A practical selection guide for engineering and sourcing teams: ductile iron vs gray iron: ductile (EN-GJS) vs gray iron (EN-GJL) compared across design rules, cost, machinability, leak-tightness, and service life, with ASTM/EN grade maps, realistic wall-thickness/tolerance tables and a PPAP-ready QC checklist.
Executive summary: YBmetal helps you to choose each material
Choose ductile iron (spheroidal graphite, EN-GJS / ASTM A536) if:
- You need higher strength and ductility (UTS ~ 400–700 MPa; elongation 5–14%).
- The part is pressure-tight, impact- or fatigue-loaded (pump/valve bodies, steering knuckles, brackets).
- You want lower leakage risk and better tolerance to shock loads.
Choose gray iron (lamellar graphite, EN-GJL / ASTM A48) if:
- You prioritize damping, dimensional stability and machinability (machine beds, housings, stove/boiler parts).
- The duty is non-pressure / low-impact, and cost sensitivity is high.
- You need thin-wall sections with excellent vibration damping.
Cost index (same geometry, same process, same volume):
Let gray iron = 1.00. Ductile iron typically falls 1.20–1.40 due to Mg treatment, inoculation, and yield. If ductile lets you reduce conservative wall or replace weldments/steel, the total life-cycle cost per service-hour can still be lower.
Standards and grade map (so drawings are unambiguous)
Common grade cross-reference (ASTM ↔ EN)
| Function | Gray Iron (ASTM A48 ↔ EN-GJL) | Ductile Iron (ASTM A536 ↔ EN-GJS) |
|---|---|---|
| General utility | Class 30 ≈ EN-GJL-200 | 60-40-18 ≈ EN-GJS-400-18 |
| Balance strength/machining | Class 40 ≈ EN-GJL-250 | 65-45-12 ≈ EN-GJS-450-12 |
| Higher strength | Class 50 ≈ EN-GJL-300 | 80-55-06 ≈ EN-GJS-500-7 |
| High strength / lower ductility | Class 60 ≈ EN-GJL-350 | 100-70-03 ≈ EN-GJS-700-2 |
Name the standard and revision year on the print: ASTM A48/A536, EN 1561 (EN-GJL), EN 1563 (EN-GJS).
Typical room-temperature properties (indicative)
| Material | UTS (MPa) | 0.2% YS (MPa) | Elong. (%) | Hardness (HBW) | Notes |
|---|---|---|---|---|---|
| EN-GJL-250 | 250–300 | — | — | 170–220 | Damping, machinability |
| EN-GJL-300 | 300–350 | — | — | 190–240 | Thicker sections need control |
| EN-GJS-450-12 | 450–550 | 310–360 | 10–14 | 140–190 | Versatile structural grade |
| EN-GJS-500-7 | 500–600 | 320–420 | 6–9 | 170–220 | Strength vs machining balance |
| EN-GJS-700-2 | 700–850 | 420–550 | 2–4 | 220–270 | High strength, lower ductility |
Test plan (what we actually run in-house): spectrometer chemistry per heat; Brinell hardness each batch; tensile per heat/lot; optional impact & metallography (nodule count/shape, matrix) for PPAP or critical parts.
Design implications: wall thickness, NVH, and leak-tightness
Minimum wall thickness by process (mm / in)
| Process | Gray iron feasible | Ductile iron feasible | Guidance |
|---|---|---|---|
| Green sand (auto line) | 4–6 mm (0.16–0.24 in) | 5–7 mm (0.20–0.28 in) | Best for regular shapes/volume |
| Resin sand | 6–10 mm (0.24–0.39 in) | 7–12 mm (0.28–0.47 in) | Medium/large parts |
| Shell molding | 3–4 mm (0.12–0.16 in) | 4–5 mm (0.16–0.20 in) | Thin-wall/high accuracy |
Shop experience: thin-wall favors gray iron or shell molding; avoid isolated hot spots—use fillets and ribs to balance modulus.
NVH (noise & vibration)
Gray iron’s lamellar graphite provides high damping—ideal for machine bases and compressor housings. Ductile iron damps less but offers superior strength/ductility, preferred for structural or safety-critical parts.
Leak-tightness and pressure
- Ductile has spherical graphite and typically lower inherent leak paths—better for pressure-tight bodies; combine with impregnation only if needed.
- Gray is acceptable for non-pressure or low-pressure duties and excels in thermal stability.
Tolerances, shrinkage, and machining allowance
Casting tolerances (ISO 8062–3)
- Use CT6–CT9 for most iron castings, depending on size/complexity.
- Define general tolerances for as-cast surfaces and GD&T (flatness, position, profile) for machined features.
Shrinkage allowance (pattern compensation)
- Gray iron: ~ 0.8–1.0%
- Ductile iron: ~ 1.0–1.2%
- Calibrate per geometry and process; we confirm on first-off with CMM/3D scan and update tooling if required.
Typical machining allowance (both units)
| Casting size (rough weight) | Green sand | Resin sand | Shell molding |
|---|---|---|---|
| ≤ 5 kg (≤ 11 lb) | 1.0–1.5 mm (0.04–0.06 in) | 1.5–2.0 mm (0.06–0.08 in) | 0.8–1.2 mm (0.03–0.05 in) |
| 5–20 kg (11–44 lb) | 1.5–2.5 mm (0.06–0.10 in) | 2.0–3.0 mm (0.08–0.12 in) | 1.0–1.5 mm (0.04–0.06 in) |
| ≥ 20 kg (≥ 44 lb) | 2.5–3.5 mm (0.10–0.14 in) | 3.0–5.0 mm (0.12–0.20 in) | 1.5–2.0 mm (0.06–0.08 in) |
Machinability & surface finish
Casting tolerances (ISO 8062–3)
- Gray iron cuts “clean” thanks to free graphite—longer tool life and easier chip control.
- Ductile iron is tougher; use appropriate inserts, speeds/feeds and coolant filtration to manage graphite/sludge.
Shrinkage allowance (pattern compensation)
- Gray iron: ~ 0.8–1.0%
- Ductile iron: ~ 1.0–1.2%
- Calibrate per geometry and process; we confirm on first-off with CMM/3D scan and update tooling if required.
Typical as-cast surface roughness (Ra):
- Green sand: Ra 6.3–12.5 μm (250–500 μin)
- Resin sand: Ra 3.2–6.3 μm (125–250 μin)
- Shell molding: Ra 1.6–3.2 μm (63–125 μin)
Finishing: blast media selection (steel shot vs grit) alters Ra; prepare surfaces per coating specs (e.g., ISO 12944 systems).
Durability and service life
- Fatigue & impact: ductile iron’s higher elongation and nodular graphite deliver better fatigue life and impact resistance—key for hubs, brackets, pressure parts.
- Thermal cycling: gray iron offers thermal stability and damping, good for stove/boiler parts and machine structures.
- Corrosion: choose coating systems (zinc-rich epoxy, powder) to match the environment; pack with VCI for export.
Cost, yield, and lead-time: what really moves the number
| Driver | Gray iron | Ductile iron | Notes |
|---|---|---|---|
| Raw metal & treatment | Lower | Higher (Mg/inoculants) | Ductile adds Mg treatment + inoculation control |
| Yield & risering | Good | Slightly lower | Ductile needs more conservative feeding on heavy sections |
| Tooling | Similar | Similar | Tool compensation differs (shrinkage/warpage) |
| Machining | Easier | Harder | Tooling & cycle time typically ↑ for ductile |
| NDT/pressure test | Often lighter | Often heavier | Tooling & cycle time are typically ↑ for ductile |
| Lead-time (prototype→PPAP) | 3–6 wks tooling + pilots | +1 wk typical | Depends on process and part size |
Break-even tips: if ductility allows thinner walls or consolidates a weldment into one casting, it often wins on total landed cost + reliability.
YBmetal Quality plan (what a robust PPAP looks like)
YBmetal adopts PPAP for projects to ensure quality:
- Incoming & melt: spectrometer chemistry per heat; charge tracking and traceability.
Process control: sand system KPIs (AFS GFN, moisture, LOI), Mg/inoculation records, pouring temperature, yield.
NDT: UT/MT for critical sections; X-ray (for steel and select ductile areas); leak test per drawing pressure.
Dimensional: FAI + CMM on all datums/profiles; 3D scan for thin-wall or complex shapes.
Mechanical: tensile & hardness per standard; optional impact; microstructure with nodule count and ferrite/pearlite.
Documentation: Control Plan, PFMEA, PSW, layout report, material certs
RFQ checklist (send this with your drawings)
- Drawing + standard and grade (e.g., EN-GJS-500-7 / ASTM A536 80-55-06).
- Annual usage/batch size, target price or cost goal (optional).
- Process preference (green/resin/shell) or performance targets (tolerance, Ra).
- Critical features & GD&T; machining drawing or datum scheme.
- Leak/pressure requirements, NDT coverage, and sample plan.
- Coating/packaging specs; export destination.
- Required documents (PPAP level, FAI, certs).
FAQs
Ready to Start Your Iron Casting Project With YBmetal?
Send us your drawings or part photos, and we’ll prepare a tailored sourcing plan, including foundry recommendations, a cost estimate, and a sampling lead time.