Austenite is what?

Heat treatment of steel is a powerful mechanism of influence on its structure and properties. It is based on the modifications of crystal lattices depending on the game of temperatures. In various conditions, iron, carbon, perlite, cementite and austenite can be present in the iron-carbon alloy. The latter plays a major role in all thermal transformations in steel.

Definition

Steel is an alloy of iron and carbon, in which the carbon content is up to 2.14% theoretically, but technologically it contains no more than 1.3%. Accordingly, all the structures that are formed in it under the influence of external influences, are also a variety of alloys.

The theory represents their existence in four variations: a solid penetration solution, a solid exclusion solution, a mechanical grain mixture or a chemical compound.

Austenite is a solid solution of the penetration of a carbon atom into a face-centric cubic crystal lattice of iron, referred to as γ. The carbon atom is introduced into the cavity of the γ-lattice of iron. Its dimensions exceed the corresponding pores between the Fe atoms, which explains the limitations of their passage through the "walls" of the basic structure. It is formed in the processes of temperature transformations of ferrite and perlite when the heat is raised above 727 ° C.

Diagram of iron-carbon alloys

The graph, called the iron-cementite state diagram, constructed by experiment, is a visual demonstration of all possible variants of transformations in steels and cast irons. Specific temperature values for a certain amount of carbon in the alloy form critical points in which important structural changes occur in the heating or cooling processes, they also form critical lines.

The GSE line, which contains the points Ac ₃ and Ac _m , displays the level of solubility of the carbon when the heat level rises.

Features of education

Austenite is a structure that forms during the heating of steel. When the critical temperature is reached, perlite and ferrite form an integral substance.

Heating options:

1. Uniform, until the required value, a short time, cooling. Depending on the characteristics of the alloy, austenite can be either fully formed or partially formed.
2. A slow rise in temperature, a long period of maintaining the achieved heat level in order to obtain pure austenite.

Properties of the resulting heated material, as well as the one that will take place as a result of cooling. Much depends on the level of heat achieved. It is important not to overheat or split.

Microstructure and properties

Each of the phases characteristic of iron-carbon alloys is characterized by the intrinsic structure of lattices and grains. The structure of the austenite is plate-like, having shapes close to both acicular and flocculent. With complete dissolution of carbon in γ-iron, the grains have a light form without the presence of dark cementite inclusions.

The hardness is 170-220 HB. Thermal conductivity and electrical conductivity are an order of magnitude lower than that of ferrite. Magnetic properties are absent.

Variants of cooling and its speed lead to the formation of various modifications of the "cold" state: martensite, bainite, troostite, sorbitol, perlite. They have a similar acicular structure, but they differ in the dispersion of particles, in the size of grains and in cementite particles.

Effect of cooling on austenite

The decomposition of austenite occurs at the same critical points. Its effectiveness depends on the following factors:

1. Cooling speed. It affects the nature of carbon inclusions, the formation of grains, the formation of the final microstructure and its properties. Depends on the medium that is used as a coolant.
2. The presence of an isothermal component at one of the decay stages - when the temperature is lowered to a certain temperature level, stable heat is maintained for a certain period of time, after which rapid cooling continues, or it occurs together with the heating device (furnace).

Thus, continuous and isothermal transformations of austenite are isolated.

Characteristics of the nature of the transformations. Diagram

C-shaped graph, which shows the nature of changes in the microstructure of the metal in the time interval, depending on the degree of temperature change - is a diagram of the transformation of austenite. Real cooling is continuous. Only some phases of forced heat retention are possible. The graph describes isothermal conditions.

The character can be diffusive and diffusionless.

At standard rates of heat reduction, the change in austenitic grain occurs diffusively. In the zone of thermodynamic instability, atoms begin to move among themselves. Those that do not have time to penetrate the iron lattice, form cementitious inclusions. They are joined by neighboring carbon particles, released from their crystals. Cementite is formed on the boundaries of disintegrating grains. Purified ferrite crystals form the corresponding plates. A dispersed structure is formed-a mixture of grains whose size and concentration depend on the cooling rate and carbon content in the alloy. Perlite and its intermediate phases are formed: sorbitol, troostite, bainite.

At significant rates of temperature decrease, the decomposition of austenite is not of a diffusion nature. Complex distortions of crystals take place, within which all atoms are simultaneously displaced in the plane, without changing their location. The absence of diffusion contributes to the generation of martensite.

Effect of quenching on the features of the decay of austenite. Martensit

Hardening is a kind of heat treatment, the essence of which is the rapid heating to high temperatures above the critical points Ac ₃ and Ac _m , followed by rapid cooling. If the temperature drops with water at a rate greater than 200 ° C per second, a solid needle phase, called martensite, is formed.

It is a supersaturated solid solution of the penetration of carbon into iron with a crystal lattice of the α type. Due to the powerful displacements of atoms, it distorts and forms a tetragonal lattice, which is the cause of hardening. The formed structure has a larger volume. As a result, the crystals, bounded by the plane, are compressed, needle-like plates are generated.

Martensite - strong and very firm (700-750 HB). It is formed solely as a result of high-speed quenching.

Hardening. Diffusion structures

Austenite is a formation from which bainite, troostite, sorbitol and perlite can be artificially produced. If quench cooling occurs at lower rates, diffusion transformations are carried out, their mechanism is described above.

Troostite is perlite, which is characterized by a high degree of dispersion. Formed with a decrease in heat of 100 ° C per second. A large number of fine grains of ferrite and cementite is distributed throughout the plane. "Hardened" is characterized by cementite lamellar form, and troostite, obtained as a result of subsequent release, has a grainy visualization. The hardness is 600-650 HB.

Bainite is an intermediate phase, which is an even more dispersed mixture of crystals of high-carbon ferrite and cementite. According to its mechanical and technological properties it is inferior to martensite, but it exceeds troostite. It forms in temperature intervals, when diffusion is impossible, and the forces of compression and displacement of the crystal structure for transformation into a martensitic structure are insufficient.

Sorbitol is a large-sized needle-shaped variety of pearlite phases with cooling at a speed of 10 ° C per second. Mechanical properties occupy an intermediate position between perlite and troostite.

Perlite is a collection of grains of ferrite and cementite, which can be granular or plate-shaped. Formed as a result of a smooth decomposition of austenite with a cooling rate of 1 ° C per second.

Beytite and troostite are more related to quenching structures, whereas sorbitol and perlite can be formed even during tempering, annealing and normalization, the characteristics of which determine the shape of the grains and their size.

Effect of annealing on the features of the decay of austenite

Virtually all types of annealing and normalization are based on the mutual reversal of austenite transformation. Complete and incomplete annealing is applied to the pre-eutectoid steels. The details are heated in the furnace above the critical points Ac ₃ and Ac _1, respectively. The first type is characterized by the presence of a long holding period, which provides a complete transformation: ferrite-austenite and perlite-austenite. After that, slow cooling of the blanks in the furnace follows. At the output, a finely dispersed mixture of ferrite and perlite is obtained, without internal stresses, plastic and strong. Incomplete annealing is less energy-intensive, changes only the structure of perlite, leaving ferrite practically unchanged. Normalization implies a higher rate of temperature reduction, but a more coarse-grained and less plastic structure at the outlet. For steel alloys with a carbon content of 0.8 to 1.3%, cooling under normalization leads to a disintegration in the direction: austenite-perlite and austenite-cementite.

Another type of heat treatment, which is based on structural transformations, is homogenization. It is applicable for large parts. It implies the absolute achievement of austenitic coarse-grained state at temperatures of 1000-1200 ° C and holding in the furnace for up to 15 hours. Isothermal processes continue with slow cooling, which facilitates the alignment of metal structures.

Isothermal annealing

Each of these ways of influencing the metal for ease of understanding is considered as an isothermal transformation of austenite. However, each of them only has specific features at a certain stage. In reality, the changes occur with a stable decrease in heat, the speed of which determines the result.

One of the methods closest to ideal conditions is isothermal annealing. Its essence also consists in heating and aging until the complete breakdown of all structures into austenite. Cooling is realized in several stages, which contributes to a slower, longer and more thermally stable decay.

1. Rapid decrease in temperature to a value of 100 ° C below the point Ac ₁ .
2. Forced retention of the achieved value (by placing in the furnace) for a long time until the completion of the formation of ferritic-pearlitic phases.
3. Cooling in calm air.

The method is also applicable to alloyed steels, which are characterized by the presence of residual austenite in the cooled state.

Residual austenite and austenitic steels

Sometimes incomplete decomposition is possible, when residual austenite takes place. This can happen in the following situations:

1. Too rapid cooling, when complete decay does not occur. It is a structural component of bainite or martensite.
2. High-carbon or low-alloy steel, for which the processes of austenite dispersed transformations are complicated. Requires the use of special methods of heat treatment, such as, for example, homogenization or isothermal annealing.

For highly doped - there are no processes of the described transformations. The alloying of steel with nickel, manganese, chromium promotes the formation of austenite as the main stable structure, which does not require additional influences. Austenitic steels are distinguished by their high strength, corrosion resistance and heat resistance, heat resistance and resistance to complex aggressive working conditions.

Austenite is a structure without the formation of which no high-temperature heating of steel is possible and which participates in practically all methods of its heat treatment in order to improve mechanical and technological properties.