NON-MAGNETIC TIMEPIECE PARTS AND THERMOMECHANICAL TREATMENT METHOD FOR OBTAINING SAID PARTS

Abstract

A non-magnetic part including an austenitic alloy, the austenitic alloy including between 50 and 85 wt % of iron, one or more gammagene elements the weight percentage or the total weight percentages of which amount to between 15 and 35 wt %, and less than 2 wt % of nitrogen. The austenitic alloy has a crystallographic structure including a predominantly cubic crystal structure and the presence of a hexagonal crystal structure. The magnetic part includes a hardness gradient in the direction extending radially from the surface of the at least one portion of the non-magnetic part to the inside of the non-magnetic part, the hardness gradient having a value greater than or equal to 100 HV.

Claims

1. A non-magnetic part comprising an austenitic alloy, said austenitic alloy comprising, in weight percentage, iron between 50 and 85%, one or more gammagenic elements whose weight percentage or the sum of the weight percentages is between 15 and 35% and nitrogen at a weight percentage between 0.1% and 2%; said austenitic alloy has a crystallographic structure comprising a majority cubic crystal structure and a presence of a hexagonal crystal structure; and the non-magnetic part comprises a hardness gradient along the direction radially extending from the surface of at least one portion of the non-magnetic part inwardly of the non-magnetic part, said hardness gradient having a value greater than or equal to 100 HV where HV is Vickers hardness.

2. The non-magnetic part according to claim 1, wherein at least one portion of a surface of the non-magnetic part has a hardness greater than or equal to 700 HV.

3. The non-magnetic part according to claim 1, wherein the surface layer radially extends from the at least one portion of the surface inwardly of the non-magnetic part over a distance, referred to as the surface layer thickness, of less than 30 m.

4. The non-magnetic part according to claim 1, comprising a central portion extending from the surface layer inwardly of the non-magnetic part, said central portion having a hardness less than or equal to 600 HV.

5. The non-magnetic part according to claim 1, wherein the non-magnetic part is a precision timepiece.

6. The non-magnetic part according to claim 5, wherein the timepiece is a balance wheel, a pallet staff or an escape pinion.

7. A use of the non-magnetic part according to claim 1, for its non-magnetic and/or hardness and/or tribological and/or fracture resistance and/or resilience properties.

8. A method for manufacturing a non-magnetic part according to claim 1, said method comprising: a step of obtaining a mechanical part, at least one portion of a surface of the mechanical part having a hardness greater than 350 HV where HV is the Vickers hardness; a surface cold working step to form a surface layer radially extending from the at least one portion of the surface of the mechanical part inwardly of the mechanical part; the surface layer comprises a cold working rate gradient, along the direction radially extending from a surface of at least one portion of the non-magnetic part inwardly of the non-magnetic part, having a value greater than 14%; and a step of heating the at least one portion of the surface of the cold worked mechanical part to a temperature of between 350 C. and 700 C. to harden the cold worked portion or portions of the mechanical part; the surface layer, after heating, has a hardness gradient, along the direction radially extending from the surface of the at least one portion of the non-magnetic part inwardly of the non-magnetic part, having a value greater than or equal to 100 HV.

9. The method according to claim 8, wherein the heating step: is implemented for a duration of between 10 minutes and 400 hours, and/or comprises a temperature gradient of between 4 C./min and 400 C./min, and/or is implemented under ambient conditions.

10. The method according to claim 8, wherein the step of obtaining the mechanical part comprises: a step of bar turning at least one portion of a turning bar to form the mechanical part, or a step of cold working at least one portion of a raw bar to form the mechanical part.

11. The method according to claim 8, wherein the step of obtaining the mechanical part comprises: a step of bar turning at least one portion of a turning bar followed by a step of cold working the at least one turned portion of the turning bar to form the mechanical part, or a step of cold working at least one portion of a raw bar followed by a step of bar turning at least one portion of the cold worked raw bar to form the mechanical part.

12. The method according to claim 10, wherein the step of cold working the at least one portion of the raw bar or the at least one portion of the turning bar or the at least one turned portion of the turning bar is a drawing step to decrease a diameter of the at least one portion of the raw bar or of the at least one portion of the turning bar or of the at least one turned portion of the turning bar.

13. The method according to claim 8, comprising a smoothing step to decrease a roughness of the at least one portion of the surface of the mechanical part.

14. The method according to claim 13, wherein the smoothing step and the surface cold working step are carried out simultaneously in a single step.

15. The method according to claim 13, wherein the surface cold working step and the smoothing step are a roll bending or roller burnishing.

Description

DESCRIPTION OF THE FIGURES

[0202] Further advantages and features of the invention will become apparent upon reading the detailed description of non-limiting implementations and embodiments, and the following appended drawings:

[0203] FIG. 1 shows scanning electron microscopy images of a balance staff,

[0204] FIG. 2 is a diagram illustrating the hardness of bars made of 20AP and FINEMAC steels before and after implementation of the method for manufacturing hardened precision timepieces in the state of the art for an applied load of 0.5 kg,

[0205] FIG. 3 illustrates the course of the induced moment in an annealed 316L steel part 513, in a cold worked 316L steel part 511, in an annealed nickel-free alloy A1 514 and in a cold worked nickel-free alloy A1 512 as a function of the magnetic field applied,

[0206] FIG. 4 illustrates a magnification of the curves 511, 512 and 514 of FIG. 3,

[0207] FIGS. 5a and 5b are, respectively, scanning electron microscopy images of a bar turned part and of a bar turned and smoothed part,

[0208] FIG. 6 is a diagram representing the hardness, for an applied load of 1 kg, of raw bars made of alloys A1 cold worked to a cold working rate of 85% and the hardness 612 of raw bars made of alloys A2 cold worked to a cold working rate of 85% as a function of the heating temperature and for a heating time of one hour,

[0209] FIG. 7 illustrates the course of hardness, for an applied load of 1 kg, as a function of the cold working rate of bars comprised of the alloy A1 for different bar heating temperatures.

[0210] FIG. 8 illustrates the course of the hardness, for an applied load of 1 kg, of a bar comprised of alloy A1 cold worked to a cold working rate of 85% as a function of the heating time, for a heating temperature of 575 C.,

[0211] FIGS. 9a and 9b set forth, respectively, the equivalent hardness HV1 measured at the surface and at different depths in an alloy A1 bar cold worked and then heated to a temperature of 525 C. and respectively in an alloy A1 bar cold worked by drawing and then surface cold worked by machining and then heated to a temperature of 525 C.,

[0212] FIG. 10 shows bright-field transmission electron microscopy images and crystallographic analysis of a part manufactured by the method according to the invention,

[0213] FIG. 11 is a graph illustrating the course of the hardness of a non-magnetic part, obtained by the method according to the invention, as a function of the cold working rate of the mechanical part, from which the non-magnetic part is obtained, and of the duration of the heating step,

[0214] FIG. 12 is a scanning electron microscopy image of a cross-section of a non-magnetic part according to the invention, on which the surface layer and the central portion of the non-magnetic part are visible,

[0215] FIG. 13 is a scanning electron microscopy image of a cross-section of the surface portion of a non-magnetic part according to the invention, showing the structure of the surface layer which has been cold worked and then heated,

[0216] FIG. 14 is a scanning electron microscopy image in thin-film backscattered electron diffraction mode of a cross-section of the surface portion of a non-magnetic part according to the invention, on which the reformed austenitic domains, the superstructures and the nitride precipitates are visible.

DESCRIPTION OF THE EMBODIMENTS

[0217] As the embodiments described below are by no means limiting, it will be possible especially to consider alternatives to the invention comprising only a selection of the characteristics described, isolated from the other characteristics described (even if this selection is isolated within a sentence comprising these other characteristics), if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention from the state of prior art. This selection comprises at least one characteristic, preferably functional without structural details, or with only part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention from the state of prior art.

[0218] The embodiment set forth is directed to the manufacture of a non-magnetic part of revolution 1. In a non-limiting illustration, the part manufactured may be a clockwork balance wheel 1 or balance staff 1 as represented in FIG. 1. FIG. 1 sets forth an image of a conventional balance wheel 1. A balance wheel 1 is a part of revolution comprising an axis of revolution 2. Each of the two ends 112 of the balance wheel 1 forms a pivot zone 112 for forming a friction zone 112. The diameter of the pivot zones 112, radially measured with respect to the axis of revolution 2, is approximately 60 m.

[0219] For the manufacture of the balance wheel 1 and other precision timepieces that should have particular mechanical properties, in particular good resistance to impact, fracture, deformation and wear, steel with the trade name with the abbreviation DIN 1.1268+Pb is known in the state of the art, comprising, in weight percentage, 1% carbon, 0.4% manganese, 0.2% silicon, sulphur, 0.2% lead, less than 0.03% phosphorus and the balance iron. Steel with the trade name FINEMAC and the abbreviation DIN 1.1268, which is an alternative to 20AP and which comprises, in weight percentage, 1% carbon, 0.5% manganese, 0.27% silicon, 0.1% sulphur, containing no lead, less than 0.03% phosphorus and the balance iron is also known.

[0220] In the state of the art, the usual method for manufacturing the balance wheel 1 and other precision timepieces is known, comprising machining a raw bar made of 20AP or FINEMAC steel, followed by a hardening heat treatment. The hardening heat treatment comprises heating to a temperature above 700 C., typically in the order of 800 C., for 15 minutes followed by water quenching the part followed by tempering at a temperature below 300 C., typically 175 C. for 30 minutes to adjust the hardness and relax the stresses generated during quenching. This hardening heat treatment is followed by a final step of smoothing the part manufactured, for example by roll bending aiming at improving the surface finish of the part.

[0221] FIG. 2 shows a diagram illustrating the Vickers hardness (HV) measured on 2 mm diameter bars made of 20AP and FINEMAC steels before 441, 442 and after 443, 444 implementing the state-of-the-art hardening heat treatment for an applied load of 0.5 kg. Bars 441 and 442 respectively illustrate hardness of the raw bar of 20AP steel, before implementation of the state of the art hardening heat treatment, and respectively the hardness of the raw bar of FINEMAC steel, before implementation of the hardening heat treatment. These measurements have been obtained from the data (time-temperature) set out in the state of the art. Bars 443 and 444 in FIG. 2 respectively illustrate the hardness of the 20AP steel balance obtained by implementing the hardening heat treatment of the state of the art and the hardness of the FINEMAC steel balance obtained by implementing the hardening heat treatment of the state of the art respectively. The hardness values of the raw bars are in the order of 300 HV.sub.0.5 and the hardness of the balance wheels is less than or equal to 700 HV.sub.0.5.

[0222] Alloys of the state of the art have been ruled out due to the excessive residual magnetisation that appears after cold working these alloys. In particular, the current standard stipulates that a watch should not have its chronometric quality degraded when exposed to magnetic fields of 60 Gauss. However, electromagnetic pollution has increased steadily in recent decades and our apparatuses and watches are now constantly exposed to strong magnetic fields, for example a smartphone now emits an average of 80 gauss. There is therefore a need to find an alternative to state-of-the-art alloys.

[0223] In the course of this research, the inventors observed that some austenitic alloys can be used, in a counter-intuitive way, when used under the conditions of the method according to the invention, for the manufacture of parts requiring significant machining and/or hardening. Indeed, austenitic alloys are known to be difficult to machine and are therefore not used when extensive machining and/or several machining steps are required. According to the invention, the austenitic alloys chosen to make up the precision timepiece comprise, in weight percentage, iron between 50 and 85%, one or more gammagenic elements whose weight percentage or the sum of the weight percentages is between 8 and 38%.

[0224] The effect of cold working on the residual magnetisation of an austenitic alloy with the trade name 316L has been evaluated. This effect is set forth in FIGS. 3 and 4. FIGS. 3 and 4 illustrate the magnetic susceptibility of austenitic alloys, that is the course of the induced magnetic moment in emu/g as a function of the applied field in Tesla, and the residual magnetisation of these austenitic alloys. The 316L alloy comprises, in weight percentage, between 16 and 19% chromium, between 9 and 13% nickel, between 1.5 and 3% molybdenum, less than 2% molybdenum, less than 0.01% manganese, less than 0.03% carbon, less than 0.005% sulphur, less than 0.003% nitrogen, less than 0.002% oxygen and the balance iron. FIGS. 3 and 4 illustrate the course of the induced magnetic field of 316L after annealing 511 at a temperature of 1050 C. for 30 minutes and of drawn 316L 513 at a cold working rate of 60%. The relative permeability, noted .sub.r, of 316L after drawing 513 at a cold working rate of 60% is 8.8 and the relative permeability of 316L after annealing 511 at a temperature of 1050 C. for 30 minutes is 1.08. It is noticed that the value of the residual magnetisation is greater than ten emu/g for cold worked 316L 513. These residual magnetisation values are incompatible with applications in the watchmaking field and do not allow this type of alloy to be used as a non-magnetic part and, in particular, as a precision timepiece.

[0225] According to the invention, austenitic alloys are used, counter-intuitively and surprisingly, when implemented under the conditions of the method, to manufacture parts with good mechanical properties, in particular good resistance to impact, fracture, deformation and wear. Indeed, it is known that the hardening heat treatments of the state of the art detailed above (heating to a temperature above 750 C. followed by quenching and tempering) are not effective on austenitic alloys.

[0226] It is known in the state of the art that the mechanical properties of iron-based alloys, in particular good resistance to impact, fracture, deformation and wear, are conferred by the presence of nickel in the alloy. According to the invention, the inventors have observed, surprisingly and counter-intuitively, that austenitic alloys not comprising nickel can be used for the manufacture of parts which need to have good mechanical properties when they comprise nitrogen in a weight percentage greater than 0.1% and less than 2% and when they are used under the conditions of the method according to the invention.

[0227] In accordance with a non-limiting embodiment set forth, two particular alloys have been chosen to study effect of the method and to study the alloy and part manufactured by the method according to the invention: an austenitic alloy, referred to as A1, comprising, in weight percentage, between 0.15 and carbon, between 9.5 and 12.5% manganese, 16.5% chromium, between 0.45 and 0.55% nitrogen, 2.7% molybdenum and the balance iron and an austenitic alloy, referred to as A2, comprising between 21 and 24% manganese, between 19 and 23% chromium, between 0.5 and 1.5% molybdenum, 0.9% nitrogen, less than 0.08% carbon and the balance iron. The method according to the invention does not cause any significant change in the composition of the alloy making up the mechanical part or the raw bar used for implementing the method. Consequently, the precision timepiece obtained by implementing the method according to the invention comprises the same composition as that of the alloy making up the mechanical part or the raw bar used (A1 and A2 according to the non-limiting embodiment set forth).

[0228] The effect of cold working on the residual magnetisation of the A1 and 316L alloys is set forth in FIGS. 3 and 4. Curves 512 and 514 represent the respective course of the magnetic moment induced in the alloy A1 after annealing at a temperature of 1050 C. for 30 minutes and drawing at a cold working rate of 72%. The relative permeability, .sub.r, of the alloy A1 after drawing at a cold working rate of 72% is 1.006 and the relative permeability, .sub.r of the alloy A1 after annealing at a temperature of 1050 C. for 30 minutes is 1.002. It is noticed in FIG. 4 that the residual magnetisation values for the annealed A1 512 alloy and the cold worked A1 514 alloy are less than 1.Math.10.sup.2 emu/g. These residual magnetisation values, .sub.r equal to 1.006 and 1.002, are better than those obtained with the 316L alloy, .sub.r equal to 8.8 and 1.08, and make the austenitic alloys according to the invention good candidates for use as non-magnetic parts and, in particular, as precision timepieces.

[0229] In accordance with a preferred but non-limiting embodiment of the invention, the method comprises a step of obtaining a mechanical part, at least one portion of a surface of which has a hardness greater than 350 HV. The mechanical part is a part of revolution, in particular a solid rod. The obtaining step is followed by a surface cold working step aiming at forming a surface layer radially extending from the surface of the mechanical part towards the axis of rotation (and symmetry) of the mechanical part. The surface layer is typically less than 30 m thick. The surface layer exhibits a cold working rate gradient along the direction radially extending from the surface of the cold worked mechanical part inwardly of the cold worked mechanical part. The variation in cold working rate along the thickness of the superficial layer is greater than 18%. In other words, the difference between the cold working rate of the surface of the mechanical part and the cold working rate of the central portion of the mechanical part is greater than 18%. Furthermore, the cold working rate of the surface of the cold worked mechanical part, obtained by implementing the surface cold working step, is greater than 100%. The surface cold working step is followed by a step of heating the cold worked mechanical part to a temperature of between 350 C. and 700 C. to harden the cold worked parts of the mechanical part.

[0230] According to the non-limiting embodiment set forth, the surface cold working step is a turning step which, in addition to surface cold working the mechanical part, has the effect of decreasing roughness of the surface of the mechanical part. FIG. 5b is an image of one end of the raw bar turned, then surface cold worked and simultaneously smoothed by roll bending, which is a particular turning method. The centre line average roughness of the cold worked and smoothed mechanical part obtained is in the order of 0.05 m.

[0231] According to the non-limiting embodiment set forth, the obtaining step of the method comprises manufacturing the mechanical part from a raw bar made of alloy A1 or A2. The obtaining step comprises a step of cold working at least one portion of the raw bar followed by a step of bar turning at least one portion of the cold worked raw bar. The purpose of this cold working step is to increase density of dislocations in the cold worked raw bar, and therefore in the mechanical part. The cold worked raw bar is referred to as the turning bar, and the turned bar, that is the raw bar cold worked and then turned, corresponds to the mechanical part. A bar turned end of the raw bar is set forth in FIG. 5a. The raw bar has the form of a wire (or tube or rod) 2 to 4 mm in diameter, typically 3 mm, and has a hardness in the order of 280 HV. It is worth noting that the raw bar, or the cold worked raw bar, should not have too high a cold working rate, typically less than 50%, for bar turning to be carried out correctly.

[0232] According to the non-limiting embodiment set forth, the raw bar. The cold working step is a drawing step aiming at increasing the hardness of the raw bar. The effect of the cold working step, in this case drawing, is to cold work the raw bar to a cold working rate greater than 30%.

[0233] According to the non-limiting embodiment set forth, the bar turning step is carried out in such a way as to obtain the particular shape of the balance staff 1 as represented in FIG. 1. In particular, the bar turning step is intended to obtain a turned bar with a diameter ranging from 20 to 60 m at the ends 112, corresponding to the pivot zones 112 of the balance wheel 1, and 1.4 mm for the part 113 of the raw bar that has been cold worked and turned and having the largest diameter. The bar turning step further cold works the turning bar (the bar obtained after the drawing step) and substantially modifies the cross-section of the bar turned. Thus, the mechanical part (cold worked and turned bar) has a cross-section that varies along its axis 2 of revolution.

[0234] According to the non-limiting embodiment set forth, the heating step is implemented for a duration of one hour at a temperature below 700 C. with a temperature rise ramp of 50 C./min under ambient conditions. The method according to the invention makes it possible to obtain mechanical properties similar or even better than those obtained by state-of-the-art hardening heat treatments, while eliminating the quenching step required in state-of-the-art hardening heat treatments. The fact that this heating step according to the invention is carried out at low temperature, in particular compared with the temperatures of the state-of-the-art hardening heat treatments, means that there is no stress concentration in the part after the heating step according to the invention. The method according to the invention therefore does not require tempering after the heating step.

[0235] With reference to FIG. 6, the effect of heating on the hardening of alloy A1 and A2 raw bars cold worked by drawing at a rate of 85% is illustrated. FIG. 6 is a diagram in which the hardness 611 of alloy A1 raw bars cold worked at a cold working rate of 85% and the hardness 612 of alloy A2 raw bars cold worked at a cold working rate of 85% is set forth as a function of the heating temperature. The heating time is one hour. A decrease in the effect of heating on hardness 611 and 612 is observed above 520 C. for the alloy A2 and above 650 C. for the alloy A1. It is also noticed that the preferred temperature is between 450 C. and 640 C. and that the optimum temperature is between 500 C. and 600 C.

[0236] With reference to FIG. 7, the effect of the cold working rate on the hardness obtained after heating is illustrated. FIG. 7 represents the course of the hardness as a function of the cold working rate of a bar with a diameter of 3 mm made of alloy A2 for different heating temperatures. The heating time is one hour. The cold working has been carried out by drawing a raw (not cold worked) bar of alloy A1. It is noticed that the higher the cold working level of the bar before heating, the greater the hardening of the cold worked bar. Consequently, to obtain the highest possible hardness in the mechanical part, it is appropriate to cold work the part as much as possible before implementing the heating step, that is heating a part has the highest possible cold working rate. Furthermore, this also implies that the heating step should preferably be implemented as the last step of the method.

[0237] With reference to FIG. 8, the course of the hardness HV1 of an alloy A1 composite bar cold worked to a cold working rate of 85% is illustrated as a function of the heating time for an applied load of 1 kg. The bar is heated to a temperature of 575 C. It is noticed that the hardness is highest for times between 100 and 300 hours. The hardness is greater than 800 HV after 45 hours of heating and 700 HV after 3 hours of heating. The hardness obtained is a function of the cold working rate of the bar before heating and the hardness of the bar before heating. For a given temperature and heating time, the higher the cold working of the bar before heating, the higher the hardness of the bar obtained after heating. Similarly, for a given temperature and heating time, the higher the hardness of the bar before heating, the higher the hardness of the bar obtained after heating.

[0238] FIGS. 9a and 9b illustrate the course of the hardness of alloy A2 bars as a function of the hardness measurement depth. The measurement depth corresponds to the distance measured radially from the outer surface of the bars towards the axis of rotation (or centre) of the bar. The hardness indicated is an equivalent HV1 hardness, that is for a load of 1 kg, calculated from ultranonindentation hardness measurements with an indentation size in the order of 1.5 m.

[0239] FIG. 9a illustrates the course of the hardness of an alloy A2 raw bar cold worked to a 30% rate by drawing and then heated to a temperature of 525 C. for 1 hour. It is noticed that the hardness of the bar is constant and homogeneous over the entire depth explored. The hardness of the bar is approximately 600 HV1.

[0240] FIG. 9b sets forth several series of measurements carried out on an alloy A1 bar cold worked to a 30% rate by drawing, then surface cold worked by machining and then heated to a temperature of 525 C. for 1 hour. After heating the bar, it is noticed that the cold worked and heated part comprises a surface layer having a hardness gradient which decreases along the direction radially extending from the external surface of the part towards the central portion of the part. According to the non-limiting embodiment set forth, the surface layer has a thickness of less than 20 m, the hardness of the surface of the cold worked and heated part is greater than 700 HV1, the central portion has a hardness less than or equal to 400 HV1 and the hardness gradient in the surface layer is greater than 200 HV1. This demonstrates that surface cold working, when followed by heating according to the invention, makes it possible to obtain a surface layer that has a hardness gradient. It also demonstrates that surface cold working generates an average cold working rate gradient in the surface layer that is greater than 18%. The average cold working rate of the central portion is identical to that of the bar not surface cold worked (by machining), that is less than or equal to 85%, it is in the order of 30% according to the embodiment. The average surface cold working rate is greater than 85%. The machining parameters are not optimal and more effective surface cold working can be achieved.

[0241] The fact that the part manufactured according to the method has such a hardness gradient means that the surface hardness of the part is much higher than the hardness of the central portion of the part. The method therefore makes it possible to obtain a part whose central portion retains some ductility and therefore gives the part better resistance to impact, fracture and deformation than a part with a uniform and constant hardness throughout the part.

[0242] Furthermore, the method according to the invention makes it possible to modulate hardness of the central portion of the manufactured part according to the application by modulating the cold working rate of the mechanical part resulting from the obtaining step. It is therefore possible to give the part better resistance to impact, fracture and deformation by having a central portion that is more ductile than the surface of the part.

[0243] Furthermore, it is possible to provide for the step of obtaining the mechanical part to include cold working of the part as a whole, for example by drawing, at a high cold working rate, for example greater than or equal to 85%, to further increase the hardness of the manufactured part.

[0244] The method also makes it possible to obtain a part with very good surface hardness and therefore better resistance to impact and wear.

[0245] Furthermore, when the surface cold working step is carried out using a turning operation to smooth and surface cold work the part, in particular roll bending or roller burnishing, this saves considerable time and energy. In addition, using turning to carry out the surface cold working step also makes it possible to take advantage of the cold working of the part generated by smoothing to further increase the hardness of the part after heating.

[0246] A bright field transmission electron microscopy analysis has been carried out on the parts manufactured by the method according to the invention. With reference to FIG. 10, it has been identified that the manufactured part comprises a predominantly face-centred cubic crystal structure and furthermore comprises the presence of a hexagonal close-packed crystal structure, whereas the A1 and alloys A2 making up the roll bent mechanical part, prior to heating, comprised a single face-centred cubic crystal structure. In particular, this hexagonal close-packed crystallographic structure corresponds to the crystal structure of crystal precipitates, within the face-centred cubic structure, whose Feret diameter is typically between 5 and 80 nm. It therefore appears that the heating step carried out under the conditions of the method according to the invention induces a change in the crystal structure of at least some of the grains of the austenitic alloy making up the mechanical part from a face-centred cubic structure to a hexagonal close-packed structure. The advantages and effects of the alloy according to the invention, particularly with regard to mechanical properties, are, at least in part, conferred by the observed modifications in crystallographic structure.

[0247] The inventors have also observed the presence of nitrogen atoms surrounding the dislocations of the austenitic alloy making up the part manufactured by the method according to the invention. The advantages and effects of the alloy according to the invention, in particular relating to the mechanical properties, are, at least in part, conferred by the decreased mobility of the dislocations in the part manufactured due to the presence of nitrogen atoms about the dislocations.

[0248] The results set forth in FIGS. 11 to 14 have been obtained from non-magnetic parts obtained according to the method described in the invention. The mechanical parts used to obtain the non-magnetic parts are 3.2 mm diameter bars made of an alloy found under the trade names CHRONIFER 108 and BIODUR 108. CHRONIFER 108 UNS S29108 is sold by KLEIN company. CHRONIFER 108 consists, in weight percentage, of manganese between 21 and 24%, chromium between 19 and 23%, molybdenum between 0.5 and 1.5%, nitrogen at 0.9%, copper at 0.25%, carbon at a weight percentage of less than 0.08%, silicon at a weight percentage of less than 0.75, phosphorus at a weight percentage less than 0.03%, sulphur less at a weight percentage than 0.1%, nickel at a weight percentage less than 0.1% and iron, the weight percentage of which completes the composition to yield a total of 100%. The BIODUR 108 alloy is sold by CARPENTER. BIODUR 108 consists, in weight percentage, of manganese between 21 and 24%, chromium between 19 and 23%, molybdenum between 0.5 and 1.5%, 0.9% nitrogen, 0.01% sulphur, copper, 0.1% nickel, 0.75% silicon, 0.08% carbon, 0.03% phosphorus and iron, the weight percentage of which completes the composition to yield a total of 100%. Identical results have been obtained for each of the CHRONIFER 108 and BIODUR 108 alloys.

[0249] FIG. 11 illustrates the course of the hardness HV1 of a non-magnetic part obtained by using the method according to the invention. The abscissa axis shows the duration of the heating step in hours and the ordinate axis shows the hardness HV1 of the magnetic part obtained. The heating step is carried out at 575 C. FIG. 11 illustrates the effect of the cold working rate of the mechanical part and the effect of the heating time on the hardness of the non-magnetic part obtained. Three cold working rates have been studied: 25%, 42% and 85%. It is noticed that the higher the cold working rate of the mechanical part, the greater the hardness of the resulting mechanical part.

[0250] The cold working step generates dislocations. These dislocations form intra- and inter-granular nucleation sites for nitride precipitates. Furthermore, these dislocations accelerate the precipitation of nitride precipitates during the heating step. The dislocations therefore contribute to increasing the hardening of the alloy.

[0251] Furthermore, according to the invention, cold working prior to heat treatment makes it possible to obtain precipitation at temperatures necessarily below 700 C., preferably at temperatures of 650 C. or less. Usually, this precipitation is observed at temperatures well above 700 C. Furthermore, cold working prior to heat treatment makes it possible to obtain substantial precipitation for shorter heating times.

[0252] Furthermore, it has been observed that for a cold working rate of 42% and a heating time of 48 hours at 575 C., the ratio of the volume of deformed austenitic phase to the volume of reformed austenitic phase is in the order of 50%. It has also been observed that for a cold working rate of 85% and a heating time of 978 h at 575 C., the ratio of the volume of deformed austenitic phase to the volume of reformed austenitic phase is 0%. This shows that under such conditions there is no longer any cold worked austenitic phase.

[0253] Furthermore, it has been observed that for a heating time of one hour at 575 C., significant hardening of the part is already measurable. This is due, in particular, to the precipitation of nitrides and the presence of reformed austenite.

[0254] In FIG. 12, the surface layer and the central portion of the non-magnetic part is observed. These two portions are clearly visible and distinguishable. The presence of chromium nitride precipitates within the reformed austenite domains is also observed therein. The method according to the invention thus makes it possible to obtain a non-magnetic part with a ductile central portion and a hard superficial layer.

[0255] FIG. 13 illustrates the reformed austenitic domains 3, comprising the and phases, and the deformed austenitic domains 4, comprising the phase.

[0256] FIG. 14 illustrates the microstructure of the non-magnetic part obtained by the method according to the invention with cold working at a cold working rate of 85% followed by heating at 575 C. for 978 hours. Reformed domains 5, of the phase, having a depleted nitrogen concentration, typically less than compared with the nitrogen composition of the mechanical part, can be observed. The presence of precipitates 6 of Cr.sub.2N.sub.0.91 nitrides can also be observed. Finally, the presence of superstructures 7 of the phase is also noticed.

[0257] It is also observed that the alloy grains have a size smaller than 1 m. It is also noticed that the size of the nitride precipitates 7 is less than 100 nm.

[0258] Of course, the invention is not limited to the examples just described and many alterations can be made to these examples without departing from the scope of the invention.

[0259] Thus, in combinable alternatives to the previously described embodiments: [0260] the step of obtaining the mechanical part comprises: [0261] a step of bar turning at least one portion of the turning bar or at least one portion of the raw bar to form the mechanical part, or [0262] a step of cold working at least one portion of the raw bar or at least one portion of the turning bar to form the mechanical part, and/or [0263] the non-magnetic part is a precision timepiece, and/or [0264] the non-magnetic part is a pallet staff or an escape pinion, and/or [0265] the invention provides for a use of the non-magnetic part for its non-magnetic and/or hardness and/or tribological and/or fracture resistance properties, and/or [0266] the obtaining step comprises a step of bar turning at least one portion of a turning bar followed by a step of cold working the at least one turned portion of the turning bar to form the mechanical part, [0267] the smoothing step is a roll bending or roller burnishing step, and/or [0268] the austenitic alloy comprises chromium in a weight percentage greater than 8%, and/or [0269] the austenitic alloy comprises nitrogen in a weight percentage greater than and/or [0270] the gammagenic element(s) of the austenitic alloy comprise(s), in weight percentage, manganese between 8 and 30% and/or cobalt between 0 and 10%, [0271] the austenitic alloy comprises one or more non-gammagenic elements, the weight percentage or sum of the weight percentages of which is between 10 and 35%, [0272] the non-gammagenic element(s) of the austenitic alloy comprise(s), in weight percentage, chromium between 0 and 35% and/or molybdenum between 0 and 8% and/or silicon between 0 and 2% and/or titanium between 0 and X and/or niobium between 0 and X % and/or tungsten between 0 and X % and/or sulphur between 0 and 1.5%, and/or [0273] the heating step: [0274] is implemented for a duration between 10 minutes and 400 hours, and/or comprises a temperature gradient of between 4 C./min and 400 C./min, and/or [0275] is implemented under a controlled atmosphere, and/or [0276] the hardness gradient has a value greater than or equal to 100 HV, and/or [0277] the turning step is a roll bending or roller burnishing step.

[0278] Furthermore, the different characteristics, forms, alternatives and embodiments of the invention may be associated with one another according to various combinations insofar as they are not incompatible or exclusive of one another.