Gray cast iron
Who this helps: Design Engineers / Buyers selecting gray/grey iron for housings, bases, brake drums, pulleys and other non-pressure parts where NVH damping, machinability and value matter.
What you’ll get: a practical gray iron casting guide—process options, ASTM⇆EN⇆ISO grade mapping, surface finish & CT notes, design rules, copy-paste drawing text, and when to upgrade/downgrade.
Prepared by YB Metal Solution. Share your drawing via /rfq—YB Metal will return a part-specific material choice, process window and test plan.
Author: YB Metal Solution Engineering Team (hereafter YB Metal)
Table of contents
- What gray/grey iron is—and why it damps so well
- Casting processes for gray iron (capability, cost & finish)
- Grades & standards: ASTM A48 ⇆ EN 1561 ⇆ ISO 185
- Where gray iron shines (and where it struggles)
- Dimensional control: ISO 8062 CT, stock & GD&T
- Surface finish (Ra) targets you can actually hit
- Design for castability & NVH
- QA & acceptance (what to verify at PPAP)
- What to put on the drawing
- What YB Metal delivers
- FAQs
What gray/grey iron is—and why it damps so well
Gray iron is a cast iron with flake (lamellar) graphite in a ferrous matrix. Flakes interrupt elastic waves and add internal friction, giving gray iron its superior vibration damping vs ductile iron or steel—critical for NVH-sensitive housings, machine bases and brake rotors.
- Acceptance concept (ASTM A48): classes are defined by tensile strength on separately cast bars; chemical composition is subordinate to tensile. That’s why two Class-35 parts can machine differently (different pearlite/ferrite) and still both be “in-spec.” ASTM International | ASTM+1
For a deeper NVH comparison: Cast Iron for Noise & Vibration Damping: Why Gray Iron Still Wins
Casting processes for gray iron (capability, cost & finish)
Pick the molding route that fits your geometry, volumes and surface/CT needs:
| Process | When it fits | Typical notes |
|---|---|---|
| Green Sand Casting | Medium–large volumes, robust ribs/junctions | Best cost; typical Ra higher; CT mid-range |
| Resin Sand vs Green Sand (accuracy & finish) | Larger parts, tight profiles, deep cores | Better dimensional hold & surface than green; slower cycle |
| Shell Molding for Thin-Wall | Thin wall (<5–6 mm / 0.2–0.24 in), crisp features | Best as-cast surface (low Ra); tooling cost ↑ |
Soundness comes first: engineer directional solidification (risers/chills) at T/Y/X junctions to avoid shrinkage. If you need a refresher: Fix Shrinkage Porosity
Grades & standards: ASTM A48 ⇆ EN 1561 ⇆ ISO 185
Use the table for design approximation; set acceptance by the target standard on your drawing/PO.
| ASTM A48 class (UTS min) | Nearest EN 1561 (EN-GJL) | Nearest ISO 185 | Typical use cues |
|---|---|---|---|
| Class 20 (138 MPa / 20 ksi) | EN-GJL-150 | 200 / 150 | High damping, low stress housings, bases |
| Class 30 (207 MPa / 30 ksi) | EN-GJL-200 | 200 | General housings, frames |
| Class 35 (241 MPa / 35 ksi) | EN-GJL-250 | 250 | Pulleys, drums, stronger housings |
| Class 40 (276 MPa / 40 ksi) | EN-GJL-300 | 300 | Wear/strength-biased, thinner ribs |
| Class 50 (345 MPa / 50 ksi) | EN-GJL-350 | 350 | Higher strength; machinability ↓ slightly |
- A48 classifies on tensile of separate bars; EN 1561 and ISO 185 classify by tensile and (optionally) hardness bands, which can help consistency in machining.
Cross-reference deep dive: Gray Iron Grades: ASTM A48 vs EN-GJL Cross-Reference
Where gray iron shines (and where it struggles)
Best fit
- NVH-sensitive housings & bases: motors, gearboxes, compressors
- Thermal mass + heat dissipation: brake drums/rotors, pump frames (non-pressure)
- Machining efficiency: high graphite lubricity, predictable chip
Use with caution / consider alternatives
- Pressure-tight parts: choose ductile iron or add impregnation + leak-test plan; for selection logic: Selecting Iron for Pressure Parts
- High impact/shock: ductile iron or cast steel
- Corrosion exposure: treat with coatings (see §6) or upgrade material
Dimensional control: ISO 8062 CT, stock & GD&T
- Set tolerances per ISO 8062-3 (GPS)—define CT grade by size band and keep machining stock on datum faces.
- For callout examples: ISO 8062 Casting Tolerances Explained
- Convert critical requirements into GD&T that respects cast variability: Open GD&T Guide
Surface finish (Ra) targets you can actually hit
Typical as-cast Ra ranges; finish after blast depends on media/coverage. Use dual units µm/µin.
| Route | As-cast Ra (typical) | After shot-blast (prep for paint) |
|---|---|---|
| Green sand | 12–25 µm (470–980 µin) | 6–12 µm (240–475 µin) |
| Resin sand | 6–12 µm (240–475 µin) | 4–8 µm (160–315 µin) |
| Shell molding | 3.2–6.3 µm (125–250 µin) | 3–5 µm (120–200 µin) |
More detail by process: Surface Finish by Process: Green vs Resin vs Shell
Design for castability & NVH
- Wall & uniformity: avoid thin-to-thick jumps; follow DFM rules: Wall Thickness Rules
- Fillets & transitions: add R to cut hot-spots and stress—see Fillet & Radius Rules
- Ribs & pads: balance stiffness vs shrink risk; stagger ribs, avoid isolated pads
- NVH: if vibration is critical, gray iron often out-damps ductile/steel; choose the lowest class that meets strength to preserve damping
QA & acceptance (what to verify at PPAP)
- Material: A48 class (tensile on separate bars per A/B/C/S bar type).
- Dimensional: CT per ISO 8062-3; GD&T on machined datums.
- Surface & coating (if outdoor): blast profile Ra 6–12 µm; DFT per stack; adhesion check
- Soundness: riser/chill plan; pilot sectioning or NDT for hot-spot verification
- Documentation: tensile logs, optional hardness snapshot (if you specify an HBW band), CMM FAI on datum stack, coating DFT/adhesion (if coated)
- See PPAP Levels & Docs
What YB Metal delivers
YB YB Metal Solution quotes with a gray iron casting plan attached:
- The right class (A48 ⇆ EN-GJL ⇆ ISO 185) for your loads & NVH.
- Process route (green/resin/shell), CT grade, and stock on critical faces.
- Coating stack (if outdoor) with DFT/cure & adhesion tests.
- Pilot evidence pack: tensile logs, hardness snapshot (if used), micrographs (graphite type), and CMM FAI.
Need a part-specific plan? Upload your drawing at /rfq—we’ll return recommendations and a quote.
FAQs
CTA — specify with proof, not guesses
Cut cycle time without burning inserts. Upload your drawing to /rfq—YB Metal will send cutting data, tool list, and a coolant/filtration plan tailored to your part.