Cast Iron vs Steel: When Carbon Content Changes Everything

## The Carbon Boundary The iron-carbon phase diagram defines the dividing line: below 2.14 wt% carbon is steel; above 2.14% is cast iron. Most steels contain 0.05-1.0% carbon, while cast irons contain 2.5-4.0% carbon. ## Types of Cast Iron | Type | Carbon Form | Tensile Strength | Elongation | Key Feature | |------|------------|------------------|------------|-------------| | Gray Iron | Flakes | 150-400 MPa | <1% | Vibration damping, machinability | | Ductile Iron | Nodules | 400-900 MPa | 2-18% | Strength + castability | | White Iron | Cementite | 275-550 MPa | <1% | Extreme wear resistance | | Malleable Iron | Temper nodules | 345-690 MPa | 5-18% | Ductility via heat treatment | ## Castability Cast iron has dramatically better castability than steel: lower melting point (1150-1200 vs 1425-1540 degrees C), better fluidity into complex shapes, and less shrinkage (1% vs 2-3%). ## Vibration Damping Gray cast iron has 10-20 times better vibration damping capacity than steel. The graphite flakes absorb vibrational energy through internal friction. This makes gray iron the material of choice for machine tool beds and engine blocks. ## Weldability Steel is readily weldable. Cast iron is difficult due to its high carbon content, requiring preheating to 200-400 degrees C and nickel-based filler (ENi-CI). Even with precautions, weld repairs in cast iron are less reliable. ## When to Choose Each **Choose cast iron when**: The shape is complex and favors casting, vibration damping is needed, machinability is important, compressive loading dominates, or cost is critical for large production runs. **Choose steel when**: Ductility and impact toughness are required, the component is welded, tensile or cyclic loading dominates, or thin-wall sections are needed.