EP2757423B1 - Part for clockwork - Google Patents

Description

Field of the invention

The invention relates to a piece for a watch movement and in particular to a non-magnetic pivoting axis for a mechanical clockwork movement and more particularly to a balance shaft, an anchor rod and a nonmagnetic escape pinion.

Background of the invention

The manufacture of a clock pivot axis consists, from a bar of hardened steel, to perform machining operations to define different active surfaces (scope, shoulder, pivots etc.) and then to submit the axis décolleté to heat treatment operations comprising at least one quench to improve the hardness of the axis and one or more income to improve toughness. The heat treatment operations are followed by a rolling operation of the pivots of the axes, an operation consisting in polishing the pivots to bring them to the required dimensions. During the rolling operation the hardness as well as the roughness of the pivots are further improved. Note that this rolling operation is very difficult or impossible to achieve with materials whose hardness is low, that is to say less than 600HV

The pivot axes, for example the balance shafts, conventionally used in mechanical watch movements are made in grades of free cutting steels which are generally carbon martensitic steels including lead and manganese sulphides to improve their performance. machinability. A steel of this type is known, designated 20AP is typically used for these applications.

This type of material has the advantage of being easily machinable, in particular to be able to bar-turning and has, after treatments of quenching and tempering, high mechanical properties very interesting for the realization of horological pivot axes. In particular, these steels exhibit high wear resistance and hardness after heat treatment. Typically the hardness of the pivots of an axis made of steel AP may reach a hardness exceeding 700 HV after heat treatment and rolling.

Although providing satisfactory mechanical properties for the horological applications described above, this type of material has the disadvantage of being magnetic and of being able to disrupt the running of a watch after being subjected to a magnetic field, and in particular when this material is used for producing a balance shaft cooperating with a balance spring of ferromagnetic material. This phenomenon is well known to those skilled in the art and is for example described in Swiss Annual Chronometry Newsletter Vol. I, pages 52 to 74 . It should also be noted that these martensitic steels are also susceptible to corrosion.

Attempts to overcome these drawbacks have been carried out with austenitic stainless steels which have the particularity of being non-magnetic, that is to say of the paramagnetic or diamagnetic or antiferromagnetic type. However, these austenitic steels have a crystallographic structure which makes it impossible to not to soak them and achieve hardnesses and therefore wear resistances compatible with the requirements for the realization of clock axes of rotation. One way to increase the hardness of these steels is work hardening, however this hardening operation does not allow to obtain hardnesses greater than 500 HV. Therefore, in the context of parts requiring high resistance to frictional wear and having pivots having little or no risk of deformation, the use of this type of steel remains limited.

Another approach to try to overcome these disadvantages has been to deposit on the pivot axes of the hard layers of materials such as the amorphous carbon known as diamond like carbon (DCL). However, there have been significant risks of delamination of the hard layer and therefore the formation of debris that can circulate inside the watch movement and come to disrupt the operation of the latter, which is not satisfactory.

Yet another approach has been considered to overcome the disadvantages of austenitic stainless steels, namely the surface hardening of these pivot axes by nitriding, carburizing or nitrocarburizing. However, these treatments are known to cause a significant loss of corrosion resistance due to the reaction of nitrogen and / or carbon with the chromium of the steel and the formation of chromium nitride and / or carbide. chromium causing a localized depletion of the matrix in chromium, which is detrimental to the desired watchmaking application. The document CH 694 465 describes a method of hardening the teeth of a wheel by ion implantation. We also know of the document CH 554 501 an axis for a measuring device with improved resistance to wear and corrosion. To do this, it is proposed to replace pins in stainless steel or cobalt alloy, hard metal or ruby by a soft iron axis having a hard surface layer of titanium nitride.

The document CH 477 718 describes a process for carbonitriding an austenitic steel for a watch case.

Summary of the invention

The aim of the present invention is to overcome all or some of the aforementioned drawbacks by proposing a pivot axis that makes it possible at the same time to limit the sensitivity to magnetic fields and to to obtain an improved hardness compatible with the requirements of resistance to wear and shocks in the field of watchmaking.

The invention also aims to provide a non-magnetic pivot axis having improved corrosion resistance.

The invention also aims to provide a non-magnetic pivot axis that can be manufactured simply and economically.

For this purpose, the invention relates to a metal pivot pin for a watch movement according to claim 1 of the patent.

As a result, a surface area where the entire axis is hardened, i.e. the core axis may remain little or not changed. By this selective hardening of portions of the axis, the pivot axis can accumulate advantages such as low sensitivity to magnetic fields, and in the main stress zones, hardness, in addition to good corrosion resistance while maintaining a good general tenacity. Moreover, the use of such austenitic steel is advantageous in that it has a high machinability.

• la profondeur prédéterminée représente entre 5% et 40% du diamètre d total du pivot, typiquement entre 5 et 35 microns;
• la surface externe durcie comporte des atomes diffusés d'un élément chimique, ledit au moins élément chimique étant un non-métal et de préférence de l'azote et/ou du carbone ;
• la surface externe durcie comporte une dureté supérieure à 1000 HV ;

According to other advantageous features of the invention:

• the predetermined depth is between 5% and 40% of the total diameter of the pivot, typically between 5 and 35 microns;
• the cured outer surface comprises diffused atoms of a chemical element, said at least one chemical element being a non-metal and preferably nitrogen and / or carbon;
• the cured outer surface has a hardness greater than 1000 HV;

In addition, the invention relates to a watch movement, characterized in that it comprises a pivot axis according to one of the preceding variants, and in particular a balance shaft, an anchor rod and / or an escape pinion comprising an axis according to one of the preceding claims.

Finally, the invention relates to a method of manufacturing a pivot axis according to claim 9 of the patent.

Therefore, by diffusion of atoms in the steel or in the alloy of cobalt or nickel, a surface area or all of the pivots is hardened without having to deposit a second material over the pivots. Indeed, the hardening is carried out directly in the material of the pivot axis which advantageously allows according to the invention to avoid any subsequent delamination as may occur in the case of the deposition of a hard layer on the axis .

In addition, this thermochemical treatment which aims to diffuse carbon and / or nitrogen atoms in the interstitial sites of the alloy does not form in principle carbides and / or nitrides that could degrade the corrosion resistance of the axes of the alloy. pivoting.

• la profondeur prédéterminée représente entre 5% et 40% du diamètre d total du pivot ;
• les atomes comportent au moins un élément chimique, ce dernier étant de préférence un non-métal comme de l'azote et/ou du carbone ;
• l'étape b) effectue un traitement thermochimique de diffusion ;
• l'étape b) effectue un processus d'implantation ionique et de traitement de diffusion ;
• les pivots sont roulés ou polis après l'étape b).

According to other advantageous features of the invention:

• the predetermined depth is between 5% and 40% of the total diameter of the pivot;
• the atoms comprise at least one chemical element, the latter preferably being a non-metal such as nitrogen and / or carbon;
• step b) performs a thermochemical diffusion treatment;
• step b) performs a process of ion implantation and diffusion processing;
• the pivots are rolled or polished after step b).

Brief description of the drawings

• la figure 1 est une représentation d'un axe de pivotement selon l'invention ;
• la figure 2 est une coupe partielle d'un pivot d'axe de balancier selon l'invention après l'opération de traitement de diffusion et avant l'opération de roulage ou de polissage.
• la figure 3 est une coupe partielle similaire à celle de la figure 2 illustrant un pivot après l'opération de traitement de diffusion et après l'opération de roulage ou de polissage.
• Les figures 4 et 5 sont des graphes illustrant le profil de dureté du bord en direction du coeur d'un pivot d'axe de balancier selon l'invention, après l'opération de diffusion, respectivement avant et après l'opération de roulage ou polissage.

Other particularities and advantages will emerge clearly from the description which is given hereinafter, by way of indication and in no way limiting, with reference to the appended drawings, in which:

• the figure 1 is a representation of a pivot axis according to the invention;
• the figure 2 is a partial section of a balance shaft pivot according to the invention after the diffusion treatment operation and before the rolling or polishing operation.
• the figure 3 is a partial section similar to that of the figure 2 illustrating a pivot after the diffusion treatment operation and after the rolling or polishing operation.
• The Figures 4 and 5 are graphs illustrating the hardness profile of the edge in the direction of the core of a balance shaft pivot according to the invention, after the diffusion operation, respectively before and after the rolling or polishing operation.

Detailed Description of the Preferred Embodiments

The invention relates to a piece for a watch movement and in particular to a non-magnetic pivoting axis for a mechanical clockwork movement.

The invention will be described below in the context of an application to a non-magnetic balance shaft 1. Of course, other types of clockwise pivot axes can be envisaged, such as, for example, axes of watchmakers, typically escape gears, or else anchor rods.

Referring to the figure 1 we see a balance shaft 1 according to the invention which comprises a plurality of sections 2 of different diameters defining typically spans 2a and shoulders 2b arranged between two end portions defining pivots 3. These pivots are each intended to rotate in a bearing typically in an orifice of a stone or ruby.

With the magnetism induced by the objects encountered on a daily basis, it is important to limit the sensitivity of the pendulum axis 1, otherwise it will influence the operation of the timepiece in which it is incorporated.

Surprisingly, the invention solves both problems at the same time without compromise and providing other benefits. Thus, the metal 4 of the axis 1 is of the steel of the austenitic type and, preferably, stainless, in order to advantageously limit its sensitivity to magnetic fields. In addition, at least the outer surface 5 of the pivots ( Figures 2 and 3 ) is hardened relative to the rest of the balance shaft to a predetermined depth to provide, advantageously according to the invention, a high hardness at said outer surface while maintaining a high tenacity.

Indeed, according to the invention, we have been able to obtain hardening of the external surface of the pivots 3 greater than 1000 HV. The above values were obtained from austenitic stainless steel nickel chromium comprising at least 16.5% of Cr and 10% Ni of type 316L (DIN X2CrNiMo17-12-2 + Su + Cu) with addition of sulfur and manganese sulfide. Of course, other austenitic steels are possible since the proportion of their component gives them paramagnetic, diamagnetic or antiferromagnetic properties and good machinability.

It has been shown empirically that a curing depth of between 5% and 40% of the total diameter of the pivots 3 is sufficient for application to a balance shaft. For example, if the radius d / 2 is 50 μm , the depth of hardening is preferably around 15 μm around the pivots 3. Of course, depending on the application, a different depth of hardening between 5 % and 80% of the total diameter d can be provided.

Preferably, according to the invention, the hardened outer surface of the pivots 3 comprises diffused atoms of at least one non-metal such as nitrogen and / or carbon. Indeed, as explained below, by interstitial supersaturation of atoms in the steel 4, a surface zone 5 is cured without having to deposit a second material over the pivots 3. In fact, the curing is carried out directly in the material 4 pivots 3 which advantageously according to the invention to avoid any subsequent delamination during use.

Consequently, at least one surface zone 5 is hardened, that is to say the core of the pivots 3 and / or the remainder of the axis, can remain little or not modified without significant modification of the mechanical properties of the axis. 1. By this selective hardening of the pivots 3 of the balance shaft 1 allows to cumulate the advantages as the low sensitivity to magnetic fields, a hardness is a high tenacity, in the main stress zones while having a good resistance corrosion and fatigue.

1. a) former un axe de balancier 1 à base d'un acier du type austénitique pour limiter sa sensibilité aux champs magnétiques et comportant un pivots 3 à chacune de ses extrémités;
2. b) diffuser des atomes selon une profondeur prédéterminée au moins à la surface externe 5 des pivots 3 afin de durcir les pivots au niveau des zones de contraintes principales.

The invention also relates to the method of manufacturing a balance shaft as explained above. The process advantageously comprises according to the invention the following steps:

1. a) forming a balance shaft 1 based on an austenitic type steel to limit its sensitivity to magnetic fields and having a pivots 3 at each of its ends;
2. b) scattering atoms at a predetermined depth at least to the outer surface 5 of the pivots 3 in order to harden the pivots at the main stress zones.

According to a first preferred embodiment, the pivots 3 are rolled or polished after step b) in order to reach the desired final dimensions and surface condition for the pivots 3. This rolling operation after treatment makes it possible to obtain pins having wear resistance and shock improved compared to axes whose pivots have only undergone the hardening operation.

By observing the graphs illustrated in Figures 4 and 5 , which were made on the basis of balance shafts which have undergone the diffusion treatment of step b) on all of its surfaces it is found that the surface hardness of the axis, including the surface of its pivots 3, reaches a hardness of the order of 1300HV (curve A figure 4 ). It is also unexpectedly found that the rolling operation which has removed part of the surface layer 5a (the dark layer at figure 2 ) also eliminated the hardest part of the superficial layer 5 of the pivots 3 but the surface hardness of the pivots 3 (curve B Figure 5 ) remains advantageously greater than 1000HV, which gives the pivots 3 very satisfactory wear resistance properties for the application in question.

Advantageously according to the invention, whatever the embodiment, the method can be applied in bulk. Thus, step b) may consist of a thermochemical treatment such as a carburizing or nitriding of several balance shafts and / or several blanks of balance shafts. It is understood that step b) may consist in interstitially diffusing in the steel 4, atoms of a chemical element preferably a non-metal such as nitrogen and / or carbon. Finally, advantageously, it has been found that the compressive stresses of the process improve the fatigue strength and the impact strength.

Step b) could also consist of an ion implantation process and / or diffusion heat treatment. This variant has the advantage of not limiting the type of diffused atoms and allowing an interstitial as well as a substitutional diffusion.

Of course, the present invention is not limited to the illustrated example but is susceptible of various variations and modifications that will occur to those skilled in the art. In particular, it may be possible to totally or almost totally treat the pivots 3, that is to say treat a percentage greater than 80% of the diameter d pivots 3 even if this is not necessary for the application to pivot axes such as axes of watchmakers' pendulums.

The invention also provides that the base material for producing a pivot axis may also be a cobalt alloy of the austenitic type comprising at least 39% cobalt, typically an alloy known under the designation DIN K13C20N16Fe15D7 typically having 39% of Co, 19% Cr, 15% Ni and 6% Mo, 1.5% Mn, 18% Fe and the balance of additives or an austenitic nickel alloy comprising at least 33% nickel typically one alloy known as MP35N® typically having 35% Ni 20% Cr, 10% Mo, 33% Co and the balance of additives.

Claims (13)

1. Metal pivot pin (1) for a timepiece movement including at least one pivot (3) at at least one of the ends thereof at least the outer surface (5) of said at least one pivot (3) is hardened to a predetermined depth relative to the core of the pin via atoms, characterized in that the metal is an austenitic steel, an austenitic cobalt alloy or an austenitic nickel alloy so as to limit the sensitivity of the pin to magnetic fields and in that the atoms are carbon or nitrogen atoms in interstitial diffused sites of the alloy.
2. Pivot pin (1) according to claim 1, characterized in that the predetermined depth represents between 5% and 40% of the total diameter ( d ).
3. Pivot pin (1) according to one of the preceding claims, characterized in that the hardened outer surface (5) has a hardness of more than 1000 HV.
4. Pivot pin (1) according to one of the preceding claims, characterized in that the metal forming the pin (1) is selected from among the group comprising austenitic chromium-nickel stainless steels including at least 16.5% Cr and 10% Ni, austenitic cobalt steels including at least 39% cobalt, and austenitic nickel steels including at least 33% nickel.
5. Pivot pin (1) according to claim 4, characterized in that the metal forming the pin is selected from among the group including X2CrNiMo17-12-2+Su+Cu austenitic steel, K13C20N16Fe15D7 austenitic cobalt alloy, and the austenitic nickel alloy having a composition of 35% Ni 20% Cr, 10% Mo, 33% Co and the remainder comprised of additives.
6. Pivot pin (1) according to one of the preceding claims, characterized in that the pin has two pivots (3).
7. Movement for a timepiece characterized in that the movement includes a pivot pin (1) according to one of the preceding claims 1 to 6.
8. Movement for a timepiece according to claim 7 characterized in that the pivot pin is a balance staff (1), a pallet staff and/or an escape pinion.
9. Method of fabricating a pivot pin (1) comprising the following steps:

   a) forming a pivot pin (1) from a base of austenitic steel (5), an austenitic cobalt alloy or an austenitic nickel alloy, to limit the sensitivity of the pin to magnetic fields, including at least one pivot (3) at one end of the pin;

   b) diffusing carbon and/or nitrogen atoms in interstitial sites to a predetermined depth at least on the outer surface of said at least one pivot (3) in order to harden the pivot (3) in the main areas of stress while maintaining a high toughness.
10. Method according to claim 9, characterized in that the predetermined depth represents between 5% and 40% of the total diameter (d) of the pivots (3).
11. Method according to one of claims 9 to 10, characterized in that step b) consists of a thermochemical diffusion treatment.
12. Method according to one of claims 9 to 11, characterized in that step b) consists of an ionic implantation process which may or may not be followed by a diffusion treatment.
13. Method according to one of claims 9 to 12, characterized in that the pivots (3) undergo a rolling/polishing step after step b).

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