Precautions for tempering operation of muffle furnace

The tempering operation of muffle furnace is a key link in the heat treatment process, which directly affects the material properties and product quality. To ensure the tempering effect and avoid safety hazards, strict control is required from the aspects of pre operation preparation, temperature control, atmosphere management, cooling treatment, and safety protection. The following are specific precautions and operating points:

1. Preparation before operation
Equipment inspection
Heating element: Check whether the resistance wire or silicon carbide rod is aging or broken to avoid uneven furnace temperature caused by local overheating.
Temperature control system: Verify the accuracy of the PID controller (recommended within ± 1 ℃), calibrate the position of the thermocouple, and ensure that the temperature measurement point represents the actual temperature of the workpiece.
Furnace sealing: Check whether the sealing strips of the furnace door and observation window are intact to prevent atmosphere leakage or external air infiltration.
Safety device: Test the over temperature alarm and power-off protection functions to ensure that the power can be automatically cut off in emergency situations.
Workpiece pre-processing
Cleaning: Remove oil stains and rust from the surface of the workpiece to prevent carbon deposition or oxidation during tempering.
Pre cooling: The quenched workpiece needs to be cooled to room temperature before being loaded into the furnace to avoid cracking caused by thermal stress accumulation.
Placement: Leave a gap of ≥ 50mm between workpieces to ensure even circulation of hot air flow; Large workpieces need to be supported by refractory bricks to prevent deformation.
Tools and Protection
High temperature resistant gloves: Choose gloves that can withstand temperatures above 300 ℃ to prevent burns when handling workpieces.
Long handled fixture: Use stainless steel or ceramic material fixtures to avoid deformation of metal tools at high temperatures.
Protective face mask: Wear a heat-resistant radiation mask to reduce the damage of high temperature to the eyes.

2. Key points of temperature control
heating rate
Low temperature tempering (<300 ℃): heating rate ≤ 50 ℃/h to avoid thermal stress concentration.
High temperature tempering (≥ 500 ℃): heating rate ≤ 30 ℃/h, ensuring consistent temperature inside and outside the workpiece.
Example: When tempering GCr15 bearing steel, it takes ≥ 3.6 hours to rise from room temperature to 180 ℃ to prevent excessive temperature difference between the surface and the core.
holding time
Formula calculation: insulation time (min)=effective thickness of workpiece (mm) × coefficient (usually 1-2).
Experience score:
Thin piece (<10mm): 1-2 hours Medium thickness (10-50mm): 2-4 hours Thick parts (>50mm): 4-6 hours
Case: A 45 steel shaft component with a diameter of 50mm needs to be tempered at 500 ℃ for 4 hours to ensure sufficient spheroidization of carbides.
temperature uniformity
Furnace partition: Place the workpiece in the middle of the furnace, avoiding the direct radiation area of the heating element.
Mixing device: The convection muffle furnace can turn on the fan to promote the circulation of hot air, and the temperature uniformity can reach ± 2 ℃.
Temperature measurement points: Install thermocouples at the top, bottom, front, and back positions of the workpiece stack to monitor temperature differences.

3. Atmosphere management
Inert atmosphere protection
Nitrogen/argon: used to prevent oxidation, flow rate controlled at 5-10L/min, oxygen content ≤ 10ppm.
Vacuum environment: For high-precision bearings, vacuum tempering (pressure ≤ 10 ⁻ ² Pa) can be used to avoid hydrogen embrittlement.
Example: When tempering silicon nitride ceramic bearings, a nitrogen flow rate of 8L/min can prevent material decomposition.
Reducing atmosphere
Hydrogen gas mixture: used for decarbonization treatment, the hydrogen ratio must be strictly controlled below 4% (explosion-proof limit).
Ammonia decomposition gas: provides active nitrogen atoms, suitable for tempering nitriding parts, with a higher decomposition rate.
Atmosphere monitoring
Oxygen probe: Real time display of oxygen content in the furnace, automatically replenishing inert gas when exceeding the standard.
Dew point meter: monitors the humidity of the atmosphere to prevent hydrogen embrittlement of the workpiece caused by water vapor.

4. Cooling treatment specifications
Cooling method selection
Air cooling: suitable for most carbon steel and alloy steel, with moderate cooling rate and small hardness change.
Oil cooling: Used for high-speed steel and mold steel, it is necessary to control the oil temperature (60-80 ℃) to prevent cracking.
Air cooling: For thin-walled parts or precision bearings, compressed air cooling is used with controllable cooling rate.
Cooling rate control
Low temperature tempered parts: naturally cool after being taken out of the furnace to avoid secondary hardening caused by rapid cooling.
High temperature tempered parts: can be quickly cooled to below 300 ℃, and then air-cooled to room temperature.
Example: W18Cr4V high-speed steel is tempered at 560 ℃ and then oil cooled to below 100 ℃ before being removed to prevent cracking.
Cooling post-treatment
Stress relief: For large workpieces, low-temperature stress relief treatment at 100-150 ℃ can be performed after cooling.
Rust prevention: Apply rust proof oil on the surface of the workpiece to prevent oxidation after tempering.

5. Safety protection measures
personal protection
Wear: heat-resistant clothing, anti smashing shoes, goggles to avoid injury from high temperature splashes.
Operation: Do not touch the workpiece or furnace door directly with your hands, use fixtures to take and place.
Equipment safety
Grounding: Ensure reliable grounding of the muffle furnace casing to prevent electrical leakage.
Ventilation: Install exhaust devices near the furnace body to promptly discharge harmful gases (such as hydrogen gas leaks).
Fire prevention: Dry powder fire extinguishers should be equipped next to the furnace, and flammable materials should not be stored.
emergency response
Overtemperature alarm: Immediately cut off the power and open the furnace door for natural cooling.
Atmosphere leakage: Close the gas valve, turn on the exhaust, and evacuate personnel to a safe area.
Workpiece cracking: Record the cracking location and process parameters, adjust the tempering temperature or insulation time.

6. Common Problems and Solutions
Insufficient tempering (high hardness)
Reason: Low temperature or insufficient insulation time.
Solution: Re temper, increase the temperature by 10-20 ℃ or extend the insulation time by 30 minutes.
Excessive tempering (low hardness)
Reason: The temperature is too high or the insulation time is too long.
Solution: Shorten the holding time or lower the tempering temperature, and re quench if necessary.
Temper brittleness
Phenomenon: The impact toughness significantly decreases.
solve:
Avoid tempering in the brittle range of 250-400 ℃.
Adding alloying elements such as Mo and W to improve the resistance to tempering brittleness.
surface decarburization
Reason: Insufficient atmosphere protection or inadequate sealing of the furnace door.
Prevention: Check the sealing strip and increase the nitrogen flow rate to 12L/min.