One of the most critical parts in the heat treatment of a metal part is the quench, or the rapid cooling of the part to achieve specified properties.
No quenching medium is perfect. There are benefits and drawbacks of each, whether it’s salt, oil, gas or caustic. In a game of trade-offs, oil is the most popular quenching medium because it offers the widest array of benefits to the widest array of parts.
To get a grip on why oil is a popular quenchant, it’s important to understand what happens in a quench.
The succession of heating and then quickly cooling parts via quenching is a way to achieve added hardness to a part that otherwise wouldn’t have been possible. The heating causes changes in the crystalline structure of a metal part’s surface; the rapid cooling “freezes” those changes in place and makes the surface harder.
The first stage in a quench is known as the vapor stage. Because the submerged part is so much hotter than the quenchant, a vapor blanket forms around the part. Cooling of the part occurs during this stage, but it is impeded by the vapor, which acts as an insulator.
The second stage is the boiling stage, which is characterized by the violent boiling of the quenchant. Parts cool fastest in this stage because the temperature of the part has decreased enough during the previous stage for the vapor blanket to dissipate. With the quenchant able to contact the part unimpeded, it can carry away the most heat through boiling.
The third stage is the convective stage, during which convection and conduction further carry heat away from a part. Convection refers to the movement of a liquid due to the tendency of hotter, less dense liquids to rise while cooler, denser liquids sink. Conduction refers to the tendency of heat to dissipate throughout a substance when there are temperature differences in the liquid. Oils are strongly agitated during quenches, forcing it to flow upward through a workload. For this reason, natural convection does not occur.
Why quench in oil?
Oil quenching is popular because of its severity; that is, it transfers heat more quickly compared to other quenching media like molten salt or gas. Water-based caustic quenchants actually quench parts even more quickly than oil, but the severity with which caustics quench parts can cause significant distortion or even cracking for some materials.
In addition, the temperature, viscosity and other chemical properties of oils can be adjusted to achieve different outcomes. This controllability is useful because it means many different types of parts can be quenched with oil, keeping operations efficient.
Oil is a versatile quenchant because oil formulas can be manipulated to suit various intended end results. A wide range of materials can take on a wide range of properties when oil quenches are used.
Types of oil
While there are many different types of oil used in quenching, two commonly used oils are fast and hot oils.
Items cooled in fast oils cool more quickly. While cooling speed in fast oils depends on the specific properties of the oil, the main reason these oils cool parts more quickly is because they’re formulated to reduce the length of the vapor stage and extend the length of the boiling stage—the fastest of the three cooling stages.
Parts made of low-carbon steel and low-hardenability alloys quench better in fast oils.
Hot oils are kept at much higher temperatures and are used to ensure that a part’s core temperature and surface temperature do not vary too greatly during a quench. This controls distortion and reduces the risk of cracking.
The trade-off with hot oils is that while they offer more uniform cooling throughout a part’s cross-section, it takes longer to achieve. Highly hardenable alloys quench better in hot oils.
Quenching is about trade-offs, and while the chemistry behind quenching can be complicated, getting the process right ensures parts will perform as designed.
Metallurgists at Paulo understand the trade-offs that come with making quenching decisions and can help you find the best heat treating solutions for your parts. Find out more about the role quenching plays in the heat treatment process by downloading the guide below.