Annealing: Process, Types, Limitations Guide

Annealing, a type of heat treatment, modifies the properties of a material to enhance its workability.

Although there are various heat treatment processes, annealing is one of the most widely used processes in industries such as automobile, aircraft manufacturing, etc.

But what exactly is annealing? How do you perform annealing and what are the factors that affect the annealing process?

Annealing is a heat treatment process that increases the ductility and workability of material while reducing its hardness by altering its physical and, in some cases, chemical properties. Furthermore, annealing can also be performed to relieve unwanted stresses developed in the material due to machining operations.

This article provides a detailed guide on annealing by going through its process, types, advantages, and limitations.

In the end, I've also discussed the difference between annealing and normalizing.

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What is Annealing? Explained

Annealing is one of the most commonly used heat-treatment to process metal workpieces and alter their mechanical properties, making them suitable for different applications.

Unlike other heat treatment processes, like tempering and normalizing, annealing involves heating the material beyond its recrystallization temperature for a particular interval of time.​

​The material is then allowed to cool, and the type of material being annealed determines the cooling rate.

Ferrous metals such as iron and steel are generally brought to room temperature in still air, whereas non-ferrous metals like copper and brass can either be quenched in water or slowly cooled at room temperature.

Heating the material causes the grain structure to change and reorients the grains to reduce any irregularities and defects (dislocations). This increases ductility and reduces hardness.

Therefore, the heating and cooling rates during the annealing process determine the material properties.

This induced ductility prevents brittle failure, and prepares the material for further cold-working, like forming, shaping, and stamping.

Alloys like brass and metals such as aluminum and copper are some examples that can be annealed. Apart from that, plastic annealing can also be performed to relive internal stresses.

Generally, cast aluminum is annealed to relieve the internal stresses produced during the casting operation and enhance its ductility.

The annealing process is performed by heating the material in an annealing furnace, followed by heat preservation for a specific time interval, and then turning off the heat source to allow gradual cooling of the material inside the furnace.

Annealing is generally used in industries to relieve the unwanted stresses developed in the workpiece during different machining operations like rolling, drawing, welding, forging, etc.

You can easily perform annealing operations if your workshop is equipped with metal melting tools, such as furnaces, tongs, crucibles, and other safety gear.

Annealing: Understand the Process

The annealing process is divided into three sequential stages: The recovery stage, the Recrystallization stage, and the Grain growth stage.

Recovery stage

In the recovery stage, the material is heated in a furnace to minimize the irregularities and defects caused by dislocations, eliminating internal stresses.

The two principal mechanisms for this stage are annihilation and rearrangement.

Annihilation is when dislocations merge and reduce in number. During the heating process, the kinetic energy increases, thereby increasing the probability of interaction of dislocations.

This process is followed by rearrangement of dislocations in such a way that it releases internal stress, reducing the overall stored energy of the material, without altering the grain shape or size.

A higher temperature leads to more effective interaction between the dislocations, increasing the efficiency of the recovery stage.

Recrystallization stage

The second stage is the recrystallization stage, which involves heating the workpiece beyond its recrystallization temperature without melting it. 

For annealing steel, the heating is in the temperature range of 400 to 700°C, while metals like iron have a recrystallization temperature of 450°C.

New grains gradually replace the deformed grains, thereby reducing the strength and hardness of the material while enhancing its ductility.

The recrystallization process is affected by factors such as the annealing time, extent of cold-working, material composition, and grain size.

An increase in the value of these factors will result in a decrease in the recrystallization temperature and vice versa.

With a lower temperature, the heating requirements will be reduced, and the process will be more convenient, meaning that the faster the recrystallization rate, the quicker the grain growth stage will be achieved.

It must be noted that the recrystallization temperature is higher for alloys than for pure metals.

Grain growth stage

This stage involves cooling the material to facilitate the completion of grain growth, initiated during the recrystallization stage.

Fully grown grains reduce the overall grain boundary area, thereby reducing the internal energy of the material.

A higher cooling rate leads to a smaller grain size due to undercooling effects, while a higher annealing temperature leads to greater grain size.

Larger grain size can lead to faster crack initiation and propagation, and consequent failure in the material.

Whereas faster cooling rates can increase the brittleness of the material, thereby increasing its probability of failure.

With an increase in annealing time, the grain size distribution first reaches a stationary state, then increases exponentially.

Factors Affecting Annealing Process

Temperature variations have a more dominant effect on annealing compared to time variations. 

The annealing temperature depends on the thickness, geometry, and composition of the material. For example, alloys have a higher annealing temperature compared to pure metals.

Thicker material will also demand more heat and thus will have a higher annealing temperature. 

Similarly, materials with a greater surface area and volume will have a higher annealing temperature.

Types of Annealing

The type of annealing process to be performed determines whether the material is to be heated below or above its critical temperature.

This critical temperature is the point where the solid phase of material transitions into the liquid phase. 

Based on their heating temperatures and cooling rates, there are seven types of annealing processes.

Type of Annealing	Extent of Heating	Heat Preservation Period	Cooling Rate
Complete	Above critical temperature	Moderate	Moderate
Isothermal	Above critical temperature	Moderate	Moderate
Incomplete	Above critical temperature	Short	Slow
Diffusion	Above critical temperature	Long	Slow
Spherification	Above critical temperature	Short	Slow
Recrystallization	Below critical temperature	Moderate	Slow
Stress	Below critical temperature	Moderate	Moderate

All annealing processes reduce hardness and improve ductility, but each results in specific properties being more dominant.

The main applications of complete annealing and isothermal annealing include reducing hardness, obtaining a fine and uniform grain structure, eliminating internal stresses, and improving machinability.

Whereas incomplete annealing mainly reduces hardness, eliminates internal stresses, and improves machinability without affecting the grain structure.

Diffusion annealing and spherical annealing are mainly used for obtaining a fine and uniform grain structure in the material.

Moreover, spherical annealing also achieves lower hardness and improved machinability.

On the other hand, recrystallization and stress annealing is mainly performed to reduce internal stresses in the material.

These distinctive characteristics of these annealing processes make them ideal for different materials.

For example, complete annealing is most suitable for hypo eutectoid steel (Carbon fraction less than 0.76%), while hypereutectoid steel (Carbon fraction more than 0.76%) requires spherical annealing.

Advantages of Annealing

The annealing process offers various advantages, making it ideal for industrial applications.

Increased Ductility/Reduced Brittleness

Annealing increases the ductility of the material while reducing its hardness. It is beneficial in cases where a structure is prone to fatigue or heavy loading. 

For instance, a truss bridge comprising steel bars will successfully resist deformation due to fatigue or heavy loads if its ductility is sufficiently high. 

Whereas, low-ductility steel bars will have a higher probability of failure and subsequent bridge collapse.

Improved Machinability

The process of annealing improves the uniformity of the grain structure, thereby improving the machinability of a material.

This provides the ability to work with hard materials, which otherwise would be extremely difficult to machine.

For example, machining a complex steel component, such as a spur gear.

Annealing improves the machinability of steel, thereby allowing you to perform the operation with high cutting speed and low tool wear.  

Elimination of Internal Stresses

Internal stresses can cause loss of tolerance, cracking, and deformations, which can lead to component failure during operation. 

However, by eliminating them, the overall mechanical strength of a material increases, thereby lowering the probability of failure.

A steel bar possessing internal stresses along with minute voids or cracks is bound to fail. However, if the same bar is annealed, it will be mechanically stronger and have a longer life.

Improved Magnetic Properties

Annealing reorients the grain structure, thereby improving its magnetic properties.

These enhanced magnetic properties make the material ideal for applications such as electromagnetic sensors, magnetic shielding, or electromagnets.

Limitations of Annealing

Despite its numerous advantages, the annealing process is accompanied by some disadvantages as well.

High Cost

The process of annealing requires a special annealing furnace, which increases its initial cost when compared to other similar processes, like normalization.

Furthermore, the running cost of the furnace and the need for skillful workers to precisely control the heating and cooling rates further increase the cost of annealing.

Coarse/Non-uniform Grain Structure

Coarse-grain structures have low strength as they have a smaller ratio of grain boundary to dislocations, due to which they have a higher likelihood of fracturing.

The properties in non-uniform grained structures may vary in different directions.

Industries where fatigue and heavy loads are common, like the construction industry, can not afford to use such materials as they may prove catastrophic. 

Time Consuming

The annealing process consists of a series of stages where the material undergoes different operations to enhance its properties.

However, this increases the overall time required to complete the process.

This time-consuming nature of annealing can be a drawback for large-scale industries where quick cycle time is of utmost importance.

Types of Metals Suitable for Annealing

Steel (alloy) and cast iron are the most commonly used metals for the annealing process. Other metals include aluminum, copper, and brass.

Materials like steel, cast iron, or aluminum are annealed for increased ductility and reduced hardness/brittleness for use in aircraft bodies, automobile bodies, etc.

Annealing copper improves its magnetic properties, making it suitable for electrical applications, like drawing into wires, etc.

Similarly, annealed brass has improved machinability and minimal internal stresses, making it suitable for use as stock in manufacturing gears and bearings.

Annealing vs Normalizing

The following table shows the differences between annealing and normalizing.

Annealing	Normalizing
Mainly relieves internal stresses	Mainly improves mechanical properties
Results in reduced hardness with enhanced strength and toughness	Results in slightly enhanced hardness with reduced strength and toughness
Grain microstructure is coarse	Grain microstructure is fine
The cooling rate is slower	The cooling rate is rapid
More time consuming	Less time consuming

Normalizing and annealing are similar in that they both involve heating a material above the recrystallization temperature followed by cooling, ​resulting in a microstructure that offers improved ductility.

However, both the processes differ in terms of the parameters, such as process time and cooling rates, with annealing being the slower process and having a slower cooling rate.

While annealing focuses primarily on relieving the internal stresses of the material, the normalizing process is performed to improve the mechanical properties of the material.

Apart from that, the material is rapidly cooled in the normalizing process, whereas annealing follows a gradual cooling of the material.

This difference in their cooling rate results in different grain structures. While annealing results in coarse grain structure, normalizing produces a fine grain microstructure.

The major difference between the two processes is the resulting material properties.

The annealing process enhances the ductility of the material, whereas the normalizing process is recommended for applications where enhanced hardness is desirable, at the cost of ductility.

Frequently Asked Questions (FAQ)

Is annealed steel stronger?

Yes, annealed steel is stronger as it is more ductile, making it softer and more machinable. 

Will annealed steel rust?

Yes, annealed steel is more prone to rust due to reduced corrosion resistance, as annealing alters its grain structure.

Can you anneal glass?

Yes, glass can be annealed. For most types of glass, the annealing temperature is in the range of 454 to 482°C.