ITMI980131A1 - PROCEDURE TO PROVIDE DIRECT PROTECTION AGAINST WEAR CORROSION TO METAL PIECES - Google Patents

Abstract

In a process suitable to give a direct protection against the corrosion of metallic pieces, a nitrogen diffusion area is realised on the outer layer with a maximum depth of 0.1 mm, and with a nitrogen weight % lower than 2-4%, depending on the steel to he treated. The nitrogen is diffused into the piece and fills the imperfections and the empty areas of the crystal lattice. The temperature at which this first phase takes place goes from 480 to 525 DEG C depending from the steel type and the maximum duration is of 10 hours. Then, both the temperature values and the gas mixture, by introducing an oxidising atmosphere, are changed and the nitrogen diffusion is fully stopped by the oxygen action of the oxidising atmosphere against the iron atoms of the surface. Layers of Fe3O4 at 95-99% content are obtained, practically FeO/Fe2O3 oxides free, which are formed at temperatures comprised between 505 and 545 DEG C and which are comprised between 2 and 4 mu m and containing an oxygen weight % between 25% and 30%. A strong barrier and insulation effect is thereby obtained directly within the steel.

Description

DESCRIPTION of the industrial invention

The present invention relates to a process for conferring direct protection against wear corrosion on metal parts, in particular without producing polluting materials.

Processes and treatments are known for providing protection against corrosion on metal parts in general, such as for example the processes described in the prior patents FR 2 672 059, US 5,346,560 and GB 2298 434.

In general, these systems involve the production of layers of magnetite on the steel to be protected. These known processes use immersion salt baths in the presence of cyanides, cyanates and post oxidation.

For example, the French patent 2 672 059 reports salt baths or environments with an oxidizing ionized atmosphere (ionic-plasma environments) as known and available systems or treatments for increasing the corrosion resistance of metal parts, in particular on page 2 of the description.

In this way it is not possible to create a barrier that resists wear and corrosion that is particularly effective and resistant, above all it is limited in size and geometry. The barrier is in fact obtained by resorting to immersion of the metal parts to be treated in molten salt baths containing carbonates - nitrides - hydroxides and oxygenated alkali metals (for oxidation phase).

The morphology of the protective layers thus obtained is however of poor compactness due to the high presence of pores (up to 50%). For a perfect insulation, this presence of pores currently requires saturation with polymeric resins or waxes that are able to close such porosities; otherwise there are preferential routes that determine the initiation of corrosion.

The thermochemical means, cited above and used up to now, are not capable of achieving a thickness with a defined and constant chemical composition.

Such protections are particularly weak and not very effective in protecting against corrosion, especially in aqueous environments containing salt, from immersion in solutions of fused light alloys and have a very low wettability combined with layers jointly subjected to hardening by precipitation of nitrides.

Furthermore, the limits of these known processes and treatments are considerable and also concern, in particular, the dimensions of the steel elements or products to be treated. In fact, through the aforementioned processes it is possible to process only products with reduced dimensions and simple geometry without cavities or with deep holes.

Furthermore, the relevant environmental limits and problems that arise in terms of storage and disposal of saline compounds with high toxic emissions and exhalations that derive and develop from known processes must be pointed out.

The object of the present invention is that of identifying a process which allows to solve the aforementioned technical problems.

This object according to the present invention is achieved by providing a process for conferring direct protection against wear corrosion on metal parts as set forth in claim 1 in the subsequent claims.

The characteristics and advantages of a process for conferring direct protection against corrosion to metal parts according to the present invention will become more evident from the following, exemplary and non-limiting description.

In a process for conferring direct protection against corrosion to metal pieces according to the present invention, as will be seen in detail below, a layer of pure magnetite is made for at least the first 3-5 µm on the surface of a metal piece to be treat, for example on tool or tempering steel and on low alloy steels as well as on sheet metal.

It must be emphasized that the process does not require any particular prior preparation of the pieces to be treated and furthermore that the oxide layer that is produced is produced without the release or production of any toxic or polluting residue.

The process for conferring direct protection against corrosion to metal pieces according to the present invention is a process which in a new way is of the gaseous type. And this fact allows to eliminate any geometric and / or dimensional restriction since the convection brings the reactant species into contact with the whole surface of the piece to be treated interested in the protection.

In particular, the invention allows the application of a layer of Fe oxide, Fe3O4, even on parts with a length of up to 10 meters vertically or with a diameter of 2.5 meters.

The residual gases of the process are conveyed to an afterburner which releases only N2 - H2 -02 in completely neutral gases into the environment.

The surface layer which is produced with the process of the invention realizes a nitrogen diffusion zone for a maximum depth of 0.1 mm. This diffusion must have a percentage by weight of nitrogen of less than 2%, which is different according to the steel of the treated piece. Nitrogen at this stage diffuses across the grain boundary and occupies the vacant or vacant areas represented by crystal lattice imperfections. Alternatively, an entrapment of the nitrogen atoms in the free interstitial spaces of the surface of the piece to be treated is obtained.

The temperature at which this first phase of the procedure takes place varies between 480 - 505 ° C for quenched and tempered steels, and between 500 - 525 ° C for tool steels.

The concentration by weight of nitrogen then gradually decreases towards the inside of the surface to almost vanish at depths greater than 0.1 mm. The duration of this first phase is a maximum of 10 hours.

Subsequently, both the composition of the gaseous mixture, with the introduction of an oxidizing atmosphere, and the temperature values changed.

In this second phase, the nitrogen diffusion is completely interrupted by the antagonistic action of the oxygen of the oxidizing atmosphere towards the iron atoms of the surface.

95 - 99% Fe3O4 layers are obtained, which are therefore practically free from FeO / Fe2O3 oxides with a higher oxygen concentration. In fact, the latter oxides would be particularly inconvenient in a protective and anti-corrosion role as they are oxides with a high growth rate and therefore easy to cleave.

According to the invention, thicknesses of iron oxide Fe304 comprised between 2 and 4 μm are formed at temperatures between 505 and 545 ° C, containing percentages by weight of oxygen varying between 25% and 30%.

A factor of great importance is the stability of the above mentioned concentrations of oxygen inside the steel and inside the entire thickness formed.

The transition from the underlying layer is generated through a sharp drop in the percentage of oxygen from 25 - 30% to zero over a maximum amplitude of about 1 μm.

Therefore, the integration of a layer with a strong chemical stability, which makes it a stable compound in a steel matrix previously hardened by a nitrogen diffusion for about 0.1 not, makes available the barrier and insulation effect of a film. protection exercised directly from the inside of the steel.

In practice, it is like having two different compounds integrated into each other exercising a mutual consolidation action. In fact, there is steel on the oxide, making it compact and supported, as well as oxide on the steel protecting and insulating it.

This makes it possible to combine a chemical barrier with resistance to oxidation at high temperatures with characteristics of compactness, resistance to wear, abrasion and gluing (adhesive wear).

In fact, the repellence of magnetite to all liquid solutions in general is given to it by the compactness and degree of compression of the more superficial layers.

This compression derives from the difference in lattice steps of the oxide with respect to the iron cell (body-centered cubic). If the ratio (between the iron reticular pitch and the oxide reticular pitch) is less than 1, the surface clearly tends to compress the lower layers; in this way, if the fragility of these layers is not so high as to crack and peel, we have a mechanical closing action towards any corrosive external environment or alloying solution or light alloy mixtures (Al, Pb, brass) themselves.

It must be emphasized that magnetite has the following most interesting characteristics for the purposes of protection from environmental corrosion and / or from molten metals:

low electrical conductivity which reduces the migration of ionized active species which lead to the further growth of the oxide layer with consequent crumbling;

- high bonding energy that requires a greater amount of heat to destabilize the surface E = KT resists up to temperatures of 900 ° C. This ensures that use at high temperatures is allowed, despite the fact that uses where the operating temperature is high are often accompanied by degenerative phenomena such as wear and abrasion; - at this point another characteristic of magnetite also becomes interesting, such as its compactness and chemical stability which results in a high hardness (850HV = 75 HRC);

the hexagonal reticular structure further confers the possibility of a parallel sliding of the atomic planes one on the other; where there are plans with greater atomic compactness (Closed Packet).

What this invention produced was the realization of a layer of magnetite in a gaseous environment which leads to the conversion into oxide of a thickness of 2-4 μm previously hardened by nitrogen diffusion.

The technical solutions and the great advantages are to be able to protect the steel product from the inside, without the application of film or cover, without limits of size and / or shape.

For example, the process of the invention is generated through a progressive and systematic control of the process parameters as a function of the chemical analysis of a differential quantometer GDS towards the depth combined with more sporadic X-ray checks.

A further parameter to be taken into account is the ratio between the linear thermal expansion coefficients of the oxide and the metal adjacent to it. For example, a FeO / Fe value of 1.25 is passed up to temperatures of 1000 ° C at a Fe203 / Fe value of 1.03.

The closer to 1 this value is, the safer the oxide - metal combination will be even in the presence of thermal stresses and tensions such as those used for die casting and hot forging.

It has previously been pointed out that the thermochemical means used up to now have a thickness with a chemical composition that is very different from that defined by the percentage by weight of oxygen between 21 and 25% and constant on the first 4 - 5 μm as in the present invention.

The process of the invention produces this chemical identity barrier of defined and constant composition without encroaching on the transition composition with the base metal.

With the process of the invention there are no porosities since the enrichment in gaseous species occurs in a very diluted way so as to never determine high localized pressures. In fact, the high localized pressures, due to the coalescence of gaseous atoms inside the host matrix, would be such as to overcome the breaking load of the steel and therefore produce an opening towards the outside.

The extension of the above to details of any size (up to 11 meters in height) and shape is a clear condition for which the patenting of the formation of a layer of Fe304 magnetite in gaseous way is requested, i.e. using convective heating systems inside of which there is a slight overpressure of 25 - 30 mbar and the steel acts as a reaction catalyst.

EXAMPLE 1

We treated X38CrMoV 5.1 steel samples in a first phase by diffusion for a time of 10 hours at a temperature of 525 ° C.

In this way, a diffusion thickness of N2 for about 100 μm was achieved.

Then, we suspended the first phase of nitrogen diffusion and introduced in a second phase an atmosphere containing oxygen for a time of 5-6 hours at a temperature of 545.

In this way, a thickness of Fe3O4 of about 3 μm was achieved.

A bar sample was thus obtained having characteristics of perfect bonding resistance and consequently liquid Al corrosion at 700 ° C. If this bar had been treated with a known method, such as that according to French patent 2672 059, the bar would have been subjected to a salt bath for a time of 10 hours.

Then the bar would be treated with an oxidizing bath. A bar sample would have been obtained having melting characteristics of the surface with the formation of Fe-Al intermetallic phases with a fast disintegrating action of the underlying matrix.

The advantages obtained with the process according to the present invention are therefore evident since there is a net increase in the preservation of the surface towards the combined wear-corrosion action caused by Al alloys with suspended silicates.

In fact, the oxidation according to the present invention does not compromise the previously acquired characteristics in terms of resistance to wear and seizure; on the contrary, especially in reference to this latter property, it considerably improves the coefficient of friction and plasticity of hard layers thanks to the morphology of the hexagonal crystal lattice.

Claims (9)

CLAIMS 1. Process for conferring direct protection against wear corrosion to metal pieces in which each metal piece is subjected to nitriding with subsequent and integrated formation of magnetite (Fe3O4), characterized in that said metal piece is initially treated in a gaseous environment with nitrogen diffused at a temperature between 480 - 525 ° C and for a duration of 10 hours until reaching percentages by weight of N2 not exceeding about 4% in a diffusion layer of said metal piece and in a second phase, once the nitrogen supply is suspended, it is treated in an oxidizing gaseous environment at a temperature between 505 - 545 ° C. 2. Process according to claim 1, characterized in that said percentage by weight of N2 is around 4% for steels containing Al or Cr. 3. Process according to claim 1, characterized in that said percentage by weight of N2 is around 2% for low-alloyed and quenched and tempered steels. 4. Process according to claim 1, characterized in that a layer of magnetite (Fe3O4 distributed on the surface of said metal piece with a thickness of 3 - 5 μm is formed, in which the transition gradient between Fe3O4 (O2 = 25-30% ) and diffusion of N2 (O2 = 0%) is included on thicknesses of ≤ 1 μm so as to guarantee the chemical identity of the two zones with complementary and integrated anti-corrosion anti-wear action. 5. Process according to claim 1, characterized in that it provides a primary diffusion blanket with a thickness of between 0.05 and 0.1 mm by using atmospheres with controlled nitriding potential with values of this between 3 and 5. 6. Process according to claim 1, characterized in that the percentage by weight of O2 in the first 3-5 μm is stably settled between 25 and 30% with the remainder represented by iron. 7. Process according to claim 1, characterized in that the protective action of Fe304 can be easily restored whenever it is necessary to reintegrate the surface exposed in operation to corrosion and wear. 8. Process according to claim 1, characterized by the fact that the integrated magnetite - diffusion action of N2 can also be applied to parts already in advanced operation after a surface mechanical cleaning and activation. 9. Process for conferring direct protection against wear corrosion to metal parts substantially as previously described.