Patent EA-001063-B1 (EA)
Title: Self-compensating balance spring for a mechanical oscillatorof a balance-spring/balance assembly of a watch movement, and process for manufacturing this balance-spring
Inventor: WALDER PIERRE-ALAIN
IPC: G04B17/06
Filed: 1998-06-19
Abstract: 1. A self-compensating balance-spring for a balance-spring/balance assembly of a mechanical oscillator of a precision instrument in particular a horological movement, made of a paramagnetic Nb-Zr alloy containing between 5% and 25% by weight of Zr and having a Young's modulus whose temperature coefficient (TCY) is such that it can substantially nullify the expression :   where E-Young's modulus of the oscillator spring;  = TCY = temperature coefficient of the oscillator spring's Young's modulus; αs - coefficient of thermal expansion of the oscillator's spring; and  αb - coefficient of thermal expansion of the oscillator's balance, characterized by the fact that it contains at least 500 ppm by weight of an interstitial doping agent at least partly formed of oxygen. 2. The balance-spring according to claim 1, characterised in that it comprises between 5% and 20% by weight of Zr and at least 600 ppm by weight of said interstitial doping agent. 3. The balance-spring according to claim 1, characterised in that the amount of said interstitial doping agent varies from 600 to 2000 ppm by weight for a concentration of 20% by weight of Zr to 500 to 800 ppm by weight for a concentration of 25% by weight of Zr. 4. The balance-spring according to any preceding claim, characterised in that the proportion of oxygen in said interstitial doping agent is comprised between 20% and 100% by weight. 5. The balance-spring according to any preceding claim, characterised in that it further comprises an amount of at least one hardening doping agent selected from the following elements : oxygen, nitrogen, carbon, boron and phosphorous. 6. The balance-spring according to any preceding claim, characterised in that it further comprises between 0.01% and 5% by weight of at least one element selected from: Be, Al, Si, Ge, Sc, Y, La, Ti, Hf, V, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au. 7. Process for manufacturing a self-compensating balance-spring according to claim 1, wherein a bar is formed from said alloy, this bar is transformed into a wire having a diameter comprised between 0.05 and 1.5 mm by cold rolling or cold drawing in the absence of oxygen, the diameter of this wire is reduced by cold rolling or cold drawing and shaped into a ribbon suitable for the balance-spring, this ribbon is wound into the shape of a spiral and submitted to at least one heat treatment under a controlled pressure and/or controlled atmosphere to reduce the temperature coefficient of the Young's modulus (TCY) by the controlled precipitation of Zr rich phases and to define the shape of the balance-spring, characterised in that the wire contains an interstitial agent in an amount producing the controlled precipitation of Zr rich phases, and the wire thus obtained is heated between 650 degree C and 880 degree C for 24h, to adjust the TCY to the desired value. 8. The process according to claim 7, characterised in that the amount of said interstitial agent in said wire is adjusted by doping with at least 600 ppm in an oxygen-containing atmosphere. 9. The process according to any one of claims 7 to 8, characterised in that said ribbon wound into a spiral shape is placed under vacuum to carry out said heat treatment. 10. The process according to any one of claims 7 to 8, characterised in that after heat treatment to adjust the TCY and define the shape of the self-compensating balance-spring, said spring undergoes a hardening heat treatment at a temperature below 650 degree C in an atmosphere containing a partial pressure of a gas containing at least one element capable of diffusing into the balance-spring. 11. The process according to claim 10, characterised in that said elements are selected from : oxygen, nitrogen, carbon, boron and phosphorous.