This article will present a fairly detailed account of what constitutes recrystallization annealing. In addition, for familiarization, other types of work with steel will be considered, which improve its structure and metal workability, reduce hardness and relieve internal stresses. All the basic properties of the alloy depend on the structure of the alloy, and heat treatment is a method that changes the structure. Recrystallization annealing and many other types of heat treatment were developed by D.K. Chernov, further developed by G.V. Kurdyumov, A.A. Bochvar, A.P. Gulyaev.
Heat treatment
This is a combination of various heating operations using special equipment and using special technology, with exposure and cooling, which are performed purely in a certain sequence and under precise conditions to change the internal structure of the alloy and obtain the desired properties. Heat treatment is divided into several types. Annealing of the first kind, which is used for absolutely any metals and alloys, does not bring phase transformations in the solid state. Recrystallization annealing is used to achieve the following characteristics.
When annealing of the first kind is heated, the mobility of atoms increases, the chemical inhomogeneity is completely or partially eliminated, and the internal stress decreases. It all depends on the heating temperature and holding time. Slow cooling is characteristic here. Varieties of this method are annealing to relieve stress after casting, welding or forging, diffusion and recrystallization annealing.
Annealing of the second kind
This annealing is also intended for metal and alloys that undergo phase transformations during annealing in the solid state, both during heating and cooling. Here, the goals are somewhat wider than those pursued by the recrystallization annealing of steel. During annealing of the second kind, a more equilibrium structure is obtained for further processing of the material. Granularity disappears, is crushed, viscosity and ductility increase, hardness and strength are significantly reduced. Such metal can already be cut. Heating is carried out to temperatures far above critical, and cooling occurs with the furnace - very slowly.
Heat treatment also includes quenching of alloys for strength and hardness. Here, on the contrary, the structure is formed non-equilibrium, increasing these parameters due to sorbitol, troostite, martensite. Temperatures are also used significantly above critical, but cooling occurs at very high speeds. The fourth type of heat treatment is tempering, which relieves internal stresses, reduces hardness and increases the viscosity and ductility of hardened steels. When heated to temperatures below critical, the cooling rate can be any. Transformations reduce the disequilibrium of the structure. So does the recrystallization annealing of steel.
Mode selection
Heat treatment can be preliminary and final. The first is used to prepare the properties of the material and its structure for further technological operations (improvement of machinability, cutting, pressure treatment). The final heat treatment forms all the properties of the finished product. How the recrystallization annealing mode is selected depends on the process and goals of the heat treatment.
It is understood that the alloy or metal is heated above the crystallization temperature, and not less than one hundred or two hundred degrees. The following is an exposure at this temperature for the right time. Cooling is the final stage of this process. This technology is divided into full, incomplete and textured annealing, and the choice depends on what the purpose of the recrystallization annealing is.
Full annealing
In practice, we most often use full annealing, but here you need to pay attention to the fact that steel annealing and its hardening are different processes. In the process of recrystallization annealing, certain procedures are performed that precede the cold processing of metal under pressure to facilitate further work with it, or annealing is the output type of heat treatment when the finished product or semi-finished product obtains the desired characteristics. Or this is an intermediate operation, for example - to effectively remove hardening.
For uniform dissolution in the matrix of alloying elements and in order to obtain a homogeneous microstructure with the same material properties, annealing is carried out in a special solution. Ferrous metals require recrystallization annealing at temperatures from 950 to 1200º C using a solution of Durferrit Glühkohle or Durferrit GS 960 salts. Non-ferrous metals are annealed at temperatures from 460 to 540º C using A. Durferrit salt, followed by subsequent processing to increase strength .
Goals
Most often, the recrystallization annealing of steels is carried out in order to bring the structure of the material to the desired parameters, which are necessary for further work. It is used after pressure treatment, if the slow recrystallization has not completely passed, and this does not allow to remove hardening.
A similar technology is usually used for hot rolled coils of alloys, where the basis is aluminum, as well as after cold rolling of sheets, strips, foils of various alloys and non-ferrous metals (nickel recrystallization annealing should be mentioned here), rods and wire, cold forming steels and pipes cold drawing. A separate procedure is annealing in the manufacture of semi-finished products and products from non-ferrous metals (including nickel).
Temperature conditions
Different materials require different heat treatment modes. Usually the entire process lasts no more than one hour to complete the recrystallization annealing, but the temperature regime for each alloy is its own. So, from 300 to 400 ° C require magnesium-based alloys, from 800 to 1150 ° C - nickel alloys, from 650 to 710 ° C - a carbon group of steels for which recrystallization annealing is necessarily carried out. The melting point, of course, is not achieved.
Aluminum alloys do not need so much, from 350 to 430 ° C is enough, and pure aluminum recrystallizes at temperatures from 300 to 500 ° C. From 670 to 690 ° C requires titanium for recrystallization, from 700 to 850 ° C - a composition of copper with nickel, from 600 to 700 ° C requires bronze and brass, and even less pure copper, it starts to recrystallize from 500 ° C. Such recrystallization annealing modes are required for individual metals and alloys.
Diffusion processing of metals
This type of annealing is otherwise called homogenizing, and it is carried out in order to eliminate the effects of dendritic segregation. Diffusion annealing is needed for alloy steel, where the ductility and toughness are reduced due to intracrystalline segregation, which leads to layered or brittle fractures. It is necessary to achieve an equilibrium structure, and therefore diffusion processing of cast metal is necessary. In addition, it improves the mechanical characteristics, and increases the uniformity of properties in the entire volume of the finished product.
This is what happens during this process: the excess phases dissolve, the chemical composition is leveled, pores appear and grow, and grain size increases. This type of heat treatment requires a long exposure of the metal at temperatures above critical (here we can talk about 1200 degrees Celsius).
Isothermal Heat Treatment
Such annealing is recommended for alloy steel, where at a constant temperature, austenite decomposes into ferrite and cementite in a mixture. Such decay can occur during other types of annealing if gradual cooling occurs due to a constant and successive decrease in temperature. Thus, the homogeneity of the structure is achieved, the time for heat treatment is reduced.
The scheme of isothermal annealing is as follows: first, heating to an indicator that will exceed the upper critical point by 50-70 degrees, then the temperature decreases by 150 degrees. After that, the heated part is transferred to a furnace or bath, where a temperature of not more than 700 ° C is maintained. The duration of the procedure will depend on the composition of the metal and the geometric dimensions of the part. Alloy compounds may require several hours, and hot rolled carbon steel sheets may take several minutes.
Differences
With complete annealing, the recrystallization of steel is ensured, which relieves the metal of various structural defects. Steel gets its most important and characteristic properties, it softens for subsequent cutting. First you need to heat it to a temperature above Ac3 by 30-50 degrees, warm it, then cool it slowly.
Most often, exposure lasts at least half an hour, but not more than an hour per ton of steel with a heating rate of 100 degrees Celsius per hour. The cooling rate varies - it depends on the composition of the steel and on the stability of austenite. If cooled rapidly, a too hard ferrite-cementite disperse structure may form.
Cooling down
The cooling rate is controlled by the cooling of the furnace with its gradual shutdown and door opening. With complete annealing, the main thing is not to overheat the alloy. Incomplete annealing is performed at temperatures below Ac3, but slightly higher than Ac1.
Then the steel is partially recrystallized, and therefore it will not get rid of defects. This is how steels with a lack of ferrite banding are treated if they need only to be softened before further processing and cutting. In addition to full and incomplete, there is also a texturing recrystallization annealing.
Application
Sometimes annealing supplements hot working with pressure (hot rolled coils, for example, of aluminum alloys, are annealed before cold rolling in order to remove hardening, which is necessarily formed as a result of hot rolling).
Annealing of this type is used much more widely in the production of products and semi-finished products from alloys and pure non-ferrous metals. This is an independent heat treatment operation. Compared to steels, a huge amount of non-ferrous metals is subjected to cold pressure treatment, after which recrystallization annealing is necessary.
In industry
If a granular form of cementite is required, aging of the alloy during annealing until complete recrystallization can last a long time - several hours. For cold deformation, which usually follows annealing, the granular form of cementite, which occurs during recrystallization during the nucleation and growth of undeformed grains, is most favorable, and this requires heating to a certain temperature.
Recrystallization annealing in industry is the initial operation to give an alloy or metal ductility before cold forming. No less often, it is present in the interval between cold deformation operations to remove hardening, and also as the final output heat treatment process so that the product or semi-finished product acquires the properties they need.
How does this happen
When heated, a deformed metal increases the mobility of atoms. Old grains are elongated, become vulnerable, intensively nucleate and grow new, already balanced and free from stress grains. They collide with the old, elongated, absorbing into their growth until they disappear completely. Recrystallization of steel and alloys is the main goal of recrystallization annealing. When heated after reaching the required temperature, the yield strength and strength of the material decrease quite sharply.
But plasticity increases, this works to improve machinability. The onset temperature of recrystallization is called the recrystallization threshold. Upon reaching it, the metal softens. The temperature cannot be constant. For a particular alloy or metal, the duration of heating, the degree of preliminary deformation, the initial grain size and much more play equally important roles.