Silica ramming mass is widely applied for linings of induction furnaces and heating furnaces. Excessively fast heating or insufficient moisture removal during baking readily causes cracks and surface spalling, greatly shortening lining service life. Quartz undergoes crystal transformation with abrupt volume change at 573 ℃, which easily generates thermal stress. In view of this material property, standardized furnace-baking control points are summarized as follows:
1. Pre-baking Basic Inspection
After ramming completion, carry out natural curing for no less than 24 hours to fully release free water. Inspect the lining for loose interlayers; reserve expansion joints per drawings and fill them with cushion filler. Unblock smoke-vent passages to avoid lining blistering and cracking caused by accumulated water vapor.
2. Segmented Temperature-controlled Baking
Heat slowly and hold constant temperature from room temperature to 200℃ with continuous ventilation to remove free water. Slow down the heating rate and extend holding time from 200℃ to 573 ℃, so as to smoothly pass the crystal-transformation range and release internal thermal stress. A moderate heating-rate increase is allowed above 573℃. Hold sufficient constant-temperature soaking after reaching working temperature before putting into service. Maintain slight negative-pressure smoke exhaust throughout the whole process for uniform furnace heating.
3. Key Anti-cracking Precautions
Avoid intense local firing to reduce temperature difference inside and outside the lining. Keep furnace doors partially open for moisture dissipation at low-temperature stages; close them only after high-temperature conditions are achieved. Never open doors for rapid cooling upon shutdown. Cool down gradually together with the furnace to prevent through-cracks induced by thermal shock.
Furnace-baking cracking mainly stems from three causes: inadequate moisture removal, improper temperature control within the crystal-transformation range, and sharp temperature swings. Strict implementation of pre-service curing, segmented gentle baking, steady moisture discharge and gradual-cooling procedures can effectively prevent cracking defects, extend lining service cycles and cut enterprise costs from maintenance and production shutdowns.
