Annealing is a heat treatment process that changes the physical and chemical properties of the material to increase the ductility; this is to formulate the work piece more workable. Above the transition temperature heating is given to the work piece, this is to maintain a suitable temperature and then cooling.
Annealing treatment is preferable for the welded components, homogeneous, relieve residual stress and increase cold working properties. Based on the condition of the temperature, phase transformation occurred in the shapes by eliminating the physical or chemical non-homogeneous mixture.Advertisement:
Coming to silver, copper, brass and steel the annealing process done by heating the work piece and allow it for cooling to the room temperature. The materials are slowly cooled in the air or quenching in water. The steel must be chilled slowly to strengthen. The material should be soft and used for stamping, forming and shaping.
If the temperature of the material is increased then the annealing process is done in three stages. They are
- Re crystallization
- Grain growth
- In this stage the work piece material should be softened by removing the linear defects, that is known as dislocations
- Due to the dislocation internal stress are occur in the work piece.
- During the time of annealing process we can observe the recovery stage at low temperatures and earlier the arrival of new strain grains.
- The size and shape of the grain does not change
- When the new grains are strain free nucleate, in this process new grains are developed to replace the internal stress.
- After the completion of the re-crystallization the annealing process is allowed for grain growth.
- The micro-structure in the grain growth starts to coarsen and cause the metal to miss a significant part of its unusual strength.
- With the help of hardening it can regain the original strength.
Annealing consists of some more variants they are
- Conventional annealing
- Isothermal annealing
- Diffusion annealing
- Re crystallization annealing
- Partial annealing
- Stress relief annealing
- Spheroidise annealing
- Process annealing
It is also known as full annealing process. In this process initially the temperature slowly rises near to C above the austenitic temperature line. In the steel components the hypoeutectoid steels is heating above and for hypereutectoid steel the temperature is above (30 – 500 C). In the second stage the steel component must be held at this temperature for the soaking period of 20 minutes per centimeter, this is to the thickness to spread the temperature through the cross section of the components and to complete the austenization. The work piece (hot steel) is placed in a furnace and cooled to the room temperature, where this process is known as furnace cooling.
- The full annealing is used to release internal stress induced due to welding and cold working.
- It is also used for increase ductility and reduces hardness
- Grain structure must be refined
- It increases the evenness of phase distribution
- It increases Machinability
- With respect to the chemical composition, it makes the material homogeneous.
In the isothermal annealing the heat treatment of the steel components is similar to the full annealing. Above the upper critical temperature heat the hypoeutectoid steel, and at this temperature is held for some time. This process is done in order to get an austenization structure, and eliminate the temperature gradient present in the steel component. The steel is rapidly cooled to the lower critical temperature. The temperature is ranges from 600 to 700 . At the chosen temperature, fast cooling can be achieved by transferring the steel into the other furnace. Within the peralitic region the super cooled austenite required temperature is made to maintain minimum stability. All the austenite transformed to pearlite when the steel is seized at this temperature. Steel is cooled in the air when the austenite is transfer to lamellar pearlite. At this temperature there is no further cooling. In the steel the internal stress are developed they are differ with the cooling rate. The main advantage of this process is the heat treatment is earlier than the full annealing. This is due to the shot process of heat treatment, where the cost is also low.
Above the temperature the eutectoid steel is heated, and lowers the temperature that is cooled rapidly. Transformation of austenite to pearlite is completed at which the steel is held at this temperature. Then steel is cooled in the presence of air. Hypereutectoids steel is not exposed to the heat treatment. In the steel constant temperatures transformations takes place with the homogeneous structure.
This process results that good surface finish and improves machinability. Mainly the process is for steel alloys where the steel posses low cooling rate. This is suitable for the small sized components.
Diffusion annealing is also known as homogenizing annealing. It is used to remove the erratic structures. Columnar grains, chemical in-similarities and dendrites are normally detected in case of heavy carbon steel and high alloy steel castings and ingots. The defects that cause ductility is reduced, toughness and develop brittleness in steel is seen. In this process steel is heated above the upper critical temperature and is seized at this temperature for long periods. Slow cooling is done in 10 to 20 hours, at the temperature of 1000 to 1200. Isolated zones are eliminated and homogeneous chemical steel is obtained by this heat treatment as a final result of diffusion. Heat the material at the high temperatures that result in hardening of austenitic grains and large scale deformation. The obtained austenitic grains are transferred into coarse perlite due to cooling where the mechanical properties are reduced.
Coarse grained structures are developed for the plastic working, or second heat treatment is applied for the casting purpose. In this process we observe the slow cooling rates, longer holding periods, higher temperatures and excessive scaling is necessary. Second heat treatment process is highly expensive.
Re crystallization Annealing
Re crystallization annealing treatment is subjected to steel which is deeply cold worked. In this process steel is heated above the re-crystallization temperature and then cooled. So, we observe the material that increase in ductility and decrease in strength and hardness. The decrease in the cross sectional area is possible with the acceptance of cold working recrystallization cycle. In middle and in the final process the re-crystallization treatment is used. These processes are frequently used in the manufacturing industries like strips, steel wires etc. The original grains are deformed, where the strain free grains are produced. Re-crystallization temperature is not fixed; it depends on the composition of the chemical, holding time and primary grain size. The small amount of the deformation causes the re-crystallization that causes lattice defects which are develop on the crystal. These defects are the main reason for the production of grains. The deformation is high at that condition when the re-crystallization temperature is low, and it achieves high holding time. Heating the temperature above the re-crystallization temperature, results in developing a fine structure. High re-crystallization temperature is required in the alloy steel and high carbon steels; it results in the collection of carbide particles.
Partial annealing is also known as inter-critical or incomplete annealing. In this the steel is heated in between the lower () and upper () critical temperatures. Heat treatment is subjected to the hypereutectoid steels. As it results, instead of coarse pearlite and cementite at grain boundaries, the microstructure consist of cementite and fine pearlite in the situation of full annealing process. The main reason for this is that the grain refinement takes place at the 10 to 30 temperature and above temperature hypereutectoid steels. This process is less expensive than the full annealing; this is because it involved at low temperatures, where as the full annealing involves high temperatures. To improve the machine-ability the hypoeutectoid steels are subjected to heat treatment. Coarse structures of pearlite and ferrite present in the steel are not suitable for this process. We can see only the incomplete phase transfer in this process. During the process the untransformed ferrite results in poor machinability.
Stress relief annealing
Stress relief annealing is conducted at moderately low temperature process. This process is mainly used to reduce the internal stress related to mechanical effects caused by welding, casting and cold working. In this process, the atoms move into the crystal lattice to obtain a stable position. In this process interstitial defects are eliminated and dis-locations take place at the sometimes. Heat treatment is used for preventing the distortions and stress corrosion cracking caused by the internal stress.
In the Spheroidise annealing structure, it consists of spheroids of carbides or bubbles in the surrounding substance of ferrite. The speroidization depends on the temperature and the hold time. By increasing the temperature the process will be completed in very short time. Selection temperature plays a key role in the heat treatment process. During cooling the dissolved carbides are regenerates as lamella with the help PF high spheroidization temperatures.
In the process annealing, heat treatment is given to the steel at below the lower critical temperature; sufficient time is provided and then allowed to cool. Cooling rate plays an important role. The main aim of this process is to reduce the hardness and ductility which is to be increased, and remaining work must be carried out easily. Compare with the re-crystallization process annealing is less expensive.
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