For centuries, blacksmiths and ironworkers have employed quenching to improve the performance of various weapons and tools. Although the metallurgical processes involved may not have been fully understood at the time, these workers would have quickly realized that the heating and subsequent cooling process affected the strength and brittleness of iron.
In modern times, the quenching of metals is used in the manufacture of various types of steel for a variety of uses from structural steel for the construction industry to stainless steel kitchen knives.
In this article we shall explore, in detail, the metallurgical processes involved in quenching, how quenching enhances a metal's properties and how the rate of quenching affects the hardness and brittleness of the final product.
What is Quenching and How Does it Work?
Quenching is the rapid cooling of a heated metal in a quenching medium such as water, oil or air in order to obtain desirable material properties. In metallurgy, quenching is one of the critical steps in the heat treatment of a metal and is typically used to harden the final steel product.
Metals (e.g., steel) are unique in that their molecular structure can be altered by heating and subsequent cooling of the specimen. By adjusting the rate and intensity of the heating and cooling process, the molecules can be transformed and shaped in ways that affect the strength, durability and ductility of the metal. (For more on these characteristics, see 6 Tests to Measure a Material's Strength.)
On the surface, the quenching process is relatively simple. The metal is heated to a very high temperature and then quickly cooled to make it harder. However, the chemical processes involved are complex, often requiring qualified metallurgists trained in determining which quenching medium and heating temperatures will achieve specific properties.
Metals, on a molecular level, have their atoms arranged in a crystalline lattice structure. They also possess a unique property called polymorphism, which means that metals can exist in different crystalline forms depending on the amount of heat and pressure applied to them. As the metal is heated above a critical temperature (815oC – 900oC for most metals), the metal takes on a different lattice structure called austenite. Austenite has a face-centered cubic (FCC) lattice structure that readily accepts and dissolves additional carbon atoms. Sudden cooling of the metal in its austenitic stage causes the dissolved carbon atoms to remain trapped in the lattice, putting the metal under a state of tensile stress. The metal, now extremely hard and brittle, exists in a form known as martensite.
Tempering the Metal after Quenching
As previously mentioned, quenched martensite is highly stressed, very hard and brittle. To improve its workability, some of the excess stress and hardness in the metal is relieved by heating the metal to a temperature below the critical point for a specified time and allowing it to cool in air. This process of reducing the hardness and stress by reheating and cooling the metal is known as tempering.
Quenching and tempering of metals are done using a variety of media, temperatures, exposure times, rates and methods to achieve different desired results. It is ultimately up to the metallurgist to decide which parameters are suitable when quenching to ensure that the final product meets its intended specifications. (Learn why this is important in the context of corrosion in the article Effect of Corrosion on a Material's Tensile Strength and Ductility.)
How the Quenching Rate Affects the Metal's Performance
Metals are manufactured to meet various specifications for use in different situations and applications. The strength (hardness) and the ductility (brittleness) of the metal can be determined by the rate of quenching dictated by the use of different quenching mediums. Low-hardenability components made from carbon steel and low-alloy steel generally require more severe quenching to achieve the desired level of hardness. On the other hand, high-alloy steels that possess higher hardenability are quenched in less severe media. Some of the quenching media commonly used are:
- Caustics - This group of quenchants include water, brines and caustic sodas. They have the ability to remove heat from metal components the fastest, therefore providing the most severe quenches. Though this may often yield the most efficient results where maximum hardness is required, the drastic change in temperature can result in brittleness, cracking and warping.
- Oils - Where hardness can be sacrificed to increase ductility and workability, oils are often used as the quenching medium. These oils oxidize and form a sludge when quenching, thereby lowering the quenching rate and overall efficiency of the process. Oil has become one of the most popular quenching mediums since it can be classified as relatively severe but minimizes the possibility of cracking and warping. The chemical composition of the oil can be custom formulated to adjust the cooling rate to suit the desired end result.
- Molten salt – Quenching in molten salt further reduces the risk of cracking and distortion since the molten salt provides a slower cooling rate than heated oils and caustic solutions. Similar to oils, different compositions of this quenchant can be formulated to adjust the rate of cooling, therefore adding versatility and control to the process.
- Gas – Using gas as a quenching medium is suitable for applications where hardness must be achieved with a minimum risk of cracking and distortion. During this method, the metallic components are placed in a sealed vacuum and blasted with high-velocity gases. Operators can adjust the cooling rate of the quenching process by adjusting the speed and pressure of the gases in the chamber. Nitrogen, helium and argon are some of the common gases used as quenching media. High-alloy tool steel and jet engine turbines are examples of components typically quenched with gas.
The quenching of metals, though a relatively simple concept on the surface, is an exact science that allows metallurgists to improve a metal's performance while still being able to custom tailor its properties for use in specific applications. The heating temperatures, cooling rates, quenching media and tempering methods are all factors that determine the final metal properties and should be specified by qualified professionals to ensure maximum efficiency and quality of the end product.