EP3796101A1 - Hairspring for clock movement - Google Patents

Abstract

The present invention relates to a spiral spring (1) intended to equip a balance of a timepiece movement, characterized in that the spiral spring (1) is made of an alloy of niobium and titanium consisting by weight of:
- niobium: balance at 100%;
- titanium with a percentage greater than or equal to 1% and less than 40%;
- traces of other elements chosen from O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said elements being between 0 and 1600 ppm of the total by weight and the sum of said traces being less or equal to 0.3% by weight.

The present invention also relates to its manufacturing process.

Description

Field of the invention

The invention relates to a spiral spring intended to equip a balance of a clockwork movement. It also relates to the method of manufacturing this spiral spring.

Background of the invention

• nécessité d'obtention d'une limite élastique élevée,
• facilité d'élaboration, notamment de tréfilage et de laminage,
• excellente tenue en fatigue,
• stabilité des performances dans le temps,
• faibles sections.

The manufacture of spiral springs for watchmaking must face constraints that are often incompatible at first glance:

• need to obtain a high elastic limit,
• ease of production, especially wire drawing and rolling,
• excellent fatigue resistance,
• performance stability over time,
• low sections.

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

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

Summary of the invention

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

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

The invention also relates to the method of manufacturing this clockwork spiral spring as claimed in the appendix.

Brief description of the drawings

• la figure 1 représente, de façon schématisée, un ressort spiral réalisé avec un alliage Nb-Ti selon l'invention ;
• la figure 2 représente les courbes d'évolution du module de Young en fonction de la température rapporté sur le module de Young à 20°C pour respectivement le Nb pur et un alliage Nb-Ti selon l'invention avec 30% en poids de Ti.

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

• the figure 1 represents, schematically, a spiral spring produced with an Nb-Ti alloy according to the invention;
• the figure 2 represents the evolution curves of the Young's modulus as a function of the temperature relative to the Young's modulus at 20 ° C. for respectively pure Nb and an Nb-Ti alloy according to the invention with 30% by weight of Ti.

Detailed description of the preferred embodiments

The invention relates to a clockwork spiral spring made from a binary type alloy comprising niobium and titanium.

• du niobium : balance à 100% ;
• du titane dans un pourcentage supérieur ou égal à 1% et inférieur à 40%. Plus particulièrement, cet alliage comporte une proportion en poids de titane comprise entre 5 et 35%, de préférence entre 15 et 35% et plus préférentiellement entre 27 et 33% ;
• des traces d'autres éléments choisis parmi O, H, C, Fe, Ta, N, Ni, Si, Cu et/ou Al, chacun desdits éléments étant compris entre 0 et 1600 ppm du total en poids, et la somme de ces traces étant inférieure ou égale à 0.3%. En d'autres mots, le total des pourcentages en poids du titane et du niobium est compris entre 99.7% et 100% du total.

According to the invention, this alloy comprises by weight:

• niobium: 100% balance;
• titanium in a percentage greater than or equal to 1% and less than 40%. More particularly, this alloy comprises a proportion by weight of titanium of between 5 and 35%, preferably between 15 and 35% and more preferably between 27 and 33%;
• traces of other elements chosen from O, H, C, Fe, Ta, N, Ni, Si, Cu and / or Al, each of said elements being between 0 and 1600 ppm of the total by weight, and the sum of these traces being less than or equal to 0.3%. In other words, the total of the percentages by weight of titanium and niobium is between 99.7% and 100% of the total.

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 tantalum is less than or equal to 0.10% 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 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 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 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 nickel is less than or equal to 0.01% of the total, in particular less than or equal to 0.16% 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.

Advantageously, this spiral spring has a two-phase microstructure comprising niobium in the centered cubic beta phase and titanium in the compact hexagonal alpha phase.

To obtain such a microstructure, and suitable for the development of a spring, it is necessary to precipitate part of the alpha phase by heat treatment.

Les variables M et T sont respectivement la marche et la température. E est le module de Young du ressort spiral, et, dans cette formule, E, β et α s'expriment en °C^-1.
CT est le coefficient thermique de l'oscillateur, (1/E. dE/dT) est le CTE de l'alliage spiral, β est le coefficient de dilatation du balancier et α celui du spiral. L'alliage en phase bêta écroui présente un CTE fortement positif, et la précipitation de la phase alpha qui possède un CTE fortement négatif permet de ramener l'alliage biphasé à un CTE proche de zéro, ce qui est particulièrement favorable. Cependant, comme mentionné plus haut, un pourcentage trop élevé de titane mène à la formation de phases fragiles. Un pourcentage de titane inférieur à 40% en poids permet d'obtenir un bon compromis entre les différentes propriétés recherchées. Il est par ailleurs supposé que l'interaction entre les dislocations et les interstitiels C, H, N, O présents dans l'alliage de même que l'interaction entre les dislocations et les précipités de titane alpha jouent également un rôle favorable sur le CTE. La mise en mouvement des dislocations en fonction de la température provoque une diminution du module de Young du ressort spiral qui contrecarre l'anomalie positive de la phase bêta.The higher the titanium content, the higher the maximum proportion of alpha phase which can be precipitated by heat treatment, which encourages the search for a high proportion of titanium. But conversely, the higher the titanium content, the more difficult it is to obtain only a precipitation of the alpha phase at the grain boundaries. The appearance of intragranular Widmastätten alpha-Ti type precipitates or the intragranular w phase makes the deformation of the material difficult, if not impossible, which is then not suitable for the production of a spiral spring, and it is then advisable not to incorporate too much titanium in the alloy. In addition, the application of this alloy to a spiral spring requires properties capable of guaranteeing the maintenance of chronometric performance despite the variation in 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, a CTE of +/- 10 ppm / ° C must be achieved. The formula which links the CTE of the alloy and the coefficients of expansion of the hairspring and balance is as follows: $$\mathit{CT}=\frac{\mathit{dM}}{\mathit{dT}}=\left(\frac{1}{2E}\frac{\mathit{of}}{\mathit{dT}}-\beta +\frac{3}{2}\alpha \right)\times 86400\frac{s}{\mathit{j\; °\; C}}$$

The variables M and T are respectively the rate and the temperature. E is the Young's modulus of the spiral spring, and, in this formula, E, β and α are expressed in ° C ^-1 .
CT is the thermal coefficient of the oscillator, (1 / E. DE / dT) is the CTE of the balance spring alloy, β is the expansion coefficient of the balance and α that of the balance spring. The hardened beta-phase alloy exhibits 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. However, as mentioned above, too high a percentage of titanium leads to the formation of brittle phases. A percentage of titanium less than 40% by weight makes it possible to obtain a good compromise between the various desired properties. It is also assumed that the interaction between dislocations and C, H, N, O interstitials present in the alloy as well as the interaction between dislocations and alpha titanium precipitates also play a favorable role on CTE. . The setting in motion of the dislocations as a function of the temperature causes a reduction in the Young's modulus of the spiral spring which counteracts the positive anomaly of the beta phase.

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 less than or equal to 120 GPa and preferably less than or equal to 110 GPa.

• élaboration d'une ébauche dans un alliage comportant du niobium et du titane et plus précisément :
  • du niobium : balance à 100% ;
  • un pourcentage en poids de titane supérieur ou égal à 1% du total et inférieur à 40% du total ;
  • des traces d'autres éléments choisis parmi O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, chacun desdits éléments étant compris entre 0 et 1600 ppm du total en poids, et la somme desdites traces étant inférieure ou égale à 0.3% en poids;
• une trempe de type bêta de ladite ébauche, de façon à ce que le titane dudit alliage soit essentiellement sous forme de solution solide avec le niobium en phase bêta ;
• application audit alliage de séquences de déformation suivie d'un traitement thermique. On entend par déformation une déformation par tréfilage et/ou laminage. Le tréfilage peut nécessiter l'utilisation d'une ou plusieurs filières lors d'une même séquence ou lors de différentes séquences si nécessaire. Le tréfilage est réalisé jusqu'à l'obtention d'un fil de section ronde. Le laminage peut être effectué lors de la même séquence de déformation que le tréfilage ou dans une autre séquence. Avantageusement, la dernière séquence appliquée à l'alliage est un laminage de préférence à profil rectangulaire compatible avec la section d'entrée d'une broche d'estrapadage. Ces séquences mènent à l'obtention d'une microstructure bi-phasée comportant du niobium bêta et du titane alpha, avec une limite élastique supérieure ou égale à 500 MPa et un module d'élasticité inférieur ou égal à 120 GPa et de préférence à 110 GPa.
• estrapadage pour former un ressort spiral, suivi d'un traitement thermique final.

The invention also relates to the method of manufacturing the clockwork spiral spring, characterized in that the following steps are successively implemented:

• preparation of a blank in an alloy comprising niobium and titanium and more precisely:
  • niobium: 100% balance;
  • a percentage by weight of titanium greater than or equal to 1% of the total and less than 40% of the total;
  • traces of other elements chosen from O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said elements being between 0 and 1600 ppm of the total by weight, and the sum of said traces being less or equal to 0.3% by weight;
• beta-type quenching of said blank, so that the titanium of said alloy is essentially in the form of a solid solution with the niobium in the beta phase;
• application to said alloy of deformation sequences followed by heat treatment. By deformation is meant a deformation by wire drawing and / or rolling. Wire drawing may require the use of one or more dies during the same sequence or during different sequences if necessary. The wire drawing is carried out until a wire of round section is obtained. Rolling can be done in the same strain sequence as wire drawing or in a different sequence. Advantageously, the last sequence applied to the alloy is a rolling preferably with a rectangular profile compatible with the entry section of a stepping spindle. These sequences lead to obtaining a two-phase microstructure comprising beta niobium and alpha titanium, with a limit elastic greater than or equal to 500 MPa and an elastic modulus less than or equal to 120 GPa and preferably 110 GPa.
• slipping to form a spiral spring, followed by a final heat treatment.

In these coupled sequences of strain-heat treatment, each strain is performed with a given strain rate between 1 and 5, this strain rate corresponding to the classic formula 2ln (d0 / d), where d0 is the diameter of the last beta hardening, and where d is the diameter of the hardened wire. The global accumulation of the deformations over the whole of this succession of sequences brings a total rate of deformation of between 1 and 14. Each coupled sequence of deformation-heat treatment comprises, each time, a heat treatment of precipitation of the alpha Ti phase. .

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 by a 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 at a temperature between 400 ° C and 600 ° C.

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

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

More particularly, each coupled sequence of heat-treatment-strain, other than the first, comprises a strain 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 is added to the blank, taken from among copper, nickel, cupro- nickel, cupro-magnanese, gold, silver, nickel-phosphorus Ni-P and nickel-boron Ni-B, or the like, to facilitate forming into a wire shape during deformation. And, after the deformation-heat treatment sequences or after the stripping step, the wire is freed from its layer of ductile material, in particular by chemical attack.

In a variant, the surface layer of ductile material is deposited so as to constitute a spiral spring, the pitch of which is not a multiple of the thickness of the blade. In another variant, the surface layer of ductile material is deposited so as to constitute a spring whose pitch is variable.

In a particular horological application, ductile material or copper is thus added at a given moment to facilitate the shaping in the form of wire, so that a thickness of 10 to 500 micrometers remains on the wire. with a final diameter of 0.3 to 1 millimeters. The wire is stripped of its layer of ductile or copper material in particular by chemical attack, then is rolled flat before the manufacture of the spring proper by slipping.

The supply of ductile or copper material can be galvanic, or else mechanical, it is then a jacket or a tube of ductile or copper material. which is fitted to a large diameter niobium-titanium alloy bar, and then which is thinned during the deformation steps of the composite bar.

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

The layer can be removed in particular by chemical attack, with a solution based on cyanides or based on acids, for example nitric acid.

By a suitable combination of deformation and heat treatment sequences, it is possible to obtain a very fine two-phase lamellar microstructure, in particular nanometric, comprising or composed of beta niobium and alpha titanium. This alloy combines a very high elastic limit, greater than at least 500 MPa, and a very low modulus of elasticity, of the order of 80 GPa to 120 GPa. This combination of properties works well for a spiral spring. The alloy after the deformation-heat treatment sequences exhibits a <110> texture. In addition, this niobium-titanium alloy according to the invention can easily be covered with ductile material or copper, which greatly facilitates its deformation by drawing.

A binary type alloy comprising niobium and titanium, of the type selected above for the implementation of the invention, also exhibits an effect similar to that of "Elinvar", with a thermoelastic coefficient practically zero. within the temperature range of usual use of watches, and suitable for the manufacture of self-compensating balance springs.

• la texture cristallographique de l'alliage qui est influencée par le taux de réduction depuis la trempe bêta,
• la densité de dislocations ajustée via les traitements thermiques qui induisent des phénomènes de restauration, voire de recristallisation,
• la concentration en interstitiels qui vont interagir avec les dislocations,
• le pourcentage de Ti en phase alpha
• la densité de précipités dans l'alliage (nombre de précipités Ti en phase alpha par unité de volume) .

More precisely, if we compare to the figure 2 , the change in Young's modulus (E (T) / E _{20 ° C} ) as a function of temperature for pure Nb and an Nb-Ti alloy according to the invention with 30% by weight of Ti, it is observed that the two curves are in S with the notable difference that the presence of Ti makes it possible to greatly reduce the difference between the minimum and the maximum of the curve along both the x-axis and the y-axis. More precisely, the presence of Ti in the alloy and the manufacturing method according to the invention attempt to smooth the curve by reducing the maximum of the curve. This positive effect on the reduction of the maximum with the alloy according to the invention is attributed to a set of factors which are:

• the crystallographic texture of the alloy which is influenced by the rate of reduction since the beta quenching,
• the dislocation density adjusted via heat treatments which induce restoration or even recrystallization phenomena,
• the concentration of interstitials that will interact with the dislocations,
• the percentage of Ti in the alpha phase
• the density of precipitates in the alloy (number of Ti precipitates in alpha phase per unit volume).

Claims (18)

Spiral spring (1) intended to equip a balance of a timepiece movement, characterized in that the spiral spring (1) is made of an alloy of niobium and titanium consisting by weight of: - niobium: balance at 100%; - titanium with a percentage greater than or equal to 1% and less than 40%, - traces of other elements chosen from O, H, C, Fe, Ta, N, Ni, Si, Cu and / or Al, each of said elements being between 0 and 1600 ppm of the total by weight and the sum of said traces being less than or equal to 0.3% by weight. Spiral spring (1) according to Claim 1, characterized in that the said alloy comprises a percentage by weight of titanium of between 5 and 35%. Spiral spring (1) according to Claim 1, characterized in that the said alloy comprises a percentage by weight of titanium of between 15 and 35%. Spiral spring (1) according to Claim 1, characterized in that the said alloy comprises a percentage by weight of titanium of between 27 and 33%. Spiral spring (1) according to one of the preceding claims, characterized in that it has a two-phase microstructure comprising niobium in the beta phase and titanium in the alpha phase. Spiral spring (1) 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 less than or equal to 120 GPa, preferably less than or equal to 110 GPa. Method of manufacturing a spiral spring (1) intended to equip a balance of a timepiece movement, characterized in that it comprises successively: - a stage of preparation of a blank in an alloy of niobium and titanium constituted by weight of: - niobium: balance at 100%; - titanium with a percentage greater than or equal to 1% and less than 40%, - traces of other elements chosen from O, H, C, Fe, Ta, N, Ni, Si, Cu and / or Al, each of said elements being between 0 and 1600 ppm of the total by weight and the sum of said traces being less than or equal to 0.3% by weight; a beta-type quenching step of said blank, so that the titanium of said alloy is essentially in the form of a solid solution with the niobium in the beta phase, - a step of applying to said alloy a succession of deformation sequences followed by an intermediate heat treatment, - a stepping step to form the spiral spring (1), - a final heat treatment step. Method of manufacturing a spiral spring (1) according to Claim 7, characterized in that the deformation during each sequence is carried out by drawing and / or rolling. Method of manufacturing a spiral spring (1) according to Claim 8, characterized in that the deformation of the last sequence is carried out by flat rolling. Method of manufacturing a spiral spring (1) according to one of claims 7 to 9, characterized in that the deformation of each sequence is carried out with a given deformation rate of between 1 and 5, the overall sum of the deformations on the whole of said succession of sequences leading to a total rate of deformation of between 1 and 14. Method of manufacturing a spiral spring (1) according to one of claims 7 to 10, characterized in that the beta-type quenching is a solution treatment, with a duration of between 5 minutes and 2 hours at a temperature between 700 ° C and 1000 ° C, under vacuum, followed by gas cooling. A method of manufacturing a spiral spring (1) according to one of claims 7 to 11, characterized in that the beta-type quenching is a treatment of dissolving for 1 hour at 800 ° C under vacuum, followed by 'gas cooling. Method of manufacturing a spiral spring (1) according to one of claims 7 to 12, characterized in that the final heat treatment as well as the intermediate heat treatment of each sequence is a treatment of precipitation of Ti in the alpha phase of a period of between 1 hour and 200 hours at a temperature of between 300 ° C and 700 ° C. Method of manufacturing a spiral spring (1) according to one of claims 7 to 13, characterized in that the final heat treatment as well as the intermediate heat treatment of each sequence is a treatment of precipitation of Ti in the alpha phase of a period of between 5 hours and 30 hours at a temperature between 400 ° C and 600 ° C. A method of manufacturing a spiral spring (1) according to one of claims 7 to 14, characterized in that said method comprises between one and five said deformation sequences followed by an intermediate heat treatment. Method of manufacturing a spiral spring (1) according to one of claims 7 to 15, characterized in that the first said deformation sequence followed by an intermediate heat treatment comprises a first deformation with at least 30% reduction in section. Method of manufacturing a spiral spring (1) according to Claim 16, characterized in that each said sequence of deformation followed by an intermediate heat treatment, other than the first, comprises a deformation between two intermediate heat treatments with at least 25 % section reduction. Method of manufacturing a spiral spring (1) according to one of claims 7 to 17, characterized in that , after the step of producing the alloy blank, and before the step of applying a succession of sequences, a surface layer of ductile material taken from among copper, nickel, cupro-nickel, cupro-magnanes, 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 stranding step, said wire is freed from its layer of said ductile material by chemical etching.

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