Induction Furnace Melting Process and Quality Control

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

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

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

• Classification & Management:
  • Chemical Segregation: Strictly separate Carbon Steel, Low-alloy Steel, Stainless Steel, and Cast Iron returns. Trace amounts of Cr or Cu mixing into plain carbon steel can cause mechanical property failures (e.g., 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.
• Cleanliness:
  • Rust & Sand Removal: Heavy rust ( Fe2O3 · 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.
• Preheating & Drying:
  • Safety: Eliminate moisture to prevent steam explosions.
  • Energy Efficiency: Preheating to 300°C–500°C can significantly reduce electrical consumption.

2. Effective Deslagging: Metal Purity & Lining Protection

Core Concept: 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.
  • Warning: Avoid fluxes with high Sodium (Na) content. While they fluidize the slag, they aggressively attack Acidic (Silica) 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” Strategy: In specific processes, maintaining a very thin layer of slag can prevent the molten steel from aspirating gases (N, H), 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 (high CaO); Basic linings suffer from acidic slag (high SiO2).

3. Precise Temperature Control: The Dual Risks

Core Concept: 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:
  • Overheating:
    • Gas Pickup: Solubility of H2 and N2 increases exponentially with every 100°C rise.
    • Lining Erosion:SiO2 + 2C → Si + 2CO (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. Metallurgical Reactions: Deoxidation & Alloying

Core Concept: 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).
  • Final Deoxidation: Stream inoculation or addition of final deoxidizers (e.g., Ca-Si, Rare Earths) during tapping to modify inclusion morphology.
• Alloying Strategy:
  • Addition Sequence:
    • Refractory elements (e.g., Mo, Cr) should be added early.
    • Oxidizable elements (e.g., Ti, Al, B) should be added late, after good pre-deoxidation.
    • Carbon Adjustment: Recarburizers (Graphite) should be added with the solid charge. Late additions have very poor recovery (<60%).
• Element Control:
  • Silicon Pickup: At high temperatures, Carbon reduces the Silica lining, potentially causing unintended spikes in Si content during long holds.
  • Mn/S Ratio: Maintain Mn/S > 10 to prevent hot tearing/shortness.

5. Troubleshooting: Reverse Engineering from Defects

Core Concept: The casting is the “Black Box” that records melting quality.