CH716622A2 - Spiral spring for watch movement made of niobium and titanium alloy and process for its manufacture. - Google Patents

Abstract

The present invention relates to a spiral spring (1) intended to equip a balance wheel of a timepiece movement, characterized in that the spiral spring (1) is made of an alloy of niobium and titanium consisting by weight of: niobium: scale at 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 method of manufacture.

Description

Description

Field of the invention

[0001] The invention relates to a spiral spring intended to equip a balance wheel with a clock movement. It also relates to the process for manufacturing this spiral spring.

Background of the invention

[0002] The manufacture of spiral springs for watchmaking must face constraints that are often incompatible at first sight:

- need to obtain a high elastic limit,

- ease of production, in particular drawing and rolling,

- excellent fatigue resistance,

- stability of performance over time,

- small sections.

[0003] The production of spiral springs is also centered on the concern for thermal compensation, so as to guarantee regular chronometric performances. This requires a thermoelastic coefficient close to zero.

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

Summary of the invention

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

To this end, the invention relates to a clockwork spiral spring made of an alloy of niobium and titanium. According to the invention, the titanium content is comprised by weight between 1% (limit included) and 40% (limit not included). Advantageously, it is comprised by weight between 5 and 35% (limits included), preferably between 15 and 35% (limits included) and more preferably between 27 and 33% (limits included). The remainder consists of niobium and impurities including interstitials such as H, C, N and/or O, the percentage of impurities being less than or equal to 0.3% by weight.

[0007] The invention also relates to the process for manufacturing this clockwork spiral spring as claimed in the appendix.

Brief description of the drawings

Other characteristics and advantages of the invention will appear on reading the detailed description which follows, with reference to the appended drawings, where:

- Figure 1 shows, schematically, a spiral spring made with an Nb-Ti alloy according to the invention;

- Figure 2 represents the curves of evolution of the Young's modulus as a function of the 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

[0009] The invention relates to a clockwork spiral spring made of a binary-type alloy comprising niobium and titanium.

According to the invention, this alloy comprises by weight:

- niobium: balance at 100%;

- titanium in a percentage greater than or equal to 1% and less than 40%. More particularly, this alloy comprises a proportion by weight of titanium of between 5 and 35%, preferably between 15 and 35% and more preferably between 27 and 33%;

- traces of other elements chosen from O, H, C, Fe, Ta, N, Ni, Si, Cu and/or Al, each of the 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 weight percentages of titanium and niobium is between 99.7% and 100% of the total.

[0011] 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.

[0012] The percentage by weight of tantalum is less than or equal to 0.10% of the total.

[0013] 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 even less than or equal to 0.0175% of the total.

[0014] The percentage by weight of iron is less than or equal to 0.03% of the total, in particular Interior or equal to 0.025% of the total, or even even less than or equal to 0.020% of the total.

[0015] 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.

[0016] 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 even less than or equal to 0.0005% of the total.

[0017] The percentage by weight of nickel is less than or equal to 0.01% of the total.

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

[0019] 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.

[0020] 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.

[0021] The percentage by weight of aluminum is less than or equal to 0.01% of the total.

[0022] Advantageously, this spiral spring has a bi-phase microstructure comprising niobium in the central betacubic phase and titanium in the compact hexagonal alpha phase.

[0023] 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.

[0024] The higher the titanium content, the higher the maximum proportion of alpha phase which can be precipitated by heat treatment, which encourages the search for a high proportion of titanium. But on the contrary, the higher the titanium content, the more difficult it is to obtain only a precipitation of the alpha phase at the grain boundaries. The appearance of precipitates of the intragranular Widmastätten alpha-Ti type or the co-intragranular phase makes the deformation of the material difficult, if not impossible, which is then not suitable for the production of a spiral spring, and it is then advisable not to incorporate too much of titanium in the alloy. In addition, the application of this alloy to a spiral spring requires properties capable of guaranteeing the maintenance of chronometric performance despite the variation in the temperatures of use of a watch incorporating such a spiral spring. The thermoelastic coefficient, also called GTE of the alloy, is then of great importance. To form a chronometric oscillator with a CuBe or nickel silver balance wheel, a GTE of +/- 10 ppm/°C must be achieved. The formula that links the GTE of the alloy and the expansion coefficients of the hairspring and the balance wheel is as follows:

1 dE

H~dT

d.\[ dT

-ß + -a\' S6400—

2 EC

CT =

[0025] The variables M and T are the operation 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.

[0026] CT is the thermal coefficient of the oscillator, (1/E. dE/dT) is the CTE of the hairspring alloy, β is the coefficient of expansion of the balance and a that of the hairspring. The hardened beta phase alloy has a strongly positive CTE, and the precipitation of the alpha phase which has a strongly negative CTE makes it possible to bring the biphase alloy back 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 of less than 40% by weight makes it possible to obtain a good compromise between the various properties sought. 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 setting in motion of the dislocations as a function of the temperature causes a decrease in the Young's modulus of the spiral spring which counteracts the positive anomaly of the beta phase.

[0027] The spiral spring produced with this alloy has a yield strength 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.

The invention also relates to the process for manufacturing the watch spiral spring, characterized in that the following steps are successively implemented:

- development of a blank in an alloy comprising niobium and titanium and more precisely:

- niobium: balance at 100%;

- a percentage by weight of titanium greater than or equal to 1% of the total and less than 40% of the total;

- 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 lower 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 the beta phase;

- Application to said alloy of deformation sequences followed by heat treatment. By deformation is meant a deformation by drawing and/or rolling. Wire drawing may require the use of one or more dies during

of the same sequence or during different sequences if necessary. The trellising is carried out until a round cross-section yarn is obtained. Rolling can be carried out during the same deformation sequence as latticework 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 strapping arm. These sequences lead to obtaining a bi-phase microstructure comprising beta niobium and alpha titanium, with an elastic limit greater than or equal to 500 MPa and an elastic modulus less than or equal to 120 GPa and preferably 110 GPa .

- strapping to form a spiral spring, followed by a final heat treatment.

[0029] In these coupled deformation-heat treatment sequences, each deformation is carried out with a deformation rate given between 1 and 5, this deformation rate corresponding to the conventional formula 2ln(d0/d), where dO is the diameter of the the last beta quench, and where d is the diameter of the hardened wire. The global accumulation of the deformations on the whole of this succession of sequences leads to a total rate of deformation comprised between 1 and 14. Each sequence coupled with deformation-heat treatment includes, each time, a heat treatment of precipitation of the alpha phase Ti.

[0030] The beta quenching prior to the deformation and heat treatment sequences 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 gas cooling.

[0031] Even more particularly, this beta quenching is a solution heat treatment for 1 hour at 800° C. under vacuum, followed by cooling under gas.

To return 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 particularly, the duration is between 5 hours and 30 hours at a temperature between 400°C and 600°C.

More particularly, the method comprises between one and five coupled deformation-heat treatment sequences.

[0034] More particularly, the first coupled deformation-heat treatment sequence comprises a first deformation with at least 30% reduction in section.

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.

[0036] More particularly, after this production of said alloy blank, and before the deformation heat treatment sequences, in an additional step, a surface layer of ductile material taken from among copper, nickel, cupro is added to the blank. -nickel, cupro-magnanese, gold, silver, nickel-phosphorus Ni-P and nickel-boronNi-B, or the like, to facilitate forming into wire form during deformation. And, after the deformation-heat treatment sequences or after the strapping step, the wire is stripped of its layer of ductile material, in particular by chemical attack.

[0037] In a variant, the surface layer of ductile material is deposited so as to constitute 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 constitute a spring whose pitch is variable.

[0038] In a particular horological application, ductile material or copper is thus added at a given moment to facilitate shaping in the form of a wire, such that a thickness of 10 to 500 micrometers remains. on the wire to the final diameter of 0.3 to 1 millimeters. 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 actual spring by strapping.

[0039] The supply of ductile material or copper can be galvanic, or else mechanical, it is then a jacket or a tube of ductile material or copper which is adjusted on a bar of niobium-titanium alloy with a fat diameter, then which is thinned during the stages of deformation of the composite bar.

[0040] A diffusion barrier layer, for example nb, can be added between the nb-Ti and the Cu in order to prevent the formation of intermetallics harmful to the deformability of the material. The thickness of this layer is chosen so as to correspond to a thickness of 100 nm to 1 μm on the 0.1 mm diameter wire.

[0041] The removal of the layer can be carried out in particular by chemical attack, with a solution based on cyanides or based on acids, for example nitric acid.

[0042] By a suitable combination of deformation and heat treatment sequences, it is possible to obtain a very fine lamellar bi-phase microstructure, in particular nanometric, comprising or composed of beta niobium and alpha titanium. This alloy combines a very high elastic limit, greater than at least 500 MPa and a very low modulus of elasticity, of 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 presents a <110> texture. In addition, this niobium-titanium alloy according to the invention can easily be covered with ductile material or copper, which greatly facilitates its deformation by latticework.

Claims (18)

[0043] 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 nil in the range of temperatures of usual use of watches, and suitable for the manufacture of self-compensating hairsprings. [0044] More precisely, if we compare to FIG. 2, the evolution of the Young's modulus (E(T)/E2o-c) as a function of the temperature for pure Nb and an Nb-Ti alloy according to the invention with 30% by weight of Ti, it is observed that the two curves are S-shaped with the notable difference that the presence of Ti makes it possible to greatly reduce the difference between the minimum and the maximum of the curve according to both the abscissa tax and the tax ordinates. 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 rate of reduction since beta quenching, - the density of dislocations adjusted via heat treatments which induce phenomena of restoration, even of recrystallization, - the concentration of interstitials which will interact with the dislocations, - the percentage of Ti in the alpha phase - the density of precipitates in the alloy (number of Ti precipitates in the alpha phase per unit volume). Claims 1. Spiral spring (1) intended to equip a balance wheel with a clock movement, characterized in that the spiral spring (1) is made of an alloy of niobium and titanium consisting by weight of: - niobium: 100% balance; - titanium with a percentage greater than or equal to 1% and less than 40%, - 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 said traces being less than or equal to 0.3% by weight. 2. Spiral spring (1) according to claim 1, characterized in that said alloy comprises a percentage by weight of titanium of between 5 and 35%. 3. Spiral spring (1) according to claim 1, characterized in that said alloy comprises a percentage by weight of titanium of between 15 and 35%. 4. Spiral spring (1) according to claim 1, characterized in that said alloy comprises a percentage by weight of titanium of between 27 and 33%. 5. Spiral spring (1) according to one of the preceding claims, characterized in that it has a bi-phase microstructure comprising niobium in the beta phase and titanium in the alpha phase. 6. Spiral spring (1) according to one of the preceding claims, characterized 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 at 110 GPa. 7. Process for manufacturing a spiral spring (1) intended to equip a pendulum with a clockwork movement, characterized in that it successively comprises: - a step for producing a blank in an alloy of niobium and titanium, consisting by weight of: - niobium: 100% balance; - titanium with a percentage greater than or equal to 1% and less than 40%, - 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 said traces being less than or equal to 0.3% by weight; - a step of 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 the beta phase, - a step of applying to said alloy a succession of deformation sequences followed by an intermediate heat treatment, - a strapping step to form the spiral spring (1), - a final heat treatment step. 8. A method of manufacturing a spiral spring (1) according to claim 7, characterized in that the deformation during each sequence is carried out by drawing and/or rolling. 9. Process for manufacturing a spiral spring (1) according to claim 8, characterized in that the deformation of the last sequence is carried out by flat rolling. 10. Process for manufacturing a spiral spring (1) according to one of claims 7 to 9, characterized in that the deformation of each sequence is carried out with a deformation rate given 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. 11. Process for manufacturing a spiral spring (1) according to one of claims 7 to 10, characterized in that the beta-type quenching is a solution treatment, with a duration of between 5 minutes and 2 hours. at a temperature between 700°C and 1000°C, under vacuum, followed by cooling under gas. 12. Process for manufacturing a spiral spring (1) according to one of claims 7 to 11, characterized in that the beta-type quenching is a solution treatment for 1 hour at 800° C. under vacuum, followed by cooling under gas. 13. Process for manufacturing a spiral spring (1) according to one of claims 7 to 12, characterized in that the final heat treatment as well as the intermediate heat treatment of each sequence is a precipitation treatment of Ti in alpha phase lasting between 1 hour and 200 hours at a temperature between 300°C and 700°C. 14. Process for manufacturing a spiral spring (1) according to one of claims 7 to 13, characterized in that the final heat treatment as well as the intermediate heat treatment of each sequence is a precipitation treatment of Ti in alpha phase lasting between 5 hours and 30 hours at a temperature between 400°C and 600°C. 15. Process for manufacturing a spiral spring (1) according to one of claims 7 to 14, characterized in that said process comprises between one and five said deformation sequences followed by an intermediate heat treatment. 16. Process for manufacturing a spiral spring (1) according to one of claims 7 to 15, characterized in that the first said sequence of deformation followed by an intermediate heat treatment comprises a first deformation with at least 30% of section reduction. 17. A method of manufacturing a spiral spring (1) according to claim 16, characterized in that each said sequence of deformation followed by an intermediate heat treatment, other than the first, comprises a deformation between two intermediate heat treatments with at least minus 25% section reduction. 18. Process for manufacturing a spiral spring (1) according to one of claims 7 to 17, characterized in that, after the step of producing the alloy blank, and before the step of applying of a succession of sequences, a surface layer of ductile material taken from copper, nickel, cupro-nickel, cupromagnanese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B, to facilitate shaping in wire form and in that, before or after the strapping step, said wire is stripped of its layer of said ductile material by chemical attack.