Welding of austenitic steels. Weldability of austenitic steels

Above a certain content of manganese, nickel or some other elements, the γ-state exists as a stable state from room temperature to the melting point. Such highly alloyed iron alloys are called austenitic steels. Unlike other iron alloys, austenitic steels (and ferritic steels) do not undergo transformations when heated and cooled. Therefore, heat treatment is not used to strengthen austenitic steels.

Cold-resistant austenitic steels also include chromium-manganese steels(austenitic steels in which nickel is completely or partially replaced by manganese); stable austenitic chromium-nickel-manganese steels with nitrogen(austenitic steels simultaneously alloyed with chromium, nickel and manganese) and metastable austenitic steels.

Lit.:

1. Gulyaev A.P. Metallurgy. - M.: Metallurgy, 1977. - UDC669.0 (075.8)
2. Ivanov V.N. Dictionary-reference book for foundry production. – M.: Mechanical Engineering, 1990. – 384 p.: ill. ISBN 5-217-00241-1
3. Solntsev Yu.P., Pryakhin E.I., Voytkun F. Materials science: Textbook for universities. - M.: MISIS, 1999. - 600 p. - UDC 669.017

Existing austenitic high-alloy steels and alloys are distinguished by the content of the main alloying elements - chromium and nickel and by the composition of the alloy base. High-alloy austenitic steels are considered to be iron-based alloys alloyed with various elements in amounts up to 55%, in which the content of the main alloying elements - chromium and nickel - is usually no higher than 15 and 7%, respectively. Austenitic alloys include iron-nickel alloys with an iron and nickel content of more than 65% with a nickel to iron ratio of 1:1.5 and nickel alloys with a nickel content of at least 55%.

Austenitic steels and alloys are classified

• according to the alloying system,
• structural class,
• properties
• and official purpose.

High-alloy steels and alloys are the most important materials, widely used in chemical, petroleum, power engineering and other industries for the manufacture of structures operating in a wide temperature range. Due to their high mechanical properties at subzero temperatures, high-alloy steels and alloys are used in a number of cases as cold-resistant steels. The appropriate selection of alloying elements determines the properties and main service purpose of these steels and alloys (Tables 1 – 3).

A characteristic feature of corrosion-resistant steels is their low carbon content (no more than 0.12%). With appropriate alloying and heat treatment, steels have high corrosion resistance at 20°C and elevated temperatures both in a gas environment and in aqueous solutions of acids, alkalis and liquid metal media.

Heat-resistant steels and alloys have high mechanical properties at elevated temperatures and the ability to withstand heating loads for a long time. To impart these properties, steels and alloys are alloyed with strengthening elements - molybdenum and tungsten (up to 7% each). An important alloying additive introduced into some steels and alloys is boron, which promotes grain refinement.

Heat-resistant steels and alloys are resistant to chemical destruction of the surface in gas environments at temperatures up to 1100 – 1150°C. They are usually used for lightly loaded parts (heating elements, furnace fittings, gas pipeline systems, etc.). The high scale resistance of these steels and alloys is achieved by alloying with aluminum (up to 2.5%) and silicon, which contribute to the creation of strong and dense oxides on the surface of parts that protect the metal from contact with the gaseous environment.

According to the alloying system, austenitic steels are divided into two main types: chromium-nickel and chromium-manganese. There are also chromium-nickel-molybdenum and chromium-nickel-manganese steels.

Depending on the basic structure obtained by cooling in air, the following classes of austenitic steels are distinguished: austenitic-martensitic, austenitic-ferritic, austenitic.

Alloys based on iron-nickel (with a nickel content of more than 30%) and nickel bases are stably austenitic in structure and do not have structural transformations when cooled in air. Currently, austenitic-boride Kh15N15M2BR1 (EP380), Kh25N20S2R1 (EP532), KhN77SR1 (EP615) and high-chromium austenitic KhN35VYu (EP568), KhN50 (EP668) steels and alloys, the main structure of which contains austenite and boride or chromium-nickel eutectic phases respectively.

After appropriate heat treatment, high-alloy steels and alloys have high strength and plastic properties (Table 4). Unlike carbon steels, these steels acquire increased plastic properties when hardened. The structures of high-alloy steels are varied and depend not only on their composition, but also on heat treatment modes, the degree of plastic deformation and other factors.

The position of phase regions on phase diagrams is determined mainly in the form of pseudo-binary sections of the iron-chromium-nickel or iron-chromium-manganese systems (Fig. 1). Iron-chromium-nickel alloys immediately after solidification have solid solutions of the following types: α And γ and heterogeneous region of mixed solid solutions α + γ . The stability of austenite is determined by the proximity of the composition to the boundary α - And γ -regions Instability can manifest itself when heated to moderate temperatures and subsequent cooling, when the austenitic structure fixed by rapid cooling partially transforms into martensitic. An increase in nickel content in these alloys contributes to a decrease in temperature γ → α (M)-transformations (Fig. 2).

Rice. 1. Vertical sections of the phase diagrams of iron–chromium–nickel (a) and iron–chromium–manganese (b)

Rice. 2. Changes in the temperature of martensitic transformation of iron-chromium-nickel alloys depending on alloying

Instability manifests itself during cold deformation, when steels of type 18-8, depending on the degree of deformation, change their magnetic and mechanical properties (Fig. 3). In addition, the instability of austenitic steels can be caused by the release of carbides from the solid solution when the temperature changes, accompanied by a change in the concentration of carbon and chromium. This causes a disruption of the equilibrium state and the transformation of austenite into ferrite and martensite mainly along the grain boundaries, where the greatest depletion of chromium and carbon in the solid solution is observed.

Rice. 3. Change in the mechanical properties of chromium-nickel steel (18% Cr, 8% Ni, 0.17% C) depending on the degree of cold deformation (compression)

In the ternary system of iron-chromium-manganese alloys, after solidification, a continuous series of solid solutions with γ -lattice and during further cooling, depending on the composition of the alloy, various allotropic transformations occur. Manganese is one of the elements that expands γ - area, and in this respect is similar to nickel. With sufficient concentrations of manganese (>15%) and chromium (<15%) сталь может иметь однофазную аустенитную структуру. Сопоставление фазовых диаграмм систем железо – хром – никель и железо – хром – марганец при высоких температурах и 20°С показывает, что аустенитная фаза в системе с никелем имеет значигельно большую площадь.

During the crystallization of chromium-nickel steels, crystals of chromium-nickel ferrite, which has a δ-iron lattice, first begin to fall out of the melt (Fig. 4). As it cools, δ-ferrite crystals form chromium-nickel austenite, which has a lattice γ -iron, and the steel acquires an austenitic structure. Carbon in austenitic-ferritic and austenitic steels at temperatures above line S.E. is in solid solution and in the form of interstitial phases. Slow cooling of steel below the line S.E. leads to the release of carbon from the solid solution in the form of a chemical compound - chromium carbides of the Cr 23 C 6 type, located mainly along the grain boundaries. Further cooling below the line S.K. promotes precipitation of secondary ferrite along the grain boundaries. Thus, when slowly cooled to 20°C, steel has a ustenitic structure with secondary carbides and ferrite.

Rice. 4. Pseudo-binary phase diagram depending on the carbon content for the alloy 18% Cr, 8% Ni, 74% Fe

During rapid cooling (quenching), the decomposition of the solid solution does not have time to occur, and austenite is fixed in a supersaturated and unstable state.

The amount of precipitated chromium carbides depends not only on the cooling rate, but also on the amount of carbon in the steel. When its content is less than 0.02 - 0.03%, i.e. below the limit of its solubility in austenite, all carbon remains in solid solution. In some compositions of austenitic steels, accelerated cooling can lead to the fixation of primary δ-ferrite in the structure, preventing hot cracks.

A change in the content of alloying elements in steel affects the position of the phase regions. Chromium, titanium, niobium, molybdenum, tungsten, silicon, vanadium, being ferritizers, contribute to the appearance of a ferritic component in the steel structure. Nickel, carbon, manganese and nitrogen maintain the austenitic structure. However, the main alloying elements in the steels under consideration are chromium and nickel. Depending on their ratio, steels are sometimes divided into steels with a small (%Ni/%Cr)≤1 and a large (%Ni/%Cr)>1 austenitic reserve.

In austenitic chromium-nickel steels alloyed with titanium and niobium, not only chromium carbides are formed, but also titanium and niobium carbides. When the content of titanium Ti > [(%C–0.02)*5] or niobium Nb > (%C*10) all free carbon (above the limit of its solubility in austenite) can be released in the form of titanium or niobium carbides, and austenitic steel becomes not prone to intergranular corrosion. The precipitation of carbides increases the strength and reduces the plastic properties of steels. This property of carbides is used for carbide hardening of heat-resistant steels, carried out in combination with intermetallic hardening with Ni 3 Ti particles; Ni 3 (Al, Ti), Fe 2 W, (N, Fe) 2 Ti, etc. Intermetallic compounds also include the σ-phase, which is formed in chromium-nickel steels during prolonged heating or slow cooling at temperatures below 900 - 950 ° C . It has limited solubility in α - And γ -solid solutions and, being released mainly along the grain boundaries, strengthens the alloy and at the same time sharply reduces the plastic properties and impact strength of the metal. Increased concentrations of chromium (16–25%) and ferritizing elements (molybdenum, silicon, etc.) in steel contribute to the formation of the σ phase at 700–850°C. The separation of this phase occurs predominantly with the formation of an intermediate phase of ferrite ( γ →α→ σ ) or δ-ferrite transformation (δ → σ ). However, it is also possible to isolate it directly from a solid solution ( γ → σ ).

In chromium-manganese steels with a high content of chromium and manganese, precipitation is also observed during slow cooling. σ -phases. Carbon in chromium-manganese and chromium-manganese-nickel steels leads to dispersion hardening of steels after appropriate heat treatment, especially when combined with carbide-forming elements (vanadium, niobium and tungsten).

Strengthening of austenitic boride steels occurs mainly due to the formation of borides of iron, chromium, niobium, carbon, molybdenum and tungsten. In accordance with these processes, austenitic steels are divided, depending on the type of hardening, into carbide, boride and intermetallic hardening. However, in most cases, due to the content of a large number of different alloying elements in steels and alloys, their strengthening occurs due to the complex influence of dispersed phases and intermetallic inclusions.

Table 1. Composition of some corrosion-resistant austenitic steels and alloys, %

Table 2. Composition of some heat-resistant austenitic steels and alloys, %

Table 3. Composition of some heat-resistant austenitic steels and alloys, %

Table 4. Typical mechanical properties of some grades of high-alloy austenitic and austenitic-ferritic steels and alloys

E. G. NAZAROV, S. B. MASLENKOV
TSNIICHERMET
ISSN 0026-0819. “Metal science and heat treatment of metals”, No. 3, 1970

Heat treatment affects the structure (grain size, block size, size and quantity of dispersed phases, the nature of their distribution), and also shapes the state of grain boundaries and the directed release of strengthening phases, which significantly increases the properties of heat-resistant materials.

Mechanical treatment usually precedes heat treatment, but is often used after heat treatment, as well as before and after it.

Parts and semi-finished products are subjected to heat treatment before operation, but sometimes (in whole or in part) they are processed during operation.

Austenitic precipitation-hardening steels and alloys are subjected to various types of heat treatment: annealing, hardening, tempering (aging or precipitation hardening) and stress-relieving tempering.

During machining or other operations, the metal becomes embrittled. To eliminate brittleness and reduce the hardness of alloys, annealing is used. When annealing, alloys are heated to high temperatures ~1000-1250 °C (depending on the chemical composition of the alloy), held for 0.5 to several hours (depending on the mass of the workpiece or part) and cooled at the highest possible speed. For less alloyed alloys, cooling in water is allowed, but for highly alloyed complex alloys, cooling in air in oil and other mild cooling media is preferable, since cooling in water can lead to thermal cracks.

To achieve high strength properties and heat resistance, heat-resistant steels and alloys are subjected to double processing consisting of hardening and subsequent aging.

For the alloys under consideration, the hardening operation differs in its effect from the hardening of carbon steels and is carried out with the aim of dissolving carbide and intermetallic phases in a solid solution, i.e. to obtain a homogeneous solid solution with minimal hardness. In the USA and England, hardening of ordinary carbon steels is called “hardening”, i.e. acquiring hardness; hardening of heat-resistant alloys is called “solution treating,” i.e. processing into a (solid) solution.

For all dispersion-hardening heat-resistant steels and alloys, the heating temperature for hardening is approximately the same as the annealing temperature.

By holding at high temperatures, excess phases are dissolved in a solid solution and grains of the required size are obtained. The grain size of steels and alloys depends on the heating temperature and holding time.

Often, after quenching, it is recommended to conduct faster cooling to prevent the precipitation of excess phases. However, as will be shown below, this is unnecessary, especially when processing complex austenitic alloys, in which, even with relatively rapid cooling, catathermic hardening occurs, i.e., the release of strengthening phases when cooled from a high temperature. This process depends on the tendency of the alloys to dispersion hardening, so it is necessary to dwell on this important phenomenon.

Dispersion hardening or aging of steels and alloys can be: anathermic, catathermic and isothermal. Diathermic aging occurs in the process of heating a steel or alloy at a continuously increasing temperature, catathermal aging occurs in the process of cooling a steel or alloy at a continuously decreasing temperature. Isothermal aging occurs at a constant temperature

There are weakly, moderately and strongly dispersion-hardening alloys. There is no sharp distinction between them, however, it is easy to separate these groups of alloys based on the intensity of dispersion hardening processes. According to this principle, for the first time in the work, and later in the work, dispersion-hardening alloys were divided into three groups.

Highly precipitation-hardening steels and alloys are generally effectively strengthened due to hardening during catathermal aging. These alloys contain 5-7% or more of the strengthening phase. Additional aging of these alloys leads to little or almost no increase in hardness and strength, for example, such alloys as: NH35VTYu (EI787), EI929, EI867, Yudimet 700, Nin-109, Nin-115, etc. The chemical composition of the alloys is given in Table. 3 and 4.

Moderately dispersion-hardening alloys are strengthened during catathermal and, to a greater extent, during isothermal aging. These alloys KhN35VT (EI612), EI612K, KhN35VTR (EI725), EP164, A-286, Discaloy-24 contain 2-5% of the strengthening phase.

Weak or low dispersion-hardening alloys are strengthened only during artificial isothermal aging. These steels and alloys are not subject to catathermal aging and contain a small amount of a strengthening phase (up to 2%). This group includes alloys: EI813, Kh25N16G7AR (EI835), EI435, Nim-75, V-480S, etc.

Thus, there is no need to ensure rapid cooling of the alloys after high-temperature heating. The necessary strengthening of alloys of one or another group can be achieved as a result of natural catathermal or artificial isothermal aging, or, finally, as a result of their combinations.

Double hardening. For some alloys, especially those containing a significant amount of the strengthening phase, the best combination of mechanical properties is obtained after double hardening (normalization). The first high-temperature normalization (1170-1200 °C) ensures the formation of a homogeneous solid solution and relatively coarse grains, which contribute to the highest creep resistance. The second low-temperature normalization (1000-1100 °C) leads to the predominant precipitation of carbides along the grain boundaries and the formation of a strengthening phase of varying dispersion. Larger γ'-phase precipitates are formed upon cooling from 1050 °C in air. For many alloys - KhN70VMTYu (EI617), EI929, KhN35VTYu (EI787), the "Nimonic" series - after double normalization followed by aging, the heat-resistant and plastic properties significantly increase.

Dispersion hardening (aging). To obtain high strength properties, almost all heat-resistant alloys are subjected to dispersion hardening (separation of dispersed phases from a solid solution) before use. The composition and nature of the strengthening phases determine the aging temperature regimes for a given alloy.

Heat-resistant alloys based on nickel-chromium, iron-nickel-chromium and cobalt-nickel-chromium bases contain:
a) primary carbides (TiC, VC, TaC, ZrC, NbC, etc.), having a very high dissociation temperature;
b) secondary carbides (M 23 C 6; M 6 C; M 7 C 3), released from the solid solution. Carbide M 23 C 6 is formed in alloys with 5% Cr or more;
c) the main strengthening intermetallic γ’-phases (Ni 3 Ti, Ni 3 Al, Ni 3 Nb, etc.). Due to the fine dispersion of these phases and coherence with the solid solution, alloys during their formation acquire maximum heat resistance.

Carbide-hardened steels and alloys are used at lower temperatures than intermetallic-hardened alloys. Carbides are less dispersed, more prone to coagulation, and distributed less uniformly in the alloy matrix than the γ' phases. However, to achieve average heat resistance, one carbide strengthening is sufficient. Carbide phases additionally strengthen alloys that harden as a result of the precipitation of the γ'-phase.

The morphology of particles of γ'-phases and carbides largely depends on heat treatment and its duration and regulates the properties of alloys. The duration of thermal exposure leads to enlargement of the particle sizes of the γ’-phase and causes reactions that occur primarily at grain boundaries. To understand the processes occurring in alloys during heat treatment and predict their properties during long-term service, it is very important to know the exact composition of the γ'-phase at any temperature and various holding times at this temperature, as well as the chemical composition of the matrix solid solution. The rates of transformation of carbide and intermetallic phases and their reactions can be additionally assessed using data from the kinetics of changes in hardness, physical and mechanical properties. In the most common, heat-resistant nickel-based alloys containing chromium and cobalt, alloyed with aluminum, titanium and molybdenum, the transformation reactions can be expressed as an equation: MS+γ→ M 6 S+γ+γ’+ MS, Where M elements: Cr, Ti, Ta and others; M'- the same carbide-forming elements as in M. Approximately half the amount of carbon, according to the work, remains in carbides MS, which we conventionally called M'S; γ’-phase (Ni 3 M) - a compound of excess titanium and aluminum in a solid γ-solution with nickel.

Carbides M 6 S are formed at 980-1150 °C, while the carbide reaction MS→M 23 S 6 occurs at 760-980 °C. It has been established that if the alloy contains molybdenum and tungsten in the amount of >6%, then carbides will mainly be released in the form M 23 S 6, however, it is indicated that this provision appears to be inaccurately substantiated. This obviously depends on the carbon content.

Studies carried out on the V-1900 alloy have established the reactions that occur in it after heat treatment (1080 °C 4 h, air+899 °C 10 h, air) and during long-term aging up to 2400 h at 980 °C. They are expressed by the equation:
MS + γ + γ’ → M 6 S+ γ + remainder γ’.

Carbides MS (A= 4.37 Å) are rich in titanium and tantalum, and carbides M 6 S (A= 11.05 Å) are rich in molybdenum, nickel and cobalt. Carbides M 6 S are observed in two forms: globular and lamellar. Over time, the globules and plates of carbides become larger. The γ'-phase precipitates are initially globular, then the γ'-phase appears in the form of plates; over time, at high temperatures, they grow, agglomerate, and elongate in size. At the same time, the γ'-phase precipitates surround all carbides and grain boundaries in the form of a shell. The application of voltage significantly accelerates the carbide transition process MS into carbides M 6 S and intermetallic changes. In alloys with higher chromium content, carbides are mainly formed M 23 S 6.

The reaction rate of the γ'-phase transformation is greater when stresses are applied during heat exposure than when stresses are previously obtained. Stresses lead to selective processes of precipitation and transformations and contribute to the thickening of grain boundaries, causing elongation and coalescence of strengthening phases, as was shown in the works. Grain coarsening helps accelerate the reactions of transformations of carbide and intermetallic phases occurring in the boundary zones. For example, the appearance of a high-temperature lamellar phase in alloys is detected much earlier in coarse-grained alloys.

The work established the formation of an intermetallic phase Ni 2 -Al, Ti in the 15 Cr-25 Ni-3 Al-2.5 Ti alloy, along with the γ’-phase Ni 3 (Al, Ti). The Ni 2 Al, Ti phase is formed during aging at 700 °C and has the form of plates, the size of which increases with aging time. This phase is released mainly in areas free from the γ’ phase, as well as along grain boundaries. It is incoherent with the solid solution, so microvoids before the destruction of the alloy are formed primarily near its precipitates.

Laves phases(AB 2) - slightly strengthen alloys due to their incoherence with the solid solution and thermal instability. But in the presence of a γ’-phase in the structure, Laves phases make it possible, due to the inherent duration of the incubation period of precipitation, to extend the service life of alloys at temperatures not higher than 750 °C.

Boride phases- type M 3 AT 2 , M 3 IN, M 5 The 5 different boron alloys have complex chemical compositions. For example, in this work, such phases correspond to the compound (Mo 0.5 Cr 0.25 Ti 0.15 Ni 0.10) 3 B 2

Depending on the presence of certain phases and the state of the alloy (cast, deformed), dispersion hardening modes are prescribed. The aging temperature should not cause dissolution of the strengthening phases and coagulation or coalescence. Although in some cases, to obtain the desired properties, it is necessary to deliberately apply high temperatures, causing coagulation of particles and their release in a less dispersed form. Typically, aging of alloys with carbide hardening is carried out at 600-800 °C, with intermetallic hardening at 700-1000 °C, depending on the number and composition of excess phases. With an increase in the amount of the strengthening phase (the sum of titanium and aluminum) in the alloys, the aging temperature also increases (see Fig. 1). Alloys containing more than 8% (Ti+Al) are only heated to 1050-1200 °C and cooled in air. As a result of catathermal aging, such alloys acquire maximum hardening (for example, alloys ZhS6-K and EI857). Rene 100 and IN-100 alloys with 9-10.5% (Ti+Al) are aged at ~1000 °C, but this is essentially a second hardening, not aging. Apparently, for such alloys this high-temperature aging is unnecessary; they are even more susceptible to catathermal aging, and for them cooling in air from normalization temperatures is quite sufficient, as, for example, shown in the figure for the IN-100 alloy

Fig.1.

Aging modes can be changed depending on the required properties of the alloy. There are stepwise aging regimes - double and more complex, but they are not very practical. For short-term service life and especially for long-term service life, the use of multi-stage aging modes is completely unjustified, since the resulting structures in the process of complex heat treatments inevitably change under conditions of long-term operation, under the influence of temperature and load. Aging processes in alloys continue to occur regardless of the initial structural state. Particles of the strengthening phase coagulate, coalesce, and unstable particles dissolve in the solid solution, repeated and repeated releases of new more equilibrium (at this stage) particles occur, these processes occur simultaneously. Depending on temperature conditions, one or another process may predominate. After exposure (usually from 4 to 16 h) at aging temperatures, the alloys are cooled in air.

Typical heat treatment regimes for foreign alloys are presented in table. 1. and for domestic ones - in table. 2. The chemical compositions of these alloys are given in table. 3 and 4. It should be noted that we almost never use annealing for these alloys, and annealing from quenching (normalization) differs very little (see Table 1).

Table 1

table 2

Table 3

Table 4

The hardening temperature abroad is lower and the holding time is much shorter (almost 2 times) than the hardening temperature used in the USSR. As a result, foreign alloys are finer-grained than those used in our country. Second hardening is not used abroad, whereas in our country it is successfully used for many alloys.

Given in table. 1 and 2 typical heat treatment modes can be changed depending on requirements. It is known that alloys with coarse grains, obtained by heating to high temperatures, have higher creep resistance than fine-grained ones. Coarse-grained alloys (2-3 points) also have significantly higher long-term strength at high temperatures. However, in the case of moderately high temperatures (600-700 °C), alloys with an average grain size of 4-5 points have higher heat resistance. The fine-grained structure, due to the higher surface energy of branched grain boundaries, is more unstable, especially at elevated operating temperatures, therefore the grain size of heat-resistant alloys, especially those intended for long-term service, must correspond to 3-4 points on the standard scale. This grain size is common after heating to 1100-1120 °C, and for complex alloys at 1150-1170 °C.

Abroad, most industrial alloys are heated at these temperatures.

To obtain high strength properties at room and low temperatures (~550 °C), normalization should be carried out at 950-1050 °C and aging at lower temperatures, as a result of which the alloys are fine-grained (5-6th point), strengthened by finely dispersed γ precipitates '-phases.

Thus, the choice of heat treatment mode is determined by the required mechanical properties. When using highly dispersion-hardening alloys for operation at temperatures exceeding the temperature range of dispersion hardening (for example, at 900-950 °C), they are subjected to only one normalization. When heated to operating temperatures, intensive hardening of the alloys occurs during the heating process (anathermic aging), they receive maximum hardening in the operating temperature zone and can successfully withstand loads for a certain time. However, the same alloys, pre-aged, have less resistance to temperatures and loads and, therefore, are less efficient. Weakly dispersion-hardening alloys (EI813, EI435, Inconel-600, etc.) are not subjected to aging, since their dispersion hardening has little effect and occurs during operation. To ensure long-term stability of alloys, a moderate content of strengthening phases in their structure is necessary (i.e., the use of moderately dispersion-hardening alloys). It is very important to obtain a uniform and maximum separation of finely dispersed intermetallic and carbide phases, which was provided for by stepwise processing modes. Stepped aging regimes, although they lead to a loss of strength properties, significantly increase the plastic properties and reduce the tendency of alloys to thermal brittleness. However, later experiments showed the inappropriateness of this method. Thus, on the highly dispersion-hardening alloy KhN35VTYu (EI787), complex heat treatment regimes were tested simultaneously with the simplest regime, consisting of only one aging at 750 °C. The tendency to thermal brittleness was assessed at exposures up to 10’000-20’000 h and temperature 700 °C. The results (Table 5) show that, regardless of the complexity of the preliminary heat treatment regime, the alloy becomes embrittled. Increasing the number of tempering stages or holding duration affects only the initial values ​​of impact strength. During the aging process it decreases, and to a lesser extent after heat treatment consisting of aging alone.

As previously indicated, the processes of dispersion separation, coalescence and dissolution of thermodynamically unstable particles of the second phase occur continuously. These processes occur regeneratively, cycle after cycle is repeated, therefore, no matter how much the alloy is pre-aged and heat treatment regimes are complicated, it will change its properties during long-term heat exposure and become embrittled as a result of the constant release of particles of the strengthening phase and changes in the structural state.

We should focus on the original and simple mode of heat treatment of dispersion-hardening hot- or cold-deformed alloys, which consists of single aging (without pre-hardening).

This mode allows you to obtain the best strength properties and ductility in a wide temperature range, as well as the highest heat resistance and fatigue resistance at temperatures up to 750 ° C. In addition, this mode provides better resistance to thermal embrittlement and insensitivity to cuts. A processing regime consisting of aging alone has been tested on some alloys and has been successfully introduced into production. There is no information yet on the use of such regimes abroad.

Another important condition for ensuring long-term stability of alloys is achieving high thermal stability of the strengthening phases. This is achieved by complicating the composition of the strengthening phases, by introducing into the alloy elements that are partially included in the composition of the strengthening γ’-phase. The most effective strengthening γ phases - Ni 3 Al and Ni 3 Ti and their combination - Ni 3 (A1, Ti) can be complicated by: niobium, tantalum, tin, silicon, magnesium, beryllium, ruthenium, molybdenum and other elements that provide dispersion hardening of nickel alloys. Of these, elements with a slightly larger atomic diameter, such as tin, are of particular interest.

The atomic diameters of some elements that form γ’ type phases with nickel are as follows:

Relieving stress. Tempering is often used to relieve stress and stabilize the dimensions of parts. Internal stresses can arise as a result of machining, welding or during operation. Finished products made of heat-resistant alloys are tempered at 400-700 °C with exposure depending on the dimensions of the product; after the holiday, slow cooling. At higher tempering temperatures, aging processes begin to occur, and for many alloys tempering can be combined with conventional aging, therefore, as a final treatment before operation, it is advisable to carry out aging, which allows the internal stresses to be completely removed.

New research. A patent was issued in the USA for a method for increasing the hardness, strength characteristics, creep resistance and heat resistance of austenitic heat-resistant alloys on nickel, nickel-cobalt and other bases (US Patent No. 3329535 dated July 4, 1967). This method consists of solution processing with air cooling under the application of high hydrostatic pressure (10’000-50’000 atm), which noticeably reduces the solubility of carbon in solid solution (holding under pressure 1-10 min). As a result of high pressure, carbon atoms or carbides are “squeezed out” from the matrix into coherent precipitates and are arranged in the form of a network, while particles of coherent phases do not fall out, as usual, along grain boundaries. With subsequent aging (650-980 °C), carbides precipitate around uniformly distributed cellular formations of the solid solution.

Of interest are studies carried out in the USA on the Inconel-718 alloy. Strengthening of this alloy is achieved by precipitation of the γ'-phase based on Ni 3 Nb, the composition of which corresponds to the Ni 3 compound (Nb 0.8 Ti 0.2), . The Incone1-718 alloy is slowly dispersion-hardening and, as a result, high-tech and well weldable. It is suitable for operation up to 760 °C. Its high strength (σ 0.2 to 120-145 kg/mm ​​2) combined with good corrosion resistance. Noteworthy is the low normalization temperature of 955 °C (see Table 1), which provides high strength values. The influence of niobium on the properties of this alloy is beneficial and effective. Titanium also has an increasing effect on the properties of the Inconel-718 alloy, no less than niobium. The effect of aluminum is less significant, causing a slight increase in strength with a variable effect. Silicon is similar in influence to niobium with minor deviations. The paper presents the results of studies of binary (Ni+Si) and ternary (Ni+Si+Ti) alloys. The formation of the β-phase has been established: Ni 3 S and Ni 3 (Si, Ti), in alloys containing ~12-13% Si and 6-10% Si and 1-4% Ti, respectively. The X-ray diffraction method established that the Ni 3 (Si, Ti) phase is similar to the γ’-phase Ni 3 (Al, Ti); Ni 3 Si, or β-phase in binary alloys is formed as a result of a peritectoid reaction at temperatures below 1040 °C. It has significant plasticity, like the corresponding Ni 3 (Si, Ti) phase. The addition of titanium to the binary alloy (~2%) eliminates the peritectoid β-formation, and the resulting Ni 3 (Si, Ti) phase has the same melting point as the Ni 3 Ti compound (1380 °C). Alloys containing silicon and titanium in the indicated quantities have fairly high strength properties and ductility. The maximum tensile strength and yield strength of cast alloys at room temperature are respectively: 55-57 and 25-28 kg/mm ​​2, and the minimum elongation is in the range of 15-30%. Other properties of these dispersion-hardening alloys are not given.

Harmful phases. During long-term heat treatment or during service, σ-, μ- and other phases are released in many heat-resistant alloys, which do not have a strict stoichiometric ratio and are solid solutions of variable composition. These phases cause a decrease in the plastic properties of steels and alloys. The formation of the σ-phase can be greatly facilitated by chromium, tungsten, molybdenum, etc. Small additions of cobalt (up to 5%) can reduce the process of σ-formation. At the same time, it is part of the strengthening phase Ni 3 M and releases chromium into a solid solution. Cobalt content above 5% actively affects σ-formation, especially when there is a deficiency of chromium in the alloy. There are methods for calculating the time of formation of the σ phase in alloys. These are calculations of the so-called N v point - the point of density of electron vacancies, however they are not always accurate. There are alloys that have a dangerous point Nv, but do not form a σ-phase. The σ phase was discovered in the alloys Ud-700, Ud-500, Ud-520, IN-713C, and Rene-41. Although the σ phase reduced the performance of Ud-700 and IN-100 alloys, it had little or no effect on the strength of other alloys. Studies of high-strength cast alloys have established that the presence of the σ-phase does not affect the decrease in properties.

Nickel-based alloys resist oxidation well up to temperatures of 850-950 °C. At higher temperatures (heating temperatures for quenching), they are oxidized from the surface and along the grain boundaries, therefore, for heat treatment of heat-resistant alloys at high temperatures, according to the work, it is desirable to have vacuum or hydrogen furnaces. Cooling of the metal at the end of the exposure is achieved using a jet of inert gas. If oxidation is unacceptable, ovens with a protective atmosphere must be used. Heating in salt baths is undesirable, since the chlorides in the bath can react with the metal surface during the heating process, even at aging temperatures. Thermal furnaces for aging can be conventional with an air atmosphere and heated by gas. A dilute exothermic atmosphere is relatively safe and economical. An endothermic atmosphere is not recommended. If oxidation is unacceptable, then an argon atmosphere is used. The accuracy of temperature control during heat treatment should be within 4-5 °C for wrought alloys, and 8-10 °C for cast alloys.

Bibliography:

1. Nazarov E. G., Latyshov Yu. V. Improving the properties of dispersion-hardening heat-resistant steels and alloys. M., GOOINTI, 1964, No. 23-64-1349/26.
2. Borzdyka A. M., Tseitlin V. 3. Structure and properties of heat-resistant alloys in connection with heat treatment of NTO MASHPROM M., “Machine Building”, 1967.
3. Belikova E.I., Nazarov E.G. “MiTOM”, 1962, No. 7.
4. Betterige W., Franklin A. "J. of the Institute of Metals", 1957, v. 85.
5. Betteridge W. Smith. Heat-resistant metal materials. Foreign publishing house lit., 1958.
6. Belyatskaya I. S., Livshits B. G. “News of universities. Ferrous metallurgy", 1960, No. 7.
7. Estulin G.V. Supplement to the magazine "Steel", 1958.
8. Livshits D. E., Khimushin F. F. Research on heat-resistant alloys. USSR Academy of Sciences, 1957.
9. Danesi W., Donachie M., Radavich J. “TASM”, 1966, v. 59.
10. Danesi W., Donachie M. “J. of the Institute of Metals", 1969, v. 97.
11. Cowan T. "J. of Metals", 1968, v. 20, no. 11.
12. Nazarov E. G., Pridantsev M. V. “MiTOM”, 1963, No. 11.
13. Nazarov E. G. “MiTOM”, 1969, No. 8.
14. Sims S. "J. of Metals", 1966, No. 10.
15. Levin E. E., Pivnik E. M. Progressive methods of heat treatment of high-alloy heat-resistant alloys. Series “Metal science and heat treatment”. Vol. 4. Leningrad, 1963.
16. Gulyaev A. P., Ustimenko M. Yu, “Izvestia of the USSR Academy of Sciences “Metals”, 1966, No. 6.
17. Ulyanin E. A. “MiTOM”, 1966, No. 10.
18. Williams K. "J. of the Institute of Metals", 1969, v. E7.
19. Murphy H., Sims C. Beltran A. "J. of Metals", 1968, v. 20, no. 11.
20. Burger J., Hanink D. “Metal Progress” 1967, v. 92, no. 1.
21. Wagner H., Prock J “Metal Progress”, 1967, v. 91, no. 3.
22. Mihalisin I., Bicber C., Grant R. “Trans, of Metallurgical Society of A1ME”, 1968, v. 242.
23. Khimushin F. F. Heat-resistant steels and alloys. M. "Metallurgy", 1969.
24. Ozel M., Nutting I. "J. Iron and Steel Institute", 1969, v. 207.

Austenitic steels (see Tables 1 and 2) are used for the manufacture of gas turbine parts, engine valves, tanks, pipes and other parts operating at temperatures of 500-700°C. Heat-resistant steels are alloyed with chromium and nickel and are at the same time corrosion-resistant.

Austenitic steels are divided into austenitic steels that are not hardened by heat treatment (nonaging) and austenitic steels that are hardened by heat treatment (aging).

Non-aging austenitic heat-resistant steels include steel types 12Х18Н9Т, 12Х18Н10Т, 12Х18Н12Т.

Aging austenitic steels are usually more complexly alloyed, for example: 37Х12Н8Г8МФ5 (ЭИ481), 45Х14Н14В2М (ЭИ69), etc.

According to the method of hardening, they are divided into austenitic steels with carbide hardening and austenitic steels with intermetallic hardening.

In non-hardening austenitic steels, chromium is introduced to impart corrosion resistance (a dense oxide film of CrO is formed on the surface), nickel to obtain an austenitic structure, and titanium to prevent intergranular corrosion, which disrupts the bond between grains and makes the steel unsuitable for use. If the steel does not contain titanium (or niobium), then chromium carbides are formed in it, which, when the hardened steel is heated to 500-700°C, are released along the grain boundaries, and the steel’s resistance to corrosion decreases. When titanium is introduced, titanium carbides TiC are formed, which eliminates the release of chromium carbides and the occurrence of intergranular corrosion. Heat treatment of steels involves heating to temperatures of 1050-1100°C followed by cooling in water. The structure after quenching consists of austenite grains with the presence of twins and a small amount of TiC carbides. Steel is used for parts of exhaust systems, pipes, as well as semi-finished products in the form of sheets and grade steel. Temperature of scale formation is 850°C. The long-term heat resistance of steel at 600°C and exposure for 100,000 hours is 110 MPa.

The chemical composition of austenitic steels with carbide strengthening 37Х12Н8Г8МФВ, 45Х14Н14В2М is given in table. 1.

Chromium and molybdenum increase the recrystallization temperature and, therefore, the heat resistance of steel due to the formation of carbides and protect the steel from oxidation. Nickel, expanding the -region, is introduced to obtain an austenitic structure. Manganese is used as an austenite-forming agent to partially replace nickel and increase the stability of austenite.

Niobium and titanium are very effective carbide formers. The niobium content is usually low (0.1-0.2%). Titanium and niobium are used to bind carbon, as in chromium-nickel stainless steels, to avoid intergranular corrosion, and to produce natural fine grains.

Long-term heat resistance at a test temperature of 600°C and exposure for 100 hours is 400 MPa. The structure of steel after quenching at 1140°C in water consists of large austenite grains and a small amount of carbides: VC, NbC. Monocarbides are most often isolated along grain boundaries in the form of large inclusions of irregular shape.

Austenitic steel with carbide hardener 45Х14Н14В2М has 2-2.6% tungsten in its structure. Tungsten, like titanium and niobium, in stainless steels prevents intermetallic corrosion. The behavior of the remaining elements is similar to the previous steel. The structure of steel after annealing at 820°C consists of small austenite grains and a large amount of carbides. The steel hardening temperature is 1175°C. The higher the quenching temperature, the more heat-resistant the steel becomes, but less ductile and tough. This is apparently due to the more complete dissolution of carbides in austenite, its greater stability, and coarse-grained structure. After quenching in water, the steel structure consists of large austenite grains and a small amount of carbides. In aircraft engine manufacturing, steel is used for the manufacture of exhaust valves of piston engines, compressor blades of the last stages of jet engines, and pipeline parts.

Heat-resistant steels with intermetallic hardener (10Х11Н20Т3Р, 10Х11Н23Т3МР) (see tables 1,2). To increase heat resistance, they are alloyed with chromium, molybdenum, tungsten with additions of aluminum, titanium or niobium and tantalum. Titanium and aluminum form the main strengthening g¢ phase (Ni Ti or Ni TiAl). Molybdenum alloys the solid solution, increasing the interatomic bond energy. Boron strengthens the boundaries of austenite grains. Strengthening heat treatment of these steels consists of hardening and aging (see Table 2). Steels are used for the manufacture of combustion chambers, turbine disks and blades, as well as welded structures operating at temperatures up to 700°C.

Austenitic steels are characterized by high ductility and weldability, but compared to pearlitic steels they are more difficult to process by pressure and cutting.

Heat-resistant alloys based on iron-nickel (KhN35VT, KhN35VTYu, etc.) are additionally alloyed with chromium, titanium, tungsten, aluminum, boron; strengthened by hardening and aging. They are used for the manufacture of turbine blades and disks, nozzle rings and other parts operating at temperatures up to 750°C.

In power engineering, chemical and oil industry enterprises, equipment elements that are in direct contact with aggressive environments must be made of a special material that can withstand negative impacts. According to modern technologies, austenitic steels are used, their grades are selected in accordance with production tasks.

This is a highly alloyed material that forms a 1-phase structure during crystallization. It is characterized by a face-centered crystal lattice, which is preserved even at cryogenic temperatures - below -200 degrees C. The material is characterized by a high content of nickel, manganese and some other elements that contribute to stabilization at different temperatures. Austenitic steels are classified into 2 groups regarding composition:

• material based on iron, in which chromium is up to 15%, and nickel is up to 7%, the total number of alloying elements should not exceed 55%;
• material based on nickel, when its content is 55% and higher, or based on iron-nickel, when the content of these components is 65% and higher, and the ratio of iron and nickel is in the proportion of 1 to 1 ½, respectively.

The nickel content in these iron alloys is necessary to increase manufacturability, resistance and strength to heat, and increase ductility parameters. Chrome increases resistance to corrosion and high temperatures. Other alloying additives can form other unique properties that austenitic stainless steel should have under certain technological conditions. Unlike other materials, this iron alloy does not undergo transformations when temperatures decrease and increase. Therefore, temperature treatment is not used.

Classification of austenitic steels by groups and grades

What steels belong to austenitic steels are usually classified into three groups:

Features of processing of austenitic steels

Austenitic steels are difficult to machine materials. Thermal effects on them are difficult, so other technologies are used. Machining of these alloys is difficult because the material is prone to work hardening and minor deformations significantly densify the material. This iron alloy produces long chips because it has high toughness parameters. Mechanical processing of austenitic steels requires energy and consumes 50% more resources compared to carbon alloys. Therefore, their processing must be performed on powerful and rigid machines. Welding, ultrasonic influence and cryogenic-deformation technology are possible.