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In China, casting primarily provides castings for mechanical products. However, casting is also a primary industry characterized by high energy consumption and low profitability. In the casting production process, energy and material inputs account for more than half of the output value. Therefore, it is an important task for the casting industry to make rational use of energy, conserve energy, and reduce consumption.
Melting is an important process in the casting production cycle. The energy consumption associated with melting accounts for approximately half of the total energy consumption in the entire casting production process. Medium-frequency induction furnaces for melting offer advantages such as high quality, high efficiency, flexibility, stability, environmental friendliness, and controllability, and are now widely used in casting production.
1. Improve Medium-frequency induction furnace Crucible life
The crucible is an essential component of the medium-frequency induction furnace equipment system. During the smelting process, the crucible is subjected to high temperatures and chemical erosion from the slag; it also endures the impact of charge materials during feeding. Particularly in intermittent production, the crucible undergoes severe thermal cycling—rapid heating and cooling—as it is repeatedly heated and allowed to cool down between batches. In severe cases, this thermal shock can cause molten metal to penetrate through the furnace lining, rendering the crucible unusable. Currently, the service life of a crucible subjected to repeated rapid heating and cooling typically ranges from 70 to 80 melting cycles. The primary failure modes leading to crucible replacement are cracking of the furnace walls and chunky spalling of the furnace bottom, which can result in molten iron leaking out of the furnace. Extending the service life of medium-frequency induction furnace crucibles not only increases the output of castings but also reduces the labor intensity for workers and lowers the overall cost of casting production. To achieve a longer crucible lifespan, the following key measures must be taken:
1) Select furnace lining materials based on the smelting material, production schedule, and furnace capacity.
2) Select refractory materials with good performance, high purity of active ingredients, few impurities, and a properly balanced particle size distribution.
3) Select and apply appropriate furnace-building techniques, strengthen process control during furnace construction, and pay close attention to the bonding, compactness, and uniformity of each part of the furnace lining.
4) Strictly adhere to the furnace baking and sintering procedure. During the sintering stage, the temperature should be 50℃ to 80℃ higher than the usual iron tapping temperature, and hold this temperature for 2 to 3 hours. Control the heating rate carefully: during the low-temperature stage—before reaching 900℃—the heating rate should be 1.2℃/min. This facilitates the release of free water and crystal water from the lining materials. After holding at 900℃ for 2 to 3 hours, quartz sand will undergo a stable phase transformation. If the heating rate is too fast, the quartz sand will undergo an abrupt phase transition, thereby shortening the furnace’s service life. Ensure sufficient baking time to allow the moisture in the furnace lining to gradually evaporate. Make sure that the first firing of a new furnace is properly carried out, aiming to achieve a reasonable three-layer structure in which the initial thicknesses of the sintered layer, semi-sintered layer, and buffer layer each account for approximately one-third of the total lining thickness.
5) Properly implement all furnace operation procedures to minimize the occurrence of defects. Regularly maintain and care for the medium-frequency induction furnace equipment system and the crucible to extend the service life of the furnace lining.
2. Energy Saving During the Operation of Medium-Frequency Induction Furnaces
As is well known, a medium-frequency induction furnace uses a medium-frequency power supply to generate a medium-frequency magnetic field, enabling... Crucible Induction eddy currents generated in ferromagnetic materials produce heat, thereby achieving the purpose of melting furnace charge. During medium-frequency induction melting, improper operation can lead to increased energy consumption. Therefore, it is essential to standardize operations and implement scientific management throughout the entire process—from preparation of furnace charge, to charging, melting, and tapping—to achieve energy savings, reduce consumption, and improve energy utilization efficiency.
2.1 Preparation of the Charge Material
The preparation of furnace charge plays a significant role in energy conservation and consumption reduction. If the furnace charge contains 4% impurities, 4% of the energy will be consumed simply to melt these impurities. Therefore, it is essential to sandblast the furnace charge as much as possible. The size of the furnace charge pieces should be selected based on the inner diameter and height of the crucible. Furnace charge pieces that are too large or too small can both adversely affect the efficiency and quality of melting. Generally, the ratio of furnace charge diameter to crucible diameter should be between 0.28 and 0.3. When melting steel and using starter blocks, the diameter of these blocks should be 20–30 mm smaller than the crucible diameter, their height should be one-third of the crucible height, and their mass should account for 40%–50% of the total crucible capacity. If scrap steel (or iron) chips are used as furnace charge, they should be baled; large and long pieces of charge should be cut into smaller sizes. Before loading the furnace charge, it should be preheated to remove moisture, oil stains, volatile substances, and other impurities. This not only ensures the safety of the melting operators but also helps save electricity and improve the quality of the molten metal. It is crucial to understand the chemical composition of various types of furnace charge, perform batch calculations, and make rational combinations of charge materials to ensure that the molten metal meets the required specifications.
2.2 Feeding
The compactness of the charge directly affects the melting rate of the furnace charge; therefore, the packing density at the bottom and middle of the crucible, as well as throughout the entire charge, should be as high as possible. When loading the charge, pack the materials tightly at the bottom and loosely toward the top, and mix large and small pieces appropriately. Small pieces or lightweight, thin scrap materials should not be added when the furnace is cold; instead, they should be introduced directly once there is molten metal present, thereby reducing energy consumption. When using the scrap steel carbon-increasing melting process, first add the carbon-increasing agent (adding it in batches to the bottom of the furnace), then add the return charge, followed by the scrap steel. If there is residual molten iron remaining, wait until the carbon-increasing agent at the bottom of the furnace and the return charge have turned red-hot before adding the remaining iron.
Large electric furnaces should be equipped with charging machines and furnace charge preheating devices to reduce charging time, shorten the melting cycle, and improve production efficiency.
2.3 Melting Operation
There should be a reasonable power supply system. Typically, when power is first applied, the power level should be around 60%. Once the inrush current has subsided, the power should be quickly adjusted to its maximum value to accelerate the melting rate. Employing proper pre-furnace operational techniques will help control the subsequent addition amounts precisely. Adopt a feeding method of adding small quantities multiple times rather than adding large amounts all at once during cold starts. If the conditions of the molten metal in the furnace permit, the charge materials can be preheated by placing them near the furnace opening. It’s crucial to observe frequently, stir the charge regularly, and add materials promptly to prevent bridging of the charge. Charge bridging can cause the molten metal in the furnace to remain heated for an extended period, leading to excessively high temperatures. Moreover, forcibly stirring or impacting the furnace lining when attempting to break up bridging can significantly shorten the crucible’s service life.
During the smelting process, whether melting steel molten metal or using scrap steel to increase carbon content and melt iron molten metal, it is necessary to perform pre-furnace chemical composition analysis. At this stage of the smelting operation, the power should be reduced, and measures such as heat preservation or temperature reduction should be adopted to minimize the erosion of the furnace lining caused by the high-temperature molten metal. Once the analysis results are available and the composition meets the specifications, the temperature should be rapidly increased to reach the required tapping temperature. The operating procedures for electric furnace smelting are as follows:
1) After confirming that the furnace lining and furnace mouth are free from any damage due to burning, you may start the medium-frequency power supply only after adding clean charge material equivalent to one-third of the crucible’s capacity. The principle for adding charge material is as follows: Place small, dense pieces at the bottom of the crucible; once a molten pool has formed, add lighter, thinner materials and larger pieces. When loading the crucible, aim to achieve the highest possible density of metal charge material.

2) Within 4 to 8 minutes after power-on, supply approximately 40% to 60% of the maximum power (i.e., 800 kW to 1200 kW), then gradually increase the power output to its maximum value.

3) Once the molten iron has melted to more than one-third of its total volume, gradually add the carbon-increasing agents. Do not add all the carbon-increasing agents at once. Note that the carbon-increasing agents must be added together with the charge materials.

4) During the melting process, frequently check the wear condition of the furnace lining, the various instruments on the intermediate-frequency power supply cabinet, and the circulation status of the cooling water. There must be no instances of charge material being suspended or bridged within the furnace chamber. If such a situation occurs, promptly tamp down the charge to prevent “bridging.”

5) When 95% of the charge material has melted, take a sample for compositional analysis and then add the remaining 5% of the charge material into the furnace to continue melting. To prevent molten iron from overflowing, maintain a distance of 100 mm between the molten iron level and the furnace rim.

6) After all charge materials have melted completely, reduce the power to 40%–60%, tilt the furnace to remove slag, and measure the temperature. Only when the molten iron meets the process requirements can it be tapped.

7) To ensure the melting rate of the molten iron in the furnace, approximately 300 kg of molten iron should be retained in the furnace, and new charge materials should be added promptly while continuing to supply power for smelting.

8) If production is not continuous, the last batch of molten iron must be completely drained; no residual molten iron is allowed. After the furnace is shut down, the cooling water must not be stopped immediately. The circulating water pump may only be turned off after the cooling water has been circulated continuously for 6 hours.
2.4 Controlling the tapping temperature of molten metal
On the premise of ensuring casting quality, reduce the superheat temperature of the molten steel (or iron) as much as possible to shorten the melting time and save energy. Promptly apply a covering agent on the surface of the ladle containing molten steel (or iron) to prevent the molten steel (or iron) from coming into contact with air, thus avoiding oxidation, gas absorption, and rapid cooling.
2.5 Extend the continuous smelting time
Specific energy consumption for electricity is related to the melting process. To optimize production, we should arrange for concentrated, continuous melting as much as possible—increasing the number of batches melted in a single furnace, extending the duration of each continuous melting cycle, and reducing the frequency of cold-furnace melts—thereby achieving the goal of energy conservation.
Through the above introduction, we have gained a general understanding of the energy-saving issues related to medium-frequency induction furnaces.