12 Kinds of Heat Treatment Methods You Need to Know

  • February 8, 2019
  • Categories: Blog

The processing method of heating, heat preservation and cooling of solid metals or alloys in order to obtain the required microstructure and properties.

1. Annealing

Operation method: Heating the steel to Ac3 + 30-50 degrees or Ac1 + 30-50 degrees or lower than Ac1 degrees and slowly cooling with the furnace temperature.

Objective:
1. Reduce hardness, improve plasticity, improve cutting and pressure machining performance;
2. Refine the grain, improve the mechanical properties, prepare for the next step of the process;
3. Eliminate the internal stress caused by cold and hot working.

Key Applications:
1. Suitable for alloy structural steel, carbon tool steel, alloy tool steel, high speed steel forgings, welds and unqualified raw materials.
2. Generally, annealing is carried out in the blank state.

2. Normalization

Operation method: the steel parts are heated to 30 °50 degrees above Ac3 or Accm, and cooled at a cooling rate slightly higher than that of annealing after heat preservation.

Objective:
1. 1. Reduce hardness, improve plasticity, improve cutting and pressure machining performance;
2. Refine the grain, improve the mechanical properties, prepare for the next step of the process;
3. Eliminate the internal stress caused by cold and hot working.

Key Applications:
Normalizing is usually used as a pre-heat treatment process for forgings, welds and carburized parts. For low carbon and medium carbon structural steel and low alloy steel with low performance requirements, they can also be used as final heat treatment. For general medium and high alloy steel, air cooling can lead to complete or local quenching, so it cannot be used as the final heat treatment process.

3. Quench

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

Objective:
In order to obtain a high-hardness martensitic structure, it is sometimes used to quench some high alloy steel (such as stainless steel, stainless steel) in order to obtain a single uniform austenite structure, so as to improve the wear resistance and corrosion resistance.

Key Applications:
1. It is commonly used in 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, it will cause great internal stress and reduce the plasticity and impact toughness of steel. So tempering should be carried out to obtain better comprehensive mechanical properties.

4. Tempering

Operation method: the quenched steel parts are reheated to a certain temperature below Ac1 and cooled in air or oil, hot water and water after heat preservation.

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

Key Applications:
1. The high hardness and the wear resistance of the steel are tempered at low temperature when the steel is quenched; the elastic and yield strength of the steel is improved by moderate temperature tempering under the condition of maintaining a certain toughness; the high impact toughness and the plasticity are maintained, and the high-temperature tempering is used when sufficient strength is used.
2. Generally, tempering of steel between 230 and 280 degrees shall be avoided as much as possible, and the stainless steel shall be avoided from tempering between 400~450 degrees, because one tempering brittleness will be produced at this time.

5. Quenching and tempering

Operation method: high temperature tempering after quenching is called tempering and tempering, that is, the steel parts are heated to a temperature of 10 degrees 20 degrees higher than that of quenching, quenched after heat preservation, and then tempered at a temperature of 400 degrees 720 degrees.

Objective:
1. Improve the cutting performance and improve the smoothness of the machined surface.
2. Reduce the deformation and cracking during quenching.
3. Good comprehensive mechanical properties are obtained.

Key Applications:
It is suitable for alloy structural steel, alloy tool steel and high-speed steel with higher hardenability; Not only can be used as the final heat treatment of various important structures, but also can be used as a pre-heat treatment of some close parts, such as lead screw and the like, in order to reduce the deformation.

6. Aging

Operation method: heat the steel parts to 80 degrees 200 degrees, heat preservation 5 hours or more for 20 hours or more, and then remove them from the furnace and cool them in the air.

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

Key Applications:
1. Suitable for all kinds of steel after quenching.
2. It is often used for compact workpieces that require no change in shape, such as compact screw, measuring tool, bed chassis and so on.

7. Cold Treatment

Operation method: the quenched steel parts are cooled to-60 ≤ 80 degrees or lower in low temperature medium (such as dry ice, liquid nitrogen). After the temperature is uniform and consistent, the average temperature is taken out to room temperature.

Objective:
All or a large part of the residual austenite in the quenching steel piece is converted into martensite, so that hardness, strength, wear resistance and fatigue limit of the steel piece are improved. The microstructure of the steel is stabilized to stabilize the shape and size of the steel.

Key Applications:
1. Cold treatment of steel parts should be carried out immediately after quenching, and then tempered at low temperature to eliminate the internal stress during low temperature cooling.
2. Cold treatment is mainly suitable for compact cutting tools, measuring tools and compact parts of alloy steel.

8. Flame heating surface quenching

Operation method: the flame burned with oxygen-acetylene mixture gas is sprayed onto the surface of the steel, heated quickly, and cooled with water as soon as the quenching temperature is reached.

Objective:
To improve the surface hardness, wear resistance and fatigue strength of steel parts, and to maintain toughness in the center.

Key Applications:
1. Most of them are used for medium carbon steel parts, and the general depth of the quenching layer is 2~6mm.
2. Applicable to large workpieces produced in single or small batches and workpieces requiring local quenching.

9. Induction heating surface quenching

Operation method: putting the steel piece into an inductor, generating an induced current on the surface layer of the steel piece, heating the steel piece to a quenching temperature in a very short time, and then spraying water for cooling

Objective:
To improve the surface hardness, wear resistance and fatigue strength of steel parts, and to maintain toughness in the center.

Key Applications:
1. Most of them are used in medium carbon steel and alloy structural steel parts.
2. Due to the skin effect, the high frequency induction quenching layer is generally 1 to 2 mm, the intermediate frequency quenching is generally 3 to 5 mm, and the high frequency quenching is generally more than 10 mm.

10. Carburizing

Operation method: the steel parts are put into the Carburizing medium, heated to 900 degrees 950 degrees and insulated, so that a certain concentration and depth of carburized layer can be obtained for the steel noodles.

Objective:
To improve the surface hardness, wear resistance and fatigue strength of steel parts, and to maintain toughness in the center.

Key Applications:
1. For low carbon steel and low alloy steel parts with 0.15% to 0.25% carbon content, the depth of Carburizing layer is 0.5 to 2.5 mm.
2. After Carburizing, it is necessary to quench the surface to get martensite in order to achieve the purpose of Carburizing.

11. Nitridation

Operation method: the active nitrogen atom decomposed by ammonia at 500 to 600 degrees is used to saturate the surface of steel parts to form nitride layer.

Objective:
To improve the hardness, wear resistance, fatigue strength and corrosion resistance of steel parts.

Key Applications:
Most of them are used in medium carbon alloy structural steel containing aluminum, chromium, Mo and other alloy elements, as well as carbon steel and cast iron. The depth of nitriding layer is 0.025mm to 0.8 mm.

12. Nitrocarburizing

Operation method: Carburizing and nitriding to the surface of steel parts at the same time.

Objective:
To improve the hardness, wear resistance, fatigue strength and corrosion resistance of steel parts.

Key Applications:
1. Used in low carbon steel, low-alloy structural steel and tool steel parts, generally the nitride layer depth is 0.02~3mm.
2. Quenching and low-temperature tempering are required after nitriding.

Above are the 12 common-used ways of heat treatment. If you like this article, please share it to your network.

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