FeSiMg Mg fade kinetics in ductile iron: why the 10-minute window is real
The 10-minute critical window from nodularization to pour. Mg fade physics, FeSiMg particle size logic, and numbers from a Turkish foundry case.
The most common production defect in ductile iron is Mg loss. The operator drops FeSiMg into the tundish, 12 minutes later the ladle reaches the mold — and by then active Mg in the melt has dropped from 0.045% to 0.025%. Nodule count per mm² collapses from 220 to 110, graphite shape shifts from Type V-VI to Type III, and Brinell hardness variance opens up by 15-20 HB.
The name for this defect is Mg fade. It's not a metallurgy problem. It's a clock problem.
Physical frame: why Mg burns off
Magnesium vapourizes at 1,150°C. Nodularization happens at 1,450-1,500°C — so Mg starts leaving as gas from second one. The second loss path is oxidation: dissolved S + O in the bath and MgO in the slag keep pulling Mg into sulphides and oxides.
Active Mg half-life (T½) in practice is 8 to 12 minutes. Tundish design, ladle cap, slag depth, and whether you dust the surface with a thermal blanket all shift this window 2-3 minutes in either direction. Field test: sample iron at 0, 5, 10, 15 minutes after FeSiMg addition and run spectro. Most foundries write their "pour by minute 10" rule from this curve.
Why FeSiMg particle size sits in 5-25 mm
Particles smaller than 3 mm float, oxidize quickly, waste Mg. Particles above 30 mm sink in the melt; by the time they fully dissolve the bath already started the fade clock. 5-25 mm was optimized not in a lab but on foundry floors — Acarer Metal in the 1990s, and today roughly 80% of Turkish foundries operate inside this band.
Standard Si 44-48% + Mg 5-7% + RE 1-2% spectrum serves three specific jobs:
- Si: the FeSiMg matrix — it delivers Mg into the melt before vapourization wins.
- Mg: the nodularizing element. 5-7% is calibrated to hit a single-shot ladle treatment without needing topping copper wire.
- RE (rare earth): the best-known fade retardant. Just 0.5-1% RE can extend the active Mg half-life from 10 to 14 minutes.
The Al ≤1% ceiling is not decorative — breach it and vermicular (worm-like) graphite starts replacing spheroidal nodules.
Field numbers: Bursa, November 2025
An automotive casting plant (name withheld) hit a reject rate of 2.8% on 600 tons/month of ductile iron. Nodule counts were bouncing 110-140/mm²; target was 250+.
What the audit found:
- Tundish-to-pour travel time averaged 14 minutes. Charging equipment delay.
- FeSiMg particle distribution showed 35% above 25 mm.
- Fix: FeSiMg consolidated to 5-20 mm, tundish lid rebuilt, pour time squeezed from 14 to 9 minutes.
Three weeks later the reject rate fell to 0.9%; nodule counts parked at 240-280/mm². FeSiMg consumption did not change — only particle distribution and timing. Estimated monthly saving: ~18,000 USD in recovered scrap and rework.
Expert read
The Xinjiang power tariff hike announced in March won't shift Erdos-line FeSi pricing until autumn — so CIF Marmara buyers should lock in FeSiMg spot during the February-May 2026 window. Pre-Chinese New Year shipments historically carry a 3-5% premium, which means inventory for October 2026 ANKIROS season should be closed by June.
Operator checklist
- Add FeSiMg 9-10 minutes before pour — never cross minute 12.
- Particle size 5-25 mm, distribution tight.
- Tundish lid + inert powder + slag depth above 60 mm — fade clock slips back 1-2 minutes.
- Every lot: verify Mg, RE, Al on COA. Reject if Mg <6% or Al >1.2%.
- Shoot spectro at minute 0-5-10-15 on the first heat of the day. Know your curve.
Arsam Metal's FeSiMg line carries Chinese (Tsingshan + Erdos) 5-25 mm particles in Gebze bonded stock. Mg 5-7% + RE 1-2% + Si 44-48% spectrum shipped with MTC + COA. Technical support is bundled with supply — get in touch via contact to run an active-Mg half-life curve for your foundry.
Arsam Technical Team
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