Quick Answer
ASTM A536 grade numbers are the properties. A grade like 60-40-18 reads: 60 ksi minimum tensile strength, 40 ksi minimum yield, 18% minimum elongation. Lower-strength grades are ferritic, tough and machinable; higher-strength grades are pearlitic or alloyed, harder and more wear resistant. The most commonly cast grades are 60-40-18, 65-45-12, 80-55-06 and 100-70-03 — and picking between them is mostly a question of whether the part needs impact toughness or wear strength.
The Five Standard Grades
| Grade | Tensile min | Yield min | Elongation min | Matrix / character |
|---|---|---|---|---|
| 60-40-18 | 414 MPa | 276 MPa | 18% | Ferritic — maximum toughness, easy machining |
| 65-45-12 | 448 MPa | 310 MPa | 12% | Mostly ferritic — general purpose |
| 80-55-06 | 552 MPa | 379 MPa | 6% | Ferritic-pearlitic — strength with some ductility |
| 100-70-03 | 689 MPa | 483 MPa | 3% | Pearlitic — wear resistance, gears and sprockets |
| 120-90-02 | 827 MPa | 621 MPa | 2% | Quenched and tempered — highest strength |
What Makes Ductile Iron “Ductile”
A magnesium treatment just before pouring forces the graphite to form as spheres instead of the flakes found in gray iron. Flake graphite acts like thousands of internal notches — that is why gray iron snaps. Nodular graphite lets the metal stretch, giving cast iron elongation numbers once reserved for steel, while keeping iron’s castability, damping and lower cost. We covered the gray iron side of the family in our ductile iron sand casting guide.
How Buyers Should Choose a Grade
- Shock and impact loads, or cold climates: 60-40-18. Its ferritic matrix keeps toughness at low temperatures — the reason it dominates wind-turbine hubs and municipal castings.
- General machinery parts with no extreme requirement: 65-45-12. The default of the family.
- Higher loads, moderate wear: 80-55-06 — brackets, housings, crankshafts and machine bases.
- Sliding wear, gear teeth, sprockets: 100-70-03, often with induction-hardened surfaces.
- Maximum strength in iron: 120-90-02, heat treated; at this point also compare price against cast steel.
Ductile Iron vs Cast Steel for the Same Part
Ductile iron pours at lower temperature, shrinks less, and machines faster — typically making it 20–40% cheaper than a cast steel equivalent. Cast steel keeps the advantage in weldability (iron welding is repair-only in practice), in impact toughness at the highest requirements, and in code-driven pressure applications that specify steel. For many housings, brackets and counterweights, 80-55-06 quietly replaces WCB steel at a better price.
Specification Checklist
- Call out ASTM A536, grade — e.g., A536 65-45-12
- State test-bar type if it matters to your code (separately cast Y-blocks are the default)
- Add hardness window if machining lines are sensitive to it
- Specify nodularity check (microstructure exam or ultrasonic velocity) for critical parts
- For safety-critical or low-temperature parts, require Charpy impact tests — A536 itself does not mandate them
FAQ
What does 80-55-06 mean exactly?
80 ksi (552 MPa) minimum tensile, 55 ksi (379 MPa) minimum yield, 6% minimum elongation. The naming is the spec.
Is ductile iron weldable?
Only with difficulty — treat it as non-weldable at the design stage. If the assembly needs welding, use cast steel or design a bolted joint.
Which A536 grade is easiest to machine?
60-40-18 and 65-45-12. Machinability drops as the pearlite fraction rises through 80-55-06 to 100-70-03.
Can the same pattern produce different grades?
Yes. Grade is controlled by chemistry and cooling/heat treatment, not tooling — useful when one design serves several load cases.