① Tempering
The tempering purpose of precision casting products includes the following aspects:
1. Reduce brittleness and eliminate or reduce internal stress. There is great internal stress and brittleness after quenching. If not tempered in time, the precision casting will often deform or even crack.
2. Obtain the mechanical properties required by the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the requirements of different properties of various workpieces, the hardness can be adjusted through appropriate tempering to reduce brittleness and obtain the required toughness and plasticity.
3. Stable workpiece size
4. For some alloy steels that are difficult to soften after annealing, high temperature tempering is often used after quenching (or normalizing), so as to properly aggregate carbides in the steel and reduce the hardness for machining.
② The production of normalized precision castings also needs normalizing
Normalizing is a heat treatment process in which the casting is heated to 30-50 ℃ above the critical temperature and cooled in still air after holding for an appropriate time, which is called normalizing. The main purpose of normalizing is to refine the structure, improve the properties of steel and obtain the structure close to equilibrium.
The main difference between normalizing and annealing process is that the cooling speed of normalizing is slightly faster, so the production cycle of normalizing heat treatment is short. Therefore, when annealing and normalizing can also meet the performance requirements of parts, normalizing shall be selected as far as possible. Most medium and low carbon steel billets are generally normalized heat treated. Generally, alloy billets are often annealed. If normalizing is used, the hardness after normalizing is high due to the fast cooling speed, which is not conducive to cutting.
