Induction Forging & Preheating
Rapid, uniform billet and bar heating to forging temperature for consistent metal flow, longer die life, and 60–80% less scale than gas furnaces.
How It Works
Billets, bars, or slugs pass through multi-turn solenoid coils and are heated to the target forging temperature. Low to medium frequency (1–10 kHz) is used to achieve through-heating — the skin depth at these frequencies is large enough to heat the full cross-section by conduction from the surface inward.
Multi-zone coil arrangements allow temperature profiling: the leading zone heats aggressively while trailing zones equalize, minimizing the surface-to-core temperature gradient. The goal is ΔT < 50°C across the cross-section at the coil exit.
Compared to gas furnaces, induction provides 60–80% less scale formation (shorter time at temperature), 20–40% energy savings, instant startup with no warm-up cycle, and precise piece-by-piece temperature control.
For progressive (conveyor) systems, billets are pushed or conveyed through the coil in a continuous stream. The dwell time equals coil length divided by throughput speed, and total power scales with the mass flow rate.
Typical Parameters
| Material | Frequency | Power Density | Temp Range | Dwell Time |
|---|---|---|---|---|
| Carbon steel billets (50–150 mm dia) | 1 – 3 kHz | 0.5 – 1.5 kW/cm² | 1100 – 1250°C | 10 – 60 s |
| Alloy steel bars (25–75 mm dia) | 3 – 10 kHz | 0.8 – 2 kW/cm² | 1050 – 1200°C | 8 – 40 s |
| Stainless steel billets | 1 – 10 kHz | 0.5 – 1.5 kW/cm² | 1050 – 1200°C | 15 – 60 s |
| Aluminium billets (50–200 mm dia) | 1 – 3 kHz | 0.3 – 0.8 kW/cm² | 400 – 500°C | 30 – 120 s |
| Titanium billets | 3 – 10 kHz | 0.5 – 1.5 kW/cm² | 900 – 1050°C | 15 – 60 s |
| Bolt / fastener blanks (< 25 mm) | 10 – 50 kHz | 1 – 3 kW/cm² | 1100 – 1250°C | 3 – 15 s |
Key Considerations
- Temperature uniformity is the primary quality metric. Core-to-surface ΔT should be < 50°C for most applications; tighter for aluminium extrusion billets (< 20°C).
- The Curie transition in steel at ~770°C causes a dip in heating rate as permeability drops from ~200 to 1. The system must supply extra power through this transition zone.
- Scale formation increases exponentially above 900°C. Minimizing time at temperature is the primary scale reduction strategy — induction’s fast heating is inherently better than gas furnaces.
- Progressive (conveyor) heating requires matching line speed to power and coil length. A 10% increase in line speed requires a 10% increase in power to maintain exit temperature.
- Die preheating (200–400°C) via induction extends die life by 2–5× and reduces thermal shock cracking at the die surface.
- End heating and partial heating of bars for upset forging requires coil designs that heat only the portion to be forged, keeping the grip end cold for robotic handling.
Common Coil Geometries
Multi-Turn Solenoid
Standard for round billets and bars. Coil bore matches billet diameter with 10–20 mm clearance. Long coils (1–3 m) for progressive heating.
Channel Coil
For rectangular cross-sections — blooms, slabs, and flat bars. The workpiece slides through the channel for uniform four-sided heating.
Transverse Flux Coil
For thin strip or sheet preheating where conventional solenoid coupling is poor. The magnetic field passes across the strip thickness rather than along it.