RU2581323C1 - High-temperature alloy - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, particularly compositions of heat-resistant alloys used for manufacture reservoirs and reaction pipes in oil-and-gas processing plants with working conditions at temperatures from plus 700 °C to plus 980 °C, and pressures up to 46 atm. Alloy contains following component ratio, wt.%: carbon 0.08-0.14, chromium 19.0-21.0, nickel 31.0-34.0, niobium 0.90-1.35, silicon 0.0005-0.79, manganese 0.5005-1.21, vanadium 0.0005-0.20, titanium 0.0005-0.10, aluminium 0.0005-0.10, yttrium > 0-0.001, oxygen >0.0005-0.028, hydrogen > 0.0005-0.0025, nitrogen > 0.0005-0.095, sulfur ≤ 0.03, phosphorus ≤ 0.03, lead ≤ 0.009, tin ≤ 0.009, arsenic ≤ 0.009, zinc ≤ 0.009, antimony ≤ 0.009, molybdenum ≤ 0.5, copper ≤ 0.2, iron - the rest. For alloy components following condition applies, wt.%: (Cr_Eq/Ni_Eq)≥0.585, where Cr_Eq is chromium equivalent, Ni_Eq is nickel equivalent, Cr_Eq = Cr+2×Al+3×Ti+V+Mo+1.6×Si+0.6×Nb, Ni_Eq = Ni+32×C+0.6×Mn+22×N+Cu.

EFFECT: higher structural stability of alloy during ageing, as well as reduced tendency to hot cracks during welding are provided.

2 cl

Description

The invention relates to metallurgy, in particular, to compositions of heat-resistant low-carbon chromium-nickel alloys of the austenitic class, and can be used in the manufacture of manifold pipes (reaction tubes) of high-temperature plants for the production of hydrogen, methanol, ammonia, etc., with operating conditions at temperatures from plus 700 ° C to plus 980 ° C and pressure up to 46 atm. In justified cases, products from these alloys (pipes, sheets, forgings, and other metal products) can be used in petrochemical and other equipment as a corrosion-resistant material.

Known heat-resistant alloy KHR32C of the Japanese company KUBOTA, having the following composition: C - 0.11 ÷ 0.16%; Si - 1.2% (max), Μn - 0.5% (max); Cr - 19.0 ÷ 20.5%; Ni - 33.5 ÷ 35.0%; Nb - 0.6 ÷ 1.6% (KUBOTA HEAT RESISTANT ALLOYS, Catalog, 1997).

Closest to the claimed technical essence and the achieved result is a heat-resistant alloy described in the patent of the Russian Federation No. 2149210, class. C22C 30/00, C22C 38/50, publ. 05/20/2002, bull. No. 14 and containing, by weight. %: carbon 0.08 ÷ 0.14; chrome 19.0 ÷ 21.0; nickel 31.0 ÷ 34.0; niobium 0.90 ÷ 1.35; iron - the rest, additionally contains, wt. %: silicon 0.0005 ÷ 0.79; manganese 0.5005 ÷ 1.21; vanadium 0.0005 ÷ 0.20; titanium 0.0005 ÷ 0.10; aluminum 0.0005 ÷ 0.10. The heat-resistant alloy may additionally contain molybdenum and copper, with the mandatory simultaneous fulfillment of two conditions:% Ni + 32 ×% C + 0.6 ×% Mn +% Cu = 34.96984 ÷ 39.406%; % Cr + 3 ×% Ti +% V +% Mo + 1.6 ×% Si + 0.6 ×% Nb = 19.5433 ÷ 23.16786%. The heat-resistant alloy may contain phosphorus, lead, tin, arsenic, zinc, molybdenum and copper in amounts not exceeding the following values, wt. %: sulfur - 0.03; phosphorus - 0.03; lead - 0.01; tin - 0.01; arsenic - 0.01; zinc - 0.01; molybdenum - 0.5; copper - 0.2. The nature of the structure of the cast structure of the known alloys: relatively large grains of austenite (from 0.1 to 0.25 mm), with significant different grain sizes (the ratio of the maximum grain area to austenite to minimum is from 2 to 7).

Known heat-resistant low-carbon chromium-nickel alloys are smelted in induction furnaces with a main lining.

Manifold pipes (reaction tubes) for oil and gas processing plants are usually made by welding tube blanks of chromium-nickel alloys obtained by centrifugal casting (ASTM [American Society for Testing and Materials], A608, Cen-trifugally Cast iron-chromium-nickel High Alloy Tubing for pressure application at high temperatures). It is possible to produce pipes from the indicated low-carbon heat-resistant alloy not only by the centrifugal cast method, but also by pressing, rolling, and other methods of plastic deformation. Centrifugally cast billets are usually pre-machined on the inner surface to remove defects of metallurgical origin (Yoshikazu Kuriyama, Yasuhisa Yamazaki, Iwao Kawashima, IHI, Engineering Review, 3, No. 5, September, 1970) and then welded to obtain the desired reaction tube lengths. Heat-resistant pipes from the considered chromium-nickel alloys can be obtained, in particular, by centrifugal casting.

Collector pipes (complete reaction tubes) are usually used in tubular reforming and cracking furnaces of hydrogen, ammonia, methanol and other oil refining, chemical and petrochemical plants. In justified cases, these reaction tubes can be used in heat exchange equipment, etc. as a corrosion-resistant material.

Given the need for welding centrifugally cast billets to obtain the reaction tubes of the required length, the latter must have high-quality welded joints.

The service life of reaction tubes welded from centrifugally cast tube billets of known alloys in oil and gas refineries operating at temperatures of 700 ÷ 980 ° C and pressure up to 46 atm is from 20,000 to 65,000 hours, after which they must be replaced, because After such a long period of operation, their strength under operating conditions (temperature, pressure, composition of the medium) decreases sharply, which can lead to emergency destruction of the reaction (welded) pipe and failure of the entire installation with unpredictable consequences.

Damage to reaction tubes (obtained by welding centrifugally cast billets) in tubular furnaces for the production of hydrogen, methanol, ammonia, etc., in particular, occurs due to the combined effect of thermal stresses in the weld zone (due to temperature differences on the outer and inner walls of the pipe) and stresses in the weld zone arising due to the high pressure of the process gas inside the pipe. The total stresses cause creep, which mainly (in the initial stage) is manifested in the volume of the heat-resistant alloy near the inner surface of the pipes in the welded zone.

It is known that the main and common feature of welding centrifugally cast billets of chromium-nickel alloys of the austenitic class is the tendency to form hot cracks in the heat-affected welded zone, which have an intergranular character. Thus, the weldability of chromium-nickel steels and alloys of the austenitic class is an important characteristic that determines the quality of these materials.

Hot cracks occur in the weld and in the heat-affected zone (HAZ) of the base material (austenitic chromium-nickel alloy), when the deformations that develop when the weld is cooled or externally exceeded the ductility of the metal in a specific weld zone. They range in size from very small (less than 1 mm) segregation cracks in the HAZ to large crystallization cracks that can extend over the entire length of the welded joints.

Nickel-chromium steels and austenitic alloys are considered to be weldable (having weldability) to a specified extent in these processes and for this purpose, when metal integrity is achieved by welding with the appropriate technological process, so that the welded centrifugally cast pipe billets meet the technical requirements both in terms of their own qualities and and in relation to their influence on the design of the reaction tube that they form (GOST 29273-92 Weldability. Definition).

Hot cracks are related to intergranular fractures and are divided into crystallization and segregation ones. Liquidation hot cracks are formed during the melting of liquids in the heat affected zone (HAZ) of the base metal (austenitic chromium-nickel alloy). The probability of crystallization cracks is determined by the nature of the change in the ductility of nickel-chromium alloys upon their deformation in the solid-liquid state. Crystallization cracks - hot cracks formed during crystallization of the liquid phase of the weld metal.

Ensuring the purity of alloys by impurities, which contribute to the formation of low-melting phases during crystallization, can increase the resistance to the formation of crystallization hot cracks of austenitic chromium-nickel alloys. These measures narrow the temperature range of brittleness and increase the margin of ductility.

To increase the resistance of austenitic chromium-nickel alloys to the formation of segregation hot cracks, it is necessary to alloy them with elements that reduce the diffusion mobility of atoms in the lattice or contribute to the creation of a fragmentary cast structure (curvature of crystallite boundaries, the formation of dispersed second phases and precipitates during crystallization during subsequent cooling), as well as ensuring purity of chromium-nickel alloys based on interstitial impurities. In addition, grinding of the metal structure (while reducing the different grain size) with the introduction of certain alloying elements can serve to increase the resistance to hot cracks.

The following most important metallurgical factors have been identified that contribute to increasing the resistance of the metal to hot cracking during welding of nickel-chromium austenitic steels and alloys: limiting the content of impurities forming low-melting phases in order to narrow the effective crystallization interval.

A useful measure can also be the limitation in the base and deposited metals of the content of harmful and liquor impurities, as well as dissolved gases.

Gases have a peculiar effect on the structure of chromium-nickel steels. The effect of gases, as well as alloying impurities, on the resistance to the formation of hot cracks is associated with their effect on the primary structure of the base metal and welds.

Nitrogen acts both as an austenitizer and as an inoculator (modifier, additive), grinding the cast structure of the base metal of a centrifugally cast billet, as well as the structure of welds.

Hydrogen does not significantly affect the primary microstructure of the base metal and the structure of the weld. Hydrogen alone does not cause hot cracks, but is able to enhance crack formation caused by other elements. In this case, hydrogen, with its excess in the weld metal, diffuses into the cracks that formed for some reason. With decreasing temperature, hydrogen atoms associate into molecules, the internal gas pressure in the cracks increases sharply, and microscopic cracks turn into macroscopic ones.

Oxygen in a relatively small concentration prevents the formation of a crushed structure (unlike nitrogen). With an increase in oxygen content, it acts similarly to nitrogen. Oxygen, oxidizing ferrite-forming impurities, promotes austenization. Nitrogen, dissolving in γ-iron, directly changes the structure of the base metal and the weld.

It is also known that with the mutual influence of hydrogen and nitrogen, the porosity of welds decreases (Shirshov I.G., Kotikov V.N. Plasma cutting.- L.: Mechanical engineering. Leningrad. Department, 1987. - 192 p. Silt.) .

If the amounts of nitrogen and oxygen are approximately equal, the base metal and the weld become disoriented.

The composition of gases (oxygen, hydrogen, nitrogen) in chromium-nickel heat-resistant steels and alloys of the austenitic class is mainly determined by their amount in charge materials that are involved in smelting, as well as their solubility in specific alloys. To increase the oxygen, hydrogen and nitrogen content in the composition of the heat-resistant alloy, special technological methods were also used, which constitute an independent “know-how” and are not described in this material. Exceeding the maximum values of the gas content in heat-resistant alloys can be controlled by well-known measures, for example, degassing, as well as using a mixture containing the minimum amount of gas dissolved in it during smelting.

Reducing the gas content in the heat-resistant alloys of the austenitic class contributes to the purging of the molten metal with argon. At the same time, each argon bubble represents a “vacuum chamber”, since the partial pressures of hydrogen and nitrogen in such a bubble are equal to zero, therefore, gases dissolved in the metal pass into the bubble and are removed to the atmosphere with it. In addition, when purging with argon there is intense mixing of the metal, averaging its composition. The oxygen content can be reduced by 2–3 times, hydrogen by 10–70% and nitrogen by 10–60%.

It is known that the harmful effects of hydrogen and nitrogen can be reduced by introducing into the metal impurities that bind hydrogen and nitrogen to strong hydrides and nitrides, for example, by introducing rare-earth metals. The introduced rare-earth metals (REM), for example, yttrium, partially interact with oxygen, which is part of the previously formed inclusions, and partially with sulfur, forming sulfides. The melting point of REM sulfides is about 2000 ° C, and they, like REM oxides, are not located at the edges of the grain during crystallization, but they themselves are crystallization centers and are located in the center of the grain. Thus, their negative impact on the properties of steel is minimized. It is also possible the formation of rare-earth oxysulfides of type Y ₂ O ₂ S (V. Kudrin. Theory and technology of steel production: Textbook for high schools. - M.: Mir, Publishing House AST, 2003. - 528 p., Ill. )

It is known that the harmful effects of arsenic, zinc, lead, antimony, and tin affect the heat resistance of alloys not only in the range of operating temperatures, but also at higher temperatures.

Sulfur with its large content leads to a sharp decrease in heat resistance, forming fusible eutectics with iron, nickel and cobalt. Sulfur is particularly detrimental in high temperature nickel alloys or high nickel steels. To reduce the harmful effects of sulfur in the finished alloys, rare earth elements (for example, yttrium) are added to bind sulfur to more refractory compounds.

In addition, sulfur has a detrimental effect on the weldability of austenitic chromium-nickel steels and alloys, causes microcracking associated with hot brittleness (Heat-resistant steels and alloys. Khimushin F.F. Metallurgy Publishing House, 1969, 2nd ed., 752 s )

It is known that microalloying with rare-earth elements (for example, yttrium) of austenitic steels (alloys) in a certain ratio improves their structural stability at operating temperatures and, as a result, increases heat resistance.

Microalloying with yttrium leads to a decrease in the diffusion mobility of hydrogen atoms in the crystal lattice of austenitic steel (alloy), and also contributes to an increase in the work of nucleation and the development of dislocation and intergrain brittle cracks under operating conditions.

At the same time, yttrium microalloying increases the corrosion resistance of steels (alloys) due to grinding of austenite grain, refining of metal and increase of surface resistance against general corrosion and stress corrosion cracking (Plant Laboratory. Diagnostics of materials. No. 2, 2010. Volume 76. Investigation of cavitation-corrosion resistance of directed steel 06Kh23N18M5 microalloyed with yttrium - A.V. Patyupkin, A.S. Rudychev, E.V. Fedoseev, A.G. Aleksandrov).

The improvement of the service characteristics of yttrium alloy steels (alloys) is due to a decrease in the size of austenitic grains and stabilization of their dispersion, because during crystallization of the metal, yttrium inhibits the growth of austenitic grain.

For the manufacture of reaction (welded) pipes, centrifugally cast billets of chromium-nickel steels and austenitic alloys should be used with tight control on the content of these impurities. When smelting in induction furnaces, special attention should be paid to the use of only pure charge materials. For retrofitting, the use of used metal and strongly oxidized components is not permissible.

Almost all chromium-nickel steels and austenitic alloys have a potential tendency to form hot cracks in all types of fusion welding.

There are numerous methods and criteria for assessing the quality of chromium-nickel steels and austenitic alloys with respect to the formation of hot cracks during welding (processes occurring at high temperatures and mechanical stresses).

In particular, the calculation and statistical method for assessing the resistance (previously investigated) of nickel chromium steels and alloys against the formation of hot cracks. This method is based on the use of parametric equations compiled using regression analysis. The parametric equations include ferrite-forming (Cr, Ti, Mo, Si, Nb) and austenite-forming chemical elements (Ni, С, Μn, Ν, Cu), where Cr, Ti, Mo, Si, Nb, Ni, C, Μn, N , Cu - designations of elements according to the periodic system of chemical elements (table of D.I. Mendeleev).

The dependence of the resistance to hot cracking of chromium-nickel steels and alloys of the austenitic class on the ratio of equivalents of chromium and nickel is also known: (Cr _E / Ni _E ), where Cr _E is the equivalent of chromium; Ni _E is the equivalent of nickel (Welding and materials to be welded: In 3 volumes of T.1. Weldability of materials. Reference ed. / Ed. By E.L. Makarov. - M., Metallurgy, 1991, p. 528). The parametric equation in the form of the equivalent of chromium: Cr _E = f (Cr, Ti, Mo, Si, Nb) is the dependence of Cr _E on ferrite-forming elements. The parametric equation in the form of a nickel equivalent: Ni _E = f (Ni, C, Mn, Ν, Cu) is the dependence of Ni _E on austenite-forming elements. The greater the ratio (Cr _E / Ni _E), the higher the resistance of nickel-chromium alloys austenitic hot cracking.

In order to increase the resistance of austenitic chromium-nickel alloys to the formation of hot cracks during welding, in particular, it is necessary to minimize the sulfur and phosphorus content (sulfur can form fusible compounds, in particular with nickel), and prevent delamination. To grind the structure of austenitic steels and alloys - they should be alloyed with small additives of rare-earth elements to grind the structure and stabilize it.

It is known that creep is caused by the displacement of certain groups of atoms in the structure of the alloy. These groups of atoms in the process of flowing through the metal mass accumulate at the grain boundaries, which leads to the appearance of voids, which subsequently line up and subsequently lead to microcracks. This negative effect is enhanced when low-melting eutectics are placed at the grain boundaries of austenite. In addition, the negative effect is amplified in alloys with a coarse-grained structure and with a structure with a large variation in the size of the authentic grain (high grain size).

The formation of defects in the structure of a heat-resistant alloy (reaction tubes) from the formation of voids to the occurrence of microcracks up to the appearance of a through fistula is considered to be as three-stage as the creep process itself.

So, with primary creep, which is fixed immediately with the start of operation of the reaction tubes, in the process of hardening of the metal (directly during high-temperature operation under the internal pressure of the tubes), the strain rate decreases. In this case, a slowdown in the movement of trace elements in the metal structure occurs. At this stage of creep, the onset of micropore formation occurs at the boundary of grains and phases.

With secondary (stable) creep, in fact, during normal aging of heat-resistant alloy (at calculated values of temperature and internal pressure in the reaction tubes), an increase in the diameter of the tubes with a constant, extremely slow speed is recorded. At the end of this creep stage, micropores grow and combine.

Tertiary creep is characterized by a high rate of deformation and the union of microcracks into cracks larger than the size of austenitic grains. In this case, defects (in the form of voids turning into cracks) in the structure of a heat-resistant alloy increase, which is equivalent to an increase in the load on defect-free pipe sections. An increasing strain rate ultimately leads to the destruction of the reaction tube from a heat-resistant chromium-nickel alloy of the austenitic class.

To increase the operability of reaction tubes made of heat-resistant chromium-nickel alloys, it is extremely important to determine the moment of termination of secondary creep, as well as to postpone the process of tertiary creep, in which voids at the grain boundaries grow up to the formation of cracks in the metal structure.

It is known that the formation of carbides in the microstructure of a heat-resistant alloy leads to creep resistance of the alloy. Carbides can be divided into two types: primary carbides, which are formed in the process of solidification in the form of a fine mesh at the boundaries of austenitic grains, and secondary carbides (which are formed even at high temperature loading of reaction tubes from heat-resistant alloys), which precipitate in the form of finely dispersed tubes during operation particles not along the boundaries, but in the austenitic grains of the heat-resistant alloy themselves (aging process). Each finely dispersed secondary carbide particle at the microstructure level acts as an obstacle that prevents the shifts characteristic of creeping deformations.

It is thanks to secondary carbides (finely dispersed particles) that creep resistance of heat-resistant chromium-nickel austenitic alloys is ensured. In the creep process, these secondary carbides do not allow metal grains to move, which prevents damage to the structure.

Moreover, if a slow increase in the size of secondary finely dispersed particles occurs during aging of the chromium-nickel alloy, the alloy will have structural stability at elevated temperatures. It is known that alloys with structural stability are less likely to form hot cracks during welding.

One of the possible reasons for the insufficiently high heat resistance of reaction tubes and the decrease in the quality of reaction tubes welded from centrifugally cast billets of known heat-resistant chromium-nickel alloys of the austenitic class is the formation of hot cracks during welding.

The main technical result achieved by the implementation of the claimed invention is to increase the structural stability of the alloy during aging (during long-term operation under load at high temperatures), as well as to reduce the tendency of the alloy to form hot cracks during welding.

The specified technical result is achieved due to the fact that the heat-resistant alloy containing carbon, chromium, nickel, niobium, silicon, manganese, vanadium, titanium, aluminum, sulfur, phosphorus, lead, tin, arsenic, zinc, molybdenum, copper and iron, characterized the fact that it additionally contains yttrium, antimony, oxygen, hydrogen and nitrogen in the following ratio of components, wt. %: carbon 0.08 ÷ 0.14; chrome 19.0 ÷ 21.0; nickel 31.0 ÷ 34.0; niobium 0.90 ÷ 1.35; silicon 0.0005 ÷ 0.79; manganese 0.5005 ÷ 1.21; vanadium 0.0005 ÷ 0.20; titanium 0.0005 ÷ 0.10; aluminum 0.0005 ÷ 0.10; yttrium> 0 ÷ 0.001; oxygen> 0.0005 ÷ 0.028; hydrogen> 0.0005 ÷ 0.0025; nitrogen> 0.0005 ÷ 0.095; sulfur ≤0.03; phosphorus ≤0.03; lead ≤0.009; tin ≤0.009; arsenic ≤0.009; zinc ≤0.009; antimony ≤0.009; molybdenum ≤0.5; iron - the rest; copper ≤0.2 under the following conditions, wt. %: (Cr _E / Ni _E ) ≥0.585, where Cr _E is the equivalent of chromium; Ni _E is the equivalent of nickel; Cr _e = Cr + 2 × Al + 3 × Ti + V + Mo + 1.6 × Si + 0.6 × Nb; Ni _E = Ni + 32C + 0.6 × Mn + 22 × N + Cu, and for the contents of sulfur and phosphorus, the condition (S + P) ≤0.025 is fulfilled.

, где $$r_{t.1}^{т}$$

- средний размер мелкодиспергированных частиц вторичных карбидов, состаренных в течение t.1 часов эксплуатации при температуре Т; $$r_{t.2}^{т}$$

- средний размер мелкодиспергированных частиц вторичных карбидов, состаренных в течение t.2 часов эксплуатации при той же температуре Т. В идеальном случае в процессе эксплуатации под нагрузкой при температуре Т в течение (t.2-t.1) часов отношение $$r_{t.1}^{т}/r_{t.2}^{т}=1$$

, т.е сплав обладает абсолютной структурной стабильностью.The structural stability of the alloy during long-term operation under load at high temperatures was evaluated by the value of r, which is defined as the ratio $$r_{t\mathrm{.one}}^t/r_{t.2}^t$$

where $$r_{t\mathrm{.one}}^t$$

- the average size of finely dispersed particles of secondary carbides aged over t.1 hours of operation at temperature T; $$r_{t.2}^t$$

- the average size of finely dispersed particles of secondary carbides aged over t.2 hours of operation at the same temperature T. In the ideal case, during operation under load at temperature T for (t.2-t.1) hours, the ratio $$r_{t\mathrm{.one}}^t/r_{t.2}^t=\mathrm{one}$$

, i.e., the alloy has absolute structural stability.

The tendency of the alloy to form hot cracks during welding was evaluated visually (using magnifying devices), after applying a load to the sample with a weld weld.

The claimed low-carbon chromium-nickel heat-resistant alloy is purely austenitic and is smelted only in induction furnaces with a main or neutral lining (acid lining leads to decarburization of the alloy) and using pure charge materials (primary charge). Waste, trimmings and other contaminated materials are not used in the smelting of the claimed alloy. The specifics of heating and melting of metal in induction furnaces without the formation of an electric arc (in contrast to electric arc furnaces) does not require slag guidance on the surface of a liquid metal with the transfer of a number of impurities to the induced slag and its subsequent removal. In addition, the application of the high-frequency heating principle in an induction furnace provides intensive mixing of alloy components during the smelting process, which further reduces the negative impact of segregation processes. Melting in an induction furnace occurs in a lined inductor. Thus, the heat-resistant alloy is protected from any contamination. The heat-resistant alloy, when melted in induction furnaces, is protected from saturation by fuel combustion products (as opposed to melting in open-hearth and other furnaces), from the carburizing effect of electrodes (as opposed to melting in electric arc furnaces) and from saturation with gases (hydrogen, oxygen and nitrogen) ) from the surrounding atmosphere (in contrast to melting in electric arc furnaces in the presence of an ultrahigh-temperature electric arc).

An induction furnace is a kind of transformer in which the metal charge to be melted is a secondary winding, and the primary winding of the transformer is formed by an inductor coil through which an alternating current of high frequency (over 1000 Hz) flows. The current induced in the metal charge heats it until it melts. This circumstance makes it possible (unlike other melting methods) to easily control the temperature of the molten metal in an induction furnace.

Products (reaction tubes) based on the declared heat-resistant alloy were obtained from centrifugally cast billets or castings made by pouring molten heat-resistant alloy into a rotating chill (for a centrifugally cast pipe) with an inner diameter equal to the outer diameter of the pipe obtained, or by pouring the molten heat-resistant alloy into a specially prepared mold (for shaped casting). In the production of the declared heat-resistant alloy, some alloying components (titanium, yttrium, etc.) are introduced into the molten metal immediately before its release (pouring it into a rotating chill mold) in order to avoid their oxidation and burning. The sequence and technology of introducing alloying components is not presented in this application and is a separate (independent) know-how on the alloying method. Subsequently, after crystallization of the heat-resistant alloy, the obtained cast billets were machined without deformation of the material structure, i.e. by removing chips.

The main research results were obtained using an alloy of the following composition, wt. %: nickel - 31.3; carbon 0.08; Manganese - 0.62; copper - 0.1; chromium - 20.8; titanium - 0.09; aluminum - 0.08; molybdenum - 0.45; silicon - 0.74; niobium - 1.3; vanadium - 0.17; sulfur - 0.008; phosphorus - 0.009; lead - 0.006; tin - 0.006; arsenic - 0.0075; zinc - 0.0054; antimony - 0.005; yttrium - 0.0008; oxygen - 0.018; hydrogen is 0.0019; nitrogen - 0.033; iron - 44.1694. Cr _e =% Cr + 2 ×% Al + 3 ×% Ti +% V +% Mo + 1.6 ×% Si + 0.6% Nb = 23.814; Ni _E =% Ni + 32% C + 0.6 ×% Mn + 22 ×% N +% Cu = 35.058; (Cr _E / Ni _E ) = 0.705≥0.585; (S + P) = 0.017≤0.025.

The average grain size (and finely divided particles of secondary carbides) was determined using a metallographic microscope (GOST 5639 "Steel. Methods for the detection and determination of grain size"). The study was conducted on samples cut from centrifugally cast pipes aged for 200, 2000 hours (at a temperature of 980 ° C and a pressure in the pipes of 30 atmospheres) with the active deposition of finely dispersed particles of secondary carbides in austenitic grains of a heat-resistant alloy.

The size of austenite grains, as well as the size of finely dispersed particles of secondary carbides, was determined using a specially developed computerized software package “Structure Analyzer”, which works with high resolution macro- and microstructure photographs. Photographs of the microstructure of an austenitic heat-resistant alloy with identified austenite grains in accordance with GOST 5639 were converted into electronic format (convenient for working in the Structure Analyzer program) and the analysis was performed according to the following algorithm. The Structure Analyzer program primarily identifies austenitic grain boundaries and all finely divided particles of secondary carbides within the boundaries of each grain. At the same time, it is possible to adjust the data obtained by a material specialist. Subsequently, all finely dispersed particles of secondary carbides trapped in the field of one austenite grain, with the exception of particles adjacent to the boundary of the austenite grain, were subjected to analysis. Using the “Structure Analyzer” software package, all austenite grains were analyzed within the prepared thin section.

In the known prototype alloy, the ratio of the average size of finely dispersed particles of secondary carbides aged over 2000 hours at a temperature of plus 980 ° C to the average size of finely dispersed particles of secondary carbides aged over 200 hours at the same temperature plus 980 ° C was 1, 83, which indicates the structural instability of the known alloy during operation.

составило 1,12, что свидетельствует о структурной стабильности заявленного сплава в процессе старения, где

- средний размер мелкодиспергированных частиц вторичных карбидов, состаренных в течение 2000 часов при температуре плюс 980°C;

- средний размер мелкодиспергированных частиц вторичных карбидов, состаренных в течение 200 часов при температуре плюс 980°C.For the claimed alloy ratio

amounted to 1.12, which indicates the structural stability of the claimed alloy in the aging process, where

- the average size of finely dispersed particles of secondary carbides aged over 2,000 hours at a temperature of plus 980 ° C;

- the average size of finely dispersed particles of secondary carbides aged for 200 hours at a temperature of plus 980 ° C.

To assess the tendency of austenitic chromium-nickel alloys to form hot cracks, we used the method GOST 54790-2011 “Tests that destroy welds of metal materials. Tests for resistance to hot cracking in welded joints. Arc welding processes. Part 3. Tests with external load. " Tests according to GOST Ρ 54790-2011 provide information on the resistance to hot cracking of basic materials, metal welds and welds. Hot cracking resistance testing involves applying an external load to the sample using test equipment.

When testing austenitic chromium-nickel alloys, samples of this material were heated by metal melting by welding in high-grade argon according to GOST 10157 (GOST 10157-79 "Gaseous and liquid argon. Technical conditions") with a non-consumable electrode (a bar with a diameter of 2 mm from lanthanum tungsten according to TU is used 48-19-27-87 "Tungsten lanthanized in the form of rods") when tested with a load acting along the weld.

The material is considered sensitive to hot cracking while it is in the range of brittleness temperatures (BTR), which is determined by the difference between the temperatures of zero strength (NST) and ductility recovery (DRT).

The most reliable criterion for assessing the sensitivity of a metal to hot cracking is the plasticity recovery coefficient (RDR), which is used to predict hot cracking in the heat-affected zone of the base material (austenitic chromium-nickel alloy).

To test the load acting along the weld, we used flat samples obtained by casting in an induction furnace and casting in a special mold, followed by machining to size: length - 150 mm, width - 75 mm, thickness - 25 mm.

The test for the tendency of the metal to form hot cracks was as follows: a) a filler weld was made on the main material with a non-consumable electrode (lanthanum tungsten) in argon - according to the diagram shown in Figure 3 GOST 54790; b) a load was applied at a fixed point in the roller (in the center of the sample) (the speed of the plunger of the hydraulic press is 1.8 mm / s), leading to bending of the sample around a special mandrel; c) upon completion of the tests, visually, at 25-fold magnification, the presence of cracks was checked.

The total length of visible cracks (L _tot ) is determined and reflected on the graph as a function of bending strain. A curve that determines the length of cracks depending on the deformation on the surface of the sample makes it possible to evaluate the resistance to the formation of hot cracks. The obtained graphic results allow us to evaluate the material from the point of view of predicting the behavior of the metal at high temperatures (during welding) and make the following conclusions about the resistance of the material to the formation of hot cracks: a) a material with a high resistance to the formation of hot cracks; b) increased risk of hot cracking; c) high risk of hot cracking.

10 samples of the known prototype alloy and 10 samples of the claimed alloy were tested. As a result of the tests, the following data were obtained. All 10 samples of the claimed alloy showed positive results and all of them correspond to a material with high resistance to hot cracking. Two out of 10 samples (20%) of the prototype alloy showed an increased risk of hot cracking.

The analysis of the test results for the tendency of the metal to form hot cracks showed that the achievement of the set technical result leads to high-quality welded joint, i.e. manufacturing high-quality reaction tubes.

To conduct studies of the heat-resistant properties of the claimed alloy, 2 nozzles 75.0 mm long were cut from the end part of the manufactured centrifugal cast billet ⌀110 × 10 mm, which were welded together. The welding method is arc welding in an argon medium with a non-consumable electrode (it is allowed to use filler materials during welding, which should be used as a material identical in chemical composition to the main material being welded in accordance with TU 14-131-994-2003 "Welding wire from high alloy alloys"). Form of prepared edges: with bevelled edges; weld nature: one-sided; the designation of the welded joint is C17 according to GOST 16037. Test samples were made from the obtained welded pipe with a length of 150 mm. The direction of the axis of the cut samples coincided (was parallel) with the direction of the axis of the centrifugally cast pipe. Welding of centrifugally cast pipe billets was carried out according to RD 3689-002-00220302 / 31-2008 “Welding of radiant pipes and their elements for reaction tube furnaces. Key Points. ”

Heat resistance was evaluated by long-term strength, i.e. voltage causing destruction at a given temperature for a given period of time.

A comprehensive assessment of the quality of the reaction tubes of the proposed alloy, including the weldability of the proposed alloy (tendency to hot cracks), serves as a test for long-term strength of welded samples. These tests were carried out on cylindrical samples with a diameter of the calculated length of 10 mm at a temperature of 980 ° C.

During long-term tests at high temperatures, the fracture (rupture) of the welded specimen occurs as a result of constant loading, which is carried out using lever loading (ND Sazonova, “Testing heat-resistant materials for creep and long-term strength, M., Mechanical Engineering, 1965) .

Technical requirements for machines for testing metals for long-term strength corresponded to GOST 15533.

The sample (type IV according to GOST 1497), installed in the grips of the testing machine and placed in the furnace, was heated to a predetermined temperature (heating time did not exceed 8 hours) and kept at this temperature for at least one hour. After heating the sample and holding at a given temperature, a load was smoothly applied to the sample to provide the required test voltage.

), при котором изделие из данного сплава разрушилось бы за определенный промежуток времени (τ, час) при заданной температуре (t, °C).The main indicator of this type of test is the time to failure at a given voltage and temperature. The results of the tests were plotted on the heat resistance graph in the coordinates logτ-logσ (where τ is the time to failure, σ is the stress). The resulting graph allows you to predict stress (long-term strength,

), in which the product from this alloy would collapse in a certain period of time (τ, hour) at a given temperature (t, ° C).

, т.е. напряжения, при котором испытуемый при температуре плюс 980°C сварной образец разрушился бы как минимум через 120 часов.In order to reduce the duration of the tests, they were carried out at relatively high voltages (long-term strength tests were carried out at a temperature of 980 ° C and high voltages in accordance with GOST 10145), which made it possible to determine specific 120-hour values from the obtained heat resistance graph (lgτ-lgσ) long durability

, i.e. voltage at which the test specimen at a temperature of plus 980 ° C would break in at least 120 hours.

The analysis of the results of tests for long-term strength showed that the achievement of the set technical result leads to an increase in its heat resistance, i.e. ultimately, leads to the manufacture of better reaction tubes that can be operated for a longer period of time.

образцов труб, изготовленных из заявленного сплава, повышается (по сравнению со сплавом - прототипом) с 22,7 до 27,0 Н/мм², что в конечном итоге приводит к увеличению на 10÷15% ресурса эксплуатации реакционных труб из заявленного жаропрочного сплава при прочих равных условиях. При этом механические свойства заявляемого сплава в исходном состоянии при комнатной температуре остаются такими же, как и у сплава-прототипа, т.е. предел прочности (σ_В) не менее 441 МПа; предел текучести (σ₀₂) не менее 176 МПа; относительное удлинение (δ₅) не менее 30%.As a result of comprehensive studies on 12 experimental swimming trunks, it was found that if all the components of the claimed alloy are within the limits specified in the claims, the expected technical result is achieved, and 120-hour long-term strength

pipe samples made from the claimed alloy increases (compared with the prototype alloy) from 22.7 to 27.0 N / mm ² , which ultimately leads to an increase of 10 ÷ 15% in the life of the reaction tubes from the claimed heat-resistant alloy ceteris paribus. In this case, the mechanical properties of the claimed alloy in the initial state at room temperature remain the same as that of the prototype alloy, i.e. tensile strength (σ _B ) not less than 441 MPa; yield strength (σ ₀₂ ) not less than 176 MPa; elongation (δ ₅ ) of at least 30%.

It was found that the influence of yttrium, oxygen, hydrogen and nitrogen separately or in pairs did not lead to structural stability of the alloy (the ratio r exceeded 1.23). The nature of the structure of the cast structure can be described as relatively large grains of austenite with significant different grain sizes. If the content of oxygen, hydrogen and nitrogen is exceeded, the maximum values specified in the formula, the technical result will not be achieved. A decrease in the content of oxygen, hydrogen and nitrogen below the minimum values specified in the formula meets significant technical difficulties, mainly due to the presence of dissolved gases in almost all charge materials. And degassing to this level is an extremely complex process without achieving a technical result. The minimum gas saturation limit (0.0005% in terms of oxygen, hydrogen and nitrogen) in the claimed alloy has been experimentally established, with a further decrease in which the liquid metal acquires a tendency to active secondary (re) oxidation and hydrogenation during casting into the mold (mold). With prolonged aging, the heterogeneity increases with increasing maximum austenite grain size, which negatively affects the tendency to form hot cracks during welding, i.e. lower weldability. When testing (according to GOST Ρ 54790-2011), two out of 10 samples of the prototype alloy showed an increased risk of hot cracking, and one sample showed a high risk of hot cracking.

увеличилась с 22,7 до 27,0 Н/мм².It was found that only a complete combination of all the distinguishing features (the presence of yttrium, oxygen, hydrogen and nitrogen in the specified alloy within the specified limits) leads to a decrease in the size of austenite grains, a decrease in the grain size of the structure (increased uniformity). Moreover, with aging, a further change in the grain size and size of finely dispersed particles of secondary carbides was practically not observed, which indicates an increased structural stability of the claimed alloy (ratio r = 1.12). In this case, the tendency of the claimed alloy to the formation of hot cracks during welding, i.e. contributed to improved weldability. When testing 10 samples of the claimed alloy according to GOST Ρ 54790-2011, all of them showed positive results and corresponded to a material with high resistance to hot cracking. In fact, this contributed to improving the quality of reaction tubes welded from centrifugally cast billets of a nickel-chromium alloy of the claimed composition. Long-term strength in testing welded samples of the claimed alloy

increased from 22.7 to 27.0 N / mm ² .

в процессе старения (эксплуатации под нагрузкой при высоких температурах). При испытании (по ГОСТ Ρ 54790-2011) 10-ти образцов заявленного сплава, все они показали положительные результаты и соответствуют материалу с высокой сопротивляемостью образованию горячих трещин. А это в свою очередь приведет к увеличению длительной прочности сварных образцов из заявленного сплава

с 27,0 до 29,0 Н/мм² (что свидетельствует о качественных сварных соединениях реакционных труб из заявленного жаропрочного сплава, т.е. в целом о качестве самих труб). Для снижения содержания серы и фосфора в составе жаропрочного сплава до заявленных пределов следует использовать один из известных способов десульфурации и дефосфорации, соответственно.The research results also showed that if the two conditions specified in the claims are fully satisfied, this will lead to the elimination of delamination during crystallization of the alloy and to the refinement of the grain structure (austenite grain size decreases) while reducing the grain size of the structure (increasing uniformity) , and also lead to a decrease in the tendency to form hot cracks with relative evenness and lead to structural stability

during aging (operation under load at high temperatures). When testing (according to GOST Ρ 54790-2011) 10 samples of the claimed alloy, all of them showed positive results and correspond to a material with high resistance to hot cracking. And this in turn will lead to an increase in the long-term strength of welded samples of the claimed alloy

from 27.0 to 29.0 N / mm ² (which indicates high-quality welded joints of the reaction pipes from the declared heat-resistant alloy, i.e., in general, about the quality of the pipes themselves). To reduce the sulfur and phosphorus content in the composition of the heat-resistant alloy to the declared limits, one of the known methods of desulfurization and dephosphorization, respectively, should be used.

с 27,0 до 20,9÷22,7 Н/мм² (что равносильно снижению ресурса эксплуатации реакционных труб на 10 15%). При этом резко ослабляется положительный эффект от аддитивного воздействия всех отличительных признаков на замедление процесса зарождения трещин, т.е. на улучшение состояния границ зерен и на нейтрализацию в жаропрочном сплаве вредных примесей, способных образовывать легкоплавкие эвтектики.In addition, it was experimentally confirmed that if the limits of the content of sulfur, phosphorus, lead, tin, arsenic, zinc and antimony specified in the claims are exceeded, the content of fusible compounds along the boundaries of austenitic grains sharply increases, and this in turn leads to delamination at crystallization of the alloy, an increase in the tendency to form hot cracks. Two of the 10 samples of the prototype alloy when tested according to GOST Ρ 54790-2011 showed an increased risk of hot cracking. Ultimately, this leads to a decrease in the quality of the reaction tubes (in fact, a decrease in the heat resistance of welded joints). This leads to a decrease in the long-term strength of the alloy.

from 27.0 to 20.9 ÷ 22.7 N / mm ² (which is equivalent to a decrease in the service life of the reaction pipes by 10 15%). At the same time, the positive effect of the additive effect of all the distinguishing features on the slowdown of the crack nucleation process is sharply weakened, i.e. to improve the state of grain boundaries and to neutralize in a heat-resistant alloy harmful impurities that can form fusible eutectics.

Thus, studies of the physical parameters of the claimed alloy showed that it is at the level of known analogues in mechanical properties at room temperature (σ _B , σ ₀₂ , δ ₅ ), and surpasses them in terms of heat resistance due to an increase in structural stability during aging, and also reducing the tendency to form hot cracks during welding at the content of components specified in the claims.

Claims (2)

1. Heat-resistant alloy containing carbon, chromium, nickel, niobium, silicon, manganese, vanadium, titanium, aluminum, sulfur, phosphorus, lead, tin, arsenic, zinc, molybdenum, copper and iron, characterized in that it additionally contains yttrium , antimony, oxygen, hydrogen and nitrogen in the following ratio of components, wt.%:
carbon 0.08-0.14
chrome 19.0-21.0
nickel 31.0-34.0
niobium 0.90-1.35
silicon 0.0005-0.79
Manganese 0.5005-1.21
vanadium 0.0005-0.20
titanium 0.0005-0.10
aluminum 0.0005-0.10
yttrium> 0-0.001
oxygen> 0.0005-0.028
hydrogen> 0.0005-0.0025
nitrogen> 0.0005-0.095
sulfur ≤0.03
phosphorus ≤0.03
lead ≤0.009
tin ≤0.009
arsenic ≤0.009
zinc ≤0.009
antimony ≤0.009
molybdenum ≤0.5
copper ≤0.2
iron rest
under the following conditions, wt.%:
(Cr _E / Ni _E ) ≥0.585,
where Cr _E is the equivalent of chromium, Ni _E is the equivalent of nickel,
Cr _e = Cr + 2 × Al + 3 × Ti + V + Mo + 1.6 × Si + 0.6 × Nb;
Ni _E = Ni + 32 × C + 0.6 × Mn + 22 × N + Cu. 2. The heat-resistant alloy according to claim 1, characterized in that the condition (S + P) ≤0.025 is fulfilled for the contents of sulfur and phosphorus.