This article will present a fairly detailed story about what recrystallization annealing is. 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 main properties of the alloy depend on the structure of the alloy, and the method that changes the structure is heat treatment. 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 with the help of special equipment and special technology, with holding and cooling, which are performed strictly in a certain sequence and under precise modes in order to change the internal structure of the alloy and obtain the desired properties. Heat treatment is divided into several types. Annealing the firstkind, 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, chemical inhomogeneity is completely or partially eliminated, and internal stress decreases. It all depends on the heating temperature and holding time. Slow cooling is characteristic here. Variations of this method are stress relief annealing after casting, welding or forging, diffusion annealing, and recrystallization annealing.
Second Annealing
This annealing is also intended for metals and alloys that undergo phase transformations during solid state annealing - both when heated and when cooled. Here, the goals are somewhat broader than those pursued by recrystallization annealing of steel. Annealing of the second kind results in a more balanced structure for further processing of the material. Graininess disappears, is crushed, viscosity and plasticity increase, hardness and strength are significantly reduced. Such metal can already be cut. Heating is carried out to temperatures much higher than the critical ones, and cooling takes place along with the furnace - very slowly.
Also heat treatment includes hardening of alloys for strength and hardness. Here, on the contrary, a non-equilibrium structure is formed, which increases these parameters due to sorbite, troostite, and martensite. The temperatures used are also much higher than the critical ones, but the cooling takes place at very high speeds. fourth kindheat treatment - tempering, which relieves internal stresses, reduces hardness and increases the toughness and ductility of hardened steels. When heated to temperatures below critical, the cooling rate can be any. Transformations reduce the nonequilibrium structure. This is how recrystallization annealing of steel works.
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.
Implies the heating of an alloy or metal above the crystallization temperature, and not less than one hundred or two hundred degrees. This is followed by exposure at this temperature for the required time. Cooling is the final stage of this process. This technology is divided into full, partial and texturing annealing, and the choice depends on what is the purpose of recrystallization annealing.
Full annealing
In practice, we most often use full annealing, but here you need to pay attention to the fact that steel annealing and hardening are different processes. During the recrystallization annealing process, certain procedures are performed that precede the cold working of the metal under pressure to facilitate further work with it, orannealing is the output type of heat treatment, when the finished product or semi-finished product receives the desired characteristics. Either this is an intermediate operation, for example - for the effective removal of cold hardening.
For uniform dissolution of alloying elements in the matrix 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 between 950 and 1200ºC using a Durferrit Glühkohle or Durferrit GS 960 s alt solution..
Goals
Most often, recrystallization annealing of steels is carried out in order to bring the structure of the material to the desired parameters that are necessary for further work. It is used after pressure treatment, if the slow recrystallization has not completely passed, and this does not allow the hardening to be removed.
Such technology is usually used for hot-rolled alloy coils, where the base is aluminum, as well as after cold rolling of sheets, strips, foils from various alloys and non-ferrous metals (here it is necessary to mention nickel recrystallization annealing), rods and wires, cold-formed steels and cold-drawn pipes. 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 whole process lasts no more than one hour to complete the recrystallization annealing, but the temperature regime for each alloy is its own. So, magnesium-based alloys are required from 300 to 400 °С, nickel alloys are required from 800 to 1150 °С, carbon steels are required from 650 to 710 °С, for which recrystallization annealing is mandatory. The melting point is naturally not reached.
Aluminum alloys do not need so much, enough from 350 to 430 °C, and pure aluminum recrystallizes at temperatures from 300 to 500 °C. From 670 to 690 °C titanium is required for recrystallization, from 700 to 850 °C copper and nickel compositions are required, from 600 to 700 °C bronze and brass are needed, and even less pure copper, it starts recrystallization from 500 °C. Such modes of recrystallization annealing are required for certain 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 consequences of dendritic segregation. Diffusion annealing is needed for alloy steels where the ductility and toughness index is reduced due to intracrystalline segregation, which leads to lamellar or brittle fractures. It is necessary to achieve an equilibrium structure, and therefore diffusion treatment of cast metal is necessary. In addition, it improves both mechanical characteristics and increases the uniformity of properties throughout the finished product.
Here's what happensprocess: excess phases are dissolved, the chemical composition is leveled, pores appear and grow, the 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
This type of annealing is recommended for alloy steels where, at a constant temperature, austenite decomposes into ferrite and cementite in the mixture. Such decomposition can occur in other types of annealing if there is a gradual cooling due to a constant and successive decrease in temperature. Thus, the uniformity of the structure is achieved, the time for heat treatment is reduced.
The isothermal annealing scheme is as follows: first, heating to an indicator that will exceed the upper critical point by 50-70 degrees, then lowering the temperature by 150 degrees. After that, the heated part is transferred to a furnace or bath, where the temperature is maintained no more than 700 °C. The duration of the procedure will depend on the composition of the metal and the geometric dimensions of the part. Alloy compounds can take hours, while hot rolled carbon steel sheets take minutes.
Differences
With full annealing, recrystallization of steel is ensured, relieving the metal of various structural defects. Steel receives its most important and characteristic properties, softens for subsequent cutting. Needfirst heat it to a temperature above Ac3 by 30-50 degrees, warm it up, then slowly cool it.
Most often, exposure lasts at least half an hour, but no more than an hour per ton of steel with a heating rate of 100 degrees Celsius per hour. The cooling rate varies depending on the composition of the steel and on the stability of the austenite. If cooled quickly, a ferritic-cementite dispersion structure that is too hard may form.
Cooling down
The cooling rate is regulated by cooling the oven with its gradual shutdown and opening the door. With full annealing, the main thing is not to overheat the alloy. Partial annealing is performed at temperatures below Ac3, but slightly above Ac1.
Then the steel will partially recrystallize, and therefore it will not get rid of defects. This is how steels with no ferritic banding are treated, if they only need to be softened before further processing and cutting. In addition to full and incomplete, there is also texturizing recrystallization annealing.
Application
Sometimes annealing complements hot working (hot-rolled coils, such as aluminum alloys, are annealed before cold rolling to remove hard work that is bound to occur as a consequence of hot rolling).
Annealing of this type is much more widely used in the production of products and semi-finished products from alloys and pure non-ferrous metals. This is already an independent heat treatment operation. Compared to steels, a huge number of non-ferrous metals are subjected to cold working, after which recrystallization annealing is necessary.
In industry
If a granular form of cementite is required, holding the alloy during annealing until complete recrystallization can last a long time - several hours. For cold deformation, which usually follows annealing, it is the granular form of cementite that is most favorable, which occurs during recrystallization in the process of nucleation and growth of undeformed grains, and this requires heating to a certain temperature.
Recrystallization annealing in industry is the initial operation to impart plasticity to an alloy or metal prior to cold working. It is no less often present in the interval between cold deformation operations to remove hardening, and also as an output final heat treatment process so that the product or semi-finished product acquires the properties it needs.
How it happens
When heated, the deformed metal increases the mobility of atoms. Old grains are stretched out, become vulnerable, new grains are intensively born and grow, already balanced and stress-free. They collide with old, elongated ones, absorbing them into their growth until their complete disappearance. 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, it works to improve machinability. The temperature at which recrystallization begins is called the threshold.recrystallization. When it is reached, the metal softens. Temperature cannot be constant. For a particular alloy or metal, the duration of heating, the degree of pre-deformation, the initial grain size and much more play an equally important role.