We heat the metal or alloy to a given temperature and maintain it for a period of time, and then cool it with a selected rate and method to obtain the required microstructure and properties. This operation process is called surface heat treatment. There are many ways for us to choose when processing.

1. Annealing

Operation method: After heating the steel pieces to Ac3+30~50 degrees or Ac1+30~50 degrees or the temperature below Ac1 (relevant information can be consulted), generally cool slowly with the furnace temperature.

Purpose: 1. Reduce hardness, improve plasticity, and improve cutting and pressure processing performance; 2. Refine grains, improve mechanical properties, and prepare for the next process; 3. Eliminate internal stress generated by cold and hot processing.

Application points: 1. Suitable for alloy structural steel, carbon tool steel, alloy tool steel, high-speed steel forgings, welded parts and raw materials that are not qualified in supply state; 2. Generally, annealing is performed in the rough state.

2. Normalizing

Operation method: heat the steel to 30~50 degrees above Ac3 or Accm, and cool it at a slightly higher cooling rate than annealing after heat preservation.

Purpose: 1. Reduce hardness, improve plasticity, and improve cutting and pressure processing performance; 2. Refine grains, improve mechanical properties, and prepare for the next process; 3. Eliminate internal stress generated by cold and hot processing.

Application points: 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 steels and low-alloy steels with low performance requirements, it can also be used as post-processing heat treatment. For general medium and high alloy steels, air cooling can lead to complete or partial quenching, so it cannot be used as a post-heat treatment process.

3. Quenching

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

Purpose: Quenching is generally to obtain a high-hardness martensitic structure. 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 points: 1. Generally used for carbon steel and alloy steel with 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 will cause great internal stress, Reduce the plasticity and impact toughness of the steel, so tempering is required to obtain better comprehensive mechanical properties.

4. Tempering

Operation method: reheat the quenched steel to a certain temperature below Ac1, and after heat preservation, cool it in air or oil, hot water or water.

Purpose: 1. Reduce or eliminate the internal stress after quenching, reduce the deformation and cracking of the workpiece; 2. Adjust the hardness, improve the plasticity and toughness, and obtain the mechanical properties required by the work; 3. Stabilize the size of the workpiece.

Application points: 1. Use low temperature tempering to maintain the high hardness and wear resistance of the steel after quenching; use medium temperature tempering to improve the elasticity and yield strength of the steel while maintaining a certain toughness; to maintain high impact toughness 2. Generally, steel should be tempered at 230~280 degrees, and stainless steel should be tempered at 400~450 degrees, because a temper brittleness will occur at this time.

5. Conditioning

Operation method: High temperature tempering after quenching is called quenching and tempering, that is, heating the steel to a temperature 10~20 degrees higher than that of quenching, quenching after heat preservation, and then tempering at a temperature of 400~720 degrees.

Purpose: 1. Improve the machining performance and improve the surface finish; 2. Reduce the deformation and cracking during quenching; 3. Obtain good comprehensive mechanical properties.

Application points: 1. Suitable for alloy structural steel, alloy tool steel and high-speed steel with high hardenability; 2. Not only can be used as post-heat treatment of various important structures, but also can be used as some tight parts, such as wire Pre-heat treatment of bars, etc., to reduce deformation.

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6. Aging

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

Purpose: 1. Stabilize the structure of the steel after quenching, reduce the deformation during storage or use; 2. Reduce the internal stress after quenching and grinding, and stabilize the shape and size.

Application points: 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.

7. Cold treatment

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

Purpose: 1. Convert all or most of the retained austenite in the quenched steel to martensite, thereby improving the hardness, strength, wear resistance and fatigue limit of the steel; 2. Stabilize the structure of steel to stabilize the shape and size of steel parts.

Application points: 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 tight knives, measuring tools and tight parts made of alloy steel.

8. Flame heated surface hardening

Operation method: use the flame of oxygen-acetylene mixed gas to spray on the surface of the steel part, heat it quickly, and spray water to cool it immediately after reaching the quenching temperature.

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

Application points: 1. It is mostly used for medium carbon steel parts, and the depth of the hardened layer is generally 2 to 6 mm; 2. It is suitable for large workpieces produced in single or small batches and workpieces requiring partial quenching.

9. Induction Heating Surface Hardening

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

Purpose: To improve the surface hardness, wear resistance and fatigue strength of steel parts, and maintain the toughness of the core.

Application points: 1. Mostly used for medium carbon steel and Zhongtang alloy structural steel parts; 2. Due to the skin effect, the hardening layer of high-frequency induction hardening is generally 1-2mm, that of intermediate-frequency quenching is generally 3-5mm, and that of high-frequency quenching is generally greater than 10mm.

10. carburization

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, and the core still maintains a tough state.

Application points: 1. It is used for low carbon steel and low alloy steel parts with carbon content of 0.15% to 0.25%, and the depth of the carburized layer is generally 0.5 to 2.5 mm; 2. After carburizing, quenching must be carried out to obtain martensite on the surface, in order to achieve the purpose of carburizing.

11. Nitriding

Operation method: use in 5. ． At ~600 degrees, the active nitrogen atoms decomposed by ammonia gas 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 the steel surface.

Application points: It is mostly used for medium carbon alloy structural steel containing aluminum, chromium, molybdenum and other alloying elements, as well as carbon steel and cast iron. Generally, the depth of the nitride layer is 0.025 ~ 0.8mm.

12. nitrocarburizing

Operation method: Carburizing and nitriding simultaneously on the surface of the steel.

Purpose: To improve the hardness, wear resistance, fatigue strength and corrosion resistance of the steel surface.

Application points: 1. Mostly used for low carbon steel, low alloy structural steel and tool steel parts, generally the depth of the nitrided layer is 0.02 ~ 3mm; 2. Quenching and low temperature tempering are required after nitriding.

The steps of the surface heat treatment process are different for different requirements, so when we conduct heat treatment, we should choose the heat treatment according to the material conditions and the use environment of the enterprise, so as to strengthen the production of the enterprise.