What is tempering process?
What is tempering process?
A metal heat treatment method that reheats the quenched workpiece to an appropriate temperature below the critical temperature, keeps it warm for a period of time, and then cools it down in a certain way. Taking Carbon steel as an example, the appropriate temperature refers to a temperature that is appropriately lower than Ac1 (eutectoid line). The cooling method can be air cooling or cooling in water, oil and other media. Tempering is a subsequent process of quenching. Quenched workpieces generally need to be tempered. The main purpose of tempering is to: reduce brittleness, eliminate internal stress, reduce deformation and cracking tendency of workpieces; adjust hardness to improve plasticity and toughness to obtain the required mechanical properties of workpieces; stabilize workpiece size; improve the stability of the internal organization of the workpiece to achieve the transformation of unstable unbalanced organization into stable balanced organization.
There are many types of tempering, which can be divided into low-temperature tempering (150-250℃), medium-temperature tempering (350-450℃) and high-temperature tempering (500-680℃) according to the temperature. The tempering temperature is usually determined according to the requirements for the mechanical properties of the workpiece. In production practice, the determination of hardness is relatively simple, and the hardness value is often used as the main reference indicator for selecting tempering temperature and process. Low temperature tempering is mainly used in various tools, molds, rolling bearings, and carburized or surface quenched workpieces to reduce internal stress and brittleness and maintain high hardness and wear resistance after quenching. Medium temperature tempering is mainly used in various springs, forging dies, impact tools and other workpieces that require high strength to obtain suitable toughness, elasticity and good yield strength. High temperature tempering is widely used in various important structural parts, connecting rods, bolts, gears and shafts, etc. It can be used as both final heat treatment and preliminary heat treatment (to provide organizational preparation for the next process). Quenching plus high temperature tempering is usually called “tempering” treatment. Although some people are currently trying to use deformation heat treatment to replace tempering treatment, tempering is still widely used in the machinery manufacturing industry.
In addition to the conventional method of heating and tempering in a furnace, some other tempering methods have been developed in production practice according to different needs; such as self-tempering and induction heating tempering. (1) Self-tempering. When the workpiece is quenched, the cooling is not allowed to proceed to the end, but the cooling is interrupted in advance. The residual heat of the core is used to heat the external hardened layer, and tempering is performed when the temperature reaches a certain tempering temperature to maintain the tempering effect of the hardened layer. It has been successfully applied to some tools, crankshafts and other workpieces. (2) Induction heating tempering. The use of industrial frequency induction heating tempering can greatly shorten the process time, easily realize automation, and improve production efficiency. For example, some simple-shaped bearing rings and shafts can adopt this type of tempering. (3) Other tempering methods. According to different heating methods, there are also flame heating tempering, electric heating tempering, etc. The aging treatment effects of these methods are similar to tempering, but they take a long time. Low-temperature aging treatment is actually to supplement the tempering process.