CH714492B1 - Spiral spring for clock movement - Google Patents

Abstract

The present invention relates to a balance spring made of a niobium and titanium alloy consisting of: – niobium: balance at 100% by weight, – titanium: between 40 and 60% by weight, – traces of elements selected from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, between 0 and 1600 ppm by weight individually, with a cumulative amount of less than 0.3% by weight. Said alloy has an elastic limit greater than or equal to 600 MPa and a modulus of elasticity less than or equal to 100 GPa and is covered with a surface layer of a ductile material such as copper.

Description

Field of invention

[0001] The invention relates to a spiral spring intended to equip a balance wheel of a clock movement.

Background of the invention

[0002] The manufacture of spiral springs for watchmaking must face constraints which are often 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, so as to guarantee regular chronometric performances. To do this, it is necessary to obtain a thermoelastic coefficient close to zero. We are also looking to produce spiral springs with limited sensitivity to magnetic fields.

[0004] Any improvement on at least one of these points, and in particular on the limited sensitivity to magnetic fields, thermal compensation, and ease of production, in particular drawing, wire drawing and rolling, therefore represents a significant advance.

Summary of the invention

[0005] The invention proposes to define a new type of spiral spring intended to equip a balance wheel of a watch movement, based on the selection of a particular material, and to develop the appropriate manufacturing process.

[0006] For this purpose, the invention relates to a spiral spring intended to equip a balance of a watch movement, the spiral spring being made of an alloy of niobium and titanium consisting of: – niobium: balance at 100% by weight, – titanium: between 40 and 60% 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 of between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being between 0% and 0.3% by weight, said alloy having an elastic limit greater than or equal to 600 MPa and a modulus of elasticity less than 100 GPa, said alloy being covered with a surface layer of a ductile material chosen from the group comprising copper, nickel, cupro-nickel, cupro-manganese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B.

[0007] A method for manufacturing such a spiral spring comprises: – a step of producing a blank in an alloy of niobium and titanium consisting of: – niobium: balance at 100% by weight, – titanium: between 40 and 60% 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 of between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being between 0% and 0.3% by weight, – a step of β-type quenching of said blank to a given diameter, so that the titanium of said alloy is essentially in the form of a solid solution with the niobium in β phase (body-centered cubic structure), the titanium content in α phase (hexagonal compact structure) being less than or equal to 5% by volume, – at least one step of deformation of said alloy alternated with at least one heat treatment step such that the niobium and titanium alloy obtained has an elastic limit greater than or equal to 600 MPa and a modulus of elasticity less than or equal to 100 GPa, a rolling step to form the spiral spring being carried out before a final heat treatment step.

[0008] The method comprises, before the deformation step, a step of depositing, on the alloy blank, a surface layer of a ductile material chosen from the group comprising copper, nickel, cupro-nickel, cupro-manganese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B, said surface layer of ductile material being retained on the final spiral spring obtained, the thermoelastic coefficient of the niobium and titanium alloy being adapted accordingly.

[0009] The spiral spring according to the invention is made of a paramagnetic niobium and titanium alloy and having the mechanical properties and the thermoelastic coefficient required for its use as a spiral spring for a balance wheel. It is obtained according to a manufacturing process which makes it possible to facilitate the shaping in the form of wire of the NbTi alloy blank, and more specifically to facilitate the drawing, wire drawing, and rolling and to obtain a perfectly regular final wire section.

Detailed description of preferred embodiments

[0010] The invention relates to a spiral spring intended to equip a balance wheel of a clockwork movement and made from a binary type alloy comprising niobium and titanium.

[0011] According to the invention, the spiral spring is made from an alloy of niobium and titanium consisting of: – niobium: balance at 100% by weight, – titanium: between 40 and 60% 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 of between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being between 0% and 0.3% by weight, said alloy having an elastic limit greater than or equal to 600 MPa and a modulus of elasticity less than 100 GPa.

[0012] According to the invention, said NbTi alloy is covered with a surface layer of a ductile material chosen from the group comprising copper, nickel, cupro-nickel, cupro-manganese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B.

[0013] The preserved surface layer of ductile material makes it possible to obtain a perfectly regular final wire section. The ductile material may preferably be copper or gold.

[0014] Advantageously, the spiral spring may comprise, on the surface layer of ductile material, a final layer of a material selected from the group comprising copper, nickel, cupro-nickel, cupro-manganese, silver, nickel-phosphorus Ni-P, nickel-boron Ni-B, gold, provided that the material of the final layer is different from the ductile material of the surface layer, Al2O3, TiO2, SiO2 and AIO. This final layer has a thickness of 0.1 µm to 1 µm and makes it possible to color the spiral or to obtain insensitivity to climatic aging (temperature and humidity).

[0015] Advantageously, the alloy used in the present invention comprises between 40 and 49% by weight of titanium, preferably between 44 and 49% by weight of titanium, and more preferably between 46% and 48% by weight of titanium, and preferably said alloy comprises more than 46.5% by weight of titanium and said alloy comprises less than 47.5% by weight of titanium.

[0016] If the titanium content is too high, a martensitic phase appears, leading to problems of brittleness of the alloy during its implementation. If the niobium content is too high, the alloy will be too soft. The development of the invention made it possible to determine a compromise, with an optimum between these two characteristics close to 47% by weight of titanium.

[0017] Also, more particularly, the titanium content is greater than or equal to 46.5% by weight relative to the total of the composition.

[0018] More particularly, the titanium content is less than or equal to 47.5% by weight relative to the total of the composition.

[0019] In a particularly advantageous manner, the NbTi alloy used in the present invention does not comprise other elements except for possible and unavoidable traces. This makes it possible to avoid the formation of fragile phases.

[0020] 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.

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

[0022] 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.

[0023] 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.

[0024] 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.

[0025] More particularly, 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 particularly, the silicon content is less than or equal to 0.01% by weight of the total.

[0027] 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.

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

[0030] The spiral spring of the invention has an elastic limit greater than or equal to 600 MPa.

[0031] Advantageously, this spiral spring has a modulus of elasticity less than or equal to 100 GPa, and preferably between 60 GPa and 80 GPa.

[0032] Furthermore, the spiral spring according to the invention has a thermoelastic coefficient, also called CTE, allowing it to guarantee the maintenance of chronometric performance despite the variation in the operating temperatures of a watch incorporating such a spiral spring.

[0033] To form a time-lapse oscillator meeting COSC conditions, the CTE of the alloy must be close to zero (± 10 ppm/°C) to obtain a thermal coefficient of the oscillator equal to ± 0.6 s/d/°C.

[0034] The formula which links the CTE of the alloy and the coefficients of expansion of the hairspring and the balance wheel is as follows:

[0035] The variables M and T are respectively the rate and the temperature. E is the Young's modulus of the spiral spring, and, in this formula, E, β and α are expressed in °C<-1>.

[0036] 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 spring.

[0037] An adequate CTE and therefore CT depending on the surface layer and the possible final layer are easily obtained when implementing the different stages of the process as will be seen below.

[0038] According to a first variant, said alloy of niobium and titanium of the spiral spring is of single-phase structure in which the titanium is essentially in the form of a solid solution with the niobium in β phase, the titanium content in α phase being less than or equal to 10% by volume, preferably less than or equal to 5% by volume, and more preferably less than or equal to 2.5% by volume.

[0039] According to a second variant, said alloy of niobium and titanium of the spiral spring is of two-phase structure comprising a solid solution of niobium with titanium in β phase and a solid solution of niobium with titanium in α phase, the titanium content in α phase being greater than 10% by volume.

[0040] A method for manufacturing a spiral spring made of a binary type NbTi alloy as defined above, comprises: – a step of producing a blank in a niobium and titanium alloy consisting of: – niobium: balance at 100% by weight, – titanium: between 40 and 60% 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 of between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being between 0% and 0.3% by weight, – a step of β-type quenching of said blank to a given diameter, so that the titanium of said alloy is essentially in the form of a solid solution with the niobium in β phase, the titanium content in α phase being less than or equal to 5% by volume, and more preferably less than or equal to at 2.5% by volume, – at least one step of deformation of said alloy alternated with at least one heat treatment step so that the niobium and titanium alloy obtained has an elastic limit greater than or equal to 600 MPa and a modulus of elasticity less than or equal to 100 GPa, a step of strapping to form the hairspring being carried out before the last heat treatment step, this last step making it possible to fix the shape of the hairspring and to adjust the thermoelastic coefficient, – and, before the deformation step, a step of depositing, on the alloy blank, a surface layer of a ductile material chosen from the group comprising copper, nickel, cupro-nickel, cupro-manganese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B, said surface layer of ductile material being retained on the hairspring, the thermoelastic coefficient of the niobium and titanium alloy being adapted accordingly.

[0041] As will be seen below, the thermoelastic coefficient of the niobium-titanium alloy can be easily adjusted by choosing the appropriate strain rate and heat treatments.

[0042] Advantageously, the thickness of the layer of ductile material deposited is chosen so that the ratio of surface area of ductile material/surface area of the NbTi alloy for a given wire section is less than 1, preferably less than 0.5, and more preferably between 0.01 and 0.4.

[0043] Such a thickness of ductile material, and in particular copper, makes it possible to easily stretch, draw and roll the Cu/NbTi composite material.

[0044] The ductile material, preferably copper, is thus deposited at a given time to facilitate the shaping of the wire by stretching and drawing, such that a thickness preferably of between 1 and 500 micrometers remains on the wire with a total diameter of 0.2 to 1 millimeter.

[0045] Furthermore, the preserved surface layer of ductile material makes it possible to obtain a perfectly regular final wire section.

[0046] The contribution of ductile material, in particular copper, can be galvanic, PVD or CVD, or mechanical, in which case it is a jacket or a tube of ductile material such as copper which is fitted onto a bar of niobium-titanium alloy to a large diameter, then which is thinned during the step(s) of deformation of the composite bar.

[0047] The ductile material may preferably be copper or gold, deposited by galvanic, PVD or CVD means.

[0048] The method may further comprise a step of depositing, on the preserved surface layer of ductile material, a final layer of a material selected from the group comprising Al2O3, TiO2, SiO2 and AIO, by PVD or CVD. A final layer of gold deposited by galvanic gold flash may also be provided if gold has not already been used as the ductile material of the surface layer. Copper, nickel, cupro-nickel, cupro-manganese, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B may also be used for the final layer, provided that the material of the final layer is different from the ductile material of the surface layer.

[0049] This final layer has a thickness of 0.1 µm to 1 µm and makes it possible to color the hairspring or to obtain insensitivity to climatic aging (temperature and humidity).

[0050] Preferably, the quenching step β is a solution treatment, with a duration of between 5 minutes and 2 hours at a temperature of between 700°C and 1000°C, under vacuum, followed by cooling under gas.

[0051] More particularly still, this beta quenching is a solution treatment, between 5 minutes and 1 hour at 800°C under vacuum, followed by cooling under gas.

[0052] Preferably, the heat treatment is carried out for a period of between 1 hour and 80 hours, or even more, preferably between 1 hour and 15 hours at a temperature of between 350°C and 700°C. More preferably, the heat treatment is carried out for a period of between 5 hours and 10 hours at a temperature of between 350°C and 600°C. Even more preferably, the heat treatment is carried out for a period of between 3 hours and 6 hours at a temperature of between 400°C and 500°C.

[0053] A deformation step generally refers to one or more deformation treatments, which may include wire drawing and/or rolling. Wire drawing may require the use of one or more dies during the same deformation step or during different deformation steps if necessary. Wire drawing is carried out until a wire of round section is obtained. Rolling may be carried out during the same deformation step as wire drawing or in another subsequent deformation step. Advantageously, the last deformation treatment applied to the alloy is rolling, preferably with a rectangular profile compatible with the entry section of a stripping spindle.

[0054] In a particularly advantageous manner, the total deformation rate, the number of heat treatments and the parameters of the heat treatments are chosen to obtain a spiral spring having a thermoelastic coefficient as close as possible to 0. Furthermore, depending on the total deformation rate, the number of heat treatments and the parameters of the heat treatments, a single-phase or two-phase NbTi alloy is obtained.

[0055] More particularly, according to a first variant, the number of heat treatment and deformation steps is limited so that the alloy of niobium and titanium of the spiral spring obtained retains a structure in which the titanium of said alloy is essentially in the form of a solid solution with the niobium in β phase (body-centered cubic structure), the titanium content in α phase being less than or equal to 10% by volume, preferably less than or equal to 5% by volume, more preferably less than or equal to 2.5% by volume.

[0056] Preferably, the total deformation rate is between 1 and 5, preferably between 2 and 5.

[0057] In a particularly advantageous manner, a blank is used whose dimensions are as close as possible to the desired final dimensions so as to limit the number of heat treatment and deformation steps and to preserve an essentially single-phase β structure of the NbTi alloy. The final structure of the NbTi alloy of the spiral spring may be different from the initial structure of the blank, for example the titanium content in α form may have varied, the essential thing being that the final structure of the NbTi alloy of the spiral spring is essentially single-phase, the titanium of said alloy being essentially in the form of a solid solution with the niobium in β phase, the titanium content in α phase being less than or equal to 10% by volume, preferably less than or equal to 5% by volume, more preferably less than or equal to 2.5% by volume. In the alloy of the blank after β quenching, the titanium content in α phase is preferably less than or equal to 5% by volume, more preferably less than or equal to 2.5% by volume, or even close to or equal to 0.

[0058] Thus, according to this variant, a spiral spring is obtained made of an NbTi alloy having an essentially single-phase structure in the form of a β-Nb-Ti solid solution, the titanium content in α form being less than or equal to 10% by volume, preferably less than or equal to 5% by volume, more preferably less than or equal to 2.5% by volume.

[0059] Preferably, the method comprises a single deformation step with a deformation rate of between 1 and 5, preferably between 2 and 5.

[0060] Thus, a particularly preferred method comprises, after the β quenching step, the step of depositing, on the alloy blank, the surface layer of ductile material, a deformation step including wire drawing using several dies then rolling, a drawing step then a final heat treatment step (called fixing).

[0061] The method may further comprise at least one intermediate heat treatment step, such that the method comprises, for example, after the β-quenching step, the step of depositing, on the alloy blank, the surface layer of ductile material, a first deformation step, an intermediate heat treatment step, a second deformation step, the strapping step and then a final heat treatment step.

[0062] The higher the strain rate after β-quenching, the more positive the thermal coefficient CT. The more the material is annealed after β-quenching, in the appropriate temperature range, by the various heat treatments, the more negative the thermal coefficient CT becomes. An appropriate choice of the strain rate and the parameters of the heat treatments makes it possible to bring the single-phase NbTi alloy to a CTE close to zero, which is particularly favorable.

[0063] According to a second variant, a succession of sequences of a deformation step alternating with a heat treatment step is applied, until an alloy of niobium and titanium with a two-phase structure is obtained comprising a solid solution of niobium with titanium in the β phase (body-centered cubic structure) and a solid solution of niobium with titanium in the α phase (compact hexagonal structure), the titanium content in the α phase being greater than 10% by volume.

[0064] To obtain such a two-phase structure, it is necessary to precipitate a portion of the α phase by heat treatments, according to the parameters indicated above, with a high deformation between the heat treatments. Preferably, however, longer heat treatments are applied than those used to obtain a single-phase spring alloy, for example heat treatments carried out for a duration of between 15 hours and 75 hours at a temperature of between 350°C and 500°C. For example, heat treatments of 75h to 400h at 350°C, of 25h at 400°C or of 18h at 480°C are applied.

[0065] In this second “two-phase” variant, a blank is used which, after β-hardening, has a much larger diameter than that of the blank prepared for the first “single-phase” variant. Thus, in the second variant, for example, a blank with a diameter of 30 mm is used after β-hardening, whereas for the first variant, a blank with a diameter of 0.2 to 2.0 mm is used after β-hardening.

[0066] Preferably, in these coupled deformation-heat treatment sequences, each deformation is carried out with a deformation rate of between 1 and 5, the overall accumulation of deformations over the whole of said succession of sequences leading to a total deformation rate of between 1 and 14.

[0067] The strain rate corresponds to the classic formula 2In(d0/d), where d0 is the diameter of the last beta quench or that of a deformation step, and d is the diameter of the work-hardened wire obtained at the following deformation step.

[0068] Advantageously, the method comprises in this second variant between three and five coupled deformation-heat treatment sequences.

[0069] More particularly, the first coupled deformation-heat treatment sequence comprises a first deformation with at least 30% reduction in section.

[0070] More particularly, each coupled deformation-heat treatment sequence, other than the first, comprises a deformation between two heat treatments with at least 25% reduction in section.

[0071] In this second variant, the work-hardened β-phase alloy has a strongly positive CT, and the precipitation of the α-phase which has a strongly negative CT, makes it possible to bring the two-phase alloy to a CTE close to zero, which is particularly favorable.

[0072] The method allows the production, and more particularly the shaping, of a balance spring made of a niobium-titanium alloy, typically containing 47% by weight of titanium (40-60%), having an essentially single-phase microstructure of β-Nb-Ti in which the titanium is in the form of a solid solution with the niobium in the β phase or a very fine lamellar two-phase microstructure comprising a solid solution of niobium with titanium in the β phase and a solid solution of niobium with titanium in the α phase. In addition, the balance spring has a perfectly regular final section, without irregularities or wires on the surface. The NbTi alloy has high mechanical properties, combining a very high elastic limit, greater than 600 MPa, and a very low modulus of elasticity, of the order of 60 GPa to 80 GPa. This combination of properties is well suited for a balance spring.

[0073] Such an alloy is known and used for the manufacture of superconductors, such as magnetic resonance imaging devices, or particle accelerators, but is not used in watchmaking.

[0074] A binary type alloy comprising niobium and titanium, of the type selected above for the implementation of the invention, also exhibits an effect similar to that of “Elinvar”, with a thermo-elastic coefficient practically zero in the temperature range of usual use of watches, and suitable for the manufacture of self-compensating balance springs.

[0075] Furthermore, such an alloy is paramagnetic.

Claims (8)

1. Spiral spring intended to equip a balance wheel of a watch movement, the spiral spring being made of an alloy of niobium and titanium consisting of: – niobium: balance at 100% by weight, – titanium: between 40 and 60% 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 of between 0 and 1600 ppm by weight, the total amount constituted by all of said elements being between 0% and 0.3% by weight, said alloy having an elastic limit greater than or equal to 600 MPa and a modulus of elasticity less than 100 GPa, characterized in that said alloy is covered with a surface layer of a ductile material chosen from the group comprising copper, nickel, cupro-nickel, cupro-manganese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B. 2. Spiral spring according to claim 1, characterized in that it comprises, on the surface layer of ductile material, a final layer of a material chosen from the group comprising copper, nickel, cupro-nickel, cupro-manganese, silver, nickel-phosphorus Ni-P, nickel-boron Ni-B, gold, chosen different from the ductile material of the surface layer, Al2O3, TiO2, SiO2 and AIO. 3. Spiral spring according to claim 1 or 2, characterized in that said alloy comprises between 40 and 49% by weight of titanium, preferably between 44 and 49% by weight of titanium, and more preferably between 46% and 48% by weight of titanium. 4. Spiral spring according to one of claims 1 to 3, characterized in that said alloy comprises more than 46.5% by weight of titanium. 5. Spiral spring according to one of claims 1 to 4, characterized in that said alloy comprises less than 47.5% by weight of titanium. 6. Spiral spring according to one of the preceding claims, characterized in that said alloy of niobium and titanium is of essentially single-phase structure, in which the titanium is essentially in the form of a solid solution with the niobium in β phase, the titanium content in α phase being less than or equal to 10% by volume. 7. Spiral spring according to claim 6, characterized in that the titanium content in α phase is less than or equal to 5% by volume. 8. Spiral spring according to one of claims 1 to 5, characterized in that said alloy of niobium and titanium is of two-phase structure comprising a solid solution of niobium with titanium in β phase and a solid solution of niobium with titanium in α phase, the titanium content in α phase being greater than 10% by volume.