Precautions for using a vacuum tempering electric furnace

The use of a vacuum tempering electric furnace requires strict control in terms of preparation before operation, monitoring during operation, process parameter control, safety protection, and maintenance. The following are specific precautions:

1. Preparation before operation
Equipment inspection
Vacuum system: Check the oil level of the vacuum pump (the oil level of the mechanical pump should be above 1/2 of the window), whether the heating power of the diffusion pump is normal, and ensure that there are no leaks in the vacuum pipeline (which can be detected by a helium mass spectrometer leak detector).
Heating system: Confirm that the heating elements (such as molybdenum strips and graphite heaters) are not broken or short circuited, and the resistance value should be close to the nominal value (such as molybdenum strip resistance deviation ≤ 10%).
Temperature control system: Check that the thermocouple model matches the temperature controller (if an S-type thermocouple requires an S-type temperature controller), and that the wiring terminals are not loose or oxidized (the contact surface can be polished with sandpaper).
Cooling system: Ensure that the pressure (usually 0.2-0.4MPa) and flow rate of the water cooler are normal to avoid equipment damage caused by insufficient cooling.
Workpiece preparation
Cleanliness: The surface of the workpiece should be free of oil stains, rust, or impurities (can be wiped with alcohol) to prevent volatilization and contamination of the furnace under vacuum.
Loading capacity: The single loading capacity shall not exceed 70% of the furnace volume, and the distance between workpieces shall be ≥ 50mm to avoid thermal circulation obstruction.
Tool rack: Use high-temperature resistant (≥ tempering temperature 200 ℃), non-magnetic tool racks (such as ceramic or stainless steel) to prevent workpiece deformation or adhesion.
Environmental Requirements
Cleanliness: There is no dust or corrosive gas within 5 meters around the furnace body to avoid contaminating the vacuum system.
Temperature and humidity: The ambient temperature is 5-35 ℃, and the humidity is ≤ 70% to prevent electrical components from being affected by moisture and short circuiting.

2. Monitoring during operation
Vacuum degree control
Vacuum pumping stage: First, start the mechanical pump to pre pump to below 10Pa, and then start the diffusion pump to pump to the process requirements (such as high temperature tempering requiring ≤ 10 ⁻ ³ Pa).
Pressure holding test: After closing the vacuum valve, the pressure should rise by ≤ 10Pa within 10 minutes, otherwise the leakage point needs to be checked.
Vacuum breaking operation: After tempering, first fill high-purity nitrogen gas (purity ≥ 99.999%) to atmospheric pressure, and then open the furnace door to avoid oxidation of the workpiece.
Temperature monitoring
Real time curve: Observe the temperature curve through a temperature controller to ensure that the heating rate meets the process requirements (such as low-temperature tempering ≤ 5 ℃/min, high-temperature tempering ≤ 3 ℃/min).
Temperature uniformity: Use an infrared thermometer or multi-point thermocouple to detect the temperature difference inside the furnace, which should be ≤± 5 ℃ (precision parts should be ≤± 3 ℃).
Exception handling: If the temperature exceeds the set value (such as 5 ℃), immediately pause heating and check thermocouples, solid-state relays, or heating elements.
Cooling system monitoring
Water flow rate: Ensure that the cooling water flow rate is ≥ 5L/min to avoid damage to heating elements or vacuum pumps due to overheating.
Water temperature: Inlet water temperature ≤ 30 ℃, outlet water temperature ≤ 50 ℃, to prevent backflow of condensed water.

3. Process parameter control
Temperature setting
Low temperature tempering (150-250 ℃): Suitable for tool steel and spring steel, strict temperature control is required to maintain high hardness (such as HRC60-62 after tempering T10A steel).
High temperature tempering (500-650 ℃): suitable for alloy structural steel and stainless steel, requiring long-term insulation (2-4 hours) to ensure sufficient precipitation of carbides.
Deep cold tempering: After deep cold treatment (-196 ℃), it is necessary to temper in stages (such as -196 ℃ → room temperature → 150 ℃ → 250 ℃) to avoid thermal stress cracking.
heating rate
General rule: Heating rate=Material thickness (mm) x coefficient (0.5-1), for example, the heating rate of 10mm thick 42CrMo steel is ≤ 5 ℃/min.
Special materials: High carbon steel or thin-walled parts require slower heating (≤ 3 ℃/min) to prevent deformation or cracking.
holding time
Calculation formula: insulation time (min)=material thickness (mm) × coefficient (1-2). For example, 15mm thick 304 stainless steel needs to be insulated for 15-30 minutes for high-temperature tempering.
Verification method: Confirm sufficient structural transformation through metallographic examination (such as complete decomposition of martensite into tempered martensite).

4. Security protection
Electrical safety
Grounding protection: The equipment casing must be reliably grounded (grounding resistance ≤ 4 Ω) to prevent electric leakage and injury.
Insulation testing: Regularly use a megohmmeter to test the insulation resistance of the heating element (normal ≥ 1M Ω) to avoid short circuits.
Emergency stop button: Ensure that the emergency stop button functions properly and can immediately cut off the power in case of an emergency.
high-temperature protection
Insulated gloves: High temperature resistant gloves (temperature resistance ≥ 300 ℃) should be worn when operating furnace doors or workpieces.
Protective face mask: When observing the furnace, it is necessary to wear a radiation shielding face mask to avoid eye burns caused by infrared radiation.
Warning signs: Set up warning signs such as “High temperature danger” and “Vacuum operation” around the furnace body.
Gas safety
Nitrogen purity: High purity nitrogen gas (purity ≥ 99.999%) should be used when filling nitrogen to break the vacuum, to prevent oxygen from entering and causing oxidation.
Gas pressure: The pressure of the nitrogen cylinder should be ≥ 2MPa to avoid slow inflation speed affecting efficiency.

5. Maintenance and upkeep
routine maintenance
Cleaning the furnace: Use a vacuum cleaner to clean the residue inside the furnace after each use to avoid contaminating the next process.
Check the sealing strip: Regularly inspect the furnace door sealing strip (such as fluororubber strip) for aging and replace it if necessary.
Lubricate rotating parts: Apply high-temperature grease (such as molybdenum disulfide) to vacuum valves, furnace door hinges, and other parts.
regular maintenance
Thermocouple calibration: Every 3 months, calibrate the thermocouple with a standard temperature source (such as a dry furnace) to ensure measurement accuracy.
Vacuum pump maintenance: Replace the mechanical pump oil (such as 1 # vacuum pump oil) and clean the diffusion pump oil chamber every 6 months.
Heating element replacement: Replace the heating element according to the usage situation (such as resistance deviation>20%) to avoid local overheating.
Long term discontinuation
Clean and dry: Clean the furnace before shutdown and fill it with dry nitrogen for protection.
Power and water outage: Turn off the power and water sources to prevent the equipment from getting damp or freezing (the cooling water needs to be drained in winter).