CH718455A2 - Spiral spring for a clock movement and process for manufacturing this spiral spring. - Google Patents

Abstract

The present invention relates to a spiral spring intended to equip a balance wheel of a watch movement, characterized in that the spiral spring is made of an alloy consisting of: - Nb, Ti and at least one element chosen from V and Ta, – optionally at least one element chosen from Zr and Hf, – optionally at least one element chosen from W and Mo, possible traces of other elements chosen from O, H, C, Fe, N , Ni, Si, Cu, Al, with the following percentages by weight: o a total content of Nb, V and Ta between 40 and 85%, o a total content of Ti, Zr and Hf between 15 and 55%, o a content for W and Mo respectively between 0 and 2.5%, o a content for each of said elements chosen from O, H, C, Fe, N, Ni, Si, Cu, Al between 0 and 1600 ppm with the sum of said traces less than or equal to 0.3% by weight. The present invention also relates to its method of manufacture.

Description

Field of the invention

The invention relates to a spiral spring intended to equip a balance wheel of a clock movement. It also relates to the manufacturing process of 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, stable performance over time, weak sections.

[0003] The alloy chosen for a spiral spring must also have properties 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 CTE, of the alloy is then of great importance. To form a chronometric oscillator with a CuBe or nickel silver balance wheel, a CTE of +/- 10 ppm/°C must be achieved.

The formula which links the CTE of the alloy and the expansion coefficients of the hairspring (α) and of the balance (β) to the thermal coefficient (CT) of the oscillator is as follows:

the variables M and T being respectively the rate in s/d and the temperature in °C, E being the Young's modulus of the spiral spring with (1/E. dE/dT) which is the CTE of the spiral alloy, the expansion coefficients being expressed in °C<-1>.

[0005] In practice, the CT is calculated as follows: with a value which must be between -0.6 and +0.6 s/d°C.

[0006] From the prior art, there are known spiral springs for watchmaking made of binary Nb-Ti alloys with percentages by weight of Ti typically between 40 and 60% and more specifically with a percentage of 47% . With a suitable deformation and heat treatment scheme, this spiral spring has a two-phase microstructure comprising niobium in the beta phase and titanium in the form of precipitates in the alpha phase. Hardened beta-phase Nb exhibits a strongly positive CTE while alpha-phase Ti possesses a strongly negative CTE allowing the two-phase alloy to be reduced to a CTE close to zero, which is particularly favorable for the CT.

[0007] There are nevertheless certain disadvantages to the use of Nb-Ti binary alloys for the spiral springs. The Nb-Ti binary alloy is particularly favorable for low CT as mentioned above. On the other hand, its composition is not optimized for the secondary error which is a measure of the curvature of the step which is approximated above by a straight line passing through two points (8°C and 38°C). The rate can deviate from this linear behavior between 8°C and 38°C and the secondary error at 23°C is a measure of this deviation at the temperature of 23°C. Typically, for an NbTi47 alloy, the secondary error is 4.5 s/d whereas preferably it should be between -3 and +3 s/d.

[0008] Another disadvantage of binary Nb-Ti alloys is related to the precipitation of titanium which takes place mainly after the strapping step during the fixing step. This step is crucial because it allows to fix the shape of the hairspring and to obtain the CT close to zero following the precipitation of the Ti. In practice, the precipitation times are very long with, for an NbTi47 alloy, times between 8 and 30 hours and on average around 20 hours, which significantly increases the production times. Apart from the problem of high production times, too high a percentage of titanium can lead to the formation of fragile martensitic phases which make it difficult, if not impossible, to deform the material for the production of a spiral spring. It is therefore advisable not to incorporate too much titanium into the alloy.

[0009] To date, there is still a need for new chemical compositions fulfilling the various criteria of absence of fragile phases, reduction of production times and reduction of secondary error while maintaining a low CT for production of spiral springs.

Summary of the invention

[0010] The object of the invention is to propose a new chemical composition for a spiral spring making it possible to overcome the aforementioned drawbacks.

To this end, the invention relates to a watch spiral spring made of an at least ternary alloy with a base of niobium and titanium. According to the invention, the Nb is partly replaced by Ta and/or V so as to reduce the secondary error. Advantageously, the Ti is partly replaced by Zr and/or Hf so as to accelerate the precipitation, or in other words to increase the driving force of precipitation, during fixing while maintaining the same CT. Furthermore, the presence of at least three elements in a significant proportion in the alloy makes it possible to improve the mechanical properties and in particular the elastic limit of the alloy.

More specifically, the present invention relates to a spiral spring intended to equip a balance wheel of a clockwork movement, said spiral spring being made of an at least ternary alloy consisting of: of Nb, Ti and at least one element chosen from V and Ta, optionally at least one element chosen from Zr and Hf, optionally at least one element chosen from W and Mo, possible traces of other elements chosen from O, H, C, Fe, N, Ni, Si, Cu, Al, the percentages by weight of the various elements being as follows: a total content of Nb, V and Ta of between 40 and 85%, a total Ti, Zr and Hf content of between 15 and 55%, a content for W and Mo respectively between 0 and 2.5%, a content for each of said elements chosen from O, H, C, Fe, N, Ni, Si, Cu, Al of between 0 and 1600 ppm with the sum of said traces less than or equal to 0.3% by weight.

[0013] Preferably, the content by weight of Nb is greater than 45% in order to obtain a sufficient percentage of beta phase having a strongly positive CTE intended to be compensated by the negative CTE of the alpha phase of Ti, Zr, Hf .

[0014] Preferably, the content by weight of Ti is at least 15%.

[0015] The invention also relates to the process for manufacturing this clockwork spiral spring.

[0016] Other characteristics and advantages of the invention will appear on reading the detailed description below.

detailed description

[0017] The invention relates to a watch spiral spring made of an at least ternary alloy comprising niobium and titanium and one or more additional elements.

According to the invention, this alloy consists of: of Nb, Ti and at least one element chosen from V and Ta, optionally at least one element chosen from Zr and Hf, optionally at least one element chosen from W and Mo, possible traces of other elements chosen from O, H, C, Fe, N, Ni, Si, Cu, Al, with the following weight percentages for a total of 100%: a total Nb, V and Ta content of between 40 and 85% with preferably an Nb content greater than 45%, or even greater than or equal to 50%, a total Ti, Zr and Hf content of between 15 and 55% with preferably a minimum Ti content of 15%, a content for W and Mo respectively between 0 and 2.5%, a content for each of said elements chosen from O, H, C, Fe, N, Ni, Si, Cu, Al of between 0 and 1600 ppm with the sum of said traces less than or equal to 0.3% by weight.

According to the invention, the Nb is partly replaced by Ta and/or V. The partial replacement of Nb by Ta and/or V is intended to reduce the secondary error. Tests have been carried out on binary Nb-V and Nb-Ta alloys to show the effect of V and Ta respectively on the secondary error. The secondary error is measured at 23°C. This is the difference in rate at 23°C with respect to the straight line linking the rate at 8°C to that at 38°C. For example, the rate at 8° C., 23° C. and 38° C. can be measured using a Witschi chronoscope-type device.

Table 1 below shows reference data for pure Nb, pure V, pure Ta and the NbTi47 alloy and the values obtained as a function of the percentage by weight of V and Ta in a binary alloy. respectively Nb-V and Nb-Ta. Pure Nb has a secondary error at 23°C of -6.6 s/d. The precipitation of Ti in the alpha phase in the NbTi47 alloy compensates for the negative effect of Nb with, however, an excessive rise with a value reaching 4.5 s/d, i.e. a delta of 11.1 s/d, following the addition of Ti. Pure vanadium and pure tantalum have a significantly more negative secondary error than pure Nb with values of -24.9 and -28.7 s/d respectively. The partial replacement of Nb by V and/or Ta makes it possible to lower the secondary error to negative values lower than −7 s/d. Thus, the increasing replacement of Nb by V or Ta ranging from 5% to 25% lowers the secondary error from about -7 s/d to -12 s/d. Nb can thus be replaced by V or Ta or by a combination of V and Ta to reach this range of values. The secondary error is between −7 s/d and −12 s/d for a content of Ta alone, of V alone or a total content of Ta and V comprised between 5% and 25% by weight. Preferably, the secondary error is between -9 and -12 s/d for a content of Ta alone, of V alone or a total content of Ta and V between 10 and 25% and more preferably between 15 and 25% in weight. The addition in the Nb-V/Ta alloy of one or more elements forming an alpha phase of precipitates during the fixing step makes it possible to compensate for this negative value and to reach a value close to 0 s/d. Preferably, the content of the element(s) forming an alpha phase of precipitates is comprised for all of these elements between 15 and 55% by weight. The element forming an alpha phase is at least Ti with preferably a minimum content of 15%. It may, in addition to Ti, be Hf and/or Zr which forms with Ti a single alpha phase of precipitates during the fixing step. When the alloy comprises Zr and/or Hf, the total content of Zr and Hf is between 1 and 40%, preferably between 5 and 25%, more preferably between 10 and 25%, even more preferably between 15 and 25% by weight. Pure niobium -6.6 s/d Pure vanadium -24.9 s/d Pure tantalum -28.7 s/d NbTi47 4.5 s/d Nb100 0% -6.6 s/d Nb95V5 5% -7, 2 s/d Nb90V10 10% -8.6 s/d Nb85V15 15% -9.0 s/d Nb80V20 20% -11.0 s/d Nb75V25 25% -11.5 s/d Nb100 0% -6, 6 s/d Nb95Ta5 5% -8.0 s/d Nb90Ta10 10% -8.7 s/d Nb85Ta15 15% -10.5 s/d Nb80Ta20 20% -11.6 s/d Nb75Ta25 25% -12, 0 s/d

Table 1

[0021] The alloy may also comprise W and Mo in a content by weight for each of between 0 and 2.5% in order to increase the Young's modulus of the alloy, which allows for a given torque of the spring to reduce the thickness of the hairspring and thereby lighten the hairspring.

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

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.

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 even less than or equal to 0.0175% by weight of the total.

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 even less than or equal to 0.020% by weight of the total.

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 even less than or equal to 0.0075% by weight of the total.

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 even less than or equal to 0.0005% by weight of the total.

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

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.

More particularly, the copper content is less than or equal to 0.01% by weight of the total, in particular less than or equal to 0.005% by weight of the total.

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

Advantageously, this spiral spring has a multi-phase microstructure comprising a single beta phase of niobium, vanadium and/or tantalum and a single alpha phase of titanium, and hafnium and/or zirconium when the The alloy comprises hafnium and/or zirconium. In the presence of tantalum and vanadium, the microstructure could also comprise an intermetallic of the TaV2 type. To obtain such a microstructure, it is necessary to precipitate the alpha phase (Ti, Hf, Zr) by heat treatment as described below.

The spiral spring produced 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 greater than or equal to 100 GPa and preferably greater than or equal to 110 GPa.

[0034] The invention also relates to the process for manufacturing the watch spiral spring, characterized in that the following steps are implemented successively: a) production or provision of a blank in an alloy consisting of: - Nb, Ti and at least one element chosen from V and Ta, – optionally at least one element chosen from Zr and Hf, – optionally at least one element chosen from W and Mo, – traces any other elements chosen from O, H, C, Fe, N, Ni, Si, Cu, Al, with the following percentages by weight for a total of 100%: o a total content of Nb, V and Ta of between 40 and 85% with preferably an Nb content greater than 45%, o a total Ti, Zr and Hf content of between 15 and 55% with preferably a minimum Ti content of 15% (limit included), o a content for W and Mo respectively between 0 and 2.5%, o a content for each of said elements chosen from O, H, C, Fe , N, Ni, Si, Cu, Al between 0 and 1600 ppm with the sum of said traces less than or equal to 0.3% by weight, b) beta-type quenching of said blank, so that the titanium, and the zirconium and hafnium when they are present, said alloy is essentially in the form of a solid solution with the niobium as well as the tantalum and/or the vanadium in the beta phase; c) application to said alloy of deformation sequences followed by heat treatment. By deformation is meant a deformation by drawing and/or rolling. The drawing may require the use of one or more dies during the same sequence or during different sequences if necessary. The drawing is carried out until a wire with a round section is obtained. Rolling can be done 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 strapping pin. d) strapping to form a spiral spring, followed by a final fixing heat treatment.

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 temper, and where d is the diameter of the work-hardened wire. The global accumulation of the deformations on the whole of this succession of sequences brings a total rate of deformation comprised between 1 and 14. Each sequence coupled with deformation-thermal treatment comprises, each time, a thermal treatment of precipitation of the alpha phase ( Ti, Zr and/or Hf).

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 between 700° C. and 1000° C., under vacuum, followed gas cooling. More particularly still, this beta quenching is a solution 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 with a holding temperature between 400°C and 600°C.

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

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

[0040] 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.

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, to facilitate shaping into wire form when 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.

Alternatively, 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.

[0043] 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, so that a thickness of 10 to 500 micrometers remains. on the wire to the final diameter of 0.3 to 1 mm. 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.

[0044] 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 an alloy bar with a large diameter, then which is thinned during the stages of deformation of the composite bar.

The removal of the layer is possible in particular by chemical attack, with a solution based on cyanides or based on acids, for example nitric acid.

The final heat treatment is carried out for a period of between 1 hour and 200 hours at a temperature of between 300° C. and 700° C. More particularly, the duration is between 5 hours and 30 hours at a holding temperature between 400°C and 600°C. During this final heat treatment, the precipitation of the alpha phase is finalized. In the presence of hafnium and/or zirconium, the final treatment time can be reduced by a few hours with typically a precipitation time between 4 and 8 hours at a holding temperature between 400°C and 600°C.

By a suitable combination of sequences of deformation and heat treatment, it is possible to obtain a very fine microstructure, in particular nanometric, composed of the beta phase of niobium, tantalum and/or vanadium and of the alpha phase of titanium, and zirconium and/or hafnium if the alloy contains one or two of these last two elements. This alloy combines a very high elastic limit, greater than at least 500 MPa and a modulus of elasticity greater than or equal to 100 GPa. This combination of properties is well suited for a spiral spring. In addition, this alloy according to the invention can easily be covered with ductile material or copper, which greatly facilitates its deformation by drawing.

An alloy of at least ternary type comprising niobium, titanium, tantalum and/or vanadium of the type selected above for implementing the invention also has 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 hairsprings.

Claims (17)

1. Spiral spring intended to equip a balance wheel of a clock movement, characterized in that the spiral spring is made of an alloy consisting of: – Nb, Ti and at least one element chosen from V and Ta, – optionally at least one element chosen from Zr and Hf, – optionally at least one element chosen from W and Mo, – any traces of other elements chosen from O, H, C, Fe, N, Ni, Si, Cu, Al, with the following weight percentages: o a total content of Nb, V and Ta of between 40 and 85%, o a total Ti, Zr and Hf content of between 15 and 55%, o a content for W and Mo respectively between 0 and 2.5%, o a content for each of said elements chosen from O, H, C, Fe, N, Ni, Si, Cu, Al of between 0 and 1600 ppm with the sum of said traces less than or equal to 0.3% by weight. 2. Spiral spring according to the preceding claim, characterized in that the Nb content is greater than 45% by weight. 3. Spiral spring according to claim 1 or 2, characterized in that the Ti content is greater than or equal to 15% by weight. 4. Spiral spring according to one of the preceding claims, characterized in that the sum of the content of V and Ta is between 5 and 25% by weight. 5. Spiral spring according to one of the preceding claims, characterized in that the sum of the content of V and Ta is between 10 and 25% by weight. 6. Spiral spring according to one of the preceding claims, characterized in that the sum of the content of V and Ta is between 15 and 25% by weight. 7. Spiral spring according to one of the preceding claims, characterized in that it comprises Zr and / or Hf with the sum of the content of Zr and Hf between 1 and 40% by weight. 8. Spiral spring according to one of the preceding claims, characterized in that it comprises Zr and/or Hf with the sum of the Zr and Hf content comprised between 5 and 25% by weight. 9. Spiral spring according to one of the preceding claims, characterized in that it comprises Zr and/or Hf with the sum of the Zr and Hf content comprised between 10 and 25% by weight. 10. Spiral spring according to one of the preceding claims, characterized in that it comprises Zr and/or Hf with the sum of the Zr and Hf content comprised between 15 and 25% by weight. 11. Spiral spring according to one of the preceding claims, having a microstructure comprising a beta phase of Nb, and of V and/or of Ta and an alpha phase of Ti, and of Zr and/or of Hf when the alloy comprises Zr and/or Hf. 12. Spiral spring according to one of the preceding claims, characterized in that it has an elastic limit greater than or equal to 500 MPa, and a modulus of elasticity greater than or equal to 100 GPa, preferably greater than or equal to 110 GPa . 13. Method of manufacturing a spiral spring intended to equip a balance wheel of a clockwork movement, characterized in that it successively comprises: – a step of producing a blank in an at least ternary alloy consisting of: o of Nb, Ti and at least one element chosen from V and Ta, o optionally at least one element chosen from Zr and Hf, o optionally at least one element chosen from W and Mo, o any traces of other elements chosen from O, H, C, Fe, N, Ni, Si, Cu, Al, with the following percentages by weight: • a total content of Nb, V and Ta of between 40 and 85%, • a total Ti, Zr and Hf content of between 15 and 55%, • a content for W and Mo respectively between 0 and 2.5%, • a content for each of said elements chosen from O, H, C, Fe, N, Ni, Si, Cu, Al between 0 and 1600 ppm with the sum of said traces 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 niobium, and vanadium and/or tantalum in the beta phase, zirconium and/or l the hafnium of said alloy also being essentially in the form of a solid solution when the alloy comprises zirconium and/or hafnium, – 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, – a final heat treatment step. 14. Process for manufacturing a spiral spring according to the preceding claim, characterized in that the beta-type quenching 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. 15. Process for manufacturing a spiral spring according to one of claims 13 to 14, characterized in that the final heat treatment as well as the intermediate heat treatment of each sequence is a precipitation treatment of Ti, and optionally of Zr and /or Hf when the alloy comprises Zr and/or Hf, in the alpha phase, with a duration of between 1 hour and 200 hours at a holding temperature of between 300°C and 700°C. 16. Process for manufacturing a spiral spring according to one of claims 13 to 15, characterized in that, when the alloy comprises Zr and/or Hf, the final heat treatment is carried out at a holding temperature of between 400°C and 600°C for a period of between 4 and 8 hours. 17. A method of manufacturing a spiral spring according to one of claims 13 to 16, characterized 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, cupro-nickel, cupro-magnanese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B, to facilitate shaping in the form of a wire and in that, before or after the strapping step, said wire is freed of its layer of said ductile material by chemical attack.