How Does Heat Treatment Change the Properties of Steel?


When steel with a medium to high carbon content is subjected to heat treatment, it can be hardened. Depending on steels carbon content, they can be heated to specific temperatures (780 oC to 850 oC) and then quenched in oil or water to cool them quickly. The higher the carbon content, the lower the temperature needed to harden the metal. This temperature is also known as the austenitic crystal phase.

This process alters the crystal structures within the metal, forming cementite (otherwise known as iron carbide), a hard, brittle material that is classified as ceramic in its pure form. Cementite has an orthorhombic crystal structure and is 6.67% carbon and 93.3% iron by weight.


Annealing changes a metal's properties by altering and realigning the grain structure using heat, making the metal softer and more ductile. In this process, the steel is heated to just above its re-crystallization point, allowing it to cool slowly. A full anneal involves leaving the metal to cool in the furnace itself.


When heating the metal to a point higher than that of the annealing process and allowing it to air cool, the grain structure stresses can be removed. Normalizing steel gives the structure more stability and the metal can be prepared for other processes.


Steel can be made more malleable by temperingit. This involves heating the metal to a predetermined temperature which depends on the level of malleability required. Tempering takes away a small amount of hardness from the steel.


Quenching refers to the rapid cooling of hot metal using oil or water. This sets the steel, making it hard but brittle. Steel is heated and quenched several times, creating a tough product that will ensure it is strong enough to support buildings, bridges, and any other structure it may be used in.

Annealing, quenching, and tempering are generally always used in the heat treatment of steel.


Over the last couple of millennia, steel technology has developed in leaps and bounds. Today, steel is used in nearly all facets of our built environment. However, untreated steel has the potential to fail, with potentially catastrophic results. Therefore, controlled heat treatment of steel is crucial to ensure that it has the desired properties necessary for its intended use, and heating and cooling in different ways cause different structural changes that can fine-tune the desired structure.

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