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Four heat treatment processes

1. Annealing

Heat the steel to a certain temperature and keep it at that temperature, then slowly cool it to room temperature

Annealing includes several types such as complete annealing, spheroidization annealing, and stress relief annealing.

a， Heating steel to a predetermined temperature, holding it for a period of time, and then slowly cooling it in the furnace is called complete annealing. The purpose is to reduce the hardness of the steel, eliminate uneven structure and internal stress in the steel

b， Heating the steel to 750 degrees Celsius, holding it for a period of time, slowly cooling it to 500 degrees Celsius, and finally cooling it in air is called spheroidization annealing. The purpose is to reduce the hardness of the steel and improve its cutting performance, mainly used for high carbon steel

c， Stress relief annealing, also known as low-temperature annealing, heats steel to 500-600 degrees Celsius, holds it for a period of time, slowly cools it in the furnace to below 300 degrees Celsius, and then cools it at room temperature. During the annealing process, the structure does not change, mainly eliminating the internal stress of the metal

2. Normalization

The heat treatment process of heating steel parts to 30-50 ℃ above the critical temperature, holding them for an appropriate period of time, and cooling them in still air is called normalizing.

The main purpose of normalizing is to refine the structure, improve the properties of steel, and obtain a structure close to equilibrium.

The main difference between normalizing and annealing processes is that the cooling rate of normalizing is slightly faster, so the production cycle of normalizing heat treatment is shorter. When annealing and normalizing can meet the performance requirements of the parts, it is recommended to use normalizing as much as possible.

3. Quenching

The heat treatment process of heating steel parts to a certain temperature above the critical point (quenching temperature of 45 steel is 840-860 ℃, quenching temperature of carbon tool steel is 760-780 ℃), holding for a certain period of time, and then cooling at an appropriate speed in water (oil) to obtain martensitic or bainitic structure is called quenching.

The main difference between quenching, annealing, and normalizing treatment in terms of process is the fast cooling rate, aimed at obtaining martensitic structure. Martensitic structure is an unbalanced structure obtained after quenching of steel, with high hardness but poor plasticity and toughness. The hardness of martensite increases with the increase of carbon content in steel.

4. Tempering

The heat treatment process of quenching steel, heating it to a temperature below the critical temperature, holding it for a certain period of time, and then cooling it to room temperature is called tempering.

Steel parts that have been quenched generally cannot be used directly and must be tempered before use. Due to the high hardness and brittleness of quenched steel, direct use often leads to brittle fracture. Tempering can eliminate or reduce internal stress, reduce brittleness, and improve toughness; On the other hand, the mechanical properties of quenched steel can be adjusted to achieve the service performance of the steel. According to the different tempering temperatures, tempering can be divided into three types: low-temperature tempering, medium temperature tempering, and high-temperature tempering.

Low temperature tempering at 150-250 degrees Celsius reduces internal stress and brittleness, while maintaining high hardness and wear resistance after quenching

B medium temperature tempering 350-500; Improve elasticity and strength

C high temperature tempering 500-650; Tempering of quenched steel parts above 500 ℃ is called high-temperature tempering. After high-temperature tempering, quenched steel parts have good comprehensive mechanical properties (both strength and hardness, as well as plasticity and toughness). So generally, medium carbon steel and medium carbon alloy steel are often treated with high-temperature tempering after quenching. Axis components are most commonly used.

Quenching and high-temperature tempering are called quenching and tempering treatment.

sketch:

Annealing is a heat treatment process in which a workpiece is slowly heated in a furnace to a certain temperature above the critical point, held for a period of time, and slowly cooled down with the furnace. (Furnace cooling)

Normalization is the process of removing the heated workpiece from the furnace and cooling it in air (air cooling)

Quenching is the process of heating a workpiece to the quenching temperature (30-50 degrees above the critical point), holding it for a period of time, and then cooling it by adding it to a quenching agent

Tempering is the process of heating and cooling a component at a lower temperature after quenching.