Quick Answer
ASTM A48 classifies gray iron purely by tensile strength: the class number is the minimum tensile in ksi. Class 20 ≈ 138 MPa, Class 30 ≈ 207 MPa, Class 40 ≈ 276 MPa, Class 60 ≈ 414 MPa. Lower classes machine easier, damp vibration better and resist thermal shock; higher classes carry more load and wear longer. Class 30–35 is the sweet spot for most machine bases and housings; Class 40+ for gears, cylinders and higher-stress parts.
The Classes at a Glance
| Class | Tensile min | Character | Typical parts |
|---|---|---|---|
| Class 20 | 138 MPa | Softest, best damping and thermal shock resistance | Counterweights, stove parts, non-critical housings |
| Class 25 | 172 MPa | Good castability in thin sections | Covers, frames |
| Class 30 | 207 MPa | The general-purpose default | Machine bases, gearbox housings, pulleys |
| Class 35 | 241 MPa | Stronger sections without machinability loss | Pump housings, brackets |
| Class 40 | 276 MPa | Higher wear and load capability | Cylinders, gears, machine tool ways |
| Class 50/60 | 345/414 MPa | Highest strength gray iron; needs careful foundry control | High-pressure parts, heavy-duty components |
What Makes Gray Iron Gray — and Useful
Graphite flakes distributed through the iron give the fracture surface its gray color and create properties steel cannot match:
- Vibration damping several times better than steel — the reason machine tool bases are still gray iron in 2026
- Self-lubricating machining: graphite breaks chips and lubricates the cut; gray iron machines faster than nearly any steel
- Excellent castability: fluid, low shrinkage, happily fills complex thin-walled molds
- Compressive strength three to four times its tensile — design gray iron parts to be loaded in compression
The same flakes are its weakness in tension: they act as internal notches, so gray iron has effectively no ductility (<1% elongation) and modest tensile numbers. When a part needs tensile ductility, the family answer is ductile iron — see our ASTM A536 grade guide.
Choosing a Class: The Buyer Logic
- Default to Class 30 unless stress analysis says otherwise — it balances cost, machinability and strength.
- Damping-critical parts (machine frames, brake components): resist the urge to over-class; damping drops as strength rises.
- Section sensitivity: gray iron strength falls as section thickness grows — a Class 40 test bar does not guarantee Class 40 properties in a 75 mm wall. Discuss the controlling section with the foundry.
- Wear surfaces: Class 40+ with controlled pearlite, or specify surface hardening.
Specification Checklist
- “ASTM A48 Class 30B” — the letter suffix (A/B/C/S) sets the test-bar size matched to your casting’s controlling section; B suits most medium castings
- Hardness range if machining lines care (typical Class 30: 170–230 HBW)
- Pressure-tightness test for fluid-carrying parts — porosity policy stated up front
- For automotive-style requirements, consider SAE J431 equivalents (G3000 ≈ Class 30)
Gray iron runs through our sand casting line at YCUMETAL, including machine bases and housings finish-machined in-house.
FAQ
Is a higher class always better?
No. Higher classes cost more, machine slower and damp less. Specify the class the load case needs, not the biggest number.
Can gray iron be welded?
Practically, no — treat it as non-weldable in design. Braze repair or mechanical joining are the accepted alternatives.
A48 vs A126 — which spec?
A126 is the valve/flange/pipe-fitting gray iron spec with chemistry limits; A48 is the general engineering spec by strength class. Your application code usually names one.