Heat treatment process of steel

The common heat treatment process of steel-like knives includes hardening, tempering, annealing, normalizing, and case hardening. Heat treatment is the process in which the properties of metals are changed to a desired one. Today you’ll get to know the heat treatment process of steel and how to heat treating knives.

Process of heat treatment of metals

Read more: 3 Stages of Heat Treatment Process


Heat Treatment Process of steel

The heat treatment of steel includes hardening, annealing,  normalizing, tempering, and case hardening.


Hardening is done by subjecting a metal to a required temperature and then quickly cooling it through a medium of quenching such as oil or water. Hardening is done in order to increase the hardness and strength of metal which is achieved by rapid cooling. The metal becomes brittle after it is rapidly cooled.


Steel is annealed when heated to a specific temperature and then allowing it to cool slowly back to room temperature. It helps in relieving the internal stresses of metals, making them more ductile, and refining their grain structures. During the cooling process, the metal is covered under the arches created when heating or if heated with a furnace, it is expected to shut it off leaving the metal inside allowing the furnace and metal to slowly cool together.

Read more: Heat Treatment of Aluminum and Aluminum Alloys

I believed you can clearly see the difference between the two processes, both are subjected to heat but their cooling process differs. In the case of hardening, it is done to increase the hardness of metals while annealing is done to soften metals.


Normalizing is done on ferrous metal that undergoes forging, machining, or welding to relieve the internal stresses and make it harder and stronger. It is done by subjecting the steel or alloys to heat at a specific temperature, soaking it until it is uniformly heated, and then cooled in the open air. This technique helps to provide uniformity in their grain size and composition. The temperature used in normalizing is higher than hardening and annealing as it gives maximum toughness to the metal and resistance to external stresses.


as steel properties are too brittle and very hard to make it applicable for most items, tempering is done to reduce the internal stresses and brittleness and help to impact the toughness of the steel. It is achieved by heating steel below the lower critical temperature usually from 400 to 1105 degrees Fahrenheit or 205 to 595 degrees Celsius depending on the result needed and at higher temperatures usually from 1,300 degrees Fahrenheit or 700 degree Celsius depending on the alloy and application. The high temperature helps in increasing the level of ductility but loses some strength. In this heat treatment, the subjected heat is lower than hardening, and the cooling is done in still air. The rate of cooling determines if is to produce soft or hard metal. High-speed steel is obtained from tampered heat treatment.

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Case hardening:

Case hardening is done in order to harden the surface of a metal and leave the inside soft and tough. It is a diffusion process using low carbon steel, usually straight carbon steel or low carbon alloy steel, exposing it to a specific heat temperature and adding an alloying element, usually nitrogen or carbon, leaving more of it on the metal surface. It is rapidly cooled giving the outer surface hard and the inner part soft but tough.

Naturalizing, tempering and case hardening seems to be completely different process, most especially case hardening being a thermo-chemical process, adding an alloying element to the surface during the healing process which brings about the two effects on the metal. And as for tempering, it uses a very high temperature of a specific amount in heating steel in order to reduce the brittleness and internal stresses. While tempering heat treating can give either hard or soft to the metal due to the low heat applied and the rate of cooling.

Read more: Heat treatment process of steel

Watch the video below to learn how to heat-treat a knife:

Read more: Heat Treatment of nonferrous metals

That is all for this article, where the heat treatment process of steel is discussed, and how to heat treat a knife is also explained in video format. I hope you get a lot from the reading, if so, kindly share with other students. Thanks for reading, see you next time!


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