Balance-spring for horological movement and method for manufacturing same

Abstract

A balance-spring intended to equip a balance of an horological movement, comprising a core made of NbTi made from an alloy consisting of: niobium: balance to 100% by weight, titanium: between 5 and 95% by weight, traces of elements chosen from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said elements being present in a quantity between 0 and 1600 ppm by weight, the total quantity formed by all of said elements being between 0% and 0.3% by weight, wherein the core made of NbTi is coated with a layer of niobium, said layer of niobium having a thickness between 20 nm and 10 ?m.

Claims

1. A balance-spring intended to equip a balance of an horological movement, comprising a core made of NbTi made from an alloy consisting of: niobium: remainder to 100% by weight, titanium: between 5 and 95% by weight, traces of elements chosen from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said elements being present in a quantity between 0 ppm and 1600 ppm by weight, the total quantity formed by all of said elements being between 0% and 0.3% by weight, wherein the core made of NbTi is coated with a layer of an austenitic stainless steel, said layer of the first material having a thickness between 200 nm and 10 ?m.

2. The balance-spring according to claim 1, wherein the layer of the austenitic stainless steel has a thickness between 300 nm and 1.5 ?m.

3. The balance-spring according to claim 1, wherein the layer of the austenitic stainless steel has a thickness between 400 nm and 800 nm.

4. The balance-spring according to claim 1, wherein the concentration of Ti is between 40% and 65% by weight, between 40% and 49% by weight or between 46% and 48% by weight.

5. The balance-spring according to claim 1, wherein the core made of NbTi has a two-phase microstructure including niobium in beta phase and titanium in alpha phase.

6. The balance-spring according to claim 1, which has an elastic limit greater than or equal to 500 MPa or to 600 MPa, and a modulus of elasticity less than or equal to 120 GPa, or less than or equal to 100 GPa.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The invention relates to a method for manufacturing a balance-spring intended to equip a balance of an horological movement. This balance-spring is made from an alloy of the binary type including niobium and titanium. It also relates to the balance-spring coming from this method.

(2) The method will be described more precisely below with niobium as a first material and copper as a second material.

(3) According to the invention, the manufacturing method includes the following steps:

(4) a) a step of making available a blank with a core made of NbTi made from an alloy consisting of: niobium: balance to 100% by weight, titanium: between 5 and 95% by weight, traces of one or more elements chosen from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu and Al, each of said elements being present in a quantity between 0 and 1600 ppm by weight, the total quantity formed by all of said elements being between 0% and 0.3% by weight,

(5) b) a step of forming a layer of niobium around the blank with the core made of NbTi,

(6) c) a step of forming a layer of copper around the blank obtained from step b),

(7) d) a step of forming the blank in several sequences comprising:

(8) d1) a succession of forming-stage steps to bring the blank obtained in step c) to a determined diameter called calibration diameter and

(9) d2) a succession of steps of flat rolling the round blank obtained in step d1),

(10) e) a step of cutting the rolled wire into blades having a determined length,

(11) f) a step of winding to form the balance-spring,

(12) g) a step of final heat treatment of the balance-spring.

(13) The method of the invention further comprises a step h) of removing said layer of copper formed in step c), at a moment of step c) at which the blank has reached a diameter such that it is still possible to pass said blank at least through one draw-plate and preferably through two draw-plates with a degree of elongation of the blank of approximately 10% at each draw-plate before the first rolling step d2) or at the latest before the last stage of step d2).

(14) The method will now be described in more detail.

(15) In step a), the core is made from an NbTi alloy including between 5 and 95% by weight of titanium. Advantageously, the alloy used in the present invention comprises by weight between 40 and 60% of titanium. Preferably, it includes between 40 and 49% by weight of titanium, and more preferably between 46% and 48% by weight of titanium. The percentage of titanium is sufficient to obtain a maximum proportion of precipitates of Ti in the form of alpha phase while being reduced to avoid the formation of martensitic phase leading to problems of fragility of the alloy during its implementation.

(16) In a particularly advantageous manner, the NbTi alloy used in the present invention does not comprise other elements except for possible and inevitable traces. This allows to avoid the formation of fragile phases.

(17) More particularly, the concentration of oxygen 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.

(18) More particularly, the concentration of tantalum is less than or equal to 0.10% by weight of the total.

(19) More particularly, the concentration of carbon 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 less than or equal to 0.0175% by weight of the total.

(20) More particularly, the concentration of iron 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 less than or equal to 0.020% by weight of the total.

(21) More particularly, the concentration of nitrogen 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 less than or equal to 0.0075% by weight of the total.

(22) More particularly, the concentration of hydrogen 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 less than or equal to 0.0005% by weight of the total.

(23) More particularly, the concentration of silicon is less than or equal to 0.01% by weight of the total.

(24) More particularly, the concentration of nickel 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.

(25) More particularly, the concentration of ductile material, such as copper, in the alloy 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.

(26) More particularly, the concentration of aluminium is less than or equal to 0.01% by weight of the total.

(27) During a step b) the core made of NbTi of the blank in step a) is coated with a layer of niobium. The addition of the layer of niobium around the core can be carried out galvanically, by PVD, CVD or mechanically. In the latter case, a tube of niobium is fitted onto a bar of the alloy made of NbTi. The assembly is formed by hammering and/or drawing to thin the bar and form the blank which was made available in step a). The thickness of the layer of niobium is chosen so that the ratio surface of niobium/surface of the core made of NbTi for a given cross-section of wire is less than 1, preferably less than 0.5, and more preferably between 0.01 and 0.4. For example, the thickness is preferably between 1 and 500 micrometres for a wire having a total diameter of 0.2 to 1 millimetre.

(28) Alternatively, the layer of niobium can be made by winding a strip of niobium around the core made of NbTi, the strip of niobium/core made of NbTi assembly being then formed by hammering and/or drawing to thin the bar and form the blank which was made available at the end of step a).

(29) The core made of NbTi of the blank obtained in step b) is coated with a layer of copper during a step c). The addition of the layer of copper around the core can be carried out galvanically, by PVD, CVD or mechanically. In the latter case, a tube of copper is fitted onto a bar of the alloy made of NbTi coated with the layer of niobium. The assembly is formed by hammering and/or drawing to thin the bar and form the blank which was made available at the end of step b). The thickness of the layer of copper is chosen in such a way that the ratio surface of copper/surface of the core made of NbTi coated with the layer niobium for a given cross-section of wire is less than 1, preferably less than 0.5, and more preferably between 0.01 and 0.4. For example, the thickness is preferably between 1 and 500 micrometres for a wire having a total diameter of 0.2 to 1 millimetre.

(30) Alternatively, the layer of copper can be made by winding a strip of copper around the core made of NbTi coated with the layer of niobium, the strip of niobium/core made of NbTi assembly being then formed by hammering and/or drawing to thin the bar and form the blank which was made available at the end of step b).

(31) According to yet another alternative, the core made of NbTi coated with the niobium strip can be inserted into a tube of copper, the assembly being hot co-extruded at a temperature of approximately 600 to 900 degrees through a draw-plate.

(32) A hardening of the beta type consisting of a solution treatment is carried out at least before the later forming steps. This treatment is carried out in such a way that the titanium of the alloy is substantially in the form of a solid solution with the niobium in beta phase. Preferably, it is carried out for a duration between 5 minutes and 2 hours at a temperature between 700? C. and 1000? C., under vacuum, followed by cooling under gas. More particularly, this beta hardening is a solution treatment at 800? C. under vacuum for 5 minutes to 1 hour, followed by cooling under gas.

(33) The step d) of forming is carried out in several sequences. Forming means forming by drawing and/or rolling.

(34) Advantageously, the forming step includes at least successively sequences of forming, preferably cold, by hammering and/or drawing and/or calibration drawing designated by step d1). Step d1) allows to bring the blank obtained at the end of step c) to a determined diameter called calibration diameter of the wire.

(35) According to the invention, the method further comprises a step h) which involves removing the layer of copper formed in step c), when during step d1), the blank has reached a diameter such that it is still possible to pass said blank at least through one draw-plate with a degree of elongation of the blank of approximately 10% before the first later rolling step d2). This step of removing the layer of copper is carried out by chemical attack in a solution containing cyanides or acids, for example in a bath of nitric acid at a concentration of 53% by weight in water.

(36) A sequence of rolling operations, preferably with a rectangular profile compatible with the input cross-section of a winding spindle, is then carried out, this sequence forming step d2).

(37) Each sequence of steps d1) and d2) is carried out with a given degree of forming between 1 and 5, this degree of forming corresponding to the conventional formula 2In(d0/d), where d0 is the diameter of the last beta hardening, and where d is the diameter of the cold-worked wire. The overall total of the forming steps over this entire succession of sequences leads to a total degree of forming between 1 and 14.

(38) At the end of step d2), the layer of niobium coating the core made of NbTi has a thickness between 20 nm and 10?m, preferably between 300 nm and 1.5 ?m, more preferably between 400 and 800 nm.

(39) The wire rolled into a blade obtained at the end of step d2) is then cut to a determined length during step e).

(40) The step f) of winding to form the balance-spring is followed by the step g) of final heat treatment of the balance-spring. This final heat treatment is a treatment of precipitation of the Ti in alpha phase having a duration between 1 and 80 hours, preferably between 5 and 30 hours, at a temperature between 350 and 700? C., preferably between 400 and 600? C.

(41) According to an advantageous alternative the method can further include, between each sequence or between certain sequences of the forming steps d1) and/or d2), an intermediate heat treatment of precipitation of the titanium in alpha phase having a duration between 1 hour and 80 hours at a temperature between 350? C. and 700? C., preferably between 5 hours and 30 hours between 400? C. and 600? C. Advantageously, this intermediate treatment is carried out in step d1) between the first drawing sequence and the second calibration-drawing sequence.

(42) The balance-spring made according to this method has an elastic limit greater than or equal to 500 MPa, preferably greater than 600 MPa, and more precisely between 500 and 1000 MPa. Advantageously, it has a modulus of elasticity less than or equal to 120 GPa, preferably less than or equal to 100 GPa.

(43) The balance-spring includes a core made of NbTi coated with a layer of niobium, said layer having a thickness between 50 nm and 5 ?m, preferably between 200 nm and 1.5 ?m, more preferably between 800 nm and 1.2 ?m.

(44) The core of the balance-spring has a two-phase microstructure including niobium in beta phase and titanium in alpha phase.

(45) Moreover the balance-spring made according to the invention has a thermoelastic coefficient, also called TEC, allowing it to guarantee the preservation of the chronometric performance despite the variation in the temperatures of use of a watch incorporating such a balance-spring.

(46) The method of the invention allows to create, and more particularly to shape, a balance-spring for a balance made of an alloy of the niobium-titanium type, typically at 47% by weight of titanium (40-60%). This alloy has increased mechanical properties, by combining a very high elastic limit, greater than 600 MPa, with a very low modulus of elasticity, approximately 60 GPa to 80 GPa. This combination of properties is well suited to a balance-spring. Moreover, such an alloy is paramagnetic.