EP3056583A1 - Method for manufacturing a part made of nitrided low-alloy steel - Google Patents

Method for manufacturing a part made of nitrided low-alloy steel Download PDF

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Publication number
EP3056583A1
EP3056583A1 EP16154536.3A EP16154536A EP3056583A1 EP 3056583 A1 EP3056583 A1 EP 3056583A1 EP 16154536 A EP16154536 A EP 16154536A EP 3056583 A1 EP3056583 A1 EP 3056583A1
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Prior art keywords
carbon
surface layer
nitriding
decarburized
alloy steel
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EP16154536.3A
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German (de)
French (fr)
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EP3056583B1 (en
Inventor
Alain Viola
Nicolas Binot
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Safran Landing Systems SAS
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Messier Bugatti Dowty SA
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/24—Nitriding
    • C23C8/26—Nitriding of ferrous surfaces
    • 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
    • C21D1/06—Surface hardening
    • 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
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • 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
    • C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
    • C21D3/02—Extraction of non-metals
    • C21D3/04—Decarburising
    • 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
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/02—Pretreatment of the material to be coated
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/80—After-treatment
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
    • C23F17/00—Multi-step processes for surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by subclass C21D or C22F or class C25

Definitions

  • the invention relates to a method for manufacturing nitrided low-alloy steel parts as well as decarburized low-alloy steel parts capable of undergoing a nitriding treatment.
  • the low-alloyed nitriding steels typically have a carbon content of between 0.20% and 0.45% which makes it possible to confer on the base material its mechanical properties at the core after heat treatment.
  • the superficial properties of steel such as hardness can be conferred by a nitriding treatment which consists in diffusing ferritic phase nitrogen and which generates the precipitation of submicroscopic nitrides from nitrurigenic elements such as Cr, V, Mo and Al, present in solid solution in the treated steel.
  • a nitriding treatment which consists in diffusing ferritic phase nitrogen and which generates the precipitation of submicroscopic nitrides from nitrurigenic elements such as Cr, V, Mo and Al, present in solid solution in the treated steel.
  • the steel in a nitriding treatment, can be treated at a temperature of about 500 ° C by ammonia, which decomposes into cracked ammonia and reacts simultaneously with the iron of the steel.
  • Ammonia causes the formation of a surface layer of iron nitrides, called the combination layer, from which the nitrogen atoms diffuse towards the core of the part to form the diffusion layer.
  • nitrurigenic elements For a weakly alloyed steel comprising nitrurigenic elements, two layers can be observed after nitriding: the surface combination layer consisting of iron nitrides and the diffusion layer in which the submicroscopic nitride precipitates at the origin of the nitride are dispersed. hardness increase observed in the nitrided layer.
  • the depth of the nitriding layer may vary, depending on the nitriding conditions and the targeted applications, between 0.05 mm and 1 mm.
  • the nitriding treatment remains a relatively long process to implement.
  • Low-alloy steel means a steel for which no alloying element is present in a mass content greater than 5.00%. In other words, in a low-alloy steel, each of the chemical elements, other than iron, is present in a mass content less than or equal to 5.00%.
  • addition element both nitrurigenic and carburigenic is meant an addition element present in the steel in the form of carbide before step c) and capable of forming in step c) a nitride.
  • addition element present in the steel in the form of carbide before step c) and capable of forming in step c) a nitride.
  • step a) The carbon content in the heart of the room does not vary during step a).
  • the surface decarburization carried out in step a) affects the carbon content only at the level of the surface layer of the part and not at the level of the core thereof.
  • the carbon mass content is measured, for example, by a Castaing microprobe in the framework established by the ASTM E1019 standard.
  • the carbon mass content at the heart of the decarburized part (corresponding to that of the part before decarburization) may, for example, be between 0.20% and 0.45%.
  • the minimum mass content of carbon in the carbon-depleted surface layer is less than or equal to 70% of the carbon mass content in the core of the decarburized part. In other words, the minimum carbon content in the carbon-depleted surface layer is less than or equal to 0.70 * (carbon mass content in the core of the decarburized part).
  • the layer obtained by nitriding does not have optimal properties if carbon is present in an excessively high content in the surface layer of the part before the nitriding treatment. In this case, there is a high content of carbides in the surface layer before nitriding which can be problematic.
  • the nitrides obtained by substitution of carbon with nitrogen in the carbides (in-situ nitride precipitation) during the nitriding treatment have a lower curing potential than the nitrides obtained by precipitation from elements nitrurigenes initially present in solid solution in the ferritic matrix.
  • the carbon initially present in the carbides of the surface layer can be released and diffuse upstream of the nitrogen diffusion front. This carbon thus released can precipitate in the form of cementite networks at the grain boundaries and thus alter the mechanical properties of the nitrided layer.
  • the invention is based on the fact of performing a decarburizing treatment of the surface of the workpiece before the nitriding treatment in order to reduce the amount of carbides present on the surface of the workpiece and thus get rid of the undesirable effects of carbon. mentioned above while maintaining good properties in the heart of the room outside this superficial layer.
  • the decarburization treatment carried out in step a) makes it possible to obtain a surface layer of optimized chemical composition for receiving the nitriding treatment because of the elimination, after treatment of decarburization, of at least a portion of the carbides which contribute, during the nitriding, to the formation of carbide / carbonitride networks detrimental to the mechanical strength of the nitriding layer.
  • the invention advantageously makes it possible to significantly increase the surface hardness of the nitrided part and to obtain more intense compression stress fields making it possible to significantly improve the fatigue strength of the steel part thus treated.
  • the invention makes it possible, in particular, to reduce or eliminate the quantity of weakening cementitious networks.
  • the invention makes it possible to significantly accelerate the apparent kinetics of nitriding due to the reduction of the amount of carbon on the surface of the part, thus resulting in a greater contribution of the nitrogen to the curing mechanism.
  • the parts treated by the process according to the invention have, for an identical nitrogen content in the nitriding layer, a greater hardness, which Apparently, it amounts to an acceleration of the nitriding kinetics (everything happens as if more nitrogen was present at a given depth in the case of the parts treated by the process according to the invention).
  • the invention can thus make it possible to rapidly obtain deep nitride layers, for example 0.7 mm deep.
  • the increase in the apparent kinetics of nitriding also advantageously makes it possible to limit the relaxation of the residual stresses in the nitriding layer because of the possibility of reducing the exposure time to the nitriding temperature.
  • the intermediate tempering and tempering step is, for its part, intended to confer its properties on the base steel.
  • the quenching (cooling) can be performed from the decarburization temperature and the income is then achieved.
  • the minimum mass content of carbon in the carbon-depleted surface layer may be less than or equal to 60% of the carbon mass content in the core of the decarburized part. In other words, the minimum mass content of carbon in the carbon-depleted surface layer may be less than or equal to 0.6 * (carbon mass content in the core of the decarburized part).
  • Nitriding can be carried out under conventional conditions by any type of nitriding process known to those skilled in the art.
  • the temperature and time atmosphere settings are defined based on the base steel and the desired characteristics.
  • the thickness of the carbon-depleted surface layer may be between 0.3 mm and 1.5 mm.
  • the minimum carbon mass content in the carbon-depleted surface layer may be greater than or equal to 0.05%.
  • the treated low alloy steel part may, for example, be made of 32CrMoV13, 35CrMo4, 15CrMoV6 or X38CrMoV5 steel.
  • the treated low-alloy steel part may preferably be 32CrMoV13 steel.
  • step a) the low-alloy steel part by an oxidizing atmosphere comprising water vapor by imposing a temperature of between 850 ° C. and 1000 ° C., the oxidizing atmosphere having a dew point temperature between -40 ° C and + 20 ° C, for example between -10 ° C and + 10 ° C, for example between -4 ° C and + 4 ° C.
  • the low alloy steel part can be treated during step a) with air at a temperature of 950 ° C. in order to achieve the desired decarburization.
  • step c it is possible to carry out, before step c), a step of machining the carbon-depleted surface layer.
  • Performing such a machining step is advantageous because it can make it possible to eliminate an undesirable oxidized layer that can be formed during decarburization.
  • This oxidized layer may typically have a thickness less than or equal to a few hundredths of a millimeter.
  • a reducing atmosphere for example comprising H 2 , before step c) .
  • the machining step of the carbon-depleted surface layer can be performed before and / or after step b).
  • step c) it is possible after step c) to perform a step of machining the nitriding layer formed in step c).
  • step a) it is possible to seek to obtain a carbon-depleted surface layer after implementation of step a) in one or more predefined zones of the surface of the part.
  • step c it is still possible to eliminate, before step c), a portion of the carbon-depleted surface layer in order to preserve it only in one or more predefined areas of the surface of the part.
  • the present invention also relates to a decarburized low-alloy steel part comprising at least one addition element which is both nitrurigenic and carburigenic and has a carbon-depleted surface layer of thickness less than or equal to 1.5 mm intended to undergo nitriding. , the minimum carbon content in the carbon-depleted surface layer being less than or equal to 70% of the carbon mass content in the core of the decarburized part.
  • Such a part corresponds to the intermediate product obtained by carrying out step a) described above before carrying out the nitriding step c).
  • the carbon-depleted surface layer may have a thickness of between 0.3 mm and 1.5 mm.
  • the low alloy steel may be 32CrMoV13 steel.
  • Figures 1 to 3 very schematically the evolution of the surface composition of a low-alloy steel part during a nitriding treatment depending on whether this part has been decarburized (“decarburized steel”) or not (“Raw tempered / tempered steel”) ).
  • the part has on its surface on the one hand chromium in solid solution 1 as well as carbides enriched in chromium 2 called chromium carbides.
  • the decarburized part comprises in its carbon-depleted surface layer 3 a smaller amount of chromium carbides 2 than the non-decarburized part.
  • the weakly alloyed steel used can make it possible to obtain a martensitic, bainito-martensitic or bainitic structure in the carbon-depleted surface layer after quenching.
  • the part is, during nitriding, for example treated with a gas stream comprising ammonia.
  • the figures 2 and 3 schematize the evolution of the surface composition of the part during nitriding and compare it to the surface composition of the part before nitriding.
  • the chromium carbides 2 are converted to chromium nitrides 4 from carbides and carbon 5 is released.
  • Chromium in solid solution 1 is, in turn, converted during nitriding chromium nitride 6 from chromium in solid solution.
  • the nitrides of chromium 4 can confer less hardness on the nitride layer than the nitrides of chromium 6.
  • the nitrided layer obtained when the part was in advance Decarburized therefore advantageously has a better hardness due to a lower proportion of chromium nitrides 4.
  • the released carbon content exceeds a certain threshold of the cementitious networks 7 may appear which can weaken the nitriding layer obtained.
  • a certain threshold of the cementitious networks 7 may appear which can weaken the nitriding layer obtained.
  • a larger number of cementitious fragilizer networks 7 are formed due to a larger amount of carbon released during the nitriding.
  • a type of nitriding steel of the type 32CrMoV13 in the annealed state was first decarburized by treatment with an oxidizing atmosphere comprising water vapor and having a dew point of + 4 ° C. under a temperature of 950 ° C. for 6 hours.
  • the figure 4 shows the evolution as a function of the depth of the carbon mass content and the hardness of the decarburized low alloy steel part thus obtained.
  • a decarburization treatment leads to obtaining a carbon content of about 0.12% on the surface of the carbon-depleted surface layer (depth 0 ⁇ m), the carbon content of the steel of base being found at a depth of about 0.85 mm.
  • the minimum carbon content in the carbon-depleted surface layer thus obtained is about 0.08%.
  • oil quenching was carried out and then a tempering treatment was carried out at a temperature of 635 ° C. for 4 hours under an N 2 + 10% H 2 atmosphere. A second oil quench was performed after the treatment of income.
  • Nitriding under an atmosphere composed of 50% NH 3 , 37.5% H 2 and 12.5% N 2 at a temperature of 520 ° C for a period between 101 hours and 113 hours was performed.
  • the figure 5 provides the microhardness measurement results for the nitrided low alloy steel part obtained in the context of the present example ("Decarburized alloy + income + nitriding").
  • the figure 5 shows the microhardness measurements obtained when the same steel is subjected to a treatment differing only in that the decarburization is not carried out ("Alloy” crude "+ nitriding”).
  • Alloy crude "+ nitriding”
  • the figure 6 shows the evolution of the difference in hardness with respect to that of the core as a function of the depth for a part obtained according to Example 1 or obtained under conditions identical to those of Example 1 except that the decarburization treatment was not performed.
  • a significant surface hardness gain of about 32% (637-436 / 637) is achieved.
  • a type of nitriding steel of the type 32CrMoV13 in the annealed state was treated under the same conditions as in Example 1. The only difference relates to the fact that the decarburization was carried out under a temperature of 980 ° C. and no longer than 950 ° C. ° C.
  • the figure 7 shows the evolution as a function of the depth of the carbon mass content and the hardness of the decarburized low alloy steel part thus obtained.
  • a decarburization treatment leads to the production of a carbon content of less than 0.10% at the surface of the carbon-depleted surface layer (depth 0 ⁇ m), the carbon content of the base steel being found at a depth of about 0.9 mm.
  • the carbon content at the surface of the carbon-depleted surface layer corresponds in this case to the minimum carbon content in the carbon-depleted surface layer.
  • the figure 8 provides the microhardness measurement results for the nitrided low alloy steel part obtained in the context of the present example ("Decarburized alloy + income + nitriding").
  • the figure 8 shows the microhardness measurements obtained when the same steel is subjected to a treatment differing only in that the decarburization is not carried out ("Alloy" crude "+ nitriding”).
  • Alloy crude "+ nitriding”
  • the figure 9 shows the evolution of the difference in hardness with respect to that of the core as a function of the depth for a part obtained according to Example 2 or obtained under conditions identical to those of Example 2 except that the decarburization treatment was not performed.
  • a significant surface hardness gain of the order of 30% (627-436 / 627) is obtained.
  • An annealed 32CrMoV13 type nitriding steel was first decarburized by treatment with an oxidizing atmosphere comprising water vapor and having a dew point of 0 ° C. under a temperature of 950 ° C. 6 hours.
  • the figure 10 shows the evolution as a function of the depth of the carbon mass content and the hardness of the decarburized low alloy steel part thus obtained.
  • a decarburization treatment leads to the attainment of a carbon content of about 0.20% on the surface of the carbon-depleted surface layer (depth 0 ⁇ m), the carbon content of the steel of base being found at a depth of about 0.85 mm.
  • the minimum carbon content in the carbon-depleted surface layer thus obtained is about 0.16%.
  • oil quenching was carried out and then a tempering treatment was carried out at a temperature of 635 ° C. for 4 hours under an N 2 + 10% H 2 atmosphere. A second oil quench was performed after the treatment of income.
  • Nitriding under an atmosphere composed of 50% NH 3 , 37.5% H 2 and 12.5% N 2 at a temperature of 550 ° C for a period of 55 hours was carried out.
  • the figure 11 shows the evolution of the difference in hardness with respect to that of the core as a function of the depth for a part obtained according to Example 3 ("Decarburized alloy + income + nitriding") or obtained under conditions identical to those of the Example 3 with the exception that the decarburization treatment was not carried out ("crude alloy” + nitriding ").
  • Crude alloy + nitriding

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Abstract

La présente invention concerne un procédé de fabrication d'une pièce en acier faiblement allié nitruré comportant les étapes suivantes : a) décarburation de la surface d'une pièce en acier faiblement allié comprenant au moins un élément d'addition à la fois nitrurigène et carburigène afin d'obtenir une pièce décarburée présentant une couche superficielle appauvrie en carbone d'épaisseur inférieure ou égale à 1,5 mm, la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone étant inférieure ou égale à 70% de la teneur massique en carbone au coeur de la pièce décarburée, b) traitement de la pièce décarburée par un traitement de trempe suivi d'un traitement de revenu, et c) nitruration de la couche superficielle appauvrie en carbone afin d'obtenir la pièce en acier faiblement allié nitruré, l'étape c) étant réalisée après l'étape b).The present invention relates to a method of manufacturing a nitrided low alloy steel part comprising the following steps: a) decarburizing the surface of a low alloy steel part comprising at least one addition element that is at the same time nitrurigenic and carburigenic in order to obtain a decarburized part having a carbon-depleted surface layer of thickness less than or equal to 1 , 5 mm, the minimum mass content of carbon in the carbon-depleted surface layer being less than or equal to 70% of the carbon mass content in the core of the decarburized part, b) treatment of the decarburized part by quenching treatment followed by a treatment of income, and c) nitriding the carbon depleted surface layer to obtain the nitrided low alloy steel part, step c) being performed after step b).

Description

Arrière-plan deBackground of l'inventionthe invention

L'invention concerne un procédé de fabrication de pièces en acier faiblement allié nitruré ainsi que des pièces en acier faiblement allié décarburées aptes à subir un traitement de nitruration.The invention relates to a method for manufacturing nitrided low-alloy steel parts as well as decarburized low-alloy steel parts capable of undergoing a nitriding treatment.

Les aciers de nitruration faiblement alliés présentent typiquement une teneur en carbone comprise entre 0,20 % et 0,45 % qui permet de conférer au matériau de base ses propriétés mécaniques à coeur après traitement thermique.The low-alloyed nitriding steels typically have a carbon content of between 0.20% and 0.45% which makes it possible to confer on the base material its mechanical properties at the core after heat treatment.

Les propriétés superficielles de l'acier telles que la dureté peuvent être conférées par un traitement de nitruration qui consiste à faire diffuser de l'azote en phase ferritique et qui génère la précipitation de nitrures submicroscopiques à partir d'éléments nitrurigènes comme Cr, V, Mo et Al, présents en solution solide dans l'acier traité.The superficial properties of steel such as hardness can be conferred by a nitriding treatment which consists in diffusing ferritic phase nitrogen and which generates the precipitation of submicroscopic nitrides from nitrurigenic elements such as Cr, V, Mo and Al, present in solid solution in the treated steel.

Concrètement, dans un traitement de nitruration, l'acier peut être traité à une température de l'ordre de 500°C par de l'ammoniac, lequel se décompose en ammoniac craqué et réagit simultanément avec le fer de l'acier. L'ammoniac provoque la formation d'un couche superficielle constituée de nitrures de fer, appelée couche de combinaison, à partir de laquelle les atomes d'azote diffusent en direction du coeur de la pièce pour former la couche de diffusion.Concretely, in a nitriding treatment, the steel can be treated at a temperature of about 500 ° C by ammonia, which decomposes into cracked ammonia and reacts simultaneously with the iron of the steel. Ammonia causes the formation of a surface layer of iron nitrides, called the combination layer, from which the nitrogen atoms diffuse towards the core of the part to form the diffusion layer.

Pour un acier faiblement allié comprenant des éléments nitrurigènes, on peut observer deux couches après nitruration : la couche de combinaison en surface constituée de nitrures de fer et la couche de diffusion dans laquelle sont dispersés les précipités de nitrures submicroscopiques à l'origine de l'augmentation de dureté constatée dans la couche nitrurée.For a weakly alloyed steel comprising nitrurigenic elements, two layers can be observed after nitriding: the surface combination layer consisting of iron nitrides and the diffusion layer in which the submicroscopic nitride precipitates at the origin of the nitride are dispersed. hardness increase observed in the nitrided layer.

La profondeur de la couche de nitruration peut varier, selon les conditions de nitruration et les applications visées, entre 0,05 mm et 1 mm. Le traitement de nitruration reste toutefois un traitement relativement long à mettre en oeuvre.The depth of the nitriding layer may vary, depending on the nitriding conditions and the targeted applications, between 0.05 mm and 1 mm. The nitriding treatment, however, remains a relatively long process to implement.

Il serait souhaitable d'améliorer encore les propriétés superficielles des couches de nitruration et de réduire la durée des traitements de nitruration.It would be desirable to further improve the surface properties of the nitriding layers and to reduce the duration of the nitriding treatments.

Il existe donc un besoin pour améliorer les propriétés superficielles, notamment la dureté, des couches de nitruration formées en surface de pièces en acier faiblement allié.There is therefore a need to improve the surface properties, especially the hardness, nitriding layers formed on the surface of low alloy steel parts.

Il existe encore un besoin pour accélérer la cinétique apparente des traitements de nitruration de pièces en acier faiblement allié, c'est-à-dire réduire le temps nécessaire pour obtenir à une profondeur donnée une augmentation de dureté significative, par exemple d'au moins 200 HV, par rapport à la dureté de l'acier à coeur.There is still a need to accelerate the apparent kinetics of the nitriding treatments of low-alloy steel parts, that is to say to reduce the time necessary to obtain at a given depth a significant increase in hardness, for example at least 200 HV, relative to the hardness of the core steel.

Objet et résumé de l'inventionObject and summary of the invention

A cet effet, l'invention propose, selon un premier aspect, un procédé de fabrication d'une pièce en acier faiblement allié nitruré comportant les étapes suivantes :

  1. a) décarburation de la surface d'une pièce en acier faiblement allié comprenant au moins un élément d'addition à la fois nitrurigène et carburigène afin d'obtenir une pièce décarburée présentant une couche superficielle appauvrie en carbone d'épaisseur inférieure ou égale à 1,5 mm, la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone étant inférieure ou égale à 70% de la teneur massique en carbone au coeur de la pièce décarburée,
  2. b) traitement de la pièce décarburée par un traitement de trempe suivi d'un traitement de revenu, et
  3. c) nitruration de la couche superficielle appauvrie en carbone afin d'obtenir la pièce en acier faiblement allié nitruré, l'étape c) étant réalisée après l'étape b).
For this purpose, the invention proposes, according to a first aspect, a method of manufacturing a nitrided low alloy steel part comprising the following steps:
  1. a) decarburizing the surface of a low alloy steel part comprising at least one addition element that is at the same time nitrurigenic and carburigenic in order to obtain a decarburized part having a carbon-depleted surface layer of thickness less than or equal to 1 , 5 mm, the minimum mass content of carbon in the carbon-depleted surface layer being less than or equal to 70% of the carbon mass content in the core of the decarburized part,
  2. b) treatment of the decarburized part by quenching treatment followed by a treatment of income, and
  3. c) nitriding the carbon depleted surface layer to obtain the nitrided low alloy steel part, step c) being performed after step b).

Par « acier faiblement allié », il faut comprendre un acier pour lequel aucun élément d'alliage n'est présent en une teneur massique supérieure à 5,00%. En d'autres termes, dans un acier faiblement allié, chacun des éléments chimiques, autres que le fer, est présent en une teneur massique inférieure ou égale à 5,00%."Low-alloy steel" means a steel for which no alloying element is present in a mass content greater than 5.00%. In other words, in a low-alloy steel, each of the chemical elements, other than iron, is present in a mass content less than or equal to 5.00%.

Par « élément d'addition à la fois nitrurigène et carburigène », on entend un élément d'addition présent dans l'acier sous forme de carbure avant l'étape c) et apte à former lors de l'étape c) un nitrure. Comme exemple d'élément d'addition à la fois nitrurigène et carburigène, on peut par exemple citer les éléments suivants : Cr, Mo et V.By "addition element both nitrurigenic and carburigenic" is meant an addition element present in the steel in the form of carbide before step c) and capable of forming in step c) a nitride. As an example of both nitrurigenic and carburigenic additive elements, mention may be made, for example, of the following elements: Cr, Mo and V.

La teneur massique en carbone au coeur de la pièce ne varie pas lors de l'étape a). En d'autres termes, la décarburation de surface réalisée lors de l'étape a) n'affecte la teneur en carbone qu'au niveau de la couche superficielle de la pièce et pas au niveau du coeur de celle-ci.The carbon content in the heart of the room does not vary during step a). In other words, the surface decarburization carried out in step a) affects the carbon content only at the level of the surface layer of the part and not at the level of the core thereof.

Il est possible qu'en fonction de la profondeur à laquelle on se situe dans la couche superficielle appauvrie en carbone, la teneur massique en carbone varie. Ainsi, par « teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone », il faut comprendre la valeur minimale atteinte par la teneur massique en carbone lorsque l'on se déplace le long de la profondeur de la couche superficielle appauvrie en carbone parallèlement au gradient de teneur massique en carbone.It is possible that, depending on the depth of the carbon-depleted surface layer, the mass content of carbon varies. Thus, by "minimum carbon content in the carbon-depleted surface layer", it is necessary to understand the minimum value attained by the carbon mass content when moving along the depth of the carbon-depleted surface layer in parallel. at the mass content gradient in carbon.

La teneur massique en carbone est mesurée, par exemple, par microsonde de Castaing dans le cadre établi par la norme ASTM E1019. La teneur massique en carbone au coeur de la pièce décarburée (correspondant à celle de la pièce avant décarburation) peut, par exemple, être comprise entre 0,20% et 0,45%.The carbon mass content is measured, for example, by a Castaing microprobe in the framework established by the ASTM E1019 standard. The carbon mass content at the heart of the decarburized part (corresponding to that of the part before decarburization) may, for example, be between 0.20% and 0.45%.

Dans l'invention, la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone est inférieure ou égale à 70% de la teneur massique en carbone au coeur de la pièce décarburée. Autrement dit, la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone est inférieure ou égale à 0,70 * (teneur massique en carbone au coeur de la pièce décarburée).In the invention, the minimum mass content of carbon in the carbon-depleted surface layer is less than or equal to 70% of the carbon mass content in the core of the decarburized part. In other words, the minimum carbon content in the carbon-depleted surface layer is less than or equal to 0.70 * (carbon mass content in the core of the decarburized part).

Les inventeurs ont constaté que la couche obtenue par nitruration ne présente pas des propriétés optimales si du carbone est présent en une teneur trop élevée dans la couche superficielle de la pièce avant le traitement de nitruration. Il y a dans ce cas présence d'une teneur élevée en carbures dans la couche superficielle avant nitruration laquelle peut poser problème.The inventors have found that the layer obtained by nitriding does not have optimal properties if carbon is present in an excessively high content in the surface layer of the part before the nitriding treatment. In this case, there is a high content of carbides in the surface layer before nitriding which can be problematic.

En effet, les nitrures obtenus par substitution du carbone par l'azote dans les carbures (précipitation in-situ des nitrures) lors du traitement de nitruration ont un potentiel de durcissement moins important que les nitrures obtenus par précipitation à partir d'éléments nitrurigènes présents initialement en solution solide dans la matrice ferritique. En outre, lors de la nitruration, le carbone présent initialement dans les carbures de la couche superficielle peut être libéré et diffuser en amont du front de diffusion d'azote. Ce carbone ainsi libéré peut précipiter sous la forme de réseaux de cémentite aux joints de grains et ainsi altérer les propriétés mécaniques de la couche nitrurée.Indeed, the nitrides obtained by substitution of carbon with nitrogen in the carbides (in-situ nitride precipitation) during the nitriding treatment have a lower curing potential than the nitrides obtained by precipitation from elements nitrurigenes initially present in solid solution in the ferritic matrix. In addition, during nitriding, the carbon initially present in the carbides of the surface layer can be released and diffuse upstream of the nitrogen diffusion front. This carbon thus released can precipitate in the form of cementite networks at the grain boundaries and thus alter the mechanical properties of the nitrided layer.

Ainsi, l'invention est basée sur le fait de réaliser un traitement de décarburation de la surface de la pièce avant le traitement de nitruration afin de réduire la quantité de carbures présents en surface de la pièce et ainsi s'affranchir des effets indésirables du carbone mentionnés précédemment tout en conservant de bonnes propriétés au coeur de la pièce en dehors de cette couche superficielle. Le traitement de décarburation réalisé lors de l'étape a) permet d'obtenir une couche superficielle de composition chimique optimisée pour recevoir le traitement de nitruration du fait de l'élimination, après traitement de décarburation, d'une partie au moins des carbures qui contribuent, lors de la nitruration, à la formation de réseaux de carbures/carbonitrures préjudiciables à la résistance mécanique de la couche de nitruration.Thus, the invention is based on the fact of performing a decarburizing treatment of the surface of the workpiece before the nitriding treatment in order to reduce the amount of carbides present on the surface of the workpiece and thus get rid of the undesirable effects of carbon. mentioned above while maintaining good properties in the heart of the room outside this superficial layer. The decarburization treatment carried out in step a) makes it possible to obtain a surface layer of optimized chemical composition for receiving the nitriding treatment because of the elimination, after treatment of decarburization, of at least a portion of the carbides which contribute, during the nitriding, to the formation of carbide / carbonitride networks detrimental to the mechanical strength of the nitriding layer.

L'invention permet avantageusement de significativement augmenter la dureté en surface de la pièce nitrurée et d'obtenir des champs de contraintes de compression plus intenses permettant de nettement améliorer la tenue en fatigue de la pièce en acier ainsi traitée. L'invention permet, en particulier, de réduire, voire d'éliminer, la quantité de réseaux de cémentite fragilisants. En outre, l'invention permet de significativement accélérer la cinétique apparente de nitruration du fait de la réduction de la quantité de carbone en surface de la pièce entraînant ainsi une plus grande contribution de l'azote au mécanisme de durcissement. Ainsi, par rapport au cas où l'étape a) n'est pas réalisée avant la nitruration, les pièces traitées par le procédé selon l'invention présentent pour une teneur en azote identique dans la couche de nitruration une dureté plus importante, ce qui de manière apparente revient à une accélération de la cinétique de nitruration (tout se passe comme si plus d'azote était présent à une profondeur donnée dans le cas des pièces traitées par le procédé selon l'invention).The invention advantageously makes it possible to significantly increase the surface hardness of the nitrided part and to obtain more intense compression stress fields making it possible to significantly improve the fatigue strength of the steel part thus treated. The invention makes it possible, in particular, to reduce or eliminate the quantity of weakening cementitious networks. In addition, the invention makes it possible to significantly accelerate the apparent kinetics of nitriding due to the reduction of the amount of carbon on the surface of the part, thus resulting in a greater contribution of the nitrogen to the curing mechanism. Thus, compared with the case where step a) is not carried out before the nitriding, the parts treated by the process according to the invention have, for an identical nitrogen content in the nitriding layer, a greater hardness, which Apparently, it amounts to an acceleration of the nitriding kinetics (everything happens as if more nitrogen was present at a given depth in the case of the parts treated by the process according to the invention).

L'invention peut ainsi permettre d'obtenir rapidement des couches nitrurées profondes, par exemple de 0,7 mm de profondeur. L'augmentation de la cinétique apparente de nitruration permet aussi avantageusement de limiter la relaxation des contraintes résiduelles dans la couche de nitruration du fait de la possibilité de réduire le temps d'exposition à la température de nitruration.The invention can thus make it possible to rapidly obtain deep nitride layers, for example 0.7 mm deep. The increase in the apparent kinetics of nitriding also advantageously makes it possible to limit the relaxation of the residual stresses in the nitriding layer because of the possibility of reducing the exposure time to the nitriding temperature.

L'étape intermédiaire de trempe et revenu est, quant à elle, destinée à conférer ses propriétés à l'acier de base. Dans un exemple de réalisation, la trempe (refroidissement) peut être réalisée depuis la température de décarburation et le revenu est ensuite réalisé. En variante, on peut tout d'abord refroidir la pièce décarburée, par exemple jusqu'à la température ambiante (20°C), puis réaliser une austénitisation et effectuer après l'austénitisation un traitement de trempe suivi d'un traitement de revenu.The intermediate tempering and tempering step is, for its part, intended to confer its properties on the base steel. In an exemplary embodiment, the quenching (cooling) can be performed from the decarburization temperature and the income is then achieved. Alternatively, it is first possible to cool the decarburized part, for example up to room temperature (20 ° C), then carry out austenitization and perform after austenitization a quenching treatment followed by a treatment of income.

Dans un exemple de réalisation, la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone peut être inférieure ou égale à 60% de la teneur massique en carbone au coeur de la pièce décarburée. Autrement dit, la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone peut être inférieure ou égale à 0,6 * (teneur massique en carbone au coeur de la pièce décarburée).In an exemplary embodiment, the minimum mass content of carbon in the carbon-depleted surface layer may be less than or equal to 60% of the carbon mass content in the core of the decarburized part. In other words, the minimum mass content of carbon in the carbon-depleted surface layer may be less than or equal to 0.6 * (carbon mass content in the core of the decarburized part).

La nitruration peut être réalisée dans des conditions conventionnelles par tout type de procédé de nitruration connu de l'homme du métier. Les réglages d'atmosphère de température et de temps sont définis en fonction de l'acier de base et des caractéristiques recherchées.Nitriding can be carried out under conventional conditions by any type of nitriding process known to those skilled in the art. The temperature and time atmosphere settings are defined based on the base steel and the desired characteristics.

Dans un exemple de réalisation, l'épaisseur de la couche superficielle appauvrie en carbone peut être comprise entre 0,3 mm et 1,5 mm.In an exemplary embodiment, the thickness of the carbon-depleted surface layer may be between 0.3 mm and 1.5 mm.

Dans un exemple de réalisation, la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone peut être supérieure ou égale à 0,05%.In an exemplary embodiment, the minimum carbon mass content in the carbon-depleted surface layer may be greater than or equal to 0.05%.

La présence d'une telle teneur minimale en carbone dans la couche superficielle appauvrie en carbone permet avantageusement d'assurer une augmentation significative de la dureté en surface de la pièce après nitruration.The presence of such a minimum carbon content in the carbon-depleted surface layer advantageously makes it possible to ensure a significant increase in the surface hardness of the part after nitriding.

La pièce en acier faiblement allié traitée peut, par exemple, être en acier 32CrMoV13, 35CrMo4, 15CrMoV6 ou X38CrMoV5. La pièce en acier faiblement allié traitée peut de préférence être en acier 32CrMoV13.The treated low alloy steel part may, for example, be made of 32CrMoV13, 35CrMo4, 15CrMoV6 or X38CrMoV5 steel. The treated low-alloy steel part may preferably be 32CrMoV13 steel.

Dans un exemple de réalisation, on peut traiter durant l'étape a) la pièce en acier faiblement allié par une atmosphère oxydante comprenant de la vapeur d'eau en imposant une température comprise entre 850°C et 1000°C, l'atmosphère oxydante ayant une température de rosée comprise entre -40°C et +20°C, par exemple comprise entre -10°C et +10°C, par exemple comprise entre -4°C et +4°C.In an exemplary embodiment, it is possible to treat during step a) the low-alloy steel part by an oxidizing atmosphere comprising water vapor by imposing a temperature of between 850 ° C. and 1000 ° C., the oxidizing atmosphere having a dew point temperature between -40 ° C and + 20 ° C, for example between -10 ° C and + 10 ° C, for example between -4 ° C and + 4 ° C.

En variante, on peut traiter durant l'étape a) la pièce en acier faiblement allié par de l'air à une température de 950°C afin de réaliser la décarburation souhaitée.Alternatively, the low alloy steel part can be treated during step a) with air at a temperature of 950 ° C. in order to achieve the desired decarburization.

Dans un exemple de réalisation, on peut réaliser, avant l'étape c), une étape d'usinage de la couche superficielle appauvrie en carbone.In an exemplary embodiment, it is possible to carry out, before step c), a step of machining the carbon-depleted surface layer.

La réalisation d'une telle étape d'usinage est avantageuse car elle peut permettre d'éliminer une couche oxydée non souhaitable qui peut être formée lors de la décarburation. Cette couche oxydée peut typiquement avoir une épaisseur inférieure ou égale à quelques centièmes de millimètres. Il existe d'autres manières de réduire la quantité d'oxydes en surface de la pièce décarburée, on peut ainsi par exemple soumettre la pièce décarburée à un traitement par une atmosphère réductrice, par exemple comprenant du H2, avant l'étape c).Performing such a machining step is advantageous because it can make it possible to eliminate an undesirable oxidized layer that can be formed during decarburization. This oxidized layer may typically have a thickness less than or equal to a few hundredths of a millimeter. There are other ways of reducing the amount of oxides on the surface of the decarburized part, it is thus possible, for example, to subject the decarburized part to a treatment by a reducing atmosphere, for example comprising H 2 , before step c) .

L'étape d'usinage de la couche superficielle appauvrie en carbone peut être réalisée avant et/ou après l'étape b).The machining step of the carbon-depleted surface layer can be performed before and / or after step b).

Dans un exemple de réalisation, on peut réaliser après l'étape c) une étape d'usinage de la couche de nitruration formée lors de l'étape c).In an exemplary embodiment, it is possible after step c) to perform a step of machining the nitriding layer formed in step c).

La réalisation d'une telle étape d'usinage peut permettre d'éliminer la couche de combinaison formée lors du traitement de nitruration, seule la couche de diffusion formée lors de ce traitement étant conservée.The realization of such a machining step can eliminate the combination layer formed during the nitriding treatment, only the diffusion layer formed during this treatment is retained.

Dans un exemple de réalisation, il est possible de ne chercher à obtenir une couche superficielle appauvrie en carbone après mise en oeuvre de l'étape a) que dans une ou une plusieurs zones prédéfinies de la surface de la pièce. Dans ce cas, il est par exemple possible de revêtir avant l'étape a) la surface de la pièce en acier faiblement allié d'un masque afin de ne réaliser la décarburation que dans une ou plusieurs zones prédéfinies de la surface de la pièce délimitées par le masque. On peut ainsi par exemple déposer un vernis formant barrière à l'oxygène sur la surface de la pièce, la ou les zones sur lesquelles le vernis n'a pas été déposé étant destinées à être décarburées lors de l'étape a).In an exemplary embodiment, it is possible to seek to obtain a carbon-depleted surface layer after implementation of step a) in one or more predefined zones of the surface of the part. In this case, it is for example possible to coat before step a) the surface of the low alloy steel part of a mask in order to perform the decarburization only in one or more predefined areas of the surface of the room delimited by the mask. For example, it is possible, for example, to deposit an oxygen barrier varnish on the surface of the part, the zone or zones on which the varnish has not been deposited being intended to be decarburized during step a).

Il est encore possible que l'on élimine, avant l'étape c), une partie de la couche superficielle appauvrie en carbone afin de ne la conserver que dans une ou plusieurs zones prédéfinies de la surface de la pièce.It is still possible to eliminate, before step c), a portion of the carbon-depleted surface layer in order to preserve it only in one or more predefined areas of the surface of the part.

La présente invention vise également une pièce en acier faiblement allié décarburée comprenant au moins un élément d'addition à la fois nitrurigène et carburigène et présentant une couche superficielle appauvrie en carbone d'épaisseur inférieure ou égale à 1,5 mm destinée à subir une nitruration, la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone étant inférieure ou égale à 70% de la teneur massique en carbone au coeur de la pièce décarburée.The present invention also relates to a decarburized low-alloy steel part comprising at least one addition element which is both nitrurigenic and carburigenic and has a carbon-depleted surface layer of thickness less than or equal to 1.5 mm intended to undergo nitriding. , the minimum carbon content in the carbon-depleted surface layer being less than or equal to 70% of the carbon mass content in the core of the decarburized part.

Une telle pièce correspond au produit intermédiaire obtenu par mise en oeuvre de l'étape a) décrite plus haut avant réalisation de l'étape c) de nitruration.Such a part corresponds to the intermediate product obtained by carrying out step a) described above before carrying out the nitriding step c).

Dans un exemple de réalisation, la couche superficielle appauvrie en carbone peut présenter une épaisseur comprise entre 0,3 mm et 1,5 mm.In an exemplary embodiment, the carbon-depleted surface layer may have a thickness of between 0.3 mm and 1.5 mm.

De préférence, l'acier faiblement allié peut être l'acier 32CrMoV13.Preferably, the low alloy steel may be 32CrMoV13 steel.

Les différentes caractéristiques décrites plus haut pour le procédé selon l'invention relatives à la couche superficielle appauvrie en carbone et à l'acier faiblement allié utilisé s'appliquent à l'aspect relatif à la pièce selon l'invention.The various characteristics described above for the process according to the invention relating to the carbon-depleted surface layer and the low-alloy steel used are applicable to the aspect relative to the part according to the invention.

Brève description des dessinsBrief description of the drawings

D'autres caractéristiques et avantages de l'invention ressortiront de la description suivante, en référence aux dessins annexés, sur lesquels :

  • les figures 1 à 3 représentent de manière très schématique l'évolution de la composition en surface d'une pièce en acier faiblement allié lors d'un traitement de nitruration selon que cette pièce ait été décarburée ou non,
  • les figures 4, 7 et 10 présentent notamment des résultats de mesure obtenus par microsonde de Castaing montrant l'évolution de la teneur massique en carbone dans une pièce dont la surface a été décarburée, et
  • les figures 5, 6, 8, 9 et 11 sont des résultats de mesure comparant les duretés obtenues pour des pièces nitrurées selon que celles-ci aient ou non subi au préalable un traitement de décarburation de surface.
Other characteristics and advantages of the invention will emerge from the following description, with reference to the appended drawings, in which:
  • the Figures 1 to 3 represent very schematically the evolution of the surface composition of a steel part weakly alloyed during a nitriding treatment depending on whether this part has been decarburized or not,
  • the figures 4 , 7 and 10 notably present measurement results obtained by a Castaing microprobe showing the evolution of the mass content of carbon in a part whose surface has been decarburized, and
  • the Figures 5, 6 , 8 , 9 and 11 are measurement results comparing the hardnesses obtained for nitrided parts according to whether or not they have previously undergone surface decarburization treatment.

Description détaillée de modes de réalisationDetailed description of embodiments

On a représenté aux figures 1 à 3 de manière très schématique l'évolution de la composition en surface d'une pièce en acier faiblement allié lors d'un traitement de nitruration selon que cette pièce ait été décarburée (« Acier décarburé ») ou non (« Acier brut trempé/revenu »).Representatives Figures 1 to 3 very schematically the evolution of the surface composition of a low-alloy steel part during a nitriding treatment depending on whether this part has been decarburized ("decarburized steel") or not ("Raw tempered / tempered steel") ).

Dans les deux cas, la pièce présente à sa surface d'une part du chrome en solution solide 1 ainsi que des carbures enrichis en chrome 2 dits carbures de chrome. La pièce décarburée comporte dans sa couche superficielle appauvrie en carbone 3 une quantité inférieure de carbures de chrome 2 par rapport à la pièce non décarburée. L'acier faiblement allié mis en oeuvre peut permettre l'obtention d'une structure martensitique, bainito-martensitique ou bainitique dans la couche superficielle appauvrie en carbone après la trempe. La pièce est, lors de la nitruration, par exemple traitée par un flux gazeux comprenant de l'ammoniac.In both cases, the part has on its surface on the one hand chromium in solid solution 1 as well as carbides enriched in chromium 2 called chromium carbides. The decarburized part comprises in its carbon-depleted surface layer 3 a smaller amount of chromium carbides 2 than the non-decarburized part. The weakly alloyed steel used can make it possible to obtain a martensitic, bainito-martensitic or bainitic structure in the carbon-depleted surface layer after quenching. The part is, during nitriding, for example treated with a gas stream comprising ammonia.

Les figures 2 et 3 schématisent l'évolution de la composition superficielle de la pièce lors de la nitruration et comparent celle-ci à la composition superficielle de la pièce avant nitruration. Comme illustré à la figure 2, les carbures de chrome 2 sont, lors de la nitruration, convertis en nitrures de chrome 4 issus de carbures et du carbone 5 est libéré. Le chrome en solution solide 1 est, quant à lui, converti lors de la nitruration en nitrures de chrome 6 issus du chrome en solution solide.The figures 2 and 3 schematize the evolution of the surface composition of the part during nitriding and compare it to the surface composition of the part before nitriding. As illustrated in figure 2 at the nitriding, the chromium carbides 2 are converted to chromium nitrides 4 from carbides and carbon 5 is released. Chromium in solid solution 1 is, in turn, converted during nitriding chromium nitride 6 from chromium in solid solution.

Comme mentionné plus haut, les nitrures de chrome 4 peuvent conférer moins de dureté à la couche nitrurée que les nitrures de chrome 6. La couche nitrurée obtenue lorsque la pièce a été au préalable décarburée présente donc avantageusement une meilleure dureté du fait d'une plus faible proportion en nitrures de chrome 4.As mentioned above, the nitrides of chromium 4 can confer less hardness on the nitride layer than the nitrides of chromium 6. The nitrided layer obtained when the part was in advance Decarburized therefore advantageously has a better hardness due to a lower proportion of chromium nitrides 4.

Par ailleurs, comme illustré à la figure 3, si la teneur en carbone 5 libéré dépasse un certain seuil des réseaux de cémentite 7 peuvent apparaître lesquels peuvent fragiliser la couche de nitruration obtenue. Dans le cas où la décarburation n'a pas été réalisée avant la nitruration, un plus grand nombre de réseaux cémentite 7 fragilisants sont formés du fait d'une plus grande quantité de carbone 5 libéré lors de la nitruration.Moreover, as illustrated in figure 3 if the released carbon content exceeds a certain threshold of the cementitious networks 7 may appear which can weaken the nitriding layer obtained. In the case where the decarburization has not been carried out before the nitriding, a larger number of cementitious fragilizer networks 7 are formed due to a larger amount of carbon released during the nitriding.

ExemplesExamples Exemple 1Example 1

Un acier de nitruration de type 32CrMoV13 à l'état recuit a tout d'abord été décarburé par traitement par une atmosphère oxydante comprenant de la vapeur d'eau et présentant un point de rosée de +4°C sous une température de 950°C pendant 6 heures.A type of nitriding steel of the type 32CrMoV13 in the annealed state was first decarburized by treatment with an oxidizing atmosphere comprising water vapor and having a dew point of + 4 ° C. under a temperature of 950 ° C. for 6 hours.

La figure 4 montre l'évolution en fonction de la profondeur de la teneur massique en carbone et de la dureté de la pièce en acier faiblement allié décarburée ainsi obtenue. Comme illustré, un tel traitement de décarburation conduit à l'obtention d'une teneur en carbone d'environ 0,12% à la surface de la couche superficielle appauvrie en carbone (profondeur 0µm), la teneur en carbone de l'acier de base étant retrouvée à une profondeur d'environ 0,85 mm. La teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone ainsi obtenue est d'environ 0,08%.The figure 4 shows the evolution as a function of the depth of the carbon mass content and the hardness of the decarburized low alloy steel part thus obtained. As illustrated, such a decarburization treatment leads to obtaining a carbon content of about 0.12% on the surface of the carbon-depleted surface layer (depth 0 μm), the carbon content of the steel of base being found at a depth of about 0.85 mm. The minimum carbon content in the carbon-depleted surface layer thus obtained is about 0.08%.

A la suite de ce traitement de décarburation, une trempe à l'huile a été réalisée puis un traitement de revenu a été effectué à une température de 635°C pendant 4 heures sous une atmosphère N2 + 10%H2. Une deuxième trempe à l'huile a été réalisée après le traitement de revenu.Following this decarburization treatment, oil quenching was carried out and then a tempering treatment was carried out at a temperature of 635 ° C. for 4 hours under an N 2 + 10% H 2 atmosphere. A second oil quench was performed after the treatment of income.

Une nitruration sous une atmosphère composée de 50% de NH3, 37,5% de H2 et 12,5% de N2 à une température de 520°C pendant une durée comprise entre 101 heures et 113 heures a été réalisée.Nitriding under an atmosphere composed of 50% NH 3 , 37.5% H 2 and 12.5% N 2 at a temperature of 520 ° C for a period between 101 hours and 113 hours was performed.

La figure 5 fournit les résultats de mesure de microdureté pour la pièce en acier faiblement allié nitruré obtenue dans le cadre du présent exemple (« Alliage décarburé + revenu + nitruration »). A titre de comparaison, la figure 5 fait apparaître les mesures de microdureté obtenues lorsque le même acier est soumis à un traitement ne différant qu'en ce que la décarburation n'est pas réalisée (« Alliage « brut » + nitruration »). On constate que la mise en oeuvre d'un procédé selon l'invention permet, du fait de la réalisation de la décarburation avant la nitruration, de significativement améliorer la dureté de surface de la pièce obtenue. Le décalage de la courbe relative à la réalisation d'un traitement de décarburation vers la droite par rapport à la courbe « Alliage « brut » + nitruration » montre aussi que la cinétique apparente de nitruration est améliorée dans le cadre de l'invention.The figure 5 provides the microhardness measurement results for the nitrided low alloy steel part obtained in the context of the present example ("Decarburized alloy + income + nitriding"). For comparison, the figure 5 shows the microhardness measurements obtained when the same steel is subjected to a treatment differing only in that the decarburization is not carried out ("Alloy" crude "+ nitriding"). It can be seen that the implementation of a process according to the invention makes it possible, by virtue of carrying out the decarburization before the nitriding, to significantly improve the surface hardness of the part obtained. The shift in the curve relating to the realization of a decarburization treatment to the right with respect to the "crude alloy" + "nitriding" curve also shows that the apparent kinetics of nitriding is improved within the scope of the invention.

La figure 6 montre quant à elle l'évolution de la différence de dureté par rapport à celle du coeur en fonction de la profondeur pour une pièce obtenue selon l'exemple 1 ou obtenue dans des conditions identiques à celles de l'exemple 1 à l'exception que le traitement de décarburation n'a pas été réalisé. On constate pour la pièce obtenue selon l'exemple 1 l'obtention d'un gain de dureté de surface significatif de l'ordre de 32% (637-436/637).The figure 6 shows the evolution of the difference in hardness with respect to that of the core as a function of the depth for a part obtained according to Example 1 or obtained under conditions identical to those of Example 1 except that the decarburization treatment was not performed. For the part obtained according to Example 1, it is found that a significant surface hardness gain of about 32% (637-436 / 637) is achieved.

Exemple 2Example 2

Un acier de nitruration de type 32CrMoV13 à l'état recuit a été traité dans les mêmes conditions que dans l'exemple 1. La seule différence concerne le fait que la décarburation a été réalisée sous une température de 980°C et non plus de 950°C.A type of nitriding steel of the type 32CrMoV13 in the annealed state was treated under the same conditions as in Example 1. The only difference relates to the fact that the decarburization was carried out under a temperature of 980 ° C. and no longer than 950 ° C. ° C.

La figure 7 montre l'évolution en fonction de la profondeur de la teneur massique en carbone et de la dureté de la pièce en acier faiblement allié décarburée ainsi obtenue. Comme illustré, un tel traitement de décarburation conduit à l'obtention d'une teneur en carbone inférieure à 0,10% à la surface de la couche superficielle appauvrie en carbone (profondeur 0µm), la teneur en carbone de l'acier de base étant retrouvée à une profondeur d'environ 0,9 mm. La teneur en carbone à la surface de la couche superficielle appauvrie en carbone correspond dans ce cas à la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone.The figure 7 shows the evolution as a function of the depth of the carbon mass content and the hardness of the decarburized low alloy steel part thus obtained. As illustrated, such a decarburization treatment leads to the production of a carbon content of less than 0.10% at the surface of the carbon-depleted surface layer (depth 0 μm), the carbon content of the base steel being found at a depth of about 0.9 mm. The carbon content at the surface of the carbon-depleted surface layer corresponds in this case to the minimum carbon content in the carbon-depleted surface layer.

La figure 8 fournit les résultats de mesure de microdureté pour la pièce en acier faiblement allié nitruré obtenue dans le cadre du présent exemple (« Alliage décarburé + revenu + nitruration »). A titre de comparaison, la figure 8 fait apparaître les mesures de microdureté obtenues lorsque le même acier est soumis à un traitement ne différant qu'en ce que la décarburation n'est pas réalisée (« Alliage « brut » + nitruration »). On constate que la mise en oeuvre d'un procédé selon l'invention permet, du fait de la réalisation de la décarburation avant la nitruration, de significativement améliorer la dureté de surface de la pièce obtenue. Le décalage de la courbe relative à la réalisation d'un traitement de décarburation vers la droite par rapport à la courbe « Alliage « brut » + nitruration » montre aussi que la cinétique apparente de nitruration est améliorée dans le cadre de l'invention.The figure 8 provides the microhardness measurement results for the nitrided low alloy steel part obtained in the context of the present example ("Decarburized alloy + income + nitriding"). For comparison, the figure 8 shows the microhardness measurements obtained when the same steel is subjected to a treatment differing only in that the decarburization is not carried out ("Alloy" crude "+ nitriding"). It can be seen that the implementation of a process according to the invention makes it possible, by virtue of carrying out the decarburization before the nitriding, to significantly improve the surface hardness of the part obtained. The shift in the curve relating to the realization of a decarburization treatment to the right with respect to the "crude alloy" + "nitriding" curve also shows that the apparent kinetics of nitriding is improved within the scope of the invention.

La figure 9 montre quant à elle l'évolution de la différence de dureté par rapport à celle du coeur en fonction de la profondeur pour une pièce obtenue selon l'exemple 2 ou obtenue dans des conditions identiques à celles de l'exemple 2 à l'exception que le traitement de décarburation n'a pas été réalisé. On constate pour la pièce obtenue selon l'exemple 2 l'obtention d'un gain de dureté de surface significatif de l'ordre de 30% (627-436/627).The figure 9 shows the evolution of the difference in hardness with respect to that of the core as a function of the depth for a part obtained according to Example 2 or obtained under conditions identical to those of Example 2 except that the decarburization treatment was not performed. For the part obtained according to Example 2, it is found that a significant surface hardness gain of the order of 30% (627-436 / 627) is obtained.

Exemple 3Example 3

Un acier de nitruration de type 32CrMoV13 à l'état recuit a tout d'abord été décarburé par traitement par une atmosphère oxydante comprenant de la vapeur d'eau et présentant un point de rosée de 0°C sous une température de 950°C pendant 6 heures.An annealed 32CrMoV13 type nitriding steel was first decarburized by treatment with an oxidizing atmosphere comprising water vapor and having a dew point of 0 ° C. under a temperature of 950 ° C. 6 hours.

La figure 10 montre l'évolution en fonction de la profondeur de la teneur massique en carbone et de la dureté de la pièce en acier faiblement allié décarburée ainsi obtenue. Comme illustré, un tel traitement de décarburation conduit à l'obtention d'une teneur en carbone d'environ 0,20% à la surface de la couche superficielle appauvrie en carbone (profondeur 0µm), la teneur en carbone de l'acier de base étant retrouvée à une profondeur d'environ 0,85 mm. La teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone ainsi obtenue est d'environ 0,16%.The figure 10 shows the evolution as a function of the depth of the carbon mass content and the hardness of the decarburized low alloy steel part thus obtained. As illustrated, such a decarburization treatment leads to the attainment of a carbon content of about 0.20% on the surface of the carbon-depleted surface layer (depth 0 μm), the carbon content of the steel of base being found at a depth of about 0.85 mm. The minimum carbon content in the carbon-depleted surface layer thus obtained is about 0.16%.

A la suite de ce traitement de décarburation, une trempe à l'huile a été réalisée puis un traitement de revenu a été effectué à une température de 635°C pendant 4 heures sous une atmosphère N2 + 10%H2. Une deuxième trempe à l'huile a été réalisée après le traitement de revenu.Following this decarburization treatment, oil quenching was carried out and then a tempering treatment was carried out at a temperature of 635 ° C. for 4 hours under an N 2 + 10% H 2 atmosphere. A second oil quench was performed after the treatment of income.

Une nitruration sous une atmosphère composée de 50% de NH3, 37,5% de H2 et 12,5% de N2 à une température de 550°C pendant une durée de 55 heures a été réalisée.Nitriding under an atmosphere composed of 50% NH 3 , 37.5% H 2 and 12.5% N 2 at a temperature of 550 ° C for a period of 55 hours was carried out.

La figure 11 montre l'évolution de la différence de dureté par rapport à celle du coeur en fonction de la profondeur pour une pièce obtenue selon l'exemple 3 (« Alliage décarburé + revenu + nitruration ») ou obtenue dans des conditions identiques à celles de l'exemple 3 à l'exception que le traitement de décarburation n'a pas été réalisé (« Alliage « brut » + nitruration »). On constate pour la pièce obtenue selon l'exemple 3 l'obtention d'un gain de dureté de surface significatif.The figure 11 shows the evolution of the difference in hardness with respect to that of the core as a function of the depth for a part obtained according to Example 3 ("Decarburized alloy + income + nitriding") or obtained under conditions identical to those of the Example 3 with the exception that the decarburization treatment was not carried out ("crude alloy" + nitriding "). For the part obtained according to Example 3, it is found that a significant gain in surface hardness is achieved.

L'expression « comportant/comprenant un(e) » doit se comprendre comme « comportant/comprenant au moins un(e) ».The expression "comprising / including a person" must be understood as "comprising / including at least one".

L'expression « compris(e) entre ... et ... » ou « allant de ... à ... » doit se comprendre comme incluant les bornes.The expression "understood between ... and ..." or "from ... to ..." must be understood as including the boundaries.

Claims (8)

Procédé de fabrication d'une pièce en acier faiblement allié nitruré comportant les étapes suivantes : a) décarburation de la surface d'une pièce en acier faiblement allié comprenant au moins un élément d'addition à la fois nitrurigène et carburigène afin d'obtenir une pièce décarburée présentant une couche superficielle appauvrie en carbone d'épaisseur inférieure ou égale à 1,5 mm, la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone étant inférieure ou égale à 70% de la teneur massique en carbone au coeur de la pièce décarburée et la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone étant supérieure ou égale à 0,05%, b) traitement de la pièce décarburée par un traitement de trempe suivi d'un traitement de revenu, et c) nitruration de la couche superficielle appauvrie en carbone afin d'obtenir la pièce en acier faiblement allié nitruré, l'étape c) étant réalisée après l'étape b). A method of manufacturing a nitrided low alloy steel part comprising the following steps: a) decarburizing the surface of a low alloy steel part comprising at least one addition element that is at the same time nitrurigenic and carburigenic in order to obtain a decarburized part having a carbon-depleted surface layer of thickness less than or equal to 1 , 5 mm, the minimum carbon content in the carbon-depleted surface layer being less than or equal to 70% of the mass carbon content in the core of the decarburized part and the minimum carbon content in the carbon-depleted surface layer being greater than or equal to 0.05%, b) treatment of the decarburized part by quenching treatment followed by a treatment of income, and c) nitriding the carbon depleted surface layer to obtain the nitrided low alloy steel part, step c) being performed after step b). Procédé selon la revendication 1, caractérisé en ce que l'épaisseur de la couche superficielle appauvrie en carbone est comprise entre 0,3 mm et 1,5 mm.Process according to Claim 1, characterized in that the thickness of the carbon-depleted surface layer is between 0.3 mm and 1.5 mm. Procédé selon l'une quelconque des revendications 1 et 2, caractérisé en ce que l'on réalise, avant l'étape c), une étape d'usinage de la couche superficielle appauvrie en carbone.Process according to either of Claims 1 and 2, characterized in that a step of machining the carbon-depleted surface layer is carried out before step c). Procédé selon l'une quelconque des revendications 1 à 3, caractérisé en ce que l'on réalise après l'étape c) une étape d'usinage de la couche de nitruration formée lors de l'étape c).Process according to any one of Claims 1 to 3, characterized in that a step of machining the nitriding layer formed in step c) is carried out after step c). Procédé selon l'une quelconque des revendications 1 à 4, caractérisé en ce que la pièce en acier faiblement allié traitée est en acier 32CrMoV13.Process according to any one of Claims 1 to 4, characterized in that the treated low-alloy steel part is made of 32CrMoV13 steel. Pièce en acier faiblement allié décarburée comprenant au moins un élément d'addition à la fois nitrurigène et carburigène et présentant une couche superficielle appauvrie en carbone d'épaisseur inférieure ou égale à 1,5 mm destinée à subir une nitruration, la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone étant inférieure ou égale à 70% de la teneur massique en carbone au coeur de la pièce décarburée et la teneur massique minimale en carbone dans la couche superficielle appauvrie en carbone étant supérieure ou égale à 0,05%.A low-alloy, decarburized steel part comprising at least one nitrurigenic and carburizing additive element and having a carbon-depleted surface layer of thickness less than or equal to 1.5 mm intended to undergo nitriding, the minimum mass content of carbon in the carbon-depleted surface layer being less than or equal to 70% of the mass carbon content in the core of the decarburized part and the minimum carbon content in the carbon-depleted surface layer being greater than or equal to 0.05% . Pièce selon la revendication 6, caractérisée en ce que la couche superficielle appauvrie en carbone présente une épaisseur comprise entre 0,3 mm et 1,5 mm.Part according to Claim 6, characterized in that the carbon-depleted surface layer has a thickness of between 0.3 mm and 1.5 mm. Pièce selon l'une quelconque des revendications 6 et 7, caractérisé en ce que l'acier faiblement allié est l'acier 32CrMoV13.Part according to any one of claims 6 and 7, characterized in that the low alloy steel is 32CrMoV13 steel.
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