Every 0.01% S costs you extra nodularizer
Most foundries focus on nodularizer price, but overlook sulfur from carbon additives – a major hidden consumer of magnesium.
The chemistry:
Mg + S → MgS
Every 0.01% S consumes ~0.008% Mg.
Real impact:
Switching from 0.02% S to 0.30% S carbon additive (1.2% addition rate) increases nodularizer use by 0.9 kg/ton iron.
For a 5,000 t/year foundry:
Extra nodularizer cost: +USD 5,400/year
Saving on cheaper additive: –USD 12,000/year
→ Looks cheaper? But hidden losses (higher rejection, energy, slag) often reverse the math.
Critical limits:
| Casting grade | Max additive S |
| Premium DI (wind power, pressure parts) | ≤0.05% |
| Standard DI (QT400-600) | ≤0.15% |
| Gray iron only | ≤0.60% |
Rule of thumb:
If your base iron already has ~0.020% S, use additive with S ≤0.05%. Otherwise nodularization becomes unstable.
Simple check:
Yellow‑green, sticky slag → too much sulfur
Dark, low‑nodularity fracture → suspect carbon additive
Conclusion:
Don't save USD 200/ton on carbon additive only to spend more on nodularizer and scrap. For ductile iron, always request CoA with sulfur value ≤0.05% and verify incoming batches.
