What Happens When Metals Undergo Heat Treatment?

Techniques for Heating and Cooling Metal

Engineer heat treats industrial gear in factory
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Before modern metal working techniques were invented, blacksmiths used heat to make metal workable. Once the metal was formed into the desired shape, the heated metal was quickly cooled. Quick cooling made the metal harder and less brittle.

What Is Heat Treatment?

Heat treatment is the process of heating and cooling metals to change their microstructure and to bring out the physical and mechanical characteristics that make metals more desirable.

The temperature metals are heated to and the rate of cooling after heat treatment can significantly change metal's properties.

The most common reasons metals undergo heat treatment are to improve their strength, hardness,  toughness, ductility and corrosion resistance. Get a better idea of the process with the examples of common heat treatments that follow.

Understanding Annealing

Annealing is a form of heat treatment that brings a metal closer to its equilibrium state. Annealing softens metal making it more workable and providing for greater ductility. In this process, the metal is heated above its upper critical temperature to change its microstructure. Afterward, the metal is slow-cooled.

What Is Quench Hardening?

Less expensive than annealing, quenching is a heat treatment method that quickly returns metal to room temperature after it is heated above its upper critical temperature. The quenching process stops the cooling process from altering the metal's microstructure.

Quenching, which can be done with water, oil and other media, hardens steel at the same temperature that full annealing does.

The Precipitation Hardening Process

Precipitation hardening is also known as age hardening. It is a heat treatment process that creates uniformity in a metal's grain structure.

This process makes the material stronger. 

The process involves heating a solution treatment to high temperatures after a fast cooling process. Precipitation hardening is usually executed in an inert atmosphere at temperatures ranging from 900 to 1150°F.  It can take anywhere from an hour to four hours to carry out this process. The length of time typically depends on the thickness of the metal and similar factors. 

Why Are Metals Tempered?

Commonly used in steelmaking today, tempering is a heat treatment used to improve hardness and toughness in steel as well as to reduce brittleness. The tempering process creates a more ductile and stable structure. The aim of tempering is to achieve the best combination of mechanical properties in metals. 

What Is Stress Relieving?

Stress relieving is a heat treatment process that decreases stress in metals after they have been quenched, casted, normalized, etc. Stress is relieved by heating metal to a temperature lower than that required for transformation. After this process, the metal is then slow cooled.

The Benefits of Cryogenic Treatments

When a metal part is cryogenically treated, it is slowly cooled with liquid nitrogen. The slow cooling process helps prevent thermal stress of the metal.

Next, the metal part is maintained at a temperature of roughly −190 °C for about a day.

When it is later heat tempered, the metal part undergoes an increase of temperature up to approximately +149 °C. This heat tempering helps to lower the amount of brittleness that may be caused when martensite forms during cryogenic treatment.