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What is Heat Treatment?

By Amy Dyslex
Updated May 17, 2024
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The process of altering the physical, chemical, and mechanical properties of a metal by applying controlled heating and cooling is known as heat treatment. It is a procedure that is applied to improve or restore a product’s manufacturability. Heat treatments are most commonly applied in metallurgy, manufacturing, hot forming, and welding.

Heat treatment might be applied to raw materials, as in the metal itself, or on finished products. The methodology involves heating materials to severe temperatures to increase internal stress. Subsequently, cooling is done at an extremely low temperature, a process that is also known as quenching. This strengthens the internal lattice structure. It also brings out finery in the grains that have an increased tensile strength.

Heat treatment is commonly carried out to soften, harden, and materially modify a product. Softening reduces the hardness of a material while improving ductility and resistance. It brings out toughness and also improves grain size. This is more commonly known as annealing or normalizing. Typically, it is carried out to restore ductility and to relax metal stresses within a material. Later techniques include tempering and quenching, which toughen and rapidly cool off the materials.

Hardening is mostly carried out on steel to increase its wear and tear properties and potency. Sufficient carbon and alloy content is required in order to carry out the hardening process. When it is present in adequate amounts, the steel is directly hardened. If there is insufficient carbon content, however, a method of carbon enrichment can be used, which increases the carbon ration in carbon steel components.

Selective hardening is a variation in the hardening process where different areas are subjected to different temperatures. It is also known as differential hardening. It is mostly applied to manufacture instruments like swords, knives, and tongs. Material modification is a heat treatment methodology that modifies the behavioral properties of a material. Mostly applied on steel, it is used to enhance aging and serving life, as well as strength and reliability.

Heat treatment is most often applied on large carbon steel components. Steel responds well to heat treatments, due to its extreme manageability, and increases in commercial efficiency after the application of a heat treatment. This makes steel heat treatment a widely popular process in the metallurgical industry. Other than steel, aluminum is another metal on which heat treatment is vastly applied. Unlike with steel, heat treatment of aluminum is carried out in specially designed furnaces under extremely controlled thermal conditions.

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Discussion Comments
By anon358153 — On Dec 09, 2013

@lluviaporos: You're only partially right. Not all gems can be heat treated. Depending on whether the air in the kiln is reducing or oxidizing, you can change the color, lighten or darken a stone depending on what kind. The heat allows for rearrangement of the crystal lattice and "heals" fractures, resulting in increased clarity.

Heat treatment is undetectable in some stones. In others, a microscope is one of several things needed. Looking at refraction patterns and reflected light using various light filters are usually better diagnostic tools.

By pastanaga — On Jun 23, 2011

This is the same process they used to use at a blacksmith. They would heat the metal over and over and then cool it in water to build up strength.

Of course there are some craftspeople who still make things in this way. And in countries without industrial means of making common objects also use this method.

It's kind of awesome that there are still real working blacksmiths out there, actually. When I was traveling in Africa I had a ring made and inscribed by a blacksmith. He used silver from old coins.

It is still one of my most precious things.

By lluviaporos — On Jun 21, 2011

I found out recently that heat treatment is also a term for a process used on gemstones that can make them look darker and more vibrant.

However some people think of it as like creating synthetic gems and prefer the natural colors of the stones, so heat treated stones are often priced lower than an equivalent natural stone. Although they are presumably priced higher than they would have been without the treatment.

That's why sometimes you'll see on websites "heat treatment free". They often won't label it if it's been treated, so remember to ask if you think it's important.

Of course, you can only tell it's been heat treated if you have a microscope, so I don't really think it matters.

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