Customized muffle furnace temperature cannot rise

The inability to increase the temperature of a customized muffle furnace may be caused by various reasons such as heating system failures, power and control issues, furnace structural defects, external interference, or improper operation. The following is a detailed investigation and solution:

1. Heating system malfunction
The heating system is the core component of the muffle furnace, and its failure can directly cause the temperature to fail to rise. The following aspects need to be checked in detail:
a. Heating element damaged
Phenomenon:
Heating elements (such as silicon molybdenum rods, silicon carbon rods, molybdenum wires, graphite rods) break or burn locally, resulting in abnormal resistance or open circuit.
Use a multimeter to measure the resistance of the heating element. If the resistance is much greater than the rated value (such as the cold resistance of a silicon molybdenum rod of about 10-20 Ω) or infinite, it indicates that the element is damaged.
reason:
Long term high temperature use leads to material aging (such as silicon molybdenum rods becoming brittle above 1600 ℃).
Frequent rapid cooling and heating can cause thermal stress cracking.
Corrosion of the atmosphere inside the furnace (such as the easy reduction of silicon carbon rods in a hydrogen reducing environment).
Solution:
Replace the heating element with the same specification and ensure good contact during installation (such as using a spring to press the ends of the silicon molybdenum rod).
Optimize process parameters to avoid temperature fluctuations exceeding 100 ℃/min.
Choose corrosion-resistant materials based on the atmosphere (such as using molybdenum wire heating in a hydrogen environment).
b. Insufficient power of heating element
Phenomenon:
The power of the heating element is lower than the design requirement (such as a design power of 10kW, but only 8kW in reality), resulting in slow heating or inability to reach the target temperature.
Measure the actual input power through a power meter, or observe whether the ammeter reading reaches the rated value (such as 10kW/220V equipment, rated current of about 45.5A).
reason:
Insufficient power supply voltage (such as nominal 380V but actual only 350V).
Heating element aging leads to an increase in resistance (such as a 30% increase in resistance after using silicon carbide rods).
Transformer or regulator failure causes a decrease in output voltage.
Solution:
Check if the power supply voltage is stable and equip a voltage regulator if necessary.
Replace aging heating elements or connect new elements in parallel to increase total power.
Inspect the transformer/regulator to ensure that the output voltage meets the requirements.
c. Unreasonable layout of heating elements
Phenomenon:
Uneven distribution of heating elements can lead to local overheating or cold zones (such as furnace corners where the temperature is more than 50 ℃ lower than the center).
Detect furnace temperature distribution through infrared thermal imager or multi-point thermometer.
reason:
During design, the spacing between heating elements is too large or critical areas are not covered.
The stacking of workpieces in the furnace hinders heat transfer (such as dense stacking of metal powder causing air flow obstruction).
Solution:
Redesign the layout of heating elements (such as adding sidewall heating or adopting a three-dimensional heating structure).
Optimize the placement of workpieces (such as using partitions to disperse workpieces and leaving airflow channels).

2. Power and control system issues
The stability of the power supply and temperature control system directly affects the heating efficiency, and the following aspects need to be carefully investigated:
a. Abnormal power supply
Phenomenon:
Voltage fluctuations (such as dropping from 380V to 340V) result in a decrease in heating power and a slower heating rate.
Use a voltmeter to monitor the power input and observe whether there are frequent fluctuations or phase loss (one phase of three-phase power is open circuit).
reason:
Voltage drop caused by excessive load on the power grid or aging of the lines.
The power cord of the device is too thin (such as a cross-sectional area of less than 4mm ²), which leads to an increase in resistance.
Solution:
Equipped with a voltage regulator or UPS power supply to ensure voltage stability within ± 5% of the rated value.
Replace the thick specification power cord (such as using 6mm ² copper core wire for 10kW equipment).
b. Temperature control instrument malfunction
Phenomenon:
The temperature control instrument displays a large deviation between the temperature and the actual value (such as displaying 1000 ℃ but only 800 ℃ in reality), or cannot output control signals.
Calibrate the instrument reading with a standard thermometer (such as a platinum rhodium thermocouple), or check the instrument output signal (such as whether the 4-20mA current is normal).
reason:
Instrument parameter setting error (such as PID parameter not optimized causing control oscillation).
Damage or poor contact of thermocouple/thermistor (such as signal attenuation caused by oxidation).
Solution:
Re calibrate the temperature control instrument or replace damaged thermocouples (such as replacing K-type thermocouples with S-type thermocouples to improve accuracy).
Adjust PID parameters (such as increasing integration time Ti to reduce overshoot).
c. Solid state relay (SSR) or contactor damaged
Phenomenon:
The contact of the SSR/contactor is burned or the coil is broken, causing the heating element to be unable to sustain power.
Use a multimeter to check if the SSR input/output terminals are conductive (e.g. the output terminal should be closed when 24V is input).
reason:
Frequent switching can cause contact arc erosion (such as short control cycle of temperature control instruments).
The load current exceeds the rated value of SSR (such as 10A SSR driving 15A load).
Solution:
Replace the high-capacity SSR or contactor (such as using 20A SSR for 10A load).
Optimize the control cycle of temperature control instruments (such as extending the sampling time to more than 1 second).

3. Furnace structure and insulation defects
The insulation performance and sealing of the furnace directly affect the heat retention efficiency, and the following steps need to be checked:
a. Aging of insulation materials
Phenomenon:
The insulation layer (such as alumina fiber, asbestos) shrinks or cracks, causing accelerated heat loss (such as furnace wall temperature rising from 50 ℃ to 100 ℃).
Disassemble the furnace shell to observe the condition of the insulation material, or use an infrared thermal imager to detect the heat loss of the furnace wall.
reason:
Long term high temperature use leads to grain growth and density reduction of insulation materials (such as a 30% decrease in performance of alumina fibers after 2 years of use at 1600 ℃).
Mechanical vibration or frequent opening of the furnace door can cause damage to the insulation layer.
Solution:
Replace with new insulation materials (such as nanoporous calcium silicate board, thermal conductivity ≤ 0.03W/m · K).
Add metal protective plates (such as stainless steel plates) outside the insulation layer to prevent mechanical damage.
b. Poor furnace sealing
Phenomenon:
The furnace door, observation window, or thermocouple interface leaks air, causing cold air to enter the furnace (such as furnace pressure rising from -50Pa to 0Pa).
Apply soapy water to the sealed area and observe for any bubbles.
reason:
Aging of sealing ring (such as hardening and cracking of fluororubber ring after 1 year of use at 250 ℃).
Loose hinges on the furnace door result in loose closure.
Solution:
Replace the high-temperature resistant sealing ring (such as silicone rubber ring used in scenarios below 300 ℃).
Adjust the furnace door hinge or add a locking device (such as a spring compression mechanism).

4. External interference factors
The external environment and operating methods may indirectly affect the heating efficiency, and the following details should be noted:
a. The ambient temperature is too low
Phenomenon:
In winter or low-temperature workshops, additional energy is required to heat the furnace body and surrounding air when starting the muffle furnace (such as extending the heating time by 20% when the ambient temperature is 0 ℃).
Solution:
Place the muffle furnace in a constant temperature workshop (such as 20-30 ℃), or add a preheating device (such as wrapping the furnace body with an electric heat tracing belt).
b. Excessive workpiece load
Phenomenon:
The total mass of the workpieces in the furnace exceeds the design load (such as a design load of 50kg but actual placement of 100kg), resulting in heat being absorbed by the workpieces and unable to be replenished in a timely manner.
Solution:
Reduce the amount of workpieces processed in a single operation, or perform heat treatment in batches.
Optimize the placement of workpieces (such as using trays to distribute weight and avoid local overheating).

5. Improper operation
User operation errors may cause temperature anomalies, and the following procedures need to be standardized:
a. The heating rate is set too fast
Phenomenon:
The temperature control instrument is set to a heating rate of 50 ℃/min, but the actual power of the heating element is insufficient, resulting in temperature stagnation.
Solution:
Re calculate the reasonable heating rate based on the power of the heating element and the heat capacity of the workpiece (for example, if a 10kW equipment processes 10kg of metal, it is recommended that the heating rate be ≤ 20 ℃/min).
b. Not subjected to preheating treatment
Phenomenon:
The direct heating of the cold furnace to high temperatures (such as from room temperature to 1600 ℃) causes excessive thermal stress on the heating element and leads to its fracture.
Solution:
Adopting a segmented heating process (such as raising the temperature to 500 ℃ for 1 hour and then raising it to the target temperature).