Tempering is performed by elevating the steel to a set point below its lower critical temperature, typically following a hardening operation. Once this temperature is reached, it is held there for a specified amount of time. The exact temperature and time depend on several factors such as the type of steel and desired mechanical properties.
To get the steel to its critical temperature, some type of heating device must be used. Common devices include gas furnaces, electrical resistance furnaces, or induction furnaces. Often, this heating is done in a vacuum or with an inert gas to protect the steel from oxidation. Once the furnace achieves the desired temperature, a dwell time occurs. Following the dwell time, the furnace is shut off and the steel is allowed to cool at predetermined rate.
Tempering steel after a hardening process allows for a middle ground of hardness and strength. While the theory behind steel tempering is simple, the entire process is incredibly precise. The exact temperature requirements, cooling temperature, cooling method and speed all require extreme care to ensure the steel heat treatment process is carried out in a stable and even manner. This entire process of heating and rapidly cooling is also called quenching or oil quenching.
Quenched steel is incredibly hard, but lacks the ductility required for many applications. Tempering steel is the process of slowly, accurately reheating the quenched metal to achieve the desired balance of ductility and hardness. Any irregularities in the steel when incorporating this process results in warped, distorted and damaged materials. There are many excellent reasons to choose tempered steel over other alloys. Depending on your industry, you can enjoy various grades and thicknesses to meet your exact standards.
Additionally, some metals are annealed in order to increase electrical conductivity. The annealing process includes three distinct stages:. The first stage in the annealing process is recovery.
During recovery, metal is heated. The common heating device used is a furnace — gas, electric, and induction furnaces can be used to anneal metal.
The goal of the recovery stage is to heat the metal to a temperature where its internal structures relax. The second stage in the annealing process is recrystallization. During recrystallization, the metal is heated even further.
The sweet spot is to reach a temperature that's greater than the metals recrystallization temperature without surpassing its melting temperature. When material is held in this sweet spot, new grains can begin to develop within the metal's internal structure without the formation of preexisting stresses. Outside of manufacturing, heat treatment of steel is carried out in metal workshops for students.
When metal is tempered, it turns different colors based on the amount of heat to which it is exposed. Metal workers may be instructed to temper steel until it becomes a certain color. While steel used for axes is tempered until it becomes purple, the steel used for woodturning tools is tempered until it becomes brown, and steel used for lathe tools for brass is tempered until it becomes pale yellow.
Typically, the deeper the color, the higher the temperature at which it was tempered. Actively scan device characteristics for identification.
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