Transforming Metal with Quenching
September 11, 2023
Quenching is an important step in many metallurgy processes. For example, steel quenching is what prepares heated steel for further transformation — it helps to preserve or alter the steel’s internal microstructure, which is essential for creating steel parts that fit their intended applications.
What is Quenching?
Quenching is a step in many heat treatment processes that involves heating the part to the required temperature and immersing it in a quenching medium to quickly cool it. Other heat treatment processes often follow quenching, such as aging, tempering, or annealing to achieve the desired results. By rapidly cooling the metal, you can prevent or encourage the formation of different microstructures within the steel to achieve the properties you need for the intended application.
The various types of crystalline microstructures include:
- Austenite: Austenite forms when iron is heated during metalworking. It is a tough yet formable microstructure that is excellent for welding. Austenitic steel has a variety of applications, but it is often further worked to create martensitic steel.
- Martensite: Martensitic transformation is what happens when you rapidly cool heated metal. While martensitic steel is hard, it is also very brittle. Working the metal after quenching helps to restore some ductility to it and relieve internal stresses.
- Ferrite: Ferrite is a soft, thermodynamically stable microstructure that forms before transforming into austenite. It mostly occurs in low-carbon steel.
- Cementite: Cementite is a thermodynamically unstable microstructure that contains iron and carbon. Its rigid microstructure results in a very hard yet brittle metal.
- Pearlite: Slow cooling results in a pearlite microstructure, which is composed of alternating layers of ferrite and cementite. This structure is strong and lightweight with high wear resistance.
- Bainite: Bainite is a hard, brittle microstructure that forms when a steel component is cooled faster than the cooling rate of pearlite but slower than martensite.
Types of Quenching Media
A quenching medium is a substance you use to quench metal during heat treating processes.
Metallurgists use several types of quenching media, each delivering different results due to their unique properties.
Cooling rate is one important property that significantly affects the final result. Rapid cooling prevents internal changes from occurring, while slower cooling dramatically alters the metal’s crystalline structure.
Gas quenching is most common in vacuum furnace applications, where atmospheric control is vital. This control improves temperature uniformity during the quenching process, which reduces the risk of distortion and residue formation.
Typical gases used for quenching include helium, argon, hydrogen, and nitrogen. Each gas has different properties that affect the quenching process:
- Hydrogen: Hydrogen has the fastest quenching rate out of all gas quenching media. Its heat transfer coefficient is nearly double that of nitrogen, which can provide quenching speeds similar to oil. Low-carbon hydrogen gas is also more sustainable than industrial hydrogen produced using conventional methods.
- Nitrogen: A controlled atmosphere containing nitrogen are essential for gas quenching. Nitrogen is an ideal quenching medium for low alloy steels because it prevents surface oxidation, which can ensure steel does not harden.
- Argon: Argon is suitable for steel parts that are sensitive to nitriding. It is effective for maintaining consistent temperature and composition in metal parts by preventing surface oxidation.
- Helium: Helium is a high-performing quenching gas that is useful for improving tolerance and quality. As an inert gas, it is a safe choice for vacuum heat treating and quenching applications.
Quenching steel in water cools the metal rapidly, making it excellent for achieving maximum hardness in certain applications. However, its cooling speed can also result in distortions and cracking due to internal stress buildup. As a result, further metallurgy processes are required to finish the part.
Like many gases, water is a sustainable quenching medium because it is readily available and has a low environmental impact. When quenching with water, steam pockets can interfere with the quenching process — if excessive steam arises, the smith will need to stir the component in the medium to prevent pockets of moist air from forming and causing impurities.
Quenching in oil is one of the most popular methods because it rapidly cools the metal without the risk of distortion. Each quenching oil has a different cooling rate and flash point, so choosing the oil with the correct properties is essential for obtaining your desired results.
Many oils can be used for quenching steel, but for commercial applications, a specialized quenching oil is the best and safest option. While these oils can be costly, they are compatible with numerous metals and alloys and are more likely to provide high-quality results.
Two methods exist for quenching steel using air:
- Forced air: Forced air quenching is an active process that involves forcing pressurized air through the metal part. This method cools the part quickly, though it is still significantly slower than other quenching media.
- Air cooling: Allowing the steel to cool to room temperature in open air is a passive form of air quenching. It is significantly slower than other quenching media, including forced air, but it is environmentally sustainable and cost-effective.
Air is generally effective as a quenching medium, but it is less efficient for many applications due to its slow cooling rate.
BrineBrine, or salt water, makes an excellent quenching medium for rapid cooling. The mixture of salt and water prevents the formation of air globules during quenching, which allows it to more completely cover the part’s surface and create a uniform temperature. Brine also accommodates a wider range of temperatures than other quenching liquids. This property makes it an advantageous choice for heat treating techniques involving extremely high temperatures. Additionally, because most salt is nonflammable, it poses less of a risk than oil quenching, which can be dangerous at high temperatures.
The Quenching Process
Quenching happens once your part has reached the required temperature and soaked for an appropriate amount of time. Quench your part by removing it from the heat source and immersing it in the quenching medium, which rapidly cools the metal to room temperature and prepares it for further work.
Quenched steel is typically harder and less ductile than it was prior to being heated, which is why quenching usually precedes other heat treating processes such as tempering, aging, annealing, or normalizing.
Selecting the right quenching medium is essential for achieving the results you need. You can rely on Linde for high-quality quenching gases, including hydrogen, argon, helium, and nitrogen. We also provide controlled atmospheres to ensure safe metalworking processes and high-quality metal products.
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