ES2201405T3 - DIRECT PROTECTION PROCEDURE AGAINST CORROSION BY WEARING METAL PARTS. - 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

Direct protection procedure against wear corrosion of metal parts.

The present invention relates to a proper procedure to offer direct protection against wear corrosion of metal parts, particularly without produce polluting materials.

Procedures are generally known and treatments for corrosion protection of metal parts, such as the procedures described in previous patents FR 2,672,059, US 5,346,560 and GB 2,298,434.

In general, these systems provide the production of magnetite layers on steel that has to be protected. These known procedures use salt pickling with cyanides, cyanates and subsequent oxidation.

For example, French Patent No. 2,672,059 sample, as treatments or systems available and known for increase the corrosion resistance of metal parts, salt baths or environment with an ionized atmosphere oxidizer (ionic plasma environment), particularly in the Page 2 of the description.

In this way, it is not possible to make a barrier that can withstand wear and corrosion in a way particularly effective and resistant; above all, said barrier is limited in dimensions and geometry. In fact, the barrier is obtained bathing the metal parts that are going to be treated in bathrooms liquid salines containing carbonates - nitrides - hydroxides and oxygenated alkali metals (for the oxidation phase).

The morphology of the protective layers as well obtained is not compact enough due to the high level of porosity (up to 50%). Such porosity usually requires, to get perfect insulation, saturation with resins polymeric or waxes to close said porosity; otherwise they are present preferred paths that will initiate corrosion.

The thermochemical means above mentioned and used to date cannot make a thickness with a constant and defined chemical composition.

Said means of protection are particularly weak and ineffective for protection against corrosion in a aqueous environment containing salts, for protection during immersion in molten light alloy solutions, and said protection means have a very low wettability associated with the layers that have been subjected to hardening to through nitride precipitation.

In addition, the limits of these procedures and known treatments are important and are also related, in particular, to the dimensions of the elements of steel or products that are going to be treated.

In fact, the procedures above mentioned can only treat products with small dimensions and With simple geometry without cavities and without deep holes.

In addition the problems have to be highlighted and important environmental limits for storage and elimination of saline compounds with large emissions and toxic emanations that derive and develop from known procedures

The document JP-A-56058963 describes a surface treatment of a steel material that involves a nitrification step in NH3 at 450-600 ° C for 1-10 h, and a second stage performed in an oxidizing atmosphere in the presence of 450 superheated steam at 550 ° C.

The document JP-A-01079362 exposes a procedure to harden a surface, free of pollution, which includes the formation of a nitrated surface mild in non-oxidizing conditions, followed by a treatment of gas oxidation.

U.S. Patent No. 2,343,418 describes a method for obtaining propeller blades for aircraft, which comprises nitriding the external surfaces of the blades by heating, undergo a gas treatment and then treat the shovel with gas and air at a suitable stabilized temperature during long enough to produce an oxidized surface not Reflective on the nitride outside of the blade.

The document EP-A-0299625 describes a method for produce a corrosion and black wear resistant finish through the steps of forming an epsilon iron nitride or layer of carbonitride, raise the temperature of the component to the gas oxidation temperature and oxidize by gaseous medium to form a dense black coating and perform a treatment surface finish.

US Patent No. 4,496,410 describes a method to produce a non-alloy steel component resistant to corrosion comprising the steps of nitriding a component of non-alloy steel and produce there a nitride surface layer of epsilon iron and heat treat the component in an atmosphere oxidizer to provide a surface layer rich in oxide and then cool the component in an oil / water emulsion with the component at a temperature such that nitrogen is retained in solid solution in the ferric matrix of the microstructure of the steel.

The objective of the present invention is to define a procedure that can solve technical problems mentioned above.

Said objective, according to the present invention, is achieves a proper procedure to offer a direct protection against wear corrosion of parts metal as described by claim 1 in the following claims.

The characteristics and advantages of a proper procedure to offer direct protection against Corrosion of metal parts according to the present invention is you will understand better from the following description, which It provides as a non-limiting example.

In a proper procedure to offer a Direct corrosion protection of metal parts according to the present invention, a layer of pure magnetite will be made, as will be shown in details below, for at least first 3-5 µm on the surface of a metal part to be treated, such as on a tool steel or on hardened and tempered steel and on a steel with low alloy content as well as steel in lock.

It should be noted that the procedure does not require no particular preventive preparation of the pieces that are going to be treated, and also that the oxide layer thus obtained is produced without releasing or producing any contaminating residue or toxic.

The right procedure to offer a Direct corrosion protection of metal parts according to the The present invention is an innovative gaseous process. It allows to eliminate any dimensional and / or geometric constraint since convection contacts the reactants with the full surface of the piece to be treated to protection.

In particular, the invention allows the application of a layer of iron oxide, Fe 3 O 4, even in parts with a vertical length of up to 10 meters or with 2.5 meters of diameter.

Waste gases from the procedure are directed to a rear combustion chamber that releases only N 2 -H 2 -O 2 in gases Totally neutral to the environment.

The surface layer produced through the method of the invention has a diffusion area of nitrogen with a maximum depth of 0.1 mm. This diffusion it must have a nitrogen weight% less than 2%, being said different percentage depending on the type of steel of the piece that It will be treated. Nitrogen, during this phase, is diffused to through the orientation of the grain and fills the empty spaces or the empty areas formed by imperfections of the crystal lattice. How an alternative, nitrogen atoms are trapped in the interstitial spaces free of the surface of the piece that is going To be treated.

The temperature of this first phase of the procedure is between 480-505 ° C for hardened and hardened steel, and between 500-525ºC for tool steel.

The wt% nitrogen decreases while going towards the inner portion of the surface and it becomes almost zero at a depth above 0.1 mm. The duration of this First phase does not last more than 10 hours.

They change then both the values of temperature as the gas mixture, introducing an atmosphere oxidizing

During this second phase, the dissemination of nitrogen is completely stopped by the adverse action of oxygen in the oxidizing atmosphere against the atoms of surface iron.

Layers of Fe 3 O 4 are obtained with a 95-99% content, almost free of oxides FeO / Fe 2 O 3 having a higher oxygen content.

In fact, these last oxides could be particularly disadvantageous as anti-corrosion agents and protectors since they have a high growth rate and therefore they can break easily.

According to the invention, layers of iron oxide Fe 3 O 4 of a thickness of 2-4 at temperatures between 505 and 545 ° C, said layers They contain a weight% of oxygen between 25% and 30%.

A very important factor is the stability of the oxygen content mentioned above within the structure of steel and within the total thickness thus obtained.

The transition from the layer below is done to through a sudden decrease in the oxygen percentage of 25-30% to zero over a maximum depth of about 1 µm.

Therefore, the integration of a layer with a high level of chemical stability, which makes the compound stable in a steel matrix previously hardened by a diffusion of nitrogen of about 0.1 mm, makes available an effect of insulation and barrier of a protective film directly from inside the steel It's like having two different compounds integrated into each other and that exert an action mutually Consolidating In fact, steel supports and makes rust compact, while rust protects and insulates steel.

This allows a barrier to be arranged simultaneously chemical and high temperature oxidation resistance with characteristics of compactness, wear resistance, abrasion and glued (adhesive wear).

In fact, magnetite repellency towards All liquid solutions are generally produced by the compactness and compression level of the outer layers.

This compression is due to the difference between the network structure of the oxide and the iron cell (cubic network body centered).

If the relationship (between the stage of the network of iron and the oxide network stage) is less than 1, then the surface clearly compresses the lower layers; in this way, if the fragility of these layers is not so great to cause cracks and peels, we will get a mechanical closing action against any corrosive environment or solution for alloy or mixtures of light alloys (Al, Pb, bronze).

It should be noted that magnetite has the following more interesting features to ensure the environmental and / or metal corrosion protection fades:

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low conductivity electric that reduces the migration of active ionized agents which also increase the oxide layer with the consequent disintegration;

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high energy of union that requires a higher amount of heat to destabilize the surface E = KT that withstands temperatures up to 900 ° C. This allows the use of high temperatures, despite the fact that the use in situations where working temperatures they are tall is sometimes accompanied by degenerative phenomena like for example wear and abrasion;

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in this point also another characteristic of magnetite makes it interesting, that is, its compactness and chemical stability that the turns into a great hardness (850HV = 75 HRC); Y

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network structure Hexagonal additionally offers the possibility of a slip parallel of the atomic planes one over the other; in which there are planes with higher atomic compactness (Closed Package).

This invention has produced the realization of a magnetite layer in a gaseous environment, and said embodiment convert a 2-4 µm thick layer previously hardened through the diffusion of nitrogen in oxide.

The technical solutions and the great advantages they consist of having the possibility of protecting from the inside, Without the application of any film or coating, the product Steel without limits due to the dimensions and / or shape.

For example, the process of the invention is carried out through a systematic and continuous control of the procedure parameters based on the chemical analysis of a GDS quantum differential meter versus depth, associated with more sporadic controls with X-rays.

An additional parameter to consider is the relationship between thermal expansion coefficients linear of the oxide and the metal adjacent to it.

For example, the FeO / Fe value reaches 1.25 a temperatures up to 1,000 ° C and at a value of 1.03 for Fe_ {O} {{3} / Fe. The closer the value is to 1, the safer It is the coupling of metal oxide even under stresses thermal and stress as those present in die casting pressure and hot melt.

It has been previously noted that the medium thermochemical used so far has a chemical composition of the layer quite different from that defined by a% by weight of oxygen between 21 and 25% and constant throughout the first 4-5 µm as in the present invention.

The process of the invention produces this barrier that has a chemical identity with a composition constant and defined without changes in a transition composition With the base metal.

Using the method of the invention there is no porosity since the increase in gaseous agents is performed in a very diluted form so that highs do not originate local pressures In fact, high local pressures, by effect of the coalescence of the gaseous atom within the matrix host, would be greater than the final tensile stress of the steel and would therefore cause an opening to the Exterior.

The extension of the procedure above mentioned to parts of any type of dimensions (up to 11 m height) and shape is the reason for this patent application to cover the formation of a layer of magnetite (Fe 3 O 4) through gaseous agents, that is, using heat plants convective in which there is a slight overpressure of 25-30 mbar and steel is the catalyst for reaction.

Example 1

X38CrMoV5.1 steel samples have been treated by diffusion during a first phase of 10 hours at a temperature of 525 ° C.

Thus, a thickness of diffusion of N 2 of approximately 100 µm.

So, the first phase of dissemination of nitrogen has stopped and, during a second phase, it has introduced an atmosphere that contains oxygen, during 5-6 hours at a temperature of 545 ° C.

Thus, a thickness of Fe 3 O 4 of about 3 µm.

Therefore, a sample has been obtained in bars that have glue resistance characteristics Perfect and corrosion resistance characteristics towards Al liquid at 700 ° C.

If this bar has been treated with a method known, such as that of the French patent 2,672,059, said bar should have been subjected to a salt bath for 10 hours.

Then the bar should have been treated with an oxidizing bath A sample would have been obtained in bars with features of molten surface and phase formation Fe-Al intermetallic that have a fast action underlying matrix rupture.

The advantages according to the procedures of the present invention are evident since there is an increase important in surface protection against action combined wear-corrosion caused by Al alloys with suspended silicates.

In fact, oxidation according to the present invention does not compromise the previously acquired characteristics in terms of wear and seizure resistance; to the On the contrary, especially with reference to this last feature, the oxidation greatly improves friction and the coefficient of plasticity of hardened layers, thanks to the morphology of the hexagonal crystal net.

Claims (5)

1. Appropriate procedure to offer a direct protection against wear corrosion of parts metal, in which each metal part is subjected to nitriding with integrated and subsequent production of a layer of magnetite (Fe_ {O} {4}), wherein said metal part, during a first phase, It is treated in a gaseous environment with nitrogen diffused at a temperature between 480-525 ° C and for no more than 10 hours, until said metal part reaches% in N2 weight not greater than about 4% in a layer of diffusion of said metal part and, during a second phase, once it has been nitrogen feed stopped, the metal part is treated in a gaseous oxidizing environment at a temperature comprised between 505 and 545 ° C, so that a layer of magnetite is formed that is distributed on the surface of said metal part with a thickness of 3-5 µm and the transition gradient between the layer of Fe 3 O 4 (O 2 = 25-30%) and the diffusion layer of N_ {2} (O_ {2} \ cong about 0%) is \ leq 1 \ mum in order to guarantee the chemical identity of the two areas that have an anti-wear anti-corrosion action integrated and complementary. 2. Method according to claim 1, characterized in that said magnetite layer (Fe3O4) has an oxygen content between 25 and 30% by weight. 3. Method according to claim 1 or 2, characterized in that said weight percentage of N2 is about 4% for steel containing Cr or Al. 4. Method according to claim 1 or 2, characterized in that said weight percentage of N2 is about 2% for hardened and hardened steels and for low alloy steels. 5. Method according to any of claims 1 to 4, characterized in that it comprises a superficial mechanical activation and preliminary cleaning of metal parts in an advanced condition of use.