Helical spring for a watch movement

Abstract

The present invention relates to a helical spring for equipping the balance wheel of a timepiece movement, characterized in that the helical spring is made of an alloy composed of the following elements: - Nb, Ti and at least one element selected from V and Ta, - optionally at least one element selected from Zr and Hf, - optionally at least one element selected from W and Mo, and possibly trace amounts of other elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al, having the following weight percentages: ◦ the total content of Nb, V and Ta is between 40% and 85%, ◦ the total content of Ti, Zr and Hf is between 15% and 55%, ◦ the contents of W and Mo are respectively between 0 and 2.5%, ◦ the content of each of the elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al is between 0 and 1600 ppm, and the sum of the trace amounts is less than or equal to 0.3% by weight. The present invention also relates to a method for manufacturing the same.

Description

Field of the Invention

[0001] The present invention relates to a helical spring for equipping a balance wheel of a watch movement. It also relates to a method for manufacturing such a helical spring. Background of the Invention

[0002] The manufacture of helical springs for watchmaking must face constraints that often seem incompatible at first glance:

[0003] - The need to obtain a high elastic limit,

[0004] - Easy to produce, especially drawing and rolling,

[0005] - Excellent fatigue resistance,

[0006] - Stable performance over time,

[0007] - Small cross-section.

[0008] The alloy selected for the helical spring must also have properties that ensure the maintenance of the timekeeping performance, even when the operating temperature of the watch including such a helical spring changes. The thermal elastic coefficient (also known as TEC) of the alloy is then of great significance. To form a timekeeping oscillator with a CuBe or nickel silver balance wheel, a TEC of + / - 10 ppm / °C must be achieved.

[0009] The equation relating the TEC of the alloy, the expansion coefficient (α) of the helix, and the expansion coefficient (β) of the balance wheel to the thermal coefficient (TC) of the oscillator is as follows:

[0010]

[0011] The variables M and T are the rate in s / d and the temperature in °C respectively, E is the Young's modulus of the helical spring, where (1 / E.dE / dT) is the TEC of the helical alloy, and the expansion coefficient is expressed in °C -1 as the unit.

[0012] In fact, TC is calculated as follows:

[0013]

[0014] where the value of which must be between -0.6 and +0.6 s / d°C.

[0015] Spiral springs for watchmaking are known from the prior art, which are made of a binary Nb-Ti alloy, where the weight percentage of Ti is typically 40 - 60%, and more particularly has a percentage of 47%. With an adjusted deformation and heat treatment diagram, such spiral springs have a two-phase microstructure, including niobium in the β phase and titanium in the form of precipitates in the α phase. Cold-worked Nb in the β phase has a strongly positive TEC, while Ti in the α phase has a strongly negative TEC, which brings the TEC of the two-phase alloy close to zero, which is particularly advantageous for TC.

[0016] However, there are some drawbacks when using binary Nb-Ti alloys for spiral springs. As mentioned above, binary Nb-Ti alloys are particularly advantageous for low TC. On the other hand, its composition is not optimized for the secondary error, which is a measure of the rate curvature, and the rate curvature is approximated above by a straight line passing through two points (8 °C and 38 °C). The rate may 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. Generally, for the NbTi47 alloy, the secondary error is 4.5 s / d, while it preferably should be between -3 and +3 s / d.

[0017] Another drawback of binary Nb-Ti alloys is related to the precipitation of titanium, which mainly occurs during the fixing step after the winding step. The fixing step is crucial because it allows the shape of the spiral to be fixed and a TC close to zero to be obtained after Ti precipitation. In practice, the precipitation time is very long, for the NbTi47 alloy, the time is 8 to 30 hours, with an average of about 20 hours, which significantly increases the production time. In addition to the problem of long production time, too high a percentage of titanium can lead to the formation of a brittle martensite phase, which makes it difficult (if not impossible) to deform the material to produce spiral springs. Therefore, it is recommended not to add too much titanium to the alloy.

[0018] So far, there is still a need to meet various criteria of non-existence of brittle phases, reduction of production time, and reduction of secondary error, while maintaining a new chemical composition with low TC (for the production of spiral springs). Summary of the Invention

[0019] The object of the present invention is to propose a new chemical composition for spiral springs that allows the above-mentioned drawbacks to be overcome.

[0020] To this end, the present invention relates to a hairspring made of at least a ternary alloy having a niobium and titanium matrix. According to the present invention, Nb is partially replaced by Ta and / or V to reduce secondary errors. Advantageously, during fixation, Ti is partially replaced by Zr and / or Hf to accelerate precipitation, or in other words, to increase the driving force for precipitation during fixation while maintaining the same TC. In addition, the presence of at least three elements in significant proportions in the alloy allows for improvement of the mechanical properties of the alloy, in particular the elastic limit.

[0021] More particularly, the present invention relates to a hairspring for equipping the balance wheel of a watch movement, said hairspring being made of at least a ternary alloy composed of the following elements:

[0022] - Nb, Ti and at least one element selected from V and Ta,

[0023] - Optionally at least one element selected from Zr and Hf,

[0024] - Optionally at least one element selected from W and Mo,

[0025] - Possibly trace amounts of other elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al;

[0026] The weight percentages of the various elements are as follows:

[0027] - The total content of Nb, V and Ta is from 40% to 85%,

[0028] - The total content of Ti, Zr and Hf is from 15% to 55%,

[0029] - The contents of W and Mo are each from 0 to 2.5%,

[0030] - The content of each of the elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al is from 0 to 1600 ppm, where the sum of the trace amounts is less than or equal to 0.3% by weight.

[0031] Preferably, the weight content of Nb is greater than 45% in order to obtain a sufficient percentage of the β phase having a strongly positive TEC, which is intended to be compensated by the negative TEC of the α phase of Ti, Zr, Hf.

[0032] Preferably, the weight content of Ti is at least 15%.

[0033] The present invention also relates to a method for manufacturing such a watch hairspring.

[0034] Other features and advantages of the present invention will become apparent upon reading the following detailed description. Detailed description

[0035] The present invention relates to a watch hairspring made of at least a ternary alloy comprising niobium, titanium and one or more additional elements.

[0036] According to the invention, such an alloy consists of the following elements:

[0037] - Nb, Ti and at least one element selected from V and Ta,

[0038] - Optionally at least one element selected from Zr and Hf,

[0039] - Optionally at least one element selected from W and Mo,

[0040] - Possibly trace amounts of other elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al,

[0041] - The following weight percentages total 100%:

[0042] - The total content of Nb, V and Ta is 40 - 85%, preferably the Nb content is greater than 45%, or even greater than or equal to 50%,

[0043] - The total content of Ti, Zr and Hf is 15% - 55%, preferably the minimum Ti content is 15%,

[0044] - The contents of W and Mo are 0 - 2.5% respectively,

[0045] - The content of each of the elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al is 0 to 1600 ppm, and the sum of the trace amounts is less than or equal to 0.3% by weight.

[0046] According to the invention, Nb is partially replaced by Ta and / or V. The partial replacement of Nb by Ta and / or V is intended to reduce the secondary error. Binary Nb - V and Nb - Ta alloys were tested to show the influence of V and Ta on the secondary error respectively. The secondary error is measured at 23°C. This is the difference between the rate at 23°C and the straight line connecting the rates at 8°C and 38°C. For example, the rates at 8°C, 23°C and 38°C can be measured using a device of the Witschi chronometer type.

[0047] Table 1 below shows reference data for pure Nb, pure V, pure Ta, and the NbTi47 alloy, as well as values obtained as a function of the weight percentage of V and Ta, respectively, in the binary alloys Nb-V and Nb-Ta. Pure Nb has a quadratic error at 23 °C of -6.6 s / d. In the NbTi47 alloy, the precipitation of Ti in the α-phase compensates for the negative effect of Nb. However, with the addition of Ti, there is an excessive increase, with the value reaching 4.5 s / d, i.e., a Δ (delta) of 11.1 s / d. Pure vanadium and pure tantalum have significantly more negative quadratic errors than pure Nb, with values of -24.9 and -28.7 s / d, respectively. Partial substitution of Nb with V and / or Ta allows the quadratic error to be reduced to a negative value less than -7 s / d. Thus, in the range of 5% - 25%, increasing the substitution of Nb with V or Ta reduces the quadratic error from approximately -7 s / d to -12 s / d. Therefore, Nb can be substituted with V or Ta or with a combination of V and Ta to achieve this value range. For a separate Ta content, separate V content, or total content of Ta and V of 5 - 25 wt%, the quadratic error is between -7 s / d and -12 s / d. Preferably, for a separate Ta content, separate V content, or total content of Ta and V of 10 - 25 wt%, more preferably 15 - 25 wt%, the quadratic error is between -9 and -12 s / d. Adding one or more elements that form a precipitated α-phase in the Nb-V / Ta alloy during the fixing step allows compensation for this negative value and achieves a value close to 0 s / d. Preferably, for all these elements, the content of the element that forms the precipitated α-phase is 15 - 55 wt%. The element that forms the α-phase is at least Ti, preferably with a minimum content of 15%. In addition to Ti, it can also be Hf and / or Zr, which form a single precipitated α-phase with Ti during the fixing step. When the alloy contains Zr and / or Hf, the total content of Zr and Hf is 1 - 40 wt%, preferably 5 - 25 wt%, more preferably 10 - 25 wt%, even more preferably 15 - 25 wt%.

[0048] Alloy: Weight % V / Ta Secondary error at 23 °C Pure niobium -6.6 s / d Pure vanadium -24.9 s / d Pure tantalum -28.7 s / d NbTi47 4.5 s / d <![CDATA[Nb 100 > 0% -6.6 s / d <![CDATA[Nb 95 V 5 > 5% -7.2 s / d <![CDATA[Nb 90 V 10 > 10% -8.6 s / d <![CDATA[Nb 85 V 15 > 15% -9.0 s / d <![CDATA[Nb 80 V 20 > 20% -11.0 s / d <![CDATA[Nb 75 V 25 > 25% -11.5 s / d <![CDATA[Nb 100 > 0% -6.6 s / d <![CDATA[Nb 95 Ta 5 > 5% -8.0 s / d <![CDATA[Nb 90 Ta 10 > 10% -8.7 s / d <![CDATA[Nb 85 Ta 15 > 15% -10.5 s / d <![CDATA[Nb 80 Ta 20 > 20% -11.6 s / d <![CDATA[Nb 75 Ta 25 > 25% -12.0 s / d

[0049] Table 1 。

[0050] The alloy may also include W and Mo, each with a respective weight content of 0 to 2.5%, to increase the Young's modulus of the alloy, which allows (for a given torque of the spring) a reduction in the thickness of the helix, thereby reducing the weight of the helix.

[0051] In a particularly advantageous manner, the alloy used in the present invention does not contain other elements except for possible and inevitable traces.

[0052] More particularly, the oxygen content is less than or equal to 0.10 wt% of the total amount, and even less than or equal to 0.085 wt% of the total amount.

[0053] More particularly, the carbon content is less than or equal to 0.04% by weight of the total amount, particularly less than or equal to 0.020% by weight of the total amount, or even less than or equal to 0.0175% by weight of the total amount.

[0054] More particularly, the iron content is less than or equal to 0.03% by weight of the total amount, particularly less than or equal to 0.025% by weight of the total amount, or even less than or equal to 0.020% by weight of the total amount.

[0055] More particularly, the nitrogen content is less than or equal to 0.02% by weight of the total amount, particularly less than or equal to 0.015% by weight of the total amount, or even less than or equal to 0.0075% by weight of the total amount.

[0056] More particularly, the hydrogen content is less than or equal to 0.01% by weight of the total amount, particularly less than or equal to 0.0035% by weight of the total amount, or even less than or equal to 0.0005% by weight of the total amount.

[0057] More particularly, the silicon content is less than or equal to 0.01% by weight of the total amount.

[0058] More particularly, the nickel content is less than or equal to 0.01% by weight of the total amount, particularly less than or equal to 0.16% by weight of the total amount.

[0059] More particularly, the copper content is less than or equal to 0.01% by weight of the total amount, particularly less than or equal to 0.005% by weight of the total amount.

[0060] More particularly, the aluminum content is less than or equal to 0.01% by weight of the total amount.

[0061] Advantageously, when the alloy comprises hafnium and / or zirconium, the helical spring has a multiphase microstructure comprising a single β-phase of niobium, vanadium and / or tantalum and a single α-phase of titanium and hafnium and / or zirconium. In the presence of tantalum and vanadium, the microstructure may also include intermetallic compounds of the TaV 2 type. In order to obtain such a microstructure, it is necessary to precipitate the α-phase (Ti, Hf, Zr) by heat treatment as described below.

[0062] The helical spring produced with this alloy has an elastic limit greater than or equal to 500 MPa, more precisely 500 - 1000 MPa. Advantageously, it has an elastic modulus greater than or equal to 100 GPa and preferably greater than or equal to 110 GPa.

[0063] The present invention also relates to a method for manufacturing a watch helical spring, characterized in that the following steps are carried out successively:

[0064] a) Producing or providing a blank made of an alloy consisting of the following elements:

[0065] - Nb, Ti, and at least one element selected from V and Ta,

[0066] - Optionally, at least one element selected from Zr and Hf,

[0067] - Optionally, at least one element selected from W and Mo,

[0068] - Possible trace amounts of other elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al,

[0069] The following weight percentages total 100%:

[0070] ◦ The total content of Nb, V, and Ta is 40% to 85%, preferably the Nb content is greater than 45%,

[0071] ◦ The total content of Ti, Zr, and Hf is 15% to 55%, preferably the minimum Ti content is 15% (including the limit value),

[0072] ◦ The contents of W and Mo are each 0 - 2.5%,

[0073] ◦ The content of each of the elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al is 0 to 1600 ppm, and the total of the trace amounts is less than or equal to 0.3 wt%,

[0074] b) Subject the billet to β - type quenching such that the titanium and zirconium and hafnium (when zirconium and hafnium are present) of the alloy are substantially in the form of a solid solution with niobium and tantalum and / or vanadium (in the β - phase);

[0075] c) Apply a deformation process (followed by a heat treatment) to the alloy. Deformation refers to the deformation caused by drawing and / or rolling. If necessary, drawing may require the use of one or more dies during the same process or during different processes. Drawing is carried out until a wire with a circular cross - section is obtained. Rolling can be carried out according to the same deformation process as drawing or according to other processes. Advantageously, the last process applied to the alloy is rolling, preferably with a rectangular profile adapted to the inlet cross - section of the winding pin.

[0076] d) Wind to form a helical spring, followed by a final fixing heat treatment.

[0077] In these coupled deformation - heat treatment operations, each deformation is carried out with a given deformation amount between 1 - 5, which corresponds to the conventional formula 2ln(d0 / d), where d0 is the diameter after the last β - quenching and d is the diameter of the cold - worked wire. The overall cumulative deformation of this series of operations brings a total deformation amount between 1 - 14. Each coupled deformation - heat treatment operation includes a heat treatment for the precipitation of the α - phase (Ti, Zr, and / or Hf) each time.

[0078] The β - quenching before the deformation and heat treatment operations is a solution treatment, which is carried out under vacuum at a temperature of 700 °C to 1000 °C for a duration of 5 minutes to 2 hours and then cooled under gas. Even more particularly, this β - quenching is a solution treatment, carried out under vacuum at 800 °C for 1 hour and then cooled under gas.

[0079] Returning to the coupled deformation - heat treatment operation, the heat treatment is a precipitation treatment at a temperature of 300 °C to 700 °C for a duration of 1 hour to 200 hours. More particularly, the duration is 5 hours to 30 hours, with the temperature maintained at 400 °C - 600 °C.

[0080] More particularly, the method includes one to five coupled deformation - heat treatment operations.

[0081] More particularly, the first coupled deformation - heat treatment operation includes a first deformation with a cross - section reduction of at least 30%.

[0082] More particularly, except for the first one, each coupled deformation - heat treatment operation includes a deformation between two heat treatments, with a cross - section reduction of at least 25%.

[0083] More particularly, after the production of the alloy billet and before the deformation - heat treatment operations, in an additional step, a surface layer of a malleable material taken from copper, nickel, copper - nickel, copper - manganese, gold, silver, nickel - phosphorus (Ni - P), nickel - boron (Ni - B), etc. is added to the billet to facilitate shaping into a wire during deformation. And, after the deformation - heat treatment operations or after the winding step, the wire is stripped of its malleable material layer, especially by chemical etching.

[0084] Alternatively, a surface layer of a malleable material is deposited to form a helical spring whose pitch is not a multiple of the thickness of a blade. In another variant, a surface layer of a malleable material is deposited to form a spring with a variable pitch.

[0085] In a particular watch application, malleable material or copper is added at a given moment so as to facilitate shaping into wire form in such a way that a thickness of 10 to 500 microns remains on a wire having a final diameter of 0.3 to 1 mm. In particular, the wire is stripped of its malleable material or copper layer by chemical etching and then flattened before the actual spring is manufactured by winding.

[0086] The provision of malleable material or copper can be by electroplating or mechanical, in which case it is a sheath or tube of malleable material or copper adjusted on an alloy bar having a large diameter and then thinned during the deformation step of the composite bar.

[0087] The removal of the layer is particularly by chemical etching and it is possible to use a solution based on cyanide or an acid such as nitric acid.

[0088] The final heat treatment is carried out at a temperature of 300 °C to 700 °C for a duration of 1 hour to 200 hours. More particularly, the duration is 5 hours to 30 hours at a holding temperature of 400 °C - 600 °C. During this final heat treatment, the precipitation of the α phase is finally determined. In the presence of hafnium and / or zirconium, the final treatment time can be reduced by a few hours, typically at a holding temperature of 400 °C - 600 °C, the precipitation time being 4 - 8 hours.

[0089] By a suitable combination of deformation and heat treatment operations, a very fine microstructure can be obtained, which is particularly nanoscale and consists of a β phase of niobium, tantalum and / or vanadium and an α phase of titanium and zirconium and / or hafnium (if the alloy contains one or both of the last two elements). Such an alloy combines a very high elastic limit of greater than at least 500 MPa and an elastic modulus of greater than or equal to 100 GPa. This combination of properties is very suitable for spiral springs. In addition, such an alloy according to the invention can be easily covered with malleable material or copper, which greatly facilitates its deformation by drawing.

[0090] At least ternary type alloys of the type selected above for implementing the invention and including niobium, titanium, tantalum and / or vanadium also have an effect similar to "Elinvar", having an almost zero thermoelastic coefficient in the temperature range commonly used in watches and being suitable for manufacturing self - compensating spirals.

Claims

1. A helical spring for a balance wheel of a timepiece movement, characterized in that the helical spring is made of an alloy composed of the following elements: - Nb, Ti and at least one element selected from V and Ta, - optionally at least one element selected from Zr and Hf, - optionally at least one element selected from W and Mo, - optionally trace amounts of other elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al, having the following weight percentages: ο The total content of Nb, V and Ta is 40% to 85%, ο The total content of Ti, Zr and Hf is 15% to 55%, ο The contents of W and Mo are 0 to 2.5% respectively, ο The content of each of the elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al is 0 to 1600 ppm, and the total of the trace amounts is less than or equal to 0.3% by weight, wherein the Nb content is greater than 45% by weight, wherein the Ti content is greater than or equal to 15% by weight, and wherein the sum of the contents of V and Ta is 5 to 25% by weight.

2. The helical spring according to claim 1, characterized in that the sum of the contents of V and Ta is 10 to 25% by weight.

3. The helical spring according to claim 1 or 2, characterized in that the sum of the contents of V and Ta is 15 - 25% by weight.

4. The helical spring according to claim 1 or 2, characterized in that it includes Zr and / or Hf, and the total content of Zr and Hf is 1 - 40% by weight.

5. The helical spring according to claim 1 or 2, characterized in that it includes Zr and / or Hf, and the total content of Zr and Hf is 5 - 25% by weight.

6. The helical spring according to claim 1 or 2, characterized in that it includes Zr and / or Hf, and the total content of Zr and Hf is 10 - 25% by weight.

7. The helical spring according to claim 1 or 2, characterized in that it includes Zr and / or Hf, and the total content of Zr and Hf is 15 - 25% by weight.

8. The helical spring according to claim 1 or 2, having a microstructure including a β-phase of Nb, V and / or Ta, and an α-phase of Ti and Zr and / or Hf when the alloy includes Zr and / or Hf.

9. The helical spring according to claim 1 or 2, characterized in that it has an elastic limit greater than or equal to 500 MPa and an elastic modulus greater than or equal to 100 GPa.

10. The helical spring according to claim 9, characterized in that it has an elastic modulus greater than or equal to 110 GPa.

11. A method for manufacturing a helical spring according to any one of claims 1 to 10 for assembling a balance wheel of a timepiece movement, characterized in that it sequentially includes: - a step of producing a blank from at least a ternary alloy composed of the following elements: ο Nb, Ti and at least one element selected from V and Ta, ο optionally at least one element selected from Zr and Hf, ο optionally at least one element selected from W and Mo, ο Optional trace amounts of other elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al, with the weight percentages being as follows: · The total content of Nb, V and Ta is 40% to 85%, · The total content of Ti, Zr and Hf is 15% to 55%, · The contents of W and Mo are 0 to 2.5% respectively, · The content of each of the elements selected from O, H, C, Fe, N, Ni, Si, Cu, Al is 0 to 1600 ppm, and the sum of the trace amounts is less than or equal to 0.3% by weight, - Subject the blank to a β-type quenching step such that the titanium in the alloy is substantially in the form of a solid solution with niobium and vanadium and / or tantalum in the β phase, and when the alloy contains zirconium and / or hafnium, the zirconium and / or hafnium in the alloy is also substantially in the form of a solid solution, - Subject the alloy to a series of deformation and then intermediate heat treatment steps, - A winding step to form a helical spring, - A final heat treatment step.

12. The method for manufacturing a helical spring according to claim 11, characterized in that the β-type quenching is a solution treatment at a temperature of 700 °C to 1000 °C under vacuum for a duration of 5 minutes to 2 hours, and then cooled under gas.

13. The method for manufacturing a helical spring according to any one of claims 11-12, characterized in that the final heat treatment and the intermediate heat treatment for each process are precipitation treatments of Ti and optionally Zr and / or Hf in the α phase. When the alloy includes Zr and / or Hf, the holding temperature is 300 °C to 700 °C and the duration is 1 hour to 200 hours.

14. The method for manufacturing a helical spring according to any one of claims 11-12, characterized in that when the alloy contains Zr and / or Hf, the final heat treatment is carried out at a holding temperature of 400 - 600 °C for a duration of 4 to 8 hours.

15. The method for manufacturing a helical spring according to any one of claims 11-12, characterized in that after the step of producing the alloy blank and before the step of applying a series of deformation processes, a surface layer of a malleable material taken from copper, nickel, copper-nickel, copper-manganese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B is added to the blank to facilitate shaping into a wire, and before or after the winding step, the wire is stripped of its malleable material layer by chemical etching.