Induction Furnace Melting Process and Quality Control

Target Audience: Metallurgical Engineers, Melting Supervisors, QA/QC Engineers

1. Charge Material is Everything: Pre-treatment & Management

แนวคิดหลัก: “Garbage In, Garbage Out.” The primary function of an induction furnace is melting, not refining; therefore, source control is non-negotiable.

• การจำแนกประเภท & Management:
  • Chemical Segregation: Strictly separate Carbon Steel, Low-alloy Steel, สแตนเลส, and Cast Iron returns. Trace amounts of Cr หรือ ลูกบาศ์ก mixing into plain carbon steel can cause mechanical property failures (เช่น, reduced elongation).
  • Size Configuration: Place small material at the bottom to protect the lining, heavy blocks in the middle, and fill gaps with smaller scrap. Packing Density directly affects coupling efficiency and melt rate.
• ความสะอาด:
  • สนิม & Sand Removal: Heavy rust ( เฟ2O3 · H2O) introduces significant Oxygen and Hydrogen, leading to increased slag and porosity.
  • Degreasing: Cutting fluids and oil on machine chips are sources of Carbon and Sulfur pickup, and are primary culprits for smoke and Hydrogen porosity.
• อุ่นเครื่อง & การอบแห้ง:
  • ความปลอดภัย: Eliminate moisture to prevent steam explosions.
  • ประสิทธิภาพการใช้พลังงาน: Preheating to 300°C–500°C can significantly reduce electrical consumption.

2. Effective Deslagging: Metal Purity & Lining Protection

แนวคิดหลัก: Slag is a “protective coat” for the melt but a “trash can” for inclusions. Improper handling makes it a “killer” of furnace linings.

• Selection of Slag Coagulants:
  • Use Perlite or specialized slag removers. High-quality coagulants should expand and bind immediately upon contact, forming a crust that is easy to skim.
  • คำเตือน: Avoid fluxes with high Sodium (Na) เนื้อหา. While they fluidize the slag, they aggressively attack Acidic (ซิลิกา) linings.
• Timing of Deslagging:
  • Post-Melt Down: Skim to remove the bulk of the dirty material once fully melted.
  • Pre-Tap: Must skim before adding expensive ferroalloys to prevent them from being trapped in the slag and oxidized.
  • “Slag Cover” กลยุทธ์: In specific processes, maintaining a very thin layer of slag can prevent the molten steel from aspirating gases (เอ็น, ชม), though this requires skilled operators.
• Lining Protection:
  • Avoid mechanical impact on the furnace walls during manual skimming.
  • Control Slag Basicity to prevent “Chemical Erosion.” Acidic linings suffer from basic slag (สูง แคลเซียมโอ); Basic linings suffer from acidic slag (สูง SiO2).

3. การควบคุมอุณหภูมิที่แม่นยำ: The Dual Risks

แนวคิดหลัก: Temperature is the lifeline. Unlike an Electric Arc Furnace (EAF), an Induction Furnace has limited refining capabilities; overheating usually causes more harm than good.

• Measurement Methods:
  • Disposable Dip Thermocouple: The Gold Standard. Ensure the tip penetrates the slag layer and reaches the center of the melt (typically ~300mm below the surface).
  • Infrared/Optical Pyrometers: Use only for trending. They are easily affected by fumes, slag cover, and emissivity changes, leading to data drift.
• Consequences of Improper Control:
  • ความร้อนสูงเกินไป:
    • Gas Pickup: Solubility of H2 และ N2 increases exponentially with every 100°C rise.
    • การกัดเซาะของซับใน:SiO2 + 2C → และ + 2บจก (Crucible reduction by Carbon at high temps).
    • Coarse Grains: Leads to reduced impact toughness in the final casting.
  • Under-heating:
    • Inclusion Retention: Stokes’ Law dictates that inclusions float slower in cooler, more viscous metal.
    • Misruns/Cold Shuts: Poor mold filling capability.

4. ปฏิกิริยาโลหะ: การทำออกซิเดชั่น & การผสม

แนวคิดหลัก: Even in “dead melting,” chemical reactions occur. Electromagnetic stirring is a double-edged sword.

• Deoxidation Regimes:
  • Precipitation Deoxidation: Must be done before tapping. Typical sequence: Weak to Strong (Mn → Si → Al).
  • ดีออกซิเดชันขั้นสุดท้าย: Stream inoculation or addition of final deoxidizers (เช่น, Ca-Si, Rare Earths) during tapping to modify inclusion morphology.
• Alloying Strategy:
  • Addition Sequence:
    • Refractory elements (เช่น, โม, Cr) should be added early.
    • Oxidizable elements (เช่น, ของ, อัล, บี) should be added late, after good pre-deoxidation.
    • Carbon Adjustment: Recarburizers (กราไฟท์) should be added with the solid charge. Late additions have very poor recovery (<60%).
• การควบคุมองค์ประกอบ:
  • Silicon Pickup: ที่อุณหภูมิสูง, Carbon reduces the Silica lining, potentially causing unintended spikes in และ content during long holds.
  • Mn/S Ratio: Maintain Mn/S > 10 to prevent hot tearing/shortness.

5. การแก้ไขปัญหา: Reverse Engineering from Defects

แนวคิดหลัก: The casting is the “กล่องดำ” that records melting quality.