CH719372B1 - Pivot axis for watch movement. - Google Patents

Abstract

The invention relates to a pivot axis for a watch movement formed in an austenitic stainless steel, characterized in that the crystallographic structure of the austenitic stainless steel has a grain size of less than 40 µm according to the cross section of the grain. The pivot pin is non-magnetic and stainless, while being easily machinable.

Description

Technical area

The present invention relates to a pivot axis, for watch movement, non-magnetic and stainless.

State of the art

[0002] The manufacture of a functional component such as a watchmaker's pivot axis consists, from a hardenable steel bar, in performing bar turning operations to define the various active surfaces (bearing, shoulder, pivots, etc.) then subjecting the turned pin to heat treatment operations comprising at least one quench to improve the hardness of the pin and one or more tempers to improve its toughness. The heat treatment operations are followed by a rolling operation of the axle pivots, an operation consisting in polishing the pivots to bring them to the required dimensions. During the rolling operation, the hardness and the roughness of the pivots are further improved. It will be noted that this rolling operation is very difficult or even impossible to carry out with materials whose hardness is low, that is to say less than 600 HV.

[0003] The pivot axes, for example the balance axes, conventionally used in mechanical watch movements are made in grades of free-cutting steels which are generally martensitic carbon steels including lead and manganese sulphides. to improve their machinability. A steel of this type is known, designated 20AP, is typically used for these applications and marketed by Sandvik.

[0004] Despite the advantages of machinability and hardness, martensitic carbon steel including lead and manganese sulphides has a major drawback: its sensitivity to magnetism. The environment in which watches operate has evolved significantly over the past few decades. Electronic devices and accessories incorporating permanent magnets have multiplied, thus exposing watches, and therefore their regulating organs, to increasingly high and increasingly frequent magnetic fields. The martensitic carbon steel including lead and manganese sulphides commonly used for making balance shafts has magnetic susceptibility and a non-negligible residual field after exposure to an external magnetic field. The proximity of the axis to the hairspring, generally made of ferromagnetic material, makes it a particularly strategic component when it is desired to improve the resistance to magnetism of watches.

[0005] It will be noted that martensitic carbon steels are also sensitive to corrosion. This drawback poses a problem mainly during the stages of manufacture and storage of the axles. In use, the pivot axes of the mechanical movement normally remain confined within the sealed zone of the watch, which does not represent a particularly constraining environment for a material, even oxidizable.

[0006] Attempts to remedy 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 that does not allow them to be quenched and to achieve hardnesses and therefore wear resistances compatible with the requirements required for the production of watchmaking pivot axes while retaining good machinability and an amagnetic character. One way to increase the hardness of these steels is hardening, however this hardening operation makes it possible to obtain hardnesses of the order of 650-700 HV without improving the machinability of the material. Consequently, in the context of parts requiring good machinability combined with high resistance to wear by friction and having to have pivots presenting little or no risk of deformation, the use of this type of steel, with processes conventional, remains limited.

Brief summary of the invention

The present invention relates to a pivot axis for a watch movement, the pivot axis being formed in an austenitic stainless steel, characterized in that the crystallographic structure of the austenitic stainless steel has a grain size of less than 40 μm according to the cross-section of the grain.

Brief description of figures

Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which: FIG. 1 schematically illustrates a rotary hammering device; FIG. 2 schematically illustrates an assembly for machining by material removal; Figure 3 reports the variation of the tangential force coefficient as a function of the cutting speed for different samples; FIG. 4 reports the variation of the coefficient of axial force as a function of the cutting speed for different samples; Figure 5 reports the variation of the tangential stress coefficient as a function of the depth of cut for different samples; and Figure 6 reports the variation of the axial force coefficient as a function of the depth of cut for different samples.

Example(s) of embodiment of the invention

The present invention relates to a functional component for a watch movement, in which the functional component comprises a pivot pin. The pivot pin is formed in an austenitic stainless steel, characterized in that the crystallographic structure of the austenitic stainless steel has a grain size of less than 40 µm according to the cross section of the grain.

[0010] A non-claimed method for obtaining the functional component intended for horological or micromechanical applications, comprises the following steps: supplying raw material formed of a bar of an austenitic stainless steel alloy and having a first hardness; performing a work hardening step of said material so as to obtain a semi-finished product having a second hardness higher than the first hardness; and shaping the semi-finished product so as to obtain said component.

[0011] The hardening step includes at least one cycle of severe plastic deformation so that the crystallographic structure of the alloy is transformed into a structure with an average grain size of less than 40 μm, or even preferentially less than 20 μm, according to the cross-section of the grain.

[0012] The hardening step comprising at least one cycle of severe plastic deformation results in a grain structure where the size of the grains is not necessarily equal in the three dimensions of the grain. In particular, the hardening step comprising at least one severe plastic deformation cycle can produce grains having an elongated shape. For example, the smallest dimension in the case of an elongated grain is its cross section while the largest dimension is its longitudinal section. In the following text, the grain size is specified relative to the cross section of the grains.

[0013] The hardening step comprising at least one severe plastic deformation cycle makes it possible to reduce the size of the grain, making it possible to harden the material and to facilitate its machinability.

According to one embodiment, the raw material is the nickel-free austenitic stainless steel alloy, in particular Biodur® 108 with the following chemical composition (% by weight): max 0.08% C, max 0.75% Si, between 21.00% and 24.00% Mn, max 0.030% P, max 0.010% S, between 19.00% and 23.00% Cr, max 0.10% Ni, between 0.50% and 1.50% Mo, 0.90% N, 0.25% Cu, the balance being the F.

It will be understood that other grades of austenitic stainless steels can also be envisaged without departing from the scope of the invention. For example, the raw material can be made of a ferromanganese-chromium alloy, an austenitic CrMnMoN Ni-free steel, a P2000® alloy composed (%wt) of 16 to 20% Cr, 12 to 16 Mn, 2.5 to 4.2% of Mo, less than 0.15% C, less than 0.25 Nb, 0.75 to 1.00% N and the remainder being Fe.

[0016] The hardness of the Biodur® 108 alloy is typically around 43 to 50 HRC and 430 to 700 HV10.

[0017] In the context of severe plastic deformation, a very strong hydrostatic stress is introduced during implementation, delaying or even preventing the localization of the deformation and therefore the appearance of cracks. The deformation is more homogeneous than for conventional techniques such as rolling or drawing, where a texture linked to the direction of deformation remains. In the usual cold deformation techniques, the hardening is generated by the creation of dislocations (Frank-Read sources) which will pile up on the initial grain boundaries, to gradually form a structure of sub-boundaries (or cells) whose walls contain a very high density of dislocations. In materials that have undergone severe plastic deformation, the deformation is such that new grains are formed, with grain boundaries sharper than the cell walls and containing few dislocations. It is the very high density of grain boundaries that induces the mechanical properties of materials that have undergone severe plastic deformation. However, the deformation introduced is extremely large. For example, the deformation (or the relative elongation) can go up to 5, or even more.

[0018] The work-hardening step includes at least one severe plastic deformation cycle which makes it possible to reduce the size of the grains, thus making it possible to facilitate machinability and to harden the material of the semi-finished product.

[0019] The grain size of a crystallographic structure of an alloy having undergone "at least one severe plastic deformation cycle" can depend on the composition of the alloy and the conditions of the severe plastic deformation method. However, grain size can be verified directly and successfully using tests and procedures known to those skilled in the art and not requiring an unreasonable amount of experimentation. For example, depending on the composition of the Biodur® 108 alloy, the number of severe plastic deformation cycles can be adjusted in order to obtain a structure with an average grain size of less than 40 µm, or even less than 20 µm.

[0020] The severe plastic deformation cycle or cycles are carried out using a rotary swaging method, also known as rotary shrinking.

[0021] Figure 1 schematically illustrates a rotary hammering device 10, capable of carrying out the severe plastic deformation cycle(s) by the rotary hammering method. Hammers 11 are positioned around the raw material, for example a bar 12. The hammers 11 perform radial movements (indicated by arrows 13) at high frequency. The radial movements 13 of the hammers 11 take place in the plane of the section of the material 12, perpendicular to the longitudinal axis 15 of the material 12. For example, the frequency of the radial movements varies from 1,500 to 10,000 per minute depending on the size of the device. The lateral stroke of the 11-stroke hammers is approximately 0.2 to 5 mm. Typically, rotary hammering device 10 comprises four hammers 11. Depending on the application and size of device 10, sets of two, three, four, six or, in special cases, can be used. up to eight hammers 11. To avoid the formation of longitudinal burrs at the interstices between the hammers 11, the device 10 can impart a relative rotational movement between the hammers 11 and the work bar 12. In this case, the hammers 11 rotate around the longitudinal axis 15, in a clockwise or counterclockwise direction. During a deformation cycle, the material 12 is advanced in the direction of the longitudinal axis 15, for example in the direction indicated by the arrow 16.

[0022] As the austenitic steel of the work bar has a fairly high coefficient of friction, it is advantageous to use suitable lubrication.

[0023] Table 1 reports various samples of bars in Biodur® 108 alloy having or not having undergone the work hardening stage comprising a number of cycles of severe plastic deformation between 3 and 10.

Samples 1 and 2 correspond to a raw material bar of Biodur® 108 alloy having an initial diameter D1 of 9.53 mm. Sample 3 corresponds to the bar of samples 1 and 2, but having undergone annealing at 1065°C for 1 hour followed by water quenching. Samples 4 to 9 correspond to a bar of raw material in Biodur® 108 alloy of different initial diameters D1 (respectively 4, 5, 6, 7, 8 and 9.53 mm), having undergone the work hardening stage comprising at least one severe plastic deformation cycle. Sample 9 also underwent annealing at 1065°C for 1 hour followed by water quenching. The hardening step comprising at least one cycle of severe plastic deformation results in a final diameter D2 reduced with respect to the initial diameter D1, that is to say before the hardening step. 1 9.53 - 0 No 3 9.53 - 0 1065°C/1h/water 4 4 3.375 3 No. 5 5 3.402 5 No. 6 6 3.410 7 No. 7 8 3.455 9 No. 8 9.53 3.546 10 No. 9 9, 53 3.553 10 1065°C/1h/water

Chart 1

[0025] The step of shaping the semi-finished product may include a machining step by removing material.

[0026] The method may also comprise heat treatment operations comprising at least one anneal to improve the hardness of the component and one or more tempers to improve its toughness.

[0027] The method may also include a component termination step.

[0028] The method may also include a step of straightening the bar.

[0029] The functional component comprises a pivot axis.

[0030] In this case, the method may also comprise a rolling operation of the pivots of said axis, an operation consisting in polishing the pivots to bring them to the required dimensions.

[0031] The functional component may comprise a hardware component, pin or bar, or any functional component intended for horological or even micromechanical applications.

[0032] Table 2 reports the yield strength values Rp0.2 corresponding to the mechanical strength at a relative elongation ε = 0.2%, the maximum tensile strength Rm, the elongation at break A, the true strain (equal to the ratio of the initial diameter D1 to the final diameter D2) and the Vickers hardness HV10 (test load of 98.07 N), for the different samples. 1 1084.1 1324.7 22.6 0 412 3 655.8 1009.3 60.6 0 281 4 1624 1726 14.0 0.17 475 5 1774 1886 8.4 0.39 510 6 1971 2083 8.8 0.56 548 7 1978 2151 8.8 0.84 566 8 2023 2203 7.4 1574 9 1978 2139 9.4 0.99 534

Chart 2

[0033] Cycle(s) of severe plastic deformation by the rotary hammering method lead to microstructural refinement and consequently improved mechanical properties, i.e. exceptionally high strength combined with still decent ductility as well as an increased fatigue limit of the semi-finished product thus obtained.

[0034] Table 3 reports the grain size values for the different samples. Grain size values are reported according to the smallest dimension, i.e. according to the cross-section (trans) as well as the largest dimension, i.e. according to the longitudinal section (long ).

[0035] Table 3 shows that the work hardening step comprising three or more cycles of severe plastic deformation by the rotary hammering method, makes it possible to obtain a crystallographic structure of the Biodur® 108 alloy having a smaller grain size. to 40 µm depending on the cross section. 1 53 84 3 63 64 4 36 59 5 30 99 6 27 144 7 19 152 8 15 281 9 23 323

Chart 3

[0036] Alternatively, the severe plastic deformation cycle(s) can be achieved using one of the methods, or a combination of these methods, including: the ECAP method, equal channel conformal angular extrusion (ECAP- conform), high pressure torsion or high pressure tube twisting, HPT or HPTT, accumulative roll bonding (ARB), repetitive corrugation and straightening (RCS), asymmetric rolling , ASR), cyclic extrusion-compression (CEC), rotary hammering, or any other suitable method to obtain said ultrafine grain structure.

[0037] Depending on the method with which the severe plastic deformation cycle or cycles are carried out, the size and the shape/geometry of the grains can change. For example, for processes derived from rolling and drawing, the grains will have an elongated shape. In all cases their size will be reduced compared to that of the raw material.

[0038] Machining tests on a lathe were carried out in order to compare the machinability of the semi-finished product corresponding to samples 1, 6 and 8 with the machinability of a 20AP free-cutting steel bar. The machining tests were carried out by removing material from the bar 20 using a cutting tool 21 (see FIG. 2). The cutting speed Vc of the bar 20 was varied and the cutting forces were measured. As illustrated in FIG. 2, the cutting force F exerted by the workpiece (bar 20) on the tool (the knife 21) admits three components according to the three orthogonal directions: the tangential force Ftdû to the cutting movement and which corresponds to the torque and the power of the spindle carrying the bar 20, the axial force Ffdû to the forward movement, and the radial force Frdû to the depth of cut f. The depth of cut f is the quantity of material that the tool 21 will take up during machining on the lathe. This varies according to the cutting speed Vcaas well as the feedrate Vf of the tool. The most important component is the tangential force Ft which is expressed by the relation: Ft= Kct r f where Kct is the tangential force coefficient which is a function of the thickness of the chip and the material being machined, f is the value of the depth of cut, and r is the value of the feed. It is also possible to define an axial force coefficient Kcf and a radial force coefficient Kcap. The axial and radial force coefficients increase sharply with tool wear.

[0039] Figure 3 reports the variation of the tangential force coefficient Kcten as a function of the cutting speed Vc for the samples (Éch) 1, 6 and 8 as well as for the steel bar of free-cutting 20AP.

[0040] Figure 4 reports the variation of the axial force coefficient Kcfen as a function of the cutting speed Vc for the samples (Éch) 1, 6 and 8 as well as for the steel bar of free-cutting 20AP.

[0041] Figure 5 reports the variation of the tangential force coefficient Kcten as a function of the depth of cut f for the samples (Éch) 1, 6 and 8 as well as for the steel bar of free-cutting 20AP.

[0042] Figure 6 reports the variation of the axial force coefficient Kcfen as a function of the depth of cut f for the samples (Éch) 1, 6 and 8 as well as for the steel bar of free-cutting 20AP.

[0043] Figures 3 to 6 show that the process makes it possible to obtain for the semi-finished product in Biodur® 108 machining characteristics which are close to those of the 20AP free-cutting steel alloy and greatly improved compared to that of Biodur® 108 which has not undergone at least one severe plastic deformation cycle.

Reference numbers used in the figures

[0044] 10 rotary hammering device 11 hammer 12 raw material 13 radial movements 15 longitudinal axis 16 direction of advance 20 bar 21 machining cutting tool f depth of cut F cutting force Fttangential force Ffaxial force Frradial force Kcfforce coefficient axial Kct tangential force coefficient r feed value Vccutting speed Vffeed speed

Claims (8)

1. Pivot axis for a watch movement, the pivot axis being formed in an austenitic stainless steel, characterized in that the crystallographic structure of the austenitic stainless steel has a grain size of less than 40 μm according to the cross section of the grain . 2. Pivot pin according to claim 1, wherein the austenitic stainless steel is nickel-free. 3. Pivot pin according to claim 1 or 2, having a hardness between 475 and 574 HV10. 4. Pivot axis according to one of claims 1 to 3, having a maximum tensile strength Rmcomprise between 1726 and 2203 MPa. 5. Pivot pin according to one of claims 1 to 4, having a yield strength value Rp0.2, corresponding to the mechanical strength at relative elongation ε=0.2%, of between 1624 and 2023 MPa. 6. Pivot pin according to one of claims 1 to 5, wherein the composition of the austenitic stainless steel is (% weight): max 0.08% C, max 0.75% Si, between 21.00% and 24.00% Mn, max 0.030% P, max 0.010% S, between 19.00% and 23.00% Cr, max 0.10% Ni, between 0.50% and 1.50% Mo, 0.90% N, 0.25% Cu, the balance being Fe. 7. Pivot pin according to one of claims 1 to 5, in which the composition of the austenitic stainless steel is (% by weight): from 16% to 20% Cr, from 12% to 16% Mn, from 2.5% to 4.2% Mo, less than 0.15% C, less than 0.25% Nb, 0.75% to 1.00% N and the balance being Fe. 8. Pivot pin according to one of claims 1 to 7, wherein the grain size is less than 20 µm according to the cross section of the grain.