Grey Cast Iron Alloying Elements: Practical Ranges & Effects

Carbon (C) · ~2.5–4.0%

Primary driver of graphite and castability. Too high for the section size risks kish graphite and reduced tensile strength.

Silicon (Si) · ~1.0–3.0%

Promotes graphite/ferrite and lowers chill; excessive Si drops strength—tune to grade.

Manganese (Mn) · ~0.2–1.0%

Ties up S and promotes pearlite (strength). Overuse → MnS blowholes, slag, and carbide tendency when Mn/S is unbalanced.

Phosphorus (P) · keep low (<0.2% unless fluidity is critical)

Raises fluidity and wear but also brittleness and micro‑shrinkage risk; can worsen finning/penetration.

Sulfur (S) · trace

Needed for nucleation in small amounts; high S stabilizes pearlite/carbides and increases slag defects.

Copper (Cu) · up to ~1.0%

Pearlite stabilizer and grain refiner; watch sources for harmful traces (Pb, Sb, Te, As).

Nickel (Ni) · up to ~0.5%

Promotes graphite and stable solidification; boosts strength/elongation. High Ni in thin walls can form bainite.

Chromium (Cr) · up to ~0.5%

Hardness & wear ↑, but strong carbide former → chill and micro‑shrinkage; use carefully.

Molybdenum (Mo) · up to ~0.1%

~+4% strength per 0.1% Mo; improves hot properties. Segregation/carbides can hurt machinability and cause microporosity with Cr/P.

Tin (Sn) · up to ~0.15%

Strong pearlite promoter; too much → undercooled D‑graphite & embrittlement.

Ductile Iron: Chemistry Levers for Nodularity & Matrix

Carbon (C) · ~3.0–4.0% (CGI: 2.5–4.0%)

Excess vs section size risks flotation and irregular graphite → strength loss.

Silicon (Si) · ~1.8–4.0% (SiMo: 4.0–6.0%)

Graphite/ferrite promoter; above ~3% helps high‑temp service (often with Mo/Ni). High Si raises DBTT and may require higher pouring temp.

Magnesium (Mg) · ~0.03–0.06%

Enables spheroidization; excess vs S → carbides, micro‑porosity and slag. Minimize turbulence during Mg treatment/pouring.

Rare Earths (Ce/La…) · ≤0.02%

Inoculation aid; neutralize subversive elements (Ti, Al, Sb, Pb, Bi, Sn). Excess → carbides in thin sections, chunky graphite in heavy sections.