Mar. 14, 25
Vacuum heat treatment is a process in which the pressure in a furnace is reduced using a vacuum pump and materials are heat treated. This process helps to suppress oxidation of metals and results in a shiny and bright appearance.
It is suitable for various heat treatment processes, including sintering, degreasing, quenching, and tempering. Vacuum heat treatment is widely used because it not only enhances specific material properties during each process but also improves overall quality.
Compared to conventional quenching, vacuum quenching prevents oxide layer formation while preserving hardness and wear resistance, making it an ideal choice for components and products that require both durability and an aesthetically pleasing finish.
Vacuum heat treatment is a process in which heat treatment is carried out under reduced pressure, typically below atmospheric pressure.
In conventional heat treatment of metals, chemical reactions such as oxidation, evaporation, and reduction occur between the metal and the surrounding atmosphere. However, by carefully controlling the atmosphere and temperature, these reactions can be minimized. The key advantage of a vacuum environment is its extremely low oxygen content, which prevents the reaction between carbon in the metal and oxygen, effectively eliminating decarburization. Additionally, vacuum heat treatment aids in removing organic substances and gases from the metal surface, resulting in a clean, bright finish that enhances the material’s natural appearance.
This process is highly versatile and can be applied to a wide range of heat treatment applications.
The processing temperature for vacuum heat treatment typically ranges from 200°C to 2000°C, with vacuum levels (pressure) classified into the following categories. Specific heat treatment processes are tailored to different types of steel:
- Low vacuum (~102 Pa)
- Medium vacuum (102–10⁻¹ Pa)
- High vacuum (10⁻¹–10⁻⁵ Pa)
- Ultra-high vacuum (10⁻⁵–10⁻⁸ Pa)
In vacuum heat treatment, medium and high vacuum conditions are most commonly used.
Additionally, an alternative method known as protective atmosphere heat treatment serves the same purpose of preventing metal oxidation. Instead of creating a vacuum inside the furnace, this process involves slightly pressurizing the furnace with an inert gas, such as nitrogen or argon, above atmospheric pressure. This prevents oxygen ingress from the outside while allowing heat treatment to proceed.
Protective atmosphere heat treatment is particularly useful when reducing the furnace pressure is impractical, such as in large-volume furnaces. Since no carbon is introduced during the process, it prevents carburization and maintains the metal’s original hardness.
Metal powders are heated to fuse into a dense structure, enhancing strength while maintaining shape. Common in metals, intermetallic compounds (e.g., rare earth magnets), and composites (e.g., superhard alloys), vacuum sintering allows higher temperatures, improving corrosion resistance.
Used to relieve internal stresses, improve ductility, and refine structure, especially in steel castings and forgings. A vacuum atmosphere prevents oxidation, ensuring a clean, bright surface while eliminating the need for acid pickling.
Quenching rapidly cools heated steel to increase hardness, while tempering restores ductility and impact resistance. Vacuum processing prevents oxidation, improving dimensional accuracy and surface quality.
Removes oil, grease, and contaminants before heat treatment or finishing. Conducted in a vacuum chamber, it prevents fire hazards, enables solvent recovery, and shortens processing time.
Introduces carbon into metal surfaces to enhance wear resistance while maintaining core toughness. Compared to conventional gas carburizing, it allows higher temperatures, shorter cycles, and prevents oxidation.
Joins metals using a low-melting alloy without melting base materials. Preferred for oxidation-sensitive metals, it eliminates the need for flux and post-brazing cleaning.
A vacuum heat treatment furnace features a sealed chamber and a vacuum system that reduces the internal pressure, allowing heat treatment to occur in a controlled, subatmospheric environment.
- Bright Heat Treatment Capability
Vacuum heat treatment prevents surface oxidation, preserving the metal’s natural luster and enhancing its overall value.
- Descaling Effect
By removing organic substances and degassing the metal surface, vacuum heat treatment eliminates the need for additional cleaning processes like pickling or degreasing, improving efficiency.
- Optimal Working Environment
Many vacuum furnaces feature a water-cooled, double-wall design that keeps the outer wall near ambient temperature. Additionally, inert gases such as nitrogen or argon are often used, ensuring a safe and stable working environment.
- Improved Mechanical Properties
The degassing effect of vacuum heat treatment enhances mechanical properties, while the surface cleaning process improves wear resistance.
- Minimal Distortion
Radiant heating ensures a gradual temperature increase from room temperature, and controlled cooling reduces thermal stress, minimizing distortion in the treated material.
Also known as a rotary pump, this is the most commonly used vacuum pump. It operates using lubricating oil for rotation, ensuring excellent sealing performance. Suitable for reducing pressure from atmospheric levels to a few pascals (Pa).
Utilizes an impeller based on the Roots principle, with high-speed synchronized rotor rotation inside the casing. This design enables high pumping speeds in a compact size. However, it cannot function independently and is typically paired with an oil rotary vacuum pump. After initial pumping by the rotary lobe pump, it can achieve vacuum levels of approximately 10 to 10⁻¹ Pa.
Employs heated oil vapor to generate a high-speed jet, effectively compressing and expelling gases. Commonly used as an auxiliary pump after an oil rotary or mechanical booster pump, it can reach vacuum levels of approximately 10⁻⁴ Pa.
Activated after an oil rotary vacuum pump or similar pump has established a preliminary vacuum, this pump uses high-speed rotating vanes to compress gas molecules and expel them. It is oil-free, making it ideal for clean environments and applications requiring high vacuum levels.
Utilizes extremely low-temperature surfaces to freeze and remove gases from the system. Similar to the turbo molecular pump, it is preferred for clean environments and can achieve vacuum levels ranging from 10 to 10⁻⁸ Pa.
Functions by applying a voltage between an anode and cathode to generate an electron discharge. Some electrons collide with gas molecules, creating ions that react chemically with titanium to further enhance vacuum levels. It is suitable for achieving ultra-high vacuum levels of approximately 10⁻⁸ Pa.
In a vacuum heat treatment furnace, a dedicated processing chamber is used for heating and cooling materials.
To minimize heat loss and conserve energy, the chamber is insulated, ensuring efficient temperature control during the heating process. Its shape can vary—cylindrical or rectangular—depending on factors such as the material’s shape, processing volume, and transportation method.
While stainless steel is commonly used in standard vacuum furnaces, smaller units may feature tube furnaces made from materials such as quartz glass or alumina.
Electric heaters are the primary heat source, offering precise temperature control to maintain consistent processing quality. The heating elements are typically made from materials such as carbon, tungsten, molybdenum, or silicon carbide (SiC), depending on the required temperature and application.
Vacuum heat treatment furnaces use convection cooling, gas cooling, and oil cooling. In single-chamber furnaces, the workpiece is cooled within the same chamber, whereas multi-chamber furnaces separate heating and cooling, allowing more efficient cooling. High-pressure gas cooling is increasingly used for uniform rapid cooling, minimizing material distortion.
The cooling rate affects steel's internal structure. The Ps curve helps suppress unwanted phase precipitation, maintaining an austenitic structure. After passing the Ms point, slow air cooling prevents martensitic transformation.
A vacuum heat treatment furnace features a control panel managing vacuum systems, heaters, and sensors. Key parameters include furnace temperature, monitored via thermocouples, ensuring adherence to treatment specifications.
PID (Proportional-Integral-Derivative) control minimizes temperature fluctuations by adjusting proportional, integral, and derivative parameters. Modern systems offer auto-tuning, analyzing temperature data to optimize PID settings automatically.
Safety circuits prevent overheating by shutting down the system if preset limits are exceeded. Sensor redundancy ensures safety even in case of failure. Additional monitoring features, such as agitator fan checks, cooling water levels, and vacuum anomalies, help maintain product quality.
