Gray Cast Iron (Class 30) vs 1018 Mild Steel
Gray cast iron and 1018 mild steel are both inexpensive ferrous materials, but they behave very differently. Gray iron is a brittle casting alloy with excellent vibration damping, machinability, and wear behavior, ideal for housings and bases. 1018 is a ductile, weldable wrought steel that is stronger in tension and far more forgiving of impact. Casting versus wrought, brittle versus ductile — that frames the choice.
The verdict
Choose gray cast iron for damped, easily machined, wear-resistant cast housings and bases where loading is compressive. Choose 1018 mild steel when you need ductility, weldability, and tensile strength in a wrought, impact-tolerant part. Iron for cheap damped castings; steel for ductile, weldable, tension-loaded parts.
Side-by-side data
| Property | Gray Cast Iron (Class 30) | 1018 Mild Steel |
|---|---|---|
| Category | Cast Iron | Steel |
| Density (g/cm³) | 7.15 | 7.87 |
| Tensile strength (MPa) | 207 | 440 |
| Yield strength (MPa) | 200 | 370 |
| Elongation (%) | 0.5 | 15 |
| Hardness | 210 HB | 126 HB |
| Max service temp (°C) | 400 | 400 |
| Machinability | ●●●●● | ●●●●● |
| Corrosion resistance | ●●●●● | ●●●●● |
| Relative cost | ●●●●● | ●●●●● |
| Thermal cond. (W/m·K) | 50 | 51 |
| Typically used for | Engine blocks & vibration damping (brittle, no true yield) | General low-carbon parts, weldable |
Which should you choose?
Choose Gray Cast Iron (Class 30) when…
- Vibration damping is needed — flake graphite makes gray iron an excellent damper
- The part is a casting — complex housings, bases, and machine frames
- Easy machining matters — gray iron rates 4.5/5 vs 1018's 3.5/5
- Good sliding wear resistance is valued (the graphite lubricates)
- Lowest cost is a priority — gray iron is ~1/5, on par with mild steel
- Loading is compressive and the ~0.5% elongation brittleness is acceptable
Choose 1018 Mild Steel when…
- Ductility and impact tolerance matter — ~15% elongation vs gray iron's ~0.5%
- The part must be welded — 1018 is readily weldable; gray iron is not easily welded
- Tensile and bending strength are needed — 1018 tensile ~440 MPa vs ~207 MPa
- A true yield strength (~370 MPa) is required for design (gray iron has none)
- The part is wrought, machined, or fabricated rather than cast
- Shock loading or safety-critical tension paths are present
Key differences that matter
- Gray iron is a brittle casting (~0.5% elongation, no yield); 1018 is ductile (~15% elongation)
- 1018 is much stronger in tension (~440 vs ~207 MPa) with a real yield (~370 MPa)
- Gray iron damps vibration far better — a key advantage for machine bases and housings
- Gray iron machines more easily (4.5/5 vs 3.5/5) and offers good sliding wear behavior
- 1018 is readily weldable; gray iron requires special, often unreliable welding procedures
- Both are very cheap ferrous materials (~1.0/5 each)
- Casting geometry favors iron; wrought, fabricated, welded parts favor steel
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Open the Material SelectorGet a Quote →Frequently asked questions
Which is stronger, gray iron or 1018 steel?
1018 mild steel in tension — ~440 MPa tensile and ~370 MPa yield versus gray iron's ~207 MPa with no true yield. Steel is also ductile (~15% elongation) where gray iron is brittle (~0.5%). Gray iron is strong in compression and stiff, but for tensile, bending, or impact loads, 1018 is the substantially tougher and stronger choice.
Can gray cast iron be welded like 1018?
Not easily. Gray iron's high carbon and brittle structure make welding difficult and prone to cracking; it needs preheating, special nickel rods, and slow cooling, and results are often unreliable. 1018 mild steel welds readily with standard processes. If your design depends on welding, 1018 is far more practical than gray iron.
Why choose gray iron over steel at all?
For castability, vibration damping, machinability, and wear. Gray iron pours into complex shapes that would be costly to fabricate from steel, damps vibration far better for machine bases and engine blocks, machines more easily (4.5/5), and resists sliding wear thanks to its graphite. Where loading is compressive, these advantages outweigh its brittleness.
Which machines better?
Gray cast iron, at 4.5/5 versus 1018's 3.5/5. The flake graphite breaks chips and lubricates the cut, giving excellent tool life and finish. 1018 is reasonably machinable but gummier. For heavily machined housings and bases, gray iron's machinability is a production advantage — provided the part is a casting and the loads suit it.
Are they similar in cost?
Yes, both sit at roughly the lowest cost tier (~1.0/5). Cost rarely decides between them. The real driver is form and loading: gray iron for cheap, damped, machinable castings under compression; 1018 for ductile, weldable, fabricated parts that must carry tension or survive impact.
Property values are typical/nominal figures for early-stage guidance only and vary by temper, grade, supplier and heat treatment. Confirm critical specifications against a certified datasheet or with an mfgiq engineer before production.