Working principle of vertical vacuum furnace

The vertical vacuum furnace achieves precise heat treatment of materials under non oxidizing and low pollution conditions through the synergistic effect of vacuum environment control, efficient heating system, gas circulation, and cooling mechanism. Its working principle can be divided into four core links: vacuum system operation, heating process control, atmosphere regulation, and cooling process. The following is a detailed analysis:

1. Vacuum system operation: creating an anaerobic environment
Collaborative work of vacuum pump group
Primary vacuum pumping: Mechanical pumps (such as rotary vane pumps) are started to rapidly reduce the pressure inside the furnace from atmospheric pressure (about 10 ⁵ Pa) to 10 ⁻¹ -10 ⁻ ² Pa, removing most of the air and moisture.
High vacuum stage: a diffusion pump or molecular pump is connected to further reduce the air pressure to 10 ⁻³ -10 ⁻⁴ Pa (ultra-high vacuum) by high-speed injection of oil vapor or molecular flow, meeting the heat treatment requirements of active metals such as titanium and zirconium.
Vacuum monitoring: The resistance gauge or ionization gauge provides real-time feedback on the furnace pressure, and the system automatically adjusts the pump power to maintain the target vacuum level.
Sealing and leakage control
Dynamic sealing: The furnace door adopts fluorine rubber O-ring or metal sealing ring, combined with water-cooled flange design, to prevent deformation and leakage of sealing material under high temperature.
Leak detection mechanism: The helium mass spectrometer leak detector regularly checks the furnace leakage rate (usually ≤ 1 × 10 ⁻⁹ Pa · m ³/s) to ensure long-term vacuum stability.

2. Heating process control: precise temperature regulation
Heating element and layout
Component type: Select resistance wire (≤ 1200 ℃), silicon carbon rod (≤ 1600 ℃) or silicon molybdenum rod (≤ 1800 ℃) according to temperature requirements, evenly distributed on the inner wall or side of the furnace.
Vertical advantage: The hot air flows naturally in the vertical direction, and with the help of forced circulation fans (such as graphite blades), the temperature difference inside the furnace is eliminated (usually ≤± 5 ℃), making it particularly suitable for uniform heating of long axis and tubular samples.
Temperature control logic
PID algorithm: Real time sample temperature is collected through thermocouples (K-type, S-type) or infrared thermometers, and the PID controller dynamically adjusts the heating power to achieve a temperature control accuracy of ± 1 ℃.
Program heating: Supports programming of temperature rise and fall curves in more than 30 stages (such as step heating, insulation, and rapid cooling) to meet complex process requirements (such as multi-stage temperature control required for carburizing treatment).

3. Atmosphere regulation: flexible control of reaction environment
Gas injection system
Multi way intake: The flow rate of oxygen, nitrogen, argon, hydrogen and other gases (0.1-10 L/min) is precisely controlled by a mass flow meter (MFC), supporting single gas or mixed gas injection.
Example of reaction process:
Carbonization treatment: Introduce a mixture of methane (CH4) and nitrogen gas, decompose carbon atoms at high temperatures, and infiltrate the metal surface to improve hardness.
Reduction reaction: Inject hydrogen gas (H ₂) to reduce metal oxides (such as CuO → Cu), with a purity of up to 99.999%.
Vacuum atmosphere switching
Quick exhaust valve: When switching the atmosphere, first exhaust the furnace gas to the vacuum pump through the exhaust valve, and then inject new gas to avoid cross contamination.
Pressure control: Some models are equipped with back pressure valves to maintain a slight positive pressure inside the furnace (such as 10-100 Pa) and prevent external air from infiltrating.

4. Cooling process: Control material phase transition
natural cooling
After turning off the heating element, the furnace naturally dissipates heat through the insulation layer, with a slow cooling rate (about 5-10 ℃/min), suitable for materials sensitive to temperature gradients (such as certain ceramics).
forced air cooling
Circulating fan: Start the air-cooled fan to accelerate the flow of inert gas (such as Ar) inside the furnace, remove heat, and increase the cooling rate to 20-50 ℃/min.
Application scenario: Metal quenching treatment, which requires rapid passing through the critical temperature of phase transformation to obtain martensitic structure and improve hardness.
Oil cooled/water-cooled (optional)
Some vertical furnaces are equipped with lifting platforms, which can quickly immerse samples in oil or water to achieve ultra fast cooling (≥ 100 ℃/min), but attention should be paid to preventing thermal stress cracking.

5. Workflow Example (Taking Metal Vacuum Annealing as an Example)
Loading: Hang the metal workpiece on the top of the furnace or place it on the fixture, close the furnace door and seal it.
Vacuum pumping: The combination of mechanical pump and diffusion pump works to reduce the air pressure to 10 ⁻⁵ Pa.
Heating: Heat up according to the program to the target temperature (such as 800 ℃) and hold for 2 hours to eliminate internal stress.
Cooling: Start the air cooling system and cool down to below 200 ℃ at a rate of 30 ℃/min.
Inflation: Introduce nitrogen gas to break the vacuum, open the furnace door and remove the workpiece.

6. Influence of key technical parameters
Vacuum degree: The higher the vacuum degree, the better the oxidation inhibition effect, but the equipment cost and maintenance difficulty increase.
Heating rate: Too fast may cause thermal stress cracking, which needs to be adjusted according to the material’s thermal expansion coefficient.
Insufficient gas flow rate affects the reaction rate, while excessive flow rate increases costs and may carry impurities.
Cooling rate: It needs to match the hardenability of the material to avoid uneven organization or cracking.

7. Application scenario extension
Aerospace: Vacuum brazing of titanium alloy blades to avoid strength degradation caused by high-temperature oxidation.
Semiconductor: Annealing of silicon chips eliminates processing stress and improves crystal quality.
New energy: sintering of lithium battery cathode material (NCM), achieving uniform grain growth through precise temperature control.
Research: Material phase transition study, observing tissue evolution under different cooling rates.

The vertical vacuum furnace provides a non oxidizing, low pollution, and high-precision heat treatment solution for high-end manufacturing through precise vacuum, heating, atmosphere, and cooling control. It is a key equipment for optimizing material properties.