A practical ADI heat treatment guide: how to choose the austenitize → isothermal quench → hold cycle, what grade ranges deliver which strength/ductility, and how to control distortion, retained austenite and hardness—with YB Metal quality gates and an RFQ checklist.
Executive summary
- ADI = Austempered Ductile Iron: ausferritic matrix that delivers high strength + toughness + wear resistance.
- Cycle levers: austenitize ~850–920 °C (1560–1690 °F) → rapid transfer → isothermal quench/hold ~250–380 °C (480–715 °F); lower hold temp ⇒ higher strength, lower elongation.
- Typical property windows (indicative): UTS 800–1400 MPa, El. 1–10 %, HBW ~ 260–480 depending on grade.
- Quality gates: base iron nodularity, retained austenite %, hardness/tensile by grade, dimensional & distortion checks.
- When to pick ADI vs steel: weight & cost reduction where fatigue, impact and wear matter; avoids alloyed steel + Q&T in many cases.
What changes from ductile iron to ADI (metallurgy & benefits)
- Base ductile iron (EN-GJS / ASTM A536) is austenitized so carbon saturates the matrix; isothermal quench forms ausferrite (acicular ferrite + high-carbon austenite).
- Outcomes: 2–3× yield vs ferritic DI, markedly improved fatigue & wear, good impact at appropriate grades, often near-net machining after heat-treat if planned.
Cycle design: austenitize, transfer, isothermal quench & hold
Austenitize
- Temperature/time: ~850–920 °C; soak to core (time scales with section size/chemistry).
- Chemistry assists: Ni/Cu/Mo improve hardenability for thick sections; limit Si/Mn within spec.
- Pre-conditions: nodularity & nodule count in base iron; pearlite kept low/controlled.
Transfer
- Fast, consistent transfer from furnace to bath (seconds, not minutes) to avoid pearlite/upper bainite.
Isothermal quench & hold
- Quench medium: molten salt bath common; oil/polymer variants exist but control is tighter in salt.
- Hold temperature bands (indicative):
- 250–300 °C (480–570 °F): highest strength, lowest ductility (e.g., “1400-class”).
- 300–340 °C (570–645 °F): balanced (e.g., “1000–1200-class”).
- 340–380 °C (645–715 °F): more ductility/impact (e.g., “800-class”).
- Hold time: typically 30–120 min to reach ausferritic transformation through section.
- Rinse/wash & cool; no conventional tempering.
Distortion control
- Load symmetry & fixturing, adequate spacing/agitation, avoid mixed section stacks.
- Machine-before strategy for high-accuracy surfaces; finish after ADI.
- Stress risers: add fillets/radii per DFM; balance sections.
ADI grade ranges (ASTM A897 / EN 1564—indicative)
Exact grade names differ (e.g., 800-10 / 900-8 / 1050-6 / 1200-3 / 1400-1). Use the property windows below to choose targets; validate with tensile & hardness.
| Grade family (typical) | UTS (MPa) | YS (MPa) | Elong. (%) | Hardness (HBW) | Typical hold band |
|---|---|---|---|---|---|
| ~800-class (ductility-leaning) | 800–900 | 500–650 | 6–10 | 260–320 | ~340–380 °C |
| ~900–1050-class (balanced) | 900–1100 | 650–800 | 4–8 | 300–380 | ~300–340 °C |
| ~1200–1400-class (strength-leaning) | 1200–1400 | 900–1100 | 1–4 | 360–480 | ~250–300 °C |
Numbers are typical production windows, not the text of the standards; final properties depend on section size, chemistry and process control.
Quench severity, section size & alloying (making the grade)
- Section thickness drives time-to-core; thicker parts need hardenability aids (Ni/Cu/Mo) and robust transfer.
- Salt bath agitation & load ratio affect uniformity; keep bath load ≤ recommended % to prevent temp drop.
- DoE before PPAP: trial small matrices of hold temps/times across your section range to lock the window.
Quality gates that keep ADI consistent (what we check)
Incoming/base iron (pre-ADT):
- Nodularity & nodule count per spec; low undesirable phases.
- Chemistry review (C, Si, Mn, Cu, Ni, Mo) vs target grade & section.
In-process:
- Furnace temperature uniformity survey; transfer time log; bath temperature & agitation log; load map.
Final (per lot/sampling plan):
- Hardness (ISO 6506 Brinell): per grade window.
- Tensile (ASTM A370 / as referenced by A897 or EN 1564): representative test bars or cast-on coupons.
- Retained austenite % (metallography or XRD when specified).
- Microstructure: ausferrite morphology & absence of carbides per acceptance.
- Dimensional/Distortion: FAI + CMM/3D scan on critical datums; leak/NDT as required.
YB Metal production snapshot
From recent ADI programs (ductile iron housings/arms):
- Cycle windows we run (typical): austenitize ~880–900 °C; hold ~280–360 °C depending on target grade.
- Sampling & verification: Brinell each batch; tensile per heat/lot; retained austenite on initial qualification or per spec; FAI + CMM on criticals.
- Distortion control: symmetric fixturing; balanced loads; pre-machined datums when required.
- Documentation: PPAP pack (Control Plan, PFMEA, PSW, FAI/CMM, material certs).
Machining & finishing notes
- Pre-machine rough datums/stock when tight GD&T is needed; finish after ADI.
- Expect tool-life changes vs ferritic DI; select inserts & coolant accordingly; verify flatness/runout post-ADT.
- Surface prep to coating spec (e.g., ISO 12944 systems) when corrosion protection is needed.
Cost & risk (what tight specs do to the quote)
| Driver | Effect when grade/controls tighten |
|---|---|
| Lower hold temp (higher strength) | More distortion/scrap risk; tighter bath control; more verification |
| Thick sections | Alloying cost (Ni/Cu/Mo); longer soaks; heavier fixtures |
| Inspection scope | XRD/metallography adds cost/time; higher sampling = more lab work |
| Reworkability | ADI is not easily re-heat-treated to fix out-of-window structures—first-time yield matters |
RFQ checklist (tolerance-ready)
- Base ductile iron grade/chem target (if fixed) and ADI grade family target.
- Section thickness map; critical features & GD&T; datum strategy.
- Required properties (UTS/YS/El., hardness) & impact/fatigue if any.
- Inspection scope: hardness, tensile, RA%, microstructure, CMM; PPAP level.
- Volume & batches; coating/packaging; destination; any NDT/leak test.
- History of prior failures (distortion, cracks, property misses) if re-sourcing.
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.