Quenching metal is one of the critical stages in the heat treatment of a metal part because it’s during that process that added hardness is locked in.
The concept is relatively simple: Heat a metal and then rapidly cool it to make it harder. But in terms of the chemistry involved, the process is complex and trade-offs abound as metallurgists must decide which quenching medium and method will achieve the specified qualities.
Severity of quenchants
The severity of a quench refers to how quickly heat can be drawn out of a part. Different quenching media have different degrees of severity.
Caustics are the most severe quenchants, followed by oils, then salts and, finally, gases.
The makeup of metal parts and the specified hardness to be achieved dictate which quenching medium is used. Generally, low-hardenability parts made from carbon steel and low-alloy steel require more severe quenches to achieve a specified hardness. High-alloy steels, which are much more hardenable, are best quenched in less severe media.
The most severe quenches are executed with water, brines and caustic sodas. While these quenchants can pull heat out of parts more quickly than other quenching media, faster isn’t always better.
Quenching in caustics dissipates heat so quickly that metal parts are at risk of cracking and warping due to the drastic variation in temperature between the part surface and its core. In addition, workers must take special precautions when using caustic materials because they’re harmful when inhaled or exposed to skin and eyes.
Quenching metal in oil is the most popular method because it is relatively severe but with a diminished risk of cracking and warping. In addition, a wide range of parts quench well in oils because the chemical makeup and temperature of a quenching oil can be adjusted to suit desired end results.
For example, if a metallurgist determines a part’s intended final properties require faster quenching to achieve, “fast” oils are used. These oils are formulated to extend the amount of time during which the highest rate of cooling takes place. Quenching in fast oils is best suited for low-carbon steels and low-alloy parts. As the method’s name indicates, these quenches do not take long.
Conversely, sometimes cooling needs to be slowed. Hot oils—which are kept at higher temperatures—cool metal surfaces, but not so quickly that a part’s core temperature and surface temperature differ too widely. High-alloy parts with intricate designs quench well in hot oils, as the method reduces the risk of warping and cracking associated with differences in surface and core temperatures. Quenching in hot oil is a slower process compared to quenching in fast oil.
Because oil is flammable, workers must know the flashpoint of the oil in use as well as the load weight and surface area of the products in the workload to avoid fires during quenching.
Quenching metal parts in molten salt (also called salt baths) comes with a further reduced risk of distortion or cracking of parts because they’re hotter than hot oils. This means cooling is more controlled and uniform compared to colder, faster and more severe quenches.
The hotter the quenchant, the less severe the quench. The less severe the quench, the lower the risk of distortion.
Different mixtures of salts have different melting points and working ranges, offering added versatility as a quenching option. Because salts are not flammable, they pose no risk of fire.
On the other end of the spectrum, some salt mixtures have high melting points and working ranges and can be used to heat parts.
Salt baths are a long-lasting heat treating and quenching solution as long as they’re properly maintained. This includes ensuring oxides are regularly removed from high-heat salts and sledging out high-heat salts that contaminate quench salts on salt-to-salt lines.
Quenching metal via gas in vacuum furnaces has become more popular for parts that require high hardness and specific finishes with significantly reduced risk of distortion.
In gas quenching, parts are sealed in a vacuum chamber before being blasted with gases. The rate of cooling of a part can be precisely controlled by adjusting the pressure and speed at which the gas is delivered. Additionally, due to the fact that gas quenches occur in vacuum chambers, parts emerge significantly cleaner compared to other quenching media.
Nitrogen is the most popular gas quenchant due to its relatively low atomic mass, wide availability and low cost. Helium and argon are also used in gas quenching. Specified finished qualities dictate which gas quenchants are to be used.
High-alloy tool steels and jet engine turbines are common examples of parts often quenched in gas.
Diligence in heat treating
Because quenching and the heat treatment process is so important to the production of strong, long-lasting metal parts, it pays to be diligent when choosing a heat treatment provider.
The heat treatment expertise and equipment available at Paulo ensure your parts meet specifications and remain strong and useful for longer. Let us know how we can help with your next heat treatment job, and view the guide below to learn more about the role of quenching in heat treating.