Common problems in magnesium oxide refilling sintering include improper temperature control, insufficient holding time, lack of atmosphere protection, uneven filling, and excessively rapid cooling. These problems can directly lead to defects in the insulation layer structure, resulting in substandard performance or breakdown during operation.
1. Improper Temperature Control
Too low a temperature (<800℃): Fails to activate magnesium oxide grain recrystallization, leading to a loose structure and insufficient insulation strength;
Too high a temperature (>1050℃): May cause magnesium oxide volatilization, metal tube deformation, or sealing material failure;
Too rapid heating rate (>5℃/min): Generates thermal stress, leading to tube cracking or internal micro-cracks.
It is recommended to use a PID intelligent temperature control system, along with calibrated thermocouples, to ensure temperature control accuracy within ±10℃.
2. Insufficient Holding Time
Holding time < 1 hour: Grain growth is insufficient, recrystallization is incomplete, affecting insulation density;
Symptoms: Cold insulation resistance barely meets standards, but drops rapidly in hot or humid conditions.
In practice, 30% of rework failures stem from shortening the holding time to meet deadlines.
3. Lack of Protective Atmosphere
Failure to use high-purity nitrogen or a vacuum environment: Magnesium oxide reacts with oxygen and moisture at high temperatures, generating conductive impurities;
High furnace humidity (RH > 30%): Leads to secondary moisture absorption, resulting in failure in humid conditions;
Insufficient gas flow: Uneven distribution of protective atmosphere, leading to localized oxidation or carbonization.
Sintering must be performed in an inert atmosphere with an oxygen content < 100 ppm to prevent material deterioration.
4. Defects in Filling and Compaction Processes
Insufficient magnesium oxide powder purity (<99.9%): Impurities such as Cl⁻ and SO₄²⁻ form electrochemical corrosion channels;
Uneven filling density (<2.8 or >3.2 g/cm³): Too low a density leads to loosening, while too high a density causes stress concentration;
Lack of high-frequency vibration compaction: Internal voids exist, leading to localized overheating and breakdown during operation.
X-ray inspection can effectively detect hidden defects such as core eccentricity and voids.
5. Incorrect Cooling Process
Forced air cooling or water cooling: Rapid cooling causes thermal stress mismatch between the metal and magnesium oxide layers, resulting in cracks;
Furnace exit temperature >200℃: Residual heat causes aging or deformation of the sealant.
Natural cooling with the furnace is necessary, reducing the temperature from 1000℃ to below 200℃ within 1–2 hours.
6. Equipment and Environment Failure to Meet Standards
Insufficient cleanroom class (>100,000): Dust contamination creates weak points in insulation;
Insufficient hydraulic clamping force: Insufficient axial pressure leads to loose filling;
Weared or misaligned molds: Cause pipe deformation or seal misalignment.
Non-professional workshops often experience batch repair failures due to rudimentary equipment.
