Quenching is one of the most popular heat treatment methods for changing the properties of steel. When a toolmaker desires to increase the hardness, toughness, or strength of the steel due to the finished product’s environment, quenching is essential to perform.
What is quenching? Well, a quick answer would be that steel is heated to its recommended temperature, which changes depending on the steel grade and type. Then, the material is submerged to cool in its quenchant; the most popular tend to be oil or water to achieve its optimal hardened structure.
Other methods such as tempering or stress relieving can help improve final hardness, but this article focuses on quenching.
Keep reading for a more detailed breakdown of quenching steel, including the process and the quenchants often used.
Why quench steel?
Quenching tool steel brings with it benefits that are needed for specific industries and environments. For example, when steel is used in an environment that may require extreme physical demand, such as demolition and recycling industries, maximising the ability of the steel to perform by increasing its toughness and making it more durable is essential.
Benefits of quenching steel include making the material tougher, less brittle and increasing overall strength.
The quenching process will differ depending on the final product and the desired hardness you wish to achieve. Quenching is one process when heat treating the material, and a basic overview of heat treatments would include pre-heating, heating, quenching and tempering.
First, pre-heating your steel is essential to avoid any possible damage to the material. Heating at a faster rate or heating to a higher temperature than recommended can cause distortion and cracks to the product. Therefore, you must read the technical sheet provided by your steel provider to understand what is required.
Next, heating the material to its complete temperature and holding it for a while allows the atomic structure to change and draw in more carbon.
Quenching can now be performed to bring the steel product back to room temperature. Start by placing the steel into a medium that is recommended for that grade of material. The process of quenching causes alterations to the structure through a function defined as diffusion. The quenching process is achieved in different mediums, such as oil, air, water and brine. Each will have a different desired hardness and toughness.
Once quenching has been performed, and the material has reached its hardened state, it is advised to proceed with tempering to remove excess hardness. The steel in its current condition will be brittle and tempering will help to remove any excess heat. Tempering will heat the steel back up to below its critical point for a recommended period. Finally, allowing the material to cool in the air will increase the toughness and stability of the material by decreasing the hardness.
Types of quenching
Although quenching can be performed with a range of different mediums, there are recommended methods to use depending on the grade of steel you’re quenching. For example, the O steel grade family, such as O1 tool steel, requires steel to be hardened by oil, whereas A series, such as A2 tool steel, needs hardening to be performed by air.
Air hardening steel is a cost-effective method of cooling. Heating steel and allowing cooling to occur naturally in the air is enough to be defined as air quenched. Cooling using compressed air is another option when using air as a medium; utilising this method allows more control of the air temperature and cooling speed and thus, the achieved hardness.
Oil hardening steel is renowned as a safe method to avoid cracking and distortion in steel. Cooling using oil offers a faster rate than natural and compressed air. Furthermore, quenching steel with oil allows for precise control of the additives absorbed by the steel. Oil carries health and safety risks and can be more costly if industrial standard oil is used.
Water hardening steel works by submerging the material in a tank of water. Using water will result in a rapid quenching method compared to other mediums since as soon as the steel comes into contact with water, it will remove much of the heat from the steel.
Provided proper agitation is done, the toolmaker can minimise the risks often associated with water hardening steel. However, other risks such as distortion and cracking can occur due to the rapid quenching process of using water.
Mixing the materials salt and water will produce a product called brine. The ratio usually contains 10% salt in the water solution. Brine is a less common quenchant than oil, water, and air but has a faster cooling speed than plain water. Be aware, brine is corrosive and must be removed from the steel by rinsing once complete.
Quenching is a process that is extremely important for allowing the steel to be able to perform and withstand some of the harsh conditions buyers will use them. Increasing hardness changes the microstructures of the steel, increasing strength and hardness but does come with some negative features such as the risk of cracking and distortion. Therefore, after quenching has been performed, tempering and annealing may be required.