RU2700347C1 - Heat-resistant alloy - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to heat-resistant chrome-nickel alloys of austenitic class and can be used in production of collectors of reaction tubes of high-temperature plants of hydrogen, methanol and ammonia. Heat-resistant alloy contains, wt%: carbon 0.05÷0.15; silicon 0.50÷1.50; manganese 0.50÷1.50; chromium 19÷23; nickel 30÷33; niobium 0.70÷1.60; titanium 0.005÷0.10; zirconium 0.005÷0.15; tungsten 0.005÷0.10; lanthanum 0.005÷0.10; cobalt 0.0005÷0.10; molybdenum ≤0.10; sulfur ≤0.03; phosphorus ≤0.03; lead ≤0.01; tin + arsenic + zinc + antimony ≤0.02; nitrogen ≤0.05; copper ≤0.1; iron – balance. Alloy has a structure consisting of an austenite matrix and Cr_(22÷56)Fe_(4÷7)Ni and Nb_(25÷35)Cr_(2.5÷3.5)(FeNiTi)_(0.9÷1.1) intermetallides distributed therein at weight ratio of austenitic matrix and intermetallides (91÷95):( 3÷8):( 1÷3).

EFFECT: providing uniform distribution of secondary carbides and intermetallides in austenite matrix – this enables to avoid carbonisation during pyrolysis of hydrocarbons and formation of hot cracks during welding of reaction tubes; alloy is characterized by high heat resistance.

1 cl, 2 ex

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 reaction pipe manifolds of high-temperature installations of hydrogen, methanol, ammonia, etc., with operating conditions up to plus 1100 ° С and pressure up to 50 atm ., as well as in petrochemical and other equipment as a corrosion-resistant material.

Collector elements for petrochemical units are made by welding tube blanks of chromium-nickel alloys obtained by centrifugal casting (ASTM [American Society for Testing and Materials], A608, Centrifugally Cast iron-chromium-nickel High Alloy Tubing for pressure application at high temperatures). Previously, their inner surface is machined to remove defects (Yoshikazu Kuriyama, Yasuhisa Yamazaki, Iwao Kawashima, IHI, Engineering Review, 3, No. 5, September, 1970) and then welded to obtain a reaction tube of the required length.

Accordingly, increased demands are placed on the quality of welding billets for the manufacture of reaction tubes of the required sizes.

The service life of collectors of known alloys in oil and gas refineries, operating at temperatures of 700 ÷ 980 ° C and pressure up to 46 atm, in most cases varies from 20,000 to 65,000 hours, after which they must be replaced, since their strength under operating conditions significantly decreases , which can lead to emergency destruction and shutdown of the unit.

It is believed that damage to the reaction tubes with welds in the reforming furnaces of ammonia, hydrogen, methanol and other aggregates is caused by a combination of factors, and above all, as a result of thermal stresses in the welded zone due to temperature differences on the outer and inner walls of the pipe, and also due to the high pressure of the converted gas. They are responsible for the manifestation of creep at the initial stage of the operation of collectors in reforming furnaces.

A serious practical problem of welding tube blanks from austenitic chromium-nickel alloys is the tendency to form hot (knife) cracks in the weld zone near the weld, which have an intergranular character.

To minimize defects, they are guided by the requirements of relevant standards, in particular GOST 29273-92 Weldability. Definition

Knife cracks are classified as intergranular fractures and are divided into crystallization and segregation. The probability of the appearance of the former is determined by the crystallization mode of the liquid phase of the weld metal and the relaxation of stresses arising from this.

The sizes of segregation cracks vary from 1 mm to larger ones and can extend to the entire length of the welded joint. Their formation is facilitated by some impurities in the raw materials.

It is known that the harmful effects of arsenic, zinc, lead, tin and antimony negatively affect the heat resistance of alloys, which leads to the introduction of technical measures to reduce their concentration in steel.

A similar effect is exerted by sulfur in the case of its increased content as a result of the formation of fusible eutectics with iron, nickel, and cobalt. This effect is enhanced in heat resistant austenitic steels with a high nickel content. To suppress it, the addition of rare earth elements such as cerium, lanthanum and some others, which bind it to the corresponding sulfides, is practiced.

To increase the resistance of austenitic chromium-nickel alloys to the formation of segregation hot cracks, it is necessary to dope them with elements that reduce the diffusion mobility of atoms in the crystal lattice. An even more important role is played by the formation of intermetallic compounds enriched in chromium and niobium, which form network reinforcing structures in the austenitic matrix.

Known heat-resistant alloy described in RU 2395608, IPC C22C 30/00, publ. 07/27/2010 and including carbon, silicon, manganese, chromium, nickel, niobium, vanadium, titanium, zirconium, cerium, tungsten, sulfur, phosphorus, lead, tin, arsenic, zinc, antimony, nitrogen and iron with the following components, wt .%: carbon> 0.1 ÷ 0.14; silicon ≤ 0.80; manganese 0.50 ÷ 1.20; chrome 22.0 ÷ 25.0; nickel 33.0 ÷ 36.0; niobium 0.90 ÷ 1.35; vanadium 0.005 ÷ 0.20; titanium 0.005 ÷ 0.10; zirconium 0.10 ÷ 0.25; cerium 0.005 ÷ 0.10; tungsten 0.005 ÷ 0.10; sulfur ≤0.025; phosphorus ≤0.025; lead ≤0.007; tin ≤0.007; arsenic ≤0.007; zinc ≤0.007; antimony ≤0.007; nitrogen ≤0.01; iron the rest.

The disadvantages of this alloy include the relatively low values of mechanical indicators at room temperature and the high sensitivity of the life of the reaction tubes to the content of impurities in steel.

Known heat resistant alloy described in RU No. 2581323, class. С22С 30/00, С22С 38/60, С22С 38/50 and including carbon, chromium, nickel, niobium, silicon, manganese, vanadium, titanium, aluminum, yttrium, oxygen, hydrogen, nitrogen, sulfur, phosphorus, lead, tin, arsenic, zinc, antimony, molybdenum, copper and iron in the following components, 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, 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, ed ≤0,2, iron - the rest.

According to well-known works, the addition of yttrium and lanthanum can increase the resistance of the heat-resistant alloy to oxidation by 3 times at a temperature of 1100 ° C. In the alloy under consideration, there is no lanthanum, and the yttrium content is more than an order of magnitude lower than the concentrations of oxygen and other impurities. Its amount is clearly not enough to bind these impurities.

For this reason, the operating temperature of the collectors based on this composition is below 1000 ° C. In view of the foregoing, it is possible to further reduce the life of the reaction tubes when the limited concentrations of oxygen, hydrogen and nitrogen are exceeded, which indicates the instability of the structure of the cast alloy in real operating conditions. Accordingly, the tendency of the metal to form hot cracks on the welds of reaction tubes in reforming furnaces of ammonia and methanol aggregates, and in oil refineries, is increased.

The closest in technical essence is a heat-resistant alloy of austenitic structure, given in RU No. 2533072, class. C22C 30/00, publ. 11/20/2014 and including carbon, chromium, nickel, niobium, cerium, zirconium, silicon, manganese, vanadium, titanium, aluminum, tungsten, iron and impurities, the rest with the following content of components, wt.%: Carbon 0.05 ÷ 0.10 , chromium 24 ÷ 27, nickel 33 ÷ 35, niobium 0.6 ÷ 1.3, cerium 0.005 ÷ 0.10, zirconium 0.005 ÷ 0.10, silicon 0.81 ÷ 1.50, manganese 0.60 ÷ 1, 20, vanadium 0.005 ÷ 0.20, titanium 0.005 ÷ 0.15, aluminum 0.001 ÷ 0.10, tungsten <0.10, molybdenum <0.2, sulfur <0.03, phosphorus <0.03, lead <0 , 01, tin + arsenic + zinc + antimony <0.01, copper <0.1, iron the rest.

The specified alloy has good performance, however, a narrow range of carbon content requires a more stringent approach to the smelting.

An object of the invention is to optimize the structure and composition of the austenitic chromium-nickel alloy in order to increase its physical and mechanical properties, heat resistance and crack resistance.

The specified technical result is achieved due to the fact that the heat-resistant alloy contains, by weight. %: austenitic matrix in the amount of 91 ÷ 95; intermetallic compound Cr _{(22 ÷ 56)} Fe _{(4 ÷ 7)} Ni - (3 ÷ 8) and intermetallic compound Nb _{(25 ÷ 35)} Cr _{(2.5 ÷ 3.5)} (FeNiTi) _{(0.9 ÷ 1.1 )} - (1 ÷ 3), with the following element content, wt.%: Carbon 0.05 ÷ 0.15, chromium 19 ÷ 23, nickel 30 ÷ 33, niobium 0.7 ÷ 1.60, zirconium 0.005 ÷ 0, 15, lanthanum 0.005 ÷ 0.10, silicon 0.50 ÷ 1.50, manganese 0.50 ÷ 1.50, titanium 0.005 ÷ 0.10, tungsten 0.005 ÷ 0.10, molybdenum ≤0.10, cobalt 0, 0005 ÷ 0.10, nitrogen ≤0.05, sulfur ≤0.03, phosphorus ≤0.03, lead ≤0.01, tin + arsenic + zinc + antimony ≤0.02, copper ≤0.1, iron rest .

Compared with the prototype, it optimized the content of most elements and additionally introduced cobalt, lanthanum and nitrogen.

The presence of the indicated concentration in the declared austenitic alloy of cobalt has a positive effect on its manufacturability by expanding the interval between the temperatures of complete dissolution of the hardening γ'-phase (T _{prγ '} ) and solidus (T _S ) and, as a result, contributes to an increase in heat resistance and ductility.

Nitrogen dissolved in the metal acts as an austenitizer, however, an increase in its concentration in excess of 0.05% wt. undesirable, as this can lead to excessive consumption of zirconium and lanthanum on its binding.

The content of chromium and nickel, which are the main elements of the inventive alloy that increase its heat resistance, is slightly lower than in the prototype, which makes it possible to introduce more refractory elements such as tungsten, cobalt and molybdenum, without the risk of the formation of embrittle compounds based on them. In this case, the heat resistance of the proposed alloy is maintained at a sufficiently high level. A further decrease in their content is impractical, since it will lead to a decrease in the life of the collectors due to a decrease in the content of intermetallic compounds in the alloy and an increase in creep. When the content of chromium and nickel is more than 23 and 33 wt.%, Respectively, it will be necessary to introduce an additional alloying component designed to increase the uniformity of austenitic grains, the structure of which will include reinforcing secondary carbides,

In the formulation of the claimed alloy, the following circumstances are taken into account:

- significantly reduced the total content of harmful impurities of tin, zinc, arsenic and antimony to 0.02 wt. % This contributes to the production of an alloy with the best complex of mechanical properties in long-term operation;

- instead of cerium, lanthanum is used in the claimed alloy and its role is as follows (see D.E. Kablov and other VIAM proceedings, No. 4 (52) 2017, p.22). Having practically zero solubility in nickel, it, as a surface-active metal, surpasses other lanthanides, including cerium, in efficiency. At elevated temperatures, lanthanum segregates at the boundaries of the γ / γ'-phases, slowing down the diffusion of atoms and vacancies on them and through them, which prevents coarsening of the structure, including the development of rafting. This element neutralizes sulfur segregation at and near the surface of the pores with the formation of refractory, chemically inert globular inclusions, restoring surface tension in the pores, inhibiting their growth and the development of cracks. Acting as a highly active refining, modifying and microalloying additive, lanthanum significantly increases the heat resistance.

The vapor pressure of lanthanum at a temperature of 1600 ° C is almost 2 times lower than that of cerium (0.88 and 1.6 Pa, respectively), i.e. under melting conditions, it will be removed from the induction furnace 2 times slower. Thus, the time spent in the melt will increase and the positive effect on the whole complex of properties of the resulting steel will be more significant;

- the proposed alloy implements three hardening mechanisms (See B. L. Gruzdev. Weldability of heat-resistant austenitic steels and alloys. Ufa. USATU. 2014. - 77 s):

1. the formation of an austenitic matrix with the introduction of elements (lanthanum) that reduce the intensity of diffusion processes;

2. the formation of the austenitic matrix with its additional hardening due to the precipitation of carbide (secondary carbides) and intermetallic phases;

3. the formation of an austenitic solid solution capable of the effect of dispersion hardening due to the release of finely dispersed intermetallic phases.

In the case of the simultaneous presence of lanthanum and zirconium in the alloy, along with a decrease in the concentration of oxygen, nitrogen, and sulfur, an increase in the mechanical properties of steel is possible. The addition of these elements allows you to adjust the size of the austenitic grains and their uniformity.

In the role of carbide-forming reinforcing elements in the claimed alloy there are molybdenum, tungsten, niobium, titanium.

The austenitic matrix and two intermetallic compounds Cr _{(22 ÷ 56)} Fe _{(4 ÷ 7)} Ni and Nb _{(25 ÷ 35)} Cr _{(2,5 ÷ 3,5)} (FeNiTi ) _{(0.9 ÷ 1.1)} enriched in chromium and niobium, inhibiting the creep of the metal under operational loads, and suppressing the appearance of cracks in pipe welding. The established mass ratio of the matrix and intermetallic compounds (91 ÷ 95) :( 3 ÷ 8) :( 1 ÷ 3) is optimal and is determined by the composition of the alloy, the conditions for its production in the induction furnace, the concentration intervals of the elements used, the properties of the austenitic matrix and the distribution of intermetallic compounds in it formations.

A similar approach to the problem of creating new-generation alloys allows us to achieve the formation of fundamentally new structures at the stages of steel smelting and pouring it into chill molds during the manufacture of reaction tubes, as well as to achieve a significant improvement in the performance of the metal at various temperatures.

It is understandable that any deviations from established patterns, for example, replacing the resulting intermetallic compounds with compounds of a different composition, will negatively affect the parameters of the reaction tubes and their resource.

The declared heat-resistant chromium-nickel alloy refers to low-carbon austenitic alloys and only induction furnaces with a main lining using pure charge materials are used for its smelting. The application of this method of melting the charge provides a good dispersion of the alloy components, which further reduces the negative impact of segregation processes.

The specified alloy is characterized as casting (it is not deformable, i.e. it cannot be pressed, forged or rolled), therefore, no additional measures are required to significantly limit the content of harmful impurities, such as sulfur and phosphorus, which sharply reduce the ductility of the alloy and do not allow it to be produced deformation without destruction. In turn, sulfur and phosphorus in the declared amounts improve the machinability of the alloy by cutting and grinding.

For the developed alloy, the technological methods have minimized the negative influence of oxygen and hydrogen dissolved in it, and the tendency of welds to form so-called hot cracks has been suppressed. The presence of small amounts of nitrogen (≤0.05% wt.) Favorably affects the maintenance of the structural homogeneity of the metal during long-term operation.

Products based on the inventive heat-resistant chromium-nickel alloy were obtained from centrifugal cast billets or castings made by pouring molten heat-resistant alloy into a rotating chill mold or in a specially prepared mold (for shaped casting) in compliance with strictly specified conditions. During its production at the final stage, certain alloying components (titanium, lanthanum, zirconium, etc.) are introduced into the molten metal in a certain sequence in order to avoid their oxidation and burning. Subsequently, after crystallization of the heat-resistant alloy with a given temperature gradient, the obtained cast billets were machined without deformation of the material structure, i.e., by boring.

The main test results were obtained using alloys of the following compositions, wt.%:

Example 1

Carbon - 0.11; silicon - 1.10; Manganese - 1.15; chrome 22.0; nickel - 32.2; niobium - 1.1; zirconium - 0.07; titanium - 0.06; tungsten - 0.07; lanthanum - 0. 004; sulfur - 0.02; phosphorus - 0.02; copper - 0.07; molybdenum - 0.05; cobalt - 0.02; lead - 0.003; tin + arsenic + zinc + antimony - 0.012; nitrogen - 0.03; iron is the rest.

The content of the austenitic matrix in the obtained pipe turned out to be 91.2% in May, and the intermetallic compounds Cr ₄₂ Fe ₄ Ni and Nb ₂₈ Cr _2.8 FeNiTi - 7.0 and 1.8 wt. %, respectively.

To conduct studies of the heat-resistant properties of the claimed alloy, a 150 mm length pipe was cut from the end part of the centrifugal cast billet, from which test samples were prepared. The direction of the axis of the cut samples coincided with the direction of the axis of the centrifugally cast pipe.

Electron microscopy and X-ray diffraction analysis were performed using a Sigma f scanning electron microscope. Karl Zeiss equipped with the analytical system f. EDAX (USA) with Apollo detector and Hikari back-scattered electron detector.

A preliminary analysis of the microstructure of the material of the metal samples on a scanning electron microscope using the detection of backscattered electrons by the AsB detector showed the presence of three phases in the microstructure of the proposed alloy: the main austenitic matrix and two intermetallic phases that differ in contrast of detection of backscattered electrons. When analyzing the experimental data, it can be concluded that the intermetallic phases form a strengthening network, due to which the tendency to creep of the metal at high temperatures is reduced. The high uniformity of the crystal structure of the alloy also contributes to this.

The average grain size was determined in the eyepiece of a metallographic microscope on frosted glass (GOST 5639 "Steel. Methods for the detection and determination of grain size").

It was experimentally established that the claimed alloy, it is equal to 254 microns, that is, almost the same as that of the alloy - the prototype (255 microns).

The heterogeneity coefficient of austenitic grains was calculated as the ratio A = R _max / R _min , where R _max and R _min are the maximum and minimum linear dimensions, respectively. In the known prototype alloy A = 1.04-1.07, and for the claimed A = 1.02-1.05, which indicates the achievement of increased uniformity of crystalline formations.

The uniformity of the distribution of finely dispersed particles of secondary carbides in austenitic grains was evaluated using a metallographic microscope on frosted glass (GOST 5639 "Steel. Methods for the detection and determination of grain size") using the coefficient K = R _max / R _min , where R _max and R _min - maximum and minimum distance between particles, respectively. For the claimed alloy, it was equal to 4.0 (4.1 for the prototype), which indicates a higher uniformity of the distribution of particles of secondary carbides.

The mechanical properties were tested at temperatures of 20 and 960 ° C on samples with a working part f5, 25 mm long in accordance with GOST 9651 on a FP-100/1 machine with a sample stretching speed of 2 mm / min.

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

In order to reduce the duration of tests for the samples installed in the machine, stresses were applied at σ - 60; fifty; 40 and 35 N / mm ² in accordance with GOST 10145), which allowed us to determine the specific values of 1000-hour long-term strength from the obtained heat resistance graph (lgτ-lgσ).

It was found that the value of the long-term strength limit of the claimed alloy at 960 ° C per 100,000 hours, obtained by extrapolation, was 19.1 MPa (prototype 18.4 MPa), and the yield strength (σ ₀₂ ) was 3% higher.

This allows you to increase the life of the reaction tubes on ammonia units (pressure 4.5 MPa, temperature 850 ° C) from one hundred thousand to 125,000 hours.

Example 2

The studies were conducted on an alloy with the following content of elements, wt. %: carbon - 0.08; silicon - 0.80; Manganese - 1.0; chromium - 20.0; nickel - 31.0; niobium - 1.4; zirconium - 0.11; titanium - 0.08; tungsten - 0.07; lanthanum - 0. 007; sulfur - 0.025; phosphorus - 0.01; copper - 0.06; molybdenum - 0.07; cobalt - 0.08; lead - 0.003; tin + arsenic + zinc + antimony - 0.015; nitrogen - 0.03; iron is the rest.

The content of the austenitic matrix turned out to be 91.3% wt., And the intermetallic compounds Cr ₃₈ Fe ₄ Ni and Nb ₃₀ Cr ₃ FeNiTi - 6.5 and 2.2 wt. %, respectively.

For the alloy under study, the coefficient of heterogeneity of austenitic grains turned out to be A = 1.01-1.04, the distribution coefficient of finely dispersed particles of secondary carbides K = 4, and the long-term strength of the claimed alloy at 960 ° C per 100,000 hours was 18.8 MPa. The yield strength was the same as in example 1, that is, slightly higher than that of the prototype.

Thus, it can be argued that an increase in the heat resistance of the studied samples was achieved by optimizing the alloy structure, the presence in the austenitic matrix of the strengthening network of intermetallic phases, and due to the additional introduction of lanthanum and cobalt.

The tendency of samples of the declared heat-resistant alloy to carburization was evaluated by the kinetics of their saturation with carbon after testing for 1000 hours.

To this end, cylindrical specimens 10 mm in diameter and 50 mm long were cut from centrifugally cast tubes made according to Examples 1 and 2 and subjected to grinding to a surface cleanliness of at least R _Z = 80 μm.

The carburization test was carried out at a temperature of (1060 ± 10) ° C in a medium of carbon black grade P 324 according to GOST 7885 by heating them for 200 hours in a heat-resistant container. The diffusion of carbon into the metal was judged by the increment in the mass of the samples, as well as by the depth of the carbonized layer using the metallographic method.

We found that if all components of the alloy, including intermetallic compounds, are within the concentration limits specified in the claims, there is no carburization. Only in this case, high values of physical and mechanical properties of the metal and an increased service life of the reaction tubes are achieved.

When analyzing welds using non-destructive testing methods, cracks in examples 1 and 2 were not identified. Putting any other elements into the alloy or deviating from the recommended concentrations will violate the established patterns and lead to a significant deterioration in the characteristics of the reaction tubes during their operation. This phenomenon is due to loosening of intermetallic structural formations and an increase in the coefficients “A” and “K”.

From the description of the invention and the examples given, it follows that according to the claimed technical solution, it is possible to obtain a heat-resistant chromonickel-left austenitic alloy with an improved distribution of secondary carbides and intermetallic hardening of the austenitic matrix, which positively affects its mechanical properties, avoids carburization during the pyrolysis of hydrocarbons and the formation of hot cracks when welding reaction tubes.

Claims (3)

Heat-resistant alloy containing carbon, silicon, manganese, chromium, nickel, niobium, titanium, zirconium, tungsten, molybdenum, sulfur, phosphorus, lead, tin, arsenic, zinc, antimony, copper and iron, characterized in that it additionally contains cobalt , lanthanum and nitrogen, in the following ratio of elements, wt.%: carbon 0.05 ÷ 0.15 silicon 0.50 ÷ 1.50 manganese 0.50 ÷ 1.50 chromium 19 ÷ 23 nickel 30 ÷ 33 niobium 0.70 ÷ 1.60 titanium 0.005 ÷ 0.10 zirconium 0.005 ÷ 0.15 tungsten 0.005 ÷ 0.10 lanthanum 0.005 ÷ 0.10 cobalt 0.0005 ÷ 0.10 molybdenum ≤0.10 sulfur ≤0.03 phosphorus ≤0.03 lead ≤0.01 tin + arsenic + zinc + antimony ≤0.02 nitrogen ≤0.05 copper ≤0.1 iron rest, Moreover, it has a structure consisting of an austenitic matrix and intermetallic compounds Cr _(22–56) Fe _(4–7) Ni and Nb _(25–35) Cr _(2.5–3.5) (FeNiTi) _{( 0.9 ÷ 1.1)} with the mass ratio of the austenitic matrix and intermetallic compounds (91 ÷ 95) :( 3 ÷ 8) :( 1 ÷ 3).