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What is Quenching?

Quenching is a metallurgical process where metal parts are rapidly cooled following heat treatment to obtain a specific hardness without making the part too brittle. This rapid cooling causes a transformation of the microstructure from austenite to martensite. There are a variety of quenching media available, each with their own benefits. However, when choosing which type of media is best for a part, cooling rate and cooling uniformity are usually the deciding factors.

Types of Quenching Media

The different types of quenching media greatly influence the final properties, uniformity, and overall quality of the treated material. More severe quenchants will generally have much faster cooling rates than less severe quenchants.

Water-Based Quenching

There are several types of water-based quenching agents: caustic, brine, water, and polymer. Caustic has the fastest cooling rate and is the most commonly used water-based quenchant. However, it’s not suitable for all parts and applications due to its severity. As best practice, steel parts with less than .35% carbon are generally safe for water quenching. If working with materials that have more than .35% carbon, you have a significantly higher risk of cracking during water quenching.

Oil Quenching

Oil quenching is a moderately severe quenching method that causes significantly less distortion than water-based solutions. Because of this it’s one of the more commonly used quenchant in commercial heat treating. Oil-quenched components tend to have a more uniform microstructure. There are different types of quenching oils, with varying temperatures, viscosity, and other chemical properties that can be formulated for specific applications.

Molten Salts

Also called salt baths, this quenching medium holds parts at a higher temperature, providing a slower and more uniform cooling rate than oil quenching.

High-Pressure Gas

Modern heat-treating processes use a vacuum environment with a controlled gas atmosphere to achieve the desired cooling rate while minimizing distortion. Many parts that are conventionally oil quenched can often be vacuum quenched to the same specs but with 50%-75% less distortion.

Quenching Media Severity

Caustic Quench vs. Oil Quench

A caustic solution is created by mixing sodium hydroxide and water, creating one of the most severe quenchants used in heat treating today. Caustic solutions stay at lower temperatures and provide faster cooling rates. That means you can harden out larger parts with caustic than you would normally be able to with oil. This is especially beneficial for large heavy truck components with thicker cross sections. 

Choosing the Right Quenching Partner

Quenching is arguably the most important aspect of the heat treating process when it comes to meeting tight tolerances. And at Paulo, we have it down to a science. Our advanced metallurgy team is equipped with the knowledge and technology to precisely bring parts into their desired hardness, ductility, toughness while minimizing cracking and distortion. To learn more about quenching for your next project, connect with a Paulo expert.

More Episodes to Check Out

Check out High-Pressure Gas vs. Oil Quenching and What Types of Steel Can You Water Quench to learn more about quenching.

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