What is tempering? Function and purpose
Tempering after quenching metal timber or parts of heating to a certain temperature, heat preservation, after a certain time in a certain way the cooling of the heat treatment process and tempering is followed by an operation, after quenching are also usually workpiece heat treatment at the end of the procedure, and thus have a combination of quenching and tempering process known as the final processing. The main purposes of quenching and tempering are:
1) reduce internal stress and brittleness. There is a great deal of stress and brittleness in quenching parts. If not tempered in time, deformation or even cracking will occur.
2) adjust the mechanical properties of the workpiece. After quenching, the workpiece has high hardness and brittleness. In order to meet the different performance requirements of various workpieces, it can be adjusted through tempering, such as hardness, strength, plasticity and toughness.
3) stabilize the workpiece size. The metallographic structure can be stabilized by tempering to ensure no deformation in the future.
4) improve the cutting performance of some alloy steels.
The role of tempering is:
To improve the stability of the organization, the workpiece will not change in the process of using, so that the geometry size and performance of the workpiece remain stable.
To eliminate internal stress in order to improve workpiece performance and stabilize workpiece geometry.
Adjust the mechanical properties of the steel to meet the requirements.
Tempering has these effects because, as temperatures rise, atoms of iron, carbon and other alloying elements in iron and steel can spread more quickly, rearranging and combining atoms, and gradually transforming unstable and unbalanced tissues into stable and balanced ones. The elimination of internal stress is also related to the reduction of metal strength when temperature increases. When steel is tempered, hardness and strength decrease and plasticity increase. The higher the tempering temperature, the greater the variation of these mechanical properties. Some alloy steels with higher content of alloying elements will release some fine metal compounds during tempering in a certain temperature range, which increases the strength and hardness. This phenomenon is called secondary hardening.
Tempering requirements: workpieces with different purposes should be tempered at different temperatures to meet the requirements in use.
(1) cutting tools, bearing, carburizing and quenching parts, surface quenching parts often under 250 ℃ low temperature tempering. After low temperature tempering, the hardness changes little, the internal stress decreases and the toughness increases slightly.
(2) spring in 350 ~ 500 ℃ under the tempering temperature, high elasticity and the necessary toughness can be obtained.
(3) in the carbon structural steel parts are usually made in 500 ~ 600 ℃ high temperature tempering, in order to obtain appropriate with good strength and toughness.
Steel at 300 ℃ tempering, often make its brittleness increases, this phenomenon is called the first kind of temper brittleness. Generally do not temper at this temperature range. Some medium carbon alloy structural steels tend to be brittle if they are tempered slowly to room temperature. This phenomenon is called tempering brittleness of the second kind. Adding molybdenum to steel or cooling in oil or water during tempering prevents the brittleness of the second type of temper. This brittleness can be eliminated by reheating the second type of temper brittle steel to its original tempering temperature.
In production, the performance of the workpiece is often based on the requirements. According to different heating temperatures, tempering is divided into low temperature tempering, medium temperature tempering and high temperature tempering. The combined heat treatment process of quenching and subsequent high temperature tempering is called tempering, which means that it has both high strength and good ductility.
1, low temperature tempering: 150-150 ℃, M back, reduce internal stress and brittleness, improve plastic toughness, higher hardness and wear resistance. Used to make measuring tools, tools and rolling bearings.
2, medium temperature tempering: 350-500 ℃, T back, has the high elasticity, has certain plasticity and hardness. Used for making spring and forging die etc.
3, high temperature tempering: 500-650 ℃, S back, has the good comprehensive mechanical properties. Used for making gear, crankshaft, etc.