Quenching and Tempering of Steel
When steel is heated to above its transition temperature and rapidly cooled, it becomes very hard but also brittle. This process is known as quenching.
Quenching can be done in water, oil or air. The best choice depends on the specific steel and the application.
Various quenching media cause different cooling rates. Specialised oils provide the fastest cooling and are generally preferred for industrial applications.
Hardening
Unlike the annealing process which focuses on delivering production-orientated properties, hardening aims to achieve high strength and toughness. The key to achieving these properties is through a controlled change in the crystal structure of the metal.
To do this, the steel must be heated to a temperature well above its critical point and then quickly cooled. This rapid cooling is known as quenching. When the steel is quenched, the carbon atoms in the crystal lattice are trapped and form a body centered tetragonal martensite microstructure, which makes the steel very hard but also extremely brittle.
A simple example of this is taking a standard file, which is typically made from a low carbon steel and heats it to a cherry red colour, then submerges it in water. The rapid change in temperature causes the steel to harden but also makes it very brittle.
The other way to achieve the same outcome is by a process called carburizing. This involves applying a source of carbon to the mild steel (typically via oxy acetylene), then heating the material at a much higher temperature, then quenching it. This produces a hard surface layer called case, while the low carbon core remains soft.
The simplest and most common method is to heat the steel to the red hot stage, then plunge it into liquid such as fuel oil or salt water (water may be used but this will require fire precautions). The quick change in temperature causes the steel to harden.
Quenching
After the steel has been hardened it quenching and tempering of steel must be tempered in order to reduce the hardness and make it more ductile. This process is done by heating the metal to a lower temperature than the quenching temperature, and then cooling it rapidly.
The heat treatment process is a complex one that requires careful attention to ensure the right results are achieved. The exact temperature, quenching method and cooling rate will vary depending on the type of alloy and the desired hardness. Using the wrong temperature can weaken the metal, or cause it to become too brittle for the intended use.
Quenching is a fast cooling process that can occur in various mediums, including water, oil and mineral baths. In vacuum furnace applications, gases such as hydrogen and nitrogen are used for quenching. This allows the cooling to take place at a controlled atmospheric pressure, improving temperature uniformity and eliminating distortion and residue formation.
When the metal is being cooled, the cooling rate must be fast enough to transform austenite into martensite and prevent it from reverting back to a less-hard phase. This is why different quenching media are used, depending on the desired hardness. For example, water is efficient for low carbon steels, while whale, cotton seed and mineral oils are preferred for higher-carbon steels to minimize the risk of distortion.
Tempering
Tempering allows metals to reach a perfect middle ground between strength and hardness. When metal is too hard, it becomes brittle and breaks instead of bending when subjected to stress. Tempering helps to reduce the brittleness, while also improving tensile strength, ductility, impact strength and toughness.
To temper, the metal is heated to a lower temperature than the quenching temperature, then quickly cooled again (called tempering). This drastically decreases the atoms’ movement, making them much more likely to bend and deform rather than break. The cooling process can be accomplished using a number of mediums, such as oil, air and water. For the best results, a special quenching fluid designed for the specific grade of steel is recommended.
The type of tempering process used Tinplate steel coils Manufacturer depends on the specific alloy, steel section size and required mechanical properties. Some processes such as austempering and normalizing create pure bainite, a transitional microstructure between pearlite and martensite. Other methods, such as quenching in a bath of molten salts or in vacuum, are used to achieve different levels of hardness.
Whether used to make a safety pin or to build an 80,000-seat stadium, the heat treatment of steel and other metal alloys is crucial for functional, long-lasting products. The correct combination of heating, cooling and quenching techniques changes the properties of the metal, just as different cooking methods change the flavor of foods.
Applications
Various iron-based alloys can be strengthened with quenching and tempering to achieve a desired balance of hardness and strength for specific applications. Quenching is a highly precise process that involves heating an alloy to its austenitizing temperature before rapidly cooling it. Cooling can be achieved using water, mineral oil and even inert gases such as nitrogen. All of these steps must be carefully controlled to prevent distortions in the resulting microstructure, known as martensite.
The sudden change in temperature of the material when it is quenched makes it extremely hard, but also very brittle as carbon atoms are not given sufficient time to diffuse into a new microstructure. This is called supersaturated martensite and can be a detrimental effect for some applications.
Tempering reduces the brittleness of the material by slowing down the formation of martensite and promoting the growth of less brittle phases such as ferrite and bainite. The tempering process can also be used to remove residual stress from the material.
Tempering is an effective method of improving a material’s resistance to deformation by increasing its toughness, particularly in applications such as gears and drills where parts are constantly rubbing together. It can also be used to improve the ability of tools to develop a sharp cutting edge by complementing a high degree of hardness with some ductility.
