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EP3249060B1 - Method for thermal treatment of austenitic steels - Google Patents

Method for thermal treatment of austenitic steels Download PDF

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Publication number
EP3249060B1
EP3249060B1 EP17167986.3A EP17167986A EP3249060B1 EP 3249060 B1 EP3249060 B1 EP 3249060B1 EP 17167986 A EP17167986 A EP 17167986A EP 3249060 B1 EP3249060 B1 EP 3249060B1
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Prior art keywords
austenitic
hns
precipitates
steel
temperature
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German (de)
French (fr)
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EP3249060A1 (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 process for the heat treatment of austenitic steels. More specifically, the present invention relates to austenitic steels alloyed with nitrogen well known under their Anglo-Saxon name Austenitic High Nitrogen Steel or austenitic steels HNS. 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 which, for convenience, we will call hereafter austenitic steels HNS
  • austenitic steels with high concentrations of interstitial atoms which will be called hereafter austenitic steels HIS have properties of hardness, corrosion resistance and hypoallergenic which make them very interesting in particular for applications in the field of watchmaking and jewelry, both for the manufacture of trim elements intended to come into contact with the skin due to their very low nickel concentration, and for the manufacture of watch movement components because they are very hard, in particular after hardening.
  • HNS austenitic steels contain interstitial nitrogen atoms in high concentrations which can extend up to 1.5% in weight depending on the composition and use of the alloy.
  • HIS austenitic steels directly derived from HNS austenitic steels, contain significant amounts of interstitial carbon atoms in addition to interstitial nitrogen atoms.
  • HNS and HIS austenitic steels exhibit in particular interesting hypoallergenic properties due to their very low nickel content and their resistance to corrosion.
  • HNS and HIS austenitic steels are very difficult to machine, especially because they have a very high elastic limit, strain hardening rate and ductility. Tests show, for example, that the machining operations are two to three times longer than for steel 1.4435 and the wear of the machining tools is very important.
  • the machining of these austenitic HNS and HIS steels which, in many respects, is similar to machining titanium, is therefore long, difficult and expensive and constitutes the main obstacle to the use of these steels, particularly in the field of watchmaking and jewelry.
  • the document US 5,714,115 describes an austenitic steel alloy having the chemical composition 17.5% Cr, 4% Mo, 11% Mn, 0.02% C, 0.88% N and 0.01% Ni, the residue consisting of Fe The alloy is remelted in an electrically conductive slag, then forged. After annealing in solution at 1150 ° C., a homogeneous austenitic alloy is obtained, free from precipitation and delta ferrite, that is to say completely non-magnetic.
  • CN103233174B discloses a process for preparing high nitrogen austenitic stainless steel for vascular stents.
  • the subject of the present invention is a process for the heat treatment of austenitic steels of the HNS and HIS type, the aim of which is to make such austenitic steels more easily machinable.
  • an austenitic steel alloy HNS is provided. or HIS that is brought to its austenitization temperature or that is sintered at the austenitization temperature, then, immediately from the austenitization temperature, the temperature of the austenitic steel alloy HNS is lowered or HIS sufficiently slowly for precipitates of the nitride, carbide or even carbonitride type of chromium and / or molybdenum to appear in the structure of the resulting austenitic steel HNS or HIS, then finally the austenitic steel HNS or HIS is brought back at room temperature.
  • step which consists in causing precipitates to appear in an HNS or HIS austenitic steel precedes the step which, after machining this austenitic HNS or HIS steel, consists in putting the precipitates back into solution.
  • the heat treatment process applies equally well to parts obtained by casting and subsequent thermomechanical treatment, as to parts obtained by powder metallurgy such as metal injection molding also known by its English name. -saxonne Metal Injection Molding or MIM.
  • powder metallurgy such as metal injection molding also known by its English name.
  • MIM powder metallurgy
  • 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, according to a method which is not part of the claimed invention, to cool slowly. the alloy in order to promote the formation of precipitates.
  • Slow cooling is understood to mean cooling which, after austenitization or sintering, promotes the appearance of precipitates in the microstructure of the austenitic HNS and HIS steels thus treated, as opposed to the classic heat treatment of quenching which consists in rapidly cooling the HNS and HIS steels. HIS after austenitization or sintering in order to avoid the formation of precipitates.
  • the austenitic steels HNS and HIS be subjected to a slow cooling heat treatment which is not part of the of the invention claimed to promote the appearance of precipitates, it goes against the usual practice of cooling the alloys as quickly as possible in order to avoid as much as possible the formation of precipitates in austenitic HNS steels and Resulting HIS.
  • 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 temperature, then quenching, it is heats the austenitic steel HNS or HIS again to a temperature and for a period of time such that precipitates of the nitride, carbide or else carbonitride type of chromium and / or molybdenum appear.
  • This second variant is the most practical because it makes it possible to be able to perfectly control the parameters of the various heat treatments.
  • the first and second implementation variants of the heat treatment process of an austenitic HNS or HIS steel are therefore more particularly intended for obtaining trim elements for timepieces or jewelry, because they promote the corrosion resistance of these steels.
  • These two variants have in common that after application of a heat treatment of austenitization to an austenitic steel HNS or HIS and subsequent machining, one can indeed bring the resulting part to the annealing temperature, then quench the latter in order to return precipitates in solution.
  • an HNS or HIS austenitic steel is brought to its annealing temperature, in other words to its austenitization temperature, then it is rapidly cooled (quenching) so that no precipitate does not form, it is cold deformed and then this austenitic HNS or HIS steel is brought to a temperature and for a period of time such that precipitates of the nitride, carbide or else carbonitride type of chromium and / or molybdenum appear.
  • the hardness of the austenitic steel HNS or HIS obtained after austenitization and cold deformation is very little affected by the precipitation treatment carried out subsequently.
  • the machinability of such steels is appreciably improved.
  • the present invention proceeds from the general inventive idea which consists in subjecting the austenitic steels HNS and HIS to a heat treatment aiming to make the precipitates which have been made to appear in such austenitic steels HNS or HIS, for example during ironing back into solution. a pre-treatment of precipitation.
  • heat treatment of precipitation means 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 of molybdenum and / or of chromium.
  • this annealing treatment will preferably but not be limited to trim elements for watches or for jewelry for which corrosion resistance and ability to polish are more important properties than hardness.
  • each HNS or HIS austenitic steel composition has a time-temperature-transformation diagram which is specific to it and which is also a function of the nature of the precipitate considered.
  • the figure 1 is a time (t) - temperature (T) - transformation diagram which illustrates the heat treatment of an austenitic HNS or HIS steel according to the implementation variant of the process which is not part of the claimed invention.
  • Tr1 be the austenitization or annealing temperature of an austenitic steel of the HNS or HIS type and let a be the curve which, on the time-temperature-transformation diagram of the figure 1 , delimits an area which corresponds to the time and temperature conditions which allow the formation of rushed.
  • the figure 2 is a time (t) - temperature (T) - transformation diagram which illustrates the heat treatment of an austenitic HNS or HIS steel according to the first implementation variant of the process.
  • 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 diagram of the figure 2 , delimits an area which corresponds to time and temperature conditions which allow the formation of precipitates.
  • Tr2 the austenitization or annealing temperature of an austenitic steel of the HNS or HIS type
  • b the curve which, on the time-temperature-transformation diagram of the figure 2 , delimits an area which corresponds to time and temperature conditions which allow the formation of precipitates.
  • 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 second implementation variant of the process.
  • Tr3 be the austenitization or annealing temperature of an austenitic steel of the HNS or HIS type and let c be the curve which, on the time-temperature-transformation diagram of the figure 3 , delimits an area which corresponds to time and temperature conditions which allow the formation of precipitates.
  • the steel in question here is an austenitic HNS or HIS steel which has been cooled sufficiently rapidly from its temperature of annealing Tr3 to room temperature in order to avoid any formation of precipitates.
  • such an austenitic HNS or HIS steel is heated according to curve 10 and maintained at a temperature and for a period of time such that precipitates appear (curve 12), then is cooled (curve 12). 14).
  • the third implementation variant of the process differs from the second variant of the same process only in that, after receiving treatment followed by quenching and before the precipitation treatment, the austenitic steel HNS or HIS is work hardened, that is, cold-deformed.
  • the heat treatment which consists in bringing an austenitic steel to a temperature and for a period of time such that precipitates are formed is therefore applied, in this fourth variant, to a material hardened beforehand by work hardening.
  • the fourth and last implementation variant of the process consists in subjecting the austenitic steel to a cold deformation treatment after heat treatment according to one of the first three implementation variants.
  • the figure 4 is a view of a metallographic section of a sample of HIS X20CrMnMoN17-11-3 steel which has been annealed at its austenitization temperature and then quenched. It is noted on examination of this figure that the grain boundaries are not very marked, which denotes 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 in accordance with the second variant implementation of the method.
  • the grain boundaries are marked, which denotes the presence of significant amounts of precipitates along these grain boundaries.
  • Some larger precipitates have grown inside the grains from the grain boundaries.
  • concentration of precipitates could be obtained by bringing, after rapid cooling from the annealing temperature, the austenitic steel HIS X20CrMnMoN17-11-3 at a temperature of 800 ° C for two hours.
  • 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, ie a reduction in diameter of 16.6%.
  • this sample was then brought to a temperature of 800 ° C. for two hours according to the temperature curve shown in figure 3 .
  • the steel has many precipitates, both at the grain boundaries and inside the grains.
  • the figure 7 is a graph which shows the evolution of the hardness of the austenitic steel HIS X20CrMnMoN17-11-3 of the figure 6 depending on the temperature to which this steel is brought to form the precipitates. It is observed that the hardness of austenitic steel without precipitation treatment and after cold working is 450 HV10 (symbol in the form of a square on the graph). The same austenitic steel is, after cold work hardening, heat treated in accordance with the third implementation variant of the process.
  • Samples of this steel are brought respectively to temperatures of 750 ° C, 800 ° C, 850 ° C, 900 ° C and 950 ° C for a period of two hours, then cooled (diamond-shaped symbols on the graph) . It is observed that, for samples heated between 700 ° C and 900 ° C, the hardness is between approximately 425 HV10 and 375 HV10. In other words, the hardness of these samples of austenitic steel heat-treated in accordance with the third variant of the process varies little with respect to the hardness of austenitic steel which has been cold-worked but which has not been subjected to a precipitation treatment.
  • the machinability of the austenitic steel samples which have undergone a precipitation heat treatment according to this third variant of the process is markedly improved. Only the austenitic steel sample heated to 950 ° C for two hours has a hardness significantly lower than that of austenitic steel without precipitation treatment (less than 350 HV10). Finally, a sample of austenitic steel HIS X20CrMnMoN17-11-3 having only undergone an annealing treatment followed by quenching (symbol in the form of a triangle 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 which is in the form of a bar whose outer diameter is reduced from 3 mm to 2 mm by cold deformation by drawing , or an even greater reduction in diameter of 33.3%.
  • This steel sample undergoes the same heat treatment as at the figure 6 by being brought to a temperature of 800 ° C. for two hours in accordance with the third variant of the implementation of the method.
  • the phenomenon of precipitation is even more pronounced since, in addition to the precipitates which form along the grain boundaries and from the grain boundaries towards the interior of the grains, there is a high concentration of precipitates within themselves. grains.
  • the figure 9 is a graph which shows the evolution of the hardness of the steel of the figure 8 depending on the time and the temperature to which this steel is heated after work hardening to form the precipitates. It is observed that the hardness of austenitic steel without precipitation treatment and after cold work hardening is between 550 HV10 and 560 HV10 (symbol in the form of a square on the graph). This hardness is greater than that at the figure 7 because the work hardening rate is higher.
  • the diamond-shaped symbols on the figure 9 correspond to samples of austenitic steel brought to respective temperatures of 700 ° C, 750 ° C, 800 ° C and 850 ° C for 45 minutes.
  • HNS and HIS steels to which the precipitation process can be applied are: X5CrMnN18-18, X8CrMnN19-19, X8CrMnMoN18-18-2, X13CrMnMoN18-14-3, X20CrMnMoN17-11-3 or even X5MnCrMoN23-21.
  • precipitates that can form during the precipitation process are: M23C, MC, M6C or even M2N, where M denotes one or more of the metallic elements of the alloy which can be combined with carbon or nitrogen. to form carbides or nitrides or carbonitrides.
  • the invention applies in particular to jewelry and to the trim elements of timepieces.
  • the present invention teaches a process for the heat treatment of a HNS or HIS austenitic steel containing precipitates, this process comprising the step which consists, after machining of parts, in particular of jewelry or watchmaking, carried out in using an HNS or HIS austenitic steel containing precipitates, to put the precipitates back into solution by bringing these HNS or HIS austenitic steel parts to their austenitization temperature, then cooling these parts sufficiently quickly, typically by quenching, to prevent the precipitates to form again.
  • machining operations is understood to mean in particular but not exclusively the operations of boring, milling, drilling, threading, tapping and cutting.

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Description

Domaine technique de l'inventionTechnical field of the invention

La présente invention concerne un procédé de traitement thermique d'aciers austénitiques. 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 process for the heat treatment of austenitic steels. More specifically, the present invention relates to austenitic steels alloyed with nitrogen well known under their Anglo-Saxon name Austenitic High Nitrogen Steel or austenitic steels HNS. 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 which, for convenience, we will call hereafter austenitic steels HNS, and austenitic steels with high concentrations of interstitial atoms which will be called hereafter austenitic steels HIS have properties of hardness, corrosion resistance and hypoallergenic which make them very interesting in particular for applications in the field of watchmaking and jewelry, both for the manufacture of trim elements intended to come into contact with the skin due to their very low nickel concentration, and for the manufacture of watch movement components because they are very hard, in particular 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 œuvre 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.HNS austenitic steels contain interstitial nitrogen atoms in high concentrations which can extend up to 1.5% in weight depending on the composition and use of the alloy. HIS austenitic steels, directly derived from HNS austenitic steels, contain significant amounts 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 HNS and HIS austenitic steels exhibit in particular interesting hypoallergenic properties due to their very low nickel content and their resistance to corrosion. However, HNS and HIS austenitic steels are very difficult to machine, especially because they have a very high elastic limit, strain hardening rate and ductility. Tests show, for example, that the machining operations are two to three times longer than for steel 1.4435 and the wear of the machining tools is very important. The machining of these austenitic HNS and HIS steels which, in many respects, is similar to machining titanium, is therefore long, difficult and expensive and constitutes the main obstacle to the use of these steels, particularly in the field of watchmaking and jewelry.

Dans l'article intitulé « Influence of heat treatment on creep of a Mn-N stabilised austenitic stainless steel » paru dans le Journal of Materials Science, publié par Kluwer Academic Publishers, BO, volume 43, n°. 15, 24 juin 2008, pages 5350-5357, Wisniewski et al. divulgue un procédé de traitement thermique consistant à maintenir un acier austénitique 21-4N à une température de 1200°C pendant une heure, puis à le refroidir dans le four à une vitesse de 6°C/min.In the article titled “Influence of heat treatment on creep of a Mn-N stabilized austenitic stainless steel” published in the Journal of Materials Science, published by Kluwer Academic Publishers, BO, volume 43, no. June 15, 24, 2008, pages 5350-5357, Wisniewski et al. discloses a heat treatment process comprising maintaining a 21-4N austenitic steel at a temperature of 1200 ° C for one hour, then cooling it in the furnace at a rate of 6 ° C / min.

Dans l'article intitulé « Stress corrosion cracking, structure, and properties of nitrogen-hardened austenitic chromium-manganese steels » paru dans Fizika Metallov I Metallovedenie, volume 65, n°. 6, 1 janvier 1988, pages 1131-1137, Goikhenberg Yu N et Al divulgue des aciers HNS. Les aciers ont subi un traitement thermique d'austénitisation à 1150°C pendant une demi-heure, avant d'être refroidi à l'air.In the article titled “Stress corrosion cracking, structure, and properties of nitrogen-hardened austenitic chromium-manganese steels” published in Fizika Metallov I Metallovedenie, volume 65, n °. 6, January 1, 1988, pages 1131-1137, Goikhenberg Yu N and A1 discloses HNS steels. The steels underwent a heat treatment of austenitization at 1150 ° C. for half an hour, before being cooled in air.

Dans l'article intitulé « Effect of microstructure on the heat affected zone toughness of high nitrogen containing Ni-free austenitic austenitic stainless steel" paru dans Transactions JWRI, volume 30, n°1, 1 janvier 2001, pages 77-84, Woo Insu, Horinouchi Tsutomu et Kikuchi Yasushi décrivent un procédé de traitement thermique consistant à chauffer rapidement un acier HNS jusqu'à 1300°C, puis à maintenir cet acier à cette température avant de le refroidir selon des vitesses de refroidissement différentes.In the article titled "Effect of microstructure on the heat affected zone toughness of high nitrogen containing Ni-free austenitic austenitic stainless steel" published in Transactions JWRI, volume 30, n ° 1, January 1, 2001, pages 77-84, Woo Insu, Horinouchi Tsutomu and Kikuchi Yasushi describe a heat treatment process consisting in rapidly heating an HNS steel up to 1300 ° C., then in maintaining this steel at this temperature before cooling it according to different cooling rates.

Le document US 5,714,115 décrit un alliage d'acier austénitique ayant la composition chimique 17,5% Cr, 4% Mo, 11% Mn, 0,02% C, 0,88% N et 0,01% Ni, le résidu étant constitué par du Fe. L'alliage est refondu sous laitier électro-conducteur, puis forgé. Après un recuit en solution à 1150 ° C, on obtient un alliage austénitique homogène, exempt de précipitations et de ferrite delta, c'est-à-dire complètement non magnétique.The document US 5,714,115 describes an austenitic steel alloy having the chemical composition 17.5% Cr, 4% Mo, 11% Mn, 0.02% C, 0.88% N and 0.01% Ni, the residue consisting of Fe The alloy is remelted in an electrically conductive slag, then forged. After annealing in solution at 1150 ° C., a homogeneous austenitic alloy is obtained, free from precipitation and delta ferrite, that is to say completely non-magnetic.

CN103233174B divulgue un procédé de préparation de l'acier inoxydable austénitique à haute teneur en azote pour stents vasculaires. CN103233174B discloses a process for preparing high nitrogen austenitic stainless steel for vascular stents.

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 HNS and HIS steels 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 subject of the present invention is a process for the heat treatment of austenitic steels of the HNS and HIS type, the aim of which is to make such austenitic steels more easily machinable.

L'invention est définie par les revendications.The invention is defined by the claims.

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 une étape de:

  • avant usinage, faire apparaître des précipités du type nitrures; carbures ou bien carbonitrures de chrome et/ou de molybdène dans l'acier austénitique HNS ou HIS, ce résultat étant atteint en se munissant 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 que l'on soumet à un traitement thermique de refroidissement immédiatement depuis la température d'austénitisation, le refroidissement de l'acier austénitique HNS ou HIS résultant étant interrompu 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, l'acier austénitique HNS ou HIS étant enfin ramené à température ambiante ;
ce procédé comprenant en outre une 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.
To this end, the present invention relates to a process for the heat treatment of a HNS or HIS austenitic steel containing precipitates of the nitride, carbide or even carbonitride type of chromium and / or molybdenum, this process comprising a step of:
  • before machining, reveal precipitates of the nitride type; carbides or carbonitrides of chromium and / or molybdenum in austenitic steel HNS or HIS, this result being achieved by using an austenitic steel alloy HNS or HIS which is brought to its austenitization temperature or that is sintered at the austenitization temperature, then which is subjected to a cooling heat treatment immediately from the austenitization temperature, the cooling of the resulting austenitic steel HNS or HIS being interrupted when the temperature reaches a value at which precipitates appear, this austenitic steel HNS or HIS being maintained at this temperature and for a period such that 'precipitates appear, the austenitic steel HNS or HIS finally being brought to room temperature;
this method further comprising a step which consists,
  • after machining the austenitic steel HNS or HIS containing the precipitates, to put the precipitates back into solution by bringing the austenitic steel HNS or HIS to its austenitization temperature, then by cooling this austenitic steel HNS or HIS sufficiently quickly to avoid reform precipitates.

Ces caractéristiques se révèlent très avantageuses car elles permettent, 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.These characteristics prove to be very advantageous because they make it possible, when desired, to remove the precipitates after the parts made of austenitic steel HNS or HIS have been machined. In the particular case of timepieces, this possibility can be used in particular to eliminate precipitates in the trim elements (middle parts, watch case backs, glasses, crowns, pushers, clasps, bracelet links, etc. .) in order to make the material as homogeneous as possible and to eliminate residual stresses. The resulting steels will thus have better corrosion resistance and greater ductility. The same is true when it comes to making jewelry.

Selon une caractéristique qui ne fait pas partie de l'invention revendiquée, 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 characteristic which is not part of the claimed invention, in order to cause precipitates to appear in the austenitic steel HNS or HIS before machining, an austenitic steel alloy HNS is provided. or HIS that is brought to its austenitization temperature or that is sintered at the austenitization temperature, then, immediately from the austenitization temperature, the temperature of the austenitic steel alloy HNS is lowered or HIS sufficiently slowly for precipitates of the nitride, carbide or even carbonitride type of chromium and / or molybdenum to appear in the structure of the resulting austenitic steel HNS or HIS, then finally the austenitic steel HNS or HIS is brought back at 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 which consists in causing precipitates to appear in an HNS or HIS austenitic steel precedes the step which, after machining this austenitic HNS or HIS steel, consists in putting the precipitates back into 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, selon une méthode qui ne fait pas partie de l'invention revendiquée, de refroidir lentement l'alliage afin de favoriser la formation de précipités.It will also be noted that the heat treatment process applies equally well to parts obtained by casting and subsequent thermomechanical treatment, as to parts obtained by powder metallurgy such as metal injection molding also known by its English name. -saxonne Metal Injection Molding or MIM. In fact, 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, according to a method which is not part of the claimed invention, to cool slowly. the alloy in order to promote the formation of 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, promotes the appearance of precipitates in the microstructure of the austenitic HNS and HIS steels thus treated, as opposed to the classic heat treatment of quenching which consists in rapidly cooling the HNS and HIS steels. HIS after austenitization or sintering in order 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 qui ne fait pas partie de l'invention revendiquée pour favoriser l'apparition de précipités, on 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 that, immediately after austenitization or sintering at the austenitization temperature, the austenitic steels HNS and HIS be subjected to a slow cooling heat treatment which is not part of the of the invention claimed to promote the appearance of precipitates, it goes against the usual practice of cooling the alloys as quickly as possible in order to avoid as much as possible the formation of precipitates in austenitic HNS steels and Resulting HIS.

On 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.It has indeed been observed that by subjecting the austenitic steels HNS and HIS to the heat treatment process of the type described above, the nitrogen and carbon atoms, for example, tend to migrate towards the grain boundaries and to combine fairly easily with chromium or molybdenum atoms to form precipitates of the chromium / molybdenum nitride, carbide or even carbonitride type. However, these precipitates have very low adhesion with the matrix, so that they make the chips brittle and facilitate machining operations.

Selon encore un autre mode de mise en œuvre du procédé conforme à l'invention revendiquée, 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 method according to the claimed invention, after the austenitic steel HNS or HIS has undergone a heat treatment of austenitization or sintering at the austenitization temperature, then quenching, it is heats the austenitic steel HNS or HIS again to a temperature and for a period of time such that precipitates of the nitride, carbide or else carbonitride type of chromium and / or molybdenum appear.

Cette seconde variante est la plus pratique car elle permet de pouvoir parfaitement maîtriser les paramètres des différents traitements thermiques.This second variant is the most practical because it makes it possible to be able to perfectly control the parameters of the various heat treatments.

Les première et deuxième variantes de mise en œuvre du procédé de traitement thermique d'un acier austénitique HNS ou HIS 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 deux 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 and second implementation variants of the heat treatment process of an austenitic HNS or HIS steel are therefore more particularly intended for obtaining trim elements for timepieces or jewelry, because they promote the corrosion resistance of these steels. These two variants have in common that after application of a heat treatment of austenitization to an austenitic steel HNS or HIS and subsequent machining, one can indeed bring the resulting part to the annealing temperature, then quench the latter in order to return precipitates in solution.

Selon une troisième variante de mise en œuvre du procédé, 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 third implementation variant of the process, an HNS or HIS austenitic steel is brought to its annealing temperature, in other words to its austenitization temperature, then it is rapidly cooled (quenching) so that no precipitate does not form, it is cold deformed and then this austenitic HNS or HIS steel is brought to a temperature and for a period of time such that precipitates of the nitride, carbide or else carbonitride type of chromium and / or molybdenum appear.

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 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 carried out subsequently. On the other hand, the machinability of such steels is appreciably 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 œuvre 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 variante de mise en œuvre du procédé qui ne fait pas partie de l'invention revendiquée ;
  • la figure 2 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 œuvre du procédé ;
  • la figure 3 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 œuvre du procédé ;
  • 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 deuxième variante de mise en œuvre du procédé ;
  • 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 troisième variante de mise en œuvre du procédé ;
  • 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 troisième variante de mise en œuvre du procédé, 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 characteristics and advantages of the present invention will emerge more clearly from the detailed description which follows of an example of implementation of the process for the heat treatment of austenitic HNS and HIS steels in accordance with the present invention, this example being given in purely illustrative and non-limiting only in conjunction with the appended drawing in which:
  • the figure 1 is a schematic time-temperature-transformation diagram which illustrates the heat treatment of an austenitic HNS or HIS steel according to the implementation variant of the process which is not part of the claimed 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 first implementation variant of the process;
  • the figure 3 is a schematic time-temperature-transformation diagram which illustrates the heat treatment of an austenitic HNS or HIS steel according to the second implementation variant of the process;
  • the figure 4 is a view of a metallographic section of a sample of HIS X20CrMnMoN17-11-3 steel which has been annealed at its austenitization temperature then quenched and which does not show any 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 in accordance with the second variant implementation of the method;
  • 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 in accordance with the third variant implementation of the method;
  • the figure 7 is a graph which shows the evolution of the hardness of the austenitic steel sample HIS X20CrMnMoN17-11-3 of the figure 6 depending on the temperature to which this steel is brought to form the precipitates.
  • the figure 8 is a view of a metallographic section of a sample of austenitic steel HIS X20CrMnMoN17-11-3 having undergone a greater work hardening than the sample of austenitic steel of the figure 6 before a heat treatment in accordance with the third implementation variant of the method, and
  • the figure 9 is a graph which shows the evolution of the hardness of the austenitic steel sample HIS X20CrMnMoN17-11-3 of the figure 8 depending on the temperature to which this steel is brought 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 which consists in subjecting the austenitic steels HNS and HIS to a heat treatment aiming to make the precipitates which have been made to appear in such austenitic steels HNS or HIS, for example during ironing back into solution. a pre-treatment of precipitation. By heat treatment of precipitation means 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 of molybdenum and / or of chromium. It has in fact been observed that these precipitates are generally not very bound to the matrix of the material, so that they promote the formation and removal of chips during the machining of the parts. Thus, in accordance with the invention, it is possible, after machining of parts made of an austenitic HNS or HIS steel containing precipitates, to subject these parts to a second austenitization treatment which consists in again bringing these parts to their temperature. annealing, then quenching them so as to return the precipitates to solid solution. As the fact of bringing after machining the austenitic steels HNS and HIS a second time to their annealing temperature causes an elimination of the internal stresses in the material and therefore a reduction in its hardness, this annealing treatment will preferably but not be limited to trim elements for watches or for jewelry for which corrosion resistance and ability to polish are more important properties than 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 HNS or HIS austenitic steel composition has a time-temperature-transformation diagram which is specific to it and which is also a function of the nature of the precipitate considered.

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 variante de mise en œuvre du procédé qui ne fait pas partie de l'invention revendiquée. 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 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 HNS or HIS steel according to the implementation variant of the process which is not part of the claimed invention. Let Tr1 be the austenitization or annealing temperature of an austenitic steel of the HNS or HIS type and let a be the curve which, on the time-temperature-transformation diagram of the figure 1 , delimits an area which corresponds to the time and temperature conditions which allow the formation of rushed. We denote by 1 the rapid cooling curve which allows the austenitic steel HNS or HIS to be brought back from its annealing temperature to ambient temperature while avoiding any formation of precipitates, and by 2 the cooling curve which combines the time parameters and temperature such 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 première variante de mise en œuvre du procédé. 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 HNS or HIS steel according to the first implementation variant of the process. 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 diagram of the figure 2 , delimits an area which corresponds to time and temperature conditions which allow the formation of precipitates. We start by rapidly cooling the austenitic steel HNS or HIS from its annealing temperature Tr2 according to curve 4, then the cooling of the austenitic steel HNS or HIS is interrupted when the temperature reaches a value Tp2 at which precipitates may appear. , and this steel is maintained at this temperature Tp2 for a period of time such that precipitates appear (curve 6). Finally, the steel is brought back to ambient 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 deuxième variante de mise en œuvre du procédé. 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 deuxième variante de mise en œuvre du procédé, 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 second implementation variant of the process. Let Tr3 be the austenitization or annealing temperature of an austenitic steel of the HNS or HIS type and let c be the curve which, on the time-temperature-transformation diagram of the figure 3 , delimits an area which corresponds to time and temperature conditions which allow the formation of precipitates. The steel in question here is an austenitic HNS or HIS steel which has been cooled sufficiently rapidly from its temperature of annealing Tr3 to room temperature in order to avoid any formation of precipitates. In accordance with the second implementation variant of the process, such an austenitic HNS or HIS steel is heated according to curve 10 and maintained at a temperature and for a period of time such that precipitates appear (curve 12), then is cooled (curve 12). 14).

La troisième variante de mise en œuvre du procédé ne diffère de la deuxième variante du même procédé qu'en ce que, après traitement de reçut 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 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 third implementation variant of the process differs from the second variant of the same process only in that, after receiving treatment followed by quenching and before the precipitation treatment, the austenitic steel HNS or HIS is work hardened, that is, cold-deformed. The heat treatment which consists in bringing an austenitic steel to a temperature and for a period of time such that precipitates are formed is therefore applied, in this fourth variant, to a material hardened beforehand by work hardening.

Enfin, la quatrième et dernière variante de mise en œuvre du procédé 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 œuvre.Finally, the fourth and last implementation variant of the process consists in subjecting the austenitic steel to a cold deformation treatment after heat treatment according to one of the first three implementation variants.

Différents essais ont été menés sur l'acier austénitique HIS X20CrMnMoN17-11-3.Various tests were carried out 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 sample of HIS X20CrMnMoN17-11-3 steel which has been annealed at its austenitization temperature and then quenched. It is noted on examination of this figure that the grain boundaries are not very marked, which denotes 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 deuxième variante de mise en œuvre du procédé.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 in accordance with the second variant implementation of the method.

Comme on peut e 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.As can be seen from the examination of the figure 5 , the grain boundaries are marked, which denotes the presence of significant 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 from the grain boundaries. Such a concentration of precipitates could be obtained by bringing, after rapid cooling 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 troisième variante de mise en œuvre du procédé 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 in accordance with the third variant of implementation of the process and consisting, after annealing treatment followed by quenching and work hardening, in carrying austenitic steel HIS X20CrMnMoN17-11-3 at a temperature and for a period of time such that precipitates are formed. It has been observed that after cold deformation, the formation of precipitates is much faster. Indeed, the dislocations and vacancies induced by cold deformation create diffusion paths favorable to the germination and growth of 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 troisième variante de mise en œuvre du procédé, 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, ie a reduction in diameter of 16.6%. In accordance with the third implementation variant of the method, this sample was then brought to a temperature of 800 ° C. for two hours according to the temperature curve shown in figure 3 . We see that the 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 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 troisième variante de mise en œuvre du procédé. 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 troisième variante du procédé 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 troisième variante du procédé 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 which shows the evolution of the hardness of the austenitic steel HIS X20CrMnMoN17-11-3 of the figure 6 depending on the temperature to which this steel is brought to form the precipitates. It is observed that the hardness of austenitic steel without precipitation treatment and after cold working is 450 HV10 (symbol in the form of a square on the graph). The same austenitic steel is, after cold work hardening, heat treated in accordance with the third implementation variant of the process. Samples of this steel are brought respectively to temperatures of 750 ° C, 800 ° C, 850 ° C, 900 ° C and 950 ° C for a period of two hours, then cooled (diamond-shaped symbols on the graph) . It is observed that, for samples heated between 700 ° C and 900 ° C, the hardness is between approximately 425 HV10 and 375 HV10. In other words, the hardness of these samples of austenitic steel heat-treated in accordance with the third variant of the process varies little with respect to the hardness of austenitic steel which has been cold-worked but which has not been subjected to a precipitation treatment. On the other hand, the machinability of the austenitic steel samples which have undergone a precipitation heat treatment according to this third variant of the process is markedly improved. Only the austenitic steel sample heated to 950 ° C for two hours has a hardness significantly lower than that of austenitic steel without precipitation treatment (less than 350 HV10). Finally, a sample of austenitic steel HIS X20CrMnMoN17-11-3 having only undergone an annealing treatment followed by quenching (symbol in the form of a triangle 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 troisième variante de mise en œuvre du procédé. 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 which is in the form of a bar whose outer diameter is reduced from 3 mm to 2 mm by cold deformation by drawing , or an even greater reduction in diameter of 33.3%. This steel sample undergoes the same heat treatment as at the figure 6 by being brought to a temperature of 800 ° C. for two hours in accordance with the third variant of the implementation of the method. We see that, compared to the figure 6 , the phenomenon of precipitation is even more pronounced since, in addition to the precipitates which form along the grain boundaries and from the grain boundaries towards the interior of the grains, there is a high concentration of precipitates within themselves. 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 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 cœur de l'acier permettent de faire germer et croître les précipités.The figure 9 is a graph which shows the evolution of the hardness of the steel of the figure 8 depending on the time and the temperature to which this steel is heated after work hardening to form the precipitates. It is observed that the hardness of austenitic steel without precipitation treatment and after cold work hardening is between 550 HV10 and 560 HV10 (symbol in the form of a square on the graph). This hardness is greater than that at the figure 7 because the work hardening rate is higher. The diamond-shaped symbols on the figure 9 correspond to samples of austenitic steel brought to respective temperatures of 700 ° C, 750 ° C, 800 ° C and 850 ° C for 45 minutes. The symbols in the shape of a circle correspond to samples of austenitic steel heated to respective temperatures of 700 ° C, 750 ° C, 800 ° C and 850 ° C for two hours. If we compare the graphs of figures 7 and 9 , it is observed that the higher the work hardening rate, the more the formation of precipitates is facilitated. In fact, the mechanical tensions 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 treatment period of two hours, the hardness of the steel is all the lower the higher the precipitation temperature. 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 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.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 can be applied are: X5CrMnN18-18, X8CrMnN19-19, X8CrMnMoN18-18-2, X13CrMnMoN18-14-3, X20CrMnMoN17-11-3 or even X5MnCrMoN23-21. Finally, some examples of precipitates that can form during the precipitation process are: M23C, MC, M6C or even M2N, where M denotes one or more of the metallic elements of the alloy which can be combined with carbon or nitrogen. to form carbides or nitrides or carbonitrides. The invention applies in particular to jewelry and to the trim 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 piece of jewelry or else a wristwatch, using an austenitic steel of the HNS or HIS type containing precipitates. . However, it may 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 the machining of parts, it may be advantageous to remove these chips after machining in order to improve ductility and the resistance of these parts to corrosion. This is why the present invention teaches a process for the heat treatment of a HNS or HIS austenitic steel containing precipitates, this process comprising the step which consists, after machining of parts, in particular of jewelry or watchmaking, carried out in using an HNS or HIS austenitic steel containing precipitates, to put the precipitates back into solution by bringing these HNS or HIS austenitic steel parts to their austenitization temperature, then cooling these parts sufficiently quickly, typically by quenching, to prevent the precipitates to form again. The term “machining operations” is understood to mean in particular but not exclusively the operations of boring, milling, drilling, threading, tapping and cutting.

Claims (3)

  1. Method for heat treatment of an austenitic steel of the High Nitrogen Steel or austenitic HNS type, or of an austenitic steel of the High Interstitial Steel or austenitic HIS type, said austenitic HNS or austenitic HIS containing precipitates of nitrides, carbides or carbonitrides of chromium and/or of molybdenum, this method comprising a step of:
    • before machining, make chromium and/or molybdenum nitride, carbide or carbonitride type precipitates appear in the austenitic HNS or austenitic HIS, this result being obtained by providing an austenitic HNS or austenitic HIS alloy which is brought to its austenitizing temperature or sintered at the austenitizing temperature, then this austenitic HNS or austenitic HIS alloy is subjected to a cooling heat treatment immediately from the austenitizing temperature, the cooling of the resulting austenitic HNS or austenitic HIS being interrupted when the temperature reaches a value at which the precipitates appear, this austenitic HNS or austenitic HIS being maintained at this temperature and for a duration such that the precipitates appear, and then finally the austenitic HNS or austenitic HIS is returned to ambient temperature;
    this method further comprising a step consisting in:
    • after machining the austenitic HNS or austenitic HIS containing the precipitates, in putting again the precipitates in solution by bringing the austenitic HNS or austenitic HIS to its austenitizing temperature, then by cooling the austenitic HNS or austenitic HIS sufficiently rapidly to avoid the re-formation of precipitates.
  2. Method for heat treatment of an austenitic steel of the High Nitrogen Steel or austenitic HNS type, or of an austenitic steel of the High Interstitial Steel or austenitic HIS type, said austenitic HNS or austenitic HIS containing precipitates of nitrides, carbides or carbonitrides of chromium and/or of molybdenum, this method comprising a step of:
    • before machining, make chromium and/or molybdenum nitride, carbide or carbonitride type precipitates appear in the austenitic HNS or austenitic HIS, this result being obtained by subjecting the austenitic HNS or austenitic HIS alloy to an austenitizing heat treatment or to a sintering heat treatment at the austenitizing temperature, then by quenching the austenitic HNS or austenitic HIS alloy and reheating it again to a temperature and for a duration such that chromium and/or molybdenum nitride, carbide or carbonitride type precipitates appear;
    this method further comprising a step consisting in:
    • after machining the austenitic HNS or austenitic HIS containing the precipitates, putting again the precipitates in solution by bringing the austenitic HNS or austenitic HIS to its austenitizing temperature, then cooling the austenitic HNS or austenitic HIS sufficiently rapidly to avoid the re-formation of precipitates.
  3. Method according claim 2, wherein, after quenching and before bringing the austenitic HNS or austenitic HIS to a temperature and for a duration such that chromium and/or molybdenum nitride, carbide or even carbonitride type precipitates appear, the austenitic HNS or austenitic HIS is cold deformed.
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