CH718454A2 - 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 Zr and Hf , – optionally at least one element chosen from W and Mo, – possible traces of other elements chosen from O, H, Ta, C, Fe, N, Ni, Si, Cu, Al, with the percentages in following weights: o an Nb content between 40 and 84%, o a total Ti, Zr and Hf content between 16 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, Ta, 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. The present invention also relates to the process for manufacturing the spiral spring.

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 wheel (β) 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>.

[0004] In practice, the CT is calculated as follows between 8° C. and 38° C.: with a value which must be between -0.6 and +0.6 s/d°C.

[0005] From the prior art, there are known spiral springs for watchmaking made of binary Nb-Ti alloys with percentages of Ti typically between 40 and 60% by weight and more specifically with a percentage of Ti 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. The work-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 two-phase alloy to a CTE close to zero, which is particularly favorable for the CT.

[0006] There are nevertheless certain disadvantages to the use of binary Nb-Ti alloys for the spiral springs.

[0007] A 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. 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.

[0008] 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, which is then not suitable for producing a spiral spring. It is therefore advisable not to incorporate too much titanium into the alloy.

[0009] To date, there is still a need to develop new chemical compositions fulfilling the various criteria of absence of fragile phases and reduction of production times for the 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 Ti is partly replaced by Zr and/or Hf also able to form alpha phase precipitates. The partial replacement of Ti by Zr and/or Hf makes it possible to accelerate precipitation during fixing and therefore to reduce production times.

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 Zr and Hf, optionally at least one element chosen from W and Mo, possible traces of other elements chosen from O, H, C, Ta, Fe, N, Ni, Si, Cu, Al, with the following percentages by weight: an Nb content of between 40 and 84%, a total content of Ti, Zr and Hf of between 16 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, Ta, 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] The invention also relates to the process for manufacturing this watch spiral spring comprising successively: - a step of producing or providing a blank made of at least a ternary alloy consisting of: - Nb, Ti and 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, Ta, C, Fe, N, Ni, Si, Cu, Al, with the following weight percentages: o an Nb content of between 40 and 84%, o a total Ti, Zr and Hf content of between 16 and 55% with preferably a Ti minimum of 15%, o a content for W and Mo respectively between 0 and 2.5%, o a content for each of the said elements chosen from O, H, Ta, 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 beta type quenching step of said blank, so as to c e that the titanium of said alloy is essentially in the form of a solid solution with the niobium in the beta phase, the zirconium and/or the hafnium also being essentially in the form of a solid solution, – 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 also called fixing.

[0014] Advantageously, the final heat treatment step to finalize the precipitation of the titanium and the zirconium and/or the hafnium is carried out in a time of between 4 and 8 hours at a holding temperature of between 400° C. and 600°C.

[0015] In addition to reducing the fixing time, the partial replacement of titanium with zirconium makes it possible to reduce the secondary error as explained below.

detailed description

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: - Nb, Ti and 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, Ta, Fe, N, Ni, Si, Cu, Al, with the following percentages by weight: o an Nb content of between 40 and 84%, o a total Ti content , Zr and Hf between 16 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, Ta, 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.

Preferably, the content by weight of Nb is greater than 45%, or even greater than or equal to 50%, 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.

[0019] Preferably, the content by weight of Ti is maintained at a minimum content of 15% because Ti is more economical than Zr and Hf. In addition, it has the advantage of having a lower melting temperature than Zr and Hf, which facilitates casting.

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.

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

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

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

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

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.

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

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

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.

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

[0030] According to the invention, the Ti is partly replaced by Zr and/or Hf forming, like the Ti, alpha precipitates, so as to accelerate the precipitation during fixing and therefore to reduce the processing times. production. Advantageously, the sum of the Zr and Hf content is between 1 and 40% by weight. Preferably, the sum of the Zr and Hf content is between 5 and 25%, more preferably between 10 and 25% and even more preferably between 15 and 25% by weight.

[0031] Advantageously, the Ti is at least replaced by Zr which also makes it possible to reduce the secondary error which is a measurement of the curvature of the step which is generally approximated by a straight line passing through two points (8° C. and 38 °C). Tests were carried out on binary Nb-Ti alloys with a weight percentage of Ti of 47% (NbTi47) and Nb-Zr with weight percentages of Zr between 0 and 70% to show the effect of Ti respectively and Zr 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 the data for pure Nb, the NbTi47 alloy and the Nb-Zr alloy as a function of the weight percentage of Zr. Pure Nb has a secondary error at 23°C of -6.6 s/d. The precipitation of Ti in the NbTi47 alloy compensates for the negative effect of Nb with, however, an excessive rise with a positive value reaching 4.5 s/d. Nb-Zr alloys, on the other hand, have a negative secondary error for a Zr content greater than 0%, or even zero for Zr contents greater than or equal to 45% by weight. It follows that the partial replacement of Ti by Zr in a ternary alloy makes it possible to compensate for the excessively positive effect of Ti on the secondary error. Adding a few percent by weight of Zr already makes it possible to reduce the secondary error to a value closer to 0 than for the binary alloy NbTi47. Thus, advantageously, the Zr content is at least 5% by weight. Pure niobium 0% -6.6 s/d NbTi47 47% 4.5 s/d Nb30Zr70 70% -0.2 s/d Nb45Zr55 55% 0.0 s/d Nb50Zr50 50% 0.2 s/d Nb55Zr45 45% 0 .0 s/d Nb60Zr40 40% -3.0 s/d Nb65Zr35 35% -4.1 s/d Nb70Zr30 30% -4.8 s/d Nb80Zr20 20% -5.0 s/d Nb85Zr15 15% -5 .8 s/d Nb90Zr10 10% -6.0 s/d Nb100 0% -6.6 s/d

Table 1

[0033] 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.

Advantageously, the spiral spring according to the invention has a multiphase microstructure comprising niobium in centered cubic beta phase, and a single alpha phase of titanium and zirconium and/or hafnium.

To obtain such a microstructure, it is necessary to finalize the precipitation in the alpha phase by heat treatment during the fixing of the spring in the manufacturing process successively implementing the following steps: - a step of providing or development of a draft. For example, the blank can be made by melting the elements in an electric arc or electron gun furnace to form a billet or ingot which is hot forged then cold deformed and heat treated between deformation. The blank is made of at least a ternary alloy consisting of: – Nb, Ti and 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, Ta, C, Fe, N, Ni, Si, Cu, Al, with the following percentages by weight: o an Nb content of between 40 and 84%, o a total Ti content , Zr and Hf between 16 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, Ta, 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 under form of solid solution with the niobium in the beta phase, the zirconium and/or the hafnium also being essentially in the form of a solid solution, – an application step a said alloy of a succession of deformation sequences followed by an intermediate heat treatment. By deformation is meant a deformation by drawing and/or rolling. 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. – a strapping step to form the spiral spring, – a final heat treatment step.

[0036] 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 conventional 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-heat treatment includes, each time, a heat 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 of between 700° C. and 1000° C., under vacuum, followed by 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 3 hours and 30 hours at a temperature between 400°C and 600°C.

[0040] 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.

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, cupromagnanese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B, or the like, to facilitate forming into a 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.

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.

[0045] In a particular horological application, ductile material or copper is thus added at a given moment to facilitate shaping in the form of 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.

[0046] 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 the alloy 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 3 hours and 30 hours at a temperature between 400°C and 600°C. Advantageously, the duration is between 4 and 8 hours with maintenance at a temperature of between 400°C and 600°C. During this final heat treatment, the precipitation of the titanium as well as the hafnium and/or the zirconium in the alpha phase is finalized.

By a suitable combination of sequences of deformation and heat treatment, it is possible to obtain a very fine microstructure, in particular nanometric, comprising beta niobium and an alpha phase of titanium and hafnium and/or zirconium . 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 and preferably greater than or equal to 110 GPa. This combination of properties is well suited for a spiral spring. In addition, this at least ternary niobium-titanium-hafnium and/or zirconium alloy according to the invention can easily be covered with ductile material or copper, which greatly facilitates its deformation by drawing.

[0050] This alloy also exhibits an effect similar to that of "Elinvar", with a practically zero thermo-elastic coefficient in the range of temperatures commonly used in 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 Zr and Hf, – optionally at least one element chosen from W and Mo, – any traces of other elements chosen from O, H, Ta, C, Fe, N, Ni, Si, Cu, Al, with the following weight percentages: – a Nb content between 40 and 84%, – a total Ti, Zr and Hf content of between 16 and 55%, – a content for respectively W and Mo between 0 and 2.5%, – a content for each of said elements chosen from O, H, Ta, 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 Zr and Hf is between 1 and 40% by weight. 5. Spiral spring according to one of the preceding claims, characterized in that the sum of the content of Zr and Hf is between 5 and 25% by weight. 6. Spiral spring according to one of the preceding claims, characterized in that the sum of the content of Zr and Hf is between 10 and 25% by weight. 7. Spiral spring according to one of the preceding claims, characterized in that the sum of the content of Zr and Hf is between 15 and 25% by weight. 8. Spiral spring according to one of the preceding claims, characterized in that it comprises at least 5% by weight of Zr. 9. Spiral spring according to one of the preceding claims, characterized in that it comprises Zr and Hf. 10. Spiral spring according to one of the preceding claims, having a microstructure comprising Nb in the beta phase and Ti as well as Zr and/or Hf in the alpha phase. 11. 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 and preferably greater than or equal to 110 GPa . 12. 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 providing a blank in an at least ternary alloy consisting of: – Nb, Ti and 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, Ta, C, Fe, N, Ni, Si, Cu, Al, with the following weight percentages: o a Nb content of between 40 and 84%, o a total Ti, Zr and Hf content of between 16 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, Ta, 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, – 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, the zirconium and/or the hafnium also being essentially in the form of a solution solid, – 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. 13. 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. 14. Process for manufacturing a spiral spring according to claim 12 or 13, characterized in that the beta-type quenching is a solution treatment for 1 hour at 800° C. under vacuum, followed by cooling under gas. 15. Process for manufacturing a spiral spring according to one of claims 12 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 Zr and/or Hf in alpha phase with a duration between 1 hour and 200 hours at a holding temperature between 300°C and 700°C. 16. Process for manufacturing a spiral spring according to one of claims 12 to 15, characterized in that 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 o'clock. 17. A method of manufacturing a spiral spring according to one of claims 12 to 16, characterized in that, after the step of providing the alloy blank, and before the step of applying a succession of sequences, a superficial layer of ductile material taken from among copper, nickel, cupro-nickel, cupromagnanese, gold, silver, nickel-phosphorus Ni-P and nickel is added to said blank -boron NiB, to facilitate shaping in the form of wire and in that, before or after the strapping step, said wire is rid of its layer of said ductile material by chemical attack.