Quenching Explained

What is Quenching?

If you’ve watched medieval-themed movies and shows, this is a familiar scene: a blacksmith hammering away at an anvil, before plunging a sword into water amid huge clouds of steam. What you may not realize is that you’ve just witnessed quenching in action. While water might be the favored choice for its dramatic effects onscreen, professionals in metal working use a variety of methods ranging from air to oil to brine. But whatever the medium used, the basic concept is the same: forcing the rapid cooling of heat-treated metal.

Purpose of Quenching
During the heat treatment process, the metal is heated to a point above its recrystallization temperature but just below melting. It’s then left for a set period of time to allow the heat to completely permeate the metal. Once removed, it’s considered to be annealed metal if left to cool naturally at room temperature. However, this method can result in a drastically altered metal microstructure, which may not be the desired effect. With quenching, the rapid cooling effectively locks in the microstructure existing in the heated metal. What this means is that a harder, stronger metal is produced once it cools.

Quenching Media
The substance used in quenching is referred to as the medium, and the medium determines the quench’s severity. The more quickly you are able to draw heat out of the metal, the more severe the quench. However, faster isn’t always ¬-better. The rolling clouds of steam produced by plunging that red-hot sword into water? In reality, while water is a very effective medium for hardening metal, it can also be a risky one. If the metal is too hot and the water too cool, you may be left with cracked or distorted pieces.
The chemical composition of the metal, as well as the desired degree of hardness, also factors into the choice of quenching medium. Carbon steel and low-carbon steel alloys generally require a more severe quench in order to produce the right degree of hardness. In contrast, high-carbon steel alloys are already a harder type of metal. So lower-impact, less severe quenching media can be used to produce the hardened steel.

Here are the most commonly used types of quenching media, ranked from most to least severe:
Caustics: This involves some combination of water, salt, and caustic sodas to create a brine. However, heat dissipates so quickly in water that it can create an overload of tension between the metal’s surface and its core. When this occurs, the metal part may be left with damage such as warping. So special care is taken to properly balance the temperatures, while workers protect their eyes and skin against any exposure to the caustic solution.

Oil: The most popular medium of choice is oil, since it combines a relatively severe quenching with a lower risk of warps and cracks. It also provides more flexibility during the process, depending on the composition and temperature of the oil. If a higher degree of quenching severity is required, a “fast” oil is used – one formulated to cool the metal very quickly. Low-carbon steels do particularly well in fast oils, because it provides the rapid cooling needed to give the metal more hardness. Meanwhile, high-carbon steels and more intricately-designed metal parts benefit from a less drastic approach. An immersion in heated oil will cool the metal, but at a more controlled and gradual pace.
However, oil brings its own risks during the quenching process. Unlike water and other caustic solutions, oil is flammable. Workers must pay attention to each oil’s flashpoint to avoid any risk of fire during the process.

Salts: Also known as salt baths, this method provides a less severe quenching because the molten salt is hotter than heated oils. This level of heat allows for a slower and more uniform cooling, producing metal with fewer distortions and other defects. So when quenching precision-machined parts and tools, salt baths might be your best choice. The salt baths can also be a long-lasting and cost effective method, so long as they’re properly maintained. The removal of any remaining oxides or other contaminants from the salt bath after quenching will allow for its extended use.

Air/Gas: Air is another popular option for quenching, in large part due to its affordability. If the material is left to cool on its own down to room temperature, it is considered air-quenched (although this is generally done for annealing). For quenching purposes, forced air is used to speed the rate of cooling. However, while this is more efficient from still air, it does require an extended period of cooling – and more time ups the risk of allowing the metal’s microstructure to change. A quicker quenching can be done with gases such as nitrogen, although the trade-off is the increased cost in comparison to air.