Guides/Comparisons
Ductile iron vs gray cast iron
ductile-vs-gray-cast-ironDuctile iron and Gray cast iron at a glance
This page compares Ductile iron and Gray cast iron using only values that carry a source, a test condition and an evidence level. Ranges run across temper, heat treatment and product form, measured at room temperature where the source says so.
- Composition: Carbon (ductile iron 3.55 to 3.65%; gray iron 2.5 to 4%), Silicon (ductile iron 2.3 to 2.7%; gray iron 1 to 3%).
- Yield strength: ductile iron is higher (ductile iron 276 to 621 MPa; gray iron 165 to 228 MPa), about 2.3 times the midpoint of gray iron.
- Tensile strength: the ranges overlap (ductile iron 350 to 827 MPa; gray iron 138 to 431 MPa).
- Young's modulus: ductile iron is higher (ductile iron 169 GPa; gray iron 103 to 118 GPa), about 1.5 times the midpoint of gray iron.
"Higher" means the whole range is higher, not just one value. Where a value is a specified minimum, the table says "at least".
Side by side
| Property | Ductile iron | Gray cast iron |
|---|---|---|
| Yield strength | 276 to 621 MPa | 165 to 228 MPa |
| Tensile strength | 350 to 827 MPa | 138 to 431 MPa |
| Density | no data | 7.2 g/cm³ |
| Young's modulus | 169 GPa | 103 to 118 GPa |
| Thermal conductivity | no data | 50 W/(m·K) |
Composition
| Element | Ductile iron | Gray cast iron |
|---|---|---|
| Iron | balance | balance |
| Carbon | 3.55 to 3.65% | 2.5 to 4% |
| Silicon | 2.3 to 2.7% | 1 to 3% |
| Manganese | 0.4 to 0.6% | not listed |
| Magnesium | 0.03 to 0.045% | not listed |
| Sulfur | 0.008 to 0.015% | not listed |
| Phosphorus | up to 0.050% | not listed |
Shares are as printed in the sources, by weight.
When to use which
| Material | Best for | Weakest at |
|---|---|---|
| Ductile iron | Castings that need the ASTM A536 strengths of 60,000 to 120,000 psi, such as high-strength gears and rollers and pressure-containing valve and pump bodies (Goldens; Daycounter) | Elongation falls as the grade rises, from 18 percent at 60-40-18 to 2 percent at 120-90-02 (Goldens). Properties also depend on the heat treatment that each grade requires (ASTM A536 summary) |
| Gray cast iron | Machine bases, frames, pump housings and engine blocks, where damping, machinability and compressive load matter more than tensile strength | Tensile strength and shock resistance: the graphite flakes act as notches, so the metal breaks in a brittle way |
Where each is used
| Material | Use |
|---|---|
| Ductile iron | Gears and rollers (100-70-03 and 120-90-02 grades) |
| Ductile iron | Pressure-containing valve and pump bodies (60-40-18 grade) |
| Ductile iron | Dies, pistons, valves and moulds (EN GJS 400-15 SG iron) |
| Gray cast iron | Brake discs and rotors |
| Gray cast iron | Machine tool bases and mountings |
| Gray cast iron | Internal combustion engine cylinder blocks |
| Gray cast iron | Pump housings and valve bodies |