Classifications

• • 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
  • C23C8/06—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
  • 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

Definitions

• the present invention generally relates to the field of hardening of steel parts and more particularly that of nitriding. State of the art
• Nitriding treatments are thermochemical treatments allowing the introduction by diffusion of nitrogen at the surface of the parts to achieve surface mechanical reinforcement, typically improving hardness and fatigue resistance.
• Nitriding treatments are conventionally carried out on alloy steels previously treated by austenitization, quenching, and tempering at temperatures above 600 ° C. This then allows the steels to be nitrided at a lower temperature (conventionally at least 30 ° C lower) without affecting the underlying structural state.
• the steel is nitrided at a temperature of the order of 480 ° C to 550 ° C in an atmosphere containing ammonia, which gives up nitrogen to the surface of the steel.
• the nitrogen thus adsorbed causes the formation, on the surface, of a layer called the combination layer and consisting of iron nitrides, from which the nitrogen atoms diffuse towards the core of the part to form the diffusion layer .
• the nitrogen adsorption kinetics at the steel surface, and the thickness of the combination and diffusion layers depend on the nitriding potential, denoted KN, applied during the nitriding treatment.
• the gas nitriding of low alloy steels generates intergranular precipitation (PIG) of alloy cementite according to the following mechanism: substitution of carbides by nitrides; diffusion of carbon which is found in solid solution; precipitation at grain boundaries with mainly alloying elements (Cr, Mo, V).
• the present invention provides a gas nitriding hardening process according to claim 1.
• a nitrurable steel part, or a part thereof is subjected to a nitriding steel part, or a part thereof. initial hardening by austenitization and quenching, followed by intermediate tempering, then by a nitriding treatment in two successive stages.
• the present process is suitable for the nitriding of so-called nitriding steels, therefore alloy steels, and more particularly low alloy steels.
• the first step is a treatment with low nitriding potential, carried out at a nitriding potential KN lower than the nitriding potential KN for which a combination layer is formed on the surface of the steel part.
• a nitriding potential KN lower than the nitriding potential KN for which a combination layer is formed on the surface of the steel part.
• the second step is a treatment with a high nitriding potential, carried out at a nitriding potential KN greater than or equal to the nitriding potential for which a combination layer forms on the surface of the steel part.
• the second step is carried out until the desired depth of nitriding is reached.
• nitriding potential is defined as follows:
• PH2 where PNH3 and PH2 respectively represent the partial pressures of ammonia and of hydrogen applied during the nitriding treatment.
• KN the KN nitriding potential
• KN the nitriding potential
• the iron nitrides will not form, which results in the absence of formation of a combination layer. It is then possible to define a limit nitriding potential KN beyond which the combination layer is formed.
• the first step of the present nitriding treatment therefore takes place under nitriding conditions favoring decarburization, which is the opposite of conventional methods, which on the contrary recommend starting the treatment at a high KN nitriding potential to promote formation. rapid of a combination layer which can subsequently feed the diffusion layer. Indeed, for those skilled in the art decarburization (elimination of carbon atoms released to the atmosphere) is conventionally associated with an embrittlement of the upper layer of steel and a decrease in its hardness, and should therefore be avoided.
• the second step of the nitriding treatment of the present process takes place at a high nitriding potential, so as to form a combination layer.
• This step makes it possible to saturate the surface layer of the treated steel part (or the part of the treated steel part) with nitrogen in order to maximize the surface hardness expected following a nitriding treatment.
• the combination layer can be removed at the end of the nitriding treatment, in particular by machining / grinding.
• the nitriding treatment which is the subject of the second step of the nitriding treatment of the present process corresponds to what is conventionally done during hardening of steel by nitriding. However, unlike what happens for steels subjected to a conventional treatment, the diffusion of nitrogen atoms within the steel part treated by the method according to the invention is facilitated by the prior elimination of the atoms. of carbon.
• the diffusion layer thus obtained on the surface of a steel part treated according to the method has a surface hardness of the same order of magnitude as that obtained by a conventional treatment (surface hardness which is greater than the core hardness). But the part contains less cementite precipitated at the grain boundaries due to the prior elimination of carbon atoms.
• the part treated according to the process has, on the surface, a “healthier” microstructure, with less intergranular precipitation, allowing performance in flexural fatigue and increased contact fatigue.
• the microstructure of the surface layer is therefore generally less fragile.
• the first step of the nitriding treatment is carried out under non-oxidizing nitriding conditions so as not to induce an increase in the brittleness of the steel part subjected to the process according to the invention.
• the durations of the first and second stages are adapted according to needs. An increase in the treatment time leads, for the first step, to increased decarburization and for the second step to an increase in the depth of nitriding.
• the first and second stages may have a substantially equal duration.
• tests at different temperatures and different nitriding potentials for given steel compositions allow the calibration of the process.
• the working temperatures are generally between 480 and 580 ° C, preferably between 500 and 550 ° C.
• all of the nitriding (the two steps) can be carried out at the same temperature, called the nitriding temperature.
• the nitriding treatment is advantageously carried out in the same oven.
• the two stages are advantageously linked there without treatment or intermediate handling of the steel part to be treated. This is particularly advantageous in terms of implementation. This is especially so since for both stages, the treatment atmosphere comprises ammonia and dissociated ammonia (H2 andN2).
• the control of the nitriding potential in the furnace is done by adjusting the ammonia feed rate according to the hydrogen concentration measured by means of a probe.
• the weak and strong nitriding potentials are determined for a given temperature and a given alloy. This determination is made experimentally and on the basis of the Lehrer diagram.
• the nitriding potential K N for the first step can be between 0.15 and 0.6.
• the nitriding potential K N for the second step can be between 2 and 4.
• the ratio of strong KN to weak KN can be greater than 10, 15 or 20.
• low alloy steel is meant a steel for which no alloying element is present in a mass content greater than 5.00%.
• each of the chemical elements other than iron is present in a mass content of less than or equal to 5.00%.
• the method according to the invention is suitable for the treatment of nitriding steel parts.
• nitriding steel parts are typically low alloy steels containing elements which allow the formation of nitrides, such as chromium, molybdenum, vanadium and aluminum.
• the carbon content is preferably 0.15 to 0.9% m.
• the part is made of nitrurable steel comprising 0.15 to 0.9% m of carbon and one or more alloying elements chosen from the list comprising: chromium, nickel, molybdenum, vanadium, aluminum, silicon and manganese.
• the alloying elements are present in the following proportions: Chromium 1 to 4% m; Nickel 0.5 to 4% m; Molybdenum at 0.2 to 4% m; Vanadium 0.10 to 2% m; Aluminum 0.5 to 3% m; Silicon 0.4 to 1.5% m; and / or Manganese 1 to 2% m.
• the present process has been particularly developed for CrMoV steel grades (also noted CDV), in particular 32CrMoV13.
• the invention finds particular application for the treatment of parts in aeronautics and automobiles, of the toothing and pinion type, splines, raceways, rings and rolling elements of rolling bearings, among others, to enable them to withstand the mechanical stresses to which they are subjected, which have the particularity of being concentrated mainly on the surface (flexion fatigue, contact fatigue, fretting, wear, etc.).
• FIG. 1 a graph representing the hardness profile (hardness - depth) for 4 samples A, B, C, D;
• FIG. 2 a graph representing the number of cementite edges formed (number - depth) for the 4 samples;
• FIG. 3 a graph representing Making cementite edges formed (area - depth) for the 4 samples;
• FIG. 4 optical microscope images allowing comparison of the morphologies of the surface layers of a piece of steel subjected to a nitriding treatment according to the invention (D), a conventional treatment with high nitriding potential (A), and a treatment carried out with a nitriding potential slightly lower (B) or higher (C) than the limiting potential of formation of the combination layer.
• a variant of the present process for hardening nitrurable steel parts will be described here, in relation to the application to low-alloy steels.
• This is a process for hardening steel parts, using nitriding, which as is known consists of a surface treatment with the aim of increasing the surface hardness.
• the parts treated according to the present process therefore typically have a nitrided surface layer, the hardness of which is greater than the core hardness.
• the present process follows the traditional three-step process: initial hardening, tempering, nitriding treatment.
• the steel part to be treated (or part of it) first undergoes an initial hardening treatment, by austenitization and quenching.
• the part to be treated is brought to an austenitization temperature for a predetermined time, then undergoes quenching (controlled rapid cooling) to induce a martensitic type microstructure.
• a tempering phase follows, in which the part is heated to a tempering temperature, generally greater than 600 ° C., for a predetermined time. This tempering phase allows a softening of the part (lowering of the hardness) and an adjustment of the tensile and resilience characteristics, characteristics controlled according to the targeted applications and compatible with the machining operations.
• the processing of nitriding is carried out in order to increase the surface hardness of the steel part considered.
• gas nitriding is conventionally carried out at temperatures between 480 and 550 ° C. by applying a nitriding potential KN greater than the limiting nitriding potential for the formation of the combination layer.
• KN is generally ten times greater than the limit potential. This ensures the formation of a thick combination layer which prevents decarburization of the surface of the steel part and constitutes a reservoir of nitrogen atoms which will then diffuse to form the diffusion layer.
• the absence of decarburization is accompanied by precipitation of carbon atoms at the grain boundaries in the form of cementite edges.
• the method according to the invention implements nitriding under conditions initially allowing the absence of formation of the combination layer and then saturation of the surface layer with nitrogen atoms.
• the principle of the technical solution therefore consists here in applying a nitriding treatment in two stages: the first stage is a nitriding carried out under conditions which do not lead to the formation of a combination layer, so as to promote surface decarburization. of the room.
• the first step is carried out at low K N to reach the desired degree of decarburization.
• this nitriding step can be carried out at temperatures between 480 and 580 ° C, the nitriding potential applied being between 0.2 and 0.6.
• the second step is nitriding carried out under conditions in which the combination layer is formed, that is to say at high KN, and therefore to obtain saturation of the surface of the steel treated with nitrogen atoms.
• the second step is carried out to achieve the desired degree of nitriding.
• this nitriding step can be carried out at temperatures between 480 and 580 ° C, the nitriding potential applied being between 2 and 4.
• the nitriding potential applied being between 2 and 4.
• Such a modification of the range of the nitriding potential KN applied during the first step induces the following effects: the absence of formation of a combination layer allows decarburization of the surface layer of the steel part subjected to the treatment. the reduction in the quantity of cementite edges on the surface due to a lower content of carbides in the surface layer; the reduction in precipitation takes place without appreciable degradation of the hardness of the surface layer. the reduction in the amount of edging in the surface layer allows a healthier microstructure, with increased flexural fatigue and contact fatigue performance.
• test pieces A, B, C and D thus obtained were then subjected to different nitriding treatments, in particular (A) according to the conventional nitriding process carried out at a nitriding potential well above the limit potential K NIP ⁇ for the formation of the combination layer, (B) according to a nitriding process carried out at a nitriding potential slightly higher than this limit potential K NIP ⁇ , (C) according to a nitriding process carried out at a nitriding potential lower than this limit potential K NIP ⁇ and ( D) according to a nitriding process in accordance with the process presented above of double nitriding.
• These different treatments were carried out in a nitriding furnace at a temperature of 550 ° C. and are detailed in Table 1.
• test piece D was treated in accordance with the present process, ie a two-step nitriding treatment.
• the test piece was subjected to a first nitriding with a KN nitriding potential of 0.2 for 60 hours, then to a second nitriding with a KN nitriding potential of 3 for 60 hours.
• the nitriding treatments were carried out in a horizontal gas nitriding furnace with forced convection.
• the control of the nitriding atmosphere (ammonia + dissociated ammonia) is ensured via the regulation of the nitriding potential KN: the flow rates of ammonia and dissociated ammonia are adjusted according to the calculation of KN, based on a partial pressure measurement of dihydrogen.
• the two stages of the nitriding treatment applied to the test piece D were carried out directly one after the other in the same nitriding furnace, the transition between the first and the second stage being made by changing the value. setpoint of the nitriding potential regulation system. Cooling is provided at the end of nitriding by nitrogen convection circulating in the furnace and cooled on an internal exchanger.
• FIG. 1 shows microscopy images obtained with a x100 magnification of the surface of samples A, B, C and D. The Vickers imprints visible on these images show the depths of 100, 300 and 500 ⁇ m. Samples C and D show a sharp decrease in the number of intergranular cementite edges over the first 200 micrometers of thickness, unlike what is visible for samples A and B.
• the combination layer is removed from the treated parts by machining (grinding).
• the process according to the invention therefore makes it possible to achieve surface hardnesses similar to those obtained by the conventional process, while drastically reducing the precipitation of intergranular cementite.

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• Chemical & Material Sciences (AREA)
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• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)

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• 2019
  • 2019-12-06 FR FR1913896A patent/FR3104176B1/fr active Active
• 2020
  • 2020-12-04 EP EP20816502.7A patent/EP4069879A1/de active Pending
  • 2020-12-04 WO PCT/EP2020/084695 patent/WO2021110945A1/fr not_active Ceased

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