The Heat Treatment Process of Quenching and Tempering of Steel
The heat treatment process of quenching and tempering balances the strength, toughness and ductility of steel. This gives the material the characteristics needed for many different applications.
Quenching involves submerging hot metal in a cooling medium, such as water, oil or brine. The choice of cooling media will depend on the results desired.
Austentizing
When you heat steel, the material will begin to change shape. This is called austentizing. The process of heating and cooling to alter the structure of the metal is referred to as tempering, and it can be used to make your steel tougher while increasing its strength. It can be used to produce components like file blades that need to be extremely hard and wear-resistant.
The first step in the austentizing process is to heat your steel above its transformation temperature. This is typically around 1350 °F for steel forging blanks. Once the steel has reached its transformation temperature, it is allowed to cool for a short period of time.
A slow cooling rate is required to prevent the formation of cracks in thick sections of the heat treated part. A fast cooling rate can cause a thermal gradient that causes the outer layers of the material to shrink at a much faster rate than the inner layer, leading to a strain that generates cracks.
If the material is cooled slowly, it will return to its austenite microstructure and cannot be transformed into martensite. It is also difficult to plastically deform pure martensite because it has no slip planes. In addition, it has a very high brittleness.
Quenching
Quenching hardens steel to a high degree, but it can also make the material too brittle for quenching and tempering of steel most applications. To avoid this, the steel is often tempered to reduce some of that brittleness. This process involves heating the steel for a set amount of time at a temperature between 400deg F and 1,105deg F, depending on the application.
When this is done, the steel will be a little less hard than it was before quenching, but it’s still much harder than the raw material used to be. This makes it more durable and tough, which is especially important for parts that will see a lot of wear.
The cooling rate during this step is very important to prevent cracking or warping in the metal. The exact cooling rate depends on a variety of factors, including the quenching medium and the size and shape of the metal. Proper agitation of the quench media is also crucial to expedite the cooling process and ensure consistent material properties.
To quench steel, you’ll need a heat-safe container with enough room to submerge the metal. It’s best to use vegetable oil since it has a lower boiling point than water and will take longer to cool, reducing the chance of cracking. Be sure to wear thick gloves and a face mask during this process to avoid getting the oil or water in your eyes. You’ll also need a fireproof regulator block to keep the blade at the correct depth in the quench oil, as well as tongs or vise-grip pliers to hold it safely.
Tempering
Tempering allows the brittleness of hardened steel to be reduced, allowing it to bend before fracture and reducing stress cracking. It can also improve fatigue performance, a benefit in applications such as shafts and axles which experience repeated load cycles. It can also help reduce distortion or warping.
The tempering process involves reheating the metal to a lower temperature range and then quenching it again, usually in water or oil. This process helps to soften and re-stabilise the microstructure of the steel by changing the structure of the martensite, leaving behind soft ferrite nuclei. This process can be performed on a wide variety of steel grades and alloys, with the exact heating times, temperatures and quenching mediums depending on the required use and properties of the finished product.
In general, a high proportion of retained austenite combined with a tempered martensite results in a tougher product that has good ductility and resistance to fracturing. It also has the advantage of requiring a lower cooling rate than quenching, and it can be used for larger sections of the material.
To perform tempering, you will need a heat source, a bucket with water, and tongs or vise-grip pliers for holding the rod safely. Start by heating the steel to its hardening temperature, and then submerging it in the quenching medium. A water-based quenching Tinplate steel coils Manufacturer medium is best, but if you don’t have access to one, you can substitute vegetable oil or another heat-safe liquid. The time the rod spends in the quenching medium depends on the size of the steel and its carbon content.
Finishing
Depending on the final application, some metal products or alloys require tempering to decrease hardness, which is the opposite of what quenching does. Tempering involves heating the metal to a specific duration and temperature for it to achieve the desired properties. For example, hard tools are tempered at low temperatures so that they can maintain an increased level of hardness, while springs and other flexible mechanical parts are tempered at higher temperatures for them to obtain a decrease in hardness.
Following the austentizing process, the steel is then quenched by plunging it into a cooling bath. The temperature of the quenchant will vary, with oil being most common as it offers a wide range of potential temperatures. Other quenchants include brine, water, mineral oils and even inert gases like nitrogen or helium.
The goal of quenching is to rapidly cool the metal, locking the atoms into a highly stable state that is much harder than normal austenite. This also helps to reduce the risk of cracking in the steel, which can occur if the material is allowed to re-heat at high loads.
The process of quenching can be affected by the composition of the steel, which is why different alloying elements are used. These act to hinder carbon diffusion during lattice transformation, allowing the steel to harden more uniformly across its cross-section.
