Materials/Metals and alloys/Collections
Cast irons
cast-ironsAbout cast irons
Cast irons (gray iron, ductile iron): strength by class and grade, density and stiffness with sources.
Cast irons are iron-carbon alloys with more carbon than steel. The carbon sits as graphite, in flakes in gray iron and in nodules in ductile iron, and that shape sets the strength, ductility and how well the metal damps vibration. Each value here names its class or grade.
At a glance
- Highest yield strength: Ductile iron reaches 621 MPa. Lowest: Gray cast iron starts at 165 MPa. (2 of 2 grades have a sourced value.)
- Highest tensile strength: Ductile iron reaches 827 MPa. Lowest: Gray cast iron starts at 138 MPa. (2 of 2 grades have a sourced value.)
Ranges run across the conditions each source reports (temper, heat treatment, form), measured at room temperature where the source says so.
How they compare
The same measurements for every material in the collection. A range means the grade has values under several conditions; "at least" means the source gives a specified minimum. "no data" means no sourced value yet. For ranked tables, open a benchmark.
| Material | Yield strength | Tensile strength | Density | Young's modulus | Thermal conductivity | Melting point |
|---|---|---|---|---|---|---|
| Gray cast ironFlake graphite cast iron for housings, machine bases and brake discs | 165 to 228 MPa | 138 to 431 MPa | 7.2 g/cm³ | 103 to 118 GPa | 50 W/(m·K) | no data |
| Ductile ironspheroidal-graphite cast iron | 276 to 621 MPa | 350 to 827 MPa | no data | 169 GPa | no data | no data |
What these materials are made of
Composition by weight, as printed in the sources for each grade (the first constituents listed).
| Material | Main constituents |
|---|---|
| Gray cast iron | Fe balance · C 2.5 to 4% · Si 1 to 3% |
| Ductile iron | Fe balance · C 3.55 to 3.65% · Si 2.3 to 2.7% · Mn 0.4 to 0.6% · Mg 0.03 to 0.045% · S 0.008 to 0.015% · P up to 0.050% |
Where they are used
| Use | Grades |
|---|---|
| Brake discs and rotors | Gray cast iron |
| Machine tool bases and mountings | Gray cast iron |
| Internal combustion engine cylinder blocks | Gray cast 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) | Ductile iron |
When each grade fits
| Material | Best for | Weakest at |
|---|---|---|
| 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 |
| 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) |