← BACK TO PREVIOUS PAGE
This report evaluates Natural Gas Furnaces and Medium Frequency Induction Furnaces for pre-forging heating of brass billets to their plastic deformation temperature range of 650 – 850 °C. The selection of heating technology is a strategic decision impacting metallurgical quality, die life, energy expenditure, and production flexibility.
Superior thermal homogeneity (ΔT < 5 °C core-to-surface), ideal for large-diameter billets. Lower energy cost and excellent production buffer via soaking zone.
Heating rates up to 500 °C/min, instantaneous start-up, zero on-site combustion emissions. Suited for high-volume continuous production with tight just-in-time requirements.
Brass alloys (Cu–Zn systems, typically CW617N / CZ122 / H62) are the dominant material for hot-forged components: valve bodies, fittings, connectors, and structural hardware. Their forgeability is maximised at elevated temperatures where the face-centred-cubic (FCC) α-phase and the body-centred-cubic (BCC) β-phase coexist, reducing yield strength and improving ductility.
The heating method directly governs the thermal gradient established within the billet, the extent of surface oxidation and dezincification, and ultimately the tonnage required from the press and the service life of forging dies.
Principle: Controlled combustion of CH₄ + air transfers heat via convection, flame impingement, and radiation from refractory walls.
Design: Multi-zone: heating zone (rapid ΔT) + soaking/holding zone (temperature equalisation across billet cross-section).
Typical frequency: N/A — direct thermal radiation and convection.
Principle: AC at medium frequency (500 Hz – 10 kHz) through a copper coil induces eddy currents inside the billet; heat generated by Joule effect (I²R) directly in the metal.
Design: Through-feed coil system; billets pushed continuously through the electromagnetic field.
Typical frequency: 1 kHz – 4 kHz for brass billets (diameter-dependent).
In induction heating, current density is not uniform across the billet cross-section. It is highest at the surface and decays exponentially inward. The depth at which current density falls to 1/e (≈ 37%) of its surface value is called the reference depth or skin depth:
For a typical brass billet at 750 °C heated at 1 kHz, the skin depth δ ≈ 12–14 mm. For a 50 mm diameter billet, this means approximately 50–56% of the cross-sectional area is effectively heated by induction, with the core heated secondarily by thermal conduction. Increasing frequency to 4 kHz reduces δ to ~6–7 mm, worsening the core-to-surface temperature gradient unless adequate soak time is allowed.
Induction furnaces transfer energy directly into the workpiece with a typical electrical-to-billet efficiency of 90–95%. Gas furnaces lose significant energy through exhaust gases and refractory heat storage; their billet heating efficiency is typically 45–65%, though recuperative burner systems can raise this to ~75%.
The table below summarises characteristic heating rates and thermal response parameters for both technologies. These parameters directly determine production throughput and the feasibility of just-in-time manufacturing schedules.
The heating method has a direct influence on grain structure, surface integrity, and forging response. Three mechanisms are critical:
Selective removal of Zn from the alloy surface. Occurs above ~600 °C in the presence of oxygen. Induction-heated billets exhibit a dezincification depth of 0.1–0.3 mm vs. < 0.05 mm for reducing-atmosphere gas furnaces.
Oxidation layer (CuO, Cu₂O, ZnO). Gas furnace with slightly reducing atmosphere (λ ≈ 0.95): scale loss 0.3–0.7 % of billet weight. Open induction coil: 0.8–1.5 % weight loss (3× higher).
Prolonged soaking at > 800 °C promotes grain coarsening (ASTM grain size < 3). Gas furnaces risk this if soaking times exceed 45 min; induction eliminates this risk but may produce fine uneven grains in the core.
| CO₂ emissions | ~0.20 kg/kWh input |
| NOₓ emissions (typical) | 80–200 mg/Nm³ |
| Permitting complexity | Medium–High |
| Explosion risk | Present |
| EU ETS applicability | Yes (>20 MWth) |
| On-site CO₂ | Zero |
| NOₓ emissions | Zero (on-site) |
| Permitting complexity | Low |
| Electrical safety | Standard IEC |
| Scope 2 CO₂ (grid) | Grid-dependent |
| Parameter | 🔥 Natural Gas Furnace | ⚡ MF Induction Furnace |
|---|---|---|
| Heating Mechanism | Flame, convection, radiation (external) | Joule effect via eddy currents (internal) |
| Start-up Time | 30–60 min (refractory preheat) | < 1 min (instantaneous) |
| Heating Rate | 5–15 °C/min | 100–500 °C/min |
| Core-to-Surface ΔT (50 mm billet) | < 5 °C (after soaking) | 20–60 °C (frequency-dependent) |
| Production Buffer | Excellent — holding zone | Poor — coil must be emptied |
| Surface Oxidation / Scale Loss | 0.3–0.7% (reducing atmosphere) | 0.8–1.5% (open coil) |
| Dezincification Depth | < 0.05 mm | 0.1–0.3 mm |
| Die Life Impact | Positive — uniform material flow | Neutral / Negative — harder surface |
| Electrical Energy Efficiency | 45–65% (≈75% recuperative) | 90–95% |
| Energy Cost per Tonne (EU) | Lower — cheaper gas tariff | 25–60% higher — electricity rate |
| On-site CO₂ / NOₓ | Present — combustion gases | Zero |
| Footprint | Large (furnace + exhaust treatment) | Moderate (coil + cooling tower + capacitor bank) |
| Maintenance Profile | Lower — refractory relining every 3–7 yr | Higher — water-cooling scale, electronic components |
| Large Diameter (> 80 mm) Suitability | Excellent | Requires low-frequency & soak time |
No single technology is universally superior. Selection must be based on a structured cost-benefit analysis incorporating local utility pricing, billet geometry, production cadence, and regulatory environment.