Hairspring for clock movement

Abstract

The present invention relates to a balance spring (1) for use in a clockwork movement, characterized in that the balance spring (1) is made of a niobium-titanium alloy consisting, by weight, of: - niobium: 100%; - titanium with a percentage greater than or equal to 1% and less than 40%; - traces of other elements selected from O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said elements being between 0 and 1600 ppm of the total by weight and the sum of said traces being less than or equal to 0.3% by weight. The present invention also relates to its manufacturing process.

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 production of spiral springs is also focused on the concern for thermal compensation, in order to guarantee regular chronometric performance. This requires obtaining a thermoelastic coefficient close to zero.

[0004] Any improvement on at least one of the points, and in particular on the mechanical resistance of the alloy used, therefore represents a significant advance.

[0005] Document US 2007 / 0133355 discloses a spiral spring made from an alloy of niobium and titanium consisting of 20 to 80% by weight of niobium and 80 to 20% by weight of titanium with traces of other elements chosen from O, C, N, Fe, B, Cr, Mo, Co, Mn and Al. Summary of the invention

[0006] The invention aims to define a new type of watch spiral spring, based on the selection of a particular material, and to develop the appropriate manufacturing process.

[0007] To this end, the invention relates to a watch spiral spring made from an alloy of niobium and titanium defined by claim 1 of the patent.

[0008] The invention also relates to the method of manufacturing this watch spiral spring as claimed in the appendix. Summary description of the drawings

[0009] Other characteristics and advantages of the invention will appear on reading the detailed description which follows, with reference to the appended drawings, where: there figure 1 represents, in a schematic manner, a spiral spring made with an Nb-Ti alloy according to the invention; the figure 2 represents the curves of evolution of the Young's modulus as a function of temperature reported on the Young's modulus at 20°C for respectively pure Nb and an Nb-Ti alloy according to the invention with 30% by weight of Ti. Detailed Description of Preferred Embodiments

[0010] The invention relates to a watch spiral spring made from a binary type alloy comprising niobium and titanium.

[0011] According to the invention, this alloy comprises by weight: niobium: balance at 100%; titanium in a percentage between 27 and 33%; traces of other elements chosen from O, H, C, Fe, Ta, N, Ni, Si, Cu and / or Al, each of said elements being between 0 and 1600 ppm of the total by weight, and the sum of these traces being less than or equal to 0.3%. In other words, the total of the percentages by weight of titanium and niobium is between 99.7% and 100% of the total.

[0012] The percentage by weight of oxygen is less than or equal to 0.10% of the total, or even less than or equal to 0.085% of the total.

[0013] The weight percentage of tantalum is less than or equal to 0.10% of the total.

[0014] The percentage by weight of carbon is less than or equal to 0.04% of the total, in particular less than or equal to 0.020% of the total, or even less than or equal to 0.0175% of the total.

[0015] The percentage by weight of iron is less than or equal to 0.03% of the total, in particular less than or equal to 0.025% of the total, or even less than or equal to 0.020% of the total.

[0016] The percentage by weight of nitrogen is less than or equal to 0.02% of the total, in particular less than or equal to 0.015% of the total, or even less than or equal to 0.0075% of the total.

[0017] The percentage by weight of hydrogen is less than or equal to 0.01% of the total, in particular less than or equal to 0.0035% of the total, or even less than or equal to 0.0005% of the total.

[0018] The weight percentage of nickel is less than or equal to 0.01% of the total.

[0019] The weight percentage of silicon is less than or equal to 0.01% of the total.

[0020] The percentage by weight of nickel is less than or equal to 0.01% of the total, in particular less than or equal to 0.16% of the total.

[0021] The percentage by weight of copper is less than or equal to 0.01% of the total, in particular less than or equal to 0.005% of the total.

[0022] The weight percentage of aluminum is less than or equal to 0.01% of the total.

[0023] Advantageously, this spiral spring has a two-phase microstructure comprising niobium in the body-centered cubic beta phase and titanium in the hexagonal close-packed alpha phase.

[0024] To obtain such a microstructure, and suitable for the production of a spring, it is necessary to precipitate part of the alpha phase by heat treatment.

[0025] The higher the titanium content, the higher the maximum proportion of alpha phase that can be precipitated by heat treatment, which encourages the search for a high proportion of titanium. But conversely, the higher the titanium content, the more difficult it is to obtain precipitation of only the alpha phase at the grain boundaries. The appearance of intragranular alpha-Ti Widmastätten precipitates or the intragranular ω phase makes the deformation of the material difficult, if not impossible, which is then not suitable for the production of a balance spring, and it is therefore advisable not to incorporate too much titanium into the alloy. In addition, the application of this alloy to a balance spring requires properties capable of ensuring 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. The formula that links the CTE of the alloy and the coefficients of expansion of the hairspring and balance wheel is as follows: . CT = dM dT = 1 2 E dE dT − β + 3 2 α × 86400 s j ° C

[0026] The variables M and T are the rate and the temperature respectively. E is the Young's modulus of the spiral spring, and, in this formula, E, ß and a are expressed in °C -1< .

[0027] CT is the thermal coefficient of the oscillator, (1 / E. dE / dT) is the CTE of the balance alloy, β is the coefficient of expansion of the balance and α that of the balance. The work-hardened beta-phase alloy has a strongly positive CTE, and the precipitation of the alpha phase, which has a strongly negative CTE, allows the two-phase alloy to be brought to a CTE close to zero, which is particularly favorable. However, as mentioned above, too high a percentage of titanium leads to the formation of brittle phases. A percentage of titanium lower than 40% by weight allows a good compromise between the different desired properties to be obtained. It is also assumed that the interaction between the dislocations and the C, H, N, O interstitials present in the alloy, as well as the interaction between the dislocations and the alpha titanium precipitates, also play a favorable role on the CTE.The temperature-dependent movement of dislocations causes a decrease in the Young's modulus of the spiral spring, which counteracts the positive anomaly of the beta phase.

[0028] The spiral spring made with this alloy has an elastic limit greater than or equal to 500 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 110 GPa.

[0029] The invention also relates to the method of manufacturing the watch spiral spring, characterized in that the following steps are carried out successively: preparation of a blank in an alloy comprising niobium and titanium and more precisely: niobium: balance at 100%; a percentage by weight of titanium between 27% and 33%; traces of other elements chosen from O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said elements being between 0 and 1600 ppm of the total by weight, and the sum of said traces being less than or equal to 0.3% by weight; a beta-type quenching of said blank, so that the titanium of said alloy is essentially in the form of a solid solution with the niobium in beta phase; application to said alloy of deformation sequences followed by a heat treatment. 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. The wire drawing is carried out until a round section wire 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 a rolling preferably with a rectangular profile compatible with the entry section of a drawing spindle. These sequences lead to the production of a two-phase microstructure comprising beta niobium and alpha titanium, with an elastic limit greater than or equal to 500 MPa and a modulus of elasticity less than or equal to 120 GPa and preferably 110 GPa. drawing to form a spiral spring, followed by a final heat treatment.

[0030] In these coupled deformation-heat treatment sequences, each deformation is carried out with a given deformation rate between 1 and 5, this deformation rate corresponding to the classic formula 2ln(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 this entire succession of sequences leads to a total deformation rate between 1 and 14. Each coupled deformation-heat treatment sequence includes, each time, a heat treatment for precipitation of the alpha Ti phase.

[0031] Beta quenching prior to the deformation and heat treatment sequences is a solution treatment, lasting between 5 minutes and 2 hours at a temperature between 700°C and 1000°C, under vacuum, followed by cooling under gas.

[0032] More specifically, this beta quenching is a solution treatment, lasting 1 hour at 800°C under vacuum, followed by cooling under gas.

[0033] Returning to the coupled deformation-heat treatment sequences, the heat treatment is a precipitation treatment lasting between 1 hour and 200 hours at a temperature between 300°C and 700°C. More specifically, the duration is between 5 hours and 30 hours at a temperature between 400°C and 600°C.

[0034] More specifically, the process involves between one and five coupled deformation-heat treatment sequences.

[0035] More particularly, the first coupled deformation-heat treatment sequence comprises a first deformation with at least 30% reduction in section.

[0036] More particularly, each coupled deformation-heat treatment sequence, other than the first, includes a deformation between two heat treatments with at least 25% reduction in section.

[0037] More particularly, after this preparation of said alloy blank, and before the deformation-heat treatment sequences, in an additional step, 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, after the deformation-heat treatment sequences or after the stripping step, the wire is stripped of its layer of ductile material, in particular by chemical attack.

[0038] 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.

[0039] In a particular watchmaking application, ductile material or copper 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 or copper, in particular by chemical attack, then is rolled flat before the manufacture of the spring itself by stripping.

[0040] The contribution of ductile material or copper can be galvanic, or mechanical, in which case it is a jacket or a tube of ductile material or copper which is adjusted on a bar of niobium-titanium alloy to a large diameter, then which is thinned during the stages of deformation of the composite bar.

[0041] A diffusion barrier layer, for example nb, can be added between the nb-Ti and the Cu to prevent the formation of intermetallics that are detrimental to the deformability of the material. The thickness of this layer is chosen to correspond to a thickness of 100 nm to 1 µm on the wire with a diameter of 0.1 mm.

[0042] 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.

[0043] By an appropriate combination of deformation and heat treatment sequences, it is possible to obtain a very fine, in particular nanometric, lamellar two-phase microstructure comprising or composed of beta niobium and alpha titanium. This alloy combines a very high yield strength, at least greater than 500 MPa, and a very low modulus of elasticity, in the order of 80 GPa to 120 GPa. This combination of properties is well suited for a spiral spring. The alloy after the deformation-heat treatment sequences has a texture <110> . Furthermore, this niobium-titanium alloy according to the invention can easily be covered with ductile material or copper, which greatly facilitates its deformation by wire drawing.

[0044] A binary type alloy comprising niobium and titanium, of the type selected above for the implementation of the invention, also has an effect similar to that of “Elinvar”, with a practically zero thermo-elastic coefficient in the usual temperature range of use of watches, and suitable for the manufacture of self-compensating balance springs.

[0045] More precisely, if we compare to the figure 2, the evolution of the Young's modulus (E(T) / E 20°C) as a function of temperature for pure Nb and an Nb-Ti alloy according to the invention with 30% by weight of Ti, we observe that the two curves are S-shaped with the notable difference that the presence of Ti makes it possible to significantly reduce the difference between the minimum and the maximum of the curve along both the abscissa axis and the ordinate axis. More precisely, the presence of Ti in the alloy and the manufacturing method according to the invention attempt to smooth the curve via the reduction of the maximum of the curve. This positive effect on the reduction of the maximum with the alloy according to the invention is attributed to a set of factors which are: the crystallographic texture of the alloy which is influenced by the reduction rate since beta quenching, the density of dislocations adjusted via heat treatments which induce restoration phenomena, or even recrystallization, the concentration of interstitials which will interact with the dislocations, the percentage of Ti in alpha phase the density of precipitates in the alloy (number of Ti precipitates in alpha phase per unit of volume).

Claims

1. Balance spring (1) intended to equip a balance of a horological movement, the balance spring (1) being made of a niobium and titanium alloy containing by weight: - niobium: the remainder to 100%; - traces of other elements selected from among O, H, C, Fe, Ta, N, Ni, Si, Cu and / or Al, each of said elements being in the range of 0 to 1,600 ppm of the total weight, and the sum of said trace elements being less than or equal to 0.3 wt. %, characterised in that the niobium alloy comprises titanium in a weight percentage of between 27 and 33 wt.%.

2. Balance spring (1) according to the preceding claim, characterised in that it has a two-phase microstructure including niobium in the beta phase and titanium in the alpha phase.

3. Balance spring (1) according to one of the preceding claims, characterised in that it has a yield strength greater than or equal to 500 MPa and a modulus of elasticity less than or equal to 120 GPa, preferably less than or equal to 110 GPa.

4. Method for manufacturing a balance spring (1) intended to equip a balance of a horological movement, comprising in succession: - a step of producing a blank made from a niobium and titanium alloy containing by weight: - niobium: the remainder to 100t%; - titanium in a weight percentage between 27 and 33%, - traces of other elements selected from among O, H, C, Fe, Ta, N, Ni, Si, Cu and / or Al, each of said elements being in the range of 0 to 1,600 ppm of the total weight, and the sum of said trace elements being less than or equal to 0.3 wt%; - a step of beta-type quenching said blank such that the titanium of said alloy is essentially in the form of a solid solution with beta-phase niobium, - a step of applying, to said alloy, a succession of deformation sequences followed by an intermediate heat treatment, - a winding step for forming the balance spring (1), - a final heat treatment step.

5. Method for manufacturing a balance spring (1) according to claim 4, characterised in that the deformation during each sequence is performed by wire drawing and / or rolling.

6. Method for manufacturing a balance spring (1) according to claim 5, characterised in that the deformation of the last sequence is performed by flat rolling.

7. Method for producing a balance spring (1) according to one of claims 4 to 6, characterised in that the deformation of each sequence is performed at a given deformation rate in the range of 1 to 5, the total cumulation of the deformations over the entirety of said succession of sequences producing a total deformation rate in the range of 1 to 14.

8. Method for producing a balance spring (1) according to one of claims 4 to 7, characterised in that the beta-type quench is a dissolving treatment, the duration of which is in the range of 5 minutes to 2 hours at a temperature of between 700°C and 1,000°C, in a vacuum, followed by gas cooling.

9. Method for producing a balance spring (1) according to one of claims 4 to 8, characterised in that the beta-type quench is a dissolving treatment, lasting 1 hour at 800°C in a vacuum, followed by gas cooling.

10. Method for producing a balance spring (1) according to one of claims 4 to 9, characterised in that the final heat treatment in addition to the intermediate heat treatment of each sequence is an alpha-phase Ti precipitation treatment, the duration of which is in the range of between 1 hour to 200 hours at a temperature of between 300°C and 700°C.

11. Method for producing a balance spring (1) according to one of claims 4 to 10, characterised in that the final heat treatment in addition to the intermediate heat treatment of each sequence is an alpha-phase Ti precipitation treatment, the duration of which is in the range of 5 hours to 30 hours at a temperature of between 400°C and 600°C.

12. Method for producing a balance spring (1) according to one of claims 4 to 11, characterised in that said method includes between one and five said deformation sequences followed by an intermediate heat treatment.

13. Method for producing a balance spring (1) according to one of claims 4 to 12, characterised in that the first said deformation sequence followed by an intermediate heat treatment includes a first deformation with at least a 30% section reduction.

14. Method for manufacturing a balance spring (1) according to claim 13, characterised in that each said deformation sequence followed by an intermediate heat treatment, aside from the first, includes a deformation between two intermediate heat treatments with at least a 25% section reduction.

15. Method for producing a balance spring (1) according to one of claims 4 to 14, characterised in that after the step of producing the alloy blank, and before the step 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, the layer of said ductile material is removed from said wire by etching.

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