EP2097561B1 - Sulphuration method of ferrous alloy parts in an aqueous solution - Google Patents

Abstract

The invention relates to a superficial treatment method by electrolysis of ferrous surfaces to enhance friction or tread and seizing resistance features, wherein said surfaces provide the electrolysis anode, the electrolysis bath comprises a sulphur species, primarily contains water and also contains a chloride salt and a nitrogen species in quantities which facilitates the sulphuration reaction of said surfaces.<0}

Description

The invention relates to a method for treating metal surfaces and more generally the surfaces of ferrous alloy parts.

Such treatments are known to those skilled in the art and used in the design of mechanical members, for example when parts must rub together under severe conditions of load and pressure. These treatments can be applied, both in cases where ferrous alloy parts are intended to be lubricated (with oil, grease, etc.) or in cases where the parts are not intended to be lubricated. .

Among the various known treatment processes, mention may be made of surface oxidation processes in molten salt baths (mixtures of nitrates and nitrites) which make it possible to improve the resistance to corrosion.

Phosphating processes are also known which, by creating a surface layer of iron phosphate, make it possible to improve, in noticeable proportions, the effects of lubrication.

Sulfurization processes are also known, that is to say the production of an iron sulphide (FeS) layer on the surface of the ferrous alloy parts in order to improve their properties of resistance to seizure. The parts treated by these sulfurization processes have excellent resistance to friction, wear and seizure.

The invention relates more particularly to this last type of treatment.

Sulphidation of steels and its effects on lubrication are known to those skilled in the art and emerge, for example, from patent teaching. FR 1 406 530 , FR 2 050 754 and FR 2 823 227 .

According to the teaching of these patents, the treated metal parts are immersed for 5 to 15 minutes in a molten salt bath ionized between 200 and 350 ° C, preferably containing potassium thiocyanate. and cyanide ions, the ionization being obtained by electrolysis, the treated part being positioned at the anode. The FeS layer is obtained by modifying the surface layer of the ferrous alloy part. This electrolytic sulphurization in molten salts requires special precautions for maintaining the bath in a state of stability during the passage of the current, and requires that particular attention be paid to the recycling of the compounds used. Moreover, this process requires a large amount of salts, which is expensive.

Another solution emerges from the teaching of the patent US 6,139,973 which relates to a process for depositing iron sulfide by electrolysis of an aqueous solution containing ferric ions and thiosulphate or sulphide ions between 30 and 50 ° C. The treated part is this time positioned at the cathode. This results in real adhesion problems of the iron sulphide layer on the treated parts.

A sulfidation treatment by a purely chemical route, without the use of electrolysis, is taught in the patent FR 2 860 806 . The parts are immersed in an aqueous solution containing sodium hydroxide at a concentration of between 400 and 1000 g / l, sodium thiosulfate and sodium sulphide at a temperature above 100 ° C. for approximately 15 minutes. The major disadvantage of this process is the natural carbonation of the bath which makes it progressively unusable. This inevitable degeneration imposes both economic and ecological constraints. Also, the treatment times are long, which is detrimental.

The document Robert et al., Entropy, 10, 57, May 1974, 27-31 discloses sulfurization by anodic electrolysis in an aqueous bath at a temperature of 120 or 150 ° C. The document US 6,589,412 also discloses an anodic electrolytic sulfidation in an aqueous medium. The problem to be solved by the invention is to reduce the amount of toxic products generated by the process and reduce the energy consumption required by it, while maintaining a high anti-seizing effect and good adhesion of the iron sulphide layer on the treated parts.

To solve this problem, it has been designed and developed a surface treatment process by electrolysis of ferrous surfaces to improve their qualities of friction or resistance to wear and seizure, a process in which said surfaces form the anode of the electrolysis and the electrolysis bath contains a sulfur species which is a sulfur, a thiosulfate or a sulphite, said bath contains mainly water and contains in addition a chlorinated salt and a nitrogen species which is an amine in amounts to facilitate the sulphidation reaction of said surfaces.

The anti-seizing effect obtained is high and has a high reproducibility. The iron sulfide layer obtained has good adhesion to the surface. The amount of salts and other raw materials used is small. The production of toxic waste is limited, and the energy consumption required by the reaction is low. The process is therefore an excellent compromise combining efficiency and savings.

The bath may be an aqueous solution.

Preferably, the sulfur species is a sulphide. It may be sodium monosulfide, potassium monosulfide, ammonium monosulfide. It may also be thiosulfate, such as sodium thiosulfate, potassium thiosulfate, ammonium thiosulfate. It can also be a sulphite.

Preferably, the sulphide is introduced at a concentration equivalent to a sulphide ion concentration of between 20 g / l and 90 g / l.

Preferably, the sulphide is sodium monosulphide, introduced at a concentration of between 50 and 200 g / l.

Preferably, the chlorinated salt is a chloride, for example sodium chloride, potassium chloride, lithium chloride, ammonium chloride, calcium chloride or magnesium chloride. It may also be hypochlorite, chlorite, chlorate or perchlorate, for example sodium, potassium, lithium, ammonium, calcium or magnesium.

Preferably, the chloride is introduced at a concentration equivalent to a chloride ion concentration of between approximately 15 and 200 g / l.

Preferably, the chloride is sodium chloride, introduced at a concentration between 30 and 300 g / l.

Preferably, the nitrogen species content is between 100 mL / L and 300 mL / L approximately. The nitrogen species may comprise a nitrogen or several nitrogens. It can for example be a base, possibly low or very low. It can also be a weakened base, for example by a substituent, or conversely an enriched base. It can be an organic species.

Preferably, the nitrogen species is an amine, for example a monosubstituted or polysubstituted amine. This is for example triethanolamine, or methylamine, phenylamine, diethylamine, diphenylamine, cyclohexylamine, for example. The amine can be introduced in the form of an amino acid such as alanine, glutamic acid or proline, for example. It can also be a mixture of these compounds. The nitrogen species may also carry one or more oxygen atoms or one or more alcohol functions at a selected distance from the nitrogen or at selected distances from the nitrogen atom or nitrogen atoms. It can also carry other atoms or other functions.

Preferentially, triethanolamine is used, and the amount of triethanolamine introduced is between 100 mul and 300 mul approximately. The mechanism of action of this molecule is not eluded, and the respective roles of each of the characteristics of this molecule are not known.

If appropriate, the nitrogen species content in triethanolamine equivalent is evaluated according to conventional methods, in which case the preferred content of nitrogenous species is equivalent to a triethanolamine content of between 100 mul. and 300 mUL.

Preferably, the working temperature of the bath is less than 70 ° C. It can be room temperature, which reduces energy consumption.

Preferably, the duration of the electrolysis treatment is less than 1 hour, or in some cases less than 10 minutes, or even less than one minute.

Preferably, the electrolysis is carried out using a direct current,

According to one characteristic, the electrolysis is carried out using a pulsed current. This can be applied as a crenellated signal, or in another form.

Preferably, the pulsed current has a frequency less than 500 kHz (that is to say a period greater than 2 μs).

The duration of the pulses is less than the period of the signal, and according to one characteristic, it is less than 50 ms.

Preferably, the average current density is between 3 and 15 A / dm ² , and is for example about 8 A / dm ² , or 5 A / dm ² .

Preferably, the cathode is an inert conductive material in the solution. Preferably, it is made of stainless steel.

Finally, the invention also relates to parts whose surface is treated according to the method according to the invention.

According to a preferred embodiment, the parts to be treated are positioned in an electrolysis bath at the anode. A current density is applied between the parts and a cathode. The duration of the treatment is between a few seconds and 10 minutes, or even 20, 30 minutes or more, depending on the geometry and the surface of the parts to be treated. The treatment is typically performed at a temperature below 70 ° C.

The galling resistance resulting from the treatment process according to the invention is evaluated according to the Faville Levally machine test according to the ASTM-D-2170 standard.

In a manner known to a person skilled in the art, this test consists of treating a 6.35 mm diameter cylindrical specimen with a height of 50 mm in hardened and ground cemented 16NC6 steel. The specimen is clamped between two V-shaped jaws at 90 ° to which a linearly increasing load is applied as a function of time. The test is stopped when there is seizure or creep of the specimen. This test is characterized by a magnitude called note Faville which is the integral of the applied load with respect to the time, this note being expressed in daN.s.

Reference is made hereinafter to the examples given by way of non-limiting indication, and which show the results obtained with the characteristics of the process according to the invention, in comparison with the treatments according to the prior art.

It appeared that, when the test piece is treated according to the process according to the invention, the test piece flue and does not flu and that its note Faville is generally greater than 12 000 daN.s.

Example 1

According to this example, the Faville grade of hardened case-hardened 16NC6 steel specimens is compared, in the case of an untreated specimen (1), a phosphated specimen (2), a specimen according to the method of the invention. invention (3).

The test piece according to the invention is soaked in an aqueous solution and maintained at the anode. The cathode is made of stainless steel. On mounting the bath, the aqueous solution contains 100 g / l of sodium monosulfide, 50 g / l of sodium chloride and 200 m 2 of triethanolamine.

The treatment is carried out, according to a first variant, at ambient temperature (20 ° C.) for 10 seconds, the current is continuous and the current density applied is 8 A / dm ² .

According to a second variant, the treatment is carried out always at room temperature, but for 5 minutes, with a current pulsed at a frequency of 25 Hz (that is to say with a period of 40 ms), the duration of the pulses being 10 ms, and the current density averaged over a period of 4 A / dm ² .

We refer to the table below: 1 2 3 4 Untreated test tube Phosphate test tube (iron-manganese phosphating) Sulfurized specimen according to the invention (1 ^st alternative) Sulfurized specimen according to the invention (2 ^nd alternative) Note Faville daN.s 5000 5500 15000 15000 Stopping the test seizure seizure creep creep

It follows from this test that specimens 1 and 2 have no anti-seize property while specimens 3 and 4, according to the invention, have high anti-seizing properties.

Example 2

In this example, the note Faville of hardened, sulphurized 16NC6 steel specimens is compared by the process according to the invention (1) and by the electrolytic medium molten salt bath method, as is apparent from the teaching of the patent. FR 2.050.754 . We refer to the table below: 1 2 3 Sulfurized specimen according to the invention (1 ^st alternative) Sulfurized specimen according to the invention (2 ^nd alternative) Sulphide test piece according to FR 2.050.754 Note Faville daN.s 15000 15000 15000 Stopping the test creep creep creep

The test piece according to the invention is immersed in a bath of an aqueous solution and maintained at the anode. The cathode is made of stainless steel. On mounting the bath, the aqueous solution contains 100 g / l of sodium monosulfide, 50 g / l of sodium chloride and 200 m 2 of triethanolamine.

According to a first variant, the treatment is carried out at ambient temperature (20 ° C.) for 10 minutes, with a pulsed current at a frequency of 200 kHz (that is to say with a period of 5 μs), the duration of pulses being 2 μs, and the current density averaged over a period of 4 A / dm ² .

According to a second variant, the treatment is carried out always at room temperature, but for 10 minutes with a direct current and a current density of 5 A / dm ² .

These tests show that solutions 1, 2 and 3 have quite similar anti-seize properties.

Depending on the geometry and the surface of the parts to be treated, the skilled person will adapt the duration of the treatment, which may be between a few seconds and 30 minutes or more, being for example of the order of magnitude of 10 minutes. In will also adjust the temperature, which may be the ambient temperature, or a temperature below 70 ° C or higher. It will also adapt the current density.

• le respect de l'environnement,
• la maîtrise avec une grande précision et une grande reproductibilité, de la composition, de l'adhérence et de la continuité des couches superficielles,
• le traitement à température ambiante permettant de réduire la consommation d'énergie,
• le temps de traitement court ou très court permettant de réaliser des cycles de travail plus réduits.

The advantages stand out well from the description, in particular one underlines and recalls:

• the respect of environment,
• the control with great precision and reproducibility, of the composition, the adhesion and the continuity of the superficial layers,
• ambient temperature treatment to reduce energy consumption,
• the short or very short processing time allowing for shorter work cycles.

Claims (14)

1. Process for the surface treatment by electrolysis of ferrous surfaces in order to improve their qualities of rubbing or of resistance to wear and to jamming, during which said surfaces form the anode of the electrolysis and the bath of the electrolysis comprises a sulfur-comprising entity suitable for implementing a sulfurisation process, characterised in that the bath comprises predominantly water and comprises, in addition to the sulfur-comprising entity, which is a sulfide, a thiosulfate or a sulfite, a chlorine-comprising salt and a nitrogen-comprising entity, which is an amine, in amounts suitable for facilitating the sulfurisation reaction of said surfaces.
2. Process for the surface treatment of ferrous surfaces according to claim 1, characterised in that the sulfur-comprising entity is a sulfide and in that the sulfide is preferably introduced at a concentration equivalent to a concentration of sulfide ions of between 20 g/l and 90 g/l.
3. Process for the surface treatment of ferrous surfaces according to claim 2, characterised in that the sulfide is sodium monosulfide.
4. Process for the surface treatment of ferrous surfaces according to one of claims 1 to 3, characterised in that the chlorine-comprising salt is a chloride and in that the chloride is preferably introduced at a concentration equivalent to a concentration of chloride ions of between 15 and 200 g/l approximately.
5. Process for the surface treatment of ferrous surfaces according to claim 4, characterised in that the chloride is sodium chloride.
6. Process for the surface treatment of ferrous surfaces according to one of claims 1 to 5, characterised in that the content of nitrogen-comprising entity is between 100 ml/l and 300 ml/l approximately.
7. Process for the surface treatment of ferrous surfaces according to claim 6, characterised in that the amine is triethanolamine.
8. Process for the surface treatment of ferrous surfaces according to one of claims 1 to 7, characterised in that the operating temperature of the bath is less than 70°C and in that the operating temperature of the bath is preferably ambient temperature.
9. Process for the surface treatment of ferrous surfaces according to one of claims 1 to 8, characterised in that the electrolysis is carried out using a continuous current.
10. Process for the surface treatment of ferrous surfaces according to one of claims 1 to 8, characterised in that the electrolysis is carried out using a pulsating current and in that the pulsating current has a pulse duration preferably of less than 50 ms.
11. Process for the surface treatment of ferrous surfaces according to claim 10, characterised in that the pulsating current has a frequency of less than 500 kHz.
12. Process for the surface treatment of ferrous surfaces according to one of claims 1 to 11, characterised in that the mean current density is between 3 and 15 A/dm².
13. Process for the surface treatment of ferrous surfaces according to one of claims 1 to 12, characterised in that the cathode is made of a conducting material which is inert in the solution.
14. Process for the surface treatment of ferrous surfaces according to one of claims 1 to 13, characterised in that the cathode is made of stainless steel.