On 2018-08-08 14:48:54
heat treatment types
Metal heat treatment process can be generally divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment.
There are four basic heat treatment processes: annealing, normalizing, quenching and tempering.
1. Annealing is to heat the workpiece to a proper temperature, adopt different holding time according to the material and workpiece size, and then cool it slowly, so as to make the internal structure of the metal reach or close to the balance state, and obtain good process performance and use performance (reduce the hardness of the steel parts, facilitate cutting; eliminate residual stress, stabilize the size of the steel parts and prevent deformation and cracking; refine Grain, improve the structure, improve the mechanical properties of the steel), or prepare for further quenching. He includes complete annealing, isothermal annealing, diffusion annealing, spheroidizing annealing, stress relief annealing and recrystallization annealing.
2. The purpose of normalizing is the same as annealing, except that the temperature is higher than annealing, and it is cooled in air, the cooling speed is faster, and the microstructure obtained is finer. It is often used to improve the cutting performance of materials, to pre heat treat important parts, reduce quenching deformation and cracking of workpieces, and improve quenching quality. Sometimes it is used for the final heat treatment of parts with low requirements.
3. Quenching refers to the rapid cooling of workpiece in water, oil or other inorganic salt, organic water solution and other quenching media after heating and heat preservation. After quenching, the steel becomes hard but brittle at the same time. The purpose of quenching is to obtain m-structure, which is the most important heat treatment method for strengthening steel.
The main process parameters of quenching are quenching temperature, heating time, quenching cooling medium and cooling speed:
A. selection of quenching and heating temperature: prevent oxidation decarburization, workpiece deformation and cracking, reduce the brittleness of steel, and improve the hardness and wear resistance of steel.
The quenching temperature is related to the chemical composition of steel
Hypoeutectoid steel t = AC3 + (30-70) ° C
Eutectoid steel, hypereutectoid steel t = AC1 + (30-70) ° C
B. selection of heating time: it is related to many factors such as steel composition, original structure, workpiece shape and size, heating medium, charging method, furnace temperature, etc
T = a * D (t heating time; a heating coefficient; d effective thickness of workpiece)