EP0524037A1 - Treatment process for iron components to improve simultaneously their corrosion resistance and their friction properties - Google Patents

Abstract

The components are nitrided, preferably in a bath of molten salts based on CNO<-> cyanate ions, are oxidised, preferably in baths of molten oxidising alkali metal salts, and are then impregnated with a hydrophobic wax. According to the process the nitriding followed by the oxidation results in the formation of a layer consisting of a compact deep underlayer and of a porous surface underlayer whose thickness is between 5 and 25 mu m and exhibits open porosities between 0.2 and 3 mu m in diameter. The impregnating wax is an organic compound of high molecular weight of between 500 and 10,000, with a surface tension of between 10 and 73 mN/m in the liquid state, the contact angle between the solid phase of the surface layer and the wax in the liquid state being between 0 and 75 degrees. Applications: treatment of precision components made of ferrous metal of complex shape, simultaneously subjected to severe stresses by friction and corrosion.

Description

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

In order to give ferrous metal parts both friction and corrosion resistance properties, in general, two successive separate treatments are carried out, a first treatment to give the parts the friction properties and a second treatment of surface to ensure protection against corrosion, the latter for example by depositing zinc followed by bichromatation.

The friction and corrosion resistance properties acquired by these treatments are often sufficient for conventional parts.

However, for certain applications where technical performance is increasingly required for parts, in particular for parts subjected to severe stresses involving simultaneously several phenomena (friction, in particular abrasive wear, shock, corrosion), properties conferred by conventional methods are insufficient.

This is the case, for example, of parts intended for locksmithing mechanisms, certain types of bolts and precision screws, maneuvering screws, articulation axes, rods of jacks or shock absorbers, playing balls . The industries concerned are in particular those of the automobile, public works, handling, capital goods, household appliances, hydraulic equipment.

It is known that ferrous metal parts can be imparted by nitriding then oxidation, good friction properties and good corrosion resistance.

To this end, processes are known for nitriding ferrous metal parts, in particular nitriding by baths of cyanates and molten carbonates as described in FR-A-2 171 993 and FR-A-2 271 307, or else nitriding in ionized nitrogen atmosphere, which improve the friction properties of these parts, reducing the coefficient of friction, and increasing resistance to wear and seizure.

It is also known that if an previously nitrided part is subjected to oxidation, changes are made to the nitrided surface and this improves the corrosion resistance of these parts, while retaining the friction properties acquired by nitriding. . A particularly effective oxidation treatment is described in document FR-A-2 525 637 for this purpose, in a bath of molten oxidizing salts. Comparable corrosion resistance results are obtained by an oxidation process in an ionized atmosphere of an oxygen-containing gas.

However, this type of process gives the nitrided and then oxidized parts friction properties and corrosion resistance which are insufficient for the abovementioned applications.

It has been found that these shortcomings can be overcome by completing the nitriding and then oxidation treatment by applying a final coating.

The present invention is also based on the observation that many parts are cleaned in the usual way, in particular before they are assembled, when it is well known that they do not justify such cleaning and for which it is therefore not necessary to carry out a surface treatment resistant to conventional cleaning products.

The present invention provides a method of treating ferrous metal parts to simultaneously improve their corrosion resistance and their friction properties, method in which the parts are nitrided, oxidized and receive a final coating, characterized in that the nitriding followed by the oxidation leads to the formation of a layer consisting of a compact deep sublayer and a porous surface sublayer, said surface sublayer having a thickness of between 5 and 25 μm and having through porosities of diameter between 0.2 and 3 μm, and in that the nitrided and then oxidized parts are impregnated with a hydrophobic wax, said wax being a carbon-containing organic compound with high molecular weight of between 500 and 10,000, surface tension in the liquid state between 10 and 73 mN / m, the contact angle between the solid phase of the surface layer and the wax in the liquid state being between 0 and 75 degrees.

According to other characteristics, the surface sublayer contains more than 60% of solid phase Fe2-3N, has a hardness of between 550 and 650 HV 0.1 and a roughness of between 0.3 and 1.5 µ CLA

The impregnation wax is chosen from the list: natural waxes, or else polyethylene, polypropylene, polyesters, fluorinated waxes or else modified petroleum residues.

The method according to the invention has the advantages of being inexpensive and easy to implement even for industrial productions on series parts, of conferring on the treated parts high technical performance even when these are of complex shapes.

In addition, the protective effects of a surface treated according to this process with respect to aging by friction, by abrasive wear, by impact and by wet corrosion are multiplied in astonishing proportions compared to the commonly used solutions and make it possible to resist at length to these forms of degradation. These results will be developed further in examples.

The composition of the nitrided layer, its thickness and its hardness are adjusted so that it resists wear, without however being fragile, because then it would flake under the effect of shocks. The compact hexagonal structure of the Fe2-3N phase of the iron / nitrogen equilibrium digram provides good deformation capacity, thanks to a high atomic density on the sliding plane and is therefore particularly favorable for friction applications.

For the combination of surface roughness, molecular weight and surface tension of the wax, contact angle between solid phase and liquid phase, the intervals indicated are those which lead to maximum effectiveness of the impregnation of the wax, which cannot under these conditions be eliminated only under extreme conditions, therefore unusual and in any case exceptional for the intended applications.

Experiments have also shown that the improvement in corrosion resistance is not the only fact of the wax, because the latter applied to another surface, even reputed to favor the bonding of organic products, only leads to a much lower resistance to corrosion.

Conversely, the absence of wax decreases the effectiveness of nitriding, both from the point of view of friction properties and of impact resistance, the wax helping to minimize the rebound effects.

In a particularly advantageous arrangement of the invention, the nitriding is carried out in a bath of molten salts according to FR-A-2 171 993, consisting essentially of carbonates and cyanates of alkali metals K, Na and Li, the anion CO₃²⁻ being present for 1 to 35% by weight and the CNO⁻ anion for 35 to 65% by weight, while, in the total weight of the alkali cations, the weight proportions are 25-42.6% for Na⁺, 42, 6-62.5% for K⁺ and 11.3-17.1% for Li⁺.

More preferably, the nitriding salt bath further comprises a sulfur species in an amount such that the weight content of elemental sulfur is between 0.001 and 1%, according to FR-A-2 271 307.

It is thus possible to control with good precision the formation of the saturated surface sublayer consecutive to the diffusion of the hetero-element and to favor the appearance in significant proportion, greater than 60%, of the phase Fe2-3N of the diagram d 'iron / nitrogen balance.

The oxidation treatment makes it possible, in addition to already improving the corrosion resistance in itself, but also when it is correctly conducted, to adjust the surface properties of the solid phase, so as to lead to maximum effectiveness of the impregnation of the wax.

The inventors have good reason to believe that the presence in the combination layer of a highly electron donor or acceptor element capable of locally creating electric dipoles is highly rewarding; in this case, chemical bonding forces are created which locally reinforce the capillary forces. To this end, preference will be given to the presence of sulfur as well as oxygen in the surface sublayer.

According to preferred arrangements, the oxidation will be carried out in a bath of molten salts according to FR-A-2 525 637, generally at temperatures between 350 ° and 450 ° C.

The characteristics and other advantages of the invention will emerge more clearly from the description which follows, illustrated by experimental examples.

EXAMPLE 1

Test specimens were used, consisting of pairs comprising a ring with a diameter of 35 mm and a plate with dimensions of 30 x 18 x 8 mm, made of XC 38 steel.

These samples were nitrided in a molten salt bath, made of sodium, potassium and lithium carbonates and cyanates according to FR-A-2 171 993 and FR-A-2 271 307, with 37% by weight of cyanate ions CNO ⁻ And approximately 10 ppm of S²ions ions, the bath temperature being 570 ± 15 ° C and the immersion time of the parts being 90 min.

The sulfonitrided layer comprises, in composition by weight, about 87% of iron nitride ε (Fe2-3N), about 10% of nitride τ ′ (Fe₄N), the rest being iron oxides, sulfides and oxysulfides. Its hardness is 600 HV 0.1.

Structurally, the sulfonitrided layer has a thickness of 15 μm with a compact deep sub-layer of 8 μm thickness and a porous surface sub-layer of 7 μm, the diameter of the pores being between 1 and 2.5 μm, with a maximum pore density in the range 1.5 - 2 µm.

On leaving the nitriding bath, the parts are immersed for 20 min in a salt bath according to FR-A-2 525 637, at a temperature of 420 ± 15 ° C.

After this treatment, the nitrided layer of the parts comprises nitride ε with 6% of nitride t ′, while all the oxysulfurized compounds have been transformed into iron oxide magnetite (Fe₂O₄), with oxygen being inserted into the 2 to First 3 surface micrometers.

The surface roughness is then 0.6 µCLA.

The corrosion resistance in standard salt spray is established at 50-60 hours for the nitrided parts, and 200-250 hours for the nitrided then oxidized parts, while the parts before treatment exhibit generalized corrosion after only a few hours.

In friction test, where a rotating ring presses on a parallelepiped plate with a linearly increasing load from the initial value of 10 daN and with a sliding speed of 0.55 m / s, the duration of the test reaches 30 min for the parts nitrided, with cumulative wear of the two parts of 50 µm and a coefficient of friction of 0.40. With the nitrided, then oxidized parts, these figures pass respectively to 45 min of test duration, 40 µm of cumulative wear and 0.30 of coefficient of friction.

On the parts thus nitrided then oxidized, an impregnation is carried out, by immersing them for 2 min in polyethylene wax melted at the temperature of 150 ° C. On leaving the melted wax bath, the pieces are wiped with a clean, dry cloth.

The surface tension of the viscous phase is 32 mN / m and the contact angle between solid phase, here the sulfonitrided then oxidized layer and viscous phase is 41 degrees.

The corrosion resistance then exceeds 2,000 hours, while the friction test can continue for 50 minutes, for a cumulative wear of only 25 µm and a coefficient of friction of 0.18.

Similar experiments have been carried out on parts which have been nitrided either by gaseous route. ammonia atmosphere, either ionically in a nitrogen atmosphere: comparable results are obtained.

Likewise by carrying out a thermochemical nitrocarburization treatment in a nitrided-carbon medium, for example in a salt bath or alternatively by gas, subject however to limiting the embrittling effect of carbon, which in practice limits its rate to 3%. the surface diffusion layer.

It was also possible to verify that the oxidation subsequent to the thermochemical treatment can be carried out, in addition to a salt bath, by a simple or ionized gas route.

Finally, the final coating can be carried out by immersion of the parts, no longer in a bath of molten wax, but in a solvent containing the said wax in the dissolved state.

A series of examples of application of the invention to prototypes of parts representative of industrial reality will now be found below.

EXAMPLE 2

Nitride parts of automotive locksmith of complex shape, in stamped sheet metal, punched, folded, according to the conditions taught by FR-A-2 171 993 and FR-A-2 171 993 and FR-A-2 271 307. Au leaving the nitriding treatment, the parts have undergone an oxidation treatment under the conditions taught by FR-A-2 525 637. Finally, an impregnation treatment was carried out with a wax of the type derived from sulphonate modified petroleum, by immersing the pieces in said wax dissolved at a rate of 70 g / liter in white spirit.

Thus conditioned, the parts meet all of the specifications required by automobile manufacturers, and in particular a resistance to salt spray corrosion of at least 200 hours without "white rust" and at least 400 hours without "red rust", parts which have undergone, before exposure to salt spray, a 1 hour oven drying at a temperature of 120 ° C.

It will be noted that the solution commonly used to date for this type of application and which is a deposit of zinc or else of zinc-nickel alloy followed by bichromatation, does not satisfy the preceding specifications.

At the same time as the corrosion resistance of the parts is improved, there is a very smooth operation of the locks and an absence of wear, even after several dozen opening-closing operation of the doors, some of which are carried out with sufficient abruptness to generate effects of over-stresses and shocks.

EXAMPLE 3

Under the same conditions as in Example 2, the axes of the wipers, supports of the oscillating brushes, in carbon steel, were treated.

The same corrosion resistance specifications as in Example 1 are also observed here, and the friction in the face of sintered rings impregnated with oil takes place under very good conditions.

The reference solution used to date and which was an electrolytic nickel deposit, had two drawbacks: insufficient corrosion resistance and the existence of excess thicknesses of deposit on the sharp edges, harmful to the mounting of the parts.

EXAMPLE 4

Still under the same conditions as in the previous examples, 20 CDV 5 steel self-tapping screws were treated, intended to make holes in 30 mm of wood followed by 6 mm of steel with self-tapping of steel. .

Compared to the traditional case hardening + quenching + galvanic zinc coating, there is a significant reduction in the coefficient of friction, a significant improvement in wear resistance and a marked increase in corrosion resistance, not only visible parts of the screw after assembly, that is to say the head and the through end, but also threaded parts in contact with wood or steel.

EXAMPLE 5

It relates to axes of articulation such as can be encountered on tail lifts such as freight elevators for trucks, or for steel forklifts 35 CD 4.

Compared to the traditional solution using a heat-quenching treatment followed by a bichromated electrolytic zinc coating, there is a significant improvement in the coefficient of friction and very good resistance to corrosion in dynamic operation of the joint, this without risk of embrittlement of parts by hydrogen.

EXAMPLE 6

The method according to the invention was applied to playing balls, in particular for pétanque and Provençal games. These balls are formed of two hemispherical caps made of alloy steel type 25 CD 4, welded along a diametrical plane. After calibration correction, they are heat treated to present in the mass a specified hardness, greater than or equal to 110 daN / mm². Finally, they undergo the same surface conditioning as in the four previous examples.

Thus produced, the balls have a set of characteristics, friction coefficients and regularity of surface which, in the opinion of specialists, are particularly appreciated by high level players, participating in official tournaments and competitions and in particular: very good resistance to corrosion, very low wear rate guaranteeing compliance with the regulations which stipulate that the weight loss due to play must not exceed 15g below the marked weight, ability to withstand shock without damage, pleasant aesthetics in bright black color .

It will be appreciated that in this type of application, the final polish can be maintained by the player himself.

Claims (11)

Process for treating ferrous metal parts to simultaneously improve their corrosion resistance and their friction properties, process in which the parts are nitrided, oxidized and receive a final coating, characterized in that nitriding followed by oxidation leads to the formation of a layer consisting of a compact deep sublayer and a porous surface sublayer, said surface sublayer having a thickness of between 5 and 25 μm and having through porosities of diameter between 0.2 and 3 μm, and in that the nitrided and then oxidized parts are impregnated with a hydrophobic wax, said wax being a high molecular organic carbon compound of between 500 and 10,000, of surface tension in the liquid state between 10 and 73 mN / m, the contact angle between the solid phase of the surface layer and the wax in the liquid state being between 0 and 75 degrees. Process according to claim 1, characterized in that the surface sublayer contains more than 60% of solid phase Fe2-3N, has a hardness of between 550 and 650 HV 0.1 and a roughness of between 0.3 and 1, 5 µ CLA. Process according to Claim 1 or 2, characterized in that the nitriding is carried out according to a process chosen from nitriding in a bath of molten salts based on CNO⁻ cyanate ions and nitriding in an ionized nitrogen atmosphere. Process according to Claim 3, characterized in that the nitriding is carried out in a bath of molten salts consisting essentially of carbonates and cyanates of alkali metals K, Na and Li, the anion CO₃²⁻ being present for 1 to 35% by weight and the CNO⁻ anion for 35 to 65% by weight, while in the total weight of the alkali cations, the weight proportions are 25-42.6% for Na⁺, 42.6-62.5% for K⁺ and 11, 3-17.1% for Li⁺. Process according to claim 4 or 3, characterized in that the nitriding is carried out in the presence of a quantity of carbon compounds determined so as to obtain a carbon content in the surface sublayer of less than 3%. Process according to any one of the preceding claims, characterized in that a chemical sulfurization reaction is carried out simultaneously with the nitriding, by addition of a sulfur species in the nitriding medium. Process according to any one of the preceding claims, characterized in that the oxidation is carried out according to a process chosen from a process in baths of molten oxidizing alkaline salts, by simple gas route and by ionized gas route. Process according to Claim 7, characterized in that the oxidation is carried out in a bath of molten oxidizing alkaline salts at temperatures between 350 ° C and 450 ° C. Method according to claim 1, characterized in that the impregnation of wax is carried out by immersion of the parts, according to a method chosen from immersion in molten wax and immersion in a solution of wax dissolved in a solvent. Method according to claim 1, characterized in that the wax is chosen from natural waxes, synthetic waxes and modified petroleum residues. Process according to claim 7, characterized in that the synthetic waxes are chosen from fluorinated waxes, polyethylenes, polypropylenes, polyesters.