Alloys and materials science: iron, steel and other cast metals
Why grey cast iron and ductile iron are not the same
Introduction
Metal is not just metal. As a foundry technician you work with several families of cast iron and steel, and each has its own microstructure that determines how the finished part behaves — whether it is stiff or tough, whether it damps vibrations, and how much it can bend before it breaks.
Grey cast iron (GJL) — DS/EN 1561
Grey cast iron, abbreviated GJL, is described in the standard DS/EN 1561, which specifies the properties of unalloyed and low-alloy grey cast iron for castings made in sand moulds or moulds with equivalent thermal properties. What characterises GJL is that the graphite in the material sits as lamellae — small, flat 'flakes' scattered in a matrix of typically pearlite and ferrite.
Tough/nodular cast iron (GJS) — DS/EN 1563
Tough cast iron (also called nodular cast iron or SG iron) is described in DS/EN 1563. Here the graphite is shaped as round spheres instead of flakes — achieved by special treatment of the melt where the graphite is 'spheroidised'. The spheres make the material significantly tougher and more ductile than grey cast iron.
| Class (EN-GJS-) | Significance of the numbers |
|---|---|
| 350-22 | Approx. 350 MPa tensile strength, approx. 22 % elongation at break — very tough |
| 400-18 | Approx. 400 MPa tensile strength, approx. 18 % elongation at break — good balance between strength and toughness |
| 500-7 | Approx. 500 MPa tensile strength, approx. 7 % elongation at break — more rigid, less tough |
| 700-2 | Approx. 700 MPa tensile strength, approx. 2 % elongation at break — high strength, low toughness |
Notice the pattern: the higher the tensile strength, the lower the elongation at break. This is the classic trade-off in materials science — you can rarely get both maximum strength and maximum toughness in the same piece, and the choice of GJS grade is about hitting the right balance for the specific part you are casting.
Other metals in the foundry
- 01Steel — used where toughness and weldability weigh heavier than cast iron's damping ability
- 02Aluminum – light, corrosion-resistant, suitable for objects where weight is critical
- 03Brass and bronze — used for example for bearings, valves and items that require good sliding properties or corrosion resistance in humid environments
As a foundry technician material science is not theory for theory's sake — it is what determines whether the part you just cast can actually withstand the load it is designed for.