How Does Heat Treating Change Metal?

Steel is used in everything from vehicles and airplanes to buildings and heavy equipment. Because it’s incredibly strong and durable, it’s a versatile material. Steel is made by adding carbon to iron and heat treating it. The heat treating process makes steel harder and stronger in Gastonia, NC. Heat treating can also be used on other metals to achieve similar results. Here’s how the heat treating process works, and the effect it has on metals.

How heat treatment works

What happens during heat treatment in Gastonia, NC? When metals are heated to extremely high temperatures, the entire atomic structure changes. For example, iron has a certain crystalline structure. When it’s heated along with carbon to create steel, the crystal structure changes. Specifically, a nine-atom iron unit cell becomes a 14-atom unit cell after it’s heat treated. This change occurs when the steel is heated above its “critical temperature,” which is the term for the point at which recrystallization occurs. The iron and carbon atoms rearrange themselves into a stronger, harder metal. Sometimes additional carbon is added during the heat treating process, which is called carburizing.

Of course, you can’t simply heat the metal and let it cool down naturally. In order to preserve the new crystalline structure, the metal has to be quenched: plunged into cool oil, water or other liquids to instantly “freeze” the structure as it is. This traps the carbon atoms in the steel so it can’t diffuse out.

Heat treating methods

Here’s a closer look at some of the most common methods of heat treating:

  • Hardening: Hardening metal simply involves heating it up to a certain point, then quickly quenching it. The temperature for recrystallization varies, but generally, the more carbon content in the metal, the lower the temperature has to be.
  • Annealing: Annealing changes the grain structure of the metal, which makes it more ductile. The metal is heated up to just over the recrystallization point. Unlike in hardening, the metal is then allowed to cool down slowly, which softens the metal. Full annealing involves leaving the metal to cool while still in the furnace.
  • Normalizing: Normalizing reduces any grain stresses in the metal. This process heats the metal past the recrystallization point, then lets it air cool. It makes the metal more stable, which can make it then suitable for other metallurgical processes.
  • Tempering: Tempering makes metal more malleable, which allows it to be shaped and formed. The metal is heated to a certain temperature to achieve the effect. However, this process can make the steel softer.
  • Quenching: Quenching, as noted above, is the process in which hot metal is plunged into water or oil to stop the heat treating process. Steel is much more brittle after quenching, requiring it to be heated and quenched several times.

As you can see, there are plenty of ways to achieve different metallurgical effects through heat treatment. To find out which heat treating processes your projects require in Gastonia, NC, talk to the team at J.F. Heat Treating, Inc.

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