Surface treatment process for a steel mechanical part to improve its resistance to wear and corrosion under severe stresses.

Abstract

The invention relates to a surface treatment process for a steel mechanical part to improve its resistance to wear and corrosion, the process comprising at least one nitriding or nitrocarburizing step, of the part forming a nitrided surface layer, followed by a step of impregnating the nitrided surface layer with a hydrophobic wax, characterized in that said wax is a carbonaceous organic compound having a molecular weight of less than 500, and preferably between 200 and 500.

Description

Title of the invention: Method for surface treatment of a mechanical steel part to improve its resistance to wear and corrosion, under severe stresses Technical field

[0001] The present invention relates to the field of surface treatment of mechanical steel parts, aimed at improving their resistance to wear and corrosion. More specifically, the invention relates to a treatment method involving nitriding or nitrocarburizing, optionally followed by oxidation, and impregnation with a hydrophobic wax. Prior art

[0002] Document EP0524037 teaches a surface treatment method for improving the corrosion resistance of mechanical parts.

[0003] This document describes a technique for improving the friction and corrosion resistance properties of ferrous metal parts. The disclosed technique implements a series of treatments, including nitriding, oxidation and the application of a final coating.

[0004] This final coating is a hydrophobic wax, a high molecular weight organic carbon compound, in particular between 500 and 10,000. The application of this high molecular weight wax helps to improve the corrosion resistance of the parts and to minimize rebound effects.

[0005] These treatments have the advantage of being inexpensive and easy to implement, even for industrial production on mass-produced parts. In addition, they give the treated parts high technical performance even when these parts are of complex shapes.

[0006] Document WO2016 / 102813 presents a process using PVD coating to increase the durability of mechanical parts.

[0007] This document describes a method for treating the surface of a steel mechanical part to give it high resistance to wear and corrosion. The process comprises the following steps: - a nitriding or nitrocarburizing step to form a layer of iron nitrides at least 8 micrometers thick; - an oxidation step to generate a layer of oxides with a thickness of between 0.1 and 3 micrometers; - an impregnation step by dipping in an impregnation bath for at least 5 minutes. This bath is formed of at least 70% by weight, to within 1%, of a solvent of hydrocarbons, from 10% to 30% by weight, to within 1%, of at least one paraffin oil and at least one additive of the synthetic phenolic additive type at a concentration of between 0.01% and 3% by weight, to within 0.1%.

[0008] This method provides a substantial improvement in corrosion resistance compared to a conventional bath based on oils, acids and ethanol. In addition, the treated parts are dry to the touch, meaning they do not transfer oil to another surface and do not pick up surrounding dust. This also gives them the ability to undergo post-treatment, such as overmolding.

[0009] However, with either of the aforementioned techniques, if the treated and impregnated part is subjected to so-called severe constraints, such as high-pressure washing or repeated mechanical stresses, such as friction by a seal for example, then the corrosion resistance is degraded.

[0010] To overcome this problem, it is known to resort to the use of particular coating technologies, such as currentless electroplating of nickel or a hard chrome-type coating. However, these techniques have certain disadvantages, including: - fluctuating raw material costs: Nickel, for example, is subject to significant price fluctuations due to various economic and geopolitical factors. This can make the cost of coating operations difficult to predict and potentially more expensive; - REACH regulations: The European Regulation on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) requires prior authorisation for the use of certain materials, such as hard chrome. This can slow down the production process and increase costs. In addition, there are also environmental and health concerns associated with the use of these substances.

[0011] Therefore, although these coating technologies can improve the wear and corrosion resistance of steel mechanical parts, they have their own challenges in terms of costs and regulatory compliance. Statement of the invention

[0012] One of the aims of the invention is to overcome the drawbacks of the prior art, by proposing a method for surface treatment of a mechanical steel part to improve its resistance to wear and corrosion, which makes it possible to guarantee this resistance even when the part is subjected to aggressive and severe environmental conditions, such as repeated washing under high-pressure water jets, exposure to sand and bad weather, or repeated mechanical friction.

[0013] The present invention aims to solve the problems of the prior art by proposing a method for surface treatment of a mechanical steel part to improve its resistance to wear and corrosion. The method comprises at least one nitriding or nitrocarburizing step, forming a nitrided surface layer, consisting for example of a compact deep sub-layer and a porous surface sub-layer. The surface sub-layer contains more than 60% of Fe2-3N solid phase and has a hardness of between 550 and 650 HV 0.1. It has a roughness of between 0.3 and 1.5 micrometers, which further improves the adhesion of the wax and the resistance to corrosion. This nitriding or nitrocarburizing step is followed by a step of impregnating this layer with a hydrophobic wax.

[0014] According to the invention, this wax is a low molecular weight organic carbon compound and in particular less than 500, and preferably between 200 and 500.

[0015] This feature offers the advantage of providing effective protection against corrosion while being economically viable.

[0016] Furthermore, and contrary to the teaching of the prior art which encouraged the use of a high molecular weight organic carbon compound, the Applicant discovered, provided that the nitriding or nitrocarburizing was carried out correctly, that a low molecular weight organic carbon compound promoted the resistance of the protective wax and reduced its capacity to be washed or degraded under severe usage conditions.

[0017] Under these conditions, the invention provides superior corrosion resistance and high resistance to high-pressure washing / cleaning, exposure to sand and bad weather, or even repeated mechanical friction.

[0018] The invention considers as satisfactory a resistance to corrosion during a salt spray test greater than 1600 hours for a freshly treated part, and greater than 500 hours for a part having undergone a high pressure washing simulation after treatment.

[0019] The impregnation according to the invention does not alter the visual appearance or subsequent handling of the parts and has a dry appearance to the touch.

[0020] The method according to the invention may also comprise an oxidation step following nitriding or nitrocarburizing, which leads to the formation of a porous surface layer containing iron oxides. This oxidation layer improves the corrosion resistance of the part as well as the retention of the wax by the surface of the part.

[0021] Impregnation can be carried out in different ways, for example by dipping in a wax bath or by spraying the wax, at room temperature, which makes the process more flexible and adaptable to different part configurations.

[0022] The soaking impregnation time is at least 2 minutes, which allows a effective compromise between processing time and the quality of protection obtained.

[0023] The invention also makes it possible, in view of the observed performances, to be able to carry out the impregnation with a wax diluted to 50%, 75% or even 80% with a solvent, and also to achieve low grammage levels, while guaranteeing unequalled corrosion resistance, particularly in severe conditions. Diluted to 90%, the wax still provides good protection against corrosion under normal conditions, but is no longer suitable for severe conditions.

[0024] The method according to the invention is also advantageous in that it does not require baking, a step of air drying the impregnated wax, for at least 5 minutes and at room temperature, is sufficient and contributes to ensuring good adhesion of the wax to the surface of the part.

[0025] The wax advantageously comprises calcium sulfonate, in an amount of between 1 and 30% by mass, preferably between 1 and 20%, which makes it possible to improve the resistance to corrosion while being environmentally friendly.

[0026] Calcium sulfonate is a lubricating agent that also helps reduce friction and wear between moving surfaces. When added to wax, it improves the lubricating properties of the wax and facilitates the sliding of the contacting surfaces, and therefore reduces the scraping effect of the seal of a cylinder, for example.

[0027] The addition of calcium sulfonate also helps improve the wax's resistance to oxidation. It acts as an oxidation inhibitor, helping to prevent the wax from breaking down in the presence of oxygen. This helps extend the life of the wax and maintain its lubricating properties for longer.

[0028] Calcium sulfonate also gives the wax some stability in the presence of water. It forms a protective layer on metal surfaces, helping to prevent corrosion and maintain lubricating properties even in the presence of moisture.

[0029] According to a particular embodiment, the wax consists essentially of a C20 to C35 alkane cut and calcium sulfonate.

[0030] This offers the advantage of excellent corrosion protection, while allowing the use of economical and widely available raw materials.

[0031] The dilution of this wax can be done with a solvent containing mainly alkanes of type C8 to C12, in particular C9, C10 and Cl 1, or with a solvent of type white spirit.

[0032] The invention also relates to a mechanical part made of steel which has undergone a surface treatment to improve its resistance to wear and corrosion, the part having a nitrided surface layer impregnated with a layer of hydrophobic wax in the form of an organic carbon compound whose molecular weight is less than 500, and preferably between 200 and 500.

[0033] Preferably, and in view of the observed performances, the treated and impregnated mechanical part may have a low wax weight, and present for example a layer of wax having a surface mass of between 5 and 60 g / m2, and preferably between 30 and 60 g / m2, hence a lower consumption of product per unit of surface, which implies a lower cost price with a limited environmental impact. Detailed description of the invention

[0034] The present invention relates to a method for the surface treatment of mechanical steel parts to improve their resistance to wear and corrosion. This method uses a surface treatment based on nitriding or nitrocarburizing, followed by impregnation with a specific hydrophobic wax.

[0035] The invention finds an advantageous application for the treatment of a part requiring high resistance to corrosion, in particular evaluated according to the salt spray test according to standard ISO9227, and this under particularly severe conditions of use, for example operation in aggressive environments, such as exposure to bad weather, sand, repeated washing under high-pressure water jets.

[0036] An example of a part treated according to the invention is, for example, a tailgate lift cylinder rod arranged at the rear of the body of a truck, which is more often in the extended position than in the retracted position, thus being exposed to said aggressive and severe environmental conditions, or even to the scraping of the seal positioned at the inlet of the cylinder barrel.

[0037] The nitriding or nitrocarburizing step allows the diffusion of nitrogen or nitrogen and carbon into the surface of the steel, forming a nitrided surface layer. This layer serves to improve the wear resistance of the mechanical part.

[0038] In some embodiments, this step may be followed by an oxidation step to increase corrosion resistance. The oxidation forms a protective oxide layer on the surface of the part that provides an additional barrier against corrosive chemical reactions.

[0039] The nitriding procedure followed by oxidation results in the formation of a layer comprising a deeper compact sub-layer and a porous sub-layer on the surface. This superficial sub-layer, with a thickness of between 5 and 25 micrometers, exposes open porosities with a diameter of between 0.2 and 3 micrometers. The steel parts treated in this way are then impregnated with a specific hydrophobic wax. This wax is an organic carbon compound, with a low molecular weight of between 200 and 500, having for example a surface energy of between 29 and 35 mN / m in the solid state.

[0040] The surface energy measurement is carried out by drop angle on 3 polished pieces, immersed in wax dissolved at 50, 75 and 87.5% respectively (wax content 50, 25 and 12.5%) and left to dry for 24 hours. The surface energy depends very little on the dissolution rate.

[0041] The wax according to the invention makes it possible to better impregnate the nitrided surface layer so that it is more resistant to severe exposure conditions. The impregnation can be carried out by dipping the part in a wax bath or by spraying the wax at room temperature.

[0042] Dipping in a wax bath can be carried out for at least two minutes. The wax is usually used diluted for easier application, for example diluted to 50% or 75%, or even 80% with a solvent, such as a solvent containing mainly alkanes of type C8 to C12, in particular C9, C10 and C11, or white spirit. After impregnation, the method comprises a step of air drying the impregnated wax, for at least 5 minutes.

[0043] The wax advantageously comprises calcium sulfonate, in an amount of between 1 and 30% by mass, preferably between 2 and 20%. It is also composed of a C20 to C35 alkane cut. In certain embodiments, the wax may comprise plasticizers such as, for example, a compound of the 1,1'-Biphenyl 4,4'-dibromo type of formula C12H8Br2, in order to increase the cohesion of the wax.

[0044] After the treatment, a mechanical steel part is obtained which has a nitrided surface layer impregnated with a hydrophobic wax, significantly increasing its resistance to wear and corrosion. This treated part can be used in various mechanical applications where resistance to wear and corrosion is required, including under the severe conditions of use mentioned above.

[0045] The surface sub-layer contains more than 60% of the solid phase Fe2-3N, with a hardness between 550 and 650 HV 0.1 and a roughness between 0.3 and 1.5 micrometers CLA.

[0046] The impregnation wax is selected from natural waxes, or synthetic waxes such as polyethylene, polypropylene, polyester, fluorinated waxes or even modified petroleum residues.

[0047] The composition of the nitrided layer, its thickness and its hardness are adjusted so that it resists wear, without being fragile and risking flaking under the effect of shocks. The compact hexagonal structure of the Fe2-3N phase of the iron / nitrogen equilibrium diagram offers good deformation capacity, thanks to a high atomic density on the sliding plane and is thus particularly favorable for friction applications.

[0048] The use of the molten salt bath, as described in FR-A-2 171 993, to carry out the nitriding is a particular advantage of this invention. This bath is essen mainly composed of carbonates and cyanates of the alkali metals K, Na and Li. The weight proportions for the alkali cations are 25-42.6% for Na+, 42.6-62.5% for K+ and 11.3-17.1% for Li+ in the total weight.

[0049] The oxidation treatment not only improves the corrosion resistance but also makes it possible to best adjust the surface properties of the solid phase to maximize the effectiveness of the wax impregnation.

[0050] Finally, the oxidation is carried out in a bath of molten salts according to FR-A-2 525 637, generally at temperatures between 350° and 450°C.

[0051] Corrosion resistance tests were carried out on parts treated according to the invention and parts not in accordance with the invention, as well as such parts which had not undergone washing, and parts which had undergone high-pressure washing.

[0052] The nitriding and oxidation steps are the same for all parts, only the nature of the wax used is modified.

[0053] The conversion layer obtained by nitriding, namely the sum of the compact deep sub-layer and the porous surface sub-layer, has a thickness between 20 and 30 μm, including 5 to 10 μm of porous surface sub-layer. The oxidation layer, on the surface, has a thickness of the order of 1 to 2 μm. All the impregnations were carried out by dipping.

[0054] The high-pressure washes were carried out with a distance of 50 cm between the tested part and a nozzle of a high-pressure washing device, under a water pressure of 180 bars, cold, and for 30 seconds.

[0055] The results of these tests are compiled in the table below:

[0056] [Tables 1] Test Wax Molecular weight Weight Before g Weight After g Weight Coat eg Weight Coat eg / m2 Corrosion resistance (high pressure washing) (NSST ISO9227) Corrosion resistance (without (high pressure washing) (NSST ISO9227) 0 According to the invention, diluted to 90% with white spirit 200-500 88.73 89.054 0.324 9.5 200 > 1608 hours 0 bis According to the invention, diluted to 75% with white spirit 200-500 88.91 89.704 0.794 23.2 > 1608 hours > 1608 hours 0 ter According to the invention diluted to 50% with a C8 - C12 alkane cut 200-500 86.84 87.98 1.14 33.3 > 1608 hours > 1608 hours 1 Wax from Example 2 of patent EP0524037, diluted to 90% with white spirit 500-10.00 0 86.22 87.072 5 0.852 5 25 200 hours 700 hours 2 Example 2 of patent 500-10.00 0 86.36 89.668 3.308 97 300 hours > 1608 hours EP0524037, diluted 50% 3 Polyethylene wax, diluted 90% with white spirit 3000-10.0 00 86.18 86.998 0.818 24 144 hours 700 hours 4 Polyethylene wax, diluted 50% with white spirit 3000-10.0 00 86.44 90.089 3.649 107 300 hours > 1608 hours

[0057] For each test, tests were carried out on 10 parts and the time before corrosion appeared was measured on 50% of the parts.

[0058] It is noted that parts 0, Obis and Oter, treated according to the invention, unwashed, have improved corrosion resistance by making it possible to obtain exposure to salt spray for more than approximately 1600 hours without any signs of corrosion appearing.

[0059] The parts 0, Obis and Oter, treated according to the invention, subjected to high-pressure washing, make it possible to obtain exposure to salt spray of more than 500 hours, or even more than approximately 1600 hours without any sign of corrosion appearing, which is very satisfactory, even though the quantity of wax present on the layer is relatively low, in particular less than 60g / m2, or even less than 35g / m2.

[0060] On the other hand, it is noted that the corrosion resistance of parts treated with a wax in the form of an organic carbon compound whose molecular weight is between 500 and 10,000, see tests 1, 2, 3 and 4, after high-pressure washing, is less than 500 hours and is therefore not considered satisfactory within the meaning of the present invention.

[0061] It is also noted that increasing the weight of the wax in Examples 2 and 4 somewhat improves the corrosion resistance after high-pressure washing, but without achieving a satisfactory level of resistance.

[0062] It is clear from the above that the invention provides a method for surface treatment of a mechanical steel part to improve its resistance to wear and corrosion, under severe conditions of use, as well as such a part obtained by the method.

Claims

Claims

1. Method for surface treatment of a mechanical steel part to improve its resistance to wear and corrosion, the method comprising at least one step of nitriding or nitrocarburizing the part forming a nitrided surface layer, followed by a step of impregnating the nitrided surface layer with a hydrophobic wax, characterized in that said wax is an organic carbon compound whose molecular weight is less than 500, and preferably between 200 and 500.

2. Method according to claim 1, characterized in that the nitriding or nitrocarburizing step is followed by an oxidation step.

3. Method according to one of the preceding claims, characterized in that the impregnation is carried out by dipping in a wax bath or by spraying the wax, at room temperature.

4. Method according to claim 3, characterized in that the impregnation is carried out by dipping in a wax bath for a period of at least 2 min.

5. Method according to one of the preceding claims, characterized in that the impregnation is carried out with a wax diluted to 50% to 75% or 80% with a solvent.

6. Method according to one of the preceding claims, characterized in that it then comprises a step of air drying the impregnated wax, for at least 5 min, and at room temperature.

7. Method according to one of the preceding claims, characterized in that the wax comprises calcium sulfonate, in an amount of between 1 and 30% by mass.

8. Method according to one of the preceding claims, characterized in that the wax is essentially composed of a C20 to C35 alkane cut and calcium sulfonate.

9. Method according to one of the preceding claims, characterized in that the composition of the wax comprises 1,1'-Biphenyl 4,4'-dibromo of formula C12H8Br2.

10. Mechanical part made of steel having undergone a surface treatment to improve its resistance to wear and corrosion, the part having a nitrided surface layer impregnated with a layer of hydrophobic wax, characterized in that the wax is an organic carbon compound whose molecular weight is less than 500, and preferably between 200 and 500.

11. Mechanical part according to claim 10, characterized in that the wax layer has a surface mass of between 5 and 60 g / m2

12. ili . Mechanical part according to one of claims 10 to 11, characterized in that the wax comprises calcium sulfonate, in an amount of between 1 and 30% by mass.

13. Mechanical part according to one of claims 10 to 12, characterized in that the wax is essentially composed of a C20 to C35 alkane cut and calcium sulfonate.