EP2683845A1 - Molten-salt bath for nitriding mechanical steel parts, and implementation method - Google Patents

Abstract

The invention relates to a molten-salt bath for nitriding mechanical steel parts, essentially consisting of the following (the contents being expressed in wt %): 25 to 60 wt % of alkali-metal chlorides; 10 to 40 wt % of alkali-metal carbonates; 20 to 50 wt % of alkali-metal cyanates; and a maximum of 3 wt % of cyanide ions (formed during the use of the bath), wherein the total of the contents is 100 wt %. Preferably, the bath contains: 25 to 30 wt % of sodium cyanate; 25 to 30 wt % of sodium carbonate and lithium carbonate; 40 to 50 wt % of potassium chlorides; and a maximum of 3 wt % of cyanide ions (formed during the use of the bath), the total of the contents being 100 wt %.

Description

 FOUNDED SALT BATH FOR NITRURATION OF STEEL MECHANICAL PARTS, AND METHOD FOR IMPLEMENTING THE SAME

The invention relates to the nitriding of mechanical parts made of steel.

By mechanical parts are meant parts intended to ensure, in use, a mechanical function, which generally implies that these parts have a high hardness, good resistance to corrosion and wear; we can cite, in a non-exhaustive way:

 • wiper shafts,

 • hydraulic or gas cylinder rods,

 • combustion engine valves,

 • hinge rings.

 The range of steels in which these parts are made, at least close to their surface susceptible to friction or corrosion, is wide, ranging from unalloyed steels to so-called stainless alloys, especially chromium or nickel alloys. .

 To surface harden such parts, it is known to apply a nitriding treatment (sometimes accompanied by a carburation, in which case it is often referred to as nitrocarburizing). In fact, the concept of nitriding includes both nitriding alone, in a bath with a very low cyanide content (typically less than 0.5%), as well as nitrocarburization for cyanide contents above this threshold. In the following we group these two types of treatment under the term nitriding.

 This nitriding can be done from a gas phase or a plasma phase or from a liquid phase.

Nitriding in the liquid phase has the advantage of allowing a significant hardening to a thickness of several microns in hours of barely a few hours, but has the important disadvantage of involving the implementation of baths of molten salts, at temperatures of the order of 600 ° C (or more), containing in practice cyanides, in combination with cyanates and carbonates (the cations are in practice cations of alkali metals, such as lithium, sodium, potassium, etc ...) - In practice the cyanates are decomposed to form cyanides, carbonates and nitrogen, which is thus available to diffuse into the nitriding part. . Due to the consumption of cyanates and the enrichment of carbonates, it is necessary to provide a regeneration of baths by introduction of supplements to reduce their cyanide and cyanate contents in ranges ensuring efficiency. In the following, the contents of the baths are expressed in percentage by weight.

 However, as we know, the implementation of cyanides is dangerous for the operators as well as for the environment, so that we have been trying for decades to minimize the amount of cyanide to be nitriding processes of mechanical steel parts in molten salt baths.

 Thus, from the years 1974-75, it has been proposed to seek to minimize the cyanide content of the nitriding baths, in particular by avoiding toxic products at the time of regeneration, (FR - 2 220 593 and FR - 2 283 243 or US - 4,019,928, or GB - 1,507,904); in fact, these documents have mentioned, without particular comments, an alkaline chloride content of up to 30% (without however giving an example, for nitriding, including more than 5% by weight of NaCl in a bath containing in addition to 64% potassium cyanate, 16% potassium carbonate, 11% sodium cyanate and 4% sodium cyanide). It was considered that low cyanide baths should consist essentially of potassium or sodium cyanates, potassium and sodium carbonates, with more potassium than sodium (which lowered the temperature of the salt baths). ); the objective was to reduce the cyanide content to no more than 5% or even 3%); the decrease in cyanide content was to be offset by cyanates; there was no particular explanation for the role of chlorides apart from the fact that, in carburizing baths, barium chloride is a melting flux.

Previously (see GB-891 578 published in 1962), it had been mentioned that nitriding-carburizing baths could contain alkaline chlorides, saving cyanides and cyanates, which are much more expensive, or lowering the melting temperature; this document concerned salt baths containing from 30% to 60% of cyanides and taught to maximize the content of n-cyanates relative to isocyanates (there were no chlorides in the example described).

 It has also been mentioned (see GB-854 349 published in 1960), carburizing baths (used at temperatures of 800 ° C. to 950 ° C.) containing, by weight, from 35% to 82% of carbonate of carbonates. alkali metals, from 15% to 35% of alkali metal cyanides, from 3% to 15% of alkali metal anhydrous silicates and up to 15% of alkaline chlorides; it was indicated that it is preferable that alkaline chlorides be present, preferably up to 10%, without giving any explanation (it seems however that the presence of chlorides has contributed to the preparation of cyanides in a usable form ). On the other hand, it has been mentioned (see GB-1 052 668 published in 1966) carbonation baths, in crucibles having a composition in a well-chosen range, containing from 10 to 30% of alkali metal cyanates and at least 10% alkali metal cyanides at 600 ° C-750 ° C; a 25% content of alkali metal chloride was mentioned in connection with a starting bath (containing only cyanides (25%) and carbonates), as well as in the regeneration compound (which otherwise contained 75% cyanide). It has also been proposed (GB-1,185,640) to complete a carburation step by a short dipping step in a bath containing cyanides, cyanates, carbonates and alkali metal chlorides (without specifying ranges of levels for these latter).

For the nitriding of stainless steels, it has been proposed (US Pat. No. 4,184,899 published in 1980) a nitriding treatment in the gas phase, preceded by a thermochemical pretreatment step in a bath containing from 4% to 30% of cyanides and from 10% to 30% cyanates in combination with from 0.1% to 0.5% sulfur. It is mentioned that the rest of the pretreatment baths can be formed of carbonate or sodium chloride, without these elements being active in the treatment (about a bath to 12% cyanide and 0.3% sulfur, it is mentioned that there is initially 25% of sodium carbonate and 42.7% of sodium chloride).

 More recently, it has been proposed (see in particular US Pat. No. 4,492,604 published in 1985) a nitriding bath whose cyanide content is between 0.01% and 3%. It is indicated that, due to the strong reducing action of cyanides in nitriding baths at 550 ° C-650 ° C, while cyanates tend to release oxygen, the low cyanide nitriding baths tend to oxidize the nitriding layers and reveal unacceptable coatings. To avoid the formation of such defects, it is taught to include up to 100 ppm of selenium, in combination with a suitable composition of the crucibles (without iron).

 It has also been proposed to harden ferrous parts using a bath containing high levels of chlorides (see document EP-0 919 642 published in 1999), but this bath is actually used to complete a nitriding action, being intended for allow introduction of chromium (present in this bath in addition to the chlorides, with silica) in a previously formed nitriding layer.

 For nitriding ferrous parts, US Pat. No. 6,746,546 (published in 2004) has proposed a bath of molten salts containing alkali metal cyanates and alkali metal carbonates, with 45% to 53% cyanate ions (preferably 48% to 50%) maintained at 750 ° F to 950 ° F, i.e. 400 ° C to 510 ° C, to impart good corrosion resistance. The alkali metals were preferably sodium and / or potassium (when both were present, the potassium content was preferably 3.9: 1 relative to the sodium content); in use, this bath contained 1% to 4% cyanide (no details were given as to the presence of any other elements in the bath).

Even more recently, in order to minimize the entrainment of molten salts at the outlet of nitrided iron parts, US Pat. No. 7,217,327 has proposed a nitriding bath consisting essentially of Li, Na and K type cations and anions. carbonates and cyanates. It thus appears that various compositions of molten salt baths have been proposed in order to allow nitriding of ferrous parts without implementing significant levels of cyanide.

 However, as a general rule, nitriding treatments with a low cyanide content (typically less than 3%) should be followed by a finishing treatment as long as a low roughness is sought, which contributes to increasing the cost treatment (labor, polishing equipment) as well as the overall duration of treatment.

 A low roughness can be obtained with nitriding baths with a high cyanide content (more than 5%), but after periods of several hours (typically 4 to 6 hours), which may seem too long on an industrial scale.

 The subject of the invention is a nitriding bath with a low cyanide content capable of, at most of the order of a few hours, of nitriding mechanical parts made of iron or steel while giving them a very low roughness (ie without porosity significant), rendering unnecessary a subsequent mechanical recovery (polishing or tribofinishing), all for a moderate cost.

 The invention proposes for this purpose an essentially constituted nitriding bath (the contents are expressed by weight):

 from 25% to 60% of alkali metal chlorides,

 from 10% to 40% of alkali metal carbonates, and

 from 20% to 50% of alkaline metal cyanates,

 - a maximum of 3% of cyanide ions (formed in service), the total contents being 100%.

 It should be noted that the composition ranges are generally given for a new bath, but that one seeks in practice to stay as far as possible in these ranges; thus, there is in practice no cyanide ion in the starting bath, and it is in service that one seeks to remain at not more than 3% of cyanide ions.

The presence according to the invention of chlorine compounds in a significant amount (NaCl, KCl, LiCl, etc.) makes it possible to obtain, during nitriding, non-porous, non-powdery and therefore not very rough layers, after treatment times of barely of the order of one to two hours; the chlorides being less expensive than the other usual components of nitriding baths, a bath according to the invention is therefore more economical than a standard bath, while avoiding the need for a subsequent polishing treatment. It can be recalled that processing times of at most about two hours (2h +/- 5mn) are considered as times compatible with satisfactory yields on an industrial scale.

 It may be noted that, in baths used in the past, it had already been proposed to combine cyanates and carbonates with chlorides in nitriding baths, including when they are substantially free of cyanides, but chlorides (of which no role was recognized in nitriding) did not appear in practice at levels greater than 10-15% in the absence of cyanides (or with low levels of cyanide ions, typically less than or equal to 3%). In addition, no document suggested any correlation between the presence of chlorides and the final roughness.

 Advantageously, the alkali metal chlorides are lithium, sodium and / or potassium chlorides, which corresponds to chlorides which have been found to be effective, while having a moderate cost, and which do not require heavy stresses. handling point of view.

 Advantageously, the chloride content is between 40% and 50%, preferably at least approximately 45% (+ 1-2%, even +/- 1%). This range of contents has been found to lead, in a reasonable time, to good nitriding and low roughness.

 We understand that:

 - the cyanate content must be sufficient to allow a nitriding effect,

- the carbonate content must not become too high, as this may prevent the chemical reactions that lead to nitriding. Thus, also advantageously, the cyanate content is between 20% and 40%, or even between 20% and 35%, preferably between 20% and 30%. Even more advantageously, this content is between 25% and 40%, or even between 25% and 35%, preferably between 25% and 30%. These cyanates may in particular be sodium cyanates (or potassium cyanates).

 Also advantageously, the content of alkali metal carbonates is from 20% to 30%, preferably from 25% to 30%. These carbonates may in particular be sodium, potassium and / or lithium carbonates; it is advantageously a mixture of sodium carbonate and lithium.

 Thus, particularly advantageously, the bath of molten salts consists essentially of (+/- 2%, or even +/- 1%):

 25% to 30% of sodium cyanate,

 25% to 30% of sodium and lithium carbonates,

 40% to 50% of potassium chlorides,

 a maximum of 3% of cyanide ions (formed in service), the sum of these contents being 100%.

 Preferably, the bath of molten salts essentially consists, before formation of cyanides up to a maximum of 3, of (at +/- 2%, even +/- 1%):

 28% sodium cyanate,

 - 22% sodium carbonate,

 - 5% of lithium carbonate,

 - 45% potassium chloride,

which has proved to be a very good compromise between kinetics of nitriding, price of the constitutive mixture of the bath, surface roughness variations of the treated parts, melting point, risk of entrainment of salts on the surface of the treated parts. Of course, in use this composition may vary slightly, taking into account the reactions that take place (with in particular the formation of cyanide ions whose content is maintained at most 3%) The invention also proposes a process for nitriding mechanical parts made of iron or steel, according to which these parts are immersed in a bath of the aforementioned composition at a temperature of between 530 ° C. and 650 ° C. for at most 4 hours.

 Preferably, the parts are immersed in the bath at a temperature of between 570 ° C. and 590 ° C. for at most 2 hours.

 In practice, the duration of a nitriding treatment is conventionally of the order of 90 minutes, but it is understood that the duration of treatment depends on the nature and / or the destination of the parts; this is how one can go from some 30 minutes for valves or tool steels, up to 4 hours when one seeks to nitride on important thicknesses (layers of several tens of micrometers of thickness), or in the case of alloy steels. However, the invention is advantageously implemented with processing times of the order of 60 to 120 minutes.

 The invention also relates to mechanical parts of iron or steel nitrided according to the aforementioned method, recognizable in particular by the absence of traces of subsequent mechanical finishing process such as polishing (including the absence of fine polishing scratches).

 In the following, the compositions tested are compared with standard baths (which are the same for the various examples) which do not conform to the invention.

Example 1 (in accordance with the invention)

 Samples of annealed C45 type steel, usable for wiper shafts, hydraulic or gas cylinder rods, or hinge rings, were treated as follows.

 These samples were degreased in an alkaline solution, rinsed with water and then preheated to 350 ° C.

 They were then immersed for 60 minutes in a bath of molten salts maintained at 580 ° C. and containing:

 28% sodium cyanate,

- 22% sodium carbonate, and - 45% potassium chlorides

 - 5% lithium carbonate.

 The samples thus nitrided were then rinsed with water.

Identical samples were subjected to the same treatment, except that the nitriding treatment of 60 min at 580 ° C was done in a standard nitriding bath (not in accordance with the invention) consisting essentially of:

 58% sodium cyanate,

 - 36% potassium carbonate, and

 - 6% lithium carbonate

 In both cases, the iron nitride layer thus formed had a thickness of 10 +/- 1 μm.

 It was found that, the roughness of the samples having been initially of Ra = 0.2 micrometers, it became Ra = 0.52 micrometers after the treatment in a standard bath but Ra = 0.25 micrometers after the treatment in the bath according to the invention, c that is to say a roughness barely greater than the initial roughness.

 The composition according to the invention of this example appeared to be favorable to a good stability of the bath over time, in particular as regards the cyanide content.

 The samples thus nitrided were then oxidized in a bath of molten salts containing carbonates, hydroxides and nitrates of alkali metals. The purpose of this oxidation was to passivate the surface of the nitride layer forming an iron oxide layer of 1 to 3 μιτι thick. After oxidation, the parts were immersed in a corrosion protection oil (containing corrosion inhibitors) as is usual with nitriding processes.

The corrosion resistance (measured on 10 neutral salt spray parts according to ISO 9227) of the samples treated according to the invention was between 150 and 250 hours. The corrosion resistance (measured on 10 pieces of neutral salt spray according to ISO 9227) of the samples treated in the standard bath was between 10 and 290 hours.

 A nitriding of ferrous parts made according to the invention thus makes it possible to obtain corrosion resistance comparable to that obtained with standard bath nitriding, while improving the roughness of the surfaces, compared with a treatment in such a standard bath. .

Example 2 (not in accordance with the invention)

 Annealed C45 steel samples, prepared as above, were nitrided for 1 hour at 590 ° C in a bath containing:

 - 20% of alkali metal chlorides (NaCl, KCl)

 - 40% sodium cyanate

 - 30% potassium carbonate

 - 10% lithium carbonate

 In both cases, the layer formed has a thickness of 10 +/- 1 μιη It was found that, the roughness of the samples having been initially of Ra = 0.2 micrometers, it became Ra = 0.48 micrometers after the treatment in this bath against Ra = 0.52 microns after treatment in a standard bath.

 This leads to the conclusion that a too low chlorides content does not make it possible to lower the final roughness of the parts significantly compared to a standard bath (not in accordance with the invention).

Example 3 (not in accordance with the invention)

 He was prepared a bath containing

 - 65% sodium chloride

 - 25% potassium cyanate

 - 10% potassium carbonate.

Such a bath has proved not usable industrially since its melting temperature is greater than 600 ° C., which prevents any ferritic phase nitriding treatment (the majority of the parts are generally nitrided in the ferritic phase, ie at a temperature below 600 ° C). Only the austenitic phase nitriding is then possible, but only for temperatures above 630 ° C and with a high salt entrainment rate (high bath viscosity), which is economically unattractive.

Example 4 (in accordance with the invention)

 The treatment of annealed C45 samples, under conditions similar to those of Example 1, but in a bath containing

 - 35% sodium cyanate

 - 20% sodium carbonate

 - 20% potassium carbonate

 - 25% potassium chloride

allowed to obtain a final roughness of Ra = 0.28 μηι against Ra = 0.52 μιη in a standard bath (not according to the invention), on the surface of nitriding layers of 10 +/- 1 micrometer.

 Although satisfactory from the point of view of roughness, this composition appeared to have a higher viscosity than the composition of Example 1, which results in a greater consumption of salts.

 The degree of porosity of the nitride layers obtained according to the invention is less than 5%, whereas the degree of porosity of the nitride layers obtained with a standard bath is between 25 and 35%.

Example 5 (not in accordance with the invention)

 He was prepared a bath containing

 - 45% potassium chloride

 - 10% sodium cyanate

 - 45% sodium carbonate.

Such a bath has proved not usable for a nitriding treatment since its liquidus temperature is greater than 600 ° C. It is recalled that the temperature of the liquidus is the temperature from which the bath is fully melted and homogeneous composition (unlike the melting temperature which is the temperature from which the bath begins to be liquid, possibly in several phases.

 As explained in Example 3, such a bath can not be used industrially advantageously because it makes impossible any ferritic phase treatment and salt entrainment between 600 and 650 ° C are very important.

Example 6 (in accordance with the invention)

 Treatment of annealed C45 samples, under conditions similar to those of Example 1, but in a bath containing:

 - 45% potassium chloride

 - 30% sodium cyanate

 - 25% sodium carbonate

allows to obtain, as in example 1, a final roughness of Ra = 0.25 μιη (barely greater than the initial roughness Ra = 0.2 μ ^η τι, against Ra = 0.52 pm in a standard bath (non-compliant with the invention).

The layer of iron nitride formed in the bath following the invention is of the type ε (Fe ₂ -3n) and has a void ratio lower than 5% (measured by light microscopy) and has a hardness of 840 ± 40 HV ₀ , oi ■

 The iron nitride layer formed in the standard bath (not in accordance with the invention) is of the ε (Fe 2 -3N) type and has a porosity of between 25 and 35% (measured by optical microscopy) and has a hardness of 700 ± 40 HVo.oi- A lower apparent hardness of the layers obtained with a standard bath is explained by their higher porosity rate. Indeed, it is well known that the presence of porosity (ie holes) reduces the resistance of the layers to the penetration of the indenter used for the measurement of hardness.

 In both cases, the layer formed has a thickness of 10 +/- 1 μιτι

Example 7 (in accordance with the invention)

C45 samples machined by cold stamping and then subjected to a high frequency quenching with an initial roughness of Ra = 0.74 μ ^η ι were nitrides (after a preparation similar to that of Example 1) for two hours at 590 ° C. in a bath identical to that of Example 1, containing:

 - 28% sodium cyanate

 - 22% sodium carbonate

 - 45% potassium chloride

 - 5% lithium carbonate

 A layer of 20 +/- 1 μιτι was formed with a final roughness of Ra = 0.79 μιη. In comparison, identical samples which have been treated for the same duration, two hours, in a standard bath (not in accordance with the invention) have a final roughness layer of Ra = 1, 23 μιτι for a layer of 17 + / - 1 μηι thick.

 The degree of porosity of the nitride layers obtained according to the invention is between 5 and 10%, whereas the degree of porosity of the nitride layers obtained with a standard bath is between 55 and 65%. It is known that cold-impacted steels have a high degree of work hardening which has a detrimental effect on the porosity of the layers (the higher the degree of work hardening, the more porous the layers). The invention makes it possible to obtain layers with a low porosity rate, even for strongly hardened steels.

 The samples thus nitrided were then oxidized in a bath of molten salts containing carbonates, hydroxides and nitrates of alkali metals. The purpose of this oxidation is to passivate the surface of the nitride layer by forming an iron oxide layer 1 to 3 μητι thick. After oxidation, the parts are immersed in a corrosion protection oil (containing corrosion inhibitors) as is usual with nitro ration processes.

 The corrosion resistance (measured on 10 pieces in neutral salt spray according to ISO 9227) of the treated samples according to the invention is between 310 and 650 hours.

The corrosion resistance (measured on 10 pieces of neutral salt spray according to ISO 9227) of samples treated in a standard bath is between 240 and 650 hours. Example 8 (in accordance with the invention)

 Samples in 42CrMo4 quenched-incomes then rectified with an initial roughness of Ra = 0.34 μιη were nitrided (after a preparation similar to that of Example 1) like those of Example 7, that is to say say for two hours at 590 ° C. in a bath identical to that of Example 1, containing:

 - 28% sodium cyanate

 - 22% sodium carbonate

 - 45% potassium chloride

 - 5% lithium carbonate

 An iron nitride layer of 16 +/- 1 μm was formed with a final roughness of Ra = 0.44 μm. In comparison, identical samples that have been treated for two hours in a standard bath (not in accordance with the invention) have a layer of iron nitrides with a final roughness of Ra = 0.85 μm for a layer of 14 +/- - 1 pm thick.

The iron nitride layer formed in the bath according to the invention is of the ε (Fe2-3N) type and has a porosity of less than 5% (measured by optical microscopy) and has a hardness of 1020 ± 40 HV ₀ , IM-

The iron nitride layer formed in the standard bath is of the ε (Fe ₂ -3N) type and has a porosity of between 30 and 40% (measured by optical microscopy) and has a hardness of 830 ± 40 HV ₀ , The lower apparent hardness of the layers obtained with a standard bath is explained by their higher porosity rate. Indeed, it is well known that the presence of porosity (ie holes) reduces the resistance of the layers to the penetration of the indenter used for the measurement of hardness.

Example 9 (in accordance with the invention)

 Annealed C45 samples with an initial roughness of Ra = 0.20 μm were prepared and nitrided as in Example 1, ie for 1 hour at 580 ° C in a bath containing:

- 28% sodium cyanate - 22% sodium carbonate

 - 45% potassium chloride

 - 5% lithium carbonate

 A layer of 10 +/- 1 μηι was formed with a final roughness of Ra = 0.25 μηΊ. In comparison, identical samples that have been treated for three hours in a standard bath operating with a high cyanide level (5.2%) have a final roughness layer of Ra = 0.27 μιη for a layer of 7 +/- 1 μηη of thickness.

 It thus appears that at equivalent final roughness, although the treatment time is greater, the thickness of the layers obtained in a standard bath with a high cyanide content is lower than the thickness of the layers obtained in a bath following 'invention. This is explained by the fact that, in addition to being more polluting, a bath with a high cyanide content is also fuel, that is to say that carbon will diffuse together with nitrogen in the steel. However, carbon and nitrogen compete during diffusion because they occupy the same sites in the crystal lattice of iron. The presence of carbon will limit the diffusion of nitrogen, which will result in layers of smaller thickness.

As indicated above, the compositions indicated in the abovementioned examples define the new bath, it being specified that the indications of contents for the cyanide ions are valid in service, taking into account the reactions occurring during the nitriding (it is then sought to maintain the composition bath as stable as possible).

Claims

 1. Bath of molten salts for the nitriding of steel mechanical parts, essentially constituted (the contents are expressed by weight):  from 25% to 60% of alkali metal chlorides,  from 10% to 40% of alkali metal carbonates, and  from 20% to 50% of alkaline metal cyanates,  - a maximum of 3% of cyanide ions,  the total contents being 100%.  The molten salt bath according to claim 1, wherein the alkali metal chlorides are lithium, sodium and / or potassium chlorides.  3. Bath of molten salts according to claim 1 or claim 2, wherein the content of alkali metal chlorides is between 40% and 50%.  The molten salt bath of claim 3, wherein the alkali metal chloride content is at least approximately 45%.  A molten salt bath according to any one of claims 1 to 4, wherein the alkali metal cyanate content is between 20% and 40%.  The molten salt bath according to claim 5, wherein the alkali metal cyanate content is between 25% and 30%.  7. A bath of molten salts according to any one of claims 1 to 6, wherein the content of alkali metal carbonates is between 20% and 30%.  The molten salt bath according to claim 7, wherein the alkali metal carbonate content is between 25% and 30%.  9. Bath of molten salts according to claim 1 or claim 2, consisting essentially of:  25% to 30% of sodium cyanate,  25% to 30% of sodium and lithium carbonates,  40% to 50% of potassium chlorides, a maximum of 3% of cyanide ions, the sum of these contents being 100%.  10. bath of molten salts according to claim 9, essentially consisting, before cyanide ion formation up to a maximum of 3%, of:  28% sodium cyanate,  - 22% sodium carbonate,  - 5% lithium carbonate,  - 45% potassium chloride.  11. A method of nitriding mechanical parts of iron or steel, according to which these parts are immersed in a bath of the above composition, at a temperature between 530 ° C and 650 ° C for at most 4h.  12. The method of claim 11, wherein the parts are immersed in the bath at a temperature between 570 ° C and 590 ° C for at most 2h.  13. Nitrided steel mechanical part obtained by the method according to any one of claims 11 and 12, showing no trace of subsequent mechanical finishing process such as polishing.