The quality of the furnace lining is one of the key factors affecting the performance of coreless induction furnaces. Due to operators’ limited familiarity with induction furnaces, short lining service life is a common issue. By employing different vibrators to compact dry vibrated materials during the lining construction process, we can extend the lining’s lifespan, reduce labor intensity, and lower production costs.

Materials used in this experiment include: furnace lining ejection mechanism, furnace bottom pneumatic vibrator, Jolter pneumatic hammer, Martin pneumatic vibrator, furnace lining material, mica paper, ceramic fiber cloth, furnace lining crucible mold, furnace bottom grounding device, thermocouple, tamping and degassing tools, leveling gauge, and other tools.

1.Removal of the old furnace lining

When severe erosion of the furnace lining necessitates major repairs, after production tasks are completed, allow the electric furnace to cool to room temperature through air cooling. Tilt the furnace body to a horizontal position. Install the hydraulic ejection mechanism at the designated location on the furnace bottom. Eject the old furnace lining. Remove the furnace bottom ejection mechanism. After returning the furnace body to an upright position, clean the iron residue adhering to the inner walls of the electric furnace. Repair the furnace walls and set them aside for future use.

2.Repairing the furnace nozzle

The furnace nozzle is frequently damaged during prolonged operation due to the erosion caused by molten iron. Therefore, prior to refractory lining construction, the nozzle must first be repaired. This creates a vertical refractory interface, reducing the likelihood of metal leakage through horizontal cracks beneath the nozzle.

3.Caking on the bottom of the furnace

Remove the bottom ejector block from the old furnace lining. Use dry compressed air to blow away impurities from the block’s surface, then slowly lower it into the furnace bottom. Install the grounding device at the small hole on the ejector block and secure it tightly to the bottom. Lay a layer of ceramic fiber cloth flat on the ejector block. Place a layer of mica paper close to the furnace walls around the perimeter and secure it in place. Then evenly pour in approximately 7 bags of dry vibrated material. Place the furnace bottom pneumatic vibrator inside, maintaining a working air pressure of 0.6 MPa. After vibrating for 30 minutes, remove the vibrator. Use a leveling tool to scrape the surface of the dry vibrated material until the probe of the grounding device is exposed.

4.Selection of Furnace Crucible Molds

The crucible mold for furnace construction is fabricated from 6mm steel plate, compatible with the metal being melted. Uniformly spaced vent holes are drilled around the perimeter, with rounded edges and a slope less than 5°. The exterior surface is smooth and rust-free, with welded areas polished. The thickness must be appropriate. If the crucible mold is too thin, it will cause excessive heat dissipation, lower impact resistance, and shorten the lining life. If the thickness is excessive, it will be detrimental to induction heating and affect production efficiency.

5.Bonding and Compaction of the Furnace Body

Once the furnace bottom has been fully compacted, carefully position the furnace lining mold to ensure it is concentric with the furnace body. Then, evenly add dry vibratory material into the gap between the mold and furnace wall, adding 3-4 bags at a time while simultaneously compacting and degassing using tamping and degassing tools. Throughout the entire process, strictly prevent foreign objects such as paper scraps from contaminating the dry vibratory material.

Since the Jingya Division utilizes a configuration where one power source supplies two furnaces, to enhance the comparability of experimental results, the Jolter vibrator was used for furnace #1 and the Martin vibrator for furnace #2. Both furnace bodies were compacted by vibration for 2 hours each. Before commencing vibration, the crucible mold was secured with steel plates to prevent displacement during the process. Throughout vibration, closely monitor the settling of dry vibrated material at the furnace top. Continuously replenish dry vibrated material around the perimeter until no further settling occurs at the top, then cease vibration.

6.Sintering of the furnace body

After the furnace body has been properly sealed, load the pig iron and install the thermocouples. Then commence the furnace sintering process. The sintering process consists of three stages.

(1) First Stage. Heat the furnace charge to 1000–1100°C at a rate of 100°C/h. This stage lasts 9–10 hours, followed by a 3-hour soak within this temperature range. The soak prevents cracking in the furnace lining caused by excessive crystallization rates, thoroughly removes moisture from dry vibrated materials, and ensures uniform temperature distribution throughout the lining.

(2) Second Stage. Utilizing induction energy, melt 90% of the initial charge at a heating rate of 100°C/h. Continue feeding material before complete melting while preventing material bridging. Once the temperature reaches 1600°C, hold for 3 hours, ensuring the furnace temperature does not exceed 1625°C. This stage aims to form an initial sintered layer on the furnace lining.

(3) Third Stage. During high-temperature sintering, the structure formed by the furnace lining directly impacts its service life. Insufficient sintered layer thickness reduces longevity. After sintering is complete, power is shut off. The furnace is allowed to cool naturally to the normal iron tapping temperature before production commences.