What is heat treatment of steel?

These are controlled heating and cooling processes that cause specific changes in the metallographic structure of a metal without altering its chemical composition. These operations involve controlling factors such as temperature, time, pressure, or speed. Heat treatments of steel are used to improve hardness, mechanical strength, ductility, and more.
The time and temperature required for each process are not always the same—they depend on the material being treated, its morphology, the cooling medium, and other variables.

What are the most commonly used heat treatments for steel?
Normalizing
This can be applied to steels that have been hot-or cold- worked or have undergone defective treatments, in order to return them to a normal state. The goal is to obtain fine, homogeneous grains metallographically, and it is usually used as a preliminary step before quenching.

Annealing
This aims to soften the metal for better machinability. It facilitates the machining of high-carbon steels, maintaining dimensional stability and reducing tool wear. There are various types of annealing, such as full annealing, stress relief, homogenization, and recrystallization with prior deformation.

Quenching
This treatment increases the hardness and mechanical strength of steel by heating it to a high temperature and then cooling it rapidly. This sudden temperature change transforms the internal structure of the steel, creating a phase called martensite, which is very hard. The drawback of using this treatment alone is that the resulting piece is hard but brittle.

Tempering
A heat process typically used after quenching. This treatment improves ductility in steel at the expense of some hardness. The resulting metallographic structure is called “tempered martensite.”

THERMOCHEMICAL TREATMENTS
Carburizing
This treatment increases the carbon content in the outer layer of the steel by diffusing carbon onto the surface. After carburizing, the part undergoes quenching and tempering to harden the surface layer and increase its mechanical strength, resulting in a hard exterior with a tough core (since the chemical composition inside remains unchanged).

Nitriding
Similar to carburizing, this process enriches the surface of a metal such as steel with another element—in this case, nitrogen. The result is a hard, wear-resistant nitride layer, with significantly improved fatigue and corrosion resistance. It involves a prior heat treatment (quenching and tempering at high temperature), followed by nitriding. The result is a tough, ductile core and a very hard nitrided outer surface.

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