In the realm of induction melting, the frequency choice is not a case of “the higher the better” hoặc “the lower the better.” Medium frequency (typically between 150Hz and 8000Hz, with 500Hz – 1000Hz being the most common for melting scrap steel and special alloys) is dubbed the “golden frequency band” because it strikes the perfect physical balance between heat penetration Và động lực học chất lỏng.
For irregularly shaped scrap steel and special alloys requiring extremely high purity, medium frequency demonstrates irreplaceable metallurgical advantages. Here is an in-depth breakdown of its core mechanisms:
1. The Perfect Match of Skin Effect and Penetration Depth
Induction heating relies on the “Hiệu ứng da” — the higher the alternating current frequency, the more the induced current concentrates on the metal surface; the lower the frequency, the deeper the current penetrates.
- The Limitations of Line Frequency (50/60Hz): The penetration depth is extremely large, requiring large, dense blocks of metal to effectively form eddy currents. When faced with irregularly shaped, highly porous scrap steel, its heating efficiency is extremely low.
- The Limitations of Tần số cao (>10kHz): The penetration depth is very shallow, concentrating energy on the surface. Heat can only travel inward via conduction, easily causing the scrap surface to overheat or even oxidize and vaporize, while the inside remains unmelted.
- Các Tần số trung bình Golden Mean: The penetration depth of medium frequency perfectly cuts through loosely stacked scrap steel of varying sizes. It enables individual pieces of scrap to heat up and melt rapidly throughout, making it the best choice for processing baled scrap, steel chips, or irregular return materials.
2.Các “Just Right” Lực khuấy điện từ
Inside the induction coil, the molten metal is pushed and stirred by electromagnetic forces. The magnitude of this stirring force is inversely proportional to the square root of the frequency. This is a critical consideration when melting special alloys:
- Stirring Too Strong (Line Frequency): The molten metal surface churns violently, even creating towering “humps.” This aggressive turbulence not only severely washes and erodes the furnace lining but also exposes a large area of molten metal to the air. For special alloys containing easily oxidized elements like titanium, nhôm, or chromium, this leads to severe gas absorption, quá trình oxy hóa, and burning loss.
- Stirring Too Weak (Tần số cao): The molten metal is almost completely still. When adding alloy elements with significant specific gravity differences (like tungsten or molybdenum), severe gravity segregation occurs at the bottom, resulting in an uneven composition within the furnace.
- The Virtuous Cycle of Medium Frequency: Medium frequency generates a moderate electromagnetic stirring force, resulting in a smooth, slight bulging of the liquid surface. This benign top-to-bottom rolling action ensures that added special alloy elements melt and blend rapidly and evenly, allows non-metallic inclusions to smoothly float up into the slag, and simultaneously minimizes the oxidation area of the liquid surface.
3. Cold-Start Flexibility (Production Scheduling Advantages)
Khi khởi động, line frequency furnaces typically require retaining a portion of the molten metal from the previous heat (a molten heel) or using specially machined, large starting blocks. If the furnace is completely emptied, a line frequency furnace struggles to restart directly with cold scrap.
Medium frequency furnaces completely eliminate this restriction. They can be 100% emptied and immediately restarted at full power using cold scrap steel. For foundries that frequently change steel grades or melt different special alloy specifications, this means no cross-contamination occurs, granting immense flexibility in production scheduling.
4. High Power Density and Ultra-Fast Melting
Due to the characteristics of medium frequency power supply technology, it allows for a massive power input into a smaller furnace volume (mật độ năng lượng cao).
The significant increase in melting speed brings two direct metallurgical benefits:
- Extremely Low Burning Loss: The time the metal is exposed to high temperatures in both solid and liquid states is drastically reduced. The oxidation burning loss of expensive alloy elements is minimized, significantly improving the recovery rate.
- Maximized Hiệu suất nhiệt: Fast melting means the proportion of heat lost through the furnace body is smaller, making the overall electrical energy utilization far superior to traditional equipment.
Core Summary: Medium frequency is not a compromise, but a precise control. It utilizes a moderate penetration depth to solve the melting efficiency problem of scrap steel, and employs gentle khuấy điện từ to overcome the challenges of homogenization and oxidation resistance in special alloys.
