Alloys and materials science: iron, steel and other cast metals
Why grey cast iron and ductile iron are not the same
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 |
Læg mærke til mønsteret: jo højere trækstyrke, jo lavere brudforlængelse. Det er den klassiske afvejning i materialelæren — du kan sjældent få både maksimal styrke og maksimal sejhed i samme emne, og valget af GJS-klasse handler om at ramme den rigtige balance for netop det emne, du støber.
§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.