EP3249060A1 - Method for thermal treatment of austenitic steels and austenitic steels thus obtained - Google Patents

Method for thermal treatment of austenitic steels and austenitic steels thus obtained Download PDF

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EP3249060A1
EP3249060A1 EP17167986.3A EP17167986A EP3249060A1 EP 3249060 A1 EP3249060 A1 EP 3249060A1 EP 17167986 A EP17167986 A EP 17167986A EP 3249060 A1 EP3249060 A1 EP 3249060A1
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Prior art keywords
hns
steel
austenitic
precipitates
austenitic steel
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EP17167986.3A
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German (de)
French (fr)
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EP3249060B1 (en
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Joël Porret
Christian Charbon
Vincent Fays
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Swatch Group Research and Development SA
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Swatch Group Research and Development SA
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/02—Hardening by precipitation
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2261/00—Machining or cutting being involved

Definitions

  • the present invention relates to a heat treatment process of austenitic steels as well as the austenitic steels obtained by the implementation of this heat treatment process. More specifically, the present invention is concerned with nitrogen-austenitic steels well known by their Anglo-Saxon name Austenitic High Nitrogen Steel or austenitic HNS steels. The invention is also interested in austenitic steels with high concentrations of interstitial atoms, better known by their Anglo-Saxon name Austenitic High Interstitial Steel or austenitic steels HIS.
  • Austenitic steels alloyed with nitrogen that, for convenience, we will call later austenitic steels HNS, and austenitic steels with high concentrations of interstitial atoms which will be called hereinafter austenitic steels HIS have properties of hardness, resistant to corrosion and hypoallergenic which make them very interesting especially for applications in the field of watchmaking and jewelery, both for the manufacture of dressing elements intended to come into contact with the skin due to their very low concentration of nickel, and for the manufacture of components of watch movements because they are very hard, especially after hardening.
  • the austenitic HNS steels contain interstitial nitrogen atoms in high concentrations which can range up to 1.5% by weight depending on the composition and implementation of the alloy.
  • HIS austenitic steels directly derived from HNS austenitic steels, contain significant quantities of interstitial carbon atoms in addition to interstitial nitrogen atoms.
  • certain austenitic steels HNS and HIS exhibit particularly interesting hypoallergenic properties because of their very low nickel content and their resistance to corrosion.
  • the austenitic steels HNS and HIS are very difficult to machine, in particular because they have a very high yield strength, work hardening rate and ductility. Tests show, for example, that the machining operations are two to three times longer than for 1.4435 steel and the wear of the machining tools is very important.
  • the machining of these austenitic steels HNS and HIS which in many ways approaches the machining of titanium, is therefore long, difficult and expensive and is the main obstacle to the use of these steels especially in the field of watchmaking and jewelery.
  • the present invention relates to a heat treatment process of austenitic steels of HNS and HIS type whose purpose is to make such austenitic steels more easily machinable.
  • the present invention relates to a method of heat treatment of austenitic steel HNS or HIS containing precipitates of the type nitrides, carbides or even carbonitrides of chromium and / or molybdenum, this process comprising the step which, after machining of the austenitic steel HNS or HIS containing the precipitates, to return the precipitates in solution by carrying the steel austenitic HNS or HIS at its austenitization temperature, then cooling this austenitic steel HNS or HIS fast enough to avoid reforming precipitates.
  • This feature is very advantageous because it allows, when desired, to remove the precipitates after the austenitic steel parts HNS or HIS have been machined.
  • the resulting steels will thus have better corrosion resistance and greater ductility. The same is true when you want to make jewelry.
  • an alloy of austenitic steel HNS or HIS is provided which is heated to its austenitization temperature. or that sintered at the austenitization temperature, then, immediately from the austenitization temperature, the temperature of the austenitic steel alloy HNS or HIS is lowered sufficiently slowly so as to produce precipitates of the type nitrides, carbides or carbonitrides of chromium and / or molybdenum in the structure of the resulting austenitic steel HNS or HIS, then finally the austenitic steel HNS or HIS is brought to room temperature.
  • step of showing precipitates in an austenitic steel HNS or HIS precedes the step which, after machining of this austenitic steel HNS or HIS, consists of returning the precipitates in solution.
  • the heat treatment process applies equally well to parts obtained by casting and subsequent thermomechanical treatment, as well as to parts obtained by powder metallurgy such as metal injection molding, also known by its Anglo name. -Signs Metal Injection Molding or MIM. Indeed, immediately after sintering the alloy at its austenitization temperature in order to obtain an austenitic steel of the HNS or HIS type, it is possible to slowly cool the alloy in order to promote the formation of precipitates in accordance with the teachings of FIG. present invention.
  • Slow cooling is understood to mean cooling which, after austenitization or sintering, favors the appearance of precipitates in the microstructure of the austenitic steels HNS and HIS thus treated, as opposed to the conventional quenching heat treatment which consists of rapidly cooling the HNS steels and HIS after austenitization or sintering to avoid the formation of precipitates.
  • the austenitic steels HNS and HIS undergo a slow cooling thermal treatment to promote the appearance of precipitates
  • the invention goes against the usual practice. which consists in cooling the alloys as quickly as possible in order to avoid as much as possible the formation of precipitates in the resulting austenitic HNS and HIS steels.
  • the Applicant has indeed found that by subjecting the austenitic steels HNS and HIS to the heat treatment process of the kind described above, the nitrogen and carbon atoms for example tend to migrate towards the grain boundaries and to combine quite easily with chromium or molybdenum atoms to form precipitates of the nitride, carbide or even chromium / molybdenum carbonitride type.
  • these precipitates have a very low adhesion with the matrix, so that they make the chips brittle and facilitate the machining operations.
  • the austenitic steel HNS or HIS after the austenitic steel HNS or HIS has undergone a heat treatment of austenitization or sintering at the austenitization and quenching temperature, it is heated to again austenitic steel HNS or HIS at a temperature and for a duration such as appear nitride precipitates, carbides or carbonitrides of chromium and / or molybdenum.
  • This third variant is the most practical because it allows to perfectly control the parameters of different heat treatments.
  • the first, second and third variants of implementation of the heat treatment process of an austenitic steel HNS or HIS according to the invention are therefore more particularly intended to obtain cladding elements for timepieces or jewelery because they promote the corrosion resistance of these steels.
  • These first three variants have in common that after application of an austenitization heat treatment to austenitic steel HNS or HIS and subsequent machining, it is possible in fact bring the resulting part to the annealing temperature, then soak the latter to return the precipitates in solution.
  • an austenitic steel HNS or HIS is brought to its annealing temperature, in other words to its austenitization temperature, and then cooled rapidly (quenching) so that no precipitate is formed, it is deformed cold and then brought to austenitic steel HNS or HIS at a temperature and for a period such as appear nitrites precipitates, carbides or carbonitrides of chromium and / or molybdenum .
  • the invention also relates to a timepiece or jewelery element obtained from an austenitic steel HNS or HIS obtained by carrying out the heat treatment process according to the invention.
  • the hardness of the austenitic steel HNS or HIS obtained after austenitization and cold deformation is very little affected by the precipitation treatment according to the invention carried out subsequently.
  • the machinability of such steels is substantially improved.
  • the present invention proceeds from the general inventive idea of subjecting the austenitic HNS and HIS steels to a heat treatment aimed at ironing in solution precipitates which have appeared in such austenitic HNS or HIS steels for example during pre-treatment of precipitation.
  • precipitation heat treatment is meant a treatment which aims to place these austenitic steels HNS and HIS for a certain period of time under temperature conditions which allow the appearance of precipitates such as nitrides, carbides or carbonitrides, in particular molybdenum and / or chromium. It has indeed been observed that these precipitates are generally poorly bound to the matrix of the material, so that they promote the formation and removal of chips during machining parts.
  • this annealing treatment will preferably be reserved, but not limited to cladding elements for watches or jewelery for which the corrosion resistance and the polishing ability are properties more important than the hardness.
  • each austenitic steel composition HNS or HIS has a time-temperature-transformation diagram which is specific to it and which is also a function of the nature of the precipitate in question.
  • the figure 1 is a time (t) - temperature (T) - transformation diagram which illustrates the heat treatment of an austenitic steel HNS or HIS according to the first alternative embodiment of the method of the invention.
  • Tr1 is the austenitizing temperature or annealing of a type of austenitic steel HNS or HIS and either a curve which, on the time-temperature-transformation diagram figure 1 , defines an area that corresponds to conditions of time and temperature that allow the formation of precipitates.
  • the figure 2 is a time (t) - temperature (T) - transformation diagram which illustrates the heat treatment of an austenitic steel HNS or HIS according to the second alternative embodiment of the method of the invention.
  • Tr2 be the austenitization or annealing temperature of an austenitic steel of the HNS or HIS type and let b be the curve which, on the time-temperature-transformation chart of the figure 2 , defines an area that corresponds to conditions of time and temperature that allow the formation of precipitates.
  • the austenitic steel HNS or HIS is rapidly cooled from its annealing temperature Tr2 according to curve 4, and the cooling of the austenitic steel HNS or HIS is interrupted when the temperature reaches a value Tp2 which can appear. precipitates, and this steel is maintained at this temperature Tp2 for a period such that precipitates appear (curve 6).
  • Tp2 time-temperature-transformation chart
  • the figure 3 is a time (t) - temperature (T) - transformation diagram which illustrates the heat treatment of an austenitic steel HNS or HIS according to the third alternative embodiment of the method of the invention.
  • Tr3 is the austenitizing temperature or annealing of a HNS type austenitic steel or HIS and c is the curve, the time-temperature-transformation diagram figure 3 , defines an area that corresponds to conditions of time and temperature that allow the formation of precipitates.
  • the steel in question here is an austenitic steel HNS or HIS which has been cooled sufficiently rapidly from its annealing temperature Tr3 to room temperature in order to avoid any formation of precipitates.
  • such austenitic steel HNS or HIS is heated according to curve 10 and maintained at a temperature and for a duration such that precipitates appear (curve 12), then is cooled (curve 14).
  • the fourth alternative embodiment of the process of the invention differs from the third variant of the same process only in that, after annealing treatment followed by quenching and before the precipitation treatment, the austenitic steel HNS or HIS is hardened, that is to say deformed cold.
  • the heat treatment according to the invention which consists in wearing an austenitic steel at a temperature and for a period of time such that precipitates are formed is therefore applied, in this fourth variant, to a material that has been hardened beforehand by hardening.
  • the fifth and last alternative embodiment of the process of the invention consists in subjecting the austenitic steel to a cold deformation treatment after heat treatment according to one of the first three processing variants.
  • the figure 4 is a view of a metallographic section of a HIS X20CrMnMoN17-11-3 steel sample that has been annealed at its austenitization temperature and quenched. It is noted on examining this figure that the grain boundaries are not very marked, which indicates the absence of precipitates.
  • the figure 5 is a view of a metallographic section of a sample of austenitic steel HIS X20CrMnMoN17-11-3 having undergone a heat treatment according to the third variant of implementation of the method according to the invention.
  • the grain boundaries are marked, indicating the presence of large amounts of precipitates along these grain boundaries.
  • Some larger precipitates have grown inside the grains since the grain boundaries.
  • Such a concentration of precipitates could be obtained by carrying, after cooling rapidly from the annealing temperature, the austenitic steel HIS X20CrMnMoN17-11-3 at a temperature of 800 ° C for two hours.
  • Samples of austenitic steel HIS X20CrMnMoN17-11-3 were therefore subjected to a heat treatment process according to the fourth variant embodiment of the invention and consisting, after annealing treatment followed by quenching and hardening, to wear the austenitic steel HIS X20CrMnMoN17-11-3 at a temperature and for a time such that precipitates form. It has been observed that after cold deformation, the formation of the precipitates is much faster. Indeed, the dislocations and the gaps induced by the cold deformation create diffusion paths favorable to the germination and the growth of the precipitates.
  • the figure 6 is a view of a metallographic section of a sample of austenitic steel HIS X20CrMnMoN17-11-3 which is in the form of a bar whose outer diameter is reduced from 3 mm to 2.5 mm by cold deformation by wire drawing, a diameter reduction of 16.6%.
  • this sample was then heated to a temperature of 800 ° C. for two hours according to the temperature curve represented in FIG. figure 3 . It can be seen that steel has many precipitates, both at the grain boundaries and inside the grains.
  • the figure 7 is a graph that shows the evolution of the hardness of austenitic steel HIS X20CrMnMoN17-11-3 of the figure 6 depending on the temperature at which this steel is worn to form the precipitates. It is observed that the hardness of the austenitic steel without precipitation treatment according to the invention and after cold working is 450 HV10 (symbol in the form of a square on the graph). The same austenitic steel is, after cold working, heat-treated according to the fourth alternative embodiment of the method according to the invention.
  • Samples of this steel are respectively heated to temperatures of 750 ° C, 800 ° C, 850 ° C, 900 ° C and 950 ° C for a period of two hours, then cooled (diamond symbols on the graph) . It is observed that for samples heated between 700 ° C and 900 ° C, the hardness is between about 425 HV10 and 375 HV10. In other words, the hardness of these samples of austenitic steel heat-treated according to the fourth variant of the invention varies little with respect to the hardness of the austenitic steel hardened but not subjected to a treatment of precipitation.
  • the machinability of the austenitic steel samples having undergone a heat treatment of precipitation according to this fourth variant of the invention is clearly improved. Only the austenitic steel sample heated at 950 ° C for two hours has a hardness substantially lower than that of austenitic steel without precipitation treatment (less than 350 HV10). Finally, a sample of austenitic steel HIS X20CrMnMoN17-11-3 having undergone only annealing treatment followed by tempering (triangular symbol on the graph) has a hardness less than 250 HV10.
  • the figure 8 is a view of a metallographic section of a sample of austenitic steel HIS X20CrMnMoN17-11-3 in the form of a bar whose outer diameter is reduced from 3 mm to 2 mm by cold deformation by wire drawing a larger diameter reduction of 33.3%.
  • This steel sample undergoes the same heat treatment as at figure 6 by being heated to a temperature of 800 ° C. for two hours in accordance with the fourth variant embodiment of the invention.
  • the precipitation phenomenon is even more pronounced since, besides the precipitates which are formed along the grain boundaries and from the grain boundaries into the grains, there is a high concentration of precipitates within the grains.
  • the figure 9 is a graph that shows the evolution of the hardness of the steel of the figure 8 depending on the time and temperature at which this steel is worn after work hardening to form the precipitates. It is observed that the hardness of the austenitic steel without precipitation treatment according to the invention and after cold working is between 550 HV10 and 560 HV10 (symbol in the form of a square on the graph). This hardness is greater than that at figure 7 because the rate of work hardening is higher.
  • the diamond shaped symbols on the figure 9 correspond to samples of austenitic steel heated to temperatures of 700 ° C, 750 ° C, 800 ° C and 850 ° C respectively for 45 minutes.
  • the round symbols correspond to samples of austenitic steel heated to temperatures of 700 ° C, 750 ° C, 800 ° C and 850 ° C respectively for two hours. If we compare the graphs of Figures 7 and 9 it is observed that the higher the rate of work hardening, the more the formation of precipitates is facilitated. Indeed, the mechanical stresses at the heart of the steel make it possible to germinate and grow the precipitates.
  • HNS and HIS steels to which the precipitation process according to the invention can be applied are: XSCrMnN18-18, X8CrMnN19-19, X8CrMnMoN18-18-2, X13CrMnMoN18-14-3, X20CrMnMoN17-11-3 or still well X5MnCrMoN23-21.
  • precipitates that may form during the precipitation process are: M23C, MC, M6C or even M2N, where M denotes one or more of the metal elements of the alloy that can combine with carbon or nitrogen to form carbides or nitrides or carbonitrides.
  • the invention applies in particular to jewelery and cladding elements of timepieces.
  • the present invention teaches a method of heat treatment of an austenitic steel HNS or HIS containing precipitates, this method comprising the step which consists, after machining parts, including jewelry or timepieces, made at using a HNS or HIS austenitic steel containing precipitates, to return the precipitates in solution by bringing these austenitic steel parts HNS or HIS to their austenitization temperature, then cooling these parts sufficiently quickly, typically by quenching, to prevent precipitates to form again.
  • Machining operations include, but are not limited to, boring, milling, drilling, threading, tapping and cutting operations.

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Abstract

L'invention concerne un procédé de traitement thermique d'un acier austénitique du type High Nitrogen Steel ou acier austénitique HNS, ou bien d'un acier austénitique du type High Interstitial Steel ou acier austénitique HIS, cet acier austénitique HNS ou HIS renfermant des précipités du type nitrures, carbures ou carbonitrures de chrome et/ou de molybdène, ce procédé comprenant l'étape qui consiste, après usinage de l'acier austénitique HNS ou HIS renfermant les précipités, à remettre les précipités en solution en portant l'acier austénitique HNS ou HIS à sa température d'austénitisation, puis en refroidissant cet acier austénitique HNS ou HIS suffisamment rapidement pour éviter de reformer des précipités. L'invention concerne également différents procédés de traitement thermique permettant de faire apparaître, avant usinage, des précipités du type nitrures, carbures ou bien encore carbonitrures de chrome et/ou de molybdène dans un acier austénitique HNS ou HIS. En effet, la présence de ces précipités dans la matrice des aciers austénitiques HNS ou HIS rend les opérations d'usinage plus aisées en favorisant la formation et l'enlèvement des copeaux lors de l'usinage des pièces.The invention relates to a method for the heat treatment of a high-grade austenitic steel of the type High Nitrogen Steel or austenitic steel HNS, or of a high-strength austenitic steel of the type High Interstitial Steel or austenitic steel HIS, this austenitic steel HNS or HIS containing precipitates of the nitride, carbide or carbonitride type of chromium and / or molybdenum, this process comprising the step which, after machining of the austenitic steel HNS or HIS containing the precipitates, to return the precipitates in solution by carrying the austenitic steel HNS or HIS at its austenitization temperature, then cooling this austenitic steel HNS or HIS fast enough to avoid reforming precipitates. The invention also relates to different heat treatment processes making it possible to show, before machining, precipitates of the nitride, carbide or even carbonitride type of chromium and / or molybdenum in austenitic steel HNS or HIS. Indeed, the presence of these precipitates in the matrix of austenitic steels HNS or HIS makes machining operations easier by promoting the formation and removal of chips during machining parts.

Description

Domaine technique de l'inventionTechnical field of the invention

La présente invention concerne un procédé de traitement thermique d'aciers austénitiques ainsi que les aciers austénitiques obtenus par la mise en oeuvre de ce procédé de traitement thermique. Plus précisément, la présente invention s'intéresse aux aciers austénitiques alliés à l'azote bien connus sous leur dénomination anglo-saxonne Austenitic High Nitrogen Steel ou aciers austénitiques HNS. L'invention s'intéresse également aux aciers austénitiques à fortes concentrations en atomes interstitiels, mieux connus sous leur dénomination anglo-saxonne Austenitic High Interstitial Steel ou aciers austénitiques HIS.The present invention relates to a heat treatment process of austenitic steels as well as the austenitic steels obtained by the implementation of this heat treatment process. More specifically, the present invention is concerned with nitrogen-austenitic steels well known by their Anglo-Saxon name Austenitic High Nitrogen Steel or austenitic HNS steels. The invention is also interested in austenitic steels with high concentrations of interstitial atoms, better known by their Anglo-Saxon name Austenitic High Interstitial Steel or austenitic steels HIS.

Arrière-plan technologique de l'inventionTechnological background of the invention

Les aciers austénitiques alliés à l'azote que, pour plus de commodité, nous appellerons par la suite aciers austénitiques HNS, et les aciers austénitiques à fortes concentrations en atomes interstitiels qui seront appelés ci-après aciers austénitiques HIS présentent des propriétés de dureté, de résistance à la corrosion et hypoallergéniques qui les rendent très intéressants notamment pour des applications dans le domaine de l'horlogerie et de la bijouterie, à la fois pour la fabrication d'éléments d'habillage destinés à venir en contact avec la peau en raison de leur très faible concentration en nickel, et pour la fabrication de composants de mouvements horlogers car ils sont très durs, notamment après écrouissage.Austenitic steels alloyed with nitrogen that, for convenience, we will call later austenitic steels HNS, and austenitic steels with high concentrations of interstitial atoms which will be called hereinafter austenitic steels HIS have properties of hardness, resistant to corrosion and hypoallergenic which make them very interesting especially for applications in the field of watchmaking and jewelery, both for the manufacture of dressing elements intended to come into contact with the skin due to their very low concentration of nickel, and for the manufacture of components of watch movements because they are very hard, especially after hardening.

Les aciers austénitiques HNS renferment des atomes interstitiels d'azote en concentrations élevées qui peuvent s'étendre jusqu'à 1,5% en poids en fonction de la composition et de la mise en oeuvre de l'alliage. Les aciers austénitiques HIS, directement dérivés des aciers austénitiques HNS, renferment quant à eux des quantités importantes d'atomes interstitiels de carbone en plus des atomes interstitiels d'azote.The austenitic HNS steels contain interstitial nitrogen atoms in high concentrations which can range up to 1.5% by weight depending on the composition and implementation of the alloy. HIS austenitic steels, directly derived from HNS austenitic steels, contain significant quantities of interstitial carbon atoms in addition to interstitial nitrogen atoms.

Comme mentionné ci-dessus, certains aciers austénitiques HNS et HIS présentent notamment d'intéressantes propriétés hypoallergéniques en raison de leur très faible teneur en nickel et de leur résistance à la corrosion. Cependant, les aciers austénitiques HNS et HIS sont très difficiles à usiner, notamment car ils présentent une limite élastique, un taux d'écrouissage et une ductilité très élevés. Des essais montrent, par exemple, que les opérations d'usinage sont deux à trois fois plus longues que pour l'acier 1.4435 et l'usure des outils d'usinage est très importante. L'usinage de ces aciers austénitiques HNS et HIS qui, par bien des aspects, se rapproche de l'usinage du titane, est donc long, difficile et coûteux et constitue le principal frein à l'utilisation de ces aciers notamment dans le domaine de l'horlogerie et de la bijouterie.As mentioned above, certain austenitic steels HNS and HIS exhibit particularly interesting hypoallergenic properties because of their very low nickel content and their resistance to corrosion. However, the austenitic steels HNS and HIS are very difficult to machine, in particular because they have a very high yield strength, work hardening rate and ductility. Tests show, for example, that the machining operations are two to three times longer than for 1.4435 steel and the wear of the machining tools is very important. The machining of these austenitic steels HNS and HIS, which in many ways approaches the machining of titanium, is therefore long, difficult and expensive and is the main obstacle to the use of these steels especially in the field of watchmaking and jewelery.

Il existait donc dans l'état de la technique un besoin pour des aciers austénitiques HNS et HIS qui soient plus facilement usinables tout en conservant leurs propriétés de biocompatibilité, de dureté et de résistance à la corrosion.There was therefore in the state of the art a need for austenitic steels HNS and HIS which are more easily machinable while retaining their properties of biocompatibility, hardness and corrosion resistance.

Résumé de l'inventionSummary of the invention

La présente invention a pour objet un procédé de traitement thermique d'aciers austénitiques de type HNS et HIS dont le but est de rendre de tels aciers austénitiques plus facilement usinables.The present invention relates to a heat treatment process of austenitic steels of HNS and HIS type whose purpose is to make such austenitic steels more easily machinable.

A cet effet, la présente invention concerne un procédé de traitement thermique d'un acier austénitique HNS ou HIS renfermant des précipités du type nitrures, carbures ou bien encore carbonitrures de chrome et/ou de molybdène, ce procédé comprenant l'étape qui consiste, après usinage de l'acier austénitique HNS ou HIS renfermant les précipités, à remettre les précipités en solution en portant l'acier austénitique HNS ou HIS à sa température d'austénitisation, puis en refroidissant cet acier austénitique HNS ou HIS suffisamment rapidement pour éviter de reformer des précipités.For this purpose, the present invention relates to a method of heat treatment of austenitic steel HNS or HIS containing precipitates of the type nitrides, carbides or even carbonitrides of chromium and / or molybdenum, this process comprising the step which, after machining of the austenitic steel HNS or HIS containing the precipitates, to return the precipitates in solution by carrying the steel austenitic HNS or HIS at its austenitization temperature, then cooling this austenitic steel HNS or HIS fast enough to avoid reforming precipitates.

Cette caractéristique se révèle très avantageuse car elle permet, lorsque cela est souhaité, de faire disparaître les précipités après que les pièces en acier austénitique HNS ou HIS aient été usinées. Dans le cas particulier des pièces d'horlogerie, on pourra notamment mettre cette possibilité à profit pour faire disparaître les précipités dans les éléments d'habillage (carrures, fonds de boîtes de montres, lunettes, couronnes, poussoirs, fermoirs, maillons de bracelets etc.) afin de rendre la matière la plus homogène possible et d'éliminer les contraintes résiduelles. Les aciers résultants auront ainsi une meilleure résistance à la corrosion et une plus grande ductilité. La même chose est vraie lorsque l'on souhaite fabriquer des bijoux.This feature is very advantageous because it allows, when desired, to remove the precipitates after the austenitic steel parts HNS or HIS have been machined. In the particular case of timepieces, it will be possible to use this opportunity to remove the precipitates in the elements of dressing (middle, back of watch boxes, glasses, crowns, pushers, clasps, bracelets links etc. .) in order to make the material as homogeneous as possible and to eliminate the residual stresses. The resulting steels will thus have better corrosion resistance and greater ductility. The same is true when you want to make jewelry.

Selon une caractéristique complémentaire de l'invention, pour faire apparaître des précipités dans l'acier austénitique HNS ou HIS avant usinage, on se munit d'un alliage d'acier austénitique HNS ou HIS que l'on porte à sa température d'austénitisation ou que l'on fritte à la température d'austénitisation, puis, immédiatement depuis la température d'austénitisation, on abaisse la température de l'alliage d'acier austénitique HNS ou HIS de manière suffisamment lente pour qu'apparaissent des précipités du type nitrures, carbures ou bien carbonitrures de chrome et/ou de molybdène dans la structure de l'acier austénitique HNS ou HIS résultant, puis enfin on ramène l'acier austénitique HNS ou HIS à température ambiante.According to a complementary feature of the invention, to make precipitates appear in the austenitic steel HNS or HIS before machining, an alloy of austenitic steel HNS or HIS is provided which is heated to its austenitization temperature. or that sintered at the austenitization temperature, then, immediately from the austenitization temperature, the temperature of the austenitic steel alloy HNS or HIS is lowered sufficiently slowly so as to produce precipitates of the type nitrides, carbides or carbonitrides of chromium and / or molybdenum in the structure of the resulting austenitic steel HNS or HIS, then finally the austenitic steel HNS or HIS is brought to room temperature.

On comprendra que l'étape qui consiste à faire apparaître des précipités dans un acier austénitique HNS ou HIS précède l'étape qui, après usinage de cet acier austénitique HNS ou HIS, consiste à remettre les précipités en solution.It will be understood that the step of showing precipitates in an austenitic steel HNS or HIS precedes the step which, after machining of this austenitic steel HNS or HIS, consists of returning the precipitates in solution.

On notera également que le procédé de traitement thermique s'applique aussi bien à des pièces obtenues par coulée et traitement thermomécanique subséquent, qu'à des pièces obtenues par la métallurgie des poudres telle que le moulage par injection de métal encore connu sous sa dénomination anglo-saxonne Metal Injection Moulding ou MIM. En effet, immédiatement après frittage de l'alliage à sa température d'austénitisation afin d'obtenir un acier austénitique de type HNS ou HIS, il est possible de refroidir lentement l'alliage afin de favoriser la formation de précipités conformément aux enseignements de la présente invention.It will also be noted that the heat treatment process applies equally well to parts obtained by casting and subsequent thermomechanical treatment, as well as to parts obtained by powder metallurgy such as metal injection molding, also known by its Anglo name. -Signs Metal Injection Molding or MIM. Indeed, immediately after sintering the alloy at its austenitization temperature in order to obtain an austenitic steel of the HNS or HIS type, it is possible to slowly cool the alloy in order to promote the formation of precipitates in accordance with the teachings of FIG. present invention.

Par refroidissement lent, on entend un refroidissement qui, après austénitisation ou frittage, favorise l'apparition de précipités dans la microstructure des aciers austénitiques HNS et HIS ainsi traités, par opposition au traitement thermique classique de trempe qui consiste à refroidir rapidement les aciers HNS et HIS après austénitisation ou frittage afin d'éviter la formation de précipités.Slow cooling is understood to mean cooling which, after austenitization or sintering, favors the appearance of precipitates in the microstructure of the austenitic steels HNS and HIS thus treated, as opposed to the conventional quenching heat treatment which consists of rapidly cooling the HNS steels and HIS after austenitization or sintering to avoid the formation of precipitates.

En préconisant de soumettre, immédiatement après austénitisation ou frittage à la température d'austénitisation, les aciers austénitiques HNS et HIS à un traitement thermique de refroidissement lent pour favoriser l'apparition de précipités, l'invention va à l'encontre de la pratique habituelle qui consiste à refroidir les alliages le plus rapidement possible afin d'éviter le plus possible la formation de précipités dans les aciers austénitiques HNS et HIS résultants.By recommending immediately after austenitization or sintering at the austenitization temperature, the austenitic steels HNS and HIS undergo a slow cooling thermal treatment to promote the appearance of precipitates, the invention goes against the usual practice. which consists in cooling the alloys as quickly as possible in order to avoid as much as possible the formation of precipitates in the resulting austenitic HNS and HIS steels.

La Demanderesse a effectivement constaté qu'en soumettant les aciers austénitiques HNS et HIS au procédé de traitement thermique du genre décrit ci-dessus, les atomes d'azote et de carbone par exemple ont tendance à migrer vers les joints de grains et à se combiner assez facilement avec des atomes de chrome ou de molybdène pour former des précipités du type nitrures, carbures ou bien encore carbonitrures de chrome/molybdène. Or, ces précipités ont une très faible adhérence avec la matrice, de sorte qu'ils rendent les copeaux cassants et facilitent les opérations d'usinage.The Applicant has indeed found that by subjecting the austenitic steels HNS and HIS to the heat treatment process of the kind described above, the nitrogen and carbon atoms for example tend to migrate towards the grain boundaries and to combine quite easily with chromium or molybdenum atoms to form precipitates of the nitride, carbide or even chromium / molybdenum carbonitride type. However, these precipitates have a very low adhesion with the matrix, so that they make the chips brittle and facilitate the machining operations.

Selon un autre mode de mise en oeuvre du procédé de l'invention, pour faire apparaître avant usinage des précipités du type nitrures, carbures ou bien carbonitrures de chrome et/ou de molybdène dans l'acier austénitique HNS ou HIS, on se munit d'un alliage d'acier austénitique HNS ou HIS que l'on porte à sa température d'austénitisation ou que l'on fritte à la température d'austénitisation, puis on soumet cet alliage d'acier austénitique HNS ou HIS à un traitement thermique de refroidissement immédiatement depuis la température d'austénitisation, et on interrompt le refroidissement de l'acier austénitique HNS ou HIS résultant lorsque la température à atteint une valeur à laquelle apparaissent des précipités, cet acier austénitique HNS ou HIS étant maintenu à cette température et pendant une durée telle qu'apparaissent des précipités, puis enfin on ramène l'acier austénitique HNS ou HIS à température ambiante.According to another embodiment of the process of the invention, to reveal before machining of the nitrides, carbides or carbonitrides of chromium and / or molybdenum type precipitates in the austenitic steel HNS or HIS, one is provided with an austenitic steel alloy HNS or HIS which is heated to its austenitization temperature or sintered at the austenitization temperature, then subjected to a heat treatment of this austenitic steel alloy HNS or HIS immediately after the austenitization temperature, and the cooling of the resulting austenitic steel HNS or HIS is interrupted when the temperature reaches a value at which precipitates appear, the austenitic steel HNS or HIS being maintained at this temperature and during a time such that precipitates appear, then finally we bring the austenitic steel HNS or HIS at room temperature.

Selon encore un autre mode de mise en oeuvre du procédé de l'invention, après que l'acier austénitique HNS ou HIS a subi un traitement thermique d'austénitisation ou de frittage à la température d'austénitisation, puis de trempe, on chauffe à nouveau l'acier austénitique HNS ou HIS à une température et pendant une durée telles qu'apparaissent des précipités du type nitrures, carbures ou bien carbonitrures de chrome et/ou de molybdène.According to yet another embodiment of the process of the invention, after the austenitic steel HNS or HIS has undergone a heat treatment of austenitization or sintering at the austenitization and quenching temperature, it is heated to again austenitic steel HNS or HIS at a temperature and for a duration such as appear nitride precipitates, carbides or carbonitrides of chromium and / or molybdenum.

Cette troisième variante est la plus pratique car elle permet de pouvoir parfaitement maîtriser les paramètres des différents traitements thermiques.This third variant is the most practical because it allows to perfectly control the parameters of different heat treatments.

Les première, deuxième et troisième variantes de mise en oeuvre du procédé de traitement thermique d'un acier austénitique HNS ou HIS selon l'invention sont donc plus particulièrement destinées à l'obtention d'éléments d'habillage pour des pièces d'horlogerie ou de bijouterie, car elles favorisent la résistance à la corrosion de ces aciers. Ces trois premières variantes ont en commun qu'après application d'un traitement thermique d'austénitisation à un acier austénitique HNS ou HIS et usinage subséquent, on peut en effet amener la pièce résultante à la température de recuit, puis tremper cette dernière afin de remettre les précipités en solution.The first, second and third variants of implementation of the heat treatment process of an austenitic steel HNS or HIS according to the invention are therefore more particularly intended to obtain cladding elements for timepieces or jewelery because they promote the corrosion resistance of these steels. These first three variants have in common that after application of an austenitization heat treatment to austenitic steel HNS or HIS and subsequent machining, it is possible in fact bring the resulting part to the annealing temperature, then soak the latter to return the precipitates in solution.

Selon une quatrième variante de mise en oeuvre du procédé selon l'invention, on porte un acier austénitique HNS ou HIS à sa température de recuit, autrement dit à sa température d'austénitisation, puis on le refroidit rapidement (trempe) de façon à ce qu'aucun précipité ne se forme, on le déforme à froid puis on amène cet acier austénitique HNS ou HIS à une température et pendant une durée telles qu'apparaissent des précipités du type nitrures, carbures ou bien carbonitrures de chrome et/ou de molybdène.According to a fourth alternative embodiment of the process according to the invention, an austenitic steel HNS or HIS is brought to its annealing temperature, in other words to its austenitization temperature, and then cooled rapidly (quenching) so that no precipitate is formed, it is deformed cold and then brought to austenitic steel HNS or HIS at a temperature and for a period such as appear nitrites precipitates, carbides or carbonitrides of chromium and / or molybdenum .

L'invention concerne également un élément d'horlogerie ou de bijouterie obtenu à partir d'un acier austénitique HNS ou HIS obtenu par mise en oeuvre du procédé de traitement thermique selon l'invention.The invention also relates to a timepiece or jewelery element obtained from an austenitic steel HNS or HIS obtained by carrying out the heat treatment process according to the invention.

Grâce à ces caractéristiques, la dureté de l'acier austénitique HNS ou HIS obtenue après austénitisation et déformation à froid est très peu affectée par le traitement de précipitation selon l'invention effectué ultérieurement. Par contre, l'usinabilité de tels aciers est sensiblement améliorée.Thanks to these characteristics, the hardness of the austenitic steel HNS or HIS obtained after austenitization and cold deformation is very little affected by the precipitation treatment according to the invention carried out subsequently. On the other hand, the machinability of such steels is substantially improved.

Brève description des figuresBrief description of the figures

D'autres caractéristiques et avantages de la présente invention ressortiront plus clairement de la description détaillée qui suit d'un exemple de mise en oeuvre du procédé de traitement thermique d'aciers austénitiques HNS et HIS conforme à la présente invention, cet exemple étant donné à titre purement illustratif et non limitatif seulement en liaison avec le dessin annexé sur lequel :

  • la figure 1 est un diagramme schématique temps-température-transformation qui illustre le traitement thermique d'un acier austénitique HNS ou HIS selon la première variante de mise en oeuvre du procédé de l'invention ;
  • la figure 2 est un diagramme schématique temps-température-transformation qui illustre le traitement thermique d'un acier austénitique HNS ou HIS selon la deuxième variante de mise en oeuvre du procédé de l'invention ;
  • la figure 3 est un diagramme schématique temps-température-transformation qui illustre le traitement thermique d'un acier austénitique HNS ou HIS selon la troisième variante de mise en oeuvre du procédé de l'invention ;
  • la figure 4 est une vue d'une coupe métallographique d'un échantillon d'acier HIS X20CrMnMoN17-11-3 qui a été recuit à sa température d'austénitisation puis trempé et qui ne présente pas de précipités ;
  • la figure 5 est une vue d'une coupe métallographique d'un échantillon d'acier austénitique HIS X20CrMnMoN17-11-3 ayant subi un traitement thermique conforme à la troisième variante de mise en oeuvre du procédé selon l'invention ;
  • la figure 6 est une vue d'une coupe métallographique d'un échantillon d'acier austénitique HIS X20CrMnMoN17-11-3 ayant subi un traitement thermique conforme à la quatrième variante de mise en oeuvre du procédé selon l'invention ;
  • la figure 7 est un graphe qui montre l'évolution de la dureté de l'échantillon d'acier austénitique HIS X20CrMnMoN17-11-3 de la figure 6 en fonction de la température à laquelle cet acier est porté pour former les précipités.
  • la figure 8 est une vue d'une coupe métallographique d'un échantillon d'acier austénitique HIS X20CrMnMoN17-11-3 ayant subi un écrouissage plus important que l'échantillon d'acier austénitique de la figure 6 avant un traitement thermique conforme à la quatrième variante de mise en oeuvre du procédé selon l'invention, et
  • la figure 9 est un graphe qui montre l'évolution de la dureté de l'échantillon d'acier austénitique HIS X20CrMnMoN17-11-3 de la figure 8 en fonction de la température à laquelle cet acier est porté pour former les précipités.
Other features and advantages of the present invention will emerge more clearly from the detailed description which follows of an exemplary implementation of the heat treatment process of austenitic steels HNS and HIS according to the present invention, this example being given in FIG. purely illustrative and not limiting only in connection with the accompanying drawing in which:
  • the figure 1 is a schematic time-temperature-transformation diagram which illustrates the heat treatment of an austenitic steel HNS or HIS according to the first alternative embodiment of the method of the invention;
  • the figure 2 is a schematic time-temperature-transformation diagram which illustrates the heat treatment of an austenitic steel HNS or HIS according to the second alternative embodiment of the method of the invention;
  • the figure 3 is a schematic time-temperature-transformation diagram which illustrates the heat treatment of an austenitic steel HNS or HIS according to the third alternative embodiment of the method of the invention;
  • the figure 4 is a view of a metallographic section of a HIS X20CrMnMoN17-11-3 steel sample which has been annealed at its austenitization temperature and then quenched and which does not show precipitates;
  • the figure 5 is a view of a metallographic section of a sample of austenitic steel HIS X20CrMnMoN17-11-3 having undergone a heat treatment according to the third variant of implementation of the method according to the invention;
  • the figure 6 is a view of a metallographic section of a sample of austenitic steel HIS X20CrMnMoN17-11-3 having undergone a heat treatment according to the fourth alternative embodiment of the method according to the invention;
  • the figure 7 is a graph that shows the evolution of the hardness of the austenitic steel sample HIS X20CrMnMoN17-11-3 of the figure 6 depending on the temperature at which this steel is worn to form the precipitates.
  • the figure 8 is a view of a metallographic section of a sample of austenitic steel HIS X20CrMnMoN17-11-3 that has undergone greater work hardening than the austenitic steel sample of the figure 6 before a heat treatment according to the fourth alternative embodiment of the process according to the invention, and
  • the figure 9 is a graph that shows the evolution of the hardness of the austenitic steel sample HIS X20CrMnMoN17-11-3 of the figure 8 depending on the temperature at which this steel is worn to form the precipitates.

Description détaillée d'un mode de réalisation de l'inventionDetailed description of an embodiment of the invention

La présente invention procède de l'idée générale inventive qui consiste à soumettre les aciers austénitiques HNS et HIS à un traitement thermique visant à faire repasser en solution des précipités que l'on a fait apparaître dans de tels aciers austénitiques HNS ou HIS par exemple lors d'un traitement préalable de précipitation. Par traitement thermique de précipitation, on entend un traitement qui vise à placer ces aciers austénitiques HNS et HIS pendant une certaine durée dans des conditions de température qui permettent l'apparition de précipités tels que des nitrures, des carbures ou des carbonitrures, notamment de molybdène et/ou de chrome. On a en effet observé que ces précipités sont généralement peu liés à la matrice du matériau, de sorte qu'ils favorisent la formation et l'enlèvement des copeaux lors de l'usinage des pièces. Ainsi, conformément à l'invention, il est possible, après usinage de pièces réalisées en un acier austénitique HNS ou HIS renfermant des précipités, de soumettre ces pièces à un second traitement d'austénitisation qui consiste à porter à nouveau ces pièces à leur température de recuit, puis à les tremper de manière à remettre les précipités en solution solide. Comme le fait de porter après usinage les aciers austénitiques HNS et HIS une seconde fois à leur température de recuit provoque une élimination des contraintes internes dans le matériau et donc une diminution de sa dureté, on réservera de préférence mais non limitativement ce traitement de recuit à des éléments d'habillage pour montres ou pour bijoux pour lesquels la résistance à la corrosion et l'aptitude au polissage sont des propriétés plus importantes que la dureté.The present invention proceeds from the general inventive idea of subjecting the austenitic HNS and HIS steels to a heat treatment aimed at ironing in solution precipitates which have appeared in such austenitic HNS or HIS steels for example during pre-treatment of precipitation. By precipitation heat treatment, is meant a treatment which aims to place these austenitic steels HNS and HIS for a certain period of time under temperature conditions which allow the appearance of precipitates such as nitrides, carbides or carbonitrides, in particular molybdenum and / or chromium. It has indeed been observed that these precipitates are generally poorly bound to the matrix of the material, so that they promote the formation and removal of chips during machining parts. Thus, according to the invention, it is possible, after machining parts made of austenitic steel HNS or HIS containing precipitates, to subject these parts to a second austenitization treatment which consists in bringing these parts to their temperature again. annealing, and soaking them so as to put the precipitates in solid solution. As wearing after machining the austenitic steels HNS and HIS a second time at their annealing temperature causes an elimination of the internal stresses in the material and therefore a decrease in its hardness, this annealing treatment will preferably be reserved, but not limited to cladding elements for watches or jewelery for which the corrosion resistance and the polishing ability are properties more important than the hardness.

On comprendra que les diagrammes illustrés aux figures 1 à 3 sont des représentations schématiques simplifiées. En effet, chaque composition d'acier austénitique HNS ou HIS a un diagramme temps-température-transformation qui lui est propre et qui est également fonction de la nature du précipité considéré.It will be understood that the diagrams illustrated in Figures 1 to 3 are simplified schematic representations. In fact, each austenitic steel composition HNS or HIS has a time-temperature-transformation diagram which is specific to it and which is also a function of the nature of the precipitate in question.

La figure 1 est un diagramme temps (t) - température (T) - transformation qui illustre le traitement thermique d'un acier austénitique HNS ou HIS selon la première variante de mise en oeuvre du procédé de l'invention. Soit Tr1 la température d'austénitisation ou de recuit d'un acier austénitique de type HNS ou HIS et soit a la courbe qui, sur le diagramme temps-température-transformation de la figure 1, délimite une aire qui correspond à des conditions de temps et de température qui permettent la formation de précipités. On désigne par 1 la courbe de refroidissement rapide qui permet de ramener l'acier austénitique HNS ou HIS depuis sa température de recuit jusqu'à la température ambiante en évitant toute formation de précipités, et par 2 la courbe de refroidissement selon l'invention qui combine les paramètres temps et température de façon telle qu'en abaissant la température de l'acier austénitique HNS ou HIS suivant cette courbe 2, on permet l'apparition de précipités dans cet acier.The figure 1 is a time (t) - temperature (T) - transformation diagram which illustrates the heat treatment of an austenitic steel HNS or HIS according to the first alternative embodiment of the method of the invention. Tr1 is the austenitizing temperature or annealing of a type of austenitic steel HNS or HIS and either a curve which, on the time-temperature-transformation diagram figure 1 , defines an area that corresponds to conditions of time and temperature that allow the formation of precipitates. 1 designates the fast cooling curve which makes it possible to reduce the austenitic steel HNS or HIS from its annealing temperature to ambient temperature by avoiding any formation of precipitates, and by the cooling curve according to the invention which combines the time and temperature parameters in such a way that by lowering the temperature of the austenitic steel HNS or HIS according to this curve 2, the appearance of precipitates in this steel is allowed.

La figure 2 est un diagramme temps (t) - température (T) - transformation qui illustre le traitement thermique d'un acier austénitique HNS ou HIS selon la deuxième variante de mise en oeuvre du procédé de l'invention. Soit Tr2 la température d'austénitisation ou de recuit d'un acier austénitique de type HNS ou HIS et soit b la courbe qui, sur le diagramme temps-température-transformation de la figure 2, délimite une aire qui correspond à des conditions de temps et de température qui permettent la formation de précipités. On commence par refroidir rapidement l'acier austénitique HNS ou HIS depuis sa température de recuit Tr2 selon la courbe 4, puis on interrompt le refroidissement de l'acier austénitique HNS ou HIS lorsque la température à atteint une valeur Tp2 à laquelle peuvent apparaître des précipités, et on maintient cet acier à cette température Tp2 pendant une durée telle qu'apparaissent des précipités (courbe 6). Finalement, on ramène l'acier à température ambiante (courbe 8).The figure 2 is a time (t) - temperature (T) - transformation diagram which illustrates the heat treatment of an austenitic steel HNS or HIS according to the second alternative embodiment of the method of the invention. Let Tr2 be the austenitization or annealing temperature of an austenitic steel of the HNS or HIS type and let b be the curve which, on the time-temperature-transformation chart of the figure 2 , defines an area that corresponds to conditions of time and temperature that allow the formation of precipitates. Firstly, the austenitic steel HNS or HIS is rapidly cooled from its annealing temperature Tr2 according to curve 4, and the cooling of the austenitic steel HNS or HIS is interrupted when the temperature reaches a value Tp2 which can appear. precipitates, and this steel is maintained at this temperature Tp2 for a period such that precipitates appear (curve 6). Finally, the steel is brought back to room temperature (curve 8).

La figure 3 est un diagramme temps (t) - température (T) - transformation qui illustre le traitement thermique d'un acier austénitique HNS ou HIS selon la troisième variante de mise en oeuvre du procédé de l'invention. Soit Tr3 la température d'austénitisation ou de recuit d'un acier austénitique de type HNS ou HIS et soit c la courbe qui, sur le diagramme temps-température-transformation de la figure 3, délimite une aire qui correspond à des conditions de temps et de température qui permettent la formation de précipités. L'acier dont il est question ici est un acier austénitique HNS ou HIS qui a été refroidi suffisamment rapidement depuis sa température de recuit Tr3 jusqu'à la température ambiante pour pouvoir éviter toute formation de précipités. Conformément à la troisième variante de mise en oeuvre du procédé de l'invention, un tel acier austénitique HNS ou HIS est chauffé selon la courbe 10 et maintenu à une température et pendant une durée telles qu'apparaissent des précipités (courbe 12), puis est refroidi (courbe 14).The figure 3 is a time (t) - temperature (T) - transformation diagram which illustrates the heat treatment of an austenitic steel HNS or HIS according to the third alternative embodiment of the method of the invention. Tr3 is the austenitizing temperature or annealing of a HNS type austenitic steel or HIS and c is the curve, the time-temperature-transformation diagram figure 3 , defines an area that corresponds to conditions of time and temperature that allow the formation of precipitates. The steel in question here is an austenitic steel HNS or HIS which has been cooled sufficiently rapidly from its annealing temperature Tr3 to room temperature in order to avoid any formation of precipitates. According to the third alternative embodiment of the process of the invention, such austenitic steel HNS or HIS is heated according to curve 10 and maintained at a temperature and for a duration such that precipitates appear (curve 12), then is cooled (curve 14).

La quatrième variante de mise en oeuvre du procédé de l'invention ne diffère de la troisième variante du même procédé qu'en ce que, après traitement de recuit suivi d'une trempe et avant le traitement de précipitation, l'acier austénitique HNS ou HIS est écroui, c'est-à-dire déformé à froid. Le traitement thermique selon l'invention qui consiste à porter un acier austénitique à une température et pendant une durée telles que des précipités se forment est donc appliqué, dans cette quatrième variante, à un matériau préalablement durci par écrouissage.The fourth alternative embodiment of the process of the invention differs from the third variant of the same process only in that, after annealing treatment followed by quenching and before the precipitation treatment, the austenitic steel HNS or HIS is hardened, that is to say deformed cold. The heat treatment according to the invention which consists in wearing an austenitic steel at a temperature and for a period of time such that precipitates are formed is therefore applied, in this fourth variant, to a material that has been hardened beforehand by hardening.

Enfin, la cinquième et dernière variante de mise en oeuvre du procédé de l'invention consiste à soumettre l'acier austénitique à un traitement de déformation à froid après traitement thermique selon l'une des trois premières variantes de mise en oeuvre.Finally, the fifth and last alternative embodiment of the process of the invention consists in subjecting the austenitic steel to a cold deformation treatment after heat treatment according to one of the first three processing variants.

Différents essais ont été menés sur l'acier austénitique HIS X20CrMnMoN17-11-3.Various tests have been conducted on austenitic steel HIS X20CrMnMoN17-11-3.

La figure 4 est une vue d'une coupe métallographique d'un échantillon d'acier HIS X20CrMnMoN17-11-3 qui a été recuit à sa température d'austénitisation puis trempé. On remarque à l'examen de cette figure que les joints de grains sont peu marqués, ce qui dénote l'absence de précipités.The figure 4 is a view of a metallographic section of a HIS X20CrMnMoN17-11-3 steel sample that has been annealed at its austenitization temperature and quenched. It is noted on examining this figure that the grain boundaries are not very marked, which indicates the absence of precipitates.

La figure 5 est une vue d'une coupe métallographique d'un échantillon d'acier austénitique HIS X20CrMnMoN17-11-3 ayant subi un traitement thermique conforme à la troisième variante de mise en oeuvre du procédé selon l'invention. Comme on peut le voir à l'examen de la figure 5, les joints de grains sont marqués, ce qui dénote la présence en quantités importantes de précipités le long de ces joints de grains. On voit même (zones entourées d'un cercle sur la figure 5) que certains précipités, plus gros, ont crû à l'intérieur des grains depuis les joints de grains. Une telle concentration en précipités a pu être obtenue en portant, après refroidissement rapide depuis la température de recuit, l'acier austénitique HIS X20CrMnMoN17-11-3 à une température de 800°C pendant deux heures.The figure 5 is a view of a metallographic section of a sample of austenitic steel HIS X20CrMnMoN17-11-3 having undergone a heat treatment according to the third variant of implementation of the method according to the invention. As can be seen from the examination of the figure 5 the grain boundaries are marked, indicating the presence of large amounts of precipitates along these grain boundaries. We even see (areas surrounded by a circle on the figure 5 ) that some larger precipitates have grown inside the grains since the grain boundaries. Such a concentration of precipitates could be obtained by carrying, after cooling rapidly from the annealing temperature, the austenitic steel HIS X20CrMnMoN17-11-3 at a temperature of 800 ° C for two hours.

Pour certaines applications, comme des composants d'un mouvement horloger, il n'est pas envisageable de recuire les pièces (après traitement de précipitation) dans la mesure où l'on souhaite préserver la dureté obtenue après déformation à froid. Des échantillons d'acier austénitique HIS X20CrMnMoN17-11-3 ont donc été soumis à un procédé de traitement thermique conforme à la quatrième variante de mise en oeuvre de l'invention et consistant, après traitement de recuit suivi d'une trempe et écrouissage, à porter l'acier austénitique HIS X20CrMnMoN17-11-3 à une température et pendant une durée telles que des précipités se forment. On a observé qu'après déformation à froid, la formation des précipités est beaucoup plus rapide. En effet, les dislocations et les lacunes induites par la déformation à froid créent des chemins de diffusion favorables à la germination et à la croissance des précipités.For certain applications, such as components of a watch movement, it is not possible to anneal the parts (after precipitation treatment) insofar as it is desired to preserve the hardness obtained after cold deformation. Samples of austenitic steel HIS X20CrMnMoN17-11-3 were therefore subjected to a heat treatment process according to the fourth variant embodiment of the invention and consisting, after annealing treatment followed by quenching and hardening, to wear the austenitic steel HIS X20CrMnMoN17-11-3 at a temperature and for a time such that precipitates form. It has been observed that after cold deformation, the formation of the precipitates is much faster. Indeed, the dislocations and the gaps induced by the cold deformation create diffusion paths favorable to the germination and the growth of the precipitates.

La figure 6 est une vue d'une coupe métallographique d'un échantillon d'acier austénitique HIS X20CrMnMoN17-11-3 qui se présente sous la forme d'un barreau dont le diamètre extérieur est ramené de 3 mm à 2,5 mm par déformation à froid par tréfilage, soit une réduction de diamètre de 16,6%. Conformément à la quatrième variante de mise en oeuvre du procédé selon l'invention, cet échantillon a ensuite été porté à une température de 800°C pendant deux heures suivant la courbe de température représentée à la figure 3. On voit que l'acier présente de nombreux précipités, aussi bien aux joints de grains qu'à l'intérieur des grains.The figure 6 is a view of a metallographic section of a sample of austenitic steel HIS X20CrMnMoN17-11-3 which is in the form of a bar whose outer diameter is reduced from 3 mm to 2.5 mm by cold deformation by wire drawing, a diameter reduction of 16.6%. According to the fourth alternative embodiment of the process according to the invention, this sample was then heated to a temperature of 800 ° C. for two hours according to the temperature curve represented in FIG. figure 3 . It can be seen that steel has many precipitates, both at the grain boundaries and inside the grains.

La figure 7 est un graphe qui montre l'évolution de la dureté de l'acier austénitique HIS X20CrMnMoN17-11-3 de la figure 6 en fonction de la température à laquelle cet acier est porté pour former les précipités. On observe que la dureté de l'acier austénitique sans traitement de précipitation selon l'invention et après écrouissage à froid est de 450 HV10 (symbole en forme de carré sur le graphe). Le même acier austénitique est, après écrouissage à froid, traité thermiquement conformément à la quatrième variante de mise en oeuvre du procédé selon l'invention. Des échantillons de cet acier sont portés respectivement à des températures de 750°C, 800°C, 850°C, 900°C et 950°C pendant une durée de deux heures, puis refroidis (symboles en forme de losange sur le graphe). On observe que, pour les échantillons chauffés entre 700°C et 900°C, la dureté est comprise entre environ 425 HV10 et 375 HV10. Autrement dit, la dureté de ces échantillons d'acier austénitique traités thermiquement conformément à la quatrième variante de l'invention varie peu par rapport à la dureté de l'acier austénitique écroui mais n'ayant pas fait l'objet d'un traitement de précipitation. Par contre, l'usinabilité des échantillons d'acier austénitique ayant subi un traitement thermique de précipitation selon cette quatrième variante de l'invention est nettement améliorée. Seul l'échantillon d'acier austénitique chauffé à 950°C pendant deux heures a une dureté sensiblement inférieure à celle de l'acier austénitique sans traitement de précipitation (moins de 350 HV10). Enfin, un échantillon d'acier austénitique HIS X20CrMnMoN17-11-3 ayant subi uniquement un traitement de recuit suivi d'une trempe (symbole en forme de triangle sur le graphe) a une dureté inférieure à 250 HV10.The figure 7 is a graph that shows the evolution of the hardness of austenitic steel HIS X20CrMnMoN17-11-3 of the figure 6 depending on the temperature at which this steel is worn to form the precipitates. It is observed that the hardness of the austenitic steel without precipitation treatment according to the invention and after cold working is 450 HV10 (symbol in the form of a square on the graph). The same austenitic steel is, after cold working, heat-treated according to the fourth alternative embodiment of the method according to the invention. Samples of this steel are respectively heated to temperatures of 750 ° C, 800 ° C, 850 ° C, 900 ° C and 950 ° C for a period of two hours, then cooled (diamond symbols on the graph) . It is observed that for samples heated between 700 ° C and 900 ° C, the hardness is between about 425 HV10 and 375 HV10. In other words, the hardness of these samples of austenitic steel heat-treated according to the fourth variant of the invention varies little with respect to the hardness of the austenitic steel hardened but not subjected to a treatment of precipitation. On the other hand, the machinability of the austenitic steel samples having undergone a heat treatment of precipitation according to this fourth variant of the invention is clearly improved. Only the austenitic steel sample heated at 950 ° C for two hours has a hardness substantially lower than that of austenitic steel without precipitation treatment (less than 350 HV10). Finally, a sample of austenitic steel HIS X20CrMnMoN17-11-3 having undergone only annealing treatment followed by tempering (triangular symbol on the graph) has a hardness less than 250 HV10.

La figure 8 est une vue d'une coupe métallographique d'un échantillon d'acier austénitique HIS X20CrMnMoN17-11-3 qui se présente sous la forme d'un barreau dont le diamètre extérieur est ramené de 3 mm à 2 mm par déformation à froid par tréfilage, soit une réduction de diamètre plus importante encore de 33,3 %. Cet échantillon d'acier subit le même traitement thermique qu'à la figure 6 en étant porté à une température de 800°C pendant deux heures conformément à la quatrième variante de mise en oeuvre de l'invention. On voit que, comparé à la figure 6, le phénomène de précipitation est encore plus prononcé puisque, outre les précipités qui se forment le long des joints de grains et depuis les joints de grains vers l'intérieur des grains, on a une forte concentration de précipités à l'intérieur-même des grains.The figure 8 is a view of a metallographic section of a sample of austenitic steel HIS X20CrMnMoN17-11-3 in the form of a bar whose outer diameter is reduced from 3 mm to 2 mm by cold deformation by wire drawing a larger diameter reduction of 33.3%. This steel sample undergoes the same heat treatment as at figure 6 by being heated to a temperature of 800 ° C. for two hours in accordance with the fourth variant embodiment of the invention. We see that, compared to the figure 6 the precipitation phenomenon is even more pronounced since, besides the precipitates which are formed along the grain boundaries and from the grain boundaries into the grains, there is a high concentration of precipitates within the grains.

La figure 9 est un graphe qui montre l'évolution de la dureté de l'acier de la figure 8 en fonction de la durée et de la température à laquelle cet acier est porté après écrouissage pour former les précipités. On observe que la dureté de l'acier austénitique sans traitement de précipitation selon l'invention et après écrouissage à froid est compris entre 550 HV10 et 560 HV10 (symbole en forme de carré sur le graphe). Cette dureté est plus grande que celle à la figure 7 car le taux d'écrouissage est plus élevé. Les symboles en forme de losange sur la figure 9 correspondent à des échantillons d'acier austénitique portés à des températures respectives de 700°C, 750°C, 800°C et 850°C pendant 45 minutes. Les symboles en forme de rond correspondent à des échantillons d'acier austénitique portés à des températures respectives de 700°C, 750°C, 800°C et 850°C pendant deux heures. Si l'on compare les graphes des figures 7 et 9, on observe que plus le taux d'écrouissage est élevé, plus la formation de précipités est facilitée. En effet, les tensions mécaniques au coeur de l'acier permettent de faire germer et croître les précipités.The figure 9 is a graph that shows the evolution of the hardness of the steel of the figure 8 depending on the time and temperature at which this steel is worn after work hardening to form the precipitates. It is observed that the hardness of the austenitic steel without precipitation treatment according to the invention and after cold working is between 550 HV10 and 560 HV10 (symbol in the form of a square on the graph). This hardness is greater than that at figure 7 because the rate of work hardening is higher. The diamond shaped symbols on the figure 9 correspond to samples of austenitic steel heated to temperatures of 700 ° C, 750 ° C, 800 ° C and 850 ° C respectively for 45 minutes. The round symbols correspond to samples of austenitic steel heated to temperatures of 700 ° C, 750 ° C, 800 ° C and 850 ° C respectively for two hours. If we compare the graphs of Figures 7 and 9 it is observed that the higher the rate of work hardening, the more the formation of precipitates is facilitated. Indeed, the mechanical stresses at the heart of the steel make it possible to germinate and grow the precipitates.

On observe que, pour une même température de traitement de précipitation, la dureté des échantillons d'acier austénitique est plus faible lorsque la durée du traitement de précipitation est plus longue. On observe également que, pour une même durée de traitement de deux heures, la dureté de l'acier est d'autant plus faible que la température de précipitation est élevée. Toutefois, ces graphes montrent qu'il est possible d'obtenir des aciers avec de nombreux précipités et dont les duretés sont néanmoins proches des duretés initiales.It is observed that, for the same precipitation treatment temperature, the hardness of the austenitic steel samples is lower when the duration of the precipitation treatment is longer. It is also observed that, for the same duration of treatment of two hours, the hardness of the steel is even lower than the precipitation temperature is high. However, these graphs show that it is possible to obtain steels with many precipitates and whose hardnesses are nevertheless close to the initial hardnesses.

Il va de soi que la présente invention n'est pas limitée au mode de réalisation qui vient d'être décrit et que diverses modifications et variantes simples peuvent être envisagées par l'homme du métier sans sortir du cadre de l'invention tel que défini par les revendications annexées. Quelques exemples non limitatifs d'aciers HNS et HIS auxquels le procédé de précipitation selon l'invention peut être appliqué sont: XSCrMnN18-18, X8CrMnN19-19, X8CrMnMoN18-18-2, X13CrMnMoN18-14-3, X20CrMnMoN17-11-3 ou bien encore X5MnCrMoN23-21. Enfin, quelques exemples de précipités qui peuvent se former durant le procédé de précipitation sont : M23C, MC, M6C ou bien encore M2N, où M désigne un ou plusieurs des éléments métalliques de l'alliage pouvant se combiner au carbone ou à l'azote pour former des carbures ou nitrures ou carbonitrures. L'invention s'applique notamment aux bijoux et aux éléments d'habillage des pièces d'horlogerie.It goes without saying that the present invention is not limited to the embodiment which has just been described and that various modifications and simple variants can be envisaged by those skilled in the art without departing from the scope of the invention as defined by the appended claims. Some non-limiting examples of HNS and HIS steels to which the precipitation process according to the invention can be applied are: XSCrMnN18-18, X8CrMnN19-19, X8CrMnMoN18-18-2, X13CrMnMoN18-14-3, X20CrMnMoN17-11-3 or still well X5MnCrMoN23-21. Finally, some examples of precipitates that may form during the precipitation process are: M23C, MC, M6C or even M2N, where M denotes one or more of the metal elements of the alloy that can combine with carbon or nitrogen to form carbides or nitrides or carbonitrides. The invention applies in particular to jewelery and cladding elements of timepieces.

On a compris, de ce qui précède, qu'il est avantageux d'usiner un élément, par exemple de bijou ou bien d'une montre-bracelet, à l'aide d'un acier austénitique de type HNS ou HIS renfermant des précipités. Il peut cependant être également avantageux, après usinage, de faire disparaître ces précipités. En effet, si les précipités rendent les opérations d'usinage plus aisées en favorisant la formation et l'enlèvement des copeaux lors de l'usinage des pièces, il peut être intéressant d'éliminer ces copeaux après usinage afin d'améliorer la ductilité et la résistance de ces pièces à la corrosion. C'est pourquoi, la présente invention enseigne un procédé de traitement thermique d'un acier austénitique HNS ou HIS renfermant des précipités, ce procédé comprenant l'étape qui consiste, après usinage de pièces, notamment de bijouterie ou d'horlogerie, réalisées au moyen d'un acier austénitique HNS ou HIS renfermant des précipités, à remettre les précipités en solution en portant ces pièces en acier austénitique HNS ou HIS à leur température d'austénitisation, puis en refroidissant ces pièces suffisamment rapidement, typiquement par trempe, pour empêcher les précipités de se former à nouveau. Par opérations d'usinage, on entend notamment mais non exclusivement les opérations d'alésage, de fraisage, de perçage, de filetage, de taraudage et de découpage.It has been understood from the foregoing that it is advantageous to machine an element, for example a jewel or a wristwatch, with austenitic steel of the HNS or HIS type containing precipitates. . However, it can also be advantageous, after machining, to remove these precipitates. Indeed, if the precipitates make the machining operations easier by promoting the formation and removal of chips during machining parts, it may be interesting to remove these chips after machining to improve the ductility and the resistance of these pieces to the corrosion. Therefore, the present invention teaches a method of heat treatment of an austenitic steel HNS or HIS containing precipitates, this method comprising the step which consists, after machining parts, including jewelry or timepieces, made at using a HNS or HIS austenitic steel containing precipitates, to return the precipitates in solution by bringing these austenitic steel parts HNS or HIS to their austenitization temperature, then cooling these parts sufficiently quickly, typically by quenching, to prevent precipitates to form again. Machining operations include, but are not limited to, boring, milling, drilling, threading, tapping and cutting operations.

Claims (6)

Procédé de traitement thermique d'un acier austénitique du type High Nitrogen Steel ou acier austénitique HNS, ou bien d'un acier austénitique du type High Interstitial Steel ou acier austénitique HIS, cet acier austénitique HNS ou HIS renfermant des précipités du type nitrures, carbures ou carbonitrures de chrome et/ou de molybdène, ce procédé comprenant l'étape qui consiste, après usinage de l'acier austénitique HNS ou HIS renfermant les précipités, à remettre les précipités en solution en portant l'acier austénitique HNS ou HIS à sa température d'austénitisation, puis en refroidissant cet acier austénitique HNS ou HIS suffisamment rapidement pour éviter de reformer des précipités.Process for heat treatment of austenitic steel of the High Nitrogen Steel type or austenitic HNS steel, or of a high-strength steel or austenitic steel of the HIS type, this austenitic steel HNS or HIS containing precipitates of the nitride, carbide type or chromium and / or molybdenum carbonitrides, which process comprises the step of machining the austenitic HNS or HIS steel containing the precipitates to return the precipitates to solution by carrying the austenitic HNS or HIS steel at its austenitization temperature, then cooling this austenitic steel HNS or HIS sufficiently quickly to avoid reforming precipitates. Procédé selon la revendication 1, caractérisé en ce que, pour faire apparaître avant usinage des précipités du type nitrures, carbures ou carbonitrures de chrome et/ou de molybdène dans l'acier austénitique HNS ou HIS, on se munit d'un alliage d'acier austénitique HNS ou HIS que l'on porte à sa température d'austénitisation ou que l'on fritte à la température d'austénitisation, puis, immédiatement depuis la température d'austénitisation, on abaisse la température de l'alliage d'acier austénitique HNS ou HIS de manière suffisamment lente pour qu'apparaissent les précipités dans la structure de l'acier austénitique HNS ou HIS résultant, puis enfin on ramène l'acier austénitique HNS ou HIS à température ambiante.A method according to claim 1, characterized in that , prior to machining, nitride, carbide or carbonitride precipitates of the chromium and / or molybdenum type in HNS or HIS austenitic steel are produced. austenitic steel HNS or HIS which is heated to its austenitization temperature or sintered at the austenitization temperature, then, immediately from the austenitization temperature, the temperature of the steel alloy is lowered austenitic HNS or HIS sufficiently slowly to appear precipitates in the structure of the resulting austenitic steel HNS or HIS, then finally the austenitic steel HNS or HIS is brought to room temperature. Procédé selon la revendication 1, caractérisé en ce que, pour faire apparaître avant usinage des précipités du type nitrures, carbures ou bien encore carbonitrures de chrome et/ou de molybdène dans l'acier austénitique HNS ou HIS, on se munit d'un alliage d'acier austénitique HNS ou HIS que l'on porte à sa température d'austénitisation ou que l'on fritte à la température d'austénitisation, puis on soumet cet alliage d'acier austénitique HNS ou HIS à un traitement thermique de refroidissement immédiatement depuis la température d'austénitisation, et on interrompt le refroidissement de l'acier austénitique HNS ou HIS résultant lorsque la température à atteint une valeur à laquelle apparaissent les précipités, cet acier austénitique HNS ou HIS étant maintenu à cette température et pendant une durée telle qu'apparaissent les précipités, puis enfin on ramène l'acier austénitique HNS ou HIS à température ambiante.A method according to claim 1, characterized in that , before machining nitride precipitates, carbides or even carbonitrides of chromium and / or molybdenum in the austenitic steel HNS or HIS, it is provided with an alloy austenitic steel HNS or HIS that is brought to its austenitization temperature or that sintered at the austenitization temperature, then subjected to this alloy of austenitic steel HNS or HIS heat treatment cooling immediately since the austenitization temperature, and one interrupts the cooling of the austenitic steel HNS or HIS resulting when the temperature reaches a value at which the precipitates appear, this austenitic steel HNS or HIS being maintained at this temperature and for a time such that the precipitates appear, then finally the steel is brought back austenitic HNS or HIS at room temperature. Procédé selon la revendication 1, caractérisé en ce que, pour faire apparaître avant usinage des précipités du type nitrures, carbures ou bien encore carbonitrures de chrome et/ou de molybdène dans l'acier austénitique HNS ou HIS, on soumet un alliage d'acier austénitique HNS ou HIS à un traitement thermique d'austénitisation ou de frittage à la température d'austénitisation, puis on trempe cet alliage d'acier austénitique HNS ou HIS et on le chauffe à nouveau jusqu'à une température et pendant une durée telle qu'apparaissent des précipités du type nitrures, carbures ou bien encore carbonitrures de chrome et/ou de molybdène.A method according to claim 1, characterized in that , prior to machining nitride precipitates, carbides or even carbonitrides of chromium and / or molybdenum in the austenitic steel HNS or HIS, a steel alloy is subjected to austenitic HNS or HIS to a heat treatment of austenitization or sintering at the austenitization temperature, then this alloy of austenitic steel HNS or HIS is quenched and heated again to a temperature and for a duration such that Nitrides, carbides or even carbonitrides of chromium and / or molybdenum appear. Procédé selon la revendication 4, caractérisé en ce que, après la trempe et avant d'amener l'acier austénitique HNS ou HIS à une température et pendant une durée telles qu'apparaissent des précipités du type nitrures, carbures ou bien encore carbonitrures de chrome et/ou de molybdène, on déforme à froid l'acier austénitique HNS ou HIS.Process according to Claim 4, characterized in that , after the quenching and before bringing the austenitic steel HNS or HIS to a temperature and for a duration such as nitride, carbide or even chromium carbonitride precipitates appear. and / or molybdenum, cold deformed austenitic steel HNS or HIS. Elément d'horlogerie ou de bijouterie obtenu à partir d'un acier austénitique HNS ou HIS obtenu par mise en oeuvre du procédé de traitement thermique selon l'une quelconque des revendications 1 à 5.Timepiece or jewelery element obtained from an austenitic steel HNS or HIS obtained by carrying out the heat treatment process according to any one of Claims 1 to 5.
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