The short answer
Specify grey (gray) cast iron when the part is mainly loaded in compression or needs vibration damping and easy machining, such as bases, covers and many bearing or gearbox housings, and name a full grade such as HT250 / EN-GJL-250 / ASTM A48 Class 35. Specify ductile iron when the part needs measurable elongation, impact, tensile or fatigue strength, such as hubs, brackets and higher-duty housings, and name a full grade such as QT450-10 / EN-GJS-450-10 / ASTM A536 65-45-12. Do not swap one family for the other without design approval.
Is ductile iron the same as cast iron?
Yes, ductile iron is a type of cast iron, also called nodular or spheroidal graphite (SG) iron. Its graphite forms round nodules, so it can stretch before it breaks. In everyday RFQ language “cast iron” usually means grey iron, whose graphite forms flakes and which has almost no elongation. If a drawing says only “cast iron”, treat the material as incomplete until the family and standard are named.
What differs for the part
| Property | Grey (gray) cast iron | Ductile iron |
|---|---|---|
| Graphite shape | Flakes | Round nodules |
| Elongation | Almost none | Measurable; in QT grades the number after the dash, 3–15% |
| Minimum tensile strength (common grades) | HT200–HT300: 200–300 MPa | QT400–QT600: 400–600 MPa |
| Vibration damping | Better | Lower |
| Machining | Easier | Good, but generally slower than grey iron |
| Impact and fatigue | Lower | Higher |
| Usual cost | Lower | Higher |
Values are those implied by the common grade names; the grade charts give the full minimum properties for each standard.
Which to specify, by part type
| Part | Usual starting choice | Why |
|---|---|---|
| Machine bases, beds and covers | Grey iron | Compression loads, damping, easy machining |
| Bearing and gearbox housings | Grey iron; ductile for impact or higher stress | Damping and machinability, unless toughness is needed |
| Hubs and flanged parts | Ductile iron | Shock loads and bolted joints need elongation |
| Brackets, mounts and linkages | Ductile iron | Bending and impact loads |
These are common starting points, not design rules. The designer chooses the material; JK Casting reviews the drawing and will not substitute one family for the other.
Starting grades and where to find the full charts
- Grey iron: HT200 / EN-GJL-200 / A48 Class 30; HT250 / EN-GJL-250 / Class 35; HT300 / EN-GJL-300 / Class 40. See the gray iron grades chart.
- Ductile iron: QT400-15 / EN-GJS-400-15 / A536 60-40-18; QT450-10 / EN-GJS-450-10 / 65-45-12; QT500-7 / EN-GJS-500-7 / 80-55-06; QT600-3 / EN-GJS-600-3. See the ductile iron grades chart.
Equivalents across standards are approximate. The standard named on the drawing governs.
What to put on the drawing or RFQ
- Family, grade and governing standard, never “cast iron” alone
- Mechanical properties and hardness to be reported, and how test pieces are made
- Heat-treatment or delivery condition
- Machined surfaces, tolerances and dimensional reporting
- Surface acceptance and whether repairs are allowed
- Quantity, annual demand and raw or machined supply
For housings, the machined housing RFQ guide covers bores and datums; the sample approval checklist covers first-article approval.
How JK Casting quotes grey and ductile iron
JK Casting pours both grey (gray) iron and ductile iron as regular sand-cast materials to customer drawings in Gaomi, Shandong, China, and supplies the parts raw or CNC-machined. Regular grades are grey iron HT200–HT300 and ductile iron QT400–QT600, with their EN, ASTM and GGG equivalents; higher ductile grades are reviewed part by part. Send the drawing, the full material designation and the quantity. See ductile iron castings and gray and ductile iron castings, or send the drawing for a quotation.
Related guides: Sand casting vs investment casting · Compare casting quotations · Casting tooling costs
Frequently asked questions
Is ductile iron the same as cast iron?
Ductile iron is a type of cast iron, also called nodular or spheroidal graphite (SG) iron. In everyday RFQs “cast iron” often means grey (gray) iron, so name the family, grade and standard on the drawing.
Ductile or grey iron for a gearbox or bearing housing?
Many bearing and gearbox housings are grey iron, for damping and easy machining, for example HT250 / EN-GJL-250 / A48 Class 35. Specify ductile iron, for example QT450-10, when the housing takes impact or higher stress. The designer decides.
Can I replace grey iron with ductile iron, or the reverse?
Not without design approval. The two families have different properties and test requirements, so any change must be reviewed against the design and standard and approved in writing before production.
Are gray iron and grey iron the same material?
Yes. Gray is the American spelling and grey the British spelling of the same flake-graphite cast iron.
Does JK Casting cast both grey and ductile iron?
Yes. Grey iron (HT200–HT300 and EN-GJL / A48 equivalents) and ductile iron (QT400–QT600 and EN-GJS / A536 / GGG equivalents) are regular sand-cast materials; higher ductile grades are reviewed part by part.