Method for carrying out surface treatment on steel mechanical part to improve wear resistance and corrosion resistance of steel mechanical part under severe stress

Abstract

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

Description

Technical Field

[0001] This invention relates to the field of surface treatment of steel machine parts aimed at improving their wear resistance and corrosion resistance. More specifically, this invention relates to a treatment method comprising nitriding or nitrocarburizing, optionally followed by oxidation, and impregnation with a hydrophobic wax. Background Technology

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

[0003] This document describes a technique for enhancing the friction and corrosion resistance of ferrous metal parts. The disclosed technique involves a series of processes, including nitriding, oxidation, and applying a final coating.

[0004] The final coating is a hydrophobic wax, an organic carbon-based compound with a high molecular weight, particularly 500 to 10,000. Applying this high-molecular-weight wax helps improve the corrosion resistance of the parts and minimizes springback.

[0005] The advantages of these treatments are their low cost and ease of implementation, even for industrial-scale production of a series of parts. Furthermore, they provide high-performance treated parts, even those with complex shapes.

[0006] Document WO2016 / 102813 discloses a method for improving the durability of mechanical parts using PVD coating.

[0007] This document describes a method for treating the surface of steel mechanical parts to impart high wear resistance and corrosion resistance. The method includes the following steps:

[0008] - Nitriding or nitrocarburizing steps to form an iron nitride layer at least 8 micrometers thick;

[0009] - An oxidation step to generate an oxide layer 0.1 to 3 micrometers thick;

[0010] - The impregnation step is carried out by immersion in an impregnation bath for at least 5 minutes. The bath consists of at least 70% by weight (+ / - 1% by weight) of a hydrocarbon solvent, 10% to 30% by weight (+ / - 1% by weight) of at least one paraffin oil, and at least one synthetic phenolic additive at a concentration of 0.01% to 3% by weight (+ / - 0.1% by weight).

[0011] Compared to traditional oil-based or acid-based baths, this method achieves a significant improvement in corrosion resistance. Furthermore, the treated parts feel dry to the touch, meaning they do not transfer oil to other surfaces or attract dust. This also allows for post-processing, such as secondary molding.

[0012] However, using any of the above techniques, if the treated and impregnated parts are subjected to harsh conditions, such as high-pressure cleaning or repeated mechanical stress (e.g., friction of seals), their corrosion resistance will decrease.

[0013] To address this problem, specific coating techniques, such as electroless nickel plating or hard chrome plating, are known. However, these techniques have drawbacks, including:

[0014] - Fluctuations in raw material costs: For example, nickel prices fluctuate significantly due to economic and geopolitical factors, making coating operations unpredictable and potentially more expensive;

[0015] - REACH Regulation: The EU Regulation on the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) requires prior authorization for the use of certain materials (such as hard chrome). This may slow down production and increase costs. These substances also pose environmental and health problems.

[0016] Therefore, although these coating technologies can improve the wear resistance and corrosion resistance of steel mechanical parts, they face challenges in terms of cost and regulatory compliance.

[0017] Document FR3030578 also describes the prior art. Summary of the Invention

[0018] One object of the present invention is to overcome the shortcomings of the prior art by providing a surface treatment method for steel mechanical parts, which improves the wear resistance and corrosion resistance of steel mechanical parts, and maintains this resistance even when subjected to harsh environmental conditions such as repeated high-pressure cleaning, exposure to sand and weather, or repeated mechanical friction.

[0019] This invention addresses the problems of existing technologies by proposing a surface treatment method for steel mechanical parts. The method includes at least one nitriding or nitriding / carburizing step to form a nitrided surface layer, for example, comprising a dense inner sublayer and a porous outer sublayer. The outer sublayer contains more than 60% Fe. 2-3 The N solid phase has a hardness between 550 and 650 HV 0.1. Its roughness is between 0.3 and 1.5 micrometers, which further improves the adhesion and corrosion resistance of the wax. Following this nitriding or nitrocarburizing step is a step of impregnating the layer with a solution containing at least one hydrophobic wax.

[0020] According to the present invention, the wax is an organic carbon-based compound having a low molecular weight (specifically less than 500, preferably 200 to 500).

[0021] This feature provides effective corrosion protection while maintaining economic viability.

[0022] Furthermore, contrary to the teachings of the prior art that encourage the use of high molecular weight organic carbon compounds, the applicant has found that, provided nitriding or nitrocarburizing is performed properly, low molecular weight organic carbon compounds also enhance the retention of protective waxes and reduce their susceptibility to washing or degradation under harsh operating conditions.

[0023] Under these conditions, the present invention ensures excellent corrosion resistance and high resistance to high-pressure cleaning / washing, dust and weather exposure or repeated mechanical friction.

[0024] The present invention proposes that satisfactory corrosion resistance is greater than 1600 hours in salt spray testing for freshly treated parts and greater than 500 hours for parts that have undergone simulated high-pressure cleaning after treatment.

[0025] The impregnation performed according to the present invention does not change the visual appearance or operation of the parts and provides a dry-touch surface.

[0026] The method may also include an oxidation step following nitriding or nitrocarburizing, resulting in the formation of a porous surface layer containing iron oxide. This oxide layer enhances corrosion resistance and wax retention.

[0027] Impregnation can be performed by immersing in a bath at room temperature or by spraying a solution, which makes the method flexible and adaptable to different part geometries.

[0028] The immersion time is at least 2 minutes, which provides an effective trade-off between treatment time and protection quality.

[0029] Based on the observed performance, the impregnation solution can contain wax diluted with solvent to 50%, 75%, or 80%, achieving excellent corrosion resistance even under harsh conditions. When diluted to 90%, the wax remains protective under normal conditions but is no longer suitable for harsh conditions.

[0030] This method advantageously eliminates the need for oven drying; air drying the impregnated solution at room temperature for at least 5 minutes is sufficient, ensuring good wax adhesion.

[0031] The solution advantageously contains calcium sulfonate in a content of 1% to 30% by mass, preferably 1% to 20% by mass, thereby improving corrosion resistance while being environmentally friendly.

[0032] Calcium sulfonate also acts as a lubricant, reducing friction and wear between moving surfaces. When added to waxes, calcium sulfonate improves the wax's lubricating properties and promotes smoother movement, for example, by reducing scratching of seals against hydraulic rods.

[0033] The addition of calcium sulfonate also enhances the antioxidant properties of the wax. As an antioxidant, it prevents the wax from degrading in the presence of oxygen, thereby extending the wax's lifespan.

[0034] Calcium sulfonate also provides water stability, forms a protective layer on metal surfaces to prevent corrosion, and maintains lubrication even in humid environments.

[0035] In one embodiment, the solution comprises wax and calcium sulfonate, which are mainly composed of C20–C35 alkane components.

[0036] This provides excellent corrosion protection while using economical and readily available raw materials.

[0037] The wax can be diluted with solvents that mainly contain C8–C12 (especially C9, C10 and C11) alkanes or with petroleum solvent oil (white spirit).

[0038] The present invention also relates to a steel mechanical part having undergone such surface treatment, the steel mechanical part comprising a nitrided surface layer impregnated with a hydrophobic wax-based solution, wherein the wax is an organic carbon compound with a molecular weight of less than 500, preferably 200 to 500.

[0039] Preferably, based on observed performance, the treated and impregnated mechanical parts can have a low wax coating weight, with a unit area mass of 5 g / m². 2 Up to 60g / m 2 Preferably 30g / m 2 Up to 60g / m 2 This reduces product consumption, lowers costs, and limits environmental impact. Detailed Implementation

[0040] This invention relates to a surface treatment method for improving the wear resistance and corrosion resistance of steel mechanical parts. The method relies on a surface treatment based on nitriding or nitrocarburizing, followed by impregnation with a solution containing at least one specific hydrophobic wax.

[0041] This invention is particularly advantageous for handling parts that require high corrosion resistance (especially as evaluated by the ISO 9227 salt spray test) and are used in harsh conditions (such as exposure to weather, dust, or repeated high-pressure cleaning).

[0042] An example of a processed part is the tailgate hydraulic cylinder rod located at the rear of the truck bed, which is often exposed (protruding) to environmental stress and friction from the seals at the cylinder inlet.

[0043] Nitriding or nitrocarburizing steps allow nitrogen (or nitrogen and carbon) to diffuse onto the steel surface, forming a nitrided layer that improves wear resistance.

[0044] In some implementations, oxidation is performed after this step to further improve corrosion resistance by forming a protective oxide barrier.

[0045] The nitrided and oxidized layer has a dense inner sublayer and a porous outer sublayer (5-25µm thick) with pores of 0.2-3µm. The steel parts treated in this way are then impregnated with a hydrophobic wax-based solution with a molecular weight of 200 to 500 and a solid surface energy of 29 to 35 mN / m.

[0046] Surface energy was determined by measuring the contact angle on three polished samples that were immersed in wax solutions (50%, 75%, and 87.5%) and dried for 24 hours. This energy was almost independent of concentration.

[0047] The wax according to the invention provides excellent impregnation and resistance under harsh exposure conditions. Impregnation can be performed by immersion at ambient temperature or by spraying.

[0048] Immersion should last at least 2 minutes. The wax is typically diluted (50%–80%) with a solvent (C8–C12 alkanes or petroleum solvents). After immersion, air dry for ≥5 minutes to complete the process.

[0049] The wax solution advantageously comprises 1%-30% calcium sulfonate, preferably 2%-20%, and the wax is based on a C20–C35 alkane component. Optionally, a plasticizer, such as 1,1'-biphenyl-4,4'-dibromo(C20–C35) ... 12 H8Br2) to increase cohesion.

[0050] The resulting steel parts exhibit a nitrided layer impregnated with hydrophobic wax, which significantly improves wear resistance and corrosion resistance under harsh conditions.

[0051] The outer sublayer contains more than 60% Fe. 2-3 The nitrogen phase has a hardness between 550-650 HV 0.1 and a surface roughness between 0.3-1.5µm CLA.

[0052] The wax can be natural or synthetic (polyethylene wax, polypropylene wax, polyester wax, fluorinated wax or modified petroleum wax).

[0053] The composition, thickness, and hardness of the nitrided layer are optimized to resist wear without becoming brittle or easily peeling off. Fe 2- The close-packed hexagonal structure of 3N provides good deformability and excellent frictional properties.

[0054] A particular advantage of this invention is the use of a molten salt bath (as described in document FR-A-2 171 993) for nitriding. This bath primarily consists of carbonates and cyanates of alkali metals K, Na, and Li. The weight ratio of alkali metal cations relative to the total weight of the bath is Na. + 25%-42.6%, K + 42.6%-62.5%, Li +: 11.3%-17.1%.

[0055] Oxidation treatment also improves corrosion resistance and optimizes the surface structure for efficient wax impregnation.

[0056] As described in FR-A-2525637, oxidation is carried out in a molten salt bath, typically at 350-450°C.

[0057] Corrosion resistance tests were performed on parts treated according to the present invention and comparative parts, including cleaned and uncleaned parts.

[0058] The nitriding and oxidation steps are the same; only the type of wax differs.

[0059] The nitrided layer (dense + porous sublayer) is 20-30 µm thick (5-10 µm for porous surfaces). The oxide layer is approximately 1-2 µm thick. All impregnation is performed by immersion.

[0060] High-pressure cleaning uses a 180 bar cold water jet at a distance of 50 cm for 30 seconds.

[0061] The results of these tests are summarized in the table below:

[0062] Table 1

[0063]

[0064] For each test, the experiment was conducted on 10 parts, and the time it took for 50% of the parts to show corrosion was measured.

[0065] It was observed that parts 0, 0bis, and 0ter, which were treated according to the present invention and not cleaned, exhibited improved corrosion resistance and showed no signs of corrosion after being exposed to salt spray for more than about 1600 hours.

[0066] Parts 0, 0bis, and 0ter, treated according to the present invention and subjected to high-pressure cleaning, showed no signs of corrosion after more than 500 hours, and even more than about 1600 hours, of salt spray exposure, which is very satisfactory, despite the relatively low wax content present on the layers, i.e., less than 60 g / m². 2 Even below 35g / m 2 .

[0067] Conversely, it is noted that parts treated with waxes in the form of organic carbon-based compounds with molecular weights of 500 to 10,000 (see Tests 1, 2, 3 and 4) exhibited corrosion resistance of less than 500 hours after high-pressure cleaning, and were therefore not considered satisfactory in the sense of this invention.

[0068] It was also observed that increasing the coating weight of wax in Examples 2 and 4 improved corrosion resistance to some extent after high-pressure cleaning, but did not achieve a satisfactory level of resistance.

[0069] In summary, the present invention does indeed provide a surface treatment method for improving the wear resistance and corrosion resistance of steel mechanical parts under harsh operating conditions, and such parts obtained by the method.

Claims

1. A method for surface treating steel mechanical parts to improve their wear resistance and corrosion resistance, the method comprising: At least one nitriding or nitrocarburizing step is performed to form a nitrided surface layer, followed by impregnation of the nitrided layer with a solution containing at least one hydrophobic wax, characterized in that the wax is an organic carbon-based compound with a molecular weight of less than 500, preferably 200 to 500.

2. The method according to claim 1, wherein oxidation is performed after the nitriding or nitrocarburizing step.

3. The method according to any one of the preceding claims, wherein the impregnation is performed by immersion or spraying at ambient temperature.

4. The method of claim 3, wherein the immersion is carried out for at least 2 minutes.

5. The method according to any one of the preceding claims, wherein the impregnation solution comprises a wax diluted with a solvent to 50%, 75%, or 80%.

6. The method according to any one of the preceding claims, wherein the impregnated parts are air-dried at ambient temperature for at least 5 minutes.

7. The method according to any one of the preceding claims, wherein the solution comprises calcium sulfonate in an amount of 1% to 30% by mass.

8. The method according to any one of the preceding claims, wherein the solution comprises the hydrophobic wax and calcium sulfonate, the hydrophobic wax being primarily composed of C20–C35 alkane components.

9. The method according to any one of the preceding claims, wherein the solution further comprises 1,1'-biphenyl-4,4'-dibromo (C 12 H8Br2).

10. A steel mechanical part that has undergone surface treatment to improve its wear resistance and corrosion resistance, the steel mechanical part comprising a nitrided surface layer impregnated with a hydrophobic wax-based solution, wherein the wax is an organic carbon compound with a molecular weight of less than 500, preferably 200 to 500.

11. The mechanical part according to claim 10, wherein the coating weight of the wax base layer is 5 g / m². 2 Up to 60g / m 2 .

12. The mechanical part according to any one of claims 10 to 11, wherein the wax base layer comprises calcium sulfonate in an amount of 1% to 30% by mass.

13. The mechanical part according to any one of claims 10 to 12, wherein the wax-based solution comprises a wax consisting primarily of C20–C35 alkane components and calcium sulfonate.

Images