Hello Readers, welcome to your own website to understand each and every topic related to the manufacturing process where we transform complex content into simpler ones. In this article, we are focused to cover the meaning of annealing, process, advantages, disadvantages, and applications of annealing.
What is ANNEALING?
Annealing is a just simple term with which it becomes easy to describe the restoration of a cold-worked heat treated alloy to its original properties—for instance, to increase ductility and reduce hardness and strength, or to modify the microstructure of the alloy.
For the purpose of relieving residual stresses in a manufactured part, as well as to improve machinability and dimensional stability the annealing process is used.
Fig. 1 Heat-treating temperature ranges for plain carbon steels, as indicated on the iron–iron carbide phase diagram.
For the thermal treatment of glasses and similar products, castings, and weldments the annealing phenomenon is used.
The further steps given below are involved in annealing :
- The first step is to heat the workpiece at a specific range of temperature in a furnace.
- It needs to be held at that temperature for a particular period of time (soaking).
- After that the cooling of the workpiece should take place in air or in a furnace.
The annealing process is performed at lower temperatures to minimize or prevent surface oxidation, also it may be carried out in an inert or a controlled atmosphere.
Fig. 2 Hardness of steels in the quenched and normalized conditions as a function of carbon content.
Annealing temperature may be higher than the material’s recrystallization temperature, depending on the degree of cold work.
For example, the recrystallization temperature for copper ranges between 200° and 300°C (400° and 600°F), whereas the annealing temperature needed to fully recover the original properties ranges from 260° to 650°C
(500° to 1200°F), depending on the degree of prior cold work.
For annealing of ferrous alloys, the term full annealing is applied.
The steel is heated to above A1 or A3, and the cooling takes place slowly [typically at 10°C (20°F) per hour], in a furnace, after which it gets closed.
The structure obtained through full annealing is coarse pearlite, which is so soft and also ductile and it has small, uniform grains.
To avoid excessive softness from the annealing of steels, the cooling cycle may finish completely in still air.
For the purpose, to indicate that the part is heated to a temperature above A3 or A cm in order to transform the structure to austenite, this process is define as normalizing.
Normalizing results in somewhat higher strength and hardness, and full annealing in lower ductility.
The structure obtained is fine pearlite, with small uniform grains.
The normalizing is a process generally performer to refine the grain structure, and obtain uniform structure (homogenization), which decrease
residual stresses, and improve machinability.
Due to the spheroidizing annealing the cold workability and the machinability of steel get improved .
PROCESS OF ANNEALING
The workpiece is annealed to restore its ductility, part or all of which may have been exhausted by cold working, during the annealing process.
After the part can be worked into the final desired shape.
If the temperature is high and the time of annealing is long, grain growth may show the adverse effects on the formability of the annealed parts.
STRESS RELIEF ANNEALING
A workpiece is generally subjected to stress-relief annealing, or simply stress relieving for the purpose to reduce or eliminate residual stresses.
On the material and on the magnitude of the residual stresses present ,on both of this the temperature and time required for this process is dependent.
During the process of forming, machining, or other shaping processes, the residual stresses may have been involved or they may have been
caused by volume changes during phase transformations.
For steels, the part is not heated to high temperatures, in order to avoid
phase transformations. Slow cooling which occurs in still air, is generally employed.
Stress relieving promotes dimensional stability in situations where subsequent relaxing of residual stresses may cause distortion of the part when it is in service over a period of time. It also reduces the tendency
toward stress-corrosion cracking.
If steels are hardened by heat treatment, then tempering or drawing is used in order to reduce brittleness, increase ductility and toughness, and reduce residual stresses.
Fig. 3 Mechanical properties of oil-quenched 4340 steel as a function of tempering temperature.
The term “tempering”is also used for glasses. In tempering, the steel is heated to a specific temperature, depending on its composition, and then cooled at a prescribed rate.
Alloy steels may under go temper embrittlement, which is caused by the segregation of impurities along the grain boundaries at temperature between 480° and 590°C.
In austempering, the heated steel is quenched from the austenitizing temperature rapidly, to avoid formation of ferrite or pearlite.
It is held at a certain temperature until isothermal transformation from austenite to bainite is complete.
For the purpose to avoid thermal gradients within the part it needs to cooled at room temperature usually in still air and at moderate rate.
The quenching medium most commonly used is molten salt, at temperatures ranging from 160° to 750°C (320° to 1380°F).
In this process it involves too short time cycle, so this is economical for many applications.
In modified austempering, a mixed structure of pearlite and bainite is obtained.
First of all in this process the steel or cast iron is first quenched from the austenitizing temperature in a hot-fluid medium, such as hot oil
or molten salt.
After the above process it is held at that temperature until the temperature is uniform throughout the part; then it is cooled at a moderate rate, such as in air in order to avoid excessive temperature gradients within the part.
The structure obtained is primarily untempered martensite and because of this it is not suitable for most applications so it is then tempered.
Martempered steels have less tendency to crack, distort, or create residual stresses while heat treatment is performed on it.
The quenching temperature is lower, and due to which the cooling rate is higher in modified martempering.
For the steel which is with lower hardenability this process is much more suitable.
Ausforming process is also known as thermomechanical processing in which the steel is formed into different desired shapes within controlled range of temperature and time, to avoid formation of nonmartensitic transformation products. The part is then cooled for obtaining desired microstructure.
Ausformed parts have superior mechanical properties.
Annealing process improves the workability of a material.
It also increase the toughness.
Because of this it reduce the hardness and increases the ductility and machinability of a metal.
Heating and cooling process also reduces the brittleness of metals by enhancing their magnetic properties and electrical conductivity.
In the annealing process there is one problem that it can be a more time consuming procedure, depending on which materials are being annealed.
It can take too much time to cool, especially if they are being left to cool naturally inside an annealing furnace, when there is the requirements of materials with high temperature.
In the more functioning and more variety of industries where metals need to be worked into complex structures or worked on several time the annealing plays an important role.
For improving machinability, annealing process is used.
It is used to soften the steel.
It is also used to remove gases.
It increases the mechanical property.
We have covered all the important concepts related to annealing process. Hope you all are crystal clear with understanding all the concepts mentioned here. If you have any questions please use the comments section to get in touch with us. Till then have fun and always keep reading!