Heat treatment of steel is the most powerful mechanism for influencing its structure and properties. It is based on modifications of crystal lattices depending on the game of temperatures. Ferrite, pearlite, cementite, and austenite may be present in an iron-carbon alloy under various conditions. 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 varieties of alloys.
Theory presents their existence in 4 variations: a penetration solid solution, an exclusion solid solution, a mechanical mixture of grains or a chemical compound.
Austenite is a solid solution of carbon atom penetration 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 the limited passage of them through the "walls" of the main structure. Formed in processestemperature transformations of ferrite and perlite with increasing heat above 727˚С.
Chart of iron-carbon alloys
A graph called the iron-cementite state diagram, built experimentally, is a clear demonstration of all possible options for transformations 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 during heating or cooling processes, they also form critical lines.
The GSE line, which contains the points Ac3 and Acm, represents the level of carbon solubility as heat levels increase.
| Table of carbon solubility in austenite versus temperature | |||||
| Temperature, ˚C | 900 | 850 | 727 | 900 | 1147 |
| Approximate solubility of C in austenite, % | 0, 2 | 0, 5 | 0, 8 | 1, 3 | 2, 14 |
Features of education
Austenite is a structure that forms when steel is heated. Upon reaching the critical temperature, pearlite and ferrite form an integral substance.
Heating options:
- Uniform, until the required value is reached, short exposure,cooling. Depending on the characteristics of the alloy, austenite can be fully formed or partially formed.
- Slow increase in temperature, long period of maintaining the reached level of heat in order to obtain pure austenite.
Properties of the resulting heated material, as well as that which will take place as a result of cooling. Much depends on the level of heat achieved. It is important to prevent overheating or overheating.
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 shapes close to both acicular and flaky. With complete dissolution of carbon in γ-iron, the grains have a light shape without the presence of dark cementite inclusions.
Hardness is 170-220 HB. The thermal and electrical conductivity are an order of magnitude lower than those of ferrite. No magnetic properties.
Variants of cooling and its speed lead to the formation of various modifications of the "cold" state: martensite, bainite, troostite, sorbite, perlite. They have a similar acicular structure, but differ in particle dispersion, grain size and cementite particles.
Effect of cooling on austenite
Decomposition of austenite occurs at the same critical points. Its effectiveness depends on the following factors:
- Cooling rate. Affects the nature of carbon inclusions, the formation of grains, the formation of the finalmicrostructure and its properties. Depends on the medium used as the coolant.
- The presence of an isothermal component at one of the stages of decomposition - when 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 a heating device (furnace).
Thus, a continuous and isothermal transformation of austenite is distinguished.
Features of the character of transformations. Chart
C-shaped graph, which displays the nature of changes in the microstructure of the metal in the time interval, depending on the degree of temperature change - this is the austenite transformation diagram. Real cooling is continuous. Only some phases of forced heat retention are possible. The graph describes isothermal conditions.
Character can be diffusion and non-diffusion.
At standard heat reduction rates, the austenite grain changes by diffusion. In the zone of thermodynamic instability, atoms begin to move among themselves. Those that do not have time to penetrate into 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. Purified ferrite crystals form the corresponding plates. A dispersed structure is formed - a mixture of grains, the size and concentration of which depend on the rapidity of cooling and the contentalloy carbon. Perlite and its intermediate phases are also formed: sorbite, troostite, bainite.
At significant rates of temperature decrease, the decomposition of austenite does not have a diffusion character. Complex distortions of crystals occur, within which all atoms are simultaneously displaced in a plane without changing their location. Lack of diffusion contributes to the nucleation of martensite.
Influence of hardening on the characteristics of the decomposition of austenite. Martensite
Hardening is a type of heat treatment, the essence of which is the rapid heating to high temperatures above the critical points Ac3 and Acm, followed by rapid cooling. If the temperature is lowered with the help of water at a rate of more than 200˚С per second, then a solid acicular phase is formed, which is called martensite.
It is a supersaturated solid solution of carbon penetration into iron with an α-type crystal lattice. Due to powerful displacements of atoms, it is distorted 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 born.
Martensite is strong and very hard (700-750 HB). Formed exclusively as a result of high-speed quenching.
Hardening. Diffusion structures
Austenite is a formation from which bainite, troostite, sorbite and perlite can be artificially produced. If the cooling of the hardening occurs atlower speeds, diffusion transformations are carried out, their mechanism is described above.
Troostite is perlite, which is characterized by a high degree of dispersion. It is formed when the heat decreases 100˚С per second. A large number of small grains of ferrite and cementite is distributed over the entire plane. The “hardened” cementite is characterized by a lamellar form, and the troostite obtained as a result of subsequent tempering has a granular 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. In terms of mechanical and technological properties, it is inferior to martensite, but exceeds troostite. It is formed in temperature ranges when diffusion is impossible, and the forces of compression and movement of the crystal structure for transformation into a martensitic one are not enough.
Sorbitol is a coarse needle-like variety of pearlite phases when cooled at a rate of 10˚С per second. Mechanical properties are intermediate between pearlite and troostite.
Perlite is a combination of grains of ferrite and cementite, which can be granular or lamellar. Formed as a result of the smooth decay of austenite with a cooling rate of 1˚C per second.
Beitite and troostite are more related to hardening structures, while sorbite and perlite can also be formed during tempering, annealing and normalization, the features of which determine the shape of the grains and their size.
Effect of annealing onaustenite decay features
Practically all types of annealing and normalization are based on the reciprocal transformation of austenite. Full and incomplete annealing is applied to hypoeutectoid steels. The parts are heated in the furnace above the critical points Ac3 and Ac1 respectively. The first type is characterized by the presence of a long holding period, which ensures complete transformation: ferrite-austenite and pearlite-austenite. This is followed by slow cooling of the workpieces in the furnace. At the output, a finely dispersed mixture of ferrite and pearlite is obtained, without internal stresses, plastic and durable. Incomplete annealing is less energy intensive and only changes the structure of pearlite, leaving ferrite virtually unchanged. Normalization implies a higher rate of temperature decrease, but also a coarser and less plastic structure at the exit. For steel alloys with a carbon content of 0.8 to 1.3%, upon cooling, as part of normalization, decomposition occurs in the direction: austenite-pearlite and austenite-cementite.
Another type of heat treatment based on structural transformations is homogenization. It is applicable for large parts. It implies the absolute achievement of the 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 helps to even out the metal structures.
Isothermal annealing
Each of the listed methods of influencing the metal to simplify understandingconsidered as an isothermal transformation of austenite. However, each of them only at a certain stage has characteristic features. In reality, changes occur with a steady 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 the complete decomposition of all structures into austenite. Cooling is implemented in several stages, which contributes to a slower, longer and more thermally stable decomposition.
- The rapid drop in temperature to 100˚C below the Ac point1.
- Forced retention of the achieved value (by placing in a furnace) for a long time until the processes of formation of ferrite-pearlite phases are completed.
- Cooling in still air.
The method is also applicable to alloy steels, which are characterized by the presence of residual austenite in the cooled state.
Retained austenite and austenitic steels
Sometimes incomplete decay is possible when there is retained austenite. This can happen in the following situations:
- Cooling too fast when complete decay does not occur. It is a structural component of bainite or martensite.
- High-carbon or low-alloy steel, for which the processes of austenitic dispersed transformations are complicated. Requires special heat treatment methods such as homogenization or isothermal annealing.
For high-alloyed -there are no processes of the described transformations. Alloying steel with nickel, manganese, 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.