Annealing or Bright Annealing? Why is ...

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In the production of metal materials, heat treatment is a very important step. Annealing is the most common process in heat treatment.

Bright annealing, as the name implies, is an annealing process that can maintain the bright state of the material.

But I believe that this explanation alone will not clear your doubts. Why do materials need to be annealed? Why keep the material bright when annealed? When is bright annealing required?

In this article, we will explain the whole thing from scratch.

Why do Metals Need to be Annealed?

Work Hardening

In order to make the material into various shapes, the material needs to be processed many times. In this process, the material often needs to be deformed by extrusion, drawing and other processes. And this deformation process requires a lot of force on the material. This force is transmitted to the inside of the metal, causing the metal to generate a large and unrelieved pressure inside after deformation. This force is called stress.

Figure 1

Formation of Work Hardening

The stress generated by the material after deformation can make the material very strong. This phenomenon is called work hardening. In some cases, work hardening is a means of strengthening the material. But more often, work hardening causes the material to become more brittle, making it more prone to cracking.

Annealing

Annealing is a process in which a material is heated to a certain temperature, kept at this temperature for a period of time, and finally cooled at a certain rate. One of the main purposes of this process is to eliminate work hardening. During work hardening. Metal atoms are squeezed due to material deformation. This squeezing shortens the distance between atoms. The shortening of the distance leads to an increase in the repulsion between the atoms.

Figure 2

The Shortening of the Distance between Atoms

The process of annealing can be regarded as the process of metal atom diffusion. At room temperature, the diffusivity of metal atoms is poor. At high temperatures, atoms can diffuse sufficiently. Bring the distances between atoms back to equilibrium.

The annealed metal material will soften. At the same time, the material brittleness caused by work hardening is repaired.

Solution Annealing

For superalloys, the material often requires solution annealing. Solution annealing is also called solution treatment. It is a special annealing process. Its purpose is not just to soften materials. For superalloys, solution treatment can also play a role in solution strengthening and grain size control.

For a more detailed introduction to solution treatment, please refer to the following article:

What is Solid Solution? Why do Nickel Alloy / Superalloy need Solution Treatment?

For different types of superalloys, the solution annealing temperature, time and cooling rate are different. For specific parameters, please refer to official documents or standards.

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Why Can&#;t Ordinary Annealing Keep the Material Bright?

After the ordinary annealing process is completed, the surface of the material will turn black. This black substance is called oxide scale. You can refer to the picture below.

Formation of Oxide Scale

Due to the high temperature of the material during the annealing process, the iron element reacts very easily with oxygen in the air. Thereby generating ferrous oxide, ferric oxide, iron oxide. These are the main components of the oxide scale and are the main reason for the blackening of the surface of the material after annealing.

Figure 3

Formation of Oxide Scale

The Nature of Oxide Scale

The oxide scale is very brittle and has no elongation. There will be deep cracks on the oxide scale, which will reduce the structural stability of the internal metal material. In addition, the presence of oxide scale will reduce the corrosion resistance of the entire metal material. The larger the area of the oxide scale, the faster the corrosion rate of the metal material and the more serious the corrosion.

How to Remove Oxide Scale

Due to the various hazards of oxide scale to metal materials, the removal of oxide scale has become an important step in the processing of metal materials. Generally speaking, the main methods of removing scale are pickling and polishing.

Pickling is a chemical surface treatment method. Its principle is to allow the acid to react with the oxide scale on the metal surface, thereby corroding the oxide scale. The image below shows the surface of the material after pickling.

Polishing is a physical surface treatment method. Its principle is to wipe off the oxide skin on the metal surface by grinding. The image below shows the surface of the material after polishing.

Why Can Bright Annealing Keep Materials Bright?

Unlike ordinary annealing, no oxide scale is produced during bright annealing. That's what keeps the material shiny. As mentioned above, the formation of oxide scale is caused by the reaction of iron with oxygen. Metals must contain iron. Then the solution can only be to remove the oxygen during the annealing process.

Bright annealing is annealing in an oxygen-free environment. The gas that replaces oxygen is called the protective gas. The protective gases are generally: water vapor, ammonia, nitrogen and hydrogen. Among them, hydrogen is the most common protective gas. Because bright annealing has high requirements on the internal environment of the annealing furnace, the cost of this process is higher than that of ordinary annealing.

Why Can't it be Annealed and Then Polished?

You may have this question: since the oxide scale can be removed by pickling and polishing, why use the more expensive bright annealing process? Below we will explain each case.

Coil Tube

Coil tube is a tube with a very small diameter and a very long length. The diameter of the coil tube is generally 2 ~ 8mm. The length of the coil tube is often in the thousands. This slender and elongated shape makes the coil tube to not allow scale to form during production.

On the one hand, oxide scale does not only appear on the outside of the tube during ordinary annealing. It also spawns inside the pipe. The inner space of the coil is very small, and the generated oxide scale is very likely to cause the blockage of the pipeline. And the length of thousands of meters makes it impossible to dredge the pipeline.

On the other hand, even if the pipe is not clogged. It is also not practical to remove the scale after annealing. If the pickling method is used, the acid cannot completely flow into the pipe. This will prevent the scale inside the pipe from being removed. The polishing method is even more unable to polish the small interior of the pipe.

Wire

For wire, ordinary annealing also fails to meet the requirements. Oxide scale is formed by the reaction of oxygen and iron in the metal. The formation of oxide scale will deplete the components of the metal surface.

For a material with a very thin diameter such as wire, the consumption on the diameter of the wire cannot be ignored, which will result in the wire's tolerance not meeting the requirements. Therefore, bright annealing is often used in the production of wire.

Decarbonization Issues

Bright annealing also solves the problem of decarburization during normal annealing. Metals will decarburize at high temperatures. That is, the carbon element in the metal will be precipitated from the metal. For some metals this is not acceptable.

The degree of decarburization is also related to the annealing environment. Among them, the decarburization phenomenon is obvious in the water vapor environment. In the ordinary annealing process, moisture in the air cannot be avoided. And bright annealing acts as a complete barrier to air.

Conclusion

Metals undergo work hardening during processing, and annealing restores this hardening.

The annealing process produces oxide scale. This is formed by the reaction of oxygen with iron in the metal.

Bright annealing isolates oxygen, which ensures that no oxide scale is produced during the annealing process. This annealing process is very common on some small-sized product forms.

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While the chemical composition of a metal determines much of the mechanical properties, many metals can have their mechanical properties altered through heat treatment. There are many different types of heat treatment used today, and one of the most popular methods is annealing.

What Is Annealing?

Annealing is a heat treatment process used mostly to increase the ductility and reduce the hardness of a material. This change in hardness and ductility is a result of the reduction of dislocations in the crystal structure of the material being annealed. Annealing is often performed after a material has undergone a hardening or cold working process to prevent it from brittle failure or to make it more formable for subsequent operations.

Why Is Metal Annealed?

As mentioned above, annealing is used to reduce hardness and increase ductility. Changing these mechanical properties through annealing is significant for many reasons:

• Annealing improves the formability of a material. Hard, brittle materials can be difficult to bend or press without creating a material fracture. Annealing helps eliminate this risk.
• Annealing can also improve machinability. A material that is extremely brittle can cause excessive tool wear. Reducing the hardness of a material via annealing can reduce the wear on the tool being used.
• Annealing removes residual stresses. Residual stresses can create cracks and other mechanical complications, and it is often best to eliminate them whenever possible.

What Metals Can Be Annealed?

To perform an annealing process, a material that can be altered by heat treatment must be used. Examples include many types of steel and cast iron. Some types of aluminium, copper, brass and other materials may also respond to an annealing process.

The Annealing Process

There are three main stages to an annealing process.

1. Recovery stage.
2. Recrystallization stage
3. Grain growth stage

Recovery Stage

During the recovery stage, a furnace or other type of heating device is used to raise the material to a temperature where its internal stresses are relieved.

Recrystallization Stage

During the recrystallization stage, the material is heated above its recrystallization temperature, but below its melting temperature. This causes new grains without preexisting stresses to form.

Grain Growth Stage

During the grain growth, the new grains fully develop. This growth is controlled by allowing the material to cool at a specified rate. The result of completing these three stages is a material with more ductility and reduced hardness. Subsequent operations that can further alter mechanical properties are sometimes carried out after the annealing process.

When Are Annealed Metals Used?

Common applications for annealed metals include:

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• Work-hardened materials such as sheet metal that has undergone a stamping process or cold drawn bar stock.
• Metal wire that has been drawn from one size to a smaller size may also undergo an annealing process.
• Machining operations that create high amounts of heat or material displacement may also warrant an annealing process afterward.
• Welded components can create residual stresses in the area of the material exposed to elevated temperatures; to recreate uniform physical properties, annealing is often used.