CH718850A2 - Spiral spring for clockwork. - 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, H and possible traces of other elements chosen from O, C, Fe, N, Ni, Si, Cu and Al, with the following percentages by weight: a Ti content between 1 and 80%, an H content between 0.17 and 2%, a total content for the all other elements less than or equal to 0.3% by weight, the balance to reach 100% being made up of Nb. The present invention also relates to its method of manufacture with a step of thermochemical treatment of a blank made from an alloy of Nb and Ti in an atmosphere comprising hydrogen so as to enrich the alloy of Nb and Ti with hydrogen in the form of interstitials.

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.

[0004] The formula which links the CTE of the alloy and the expansion coefficients of the hairspring (α) and of the balance wheel (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 a solid solution of Nb and Ti in the beta phase and Ti in the form of precipitates in the alpha phase. The solid solution of hardened beta-phase Nb and Ti exhibits a strong positive CTE while the 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. It is calculated according to the following formula:

[0008] Typically, for an NbTi47 alloy, the secondary error is +4.5 s/d whereas preferably it should be between -3 and +3 s/d.

Summary of the invention

The object of the invention is to propose a new manufacturing method and a new chemical composition for a spiral spring making it possible to reduce the secondary error while maintaining a thermal coefficient close to 0.

To this end, the invention relates to a watch spiral spring made of an alloy of niobium, titanium and hydrogen. More specifically, the spiral spring is made of an alloy consisting of: of Nb, Ti, H and possible traces of other elements chosen from O, C, Fe, N, Ni, Si, Cu and Al, with the following percentages by weight: a Ti content of between 1 and 80%, an H content of between 0.17 and 2%, a total content for all other elements less than or equal to 0.3% by weight, the balance to reach 100% being made up of Nb.

The addition of hydrogen makes it possible to produce a spiral spring with a secondary error close to 0 and simultaneously with a thermal coefficient close to 0.

According to the invention, hydrogen is added to the Nb-Ti alloy by thermochemical treatment under a controlled atmosphere during the manufacturing process.

More specifically, the manufacturing process successively comprises: a) a step of producing or making available a blank made of an alloy consisting of Nb, Ti and possible traces of other elements chosen from O, C, Fe, N, Ni, Si, Cu and Al, with a Ti content between 1 and 80% and a total content for all other elements less than or equal to 0.3% by weight, the balance to reach 100 % consisting of Nb, b) a beta-type solution treatment and quenching step of said blank, so that the titanium and niobium of said alloy are essentially in the form of a solid solution in beta phase, c ) a step of applying to said alloy a succession of deformation sequences with optionally at least one heat treatment between two sequences and/or after the succession of deformation sequences, d) a strapping step to form the spiral spring, e) a heat treatment step final, called fixing, the method being characterized in that it comprises an additional thermochemical treatment step in an atmosphere comprising hydrogen, said thermochemical treatment step being carried out during the solution treatment of step b ), during a heat treatment of step c), during the final heat treatment of step e), before step b), between steps b) and c), between steps c) and d ), between steps d) and e) or after step e).

[0014] Advantageously, the thermochemical treatment is carried out on a recrystallized structure.

[0015] The spiral spring thus produced comprises hydrogen mainly or exclusively in the form of interstitials. By predominantly, as opposed to exclusively, is meant that the very local presence in a low proportion of hydrides cannot be excluded. Regarding its microstructure, it is formed of a single beta phase of Nb and Ti in solid solution.

[0016] In addition to its low secondary error and its low thermal coefficient, the spiral spring produced with the method according to the invention has a breaking load Rm greater than or equal to 500 MPa and more precisely comprised between 800 and 1000 MPa. Advantageously, it has a modulus of elasticity greater than or equal to 80 GPa and preferably greater than or equal to 90 GPa.

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

Brief description of figures

[0018] FIG. 1 represents the secondary error as a function of the thermal coefficient for Nb-Ti-H ternary shades according to the invention with 47% Ti.

[0019] FIG. 2 represents the secondary error as a function of the thermal coefficient for Nb-Ti binary shades according to the prior art with 47% Ti.

Figure 3 shows the variation of the Young's modulus with temperature for an Nb-Ti-H alloy according to the invention having been subjected to a thermochemical treatment at 652° C. for 15 minutes under 4 bar of hydrogen. In the representation, Young's modulus is normalized to Young's modulus at 23°C.

Figure 4 shows for this same alloy the X-ray diffraction spectrum (XRD spectrum).

FIG. 5 represents for the left peak (Inv) an enlargement of this XRD spectrum centered on θ=39° with the reference peak (Ref) on the right in the absence of thermochemical treatment.

detailed description

The invention relates to a watch spiral spring made of an alloy of niobium (Nb), titanium (Ti) and hydrogen (H). More specifically, the alloy consists of: of Nb, Ti, H and possible traces of other elements chosen from O, C, Fe, N, Ni, Si, Cu and Al, with the following percentages by weight: a Ti content of between 1 and 80%, an H content of between 0.17 and 2%, a total content for all the other elements present in the form of traces less than or equal to 0.3% by weight, the balance to reach 100% being made up of Nb.

Preferably, the hydrogen content is between 0.2 and 1.5% by weight, more preferably between 0.5 and 1% by weight.

Preferably, the titanium content is between 20 and 60%, preferably between 40 and 50% by weight.

The alloy used in the present invention does not include other elements than Ti, Nb and H with the exception of possible and unavoidable 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 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.

According to the invention, the alloy is enriched with hydrogen via a thermochemical treatment under an atmosphere comprising a hydrogen carrier gas.

[0036] This thermochemical treatment can be carried out at different stages of the process for manufacturing the spiral spring, the process stages being as follows: a) production or provision of a blank made of an alloy consisting of Nb, Ti and possible traces of other elements chosen from O, C, Fe, N, Ni, Si, Cu and Al, with a Ti content of between 1 and 80% and a total content for all the other elements less than or equal to 0.3% by weight, the balance to reach 100% being made up of Nb, b) solution treatment and quenching, called beta type, of said blank, so that the titanium and niobium are essentially in the form of solid solution in beta phase, c) application to said alloy of deformation sequences with optionally one or more heat treatments. 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 rolling preferably with a rectangular profile compatible with the entry section of a strapping pin, d) strapping to form a spiral spring, e) final fixing heat treatment.

According to the invention, the thermochemical treatment can be carried out during the solution heat treatment of step b), during a heat treatment of step c), during the final heat treatment for fixing the step e) or between steps a) and b), b) and c), c) and d), d) and e) or after step e). Advantageously, this treatment is carried out during step e) at the end of the manufacturing process. Carrying out the thermochemical treatment at the end of the manufacturing process makes it possible to avoid a possible release of hydrogen into the atmosphere during a subsequent step which would be carried out, for example, under vacuum. This also makes it possible to fix the geometry of the spring, the thermal coefficient and the secondary error during a single heat treatment.

The thermochemical treatment is carried out at a holding temperature of between 100 and 900° C., preferably between 500 and 800° C., more preferably between 600 and 700° C. in an atmosphere comprising hydrogen. The thermochemical treatment can be carried out in an atmosphere containing 100% H2 with an absolute pressure of between 5 mbar and 10 bar, preferably between 0.5 and 7 bar, more preferably between 1 and 6 bar, even more preferably between 3.5 and 4.5 bar . The thermochemical treatment can also be carried out in an atmosphere containing a mixture of gases, for example Ar and H2, under a total pressure of between 5 mbar and 10 bar, preferably between 0.5 and 7 bar, more preferably between 1 and 6 bar, even more preferably between 3.5 and 4.5 bar, with a volume percentage of H2 comprised between 5 and 90%. Advantageously, the thermochemical treatment is carried out for a time of between 1 minute and 5 hours.

[0039] In step b), the solution treatment and quenching, called beta type, prior to the deformation sequences is a vacuum treatment at a temperature between 600 ° C and 1000 ° C with a duration of between 5 minutes and 2 hours, followed by cooling under gas. More particularly still, the treatment is carried out at 800° C. for 1 hour under vacuum and followed by cooling under gas.

[0040] In step c), each deformation sequence is performed 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 cumulation of the deformations on the whole of this succession of sequences brings a total rate of deformation included between 1 and 14.

More particularly, the method comprises between one and five deformation sequences.

[0042] More particularly, the first sequence comprises a first deformation with at least 30% reduction in section.

More particularly, each sequence, other than the first, includes a deformation with at least 25% reduction in section.

Between the deformation sequences and/or after all of the deformation sequences, it is possible to carry out a heat treatment. This heat treatment can have several purposes: to carry out a beta-type solution treatment and quenching treatment as before, to precipitate the alpha phase of titanium or else to restore/recrystallize the structure. The beta-type solution treatment and quenching is carried out under vacuum at a temperature of between 600° C. and 1000° C. with a duration of between 5 minutes and 2 hours, followed by cooling under gas. The precipitation of the alpha phase of the titanium is carried out at a temperature of between 300 and 500° C. for a time of between 1 hour and 200 hours. The restoration/recrystallization is carried out at a temperature between 500 and 600°C for a time between 30 minutes and 20 hours.

In step e), 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.

[0046] The method can also advantageously comprise an additional step taking place after step a) of producing or making available said alloy blank, and before the deformation sequences of step c), adding at the blank a superficial layer of ductile material taken from among copper, nickel, cupro-nickel, cupro-magnanese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni- B, or similar, to facilitate forming into wire form when deforming. And, between the last deformation sequences, after the deformation sequences or after step d) of strapping, 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.

[0048] In a particular horological application, ductile material 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 millimetres. The wire is stripped of its layer of ductile material in particular by chemical attack, then is rolled flat before the manufacture of the actual spring by strapping. Alternatively, the layer of ductile material is removed after flat rolling and before strapping.

[0049] The supply of ductile material can be galvanic, or else mechanical, it is then a jacket or a tube of ductile material which is adjusted on an alloy bar to 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.

Returning to the additional step of thermochemical treatment, the addition of hydrogen is intended to reduce the secondary error. Tests were carried out on a binary Nb-Ti alloy with 47% by weight of Ti and 53% by weight of Nb. The thermochemical treatment was carried out during the final fixing heat treatment in step e) under an atmosphere comprising 100% H2 with the conditions given in Table 1 below. The thermochemical treatment was carried out either on a recrystallized structure (R) having been subjected to deformation sequences terminated by a heat treatment for recrystallization, or on a work-hardened structure (E) following deformation sequences without subsequent heat treatment of recrystallization. The secondary error (ES) was measured at 23°C according to the following formula: This is the difference in rate at 23°C with respect to the straight line connecting the rate at 8°C to that at 38° vs. For example, the rate at 8° C., 23° C. and 38° C. can be measured using a Witschi chronoscope-type device. The thermal coefficient (CT) was measured according to the following formula: with the same apparatus.

The results of the measurements are given in table 1. 01 R 10 668 4 0.01 0.0 02 E 15 652 4 -0.46 -0.8 03 E 8 668 4 -0.59 -0.8 04 E 15 652 1 0.56 1.7 R = recrystallized, E = hardened

Table 1

Samples 01 to 04 have hydrogen contents of between 0.3 and 1% by weight. All the samples have a secondary error between - 3 and + 3 s/d as desired with values close to 0 for the samples treated under a hydrogen pressure of 4 bar. The CT is also contained in the range between -0.6 and +0.6 s/d°C as desired. The optimum is obtained for sample 01 for which the thermochemical treatment was carried out on a recrystallized structure, the thermal coefficient and the secondary error being close to 0 expressed respectively in s/d°C and s/d. This sample has a hydrogen content of the order of 0.6% by weight.

The results of samples 01 to 04 are reported in Figure 1 with the secondary error (ES) as a function of the thermal coefficient (CT). In general, it has been observed that there is a direct link between CT and ES when the alloy of the spiral spring contains hydrogen, contrary to what has been observed in tests carried out in the passed on a binary alloy with 47% by weight of titanium and 53% by weight of niobium. In the latter case, as shown in Figure 2, there is no relationship between CT and ES. The representation on the same graph of these two quantities gives a cloud of points, whatever the parameters of the manufacturing process of the samples. Moreover, one never obtains points where CT=ES=0 whereas it is the case for the ternary nuances Nb-Ti-H. Thus, it was discovered that the addition of hydrogen makes it possible to control the secondary error while keeping a low CT.

The influence of temperature on the Young's modulus of sample 02 was also measured continuously using a mechanical spectrometer measuring the natural frequency of a beam in free vibration, over a range from from -20°C to +60°C (fig.3). Little influence of temperature on Young's modulus is observed.

On this same sample, an X-ray diffraction analysis (Bragg-Brentano configuration) was carried out. The diffractogram is represented in FIG. 4. The XRD spectrum between 30° and 80° does not indicate the presence of TiH2 or NbH hydride phases. By zooming in Figure 5 centered on θ=39°, which corresponds to the area of the NbTi [110] peak, we observe that it is moved to the left (peak Inv) following the thermochemical treatment with respect to the reference peak (peak Ref) without thermochemical treatment, which is the sign of an increase in the lattice parameter. It can be concluded that the thermochemical treatment makes it possible to introduce hydrogen in the form of interstitials without forming hydrides. Furthermore, no precipitation of titanium in the alpha phase is observed. The absence of titanium precipitates is attributed to the presence of hydrogen which stabilizes the beta phase of titanium.

Claims (16)

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, H and possible traces of other elements chosen from O, C, Fe, N, Ni, Si, Cu and Al, with the following weight percentages: – a Ti content between 1 and 80%, – an H content of between 0.17 and 2%, – a total content for all other elements less than or equal to 0.3% by weight, – the balance to reach 100% being made up of Nb. 2. Spiral spring according to the preceding claim, characterized in that the H content is between 0.2 and 1.5% by weight. 3. Spiral spring according to one of the preceding claims, characterized in that the H content is between 0.5 and 1% by weight. 4. Spiral spring according to one of the preceding claims, characterized in that the Ti content is between 20 and 60%, preferably between 40 and 50% by weight. 5. Spiral spring according to one of the preceding claims, characterized in that the H is present mainly or exclusively in the form of interstitials in the alloy. 6. Spiral spring according to one of the preceding claims, characterized in that the microstructure of the alloy is formed of a single beta phase of Nb and Ti in solid solution. 7. Spiral spring according to one of the preceding claims, having a thermal coefficient, called CT, of between -0.6 and +0.6 s/d°C and a secondary error, called ES, of between - 3 and + 3 s/d . 8. Method of manufacturing a spiral spring intended to equip a balance wheel of a clockwork movement, successively comprising: a) a step of producing or making available a blank made of an alloy consisting of Nb, Ti and possible traces of other elements chosen from O, C, Fe, N, Ni, Si, Cu and Al , with a Ti content between 1 and 80% and a total content for all the other elements less than or equal to 0.3% by weight, the balance to reach 100% being made up of Nb, b) a so-called beta-type solution treatment and quenching step of said blank, so that the titanium and niobium of said alloy are essentially in the form of a solid solution in the beta phase, c) a step of applying to said alloy a succession of deformation sequences with optionally at least one heat treatment carried out between two deformation sequences and/or at the end of all the deformation sequences, d) a strapping step to form the spiral spring, e) a final heat treatment step, called fixing, the method being characterized in that it comprises an additional thermochemical treatment step in an atmosphere comprising hydrogen, said thermochemical treatment step being carried out during the solution treatment of step b), during a heat treatment of step c), during the final heat treatment of step e), before step b), between steps b) and c), between steps c) and d), between steps d ) and e) or after step e). 9. A method of manufacturing a spiral spring according to the preceding claim, characterized in that the thermochemical treatment step is carried out in step e). 10. A method of manufacturing a spiral spring according to claim 8 or 9, characterized in that the thermochemical treatment step is carried out on a structure of the blank or of the spiral spring in the recrystallized state. 11. A method of manufacturing a spiral spring according to one of claims 8 to 10, characterized in that the thermochemical treatment step is carried out at a temperature between 100 and 900°C in an atmosphere comprising 100% of hydrogen under a hydrogen pressure of between 5 mbar and 10 bar, or is carried out in an atmosphere comprising a mixture of hydrogen and another gas with a volume percentage of hydrogen of between 5 and 90%, the total pressure of the mixture being between 5 mbar and 10 bar. 12. A method of manufacturing a spiral spring according to one of claims 8 to 11, characterized in that the hydrogen pressure or the total pressure of the mixture is between 0.5 and 7 bar, preferably between 1 and 6 bar , more preferably between 3.5 and 4.5 bar. 13. Process for manufacturing a spiral spring according to one of claims 8 to 12, characterized in that the temperature is between 500 and 800°C, preferably between 600 and 700°C. 14. A method of manufacturing a spiral spring according to one of claims 8 to 13, characterized in that the hydrogen pressure or the total pressure of the mixture is between 3.5 and 4.5 bar and the temperature is between 600 and 700°C. 15. Process for manufacturing a spiral spring according to one of claims 8 to 14, characterized in that the solution treatment is carried out, under vacuum, at a temperature between 600°C and 1000°C for one duration between 5 minutes and 2 hours, followed by cooling under gas. 16. A method of manufacturing a spiral spring according to one of claims 8 to 15, characterized in that, after step a) of producing or making available the alloy blank, and before step c) of applying a succession of sequences, a surface layer of ductile material chosen 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 d), said wire is stripped of its layer said ductile material by chemical attack.