Method for nitriding steel parts

Abstract

The invention relates to a method for nitriding a steel part comprising the following steps: - S1: placing the part in an enclosure; - S2: placing the enclosure under an oxidizing atmosphere at a first temperature ranging from 350°C to 470°C so as to form a layer of iron oxide; - S3: placing the enclosure under a nitrogen / ammonia / dissociated ammonia atmosphere at a second temperature greater than or equal to 480°C and less than or equal to 500°C so as to form a layer of iron nitride on the surface of the part and to initiate the formation of a diffusion layer under the layer of iron nitride and maintain a nitriding potential at a first value; and - S4: increasing the temperature of the enclosure to a third temperature ranging from 490°C to 590°C and maintaining the nitriding potential at a second value so as to form a final diffusion layer.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to the field of thermochemical treatments for the surface hardening of steel parts. The invention relates more particularly to a nitriding process for the surface hardening of parts, in particular steel parts. TECHNOLOGICAL BACKGROUND

[0002] The parts used in mechanical power transmission systems are subjected to significant stresses, particularly at the level of their surfaces which are in contact with other parts.

[0003] In the field of aeronautics, for example, the power transmission systems can be found in the engine part, in particular at the level of the engine power chain, or in the transmission systems from the engine to the lift components, in particular to the main rotor or to the tail rotor for vertical flight aircraft, or to the propulsion system, or to the lift for winged aircraft.

[0004] Generally speaking, in tribological systems, such as bearings, drive gears, or power transmissions, the various parts forming these systems must have good mechanical resistance, fatigue resistance and friction resistance characteristics, particularly at their surface level.

[0005] Furthermore, in the case of rotary wing aircraft, safety requirements require that the mechanical properties of power transmission components be maintained, particularly for an emergency period in the event of a breakdown in the lubrication function. In this type of aircraft, the mechanical properties must therefore be maintained in nominal or degraded operation and for temperature ranges of up to 450°C or 500°C.

[0006] Various heat or thermochemical treatment processes for strengthening the surface of a part, particularly a steel part, are known from the prior art. Examples include surface strengthening by quenching, carburizing, cyanidation, nitrocarburizing, carbonitriding or even nitriding.

[0007] For decades, the nitriding surface strengthening process has been used for aircraft transmission system parts. However, although this process produces steel parts with a strengthened surface, it requires optimization.

[0008] Indeed, the nitriding process allows the formation of a hardened layer in a thickness of the treated part, at the surface of the latter. This hardened layer comprises a diffusion layer which is a layer enriched in nitrogen and which has a hardness greater than the hardness of the rest of the part, in particular of a more internal part of the part. In this diffusion layer, nitrogen diffused into the part during the nitriding process reacts with compounds present in the steel forming the part, in particular with metals, which makes it possible to increase the hardness of this diffusion layer compared to the internal part of the part. However, during the process, a so-called "combination" layer consisting of iron nitrides, generally Fe 2.3 N and / or Fe 4 N, inevitably forms on at least part of the surface of the treated part.The part then comprises successively from the inside of the part to the outside of the part: the internal part having a given hardness, the diffusion layer then the combination layer, the diffusion layer and the combination layer together forming the hardened layer having a hardness greater than the hardness of the internal part.

[0009] When implementing the nitriding processes of the prior art, the thickness of the combination layer is difficult to control, which results in a combination layer that is too thick and includes protrusions, which can thus prevent optimal use of the part comprising this layer. It is thus necessary to rectify the combination layer so as to remove the combination layer and the protrusions over the entire treated part or locally in certain areas depending on the intended applications in order to obtain a functional part.

[0010] However, machining the part to grind the combination layer is a step that lengthens the manufacturing time of the part and also increases its manufacturing cost. STATEMENT OF THE INVENTION

[0011] One objective of the invention is thus to optimize the nitriding process of a metal part so as to limit or even avoid the need to machine the part to rectify the combination layer.

[0012] Another objective of the invention is to control the growth of the combination layer during the nitriding process.

[0013] The invention also aims to reduce the manufacturing cost of parts, particularly made of steel, comprising a combination layer based on iron nitrides.

[0014] To this end, the invention relates to a method for nitriding a steel part, said method comprising the following steps: S1: placing the part in an enclosure; S2: placing the enclosure under an oxidizing atmosphere at a first temperature ranging from 350°C to 470°C so as to form a layer of iron oxide on the surface of the part; S3: placing the enclosure under an atmosphere of a nitrogen (N 2 ) / ammonia (NH 3 ) mixture at a second temperature greater than or equal to 480°C and less than or equal to 500°C so as to form a layer of iron nitride on the surface of the part and to initiate the formation of a diffusion layer under the iron nitride layer and maintain a nitriding potential at a first value; and S4: increasing the temperature of the enclosure to a third temperature ranging from 490°C to 590°C and maintaining the nitriding potential at a second value lower than the first value so as to form a final diffusion layer.

[0015] The resulting part has a hardened layer comprising a diffusion layer covered with a combination layer (iron nitride layer), which has an optimized thickness and little or no protrusion. The part can therefore be used without requiring a lengthy grinding step. The nitriding process is therefore faster and the manufacturing of the part more efficient.

[0016] When it is stated that the diffusion layer is positioned below the combination layer, it means that the diffusion layer is in a more internal thickness of the part compared to the combination layer which is the outermost layer of the part.

[0017] The thickness of the final hardened layer (combination layer + diffusion) can range from a few hundredths of a millimeter to a few millimeters.

[0018] The expression "no or little growth" means a layer of iron nitride having a regular thickness, and in particular a thickness ranging from 2 to 12µm which varies by at most 20%, and preferably by at most 10%.

[0019] Furthermore, the method described above can allow a deeper diffusion layer (and therefore a thicker cured layer) to be formed without increasing the thickness of the combination layer too much. This method thus makes it possible to find a compromise between a sufficiently thick cured layer and a combination layer of limited thickness. Step S1

[0020] In step S1, the part is positioned in the enclosure which is preferably under ambient air. This step is carried out at a temperature Ti. Step S2

[0021] In step S2, the enclosure is placed under an oxidizing atmosphere, which means that the enclosure is filled with an oxidizing gas mixture, in particular a gas mixture comprising oxygen.

[0022] The gas mixture may for example be air, which means that the air represents at least 80% by volume, preferably at least 90% by volume, and even more preferably at least 95% by volume, relative to the total volume of gas present in the enclosure.

[0023] This step consists of oxidizing the iron on the surface of the part placed in the enclosure which is then under an atmosphere of an oxidizing gas mixture, which means a gas mixture comprising at least 15% oxygen by volume compared to the total volume of gas present in the enclosure.

[0024] The method may comprise a step Sa carried out between step S1 and step S2 and comprising the pumping of ambient air present in the enclosure then the injection of nitrogen. The enclosure under a nitrogen atmosphere may then be heated to the first temperature T1 then the nitrogen present in the enclosure is replaced by air.

[0025] The first temperature T1 has a value ranging from 350°C to 470°C, preferably from 400°C to 470°C, preferably from 430°C to 460°C, and even more preferably from 445°C to 455°C. The first temperature T1 may for example be 450°C.

[0026] The temperature rise to reach the first temperature T1 can be done at a rate ranging from 1°C / min to 4°C / min, preferably ranging from 1.5°C / min to 3.5°C / min, and even more preferably ranging from 2°C / min to 3°C / min. The temperature rise can be done from an initial temperature Ti which can be for example the ambient temperature up to the first temperature T1.

[0027] The pressure of the oxidizing mixture, for example air, in the enclosure can range from 800 to 1200 Pa, and preferably from 900 to 1100 Pa.

[0028] The iron oxide layer is formed on the surface of the part according to the following reaction: Reaction 1: Fe + ½ O 2 -> FeO

[0029] The duration of step S2 is determined based on the iron oxide layer forming on the surface of the part. The duration of this step can range, for example, from 15 minutes to 90 minutes.

[0030] At the end of step S2, the thickness of the iron oxide layer formed may range from 0.1 µm to 10 µm, preferably from 0.3 µm to 7 µm, and more preferably 0.5 µm to 5 µm. Step S3

[0031] In this step S3, a mixture of nitrogen (N 2 ), ammonia (NH 3 ) and dissociated ammonia is injected, which means that this mixture represents at least 80% by volume, preferably at least 90% by volume, and even more preferably at least 95% by volume, relative to the total volume of gas present in the enclosure.

[0032] Dissociated ammonia is generated from an external dissociator. Injecting dissociated ammonia from the start of step S3 allows, in particular, better control of the partial pressure of dihydrogen and therefore the nitriding potential from the start of step S3.

[0033] In this nitrogen / ammonia / dissociated ammonia mixture, the ammonia represents at least 4% by volume of ammonia, preferably at least 6% by volume, and more preferably at least 8% by volume, relative to the total volume of the mixture.

[0034] Preferably, a purging step with an inert gas, for example nitrogen, can be carried out between step S2 and step S3 so as to eliminate the oxidizing gas(es) present in the enclosure.

[0035] Furthermore, the enclosure is heated to a second temperature T2 ranging from 470°C to 525°C, preferably ranging from 480°C to 500°C, and more preferably ranging from 485°C to 495°C. The second temperature T2 may for example be 490°C.

[0036] Step S3 thus comprises a first phase P1 of increasing the temperature up to temperature T2, then a second phase P2 of maintaining temperature T2.

[0037] The temperature rise to reach the second temperature T2 can be done at a rate ranging from 1°C / min to 4°C / min, preferably from 1.5°C / min to 3.5°C / min, and even more preferably from 2°C / min to 3°C / min. The temperature rise can be done in particular from the temperature T1 to the second temperature T2.

[0038] The pressure of the nitrogen / ammonia / dissociated ammonia mixture in the enclosure can range from 1000 Pa to 1300 Pa, and preferably from 1050 Pa to 1150 Pa.

[0039] In step S3, a first reaction takes place which consists of decomposing or dissociating the ammonia by its reaction with the iron oxide formed on the surface of the part during step S2. This ammonia dissociation reaction called "cracking", also known by the Anglicism "cracking", is as follows: Reaction 2: 2FeO + 2NH 3 -> 2H 2 O + 2N + 2Fe + H 2

[0040] During this reaction, ammonia is adsorbed on the surface of the part, and in particular at the level of the iron oxide layer, which allows its dissociation. Mononuclear nitrogen N is then formed and absorbed by the part, in particular absorbed in the thickness of a sub-layer of the part, in particular a sub-layer forming in place of the iron oxide layer, this sub-layer being intended to form the diffusion layer by reaction of nitrogen with the steel. The thickness of the sub-layer of the part absorbing mononuclear nitrogen can have a thickness of up to 15µm.

[0041] Furthermore, during this stage, the gas mixture evolves since a dissociation of the ammonia occurs which allows the presence of dissociated ammonia to be maintained in the gas mixture, i.e. a mixture of dihydrogen and mononuclear nitrogen.

[0042] The hardened layer thus begins to form and a combination layer comprising iron nitrides is also formed on the surface of the hardened layer according to the following reactions: Reaction 3: N + 3Fe -> Fe 2.3 N Reaction 4: N + 4Fe -> Fe 4 N

[0043] Iron nitrides are formed on the surface of the part, forming a layer called a compound layer. This layer also forms a reservoir for nitrogen diffusion into the substrate. The nitrogen diffuses into the substrate, forming a diffusion layer by reacting the nitrogen with certain compounds present in the steel.

[0044] During this step S3, the ammonia dissociation reaction allows the formation of dihydrogen in the enclosure. The proportion of dihydrogen can thus vary during step S3, and up to 70% at the end of the second step. The proportion of dihydrogen, and in particular its partial pressure in the nitrogen / ammonia / dissociated ammonia mixture, makes it possible to calculate the nitriding potential of the mixture, which is defined according to the equation below: KN = P NH 3 / P H 2 3 / 2 , in which P(NH 3 ) is the partial pressure of ammonia in the enclosure, and P(H 2 ) is the partial pressure of dihydrogen in the enclosure. The values ​​of the partial pressures are calculated by an automaton from the results of analysis of atmospheric samples from the enclosure and the flow measurement data of the injected gases.

[0045] In step S3, the nitriding potential varies from infinity to the regulation value KN1.

[0046] The second phase of this step S3 consisting of maintaining the second temperature T2 and the nitriding potential KN1 at constant values, thus makes it possible to form a reservoir of nitrogen atoms in the sub-layer of the part and / or in the combination layer, allowing in particular the formation of the diffusion layer and also the formation of the combination layer.

[0047] The nitriding potential of the atmosphere has a first value KN1 ranging from 20 to 1. Such a value allows the nitriding potential to be high enough for iron nitrides to form, and not too high to prevent too much nitrogen from being adsorbed on the surface of the part and too thick a combination layer from forming.

[0048] The duration of step S3 is determined based on the desired combination layer thickness. The duration of this step can range, for example, from 15 minutes to 90 minutes.

[0049] At the end of step S3, the thickness of the iron nitride layer or combination layer may range from 1 µm to 20 µm, preferably from 2 µm to 18 µm, and more preferably from 5 µm to 15 µm. Step S4

[0050] In this step S4, the temperature in the enclosure is increased from the second temperature T2 to the third temperature T3 higher than the temperature T2.

[0051] During this step, the temperature T3 can range from 470°C to 590°C, preferably from 520°C to 580°C, and more preferably from 550°C to 570°C. The temperature T3 can in particular be 560°C.

[0052] Step S3 includes a first phase during which the temperature gradually increases to temperature T3, then a second phase of maintaining temperature T3 at a constant value.

[0053] This increase in temperature makes it possible in particular to accelerate the reaction of formation of iron nitrides by using the nitrogen previously adsorbed in the part.

[0054] The temperature rise to reach the third temperature T3 can be done at a rate ranging from 1°C / min to 4°C / min, preferably ranging from 1.5°C / min to 3.5°C / min, and even more preferably ranging from 2°C / min to 3°C / min. The temperature rise can be done in particular from the second temperature T2 to the third temperature T3.

[0055] During step S4, the nitriding potential has a second value KN2 ranging from 0.05 to 3. Such a value, lower than the first value of the nitriding potential, makes it possible to continue the formation of the diffusion layer by diffusion of nitrogen from the combination layer. This step S4 carried out at a temperature higher than the temperature of step S3 and at a nitriding potential lower than that of step S3, makes it possible to maintain a sufficient nitrogen reserve in the combination layer, thus making it possible to form and thicken the diffusion layer, while avoiding the formation of a combination layer that is too thick and to limit, or even avoid, growths.

[0056] The duration of step S4 is determined based on the desired effective nitriding depth. The duration of this step can range, for example, from 0 to 500 hours.

[0057] At the end of step S4, the thickness of the cured layer can range from 0.05 mm to 2.3 mm.

[0058] Furthermore, the thickness of the combination layer can range from 0.005 mm to 0.015 mm.

[0059] Following step 4, the method may include a step of purging the enclosure to remove the gas mixture present in the enclosure and injecting nitrogen into it. The final temperature called Tf may then be lowered to a value ranging from 20°C to 100°C, preferably ranging from 45°C to 80°C, and for example 70°C.

[0060] The part can then be removed from the enclosure and used with little or no machining. BRIEF DESCRIPTION OF THE FIGURES

[0061] Other characteristics and advantages of the invention will appear on reading the following exemplary embodiment, given solely by way of example and referring to the appended drawings, in which: [ Fig. 1 ] There Figure 1is a flowchart representing the steps of the method according to one embodiment of the invention; [ Fig. 2 ] There Figure 2 is a graph representing the temperature profile during the different stages of the process of the Figure 1 ; [ Fig. 3a ] There Figure 3a is a cross-sectional image of the part obtained using the nitriding process of the Figure 1 showing the hardened layer comprising the combination layer; and [ Fig. 3b ] There Figure 3b is a cross-sectional image of a part obtained according to the nitriding process of the prior art showing the hardened layer comprising the combination layer. DETAILED DESCRIPTION OF AN EXAMPLE OF IMPLEMENTATION

[0062] During step S1, a part is positioned in an enclosure which is under ambient air atmosphere and then the enclosure is closed.

[0063] During a Sa step, the ambient air present in the enclosure is pumped then nitrogen is injected.

[0064] The enclosure is then heated for a few minutes to reach an initial temperature T1 of 450°C.

[0065] In reference to the figures 1 and 2b and during a step S2, the enclosure 10 is maintained at the temperature T1 of 450°C and is purged by pumping nitrogen. Then the enclosure is again filled with ambient air.

[0066] At the end of step S2, the surface of the part which is exposed to the atmosphere of the enclosure is covered with a layer of iron oxide (FeO).

[0067] During a step Sb, the air present in the enclosure is pumped and then nitrogen is injected. During this step Sb, the temperature of the enclosure is maintained at the temperature T1 of 450°C

[0068] During step S3, a dissociated N2 / NH3 / NH3 mixture is injected into the enclosure,

[0069] Furthermore, the temperature of the enclosure is increased to reach a second temperature T3 of 490°C.

[0070] During this step S3, the previously described reaction 2 allows the formation of monomolecular nitrogen and dihydrogen. The gaseous thermodynamic equilibrium allows the atmosphere of the enclosure to reach a first nitriding potential KN1 of 18 for a period of 2 hours.

[0071] The first nitriding potential KN1 is maintained at this value and allows the formation of a hardened layer on the subsurface of the part and a layer of iron nitrides (or combination layer) on the surface of the part to be initiated.

[0072] The nitriding potential is measured using pressure probes, for example those from the company NITREX.

[0073] In a step S4, the temperature of the enclosure is increased to reach a third temperature T3 of 560°C.

[0074] During this step, the nitriding potential is lowered to a second KN2 value of 0.55 while maintaining the temperature T3 at the value of 560°C for a period of 60 hours.

[0075] The enclosure is then purged to evacuate the gas mixture present and inject nitrogen. The temperature is then lowered to a value of 70°C. Once the part has cooled to a temperature below 70°C, it is removed from the enclosure.

[0076] The total process time is 67 hours and the part has a 630 µm thick hardened layer on the subsurface of the part and a 10 µm thick combination layer. The hardened layer is then lightly ground to a thickness of 600 µm.

[0077] According to the prior art method carried out under similar conditions but with a constant nitriding potential and therefore without carrying out step S4, the method lasts 127h and the hardened layer has a thickness of 800 µm and has a combination layer with a thickness of approximately 30 to 45 µm requiring removal by grinding greater than 100 µm. The grinding is thus longer to reach a hardened layer with a thickness of 600 µm.

[0078] As visible on the Figures 3a and 3b , the combination layer is thinner and has fewer protrusions when following the method according to one embodiment of the invention compared to a method carried out according to the prior art.

[0079] The method according to this embodiment of the invention makes it possible to obtain a part having a subsurface hardened layer and a combination layer having an optimized thickness and having little protrusion. The fine control of the nitriding potential and the selection of suitable nitriding temperatures make it possible to form deep hardened layers while minimizing the thickness of the combination layer. The part can thus be used after a rapid grinding step, or even without grinding depending on the intended applications. Furthermore, the duration of the process is much shorter compared to the method according to the prior art. The nitriding process is thus faster and the manufacture of the part more efficient.

Claims

1. A method of nitriding a steel part, said method comprising the following steps: - S1: placing the part in an enclosure; - S2: placing the enclosure under an oxidizing atmosphere at a first temperature ranging from 350°C to 470°C so as to form a layer of iron oxide on the surface of the part; - S3: placing the enclosure under an atmosphere of a nitrogen / ammonia / dissociated ammonia mixture at a second temperature greater than or equal to 480°C and less than or equal to 500°C so as to form a layer of iron nitride on the surface of the part and to initiate the formation of a diffusion layer under the iron nitride layer and maintain a nitriding potential at a first value; and - S4: increasing the temperature of the enclosure to a third temperature ranging from 490°C to 590°C and maintaining the nitriding potential at a second value lower than the first value so as to form a final diffusion layer.

2. Nitriding method according to claim 1, wherein the first value of the nitriding potential is greater than or equal to 1 and less than or equal to 20.

3. Nitriding method according to claim 1 or 2, wherein the second value of the nitriding potential is greater than or equal to 0.05 and less than or equal to 3.

4. Nitriding process according to any one of the preceding claims, wherein, in step S3, the nitrogen / ammonia / dissociated ammonia mixture comprises at least 4% ammonia by volume relative to the total volume of the mixture.

5. Method according to any one of the preceding claims, in which the iron nitrides forming the iron nitride layer are chosen from Fe 2,3 N, Fe4N, and their mixtures.

6. Method according to any one of the preceding claims, further comprising, prior to step S2, a step Sa of placing the enclosure under a nitrogen atmosphere and heating the enclosure to the first temperature.

7. A method according to any preceding claim, wherein the first temperature ranges from 400°C to 470°C.

8. A method according to any preceding claim, wherein the third temperature is less than or equal to 520°C and greater than or equal to 580°C.

9. Method according to any one of the preceding claims, in which, during at least one of steps S3 and S4, dihydrogen is formed from ammonia, a measurement of the level of dihydrogen in the enclosure is used for the calculation of the nitriding potential.

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