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The service life of an electric furnace crucible is primarily affected by factors such as the refractory lining material, the furnace baking process, and the operating procedures. The following sections will introduce these aspects in detail.

The quality and performance of the furnace lining refractory materials play a fundamental role in determining the service life of the furnace.

 

 

(1) Refractory materials used for furnace construction must have a sufficiently high refractoriness, resisting the effects of high-temperature thermal loads without softening or melting. At the same time, they must also exhibit excellent dimensional stability under high-temperature thermal loads, with no shrinkage in volume and only uniform expansion.

(2) Composition of refractory materials. Impurities in refractory materials can form low-melting-point compounds at high temperatures, thereby reducing the refractoriness of the materials. Therefore, the lining sand must have a guaranteed quartz content and strictly controlled impurity composition. The compositional specifications for the lining sand are as follows: ω(SiO2) ≥ 98.0%; ω(Al2O3) ≤ 0.5%; ω(Fe2O3) ≤ 0.5%; ω(TiO2) ≤ 0.05%; ω(H2O) ≤ 0.5%.

(3) Particle Size Distribution of Refractory Materials. A reasonable particle size distribution of refractory materials facilitates the formation of a dense refractory layer during furnace construction and reduces the likelihood of defects during service. Below is a set of relatively optimal proportions: 17% for particles ranging from 3.35 mm to 5 mm, 33% for particles ranging from 0.85 mm to 1.70 mm, 20% for particles ranging from 0.1 mm to 0.85 mm, and 30% for particles smaller than 0.1 mm.

The firing process is designed to produce a sintered layer, and the quality of this sintered layer directly affects the service life of the furnace. Therefore, the firing process is an important step.

 

 

After the furnace chamber is completed, it should be immediately put through the baking process. Before starting the baking, check whether the electrical equipment and cooling water system are functioning properly. During the baking process, strictly adhere to the prescribed baking procedure. The baking procedure is a critical factor in the entire baking process; specific key points include: ① Control the heating rate carefully, especially during the early stages of baking. If the heating rate is too fast, the moisture in the furnace lining will be expelled too rapidly, easily leading to cracking and significantly shortening the furnace’s service life. ② When the furnace lining is heated to 573℃, the rapid expansion during the quartz phase transition can easily cause cracks or even spalling. Therefore, between 400℃ and 600℃, the heating rate should be slowed down. At 870℃, the furnace should be held at this temperature for 1 to 2 hours to allow for a slow and complete phase transformation. ③ The final stage of the baking process is sintering and heat preservation. The sintering temperature depends on the specific refractory material used; generally, it is desirable to obtain a sintered layer that is about 30% as thick as the furnace lining. Consequently, the sintering temperature is typically 50 to 100℃ higher than the tapping temperature.

Various processes involved in the operation of the furnace are also crucial to its service life; improper handling in any aspect can significantly shorten the furnace's lifespan. Therefore, during operation, you should pay attention to the following points.

 

 

(1) Since the sintering layer of a new furnace is relatively thin, the operating procedures for using a new furnace are particularly important. For the first batch of molten metal produced from a new furnace, it is advisable to melt 50% of the charge before starting the pour—this approach helps prevent sudden cooling of the furnace lining due to adding charge after the entire batch has been poured, thereby avoiding defects such as cracks. Whenever possible, a new furnace should undergo continuous melting to avoid cracks caused by intermittent melting that results in fluctuating temperatures. Typically, continuous melting should be maintained for at least one week.

(2) During the melting process, avoid high-temperature melting as much as possible. At high temperatures, the furnace lining will undergo a crucible reaction with the molten iron, as shown in the following equation: SiO2 + 2C → Si + 2CO. The higher the temperature, the greater the carbon content, and the lower the silicon content, the more severe the erosion of the furnace lining will become—this effect is particularly pronounced when the furnace is new. Therefore, during melting, while ensuring that the molten iron reaches the required outlet temperature, try to avoid operating at excessively high temperatures.

(3) Avoid overheating the furnace lining. Since medium-frequency furnaces heat up quite rapidly, if the melting operator is not careful, the charge material may “bridge,” causing localized high temperatures in the furnace lining—temperatures that could even exceed the lining’s refractory limit. This could potentially lead to melting and erosion of the furnace lining.

(4) During operation, if the furnace needs to be shut down for an extended period due to malfunctions or other reasons, the molten iron inside the furnace should be completely drained to prevent the cooling and solidification of the molten iron from causing cracks in the furnace lining and thereby damaging it.

(5) During operation, especially when using a new furnace, try to use clean furnace charge materials as much as possible.

(6) During furnace shutdown and cooling, to prevent the furnace lining from undergoing sudden cooling, an empty-furnace cooling procedure should be adopted. At the same time, to avoid excessive temperature differences between the top and bottom of the furnace lining during cooling, which could lead to cracking, the furnace lid should be kept closed, ensuring uniform cooling throughout the lining and thereby extending the service life of the furnace.

(7) Since vertical cracks are inevitable during furnace cooling, when starting up a cold furnace, it is advisable to first perform low-temperature baking before proceeding with melting. This allows the cracks to close up initially, thereby preventing molten iron from seeping into the cracks during melting and causing them to expand further.

(8) During furnace operation, it is important to closely monitor the furnace condition. Properly observing the furnace condition is itself a form of furnace protection. Measure the furnace bottom every three days, and inspect the furnace walls daily for each furnace, thereby ensuring the safety of the furnace lining.

(9) Perform maintenance and upkeep on the electric furnace equipment, such as regularly cleaning the coil and removing any debris from its surface to prevent coil breakdown. This helps avoid furnace disassembly caused by equipment failures and effectively extends the service life of the crucible.