12 Heat Treatment Process

01 Annealing

Operation method: After heating the steel to Ac3+30~50 degrees or Ac1+30~50 degrees or below Ac1 (relevant materials can be consulted), it is usually cooled slowly with the furnace temperature.

Intention:

Reduce hardness, improve plasticity, improve cutting and pressure processing performance; refine grains, improve mechanical performance, and prepare for the next process; eliminate internal stress caused by cold and hot processing.

Application key:

(1) Suitable for alloy structural steel, carbon structural steel, alloy structural steel, high-speed steel forgings, welding parts, and raw materials with unqualified supply conditions;

(2) Usually annealed in the rough condition

02 Normalizing

Operation method:

heat the steel to 30~50 degrees above Ac3 or Accm, and cool it at a cooling rate slightly greater than that of annealing after heat preservation.

Intention:

Reduce hardness, improve plasticity, improve cutting and pressure processing performance; refine grains, improve mechanical performance, and prepare for the next process; eliminate internal stress caused by cold and hot processing.​​

Application key:

normalizing is usually used as a pre-heat treatment process for forgings, welded parts, and carburized parts. For low-carbon and medium-carbon carbon structural steel and low-alloy steel parts with low functional requirements, it can also be used as final heat treatment. For common medium and high alloy steels, air cooling can result in complete or partial quenching, so it cannot be used as a final heat treatment process.

03 Quenching

Operation method:

heat the steel to above the phase transition temperature Ac3 or Ac1, keep it for a period of time, and then rapidly cool it in water, nitrate, oil, or air.

Purpose:

Quenching is usually to obtain a high-hardness martensite structure, and sometimes when quenching some high-alloy steels (such as stainless steel and wear-resistant steel), it is to obtain a single and uniform austenite structure to improve wear resistance. and corrosion resistance.

Application key:

(1) Usually used for carbon steel and alloy steel with a carbon content greater than 0.3%;

(2) Quenching can give full play to the strength and wear resistance potential of steel, but at the same time it will form large internal stress and reduce the plasticity and impact toughness of the steel, so tempering is required to obtain better comprehensive mechanical properties.

04 Tempering

Operation method: heat the quenched steel parts to a certain temperature below Ac1 from the beginning, and then cool them in air or oil, hot water, and water after heat preservation.

Intention:

Reduce or eliminate the internal stress after quenching, reduce the deformation and cracking of the workpiece; adjust the hardness, improve the plasticity and resistance, and obtain the mechanical properties required for the operation; stabilize the workpiece size. The key to use: To maintain the high hardness and wear resistance of the steel after quenching, use low-temperature tempering; To improve the elasticity and yield strength of the steel under the condition of maintaining a certain toughness, use medium temperature tempering; To maintain high impact toughness and high-temperature tempering is used when plasticity is dominant and there is sufficient strength; generally, steel should be tempered at 230~280 degrees and stainless steel at 400~450 degrees, because a temper brittleness will occur at this time.​​

05 Tempering

Operation method:

High-temperature tempering after quenching is called quenching and tempering. The steel is heated to a temperature 10~20 degrees higher than that of quenching, quenched after heat preservation, and then tempered at a temperature of 400~720 degrees.

Intention:

Improve the cutting performance, improve the surface finish; Reduce the deformation and cracking during quenching; Achieve excellent comprehensive mechanical performance.

Application key:

(1) Suitable for alloy structural steel, alloy structural steel, and high-speed steel with high hardenability;

(2) It can not only be used as the final heat treatment of various important parts but also can be used as the pre-heat treatment of some tight parts, such as lead screws, to reduce deformation.

Heat treatment process

06 Aging

Operation method:

heat the steel to 80~200 degrees, keep it warm for 5~20 hours or longer, and then take it out of the furnace and cool it in the air.

Purpose:

To stabilize the structure of steel parts after quenching, reduce the deformation of storage or use time; reduce the internal stress after quenching and grinding, and stabilize the shape and size.

Application key:

(1) It is suitable for all kinds of steel after quenching;

(2) It is often used for tight workpieces that require no change in shape, such as tight screws, measuring tools, bed chassis, etc.

07 Cold treatment

Operation method:

Cool the quenched steel parts in a low-temperature medium (such as dry ice, or liquid nitrogen) to -60 to -80 degrees or lower, and then take out the temperature to room temperature.

Intention:

All or most of the remaining austenite in the quenched steel is converted into martensite, and then the hardness, strength, wear resistance, and fatigue limit of the steel is improved; the arrangement of the stable steel is to stabilize the shape and size of the steel.

Application key:

(1) After quenching, steel parts should be cold-treated immediately, and then tempered at low temperature to eliminate the internal stress during low-temperature cooling;

(2) Cold treatment is mainly suitable for precision tools, measuring tools, and precision parts made of alloy steel.

08 Flame heating surface quenching

Operation method:

The flame incinerated with oxygen-acetylene mixed gas is sprayed on the surface of the steel part and heated rapidly. When it reaches the quenching temperature, it is immediately sprayed with water to cool.

Purpose:

to improve the surface hardness, wear resistance, and fatigue strength of steel parts, and the core still maintains the resistance state.

Application key:

(1) It is mostly used for medium carbon steel parts, usually the depth of the hardened layer is 2-6mm;

(2) It is suitable for large workpieces produced in single or small batches and workpieces that require partial quenching.

09 Induction heating surface quenching

Operation method: Put the steel part into the inductor, make the surface layer of the steel part generate an induced current, heat it to the quenching temperature in a very short time, and then spray water to cool it.

Purpose:

Improve the surface hardness, wear resistance, and fatigue strength of steel parts, and maintain the plastic state of the core.

Application key:

(1) Mostly used for medium carbon steel and Zhongtang alloy structural steel parts;

(2) Because of the skin effect, the hardened layer of high-frequency induction hardening is usually 1-2 mm, that of intermediate-frequency quenching is usually 3-5 mm, and that of high-frequency quenching is usually greater than 10 mm.

10 Carburizing

Operation method:

Put the steel part into the carburizing medium, heat it to 900-950 degrees and keep it warm, so that the surface of the steel part can obtain a carburized layer with a certain concentration and depth.

Purpose:

To improve the surface hardness, wear resistance, and fatigue strength of steel parts, the core still maintains the resistance state.

Application key:

(1) It is used for low carbon steel and low alloy steel parts with a carbon content of 0.15% to 0.25%, usually the depth of carburized layer is 0.5 to 2.5mm;

(2) After carburizing, quenching is necessary to obtain martensite on the surface to complete the purpose of carburizing.

11 Nitriding

Operation method:

Use the active nitrogen atoms differentiated from ammonia gas at 500 to 600 degrees to make the surface of the steel part saturated with nitrogen to form a nitride layer.

Purpose:

To improve the hardness, wear resistance, fatigue strength, and corrosion resistance of steel surfaces.

Application key:

It is mostly used for medium carbon alloy structural steel rich in alloying elements such as aluminum, chromium and molybdenum, as well as carbon steel and cast iron. Usually, the depth of the nitride layer is 0.025-0.8mm.

12 nitrocarburizing

Operation method:

Carburizing and nitriding the surface of the steel together.

Purpose:

To improve the hardness, wear resistance, fatigue strength, and corrosion resistance of steel surfaces.

Key application:

(1) Mostly used for low carbon steel, low alloy structural steel, and Donggang parts, usually, the nitrided layer is 0.02-3mm deep;

(2) Quenching and low-temperature tempering are required after nitriding.

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