What is austenite?

Heat treatment of steel is a powerful mechanism for influencing its structure and properties. It is based on modifications of crystal lattices depending on the game of temperatures. Under various conditions, ferrite, perlite, cementite, and austenite may 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 applicable contains it in an amount of not more than 1.3%. Accordingly, all the structures that are formed in it under the influence of external influences are also types of alloys.

The theory represents their existence in 4 variations: a penetration solid solution, an exception solid solution, a mechanical mixture of grains, or a chemical compound.

Austenite is a solid solution of the penetration of a carbon atom into the 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 Fe atoms, which explains their limited passage through the "walls" of the main structure. It is formed in the processes of temperature transformations of ferrite and perlite with an increase in heat above 727 ° C.

Iron Carbon Alloy Chart

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

The GSE line, which contains Ac ₃ and Ac _m points, displays the carbon solubility level with increasing heat.

Education Features

Austenite is a structure that is formed during the heating of steel. Upon reaching a critical temperature, perlite and ferrite form an integral substance.

Heating Options:

1. Uniform, until the desired value is reached, short exposure, cooling. Depending on the characteristics of the alloy, austenite can be either fully formed or partially.
2. Slow temperature increase, a long period of maintaining the achieved level of heat in order to obtain pure austenite.

The properties of the resulting heated material, as well as that which will occur as a result of cooling. A lot depends on the level of heat achieved. It is important to prevent overheating or skipping.

Microstructure and properties

Each of the phases characteristic of iron-carbon alloys has its own structure of lattices and grains. The structure of austenite is lamellar, having forms close to both a needle-like appearance and a flocculent one. With the complete dissolution of carbon in γ-iron, the grains have a light form without dark cementite inclusions.

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 needle-like structure, but differ in particle size, grain size and cementite particles.

The effect of cooling on austenite

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

1. Cooling rate. It affects the nature of carbon inclusions, grain formation, formation of the final microstructure and its properties. Depends on the environment used as a cooler.
2. The presence of an isothermal component at one of the stages of decomposition — when it is reduced 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 a heating device (furnace).

Thus, continuous and isothermal transformations of austenite are isolated.

Features of the nature of the transformations. Diagram

A C-shaped graph that displays the nature of the 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 continuously. Only a few phases of forced heat retention are possible. The graph describes isothermal conditions.

The character may be diffusion and diffusion-free.

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

At significant rates of temperature decrease, the decomposition of austenite does not have a diffusion character. Complex distortions of crystals occur, inside which all atoms are simultaneously displaced in the plane without changing their location. The lack of diffusivity contributes to the emergence of martensite.

The effect of quenching on the features of the decomposition of austenite. Martensite

Quenching is a type of heat treatment, the essence of which is rapid heating to high temperatures above the critical points Ac ₃ and Ac _m , followed by rapid cooling. If the temperature decreases with the help of water at a speed of more than 200 ° C per second, a solid needle-like phase is formed, called martensite.

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

Martensite is durable and very hard (700-750 HB). Formed solely as a result of high-speed hardening.

Quenching. Diffusion structures

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

Troostitis is perlite, which is characterized by a high degree of dispersion. It is formed with a decrease in heat of 100 ° C per second. A large number of small grains of ferrite and cementite are distributed over the entire plane. “Hardened” is characterized by plate-shaped cementite, and the troostite obtained as a result of subsequent tempering has a grainy visualization. Hardness - 600-650 HB.

Bainite is an intermediate phase, which is an even more dispersed mixture of crystals of high-carbon ferrite and cementite. It is inferior to martensite in mechanical and technological properties, but exceeds troostite. It is formed in temperature ranges when diffusion is impossible, and the compressive forces and displacements of the crystalline structure to turn into martensitic are insufficient.

Sorbitol is a coarse, needle-like variety of pearlite phases during cooling at a rate of 10 ° C per second. The mechanical properties are intermediate between perlite and troostite.

Perlite is a combination of grains of ferrite and cementite, which can be granular or lamellar in shape. It is formed as a result of smooth decomposition of austenite with a cooling rate of 1 ° C per second.

Beitite and troostite are more related to quenching structures, while sorbitol and perlite can also form during tempering, annealing, and normalization, the features of which determine the shape of the grains and their size.

The effect of annealing on the features of the decomposition of austenite

Almost all types of annealing and normalization are based on the reciprocal transformation of austenite. Complete and incomplete annealing is applied to hypoeutectoid steels. Parts are heated in a furnace above the critical points Ac ₃ and Ac _1, respectively. The first type is characterized by the presence of a long exposure period, which provides a complete transformation: ferrite-austenite and perlite-austenite. This is followed by slow cooling of the billets in the furnace. 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, only the structure of perlite changes, leaving ferrite practically unchanged. Normalization implies a higher rate of temperature reduction, but also a coarser and less plastic structure at the outlet. For steel alloys with a carbon content of 0.8 to 1.3%, when cooled as part of normalization, decomposition occurs in the direction of 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 an austenitic coarse-grained state at temperatures of 1000-1200 ° C and holding in an oven for up to 15 hours. Isothermal processes are continued by slow cooling, which contributes to the alignment of metal structures.

Isothermal Annealing

Each of these methods of influence on the metal to simplify understanding is considered as an isothermal transformation of austenite. However, each of them has characteristic features only at a certain stage. In reality, 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 holding until all structures in austenite completely decompose. Cooling is carried out in several stages, which contributes to a slower, longer, and more thermally stable decay.

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

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

Residual austenite and austenitic steels

Incomplete decomposition is sometimes possible when residual austenite occurs. This can happen in the following situations:

1. Cooling too fast when complete decay does not occur. It is a structural component of bainite or martensite.
2. Steel is high carbon or low alloy, for which the processes of austenitic dispersed transformations are complicated. It requires the use of special heat treatment methods, such as, for example, homogenization or isothermal annealing.

For highly doped - there are no processes of the described transformations. Alloying steel with nickel, manganese, and chromium contributes to the formation of austenite as the main strong structure, which does not require additional influences. Austenitic steels are characterized by high strength, corrosion resistance and heat resistance, heat resistance and resistance to difficult aggressive working conditions.

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