RU2590738C1 - Method of increasing resistance of steel pipelines against corrosion by aluminizing - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to methods of increasing resistance to corrosion and can be used in underground pipeline transport. Method of aluminising steel pipe includes cyclic heating of surface of steel pipe by pulses of electromagnetic radiation in aluminium melt above point Ac₃ with further cooling below point Ar₁ at heating and cooling rate of not less than 1 K/s, wherein heating is performed to temperature not exceeding 1,220 ± 10 K and cooling to temperature not below 820±10 K, and duration of holding at heating and cooling at extreme temperatures is determined by required depth of penetration of aluminium and its distribution uniformity in steel. Heating of surface of steel pipe is performed in protective atmosphere for depth of penetration of aluminium in steel, and specified cyclic heating in aluminium melt with further cooling is carried out not more than three times. To prevent oxidation molten aluminium is under a layer of molten cryolite. During aluminising, pressure in pipe is kept at 0.5-0.75 of working pressure generated during its operation.

EFFECT: higher efficiency and resistance of steel to high-temperature corrosion.

1 cl, 3 dwg

Description

The invention relates to methods for increasing the resistance of steel to corrosion (including high temperature) and can be used in underground pipeline transport and for protecting pipes of heating surfaces.

A known method of oxidizing steel products, including treatment with water vapor, cooling to 500 ° C in an environment of superheated steam, and then in air, characterized in that in order to increase the corrosion resistance of the coating, its continuity and ductility, as well as the intensification of the process, the oxidation is carried out thermally dissociated water vapor for 0.1-1.0 minutes (AS USSR No. 1070211, C23C 7/04).

The disadvantages of the method are:

1) the need to obtain dissociated water vapor with a temperature of from 2000 to 3050 ° C, which requires the use of an expensive plasmatron;

2) the relatively low resistance of the formed magnetite film Fe ₃ O ₄ , which under the influence of cathodic protection undergoes electrochemical reduction to iron metahydroxide III [2FeO (OH)] and iron hydroxide II [Fe (OH) ₂ ], which takes no more than 5 years , although magnetite is characterized by reduced electrochemical activity.

A known method of electrolysis cementation mainly for products from aluminum and titanium alloys, including exposure to molten alkali metal carbonates at a saturation temperature and a given current density at the cathode, characterized in that in order to increase the corrosion resistance of products by producing metal carbides on the surface, the product at 500-600 ° C and a current density at the cathode of 0.1-1.4 A / m ² in the melt of a mixture of lithium, sodium and potassium carbonates (AS USSR No. 975828, C23C 9/10).

Among the disadvantages of the method should be noted:

1) the proposed process does not provide for cementation of steel, which is the main metal for the production of pipelines;

2) the use of a melt of alkali metal carbonates as an electrolyte limits the possibilities of the method and complicates the processing of large-sized products;

3) the use of molten electrolyte at a temperature of 500-600 ° C imposes requirements for sealing the electrolysis bath in order to exclude possible emissions of toxic fumes of lithium salts.

Closest to the proposed invention relates to: a method of alloying steel with aluminum, comprising at least three cycles of heating steel parts by packs of pulses of electromagnetic radiation in a saturating medium above the Ac ₃ point, followed by cooling below the Ar ₁ point, at a heating and cooling rate of at least 1 K / s , characterized in that the heating is carried out to a temperature not higher than 1220 ± 10 K and cooling to a temperature not lower than 820 ± 10 K, the exposure time during heating and cooling at extreme temperatures is determined necessary depth of penetration of aluminum and the uniformity of its distribution in steel, and to increase the saturation rate, the processing is carried out in an aluminum melt, to prevent oxidation of which the melt surface is covered with a thin layer of coke or charcoal (RF patent No. 2431696, C23C 10/22).

The method is also not without drawbacks:

1) during operation of a pipeline operating under pressure, cracking of a highly corrosion-resistant thin film of aluminum oxide on the surface of the applied protective aluminum coating on the pipe surface is possible, which will not completely eliminate the corrosion processes.

2) a layer of coke or charcoal poorly protects the aluminum melt from oxidation, which reduces the quality of the alitization process.

The objective of the invention is to prevent corrosion processes on the outer surfaces of underground cathode-protected pipelines and pipes of high-temperature heating surfaces by applying a corrosion-resistant coating to their surface.

The task is achieved by a method of increasing the resistance of steel pipelines to corrosion by alitization, which includes cyclic heating of steel by pulses of electromagnetic radiation pulses in a saturating medium above the Ac ₃ point, followed by cooling below the Ar ₁ point at a heating and cooling rate of at least 1 K / s, when heating is performed to temperature not higher than 1220 ± 10 K and cooling to a temperature not lower than 820 ± 10 K, the exposure time during heating and cooling at extreme temperatures is determined by the required penetration depth aluminum and its uniform distribution in steel, and to increase the saturation rate, the treatment is carried out in an aluminum melt, and the surface of the steel pipe is heated in a protective atmosphere to the depth of aluminum penetration into the steel, the number of thermal cycling cycles does not exceed three; to prevent oxidation, the aluminum melt is under a layer of cryolite melt, and in the process of alitizing, a pressure of 0.5-0.75 of the working pressure created during its operation is maintained in the pipe

New significant features:

1) the steel surface is aluminized, maintaining a pressure in the pipe of 0.5-0.75 from the worker created during its operation in order to prevent cracking of a thin highly corrosion-resistant alumina film on the surface of an aluminum-saturated pipe during its operation;

2) heating the surface of the steel in a protective atmosphere to a diffusion depth of aluminum prevents the formation of iron oxides on the surface of the steel and provides a given thickness of the diffusion layer;

3) heating the steel surface to a temperature of no higher than 1220 ± 10 K and its subsequent intensive cooling after processing with aluminum melt to a temperature of at least 820 ± 10 K with a speed of at least 1 K / s ensures the implementation of phase α-Fe - γ-Fe and γ -Fe - α-Fe transitions in the shortest possible time and high productivity of the technological process due to the accelerated transport of aluminum atoms in steel and the intensification of diffusion processes;

4) conducting up to three cycles of heating the steel, processing it with molten aluminum and cooling provides high productivity of the process at a given concentration of aluminum atoms in steel at a given depth;

5) the presence of a cryolite melt on the surface of molten aluminum prevents the formation of aluminum oxide on the surface of the melt and ensures a high quality of steel aluminizing process;

6) a dense and durable alumina film protects against corrosion, including from intergranular, stess corrosion and high-temperature corrosion of aluminized steel, increasing the heat resistance of pipes of heating surfaces;

7) intermetallic compounds formed in the process of alitizing on the surface of the steel ensure tight adhesion of the aluminum layer to the steel and eliminates delamination of the coating;

8) the contact of the heated steel with the molten aluminum removes traces of iron oxides from the alitized surface and ensures a high quality process;

9) obtaining steel structures, the surface of which is saturated with aluminum to the required depth by thermal cycling, provides high performance of the process.

The above new essential features, together with the known ones, provide a technical result in all cases to which the requested amount of legal protection applies.

Obtaining the technical result of the invention is achieved using the unique properties of alalitized steels.

In most cases, in reinforced aluminum structures, the connection between the bearing steel liner and aluminum is purely mechanical. It is carried out due to the compressive force of the hardening aluminum (when the steel liner is filled with aluminum melt), or due to the pressing force (when pressing the steel liner into the manufactured part). To increase the adhesion strength on the steel bar, various protrusions, depressions, grooves are performed or the surface roughness of the steel part is increased by blasting, knurling or notching. However, in all cases, there are oxide films and an air gap at the interface between two alloys (aluminum and iron-carbon), which sharply reduce the quality of the coating.

The difficulty in obtaining a dense bimetallic contact lies in the high affinity of aluminum with oxygen and in the significant strength of aluminum oxide A1 ₂ O ₃ . The aluminum melt is located under a layer of aluminum oxide, which prevents the wetting of steel, and the rapid hardening of the boundary layer does not make it possible to remove it with oxide-dissolving fluxes and other methods.

A more efficient way to obtain bimetallic structures by aluminizing steels, as a result of which a transition layer is formed - a diffusion zone, which creates good contact of steel with aluminum melt.

Several aliasing methods are known. Alithization by immersion of steel in aluminum melt at a temperature of 820 ... 1220 K with subsequent exposure for 1 ... 15 min, at which the optimum thickness of the diffusion zone equal to 0.02 ... 0.03 mm, is considered to be the most appropriate. The main advantages of this method are simplicity, speed of the process and low cost. But to obtain a diffusion layer 0.35 mm thick, an exposure of τ = 60 minutes is required.

At the same time, during thermal cycling of steel in the temperature range (820 ... 1220) K during phase α-Fe - γ-Fe and γ-Fe - α-Fe transitions in limited volumes, the metal, as a result of the crystal lattice rearrangement, is in a pseudo-liquid state during heating and cooling, moving, as a result of phase transfer, deep into the metal, carrying with it all impurities, including alloying atoms to the core of the part. During the "crystallization" of the pseudo-melt, iron is first crystallized, and the impurity is carried by the pseudo-liquid volume of iron deep into the part, as in zone melting. The process of polymorphic transformation facilitates the penetration of alloying atoms into the pseudo-liquid volume of the metal and ensures the intensification of the process. The differential equation of mass transfer of an atom of introduction of stationary and dynamic processes of chemical-thermal treatment of steel has the form:

where: (∂m / ∂t) is the amount (mass) of the substance that passed through the site S during time t (mass transfer);

∂ ² Ф / (∂x∂t), ∂ ² T / (∂x∂t) - rates of change of the magnetic flux gradients and temperature - terms describing the accelerated electromagnetic and phase transfers of dopant atoms during polymorphic phase α-Fe - γ- Fe and γ-Fe - α-Fe transformations in the optimal temperature range during thermal cycling;

(∂c / ∂x); (∂P / ∂x); (∂T / ∂x); (∂φ / ∂x); (∂V _M / ∂x) are the gradients of concentration, pressure, temperature, potential and volume changes, which respectively describe the actual diffusion mass transfer according to the first Fick law, baric transfer along intergranular and intracrystalline spaces, which ensures the penetration of alloying atoms into steel - according to Orlov, heat transfer in accordance with the Fick-Nernst equation, Fromm and Gebhart transfer and vacancy transfer according to Mechev;

D is the diffusion coefficient of hydrogen in the metal;

M is the weight of grams - molecules of a diffusing substance;

A, B, C, Y, X - integral coefficients.

The diffusion coefficient of the alloying impurity during thermal cycling is no longer determined by the restrictions imposed on diffusion processes in a solid metal, but by the propagation velocity of a pseudo-liquid phase transformation wave in the volume of steel, which depends primarily on the rate of change of the magnetic flux gradients and temperature ∂ ² Ф / (∂x∂t ), ∂ ² T / (∂x∂t) are the terms of Eq. (1) that describe the accelerated electromagnetic and phase transfers of dopant atoms during polymorphic phase α-Fe - γ-Fe and γ-Fe - α-Fe transformations at optimal temperatures Nominal range for thermal cycling.

The accelerated transport of the alloying impurity to the metal is facilitated by the fact that during the phase transition, limited volumes of the steel surface are in a pseudo-liquid state and when the type of lattice is changed, the adsorbed impurity is transferred into the volume of steel. When heated to temperatures above 1100 K - above the GOSE line (Fig. 1), the α-Fe - γ-Fe phase transition ends and the inlet sections in the intercryllite, interblock and interfragment cavities increase, facilitating the penetration of the ligature melt into it and its transport into the metal and in metal.

Phase transformations occur not instantly throughout the volume, but gradually (depending on the degree of overheating or supercooling of the steel). A pseudo-fluid wave of polymorphic phase transformation during heating (above the GOSE line) and during cooling (below the PS line) moves with finite speed from the surface of the part from the heating source (or from the refrigerator during cooling) to its core (Fig. 1). Using the L-shaped diagram (Fig. 2) of the formation of austenite upon heating and the C-shaped diagram of the isothermal transformation of supercooled austenite (Fig. 3), it is possible to determine the exposure time of the aluminized parts at extreme temperatures to obtain a given diffusion layer thickness.

When alitizing by immersion, physicochemical phenomena occurring at a high speed, which can completely disrupt the process, acquire particular importance. Such phenomena include the formation of oxide films on the surfaces of solid and liquid metals, which interfere with the appearance of bonds between iron and aluminum atoms.

To prevent the formation of oxide films when heating steel, a protective atmosphere is used; the use of coating fluxes by introducing cryolite into a bath with molten aluminum leads to the dissolution of the film of aluminum oxide located on the surface of the bath:

The heated steel surface with traces of iron oxide is restored by liquid aluminum (the first stage of self-propagating high-temperature synthesis):

The resulting reduced iron reacts with aluminum to form the Fe _X Al _Y intermetallic compound to produce heat (the second stage of self-propagating high-temperature synthesis). The final product of chemical reactions is Fe _X Al _Y intermetallic - a transition layer (diffusion zone) between aluminum and iron with a layer thickness of 0.02-0.03 mm in a single heating-cooling cycle. Obtaining bimetallic aluminum-iron structures with diffusion bonding promotes the formation of a transition layer in the form of an intermetallic chemical compound Fe _X Al _Y under continuous process conditions.

The intense aluminothermic reaction that cleans the surface of the steel before subsequent alimony ensures high purity of the surface of the structure. The combination of the SHS process and aluminothermy during the processing of steel with aluminum melt automatically leads to the formation of a transition zone. Therefore, the basic principles of this technology can be used in various industries for the manufacture of bimetal steel-aluminum structures.

The use of this technology for producing aluminum-steel bimetallic structures with diffusion bonding formed by metallothermy and the SHS process, which has a low transient electrical resistance and high adhesion strength of steel to aluminum, ensures the design is monolithic. The diffusion layer stabilizes the electrical contact between aluminum and the steel surface, reducing the electrical resistance last and increasing the operational characteristics of bimetallic compounds.

Using the diagram of the formation of austenite during heating (Fig. 2) and the diagram of isothermal transformation of supercooled austenite (Fig. 3), it is possible to determine the holding time of aluminized parts at extreme temperatures in order to obtain a given layer thickness. If Al transport must be carried out only in a thin surface layer of the sample cross section, then temporary exposure at a temperature of 820 and 1220 K is not carried out. If it is necessary to transport Al into metal by 2-3 [mm], then the best option would be heating to a temperature of 1050 K (the upper limit of the temperature corridor) with holding at this temperature for 4 ... 5 minutes, since in this case the complete conversion of ferrite to austenite can only happen in about 16 minutes. Then, during the exposure, a pseudo-liquid polymorphic transformation wave that carries out phase transfer and carries Al will pass only a quarter of its path to the core of a part with a diameter of 20 mm, and in 4 minutes the isothermal formation of austenite will end only in the surface 2-mm layer.

From the C-shaped curve of the isothermal transformation diagram of supercooled austenite, we determine that at a temperature of 950 K the time of complete polymorphic conversion of γ-Fe to α-Fe is about 16 minutes. Then, to obtain a 2-mm diffusion layer of Al, exposure at this temperature (lower boundary of the thermal cycling temperature corridor) is required for 4 ... 5 min.

In accordance with the foregoing, a method for saturation of steel with aluminum by the pulsed method was developed and tested in laboratory conditions. Steel samples of low carbon steel in airtight containers with molten aluminum were subjected to pulsed electromagnetic fields. The upper limit of the temperature range is 1170 K, the lower limit is 820 K. The exposure at extreme temperatures was at least 5 minutes. Duration and number of cycles varied. The total processing time in any of the experiments did not exceed one and a half hours. Penetration of Al into steel was carried out over the entire cross section of the sample for 7 cycles.

The experiment confirms the possibility of obtaining steel structures, the surface of which is saturated with aluminum to the required depth, which ensures the resistance of steel to high temperature corrosion and increases the heat resistance of steel

Long (since 2009) exposure of the aluminized steel sample in water did not lead to its corrosion. The sample remains light, the surface gleams dimly, while the unaltered steel sample is covered with a “fringe” of rust, which also confirms the specified corrosion properties, including resistance to intergranular and stress corrosion, the cause of which is hydrogenation of the metal, from which aluminized steel protects thin and durable aluminum oxide film and inter-block, inter-fragment and inter-crystalline spaces “filled” with aluminum on the surface of steel, the penetration of atomic hydrogen into which yatstvuet filling their aluminum.

Claims (1)

A method of aluminizing a steel pipe, including cyclic heating of the surface of a steel pipe with bursts of pulses of electromagnetic radiation in an aluminum melt above the Ac ₃ point, followed by cooling below the Ar ₁ point at a heating and cooling rate of at least 1 K / s, while heating to a temperature of not higher than 1220 ± 10 K and cooling to a temperature not lower than 820 ± 10 K, and the exposure time during heating and cooling at extreme temperatures is determined by the necessary penetration depth of aluminum and its uniform distribution in tal, characterized in that the heating of the surface of the steel pipe is carried out in a protective atmosphere to the depth of penetration of aluminum into the steel, and the aforementioned cyclic heating in the aluminum melt with subsequent cooling is carried out no more than three times, while to prevent oxidation, the aluminum melt is under the cryolite melt layer, and in the process of alitizing in the pipe maintain a pressure of 0.5-0.75 of the working pressure created during its operation.

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