KR100215252B1 - Ferrous metal parts with simultaneously improved corrosion resistance and friction properties - Google Patents

Abstract

Precise ferrous metal parts which undergo severe friction and corrosion during use are nitridized in a molten salt bath, preferably based on CNO ^- cyanate ions, preferably oxidized in a molten oxidized alkali salt bath and then impregnated with a hydrophobic wax.

After the nitriding and oxidation step, a layer is formed of the dense deep sub-layer and the porous surface sublayer having a thickness of 5 to 25 μm and through-holes in the range of 0.2 to 3 μm in diameter. The impregnated wax is an organic compound having a high molecular weight of 500 to 10,000, a surface tension of 10 to 75 mN / m in the liquid state and a contact angle between the solid phase and the liquid wax of the surface layer is 0 to 75 °.

Description

Ferrous metal parts with improved corrosion resistance and friction

The present invention relates to a treatment method for simultaneously improving the corrosion resistance and friction characteristics of ferrous metal parts and to the resulting treated ferrous metal parts.

To provide both friction and corrosion resistance to ferrous metal parts, two separate treatments are generally performed in succession. The first treatment is to provide the friction properties to the part and the second surface treatment is to ensure corrosion resistance. For example, zinc is plated and then chromated.

The friction and corrosion resistance obtained by these treatments is often sufficient for commercial parts.

However, for some applications where increasing superior technical performance is required for parts that are heavily loaded at the same time, including various requirements (friction, wear, especially friction wear, impact, corrosion), the conventional process provides Insufficient in properties.

For example, this applies to parts intended for locking mechanisms, bolts and machine screws of any type, hinge pins, jacks and buffer rods, and game balls or bowls. Related industries are in particular automotive, public works, material handling, production, household appliances and hydraulics.

It is known that good friction properties and good corrosion resistance can be provided to the ferrous metal part by nitriding and then oxidizing.

This involves friction with known processes for nitriding ferrous metal parts, in particular with known processes for nitriding with baths of molten cyanates and carbonates as described in French Patent Application No. 2 171 993 and French Patent Application No. 2 271 307. This is achieved by nitriding in an ionized nitrogen atmosphere, which improves the frictional properties of the part by reducing the modulus and improving resistance to wear and seizure.

It is also known that if already nitrided parts are oxidized, the frictional properties obtained by nitriding are preserved while causing changes in the nitrided surface and the corrosion resistance of these parts is improved. For this purpose, French patent application No. 2 525 637 proposes a particularly effective oxidation treatment in a molten oxide bath. Comparable results on corrosion resistance are obtained by the oxidation process in an ionized atmosphere of a gas containing oxygen.

However, this type of method provides insufficient frictional properties and corrosion resistance for the above application to nitriding and then oxidized parts.

It has been found that this lack can be overcome by performing nitriding and oxidation treatments and applying the final coating.

The present invention is usually based on the finding that many parts are cleaned, especially before they are assembled, while it is well known that they do not require surface treatment to withstand conventional cleaning since they do not require such cleaning.

According to one aspect of the present invention, there is provided a method for treating ferrous metal parts for improving both corrosion resistance and friction characteristics of a part including nitriding, oxidation, and coating steps, which comprises a dense deep serve after nitriding and oxidation steps. Nitriding and oxidation to form a layer consisting of deep sub-layers and surface sub-layers with many pores, through-pores with thicknesses ranging from 5 to 25 μm and diameters ranging from 0.2 to 3 μm, with a molecular weight of 500 And a carbonaceous organic compound having a surface area of 10 to 73 mN / m in a liquid phase and impregnating a hydrophobic wax having a contact angle of 0 to 75 ° between the liquid phase wax and the liquid phase on the oxide layer.

According to another aspect of the present invention, a carbonaceous organic compound having a high density of deep sub-layers and pores after nitriding and oxidation, having a thickness of 5 to 25 μm, a through pore range of 0.2 to 3 μm, and a molecular weight of 500 to 10,000, in a liquid phase Provided is a treated ferrous metal part with a highly porous sub-layer impregnated with a hydrophobic wax with a contact angle between 0 to 75 ° and a solid surface layer with a surface tension of 10 to 73 mN / m. It is.

In other properties, the surface sub-layer contains at least 60% Fe _2-3 N (iron ε nitride) solid phase and exhibits a hardness of 550-650 HV 0.1 and a roughness of 0.3-1.5 μm CLA.

Impregnated waxes are selected from the following list:

Natural waxes, or artificial polyethylene, polypropylene, polyesters, fluorinated waxes, or variant petroleum residues.

The present invention offers the advantages of inexpensive and easy to implement tools that give a high degree of technical performance on the treated parts, even in industrial serial parts, even if the parts are complex.

Moreover, the protective effect of the surface treated by this method against aging by friction, abrasion, impact and wet corrosion has increased significantly in comparison with the solutions used in the past and helps to withstand this type of deterioration for a long time. These results will be further developed in the Examples.

The composition, thickness and hardness of the nitride layer are adjusted to withstand wear without inevitable embrittlement because the nitride layer breaks under the influence of repeated impacts in such cases. The Fe _2-3 N phase of the dense hexagonal structure of the iron / nitrogen parallelism diagram provides good deformation due to the high atomic density at the cleaved surface and thus can be ideally applied to friction.

In the combined effects of surface roughness, molecular weight and surface tension of the wax, and the contact angle between the solid and liquid phases, the indicated spacing produces the maximum effect of the wax incorporation, because in this situation the wax is only excessively or very unusual and As a result, they can only be removed as exceptional conditions for their intended application.

Experiments show that the improved corrosion resistance is not the only effect of wax, because even if the wax is applied to other surfaces, it is only known to be beneficial for the adhesion of organic compounds, but only much lower.

On the other hand, when wax is removed, the effect of nitriding is reduced in view of friction characteristics and impact strength, because the wax tends to lower the rebound effect.

In a particularly interesting arrangement of the present invention, the nitriding treatment is carried out with the carbonates and cyanates of the alkali metals K, Na, Li, i.e. from 1 to 35% by weight, as in French patent application 2 171 993. CO ₃ ^{2 -} And molten salt of 35 to 65% by weight of anionic CNO ^- and an alkali cation in which the weight ratio of alkali metal is Na ⁺ : 25-42.6%, K ⁺ : 42.6-62.5%, Li ⁺ : 11.3-17.1% Perform in bathing.

Moreover, the nitriding salt bath also contains sulfur compounds with a content of 0.001 to 1% by weight of sulfur elements, as in French Patent Application No. 2 271 307.

This has been shown to precisely control the formation of sub-layers of impregnated surfaces due to hetero-element diffusion and to promote the emergence of a significant proportion of the Fe _2-3 N phase in the iron / nitrogen parallelism, ie more than 60%. Make it possible.

In addition to improving the corrosion resistance already, if controlled correctly, the oxidation treatment can ideally adjust the surface properties of the solid phase to achieve the maximum effect of wax impregnation.

The inventor has a good reason to believe that there is a mixed layer of strong electron donors or acceptors that locally form electrical dipoles. In fact, in this case a chemical bond force occurs, which locally increases the capillary force. To achieve this, the presence of oxygen as well as sulfur is allowed in the surface sub-layer.

According to a preferred arrangement, the oxidation is carried out in a molten salt bath, generally in the temperature range of 350 to 450 ° C. as in French Patent No. 2 525 637.

The features and other advantages of the present invention will be better seen from the following description set forth by way of example.

Example 1

XC 38 steel 35 mm diameter ring and 30 × 18 × 8 mm plates are used in combination.

Nitriding these specimens in a molten salt bath, the salt bath of 37% by weight of sodium, potassium and lithium cyanates and carbonates and CNO ^- cyanate ions and about 10 ppm of S ² , as in French patent applications 2 171 993 and 2 271 307 ^It contains ions and the bath temperature is 570 ± 15 ℃ and the part immersion time is 90 minutes.

By weight composition, the sulfide layer is composed of about 87% iron ε nitride (Fe _2-3 N), about 10% τ 'nitride (Fe ₄ N), and the balance consists of iron oxide, iron sulfide, and iron oxysulfide. The hardness is 600 HV 0.1.

Structurally, the sulfonitized layer comprises an 8 μm thick deep sub-layer and a 7 μm porous surface sub-layer with a thickness of 15 μm, pore diameters ranging from 1 to 2.5 μm, and maximum pore densities from 1.5 to It is in the range of 2 m.

The parts are removed from the nitriding bath and soaked for 20 minutes in a salt bath at a temperature of 420 ± 15 ° C as in French Patent Application No. 2 525 637.

After the treatment, the nitride layer of the part consists of ε nitride containing 6% τ 'nitride and all of the oxysulfides are converted to magnetite iron oxide (Fe ₂ O ₄ ) with oxygen intercalated 2-3 microns from the surface.

The surface roughness is 0.6 μ CLA.

Corrosion resistance in standard salt mist was 50 to 60 hours for nitrided parts and 200 to 250 hours for oxidized parts after nitriding and the parts before treatment showed a flourishing corrosion after only a few hours.

In the friction test supporting the rectangular plate while linearly increasing the load from the initial value of 10 daN at a sliding speed of 0.55 m / s, the test time reached 30 minutes for the nitrided parts and the incremental wear of the two parts was 50 μm. The coefficient of friction was 0.40. For nitrided and oxidized parts, these values were changed to a test time of 45 minutes, incremental wear of 40 μm and a coefficient of friction of 0.30, respectively.

Impregnation of the oxidized parts after such nitriding was carried out by immersion in molten polyethylene wax at 150 ° C. for 2 minutes. The parts were removed from the molten wax bath and wiped with a clean, dry cloth.

The surface tension of the viscous phase was 32 mN / m and the solid phase, here the contact angle between the oxidized layer and the viscous phase after sulfonitride treatment was 41 °.

The corrosion resistance exceeded 2000h while the friction test could last 50 minutes with incremental wear only 25μm and coefficient of friction 0.18.

Experiments of the same kind have been carried out on nitrided parts by either a gaseous method in an ammonia atmosphere or an ionic method in a nitrogen atmosphere. A comparison result was obtained.

Likewise, the effect of carbon withdrawal is limited by performing a thermochemical nitrocarburizing treatment in a nitrogen / carbon medium such as in a salt bath or by a gas method, which in practice is limited to 3% in the surface diffusion layer.

It has been found that the oxidation following the thermochemical treatment can be carried out not only in the salt bath but also by a simple or ionized gas method.

The final coating can be carried out by immersing the part in a solvent containing the wax in the dissolved state rather than a bath of molten wax.

A series of embodiments will be described below applying the present invention to typical components representative of the actual industry.

Example 2

Nitriding is carried out on automotive locking parts of bent sheet metal which are complex, stamped and punched according to the conditions described in French patent applications 2 171 993 and 2 271 307.

After nitriding, the parts are oxidized according to the conditions described in French patent application No. 2 525 637. Wax impregnation is carried out using a sulfonate modified petroleum derived wax by immersing the part in the wax dissolved in a ratio of 70 g / L in white alcohol.

This controlled part meets all the requirements required by the automotive manufacturer, especially for at least 200 hours without white rust and at least 400 hours without red rust, and the treated parts are salt mist It is heated for 1 hour at a temperature of 120 ° C. before being exposed to.

It has been found that solutions commonly used to date for this type of application consisting of the application of zinc or zinc / nickel alloys followed by chrome treatment do not meet the above requirements.

At the same time, as the corrosion resistance of the parts improved, the locking device operated very smoothly after dozens of door openings and closings, no wear was observed, and some of the openings and closings were urgent enough to produce excessive stress and shock effects.

Example 3

Windscreen wiper shafts made of carbon steel that were supported by vibrating wipers under the same conditions as in Example 2 were treated.

The same requirements for corrosion resistance as in Example 1 were also met here and occurred with very satisfactory friction on the oil-impregnated sintered rings.

The current solution to plating electrolytic nickel has shown two drawbacks: insufficient corrosion and the presence of excess thick plating on sharp edges that is detrimental to the assembly of parts.

Example 4

Still in the same conditions as in the above example, a tapping screw was treated which was made of 20 CDV 5 steel and designed to drill wood 30 mm and then drill 6 mm by tapping the steel.

Compared to conventional carburizing / quenching / zinc electroplating, significant reductions in coefficient of friction and substantial improvement in wear resistance not only in the visible parts of the screw (ie the head and through ends) after assembly, but also in the screw contacting with wood and with steel A clear improvement in resistance to corrosion and corrosion was found.

Example 5

This embodiment relates to hinge pins such as those found in elevator tail gate or fork-lift trucks such as truck elevators made of 35 CD 4 steel.

Compared with the conventional solution of adopting heavy chromium zinc plating after quenching / sintering heat treatment, a significant improvement was found in the coefficient of friction and very good corrosion resistance in the dynamic operation of the hinge without the risk of hydrogen embrittlement of the part.

Example 6

The present invention applies to a game ball or bowl, especially the French game Betancque, also known as the Bosch ball or bowl. These balls or bowls are made of two hemispherical cups made of alloy steel 25 CD 4 welded along the diameter plane. After grinding to the numerical value, the heat treatment is performed to give a specific hardness of 110 daN / mm ² or more as a whole. Then, the same surface treatment is performed as in the embodiment of the foregoing 4.

According to experts, the balls or bowls thus obtained exhibit a set of features that are particularly popular with top athletes in formal championships or competitions: coefficient of friction and surface regularity. These have particularly good corrosion resistance, very low abrasion that must be met by regulation, which has an attractive appearance that can withstand impacts without damage and gloss black without losing weight by more than 15g less than the indicated weight. Means that.

For this kind of application it is clear that the final waxing can be continued by the player himself.

Claims (4)

The treated iron-based metal part is a carbonaceous organic compound having a molecular weight of 500 to 10,000 and a liquid state after the nitriding and oxidation step, with a dense deep sublayer and through pores having a thickness of 5 to 25 μm and a diameter of 0.2 to 3 μm. Characterized by having a layer of porous surface sub-layer impregnated with hydrophobic wax having a surface tension of 10 to 73 mN / m and a contact angle between the solid phase of the surface layer and the liquid wax in the range of 0 to 75 °. Iron-based metal parts. 2. The iron-based metal part according to claim 1, wherein the surface sub-layer contains at least 60% of a solid Fe _2-3 N, a hardness of 550 to 650 HV 0.1, and a range of roughness of 0.3 to 1.5 mu CLA. . The iron-based metal part according to claim 1, wherein the wax is selected from the group consisting of natural waxes, artificial waxes and petroleum residues. 4. The iron-based metal part according to claim 3, wherein the artificial wax is selected from the group consisting of fluorinated, polyethylene, polypropylene, and polyester wax.