, such as Figure 2 As shown, there is no relationship between CT and ES. Regardless of the parameters of the sample fabrication method, plotting these two quantities on the same graph yields a scatter plot. Furthermore, a point where CT=ES=0 is never obtained, unlike the case for ternary Nb-Ti-H grades. Therefore, it was found that adding hydrogen can control the mid-temperature error while maintaining a low CT.

[0093] The effect of temperature on the Young's modulus of sample O2 was also continuously measured using a mechanical spectrometer that measures the natural frequency of a freely vibrating beam, within a temperature range of -20℃ to +60℃. Figure 3 It was observed that temperature has a very small effect on Young's modulus.

[0094] X-ray diffraction analysis (Bragg-Brentano configuration) was performed on the same sample. The diffraction spectra are shown in... Figure 4 In the middle. The XRD pattern between 30° and 80° did not indicate the presence of TiH2 or NbH hydride phases. By magnification at... Figure 5 Focusing on the region at θ=39°, which corresponds to the NbTi peak

[110] , it can be seen that after thermochemical treatment, the peak shifts to the left (Inv peak) compared to the reference peak (Ref peak) without thermochemical treatment, indicating an increase in the lattice parameter. It can be concluded that thermochemical treatment allows hydrogen to be introduced in an interstitial form without the formation of hydrides. Furthermore, no α-titanium precipitation was observed. The absence of titanium precipitates is attributed to the presence of hydrogen, which stabilizes the β-phase titanium.

Claims

1. A balance wheel and hairspring for equipping a balance wheel in a watch movement, characterized in that, The balance spring is made of an alloy composed of Nb, Ti, H, and trace amounts of other elements selected from O, C, Fe, N, Ni, Si, Cu, and Al, having the following weight percentages: - Ti content is 20-60% by weight. - H content is 0.2-1.5% by weight. - The total content of all other elements is less than or equal to 0.3% by weight. - The balance up to 100% by weight consists of Nb.

2. The balance wheel and hairspring according to claim 1, characterized in that, The H content is 0.5-1% by weight.

3. The balance wheel and hairspring according to claim 1 or 2, characterized in that, The Ti content is 40-50% by weight.

4. The balance wheel and hairspring according to claim 1 or 2, characterized in that, H exists primarily or entirely in the alloy as interstitial material.

5. The balance wheel and hairspring according to claim 1 or 2, characterized in that, The microstructure of the alloy is formed by a single β phase of Nb and Ti in a solid solution.

6. The balance spring according to claim 1 or 2, having a thermal coefficient of -0.6 to +0.6 s / d °C and a mid-temperature error of -3 to +3 s / d.

7. A method for preparing a balance wheel and hairspring for equipping a balance wheel in a watch movement, comprising the following steps: a) The step of producing or supplying billets made of an alloy, said alloy being composed of Nb, Ti, H and trace amounts of other elements selected from O, C, Fe, N, Ni, Si, Cu and Al, wherein the Ti content is 20-60% by weight, the H content is 0.2-1.5% by weight, and the total content of all other elements is less than or equal to 0.3% by weight, with the balance being Nb up to 100% by weight. b) The steps of performing β-type solution treatment and quenching on the billet, so that the titanium and niobium of the alloy are essentially in the form of β-phase solid solution. c) The step of applying a series of deformation processes to the alloy, wherein optionally at least one heat treatment is performed between two deformation processes and / or at the end of all deformation processes. d) The winding step used to form the balance spring. e) The final fixation heat treatment step, The method is characterized in that it includes an additional thermochemical treatment step in an atmosphere containing hydrogen, the thermochemical treatment step being performed during the solution treatment in step b), during the heat treatment in step c), during the final heat treatment in step e), before step b), between step b) and step c), between step c) and step d), between step d) and step e), or after step e).

8. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 7, characterized in that, The thermochemical treatment step is carried out in step e).

9. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 7 or 8, characterized in that, The thermochemical treatment step is performed on the structure of the blank or balance wheel / hairspring in the recrystallized state.

10. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 7 or 8, characterized in that, The thermochemical treatment is carried out at a temperature of 100-900°C in an atmosphere containing 100% hydrogen at a hydrogen pressure of 5-10 mbar, or in an atmosphere containing a mixture of hydrogen and another gas, wherein the volume percentage of hydrogen is 5-90% and the total pressure of the mixture is 5-10 mbar.

11. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 10, characterized in that, The total pressure of the hydrogen gas or mixture is 0.5-7 bar.

12. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 10, characterized in that, The total pressure of the hydrogen gas or mixture is 1-6 bar.

13. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 10, characterized in that, The total pressure of the hydrogen gas or mixture is 3.5-4.5 bar.

14. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 10, characterized in that, The temperature is 500-800℃.

15. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 10, characterized in that, The temperature is 600-700℃.

16. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 10, characterized in that, The total pressure of the hydrogen gas or mixture is 3.5-4.5 bar, and the temperature is 600-700℃.

17. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 7 or 8, characterized in that, The solution treatment is carried out in a vacuum at a temperature of 600°C-1000°C for a duration of 5 minutes to 2 hours, followed by cooling under gas.

18. The method for preparing a balance spring for equipping a balance wheel in a watch movement according to claim 7 or 8, characterized in that, After step a) of producing or supplying the alloy billet, and before step c) of applying a series of processes, a surface layer of a ductile material selected from copper, nickel, copper-nickel alloy, copper-manganese alloy, gold, silver, nickel-phosphorus Ni-P, and nickel-boron Ni-B is added to the billet to facilitate the wire forming operation, and the surface layer of the ductile material is removed from the wire by etching before or after the winding step d).

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