Machinability Of Gray VS Ductile Iron
Who this helps: Manufacturing Engineers / CNC Programmers / Buyers planning cycle time and tooling budgets for gray/grey & ductile (SG) iron housings, hubs, brackets and covers.
What you’ll get: practical tool-life data by hardness, safe speed/feed windows (metric + imperial), coolant/filtration strategies, burr control, and ready-to-paste drawing notes.
Prepared by YB Metal Solution. Share your drawing via /rfq—YB Metal will return a machining plan with vetted cutting data, tool list and quote.
Author: YB Metal Solution Engineering Team (hereafter YB Metal)
Table of contents
- Why gray vs ductile machine differently
- Tool-life snapshot by hardness band (HBW)
- Cutting data: turning, milling, drilling, boring
- Coolant strategy: dry, MQL, flood—when each wins
- Filtration & sludge control (keep accuracy stable)
- Inserts, edge prep & geometry
- Burrs, finish & cleanliness
- Copy-paste drawing notes
- What YB Metal delivers
- FAQs
Why gray vs ductile machine differently
Graphite form:
- Gray iron (flakes) = built-in solid lubricant → short chips, low cutting forces, good damping → longer tool life, nicer finish.
- Ductile iron (nodules) = less lubrication, higher tensile strength → higher cutting forces, more burr, slightly hotter edge.
- Matrix & hardness: More pearlite/carbides (and higher HBW) ↑abrasiveness → ↓tool life.
- Chilled zones: Thin/fast-cooling edges may show local hard spots → use PCBN/ceramic only where needed.
Tool-life snapshot by hardness band (HBW)
Criteria: VB flank wear ≈ 0.30 mm (0.012 in) or finish/burr fails. Relative numbers are guides for cost models.
Material (typical matrix) | HBW | Relative tool life (turning, coated carbide) | Notes |
---|---|---|---|
Gray iron (A-flake, pearlitic) | 170–210 | 1.00 (baseline) | Smooth cut, low burr; often run dry + air. |
Gray iron (pearlite+carbide) | 220–260 | 0.7–0.8 | Watch chilled edges; ceramic at high vc if stable. |
Ductile iron EN-GJS-450-10 / 65-45-12 | 170–220 | 0.7–0.85 | Slightly more force & heat; edge hone helps. |
Ductile iron EN-GJS-500-7 / 80-55-06 | 200–250 | 0.55–0.75 | Balance speed/feed; MQL/flood reduces crater wear. |
Ductile iron EN-GJS-700-2 / 100-70-03 | 230–280 | 0.35–0.55 | Abrasive; consider tougher grades, lower vc. |
Rule of thumb: every +30–40 HBW step in cast iron can cut carbide tool life by ~20–30% at constant conditions.
Cutting data: turning, milling, drilling, boring
Start mid-range; confirm with trial coupons from your casting batch. Values assume rigid setups, CVD/PVD coated carbide unless noted.
Turning (rough → finish)
Material & HBW | vc (m/min) | vc (sfm) | f (mm/rev) | f (ipr) | ap (mm) | ap (in) |
---|---|---|---|---|---|---|
Gray 170–210 | 220–350 | 720–1150 | 0.20–0.35 | 0.008–0.014 | 2.0–4.0 | 0.08–0.16 |
Gray 220–260 | 180–280 | 590–920 | 0.18–0.30 | 0.007–0.012 | 1.5–3.0 | 0.06–0.12 |
DI 450-10 (170–220) | 180–260 | 590–850 | 0.18–0.30 | 0.007–0.012 | 1.5–3.0 | 0.06–0.12 |
DI 500-7 (200–250) | 150–230 | 490–755 | 0.16–0.28 | 0.006–0.011 | 1.5–2.5 | 0.06–0.10 |
DI 700-2 (230–280) | 120–200 | 395–655 | 0.12–0.24 | 0.005–0.009 | 1.0–2.0 | 0.04–0.08 |
Finish pass: reduce f to 0.08–0.15 mm/rev (0.003–0.006 ipr), ap 0.2–0.5 mm (0.008–0.020 in).
Face/shoulder milling (indexable, 45°/90°)
Material | vc (m/min) | fz (mm/tooth) | ap (mm) | Notes |
---|---|---|---|---|
Gray 170–210 | 250–400 | 0.10–0.20 | 1.0–3.0 | Dry or MQL; strong damping allows higher vc. |
Gray 220–260 | 200–320 | 0.08–0.18 | 1.0–2.5 | Watch edge chipping at chills. |
DI 450-10 | 180–280 | 0.08–0.18 | 0.8–2.0 | Use tougher grade, honed edge. |
DI 500-7 / 700-2 | 150–240 | 0.06–0.14 | 0.8–1.5 | Prefer PVD/tough CVD; reduce vc if chatter. |
Finishing Ra 1.6–3.2 µm (63–125 µin): fz 0.05–0.10 mm/tooth, ap ≤ 0.5–1.0 mm; consider wiper geometry.
Drilling (solid carbide or insert drill)
Material | Ø (mm) | vc (m/min) | f (mm/rev) | Notes |
---|---|---|---|---|
Gray 170–230 | 8–20 | 80–120 | 0.10–0.20 | Dry + air ok; chips are short. |
Ductile 200–260 | 8–20 | 60–90 | 0.08–0.18 | MQL/flood helps chip evacuation & edge life. |
Large Ø 20–40 | 60–90 (GI) / 50–70 (DI) | 0.12–0.30 | Ensure rigid fixturing; pilot if positional. |
Boring / reaming (tight bores)
- Boring: vc 80–160 m/min (260–525 sfm), f 0.05–0.15 mm/rev (0.002–0.006 ipr).
- Reaming: vc 30–60 m/min (100–200 sfm), f 0.05–0.12 mm/rev (0.002–0.005 ipr).
- Use flood to stabilize size and remove fines; keep coolant clean (see §5).
Coolant strategy: dry, MQL, flood—when each wins
Strategy | Where it shines | Where to avoid |
---|---|---|
Dry + air blast | Gray iron rough/finish turning & milling; keeps graphite dust dry, avoids mud; stable tool wear. | Ductile heavy cuts at higher HBW—may overheat edges and raise crater wear. |
MQL (oil mist) | DI milling/turning at mid HBW; reduces crater and built-up edge; cleaner floors than flood. | Deep drilling/boring where heat removal is critical. |
Flood coolant | Deep holes, boring/reaming, tight tolerances, multi-op cells; ductile iron finishing to control size/thermal drift. | Gray iron roughing (makes sludge); weak filtration will wreck pumps/valves. |
Setpoints (typ.)
- Soluble oil 7–10%, pH 8.5–9.2; keep tramp oil <2%.
- Air knives to keep fines off gibs/ways if running dry/MQL.
Filtration & sludge control (keep accuracy stable)
- Primary filtration: bag/cartridge 25–50 µm; add magnetic separation for Fe fines.
- Settling/drag-out trays on heavy GI lines; clean sump on schedule.
- Differential-pressure alarms for clogged filters (protects flood operations).
- Coolant monitoring: concentration, pH, bacteria; graphite + fine Fe quickly degrades coolant.
Post-machining cleanliness: vacuum + air blow, no oily graphite paste on sealing faces; specify cleanliness if critical (see §8).
Inserts, edge prep & geometry
Grade/Coating:
- Gray iron: CVD-coated carbide handles abrasion; ceramic/PCBN for high-speed finishing (stable workholding only).
- Ductile iron: tougher PVD/CVD with honed edge (K-land) to resist chipping.
- Geometry: light-to-medium negative rake, strong edge hone for DI (e.g., hone 0.02–0.06 mm / .0008–.002 in); sharper edge for GI finishing.
- Hard spots: spot-use PCBN / ceramic where chill is proven; don’t blanket the process.
Burrs, finish & cleanliness
- Burr tendency: Ductile > Gray (due to higher toughness). Add de-burr pass or use wiper on finishing.
- Finish targets: machined seals Ra 1.6–3.2 µm (63–125 µin); bores per fit. See: Surface Finish (Ra) guide
- Dust control: extract at tool–work interface; prevent build-up on linear scales & encoders.
Drawing notes
Material & hardness:
Gray iron per ASTM A48/EN-GJL, HBW target 180–230 (record per batch)
or Ductile iron per ASTM A536/EN-GJS, HBW target 180–260 (record per batch)
Machining surfaces:
Sealing faces after machining: Ra 1.6–3.2 µm (63–125 µin)
Bores per ISO fits; deburr all edges R0.2–0.5 mm (0.008–0.020 in)
Process notes:
Supplier to control coolant/filtration to maintain size stability; remove graphite fines before packing.
Critical dimensions measured at 20 ±2 °C; declare compensation if finish-machined post heat treatment.
Evidence on pilots/FAI (if required):
Tool-life record at VB=0.30 mm criterion, 3 samples
Hardness map (HBW) and microstructure snapshots
Dimensional report (CMM) post-machining on datum scheme
What YB Metal delivers
YB Metal quotes with a machining plan attached:
- Cutting data by operation (vc, f, ap), tool list and expected tool life by HBW.
- Coolant/filtration setup, burr and finish plan (Ra by zone).
- Cycle-time and cost impact of gray vs ductile options; suggestions to tweak casting (boss/rib) for shorter cycles.
Upload your drawing via —we’ll return a part-specific plan and quotation.
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.