1.Significance of heat treatment:

There are two ways to improve the properties of steel-Adjusting the chemical composition of steel, adding alloy elements, developing new materials.

The steel is heat treated. After heat treatment, the processing performance and mechanical properties of raw materials or workpieces have been greatly improved.

2. Annealing process and Application

In mechanical manufacturing, annealing and normalizing are usually used as preparatory heat treatment. Give the blank good processing performance, and prepare the structure and performance for the follow-up process.

Annealing definition:Annealing is a heat treatment process in which the steel is heated to a certain temperature and then cooled in the furnace to obtain a structure close to the equilibrium state.

3.Normalizing process and its application

Normalizing:

The heating temperature is 30 ~ 50 ℃ above AC3 or ACCM, and air cooling after heat preservation. Because the cooling rate of normalizing is higher than that of annealing, the lamellar spacing of pearlite is smaller, so the strength and hardness of normalized pearlite are higher.

Application of normalizing:

① To improve the hardness of low carbon steel (low alloy steel) and machinability.

② The network carbide of hypereutectoid steel is eliminated.

③ Some medium carbon steel and medium carbon alloy steel parts with little stress and low performance requirements can be used as final heat treatment.

4. Quenching process and Application

quench:

The heat treatment process of heating steel to AC1 or AC3, holding for a certain time, and then cooling rapidly to obtain martensite structure is called quenching.

Quenching temperature and heating time:

The principle of obtaining fine and uniform austenite grains is to obtain fine martensite structure after cooling.

The quenching temperature of hypoeutectoid steel is usually 30 ~ 50 ℃ above AC3; The quenching temperature of hypereutectoid steel is usually 30 ~ 50 ℃ above AC1.

The heating time is related to the heating temperature, heating speed, composition, shape and size of parts, charging method, etc.

5. Tempering process and its application

Tempering:

The heat treatment process of heating the quenched steel to a certain temperature below AC1, holding for a certain time and cooling to room temperature is called tempering.

Tempering purpose:

(1) Eliminate or reduce stress to prevent deformation or cracking.

(2) Adjustment performance (hardness)

(3) Stable organization, stable shape and size, ensure accuracy.

6. Surface hardening

Surface hardening:

It is a heat treatment method that the surface of workpiece is quickly heated to the quenching temperature, and then rapidly cooled, so that only the surface layer can obtain the quenching structure, while the core part still maintains the structure before quenching.

Induction surface hardening:

Induction heating is based on the principle of electromagnetic induction.

High frequency 100-500khz 0.5-2.0mm

If 2.5-8khz 2-10mm

Power frequency 50 Hz 10-15 mm