Quenching and Tempering of Steel

Quenching and tempering are two heat treatments that help balance hardness and toughness in engineering steel. When combined they provide a more versatile product that’s suitable for a range of applications.

To harden the steel it is heated to a low red or just above the point at which it becomes non-magnetic and then rapidly cooled in water, oil or air. This makes the steel very hard but also brittle.

Quenching

As a metal is heated, its atoms move around quickly, creating microscopic cracks. To prevent this, the metal is immersed in a medium that slows the atoms’ movement. This is called quenching. Many different mediums are used for different reasons, such as oil, water, brine and air convection. Each has its own advantages and tradeoffs.

The most common process is to heat the steel to its austenite temperature (also known as martensite) and then rapidly cool it. This creates a hard microstructure called martensite, which makes the steel very strong and tough. However, this microstructure is very brittle, so the steel is tempered to reduce the hardness and improve its ductility.

For example, a steel cutting tool is tempered to make it blue, which leaves the steel tough and strong but still soft enough to keep its cutting edge. Spring steel is also tempered to this colour because it is hard and durable but not so hard that it is difficult to bend.

Tempering is the process of slowly and accurately reheating the quenched steel to achieve the desired balance between hardness and ductility. This is why it takes a lot of skill and experience to do well. Without tempering, your tools and industrial parts won’t have the strength and durability you need. Without ductility, they’ll be too brittle to perform their job or break under extreme stress.

Tempering

Quenching involves rapidly cooling the metal, locking in atoms and creating an extremely hard but brittle microstructure. This is typically done by plunging the steel into a liquid, often water, but sometimes oil or brine. It’s important to use the right mixture of heat, temperature, soak time, quench medium, and cooling rate to achieve the desired effect. Different steels, component cross-sections, and applications require varying recipes.

After being quenched, the steel is reheated for a set period of time at a temperature below its critical point. Depending on the duration of the tempering process, it will transform into ferrite and unstable carbides or into various stages of tempered martensite. The microstructure can appear acicular (needle-like) or lenticular, and it has a lower impact strength than austenite but higher toughness.

Tempering is a vital step in making strong, durable materials and components. It reduces the hardness of martensite, increasing ductility and toughness. The result is a material that can withstand high stresses, strains and impacts. It’s used in a wide range of applications, including military hardware, quenching and Hardened & Tempered Steel Strip Supplier tempering of steel cranes, machinery, mining equipment and earthmoving tools. Contact a Clifton specialist for more information about how quenching and tempering can help with your application.

Annealing

Tempering reduces the extreme hardness of a metal to boost its toughness and ductility, making it much safer to work with. This is a common practice in the manufacture of steels and alloys to produce more durable, robust products that can endure harsh environments and heavy use.

This process involves heating the material to a temperature below its critical point but well above its melting point. It is then held at this tempering temperature for a period of time, known as the dwell time. The exact amount of time will depend on the materials composition and desired properties. During this time, the martensite crystals in the material reform into smaller and more stable grains, reducing internal stresses within the material. It is also during this time that carbon atoms in the metal will diffuse, turning them into more ductile phases like ferrite and cementite.

The process also softens the metal, allowing it to be shaped without cracking or breaking under mechanical stress. This allows for the manufacture of products that are strong enough for their intended uses, such as automobile drive trains that will be subject to massive amounts of pressure and torque over their lifetimes.

The tempering process will also relieve internal stresses in the material that may have been caused during quenching. These stresses can cause brittle failure in the final product, so tempering will help to avoid these problems by creating a softer, more ductile metal with a high level of strength.

Strength

In metallurgy, strength is the resistance of the material to permanent deformation or tearing. The term is often used to refer to tensile strength (resistance to bending and shear) and compressive strength (resistance to axial compression).

Quenching creates a very hard microstructure of martensite which makes the steel quite strong but also brittle. Tempering reduces the hardness by transforming the martensite into more ductile structures, which also improves toughness.

To temper, the steel is heated to a temperature below the critical point for a duration that depends on the composition of the steel and the desired mechanical properties. It is then rapidly cooled. The cooling rate is important because the material can develop temper embrittlement if it is cooled too quickly, which decreases the ductility and toughness of the temper.

For example, take a metal rod and heat it with a torch until it is deep orange in color. Then, immediately dip it in a bucket of water. The rapid quench decreases the atoms’ motion, allowing them to “relax” and move into a more stable position. This is the same principle of tempering. It also helps to avoid the stress-raising effects of quenching which can cause components to be difficult to straighten later on. This is especially a concern for large parts where the components spend more time at high temperatures.