Non-magnetic pivot axis

Abstract

The invention relates to a pivot axis for a watch movement, comprising at least one part made of pure tungsten or of a tungsten alloy with at least a portion of said part converted into a layer of tungsten carbide (5).

Description

Technical field of the invention

[0001] The present invention relates to a non-magnetic pivot axis for a mechanical clock movement and more particularly to a balance wheel axis. Technological background

[0002] The manufacture of a watch pivot axis involves machining a bar of hardenable steel to define various active surfaces (bearing, shoulder, pivots, etc.). The machined axis then undergoes heat treatment, including at least one quenching to improve its hardness and one or more tempering processes to enhance its toughness. These heat treatments are followed by pivot rolling, a process that polishes the pivots to the required dimensions. During this rolling process, the hardness and surface finish of the pivots are further improved.

[0003] Pivoting axes, such as balance staffs, commonly used in mechanical watch movements, are made from free-cutting steel grades that are generally carbon martensitic steels containing lead and manganese sulfides to improve their machinability. A steel of this type, designated 20AP, is typically used for these applications.

[0004] This type of material has the advantage of being easily machinable, particularly suitable for turning, and, after quenching and tempering treatments, exhibits high mechanical properties that are very advantageous for manufacturing watch pivot shafts. These steels, in particular, exhibit high hardness after heat treatment, resulting in excellent shock resistance. Typically, the hardness of pivots on a shaft made of 20 AP steel can reach a value exceeding 700 HV after heat treatment and rolling.

[0005] While providing satisfactory mechanical properties for the watchmaking applications described above, this type of material has the disadvantage of being magnetic and can disrupt the accuracy of a watch after being subjected to a magnetic field, particularly when used for a balance staff that interacts with a balance spring made of a ferromagnetic material. This phenomenon is well known to those skilled in the art. It should also be noted that these martensitic steels are also susceptible to corrosion.

[0006] To overcome these drawbacks, tungsten alloys are ideal materials due to their non-magnetic properties. However, the intrinsic hardness of this material is only around 400-550 HV, and there is no way to use effective hardening processes to improve its properties. This limited hardness can pose problems with the impact resistance of pivots. Summary of the invention

[0007] The invention aims to overcome the aforementioned drawbacks by proposing a pivot axis made at least partly of pure tungsten or of a tungsten alloy with a surface hardening enabling it to meet the shock resistance requirements in the watchmaking field.

[0008] To achieve this hardening, the shaft is subjected on at least part of its surface to heat treatment in a carbon-rich atmosphere so as to cause the tungsten in the shaft to react with the carbon in the atmosphere to form a layer of tungsten carbide which has the characteristic of reaching hardnesses greater than 2000 HV.

[0009] More specifically, the present invention relates to a pivot axis for a watch movement, characterized in that it comprises at least one part made of pure tungsten or of a tungsten alloy with at least a portion of said part converted into a layer of tungsten carbide.

[0010] Tungsten and its alloys combine very good corrosion resistance and very low magnetic susceptibility, while the tungsten carbide layer substantially increases the surface hardness of the pivot axes, which is an advantage for the wear and shock resistance properties of this timepiece, particularly at the pivots.

[0011] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the attached drawings. Brief description of the figures

[0012] There figure 1represents a pivot point. The figure 2 represents in partial section a pivot of a pivot axis comprising a layer of tungsten carbide according to the invention. Detailed description of the invention

[0013] In this description, the term "non-magnetic" material means a paramagnetic, diamagnetic, or antiferromagnetic material with a magnetic permeability between 0.99 and 1.01.

[0014] An alloy of an element is an alloy containing at least 50% by weight of that element.

[0015] The invention relates to a non-magnetic pivot axis for a mechanical watch movement. The invention will be described below in the context of an application to a balance staff 1. Obviously, other types of watch pivot axes are conceivable, such as axes for moving parts, typically escapement pinions, or anchor rods. Parts of this type preferably have body diameters less than 2 mm and pivots with diameters less than 0.2 mm, with a diameter that can be as small as 50 µm at the end of the pivot.

[0016] By referring to the figure 1, we can see a balance shaft 1 which has a plurality of sections 2 of different diameters, preferably formed by turning or any other machining technique by chip removal, and classically defining bearing surfaces 2a and shoulders 2b arranged between two end portions defining two pivots 3. These pivots are intended to each pivot in a bearing, typically in an orifice of a stone or ruby.

[0017] According to the invention, at least a portion of the pivot axis, and in particular at least the portion of the axis intended to enter into rotational contact with another part, is made of pure tungsten or a tungsten alloy. Also according to the invention, this portion, or a portion thereof, made of pure tungsten or a tungsten alloy is coated with a layer of tungsten carbide, which may be WC, W2C, or a non-stoichiometric WC(1-x) carbide.

[0018] In the illustrated example, at least part of the balance staff 1, namely the pivot 3, is made of pure tungsten or a tungsten alloy and at least a portion of this part is converted into a layer 5 of tungsten carbide ( figure 2 ). We thus observe a core 4 in pure tungsten or in tungsten alloy with a layer of tungsten carbide 5.

[0019] Pure tungsten is defined as a material containing tungsten in a percentage by weight greater than or equal to 99.5%, preferably greater than or equal to 99.95%. To reach 100% by weight, it may contain impurities such as molybdenum, carbon, iron, and oxygen. For example, it may contain by weight a minimum of 99.95% tungsten, a maximum of 0.020% molybdenum, and a maximum of 0.030% other impurities.

[0020] For tungsten alloys, for example, these may be alloys comprising either lanthanum oxide, rhenium, or copper and nickel.

[0021] More specifically, the tungsten alloy with lanthanum oxide, namely La₂O₃, comprises the latter in a weight percentage of between 0.2 and 2%, preferably between 0.3 and 1.5%, and more preferably between 0.5 and 1%. Advantageously, the alloy consists of tungsten, lanthanum oxide in the aforementioned percentages, and any impurities with a total content of the latter less than or equal to 1.5% by weight.

[0022] More specifically, the tungsten-rhenium alloy contains the latter in a weight percentage between 2 and 30%, preferably between 5 and 20%. Advantageously, the alloy consists of tungsten and rhenium in the aforementioned percentages and any impurities with a total content of the latter less than or equal to 1.5% by weight.

[0023] More specifically, the tungsten-copper-nickel alloy comprises copper and nickel with a total weight content of these two elements between 2 and 20%, preferably between 5 and 10%. Advantageously, the alloy consists of tungsten, copper, and nickel in the aforementioned percentages and any impurities with a total weight content of the latter less than or equal to 1.5%. Preferably, nickel is present in a predominant proportion relative to copper. This means that for the total copper + nickel content (100%), the percentage of nickel is greater than or equal to 55% and the percentage of copper is less than or equal to 45%.

[0024] The tungsten and tungsten alloys according to the invention have a hardness greater than or equal to 500 HV0.05 for pure tungsten and greater than or equal to 400 HV0.05 for tungsten alloys. Vickers hardness (HV) is defined as a hardness measured according to ISO 6507-1:2018. They have low magnetic susceptibility, typically with values ​​less than 8 × 10⁻⁵, preferably less than 7 × 10⁻⁵.

[0025] According to the invention, at least a portion of the pivot axis is made of tungsten or its alloy and then subjected to surface heat treatment to form the tungsten carbide layer. All of this at least portion of the pivot axis may be subjected to heat treatment, or only a portion of it. In the latter case, the portions not intended to react to form a tungsten carbide layer are protected, for example, by means of a ceramic or refractory metal casing that exposes only the portions to be treated.

[0026] The heat treatment is carried out in a carbon-rich gas atmosphere to facilitate the reaction between tungsten and carbon, thus forming tungsten carbide. For example, the atmosphere may contain methane (CH4) and an inert gas such as argon (Ar). The methane content can range from 5% to 20% by volume, typically 10%, with argon making up the difference. The treatment is performed at a temperature between 1000°C and 1300°C, ideally 1150°C, with a plateau duration of 5 minutes to 2 hours, ideally 15 minutes at the target temperature.

[0027] The tungsten carbide layer thus obtained has a maximum hardness of between 2000 and 2500 HV0.05. Typically, the tungsten carbide layer may exhibit small fluctuations in hardness along its thickness with values ​​ranging between 2000 and 2500 HV0.05.

[0028] The thickness of the resulting tungsten carbide layer depends on the treatment time and temperature parameters. A temperature of 1150°C and a holding time of 15 minutes yields a layer thickness of 20-30 microns. Typically, the thickness ranges from 5 to 50 microns, preferably between 20 and 35 microns.

[0029] After heat treatment, the shaft can undergo one or more tribofinishing operations to remove the carbon-rich surface layer and / or improve the surface roughness of the component.

[0030] Prior to heat treatment, the shaft is manufactured from a blank in bar form. For example, the manufacturing steps (not shown) for an entire shaft made of tungsten or a tungsten alloy are as follows: Provision of a bar made of pure tungsten or a tungsten alloy, Machining, preferably by laser ablation, to obtain the shape of the axle, Rolling of the pivots, Deburring and polishing.

[0031] It should be noted that rolling could take place after heat treatment.

Claims

1. Pivot axis for watch movement, comprising at least one part made of pure tungsten or of a tungsten alloy with at least a portion of said part converted into a layer of tungsten carbide (5).

2. Pivot axis according to the preceding claim, characterized in that the tungsten carbide layer (5) has a thickness of between 5 and 50 microns, preferably between 20 and 35 microns.

3. Pivot axis according to one of the preceding claims, characterized in that the tungsten carbide layer (5) has a maximum hardness of between 2000 and 2500 HV0.

05.

4. Pivot axis according to one of the preceding claims, characterized in that It is made from pure tungsten with a tungsten content greater than or equal to 99.5%, preferably greater than or equal to 99.95% by weight with the remainder to 100% including impurities.

5. Pivot axis according to any one of claims 1 to 3, characterized in that Tungsten alloy is an alloy with lanthanum oxide, an alloy with rhenium, or an alloy with copper and nickel.

6. Pivot axis according to claim 5, characterized in that the tungsten alloy with lanthanum oxide comprises lanthanum oxide in a percentage by weight of between 0.2 and 2%, preferably between 0.3 and 1.5%, more preferably between 0.5 and 1% and possible impurities with a total content for said possible impurities less than or equal to 1.5% by weight.

7. Pivot axis according to claim 5, characterized in that The tungsten alloy with rhenium comprises rhenium in a percentage by weight of between 2 and 30%, preferably between 5 and 20%, and possible impurities with a total content for said possible impurities less than or equal to 1.5% by weight.

8. Pivot axis according to claim 5, characterized in thatThe tungsten alloy with copper and nickel comprises copper and nickel in a total percentage by weight of between 2 and 20%, preferably between 5 and 10%, and possible impurities with a total content for said possible impurities less than or equal to 1.5% by weight.

9. Pivot axis according to one of the preceding claims, characterized in that it is a balance shaft (1) with said part formed of pivots (3) at the ends of the balance shaft (1).

10. Heat treatment process for a pivot axis comprising the following steps: - Making available the pivot axis comprising at least one part made of pure tungsten or a tungsten alloy, - Heat treatment of at least a portion of said part in a carbon-rich gaseous atmosphere at a holding temperature between 1000°C and 1300°C for a time between 5 minutes and 2 hours.

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