EP3249059A1 - 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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Publication number
EP3249059A1
EP3249059A1 EP16171672.5A EP16171672A EP3249059A1 EP 3249059 A1 EP3249059 A1 EP 3249059A1 EP 16171672 A EP16171672 A EP 16171672A EP 3249059 A1 EP3249059 A1 EP 3249059A1
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Prior art keywords
hns
steel
austenitic
temperature
precipitates
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EP16171672.5A
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German (de)
French (fr)
Inventor
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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Priority to EP16171672.5A priority Critical patent/EP3249059A1/en
Priority to EP17167986.3A priority patent/EP3249060B1/en
Priority to US15/589,067 priority patent/US11136638B2/en
Priority to JP2017101408A priority patent/JP6509944B2/en
Priority to CN201710383822.6A priority patent/CN107435087B/en
Publication of EP3249059A1 publication Critical patent/EP3249059A1/en
Withdrawn legal-status Critical Current

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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 heat treatment process of an austenitic steel HNS or HIS which consists in performing a slow cooling of this steel austenitic HNS or HIS immediately after its austenitization or sintering, so as to reveal precipitates.
  • 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.
  • HNS and HIS austenitic steels undergo a slow cooling heat treatment to promote the appearance of precipitates
  • the present invention is totally contrary to the usual practice of cooling the precipitates. alloys as quickly as possible in order to avoid as much as possible the formation of precipitates in the resulting HNS and HIS austenitic steels.
  • the Applicant has indeed found that by subjecting the austenitic steels HNS and HIS to the heat treatment process according to the invention, the nitrogen and carbon atoms, for example, tend to migrate towards the grain boundaries and to combine relatively 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.
  • the heat treatment process according to the present invention applies equally well to parts obtained by casting and subsequent thermomechanical treatment, as well as to parts obtained by powder metallurgy such as molding. by metal injection still known under the name Anglo-Saxon 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 cool slowly the alloy to promote the formation of precipitates in accordance with the teachings of the present invention.
  • the present invention relates to a heat treatment process of an austenitic steel HNS or HIS which consists of subjecting the austenitic steel HNS or HIS to cooling from the austenitization or sintering temperature, and then to interrupt the cooling austenitic steel HNS or HIS when the temperature reaches a value at which precipitates may appear, and to maintain the steel at this temperature and for a period such as appear precipitates, then finally to bring the steel temperature room.
  • the austenitic steel HNS or HIS after the austenitic steel HNS or HIS has undergone a heat treatment of austenitization or sintering then quenching, the austenitic steel HNS or HIS is heated to a temperature and for a time such that precipitates appear.
  • This third variant is the most practical because it allows to perfectly control the parameters of different heat treatments.
  • the precipitates are re-dissolved after machining by again bringing the austenitic steel HNS or HIS to its annealing temperature, and then cooling rapidly enough to avoid forming precipitates again.
  • This feature is very advantageous because it allows, when desired, to remove after machining parts precipitates that have been created through the heat treatment process according to the invention.
  • this possibility can be used to eliminate precipitates in covering elements (middle, back of watch cases, glasses, crowns, pushers, clasps, bracelets, 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.
  • clockwork components it is preferable not to subject these clockwork components to a second austenitization heat treatment, in order to preserve the hardness obtained by deformation. Cold.
  • 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, because they promote the corrosion resistance of these steels.
  • These first three variants have in common that after application of the heat treatment according to the invention to austenitic steel HNS or HIS and subsequent machining, it is indeed possible to bring the resulting part to the annealing temperature, and then soak it in order to put back 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 then this austenitic steel HNS or HIS is brought to a temperature and for a period such that appear precipitates.
  • 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 steel austenitic HNS or HIS is deformed cold after this steel has been brought back to room temperature.
  • the present invention proceeds from the general inventive idea of subjecting the austenitic HNS and HIS steels to a heat treatment of precipitation.
  • the term "heat treatment of precipitation” is intended to mean a treatment which aims at placing 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, especially molybdenum and / or chromium. It has indeed been observed that these precipitates are generally not very bound to the matrix of the material, so that they favor the formation and removal of chips when machining parts.
  • 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.
  • 1 is the fast cooling curve which allows to bring back the austenitic steel HNS or HIS since its annealing temperature to ambient temperature avoiding any formation of precipitates
  • 2 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 following this curve 2, allows the appearance of precipitates in this steel.
  • 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 rapidly from its annealing temperature Tr2 according to curve 4, and cooling of the austenitic steel HNS or HIS is interrupted when the temperature reaches a value Tp2 at which precipitates can appear. and this steel is maintained at this temperature Tp2 for a time such that precipitates appear (curve 6). Finally, the steel is brought back to room temperature (curve 8).
  • 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 time such that precipitates appear (curve 12), and 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.
  • the idea 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.
  • 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 treatment of precipitation according to the invention and after cold work-hardening 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 quenching (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 phenomenon of precipitation is even more pronounced since, besides the precipitates which are formed along the grain boundaries and from the joints of grains inward grains, there is a high concentration of precipitates inside-even 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 process Precipitation according to the invention can be applied are: X5CrMnN18-18, X8CrMnN19-19, X8CrMnMoN18-18-2, X13CrMnMoN18-14-3, X20CrMnMoN17-11-3 or alternatively 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.
  • 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 trim elements of timepieces, as well as to watch components.

Abstract

L'invention concerne un procédé de traitement thermique d'aciers austénitiques HNS ou HIS qui consiste à soumettre les aciers austénitiques HNS et HIS à un traitement thermique dont la durée et la température sont telles qu'il place ces aciers austénitiques HNS et HIS dans des conditions qui permettent l'apparition de précipités tels que des nitrures, des carbures ou des carbonitrures de molybdène ou de chrome. La présence de tels précipités, généralement peu liés à la matrice du matériau, favorise la formation et l'enlèvement des copeaux lors de l'usinage des pièces. L'invention concerne également des aciers austénitiques HNS ou HIS comprenant des précipités dans leur matrice.The invention relates to a method for thermal treatment of austenitic HNS or HIS steels which consists in subjecting the austenitic steels HNS and HIS to a heat treatment whose duration and temperature are such that it places these austenitic steels HNS and HIS in conditions which allow the appearance of precipitates such as nitrides, carbides or carbonitrides of molybdenum or chromium. The presence of such precipitates, generally little related to the matrix of the material, promotes the formation and removal of chips during machining parts. The invention also relates to austenitic HNS or HIS steels comprising precipitates in their matrix.

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, et selon une première variante, la présente invention concerne un procédé de traitement thermique d'un acier austénitique HNS ou HIS qui consiste à effectuer un refroidissement lent de cet acier austénitique HNS ou HIS immédiatement après son austénitisation ou son frittage, de façon à faire apparaître des précipités.For this purpose, and according to a first variant, the present invention relates to a heat treatment process of an austenitic steel HNS or HIS which consists in performing a slow cooling of this steel austenitic HNS or HIS immediately after its austenitization or sintering, so as to reveal precipitates.

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, les aciers austénitiques HNS et HIS à un traitement thermique de refroidissement lent pour favoriser l'apparition de précipités, la présente invention va totalement 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 that, after austenitization or sintering, HNS and HIS austenitic steels undergo a slow cooling heat treatment to promote the appearance of precipitates, the present invention is totally contrary to the usual practice of cooling the precipitates. alloys as quickly as possible in order to avoid as much as possible the formation of precipitates in the resulting HNS and HIS austenitic steels.

La Demanderesse a effectivement constaté qu'en soumettant les aciers austénitiques HNS et HIS au procédé de traitement thermique selon l'invention, 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 according to the invention, the nitrogen and carbon atoms, for example, tend to migrate towards the grain boundaries and to combine relatively 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.

On notera que, selon un avantage important, le procédé de traitement thermique conforme à la présente invention 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 be noted that, according to one important advantage, the heat treatment process according to the present invention applies equally well to parts obtained by casting and subsequent thermomechanical treatment, as well as to parts obtained by powder metallurgy such as molding. by metal injection still known under the name Anglo-Saxon 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 cool slowly the alloy to promote the formation of precipitates in accordance with the teachings of the present invention.

Selon une deuxième variante, la présente invention concerne un procédé de traitement thermique d'un acier austénitique HNS ou HIS qui consiste à soumettre cet acier austénitique HNS ou HIS à un refroidissement depuis la température d'austénitisation ou de frittage, puis à interrompre le refroidissement de l'acier austénitique HNS ou HIS lorsque la température à atteint une valeur à laquelle peuvent apparaître des précipités, et à maintenir cet acier à cette température et pendant une durée telles qu'apparaissent des précipités, puis enfin à ramener l'acier à température ambiante.According to a second variant, the present invention relates to a heat treatment process of an austenitic steel HNS or HIS which consists of subjecting the austenitic steel HNS or HIS to cooling from the austenitization or sintering temperature, and then to interrupt the cooling austenitic steel HNS or HIS when the temperature reaches a value at which precipitates may appear, and to maintain the steel at this temperature and for a period such as appear precipitates, then finally to bring the steel temperature room.

Selon une troisième variante 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 puis de trempe, on chauffe l'acier austénitique HNS ou HIS à une température et pendant une durée telles qu'apparaissent des précipités.According to a third alternative embodiment of the process of the invention, after the austenitic steel HNS or HIS has undergone a heat treatment of austenitization or sintering then quenching, the austenitic steel HNS or HIS is heated to a temperature and for a time such that precipitates appear.

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.

Selon une caractéristique complémentaire qui peut être commune aux trois variantes de mise en oeuvre du procédé de l'invention, les précipités sont remis en solution après usinage en portant à nouveau l'acier austénitique HNS ou HIS à sa température de recuit, puis en le refroidissant suffisamment rapidement pour éviter de former à nouveau des précipités.According to a complementary characteristic which may be common to the three alternative embodiments of the process of the invention, the precipitates are re-dissolved after machining by again bringing the austenitic steel HNS or HIS to its annealing temperature, and then cooling rapidly enough to avoid forming precipitates again.

Cette caractéristique se révèle très avantageuse car elle permet, lorsque cela est souhaité, de faire disparaître après usinage des pièces les précipités qui ont été créés grâce au procédé de traitement thermique selon l'invention. Dans le cas particulier des pièces d'horlogerie, on pourra 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. Au contraire, dans le cas des composants d'horlogerie (roues dentées, pignons, échappements etc.), on préférera ne pas soumettre ces composants d'horlogerie à un second traitement thermique d'austénitisation, ceci afin de préserver la dureté obtenue par déformation à froid.This feature is very advantageous because it allows, when desired, to remove after machining parts precipitates that have been created through the heat treatment process according to the invention. In the particular case of timepieces, this possibility can be used to eliminate precipitates in covering elements (middle, back of watch cases, glasses, crowns, pushers, clasps, bracelets, 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. On the other hand, in the case of clockwork components (gears, pinions, exhausts, etc.), it is preferable not to subject these clockwork components to a second austenitization heat treatment, in order to preserve the hardness obtained by deformation. Cold.

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, car elles favorisent la résistance à la corrosion de ces aciers. Ces trois premières variantes ont en commun qu'après application du traitement thermique selon l'invention 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, because they promote the corrosion resistance of these steels. These first three variants have in common that after application of the heat treatment according to the invention to austenitic steel HNS or HIS and subsequent machining, it is indeed possible to bring the resulting part to the annealing temperature, and then soak it in order to put back 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.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 then this austenitic steel HNS or HIS is brought to a temperature and for a period such that appear precipitates.

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.

Selon une caractéristique complémentaire qui peut être commune aux trois variantes de mise en oeuvre du procédé de l'invention, l'acier austénitique HNS ou HIS est déformé à froid après que l'on a ramené cet acier à température ambiante.According to a complementary characteristic which may be common to the three variant embodiments of the method of the invention, the steel austenitic HNS or HIS is deformed cold after this steel has been brought back to room temperature.

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 heat treatment according to the third variant embodiment of the process 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 de précipitation. Au sens de l'invention, on entend par traitement thermique de précipitation 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. En outre, selon un aspect complémentaire de l'invention, il est possible, après traitement thermique de précipitation et usinage des pièces, de soumettre ces pièces à un second traitement d'austénitisation en portant à nouveau ces pièces à leur température de recuit, puis en les trempant 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 ce traitement de recuit à des éléments d'habillage pour montres pour lesquels la résistance à la corrosion et l'aptitude au polissage sont des propriétés plus importantes que la dureté. Pour des composants horlogers pour lesquels on recherche avant tout la dureté, on préférera éviter le second traitement thermique de recuit afin de ne pas provoquer dans le matériau une élimination des contraintes internes qui sont apparues au cours de l'opération d'écrouissage et qui contribuent à rendre le matériau plus dur.The present invention proceeds from the general inventive idea of subjecting the austenitic HNS and HIS steels to a heat treatment of precipitation. For the purposes of the invention, the term "heat treatment of precipitation" is intended to mean a treatment which aims at placing 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, especially molybdenum and / or chromium. It has indeed been observed that these precipitates are generally not very bound to the matrix of the material, so that they favor the formation and removal of chips when machining parts. In addition, according to a complementary aspect of the invention, it is possible, after heat treatment of precipitation and machining parts, to subject these parts to a second austenitization treatment by bringing these parts back to their annealing temperature, then by soaking them so as to put the precipitates in solid solution. As 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 thus a decrease in its hardness, this annealing treatment will preferably be reserved for cladding for watches for which corrosion resistance and polishability are more important properties than hardness. For watch components for which the hardness is primarily sought, it will be preferred to avoid the second annealing heat treatment so as not to cause in the material an elimination of the internal stresses which have arisen during the work-hardening operation and which contribute to make the material harder.

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 is the fast cooling curve which allows to bring back the austenitic steel HNS or HIS since its annealing temperature to ambient temperature avoiding any formation of precipitates, and by 2 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 following this curve 2, allows the appearance of precipitates in this steel.

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. The austenitic steel HNS or HIS is rapidly cooled rapidly from its annealing temperature Tr2 according to curve 4, and cooling of the austenitic steel HNS or HIS is interrupted when the temperature reaches a value Tp2 at which precipitates can appear. and this steel is maintained at this temperature Tp2 for a time 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 variant embodiment of the method of the invention, such austenitic steel HNS or HIS is heated according to curve 10 and maintained at a temperature and for a time such that precipitates appear (curve 12), and 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 treatment of precipitation according to the invention and after cold work-hardening 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 quenching (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 phenomenon of precipitation is even more pronounced since, besides the precipitates which are formed along the grain boundaries and from the joints of grains inward grains, there is a high concentration of precipitates inside-even 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 : X5CrMnN18-18, 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, ainsi qu'aux composants horlogers.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 process Precipitation according to the invention can be applied are: X5CrMnN18-18, X8CrMnN19-19, X8CrMnMoN18-18-2, X13CrMnMoN18-14-3, X20CrMnMoN17-11-3 or alternatively 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 trim elements of timepieces, as well as to watch components.

Claims (8)

Procédé de traitement thermique d'un acier austénitique HNS ou HIS qui consiste, après austénitisation, à abaisser la température de l'acier austénitique HNS ou HIS depuis sa température d'austénitisation ou de frittage à la température d'austénitisation de manière suffisamment lente pour qu'apparaissent des précipités dans la structure de l'acier austénitique HNS ou HIS, puis enfin à ramener l'acier à température ambiante.A method of heat treatment of austenitic steel HNS or HIS which consists, after austenitization, of lowering the temperature of the austenitic steel HNS or HIS from its austenitization or sintering temperature to the austenitizing temperature sufficiently slowly to that precipitates appear in the structure of the austenitic steel HNS or HIS, then finally to bring the steel to ambient temperature. Procédé de traitement thermique d'un acier austénitique HNS ou HIS qui consiste à soumettre cet acier austénitique HNS ou HIS à un refroidissement depuis sa température d'austénitisation ou de frittage à la température d'austénitisation, puis à interrompre le refroidissement de l'acier austénitique HNS ou HIS lorsque la température à atteint une valeur à laquelle peuvent apparaître des précipités, et à maintenir cet acier à cette température et pendant une durée telles qu'apparaissent des précipités, puis enfin à ramener l'acier à température ambiante.A method of heat treating an HNS or HIS austenitic steel which comprises subjecting said austenitic HNS or HIS steel to cooling from its austenitizing or sintering temperature to the austenitization temperature, and then stopping the cooling of the steel austenitic HNS or HIS when the temperature reaches a value at which precipitates may appear, and to maintain the steel at this temperature and for a time such that appear precipitates, then finally to bring the steel to room temperature. Procédé de traitement thermique d'un acier austénitique HNS ou HIS qui consiste, après que l'acier austénitique HNS ou HIS ait subit un traitement thermique d'austénitisation ou de frittage à la température d'austénitisation puis de trempe, à chauffer l'acier austénitique HNS ou HIS à une température et pendant une durée telles qu'apparaissent des précipités, puis enfin à ramener l'acier à température ambiante.Process for the heat treatment of an austenitic steel HNS or HIS which consists, after the austenitic steel HNS or HIS has undergone a heat treatment of austenitization or sintering at the austenitization and quenching temperature, to heat the steel austenitic HNS or HIS at a temperature and for a period such as appear precipitates, then finally bring the steel to room temperature. Procédé de traitement thermique selon la revendication 3, 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, on déforme à froid l'acier austénitique HNS ou HIS.Heat treatment process according to claim 3, characterized in that , after quenching and before bringing the austenitic steel HNS or HIS to a temperature and for a period such that precipitates appear, the steel is deformed cold. austenitic HNS or HIS. Procédé de traitement thermique selon l'une quelconque des revendications 1 à 4, caractérisé en ce que les précipités sont remis en solution après usinage en portant à nouveau l'acier austénitique HNS ou HIS à sa température d'austénitisation, puis en le refroidissant suffisamment rapidement pour éviter de reformer des précipités.Heat treatment process according to any one of claims 1 to 4, characterized in that the precipitates are re-dissolved after machining by again bringing the austenitic steel HNS or HIS at its austenitization temperature, then cool rapidly enough to avoid reforming precipitates. Procédé de traitement thermique selon l'une quelconque des revendications 1 à 3, caractérisé en ce qu'il comprend en outre l'étape consistant à déformer l'acier austénitique HNS ou HIS à froid après que ledit acier a été ramené à température ambiante.A heat treatment method according to any one of claims 1 to 3, characterized in that it further comprises the step of deforming the austenitic steel HNS or HIS cold after said steel has been brought back to ambient temperature. Acier austénitique HNS ou HIS comprenant des précipités obtenu par mise en oeuvre du procédé de traitement thermique selon l'une quelconque des revendications 1 à 6.Austenitic steel HNS or HIS comprising precipitates obtained by carrying out the heat treatment process according to any one of Claims 1 to 6. Elément d'horlogerie ou de bijouterie obtenu à partir d'un acier austénitique HNS ou HIS selon la revendication 7.Timepiece or jewelery element obtained from an austenitic HNS or HIS steel according to claim 7.
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