RU2580765C1 - High-temperature alloy - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, in particular to heat resistant alloys, and may be used in manufacture of pipes, sheets, forgings, and others, metal-rolling for heat exchange, etc., equipment operating in corrosive environments, as well as vessels and vehicles operating at high pressure in temperature range from minus 196 °C to plus 450 °C. Heat-resistant alloy contains components in following ratio, wt%: carbon ≤ 0.10; chrome 20.5÷23.5; silicon ≤ 0.60; manganese ≤ 0.60; niobium 3.05÷4.20; cobalt ≤ 1.1; titanium ≤ 0.45; aluminium ≤ 0.45; yttrium > 0÷0.001; oxygen > 0.0005÷0.018; hydrogen > 0.0005÷0.0017; nitrogen > 0.0005÷0.050; sulphur ≤ 0.013; phosphorus ≤ 0.013; lead ≤ 0.009; tin ≤ 0.009; arsenic ≤ 0.009; zinc ≤ 0.009; antimony ≤ 0.009; molybdenum 7.9÷10.1; iron ≤ 5.1; nickel - balance. For content of sulphur and phosphorus condition is satisfied (S+P) ≤ 0.020, and for ratio of chromium and nickel following condition is satisfied, wt%: (Cr_e/Ni_e) ≥ 0.537, where: Cr_e - equivalent of chromium; Ni_e - equivalent of nickel; Cr_e=%Cr+2×%Al+3×%Ti+%Mo+l,6×%Si+0,6%Nb; Ni_e=%Ni+32%C+0,6×%Mn+%Co+22×%N.

EFFECT: increased structural stability of alloy in process of ageing, as well as reduced tendency to formation of hot cracks during welding.

3 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 pipes, sheets, forgings, and other metal products for heat exchange and other equipment operating in corrosive environments, as well as for vessels and apparatuses, operating at high pressure in the temperature range from minus 196 ° C to plus 450 ° C (in the state after soft, stabilizing annealing at a temperature of from plus 950 to plus 1050 ° C, preferably at 980 ± 10 ° C). Products from this alloy with a high carbon content from 0.06 to 0.1% can be used at elevated temperatures from plus 600 ° C to plus 980 ° C and pressure up to 50 atm - in the state of processing for solid solution (at heat treatment temperature - plus 1095 ° C).

Known heat-resistant alloy XH56K16MBVYUT according to GOST 5632-2014 "Alloyed stainless steels and alloys corrosion-resistant, heat-resistant and heat-resistant", having the following composition: C 0,030 ÷ 0,07%; Si ≤0.40%; Mn ≤ 0.50%; Cr 14.5 ÷ 15.5%; Ni - main; Ti 1.1 ÷ 1.4%; Al 1.4 ÷ 1.7%; W 1.2 ÷ 1.6%; Mo 4.5 ÷ 4.9%; Nb 4.2 ÷ 4.6%; S 0.015% (max); P 0.015% (max); Fe ≤2.0%; Co 15.0 ÷ 16.5%; Zr 0.6 ÷ 1.0%; B≤0.003%; Mg ≤0.03%; La ≤0.03%.

Closest to the claimed technical essence and the achieved result is a heat-resistant alloy, HAYNES® 625 alloy (UNS N06625) of the American company Haynes International, Inc., (Kokomo, Indiana, U.S.A.) containing in wt. %: carbon ≤0.10%; silicon ≤ 0.50%; molybdenum 8.0 ÷ 10.0%; cobalt ≤1.0%; Manganese ≤ 0.50%; chrome 20.0 ÷ 23.0; nickel 58.0% (min); titanium ≤0.4%; aluminum ≤0.4%; niobium + tantalum 3.15 ÷ 4.15%; iron ≤5.0%; sulfur ≤0.015%; phosphorus ≤ 0.015%.

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 usually in induction furnaces.

It is possible to make pipes from the specified low-carbon heat-resistant alloy by pressing, rolling, etc. by methods of plastic deformation, as well as by centrifugal casting, which is distinguished by its simplicity and reliability. Centrifugally cast tubular 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 further welded to obtain the pipe of the required length .

Pipes (assembled), as a rule, are used in high-temperature equipment of oil refining, chemical and petrochemical industries. In justified cases, these pipes can be used in heat exchange equipment, etc. as a heat-resistant corrosion-resistant material.

Given the need for welding pipe blanks to obtain pipes of the required length, the latter must have high-quality welded joints.

The service life of pipes welded from pipe billets of known alloys in oil and gas refineries operating at temperatures of 600 ÷ 980 ° C and a pressure of up to 50 atm is 25,000 to 60,000 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 welded pipe and failure of the entire installation with unpredictable consequences.

Damage to pipes (obtained by welding pipe billets) during operation in high-temperature equipment, in particular, occurs due to the combined effect of thermal stresses in the welded zone (due to temperature differences on the outer and inner walls of the pipe) and stresses in the welded zone caused by for high pressure 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 pipe billets made of austenitic chromium-nickel alloys 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 (with 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 pipe billets meet the technical requirements both in terms of their own qualities and regarding their influence on the design of the welded pipe 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 the 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 cracks that have 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 pp., Ill. )

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 a minimum amount of gases 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. In addition, each argon bubble represents a “vacuum chamber”, since the partial pressures of hydrogen and nitrogen in such a bubble are 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 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) by grinding austenite grains, refining of metal and increasing surface resistance against general corrosion and stress corrosion cracking (Factory Laboratory. Diagnostics of materials. No. 2, 2010. Volume 76. Investigation of cavitation-corrosion resistance of directional 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 pipes (welded from tube billets), centrifugally cast tube billets of chromium-nickel steels and austenitic alloys were 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, it is unacceptable to use metal that was in use and highly oxidized components.

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, C, Mn, N, Cu), where Cr, Ti, Mo, Si, Nb, Ni, C, Mn, N , Cu - designations of elements according to the periodic system of chemical elements (tables of D.I. Mendeleev).

The dependence of the resistance to the formation of hot cracks in 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. Ed. 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, N, 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 enhanced in alloys with a coarse-grained structure and with a structure with a large variation in the size of austenitic grains (high grain size).

The formation of defects in the structure of a heat-resistant alloy (and products from it, for example pipes) 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, during primary creep, which is fixed immediately with the beginning of high-temperature operation of pipes under pressure, during the hardening of the metal (directly during high-temperature operation under internal pipe pressure), 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 pipes), an increase in the diameter of the pipes 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. The increasing strain rate ultimately leads to the destruction of the pipe of heat-resistant chromium-nickel alloy of the austenitic class.

To increase the operability of pipes 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 until cracks in the metal structure form.

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 (formed even at high temperature loading of pipes from heat-resistant alloys), which are deposited in the form of fine particles during pipe operation not along the borders, 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 pipes and the decrease in the quality of pipes 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, silicon, manganese, niobium, titanium, aluminum, sulfur, phosphorus, molybdenum, cobalt and iron, characterized in that it additionally contains lead, tin, arsenic, zinc, antimony, yttrium, oxygen, hydrogen and nitrogen in the following ratio of components, wt. %: carbon ≤0.10; chromium 20.5 ÷ 23.5; nickel - basic; silicon ≤ 0.60; Manganese ≤0.60; niobium 3.05 ÷ 4.20; cobalt ≤1.1; titanium ≤0.45; aluminum ≤0.45; yttrium> 0 ÷ 0.001; oxygen> 0.0005 ÷ 0.018; hydrogen> 0.0005 ÷ 0.0017; nitrogen> 0.0005 ÷ 0.050; sulfur ≤ 0.013; phosphorus ≤0.013; lead ≤0.009; tin ≤0.009; arsenic ≤0.009; zinc ≤0.009; antimony ≤0.009; molybdenum 7.9 ÷ 10.1; iron ≤5.1; for sulfur and phosphorus contents, the condition (S + P) ≤0.020 is fulfilled, and for the ratio of chromium and nickel equivalents the following condition is fulfilled, wt. %: (Cr _e / Ni _e ) ≥0.537, where: Cr _e is the chromium equivalent; Ni _e is the equivalent of nickel; Cr _e =% Cr + 2 ×% Al + 3 ×% Ti +% Mo + l, 6 ×% Si + 0.6% Nb; Ni _e =% Ni + 32% C + 0.6 ×% Mn +% Co + 22 ×% N.

, где

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

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

, 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

where

- 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 a 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 in induction furnaces using pure charge materials (original 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 during melting in induction furnaces is protected from saturation of the 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 based on the declared heat-resistant alloy were obtained from centrifugally cast pipe billets or castings made by pouring molten heat-resistant alloy into a rotating chill mold (for a centrifugally cast pipe) with an inner diameter equal to the outer diameter of the pipe obtained or by pouring molten heat-resistant alloy into a specially prepared mold (for shaped castings). In the production of the declared heat-resistant alloy into molten metal, immediately before its release (pouring into a rotating chill mold), certain alloying components (titanium, yttrium, etc.) are introduced according to special conditions 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. %: carbon 0.05; chromium 22.4; nickel - 56.4544; silicon 0.58; manganese 0.25; niobium 4.0; cobalt 0.5; titanium 0.41; aluminum 0.43; yttrium 0.0009; oxygen 0.018; hydrogen 0.0017; nitrogen 0.05; sulfur 0.010; phosphorus 0.009; lead 0.008; tin 0.007; arsenic 0.006; zinc 0.008; antimony 0.007; molybdenum 9.8; iron 5.0 under the following conditions, wt. %: (Cr _E / Ni _E ) = 0.63≥0.537; (S + P) = 0,019≤0,020, where: Cr _e - equivalent chromium; Ni _E is the equivalent of nickel; Cr _e =% Cr + 2 ×% Al + 3 ×% Ti +% Mo + 1.6 ×% Si + 0.6% Nb = 37.618; Ni _E =% Ni + 32% C + 0.6 ×% Mn +% Co + 22 ×% N = 59.8044.

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 870 ° 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 translated 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. Further, all finely dispersed particles of secondary carbides that fell into the field of one austenite grain were excluded from analysis, with the exception of particles adjacent to the austenitic grain boundary. 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 at a temperature of 870 ° C to the average size of finely dispersed particles of secondary carbides aged over 200 hours at the same temperature of 870 ° C was 1.82, 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 $$r_{2000ч}^{800^oС}$$

- средний размер мелкодиспергированных частиц вторичных карбидов, состаренных в течение 2000 часов при температуре 870°C; $$r_{200ч}^{870^{o}C}$$

- средний размер мелкодиспергированных частиц вторичных карбидов, состаренных в течение 200 часов при температуре 870°C.For the claimed alloy ratio $$r=r_{2000h}^{870^o\mathrm{FROM}}/r_{200h}^{870^{o}C}$$

amounted to 1.12, which indicates the structural stability of the claimed alloy in the aging process, where $$r_{2000h}^{800^o\mathrm{FROM}}$$

- the average size of the finely dispersed particles of secondary carbides aged for 2000 hours at a temperature of 870 ° C; $$r_{200h}^{870^{o}C}$$

- the average size of finely dispersed particles of secondary carbides aged for 200 hours at a temperature of 870 ° C.

To assess the tendency of austenitic chromium-nickel alloys to the formation of hot cracks, we used the method GOST R 54790-2011 “Destructive tests on welds of metallic materials. Tests for resistance to hot cracking in welded joints. Arc welding processes. Part 3. Tests with external load. " Tests according to GOST R 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 premium 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 in accordance with TU 48 is used -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 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 R 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 (100%) 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 pipes obtained by welding.

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 welded pipes from 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 along the calculated length of 10 mm at a temperature of 870 ° 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 of 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, $${\sigma }_{\tau ,h\mathrm{but}\mathrm{from}}^{t,°C}$$

), 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 870°C сварной образец разрушился бы как минимум через 90 часов.In order to reduce the duration of the tests, they were carried out at relatively high voltages (tests for long-term strength were carried out at a temperature of 870 ° C and high voltages in accordance with GOST 10145), which allowed us to determine the specific values of the 90-hour long-term from the obtained heat resistance graph (lgτ - lgσ) strength ( $${\sigma }_{90h}^{870°\mathrm{FROM}}$$

), i.e. voltage at which the test sample at a temperature of 870 ° C would break in at least 90 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 pipes (welded from pipe billets) that can be operated over a longer period of time.

) образцов труб, изготовленных из заявленного сплава, повышается (по сравнению со сплавом-прототипом) с 65 до 75 Н/мм², что в конечном итоге приводит к увеличению на 10÷15% ресурса эксплуатации 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 830 МПа; предел текучести (σ₀₂) не менее 415 МПа; относительное удлинение (δ₅) не менее 35%.As a result of comprehensive studies on 12 experimental swimming trunks, it was revealed that if all components of the claimed alloy are within the limits specified in the claims, the expected technical result is achieved, and 90-hour long-term strength ( $${\sigma }_{90h}^{870°\mathrm{FROM}}$$

) of pipe samples made from the claimed alloy increases (compared with the prototype alloy) from 65 to 75 N / mm ² , which ultimately leads to an increase of 10-15% in the life of the pipes from the declared 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 _in ) not less than 830 MPa; yield strength (σ ₀₂ ) not less than 415 MPa; elongation (δ ₅ ) of at least 35%.

It was found that the influence of lead, tin, arsenic, zinc, antimony, yttrium, oxygen, hydrogen and nitrogen separately or in pairs did not lead to structural stability of the alloy (the ratio r exceeded 1.32). The nature of the structure of the cast structure can be described as relatively large grains of austenite with significant different grain sizes. When exceeding the maximum values of the content of lead, tin, arsenic, zinc, antimony, yttrium, oxygen, hydrogen and nitrogen, specified in the formula, the technical result was not achieved. Reducing the minimum values of lead, tin, arsenic, zinc and antimony below those specified in the formula meets significant technical difficulties, mainly due to the presence of these elements in almost all charge materials, and 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. 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 R 54790-2011), two out of 10 samples of the prototype alloy showed an increased risk of hot cracking.

) увеличилась с 65 до 75 Н/мм².It was found that only a complete combination of all the distinguishing features (the presence of lead, tin, arsenic, zinc, antimony, yttrium, oxygen, hydrogen and nitrogen in the specified range as a part of the claimed alloy) leads to an increase in the structural stability of the alloy during aging, as well as to reducing the tendency of the alloy to form hot cracks during welding. 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 R 54790-2011, all of them showed positive results and corresponded to a material with high resistance to hot cracking. In fact, this helped to improve the quality of pipes welded from centrifugally cast billets of a nickel-chromium alloy of the claimed composition. Long-term strength when testing welded samples of the claimed alloy ( $${\sigma }_{90h}^{870°\mathrm{FROM}}$$

) increased from 65 to 75 N / mm ² .

в процессе старения (эксплуатации под нагрузкой при высоких температурах). При испытании (по ГОСТ Р 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 $$({\sigma }_{90ч}^{870°С})$$

с 75 до 77 Н/мм² (что свидетельствует о качественных сварных соединениях 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 this will lead to an additional decrease in the tendency to form hot cracks, will lead to structural stability $$(r=r_{2000h}^{870^o\mathrm{FROM}}/r_{200h}^{870^{o}C}=\mathrm{one},09)$$

during aging (operation under load at high temperatures). When testing (according to GOST R 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 $$({\sigma }_{90h}^{870°\mathrm{FROM}})$$

from 75 to 77 N / mm ² (which indicates high-quality welded joints of pipes of 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 75 до 63÷65 Н/мм² (что равносильно снижению ресурса эксплуатации сварных 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 out of 10 samples (20%) of the prototype alloy, when tested according to GOST R 54790-2011, showed an increased risk of hot cracking. Ultimately, this leads to a decrease in the quality of welded pipes (in fact, a decrease in the heat resistance of welded joints). This leads to a decrease in the long-term strength of the alloy ( $${\sigma }_{90h}^{870°\mathrm{FROM}}$$

) from 75 to 63 ÷ 65 N / mm ² (which is equivalent to a decrease in the service life of welded 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 _in , 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 (3)

1. Heat-resistant alloy containing carbon, chromium, nickel, silicon, manganese, niobium, titanium, aluminum, sulfur, phosphorus, molybdenum, cobalt and iron, characterized in that it additionally contains lead, tin, arsenic, zinc, antimony, yttrium , oxygen, hydrogen and nitrogen in the following ratio, wt. %:
carbon ≤0.10 chromium 20.5 ÷ 23.5 silicon ≤0.60 manganese ≤0.60 niobium 3.05 ÷ 4.20 cobalt ≤1,1 titanium ≤0.45 aluminum ≤0.45 yttrium > 0 ÷ 0.001 oxygen > 0.0005 ÷ 0.018 hydrogen > 0.0005 ÷ 0.0017 nitrogen > 0,0005 ÷ 0,050 sulfur ≤0.013 phosphorus ≤0.013 lead ≤0.009 tin ≤0.009 arsenic ≤0.009 zinc ≤0.009 antimony ≤0.009 molybdenum 7.9 ÷ 10.1 iron ≤5.1 nickel rest 2. The heat-resistant alloy according to claim 1, characterized in that the condition (S + P) ≤0.020 is fulfilled for sulfur and phosphorus contents. 3. The heat-resistant alloy according to claim 2, characterized in that for the ratio of equivalents of chromium and nickel the following condition is fulfilled, wt. %:
(Cr _e / Ni _e ) ≥0.537,
where: Cr _e is the equivalent of chromium; Ni _e is the equivalent of nickel;
Cr _e =% Cr + 2 ×% Al + 3 ×% Ti +% Mo + l, 6 ×% Si + 0.6% Nb;
Ni _e =% Ni + 32% C + 0,6 ×% Mn +% Co + 22 ×% N.