An Introduction about heat treatment process

An Introduction about heat treatment process: Heat treatment is group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatment also used in the manufacturing of many other materials, such as glass. Heat treatment involves the use of heating or cooling. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, normalizing and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

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ANNEALING:

Annealing is heat treatment that alters the physical and sometimes chemical properties of material to increase its ductility and reduce its hardness making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a suitable amount of time and then cooling.

Annealing will restore ductility following cold working and hence allow additional processing without cracking. Annealing may also be used to release mechanical stresses induced by grinding, machining etc. hence preventing distortion during subsequent higher temperature heat treatment operations. In some cases, annealing is used to improve electrical properties.

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NORMALIZING:

Normalizing is a heat treatment process that is used to make a metal more ductile and tough after it has been subjected to thermal or mechanical hardening processes. Normalizing involves heating a material to an elevated temperature and then allowing it to cool back to room temperature by exposing it to room temperature air after it is heated. This heating and slow cooling alters the microstructure of the metal which in turn reduces its hardness and increases its ductility.

Here, three main stages to a normalizing process.

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

1. 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.

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2. 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.

3. Grain Growth Stage:

During the grain growth, the new grains fully develop. This growth is controlled by allowing the material to cool to room temperature via contact with air. 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 normalizing process.

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QUECHING:

In materials science, quenching is the rapid cooling of a workpiece in water, oil or air to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as phase transformations, from occurring. It does this by reducing the window of time during which these undesired reactions are both thermodynamically favorable, and kinetically accessible; for instance, quenching can reduce the crystal grain size of both metallic and plastic materials, increasing their hardness.

In metallurgy, quenching is most commonly used to harden steel by including a martensite transformation, where the steel must be rapidly cooled through its eutectoid point, the temperature at which austenite becomes unstable. In steel alloyed with metals such as nickel and manganese, the eutectoid temperature becomes much lower, but the kinetic barriers to phase transformation remain the same. This allows quenching to start at a lower temperature, making the process much easier. High speed steel also has added tungsten, which serves to raise kinetic barriers, which among other effects gives material properties (hardness and abrasion resistance) as though the workpiece had been cooled more rapidly than it really has. Even cooling such alloys slowly in air has most of the desired effects of quenching; high-speed steel weakens much less from heat cycling due to high-speed cutting.

Extremely rapid cooling can prevent the formation of all crystal structure, resulting in amorphous metal or “metallic glass”.

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Purpose:

Before hardening, cast steels and iron are of a uniform and lamellar (or layered) pearlitic grain structure. This is a mixture of ferrite and cementite formed when steel or cast iron are manufactured and cooled at a slow rate. Pearlite is not an ideal material for many common applications of steel alloys as it is quite soft. By heating pearlite past its eutectoid transition temperature of 727°C and then rapidly cooling, some of the material’s crystal structure can be transformed into a much harder structure known as martensite. Steels with this martensitic structure are often used in applications when the work piece must be highly resistant to deformation, such as the cutting edge of blades. This is very efficient.

(References: Heat Treatment: Principles and Techniques by C P sharma

Physical Metallurgy: Principles and Practice by V raghawan

Quenching: wikipedia)

Conclusion: In this article, you read an introduction about the heat treatment process and its purpose. Thank you for reading this article hope it may help you to understand about basics about heat treatment. And visit for more article related to bio materials, electronic materials, advance materials,  magnetic materials and metallurgical based.

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