Hairspring for clock movement

Abstract

The present invention relates to a balance spring for use in a clockwork movement, characterized in that the balance spring is made of an alloy consisting of: - Nb, Ti, H, and possible traces of other elements selected from O, C, Fe, N, Ni, Si, Cu, and Al, with the following weight percentages: - a Ti content between 1 and 80%, - an H content between 0.17 and 2%, - a total content for all other elements less than or equal to 0.3% by weight, - the remaining 100% being Nb. The present invention also relates to its manufacturing process, comprising a thermochemical treatment step of a blank made of an Nb-Ti alloy in an atmosphere containing hydrogen, so as to enrich the Nb-Ti alloy with hydrogen in the form of interstitials.

Description

Field of invention

[0001] The invention relates to a spiral spring intended to equip a balance wheel of a watch movement. It also relates to the method of manufacturing this spiral spring. Background of the invention

[0002] The manufacture of spiral springs for watchmaking must face constraints that often seem incompatible at first glance: need to obtain a high elastic limit, ease of production, particularly wire drawing and rolling, excellent fatigue resistance, stability of performance over time, small sections.

[0003] The alloy chosen for a balance spring must also have properties that guarantee the maintenance of chronometric performance despite the variation in operating temperatures of a watch incorporating such a balance spring. The thermoelastic coefficient, also known as CTE, of the alloy is then of great importance. To form a chronometric oscillator with a balance wheel made of CuBe or nickel silver, a CTE of + / - 10 ppm / °C must be achieved.

[0004] The formula that links the CTE of the alloy and the coefficients of expansion of the balance spring (α) and the balance wheel (β) to the thermal coefficient (CT) of the oscillator is as follows: CT = dM dT = 1 2 E dE dT − β + 3 2 α × 86400 s j ° C the variables M and T being respectively the rate in s / d and the temperature in °C, E being the Young's modulus of the spiral spring with (1 / E. dE / dT) which is the CTE of the spiral alloy, the expansion coefficients being expressed in °C -1< .

[0005] In practice, the CT is calculated as follows: CT = M 38 ° C − M 8 ° C 30 with a value which must be between - 0.6 and + 0.6 s / j°C.

[0006] From the prior art, spiral springs for watchmaking are known made from binary Nb-Ti alloys with weight percentages of Ti typically between 40 and 60% and more specifically with a percentage of 47%. With a suitable deformation and heat treatment scheme, this spiral spring has a two-phase microstructure comprising a solid solution of Nb and Ti in beta phase and Ti in the form of precipitates in alpha phase. The solid solution of Nb and Ti in the work-hardened beta phase has a strongly positive CTE while the Ti in alpha phase has a strongly negative CTE making it possible to bring the two-phase alloy to a CTE close to zero, which is particularly favorable for the CT.

[0007] However, there are some disadvantages to using binary Nb-Ti alloys for spiral springs. The binary Nb-Ti alloy is particularly favorable for low CT as mentioned above. However, its composition is not optimized for the secondary error, which is a measure of the curvature of the step, which is approximated above by a straight line passing through two points (8°C and 38°C). The step can deviate from this linear behavior between 8°C and 38°C, and the secondary error at 23°C is a measure of this deviation at the temperature of 23°C. It is calculated using the following formula: ES 23 ° C = M 23 ° C − M 8 ° C + M 38 ° C 2

[0008] Typically, for an NbTi47 alloy, the secondary error is + 4.5 s / d whereas preferably it should be between - 3 and + 3 s / d.

[0009] Documents CH 714 494 A2 and WO 2018 / 172164 A1 present watch spiral springs made of Nb-Ti alloy. Summary of the invention

[0010] The object of the invention is to propose a new manufacturing method and a new chemical composition for a spiral spring making it possible to reduce the secondary error while maintaining a thermal coefficient close to 0.

[0011] To this end, the invention relates to a watch spiral spring made from an alloy of niobium, titanium and hydrogen. More specifically, the spiral spring is defined by the appended claim 1.

[0012] The addition of hydrogen makes it possible to produce a spiral spring with a secondary error close to 0 and simultaneously with a thermal coefficient close to 0.

[0013] According to the invention, hydrogen is added to the Nb-Ti alloy by thermochemical treatment under a controlled atmosphere during the manufacturing process.

[0014] More specifically, the manufacturing method is defined by the appended claim 7.

[0015] Advantageously, the thermochemical treatment is carried out on a recrystallized structure.

[0016] The spiral spring thus produced contains hydrogen mainly or exclusively in the form of interstitials. By mainly, as opposed to exclusively, we mean that the very local presence in small proportion of hydrides cannot be excluded. Concerning its microstructure, it is formed of a single beta phase of Nb and Ti in solid solution.

[0017] In addition to its low secondary error and its low thermal coefficient, the spiral spring produced using the method according to the invention has a breaking load Rm greater than or equal to 500 MPa and more precisely between 800 and 1000 MPa. Advantageously, it has a modulus of elasticity greater than or equal to 80 GPa and preferably greater than or equal to 90 GPa.

[0018] Other characteristics and advantages of the invention will appear on reading the detailed description below. Brief description of the figures

[0019] There figure 1 represents the secondary error as a function of the thermal coefficient for ternary Nb-Ti-H grades according to the invention with 47% Ti. The figure 2 represents the secondary error as a function of the thermal coefficient for binary Nb-Ti grades according to the prior art with 47% Ti. The figure 3 represents the variation of Young's modulus with temperature for an Nb-Ti-H alloy according to the invention having been subjected to a thermochemical treatment at 652°C for 15 minutes under 4 bar of hydrogen. In the representation, the Young's modulus is normalized with respect to the Young's modulus at 23°C. The figure 4 represents for this same alloy the X-ray diffraction spectrum (XRD spectrum). The Figure 5represents for the left peak (Inv) an enlargement of this XRD spectrum centered on θ=39° with on the right the reference peak (Ref) in the absence of thermochemical treatment. Detailed description

[0020] The invention relates to a watch spiral spring made from an alloy of niobium (Nb), titanium (Ti) and hydrogen (H). More specifically, the alloy is made up of: of Nb, Ti, H and possible traces of other elements chosen from O, C, Fe, N, Ni, Si, Cu and Al, with the following weight percentages: a Ti content of between 20 and 60%, an H content of between 0.17 and 2%, a total content for all other elements present in trace form of less than or equal to 0.3% by weight, the balance to reach 100% being made up of Nb.

[0021] Preferably, the hydrogen content is between 0.2 and 1.5% by weight, more preferably between 0.5 and 1% by weight.

[0022] Preferably, the titanium content is between 40 and 50% by weight.

[0023] The alloy used in the present invention does not include any elements other than Ti, Nb and H except for possible and unavoidable traces.

[0024] More particularly, 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.

[0025] More particularly, 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.

[0026] More particularly, 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.

[0027] More particularly, 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.

[0028] More particularly, the silicon content is less than or equal to 0.01% by weight of the total.

[0029] More particularly, 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.

[0030] More particularly, the copper content 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.

[0031] More particularly, the aluminum content is less than or equal to 0.01% by weight of the total.

[0032] According to the invention, the alloy is enriched with hydrogen via a thermochemical treatment under an atmosphere comprising a hydrogen carrier gas.

[0033] This thermochemical treatment can be carried out at different stages of the spiral spring manufacturing process, the process steps being as follows: a) preparation or provision of a blank made from an alloy consisting of Nb, Ti and possible traces of other elements chosen from O, C, Fe, N, Ni, Si, Cu and Al, with a Ti content of between 20 and 60% and a total content for all the other elements less than or equal to 0.3% by weight, the balance to reach 100% being made up of Nb, b) solution treatment and quenching, known as beta type, of said blank, so that the titanium and niobium are essentially in the form of solid solution in beta phase, c) application to said alloy of deformation sequences with optionally one or more heat treatments. Deformation is understood to mean deformation by wire drawing and / or rolling. Wire drawing may require the use of one or more dies during the same sequence or during different sequences if necessary. Wire drawing is carried out until a wire of round section is obtained.Rolling can be carried out in the same deformation sequence as wire drawing or in another sequence. Advantageously, the last sequence applied to the alloy is rolling preferably with a rectangular profile compatible with the entry section of a drawing spindle, d) drawing to form a spiral spring, e) final fixing heat treatment.

[0034] According to the invention, the thermochemical treatment can be carried out during the solution treatment of step b), during a heat treatment of step c), during the final fixing heat treatment of step e) or between steps a) and b), b) and c), c) and d), d) and e) or after step e). Advantageously, this treatment is carried out during step e) at the end of the manufacturing process. Carrying out the thermochemical treatment at the end of the manufacturing process makes it possible to avoid a possible release of hydrogen into the atmosphere during a subsequent step which would be carried out, for example, under vacuum. This also makes it possible to fix the geometry of the spring, the thermal coefficient and the secondary error during a single heat treatment.

[0035] The thermochemical treatment is carried out at a holding temperature of between 100 and 900°C, preferably between 500 and 800°C, more preferably between 600 and 700°C in an atmosphere comprising hydrogen. The thermochemical treatment may be carried out in an atmosphere containing 100% H 2 with an absolute pressure of between 5 mbar and 10 bar, preferably between 0.5 and 7 bar, more preferably between 1 and 6 bar, even more preferably between 3.5 and 4.5 bar. The thermochemical treatment can also be carried out in an atmosphere containing a mixture of gases, for example Ar and H 2 , under a total pressure of between 5 mbar and 10 bar, preferably between 0.5 and 7 bar, more preferably between 1 and 6 bar, even more preferably between 3.5 and 4.5 bar, with a volume percentage of H 2 of between 5 and 90%. Advantageously, the thermochemical treatment is carried out for a time of between 1 minute and 5 hours.

[0036] In step b), the solution treatment and quenching, known as beta type, prior to the deformation sequences is a vacuum treatment at a temperature between 600°C and 1000°C with a duration between 5 minutes and 2 hours, followed by cooling under gas. More particularly, the treatment is carried out at 800°C for 1 hour under vacuum and followed by cooling under gas.

[0037] In step c), each deformation sequence is carried out with a given deformation rate between 1 and 5, this deformation rate corresponding to the classic formula 2In(d0 / d), where d0 is the diameter of the last beta quench, and where d is the diameter of the work-hardened wire. The overall accumulation of deformations over the entire succession of sequences leads to a total deformation rate between 1 and 14.

[0038] More specifically, the process involves between one and five deformation sequences.

[0039] More specifically, the first sequence includes a first deformation with at least a 30% reduction in section.

[0040] More specifically, each sequence, other than the first, includes a deformation with at least 25% reduction in section.

[0041] Between the deformation sequences and / or after all the deformation sequences, it is possible to carry out a heat treatment. This heat treatment can have several purposes: to carry out a solution treatment and beta-type quenching as previously, to precipitate the alpha phase of titanium or to restore / recrystallize the structure. The solution treatment and beta-type quenching is carried out under vacuum at a temperature between 600°C and 1000°C for a duration between 5 minutes and 2 hours, followed by cooling under gas. The precipitation of the alpha phase of titanium is carried out at a temperature between 300 and 500°C for a time between 1 and 200 hours. The restoration / recrystallization is carried out at a temperature between 500 and 600°C for a time between 30 minutes and 20 hours.

[0042] In step e), the final heat treatment is carried out for a period of between 1 hour and 200 hours at a temperature of between 300°C and 700°C. More particularly, the period is between 5 hours and 30 hours at a holding temperature of between 400°C and 600°C.

[0043] The method may further advantageously comprise an additional step taking place after step a) of producing or making available said alloy blank, and before the deformation sequences of step c), a surface layer of ductile material taken from copper, nickel, cupro-nickel, cupro-magnanese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B, or the like, is added to the blank to facilitate shaping into wire during deformation. And, between the last deformation sequences, after the deformation sequences or after step d) of stripping, the wire is stripped of its layer of ductile material, in particular by chemical attack.

[0044] In one variant, the surface layer of ductile material is deposited so as to form a spiral spring whose pitch is not a multiple of the thickness of the blade. In another variant, the surface layer of ductile material is deposited so as to form a spring whose pitch is variable.

[0045] In a particular watchmaking application, ductile material is thus added at a given moment to facilitate shaping into wire, so that a thickness of 10 to 500 micrometers remains on the wire with a final diameter of 0.3 to 1 millimeter. The wire is stripped of its layer of ductile material, in particular by chemical etching, then is flat-rolled before the manufacture of the spring itself by stripping. Alternatively, the layer of ductile material is removed after flat-rolling and before stripping.

[0046] The contribution of ductile material can be galvanic, or mechanical, in which case it is a jacket or a tube of ductile material which is adjusted on an alloy bar to a large diameter, then which is thinned during the stages of deformation of the composite bar.

[0047] The removal of the layer can be achieved in particular by chemical attack, with a cyanide-based solution or an acid-based solution, for example nitric acid.

[0048] Returning to the additional thermochemical treatment step, the addition of hydrogen is intended to reduce the secondary error. Tests were carried out on a binary Nb-Ti alloy with 47% by weight of Ti and 53% by weight of Nb. The thermochemical treatment was carried out during the final fixing heat treatment in step e) under an atmosphere comprising 100% H 2 with the conditions given in Table 1 below. The thermochemical treatment was carried out either on a recrystallized structure (R) having been subjected to deformation sequences terminated by a heat treatment for recrystallization, or on a work-hardened structure (E) following deformation sequences without subsequent recrystallization heat treatment. The secondary error (ES) was measured at 23°C according to the following formula: ES 23 ° C = M 23 ° C − M 8 ° C + M 38 ° C 2 This is the deviation of the rate at 23°C from the straight line connecting the rate at 8°C to that at 38°C. For example, the rate at 8°C, 23°C and 38°C can be measured using a Witschi chronoscope type device. The thermal coefficient (TC) was measured according to the following formula: CT = M 38 ° C − M 8 ° C 30 with the same apparatus. The measurement results are given in Table 1. Table 1 Sample State TT duration Temperature H2 pressure CT E ES E [min] [°C] [bar] [s / d / °C] [s / d] 01 R 10 668 4 0.01 0.0 02 E 15 652 4 -0.46 -0.8 03 E 8 668 4 -0.59 -0.8 04 E 15 652 1 0.56 1.7 R = recrystallized, E = work hardened

[0049] Samples 01 to 04 have hydrogen contents between 0.3 and 1% by weight. All samples have a secondary error between -3 and +3 s / d as desired with values ​​close to 0 for samples treated under a hydrogen pressure of 4 bar. The CT is also contained in the range between -0.6 and +0.6 s / d°C as desired. The optimum is obtained for sample 01 for which the thermochemical treatment was carried out on a recrystallized structure, the thermal coefficient and the secondary error being close to 0 expressed respectively in s / d°C and s / d. This sample has a hydrogen content of the order of 0.6% by weight.

[0050] The results of samples 01 to 04 are reported in the figure 1with the secondary error (SE) as a function of the thermal coefficient (TC). In general, it has been observed that there is a direct link between the CT and the SE when the spiral spring alloy contains hydrogen, contrary to what has been observed in tests carried out in the past on a binary alloy with 47% by weight of titanium and 53% by weight of niobium. In the latter case, as illustrated in figure 2 , there is no relationship between CT and ES. The representation on the same graph of these two quantities gives a cloud of points, whatever the parameters of the manufacturing process of the samples. In addition, we never obtain points where CT=ES=0 whereas this is the case for the ternary grades Nb-Ti-H. Thus, it was discovered that the addition of hydrogen makes it possible to control the secondary error while maintaining a low CT.

[0051] The influence of temperature on the Young's modulus of sample 02 was also continuously measured using a mechanical spectrometer measuring the natural frequency of a freely vibrating beam, over a range from -20°C to +60°C ( fig.3 ). Little influence of temperature on Young's modulus is observed.

[0052] On this same sample, an X-ray diffraction analysis (Bragg-Brentano configuration) was carried out. The diffractogram is shown in figure 4 . The XRD spectrum between 30° and 80° does not indicate the presence of TiH 2 or NbH hydride phases. By doing the Figure 5a zoom centered on θ=39°, which corresponds to the area of ​​the NbTi

[110] peak, we observe that it is shifted to the left (Inv peak) following the thermochemical treatment compared to the reference peak (Ref peak) without thermochemical treatment, which is a sign of an increase in the lattice parameter. We can conclude that the thermochemical treatment allows the introduction of hydrogen in the form of interstitials without forming hydrides. Furthermore, no precipitation of titanium in the alpha phase is observed. The absence of titanium precipitates is attributed to the presence of hydrogen which stabilizes the beta phase of titanium.

Claims

1. Balance spring intended to equip a balance of a horological movement, characterised in that the balance spring is made of an alloy consisting of: - Nb, Ti, H and possible traces of other elements selected from O, C, Fe, N, Ni, Si, Cu and Al, with the following weight percentages: - a Ti content comprised between 20 and 60 wt%, preferably between 40 and 50 wt%, - a H content comprised between 0.17 and 2 wt%, - a total content of all other elements of less than or equal to 0.3 wt%, - the remainder to 100 wt% consisting of Nb.

2. Balance spring according to the preceding claim, characterised in that the H content is comprised between 0.2 and 1.5 wt%.

3. Balance spring according to one of the preceding claims, characterised in that the H content is comprised between 0.5 and 1 wt%.

4. Balance spring according to one of the preceding claims, characterised in that H is present predominantly or exclusively in interstitial form in the alloy.

5. Balance spring according to one of the preceding claims, characterised in that the microstructure of the alloy is formed by a single beta phase of Nb and Ti in a solid solution.

6. Balance spring according to one of the preceding claims having a thermal coefficient, or CT, comprised between -0.6 and +0.6 s / d°C and a middle-temperature error, or ES, comprised between -3 and +3 s / d.

7. Method for manufacturing a balance spring intended to equip a balance of a horological movement, successively comprising: a) a step of producing or of supplying a blank made of an alloy consisting of Nb, Ti and possible traces of other elements selected from O, C, Fe, N, Ni, Si, Cu and Al, with a Ti content comprised between 20 and 60 wt%, preferably between 40 and 50 wt% and a total content of all other elements of less than or equal to 0.3 wt%, the remainder to 100 wt% consisting of Nb, b) a step of so-called beta-type solution treating and quenching said blank, so that the titanium and niobium of said alloy are essentially in the form of a beta-phase solid solution, c) a step of applying to said alloy a succession of deformation sequences optionally with at least one heat treatment carried out between two deformation sequences and / or at the end of all of the deformation sequences, d) a winding step for forming the balance spring, e) a final so-called fixing heat treatment step, the method being characterised in that it comprises an additional thermochemical treatment step in an atmosphere comprising hydrogen, said thermochemical treatment step being carried out during the solution treatment of step b), during a heat treatment of step c), during the final heat treatment of step e), before step b), between steps b) and c), between steps c) and d), between steps d) and e) or after step e).

8. Method for manufacturing a balance spring according to the preceding claim, characterised in that the thermochemical treatment step is carried out in step e).

9. Method for manufacturing a balance spring according to claim 7 or 8, characterised in that the thermochemical treatment step is carried out on a structure of the blank or balance spring in the recrystallised state.

10. Method for manufacturing a balance spring according to one of claims 7 to 9, characterised in that the thermochemical treatment is carried out at a temperature comprised between 100 and 900°C in an atmosphere comprising 100% hydrogen at a hydrogen pressure comprised between 5 mbar and 10 bar, or is carried out in an atmosphere comprising a mixture of hydrogen and another gas with a volume percentage of hydrogen comprised between 5 and 90 vol%, the total pressure of the mixture being comprised between 5 mbar and 10 bar.

11. Method for manufacturing a balance spring according to one of claims 7 to 10, characterised in that the hydrogen pressure or the total pressure of the mixture is comprised between 0.5 and 7 bar, preferably between 1 and 6 bar, more preferably between 3.5 and 4.5 bar.

12. Method for manufacturing a balance spring according to one of claims 7 to 11, characterised in that the temperature is comprised between 500 and 800°C, preferably between 600 and 700°C.

13. Method for manufacturing a balance spring according to one of claims 7 to 12, characterised in that the hydrogen pressure or the total pressure of the mixture is comprised between 3.5 and 4.5 bar and the temperature is comprised between 600 and 700°C.

14. Method for manufacturing a balance spring according to one of claims 7 to 13, characterised in that the solution treatment is carried out in a vacuum at a temperature comprised between 600°C and 1,000°C for a duration comprised between 5 minutes and 2 hours, followed by cooling under a gas.

15. Method for manufacturing a balance spring according to one of claims 7 to 14, characterised in that after step a) of producing or supplying the alloy blank, and before step c) of applying a succession of sequences, a surface layer of ductile material, taken from among copper, nickel, cupronickel, cupromanganese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B, is added to said blank to ease the wire shaping operation and in that, before or after the winding step d), the layer of said ductile material is removed from said wire by etching.

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