What is heat treatment? Heat treatment basics

What is heat treatment? Heat treatment basics

Ⅰ Introduction to heat treatment

Heat treatment is a process that changes the internal organizational structure of metal materials through heating, insulation and cooling to improve their physical, mechanical or chemical properties. Its purpose is to optimize the hardness, strength, toughness, wear resistance, corrosion resistance and processing properties of materials to adapt to different engineering applications. Heat treatment is not only applicable to steel materials, but also widely used in copper, aluminum, titanium and other non-ferrous metals and their alloys.

Ⅱ Heat treatment process type

1.Annealing

By heating to the appropriate temperature, holding for a period of time, and then slowly cooling, to eliminate the residual stress inside the material, reduce hardness, improve plasticity and toughness, and improve processing performance. Common types are complete annealing, incomplete annealing, isothermal annealing, spheroidizing annealing and so on.

2. Normalize

The steel is heated to Ac3(austenitizing temperature) above 30-50 ° C, and the air is cooled after insulation to refine the grains, improve the strength and toughness of the material, and improve the cutting performance.

3.Quench

The steel is heated to austenitizing temperature, and then cooled rapidly (such as water cooling, oil cooling) to obtain a high-hardness martensitic structure. Quenching is a key step to improve the hardness and wear resistance of steel, but it is often accompanied by increased brittleness and needs to be used with tempering.

4. Temper

After quenching, the steel is heated and held at a temperature below the critical temperature, and then cooled to room temperature to adjust the excessive hardness and brittleness generated by quenching, and obtain the required comprehensive mechanical properties. According to the different tempering temperature, it can be divided into low temperature tempering, medium temperature tempering and high temperature tempering.

5. Conditioning treatment

Tempering refers to the process of quenching and tempering the entire workpiece to achieve a uniform hardness distribution.

6. Surface hardening

Surface hardening is a treatment that hardens only the surface layer of the metal while leaving the core soft. Common methods include carburizing, nitriding, and cyaniding. These treatments can make the surface of the part have high hardness and wear resistance, while the core maintains good toughness.

7. Preheat treatment

Preheat treatment refers to the heating of metal prior to certain welding operations to reduce the risk of cracks caused by welding.

Each heat treatment process has its specific application range, and which process is chosen depends on the desired final properties as well as the type of metal. Heat treatment can be applied not only to steel materials, but also to other types of alloys. The correct heat treatment can greatly affect the functional life and reliability of metal components.

Ⅲ Special heat treatment

Surface quenching: Only the surface of the workpiece is rapidly heated and cooled to improve surface hardness and wear resistance, while maintaining good toughness and strength of the heart.

Vacuum heat treatment: Heat treatment under vacuum or inert gas protection to reduce oxidation and decarbonization of the material surface, suitable for high precision and high quality material treatment requirements.

Carburizing: low carbon steel or low carbon alloy steel is heated in the carburizing medium, so that carbon atoms penetrate the surface layer of the workpiece to improve the carbon content of the surface layer, and then quenching and tempering to obtain a surface layer of high hardness and high wear resistance.

Ammoniating: The ammonia element is infiltrated into the surface of the workpiece to form an extremely hard nitriding layer, which significantly improves the surface hardness, wear resistance and corrosion resistance, while maintaining the good toughness of the heart.

High-frequency induction heating: The use of high-frequency current generated in the workpiece eddy current for rapid heating, often used for local quenching and surface heat treatment.

Ⅳ Heat treatment process

1. Preparation

Workpiece cleaning: remove grease, dust and other pollutants from the surface of the workpiece to ensure that no harmful substances are produced during the heat treatment process. Cleaning methods may include chemical cleaning, mechanical cleaning or ultrasonic cleaning.

Workpiece inspection: Check whether the size, shape and surface quality of the workpiece meet the requirements to ensure that the workpiece is in good condition before heat treatment.

Artifact marking: Where necessary, artifacts are identified for easy tracking and management.

Fixture installation: According to the workpiece shape and heat treatment requirements, install the appropriate fixture to fix the workpiece to prevent deformation during heating and cooling.

2. Heating stage

Prepare heating equipment: select the appropriate heat treatment furnace or heating equipment, such as mesh belt furnace, box furnace, etc., and install the required control system and atmosphere control system.

Preheating: preheat the workpiece to a certain temperature to reduce thermal stress and deformation during heating. The preheating temperature and time depend on the workpiece material and heat treatment requirements.

Heating: the workpiece is sent into the furnace and heated to a predetermined temperature. The heating speed needs to be determined according to the material characteristics and the purpose of heat treatment to ensure uniform temperature inside and outside the workpiece.

3. Heat preservation stage

Insulation: Keep the workpiece at a certain temperature for a long enough time to ensure that the microstructure of the material changes. The length of holding time depends on the type of material, heating temperature and heat treatment requirements.

Atmosphere control: Depending on the requirements of heat treatment, insulation in a protective gas environment may be required to avoid oxidation or changing surface properties. The atmosphere control system is used to control the composition and concentration of the atmosphere in the furnace.

4. Cooling stage

Cooling: After the heat treatment is completed, the workpiece needs to be cooled down at an appropriate speed. The cooling method can be selected according to the need of air cooling, oil cooling or water cooling. The control of the cooling rate is essential to obtain the desired material structure and properties.

5. Post-processing stage

Check: Check the size, hardness, appearance and other aspects of the workpiece after heat treatment to verify whether the heat treatment effect meets the requirements.

Calibration: If necessary, calibration or further machining to achieve dimensional accuracy requirements.

Packaging: Proper packaging of qualified workpieces to prevent damage during storage and transportation.

Records: Keep all relevant heat treatment data and test reports for traceability and quality control.

Quality control of heat treatment

Ⅴ Temperature control

Accurate control of heating temperature, holding temperature and cooling temperature is the key to ensure the quality of heat treatment. Thermocouple, infrared thermometer and other equipment are usually used for temperature measurement and control.

1.Time control

Reasonable control of heating time, holding time and cooling time to ensure that the internal organization of the material is fully changed. Time control can be realized by timer, automatic control system, etc.

2.Cooling rate control

Different cooling methods have a great influence on the heat treatment effect. For example, the quenching cooling speed is fast, and the appropriate cooling medium and cooling method are needed to ensure that the desired structure and properties are obtained.

3.Atmosphere control

In some heat treatment processes, it is necessary to control the atmosphere in the furnace to prevent oxidation and decarbonization of materials. Commonly used atmospheres are protective atmospheres (such as nitrogen, argon, etc.) and controlled atmospheres (such as endothermic atmospheres, exothermic atmospheres, etc.).

4.Quality inspection

Quality inspection of the workpiece after heat treatment, including hardness testing, metallographic analysis, mechanical properties testing, etc., to ensure that the workpiece meets the design requirements and performance.

Ⅵ Heat treatment advantage

1.Improve mechanical properties

Improve the hardness: through the heat treatment process such as quenching, the hardness of metal materials can be significantly improved to enhance its wear resistance and scratch resistance. For example, after proper heat treatment of tool steel, the hardness can reach a very high value, so that it can maintain a sharp edge in the cutting, stamping and other processing processes, extending the service life.

Increase strength: Heat treatment can greatly increase the strength of the metal and can withstand greater loads. For example, after quenching and tempering treatment, the strength of structural steel is significantly improved, and it can be used to manufacture mechanical parts and engineering structures that can withstand heavy loads.

Improve toughness: Reasonable heat treatment process can improve the strength at the same time, maintain or improve the toughness of the material, to avoid the material is too brittle and hard and easy to break. For example, quenching stress can be eliminated by tempering, so that the toughness of the material can be restored and its impact resistance can be improved.

2.Optimized processing performance

Improve machinability: For some materials with high hardness, the hardness can be reduced by heat treatment processes such as annealing, making it easier to cut. For example, high carbon steel is spheroidized and annealed before cutting, which can spheroidize carbides, reduce the hardness and brittleness of the material, and improve the cutting performance.

Easy to cold processing: Heat treatment can improve the plasticity of metal materials, which is conducive to cold processing deformation. For example, brass is annealed before processing to improve its plasticity and facilitate cold working operations such as stretching and stamping.

3.Stable size and shape

Elimination of residual stress: Residual stress occurs during the processing and manufacturing of metals, which can cause problems such as deformation and cracking of parts. Through appropriate heat treatment processes, such as stress relief annealing, residual stress can be eliminated and the size and shape of the part can be stabilized. For example, the stress relief annealing of welded structural parts after welding can prevent the formation of welding deformation and cracks.

Improve dimensional accuracy: Heat treatment can make the organization of the material more uniform, reduce internal defects, thereby improving the dimensional accuracy of the parts. For example, after heat treatment of precision mechanical parts, higher dimensional stability can be obtained to meet the processing requirements of high precision.

4.Improve corrosion resistance

Forming a protective film: Some heat treatment processes can form a dense oxide film or other protective film on the surface of the metal to improve the corrosion resistance of the material. For example, the solid solution treatment and passivation treatment of stainless steel at high temperatures can form a stable chromium oxide protective film to enhance its corrosion resistance.

Improve the uniformity of the structure: Heat treatment can make the structure of the metal more uniform and reduce the occurrence of local corrosion. For example, after the homogenization annealing treatment of aluminum alloy, the composition of the tissue is more uniform, reducing the sensitivity of local corrosion.

5.Special properties

Obtain special physical properties: Heat treatment can change the physical properties of the metal, such as magnetism, electrical conductivity, coefficient of thermal expansion, etc. For example, the magnetic properties of electrical steel can be adjusted through heat treatment, so that it has higher permeability and lower iron loss, improving the efficiency of motors and transformers.

Produce special organizational structure: Heat treatment can make the metal form a special organizational structure, such as martensite, bainite, etc., giving the material special properties. For example, after quenching and tempering treatment, the martensite and carbide structure formed by high-speed steel makes it have high hardness, high red hardness and wear resistance, which is suitable for high-speed cutting.

Ⅶ Heat treatment application

Mechanical manufacturing: tools, molds, gears, springs, shaft parts and fasteners, etc

Automotive industry: Key components such as engine pistons, crankshafts, wheels, drive shafts, and suspension systems

Aerospace: engine blades, fuselage frames, wings, satellite frames, solar panel brackets, etc

Electronic manufacturing: wafer processing, chip packaging, resistors, capacitors, inductors, etc

Metallurgical industry: Steel production and heat treatment of non-ferrous metals can improve the workability and mechanical properties of copper alloys

Daily necessities: tableware, kitchenware, locks, hardware accessories, etc

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