Solidification and Cooling Rates: How Section Modulus Drives Microstructure

solidification cooling rates section modulus

Who this helps: Design Engineers / Buyers optimizing gray/grey iron, ductile iron and ADI parts for strength, toughness, leak-tightness and machinability.
What you’ll learn: how section (casting) modulus controls cooling rate → microstructure → properties, copy-paste formulas you can use on a napkin, and geometry/process levers to kill hot spots before they become scrap.

Prepared by YB Metal Solution. Share your drawing—YB Metal will return a part-specific modulus map, cooling notes and a castable feeder/chill plan.

Author: YB Metal Solution Engineering Team (hereafter YB Metal)

Table of contents

  • Why modulus controls microstructure
  • Clear definitions (no mix-ups)
  • Cooling time vs modulus—safe-to-paste formulas
  • What different cooling rates do in iron
  • Design levers to tune local modulus
  • Risers, chills and building a modulus gradient
  • Worked example: pad vs rib
  • Drawing notes you can copy
  • What YB Metal delivers
  • FAQs

Why modulus controls microstructure

Thicker regions (higher section modulus M) cool slower, shifting graphite form, matrix (ferrite/pearlite), hardness and shrinkage risk. Shape the M-field and you shape the microstructure & properties—before you ever touch chemistry.

Clear definitions (no mix-ups)

Values are stable continuous wall targets. Local short features can go ~0.5 mm (0.02 in) thinner if well fed. Use mm/in in drawings.

  • Casting (section) modulus, M: local volume ÷ effective cooling surface area. Higher M → slower cooling.
  • Structural section modulus (beam theory): not what we mean here.

In practice, evaluate M = V/A locally—pads, bosses, ribs, corners, spoke roots.

Cooling time vs modulus—safe-to-paste formulas

Casting modulus:
M = V / A

Chvorinov-style solidification time:
t_solid ≈ B · (V/A)^n → t_solid ≈ B · M^n
Typical sand molds: n ≈ 2 (use as a heuristic)

Relative comparison (same alloy/mold):
t1 / t2 ≈ (M1 / M2)^n

Rule of thumb: if a boss has 1.5× the modulus of a nearby wall, it can take ~(1.5)^2 ≈ 2.25× longer to solidify—plan feeding, chills or geometry accordingly.

What different cooling rates do in iron

Indicative behavior for design screening; actuals depend on chemistry, inoculation and process control.

Cooling tendency (via M)Gray/grey iron outcomeDuctile iron outcomeProperty signal
Fast (low M)Finer flakes; with weak inoculation risk chill/carbideHigh nodule count; thin walls may harden↑ HB/UTS; ↓ machinability if carbides form
Balanced (mid M)Type-A graphite, pearlite/ferrite mixGood nodularity; 450-10 / 500-7 targetsGood fatigue; balanced machining
Slow/hot spot (high M)Coarser flakes; flotation risk in very heavy sectionsLower nodules near cores/risers; softer zones↓ HB/UTS locally; ↑ shrink/leak risk

Takeaway: balance M first; then fine-tune with CE, inoculation, pouring temp and (if needed) heat treatment.

Design levers to tune local modulus

  • Uniform walls: keep adjacent wall ratio 0.7–1.3; avoid jumps > 1.5× within 10–15 mm (0.4–0.6 in).
  • Ribs & pads: ribs ≤ 0.6–0.7 × wall; taper pads so they present more cooling area A.
  • Fillets/radii: generous radii remove high-M corners and ease flow.
  • Cored pockets / lightening holes: increase A and shorten flow length without killing stiffness.
  • Boss OD & height: keep reasonable; add undercuts/vents so cores don’t choke feeding.
  • Draft: 0.5–1.0° externals; 1.0–1.5° internals—more consistent surfaces → more consistent M.

Helpful internal reads:

Wall Thickness & Uniformity Rules
Fillets & Radii

Risers, chills and building a modulus gradient

Directional solidification needs a monotonic rise in M toward the riser.

  • Toward riser: allow highest M right under/near the feeder.
  • Away from riser: step M down with thinner sections or added cooling area.
  • Chills: place at stubborn hot spots to locally increase cooling; small, targeted chills beat large risers on thin parts.
  • Coatings: zircon/alumina where heat flux is high; thinner coat at hot faces (watch penetration vs burn-on).

Worked example: pad vs rib

Part: ductile-iron housing; wall 6.0 mm (0.24 in).
Pad: 40×40×18 mm; Rib: 40×40×6 mm. Approximate effective areas shown.

Pad:
V ≈ 40·40·18 = 28,800 mm³
A_eff ≈ 2·(40·18 + 40·18) + (40·40) ≈ 3,920 mm²
M_pad ≈ 7.35 mm

Rib:
V ≈ 40·40·6 = 9,600 mm³
A_eff ≈ 2·(40·6 + 40·6) + (40·40) ≈ 2,240 mm²
M_rib ≈ 4.29 mm

t_pad / t_rib (n≈2) ≈ (7.35 / 4.29)² ≈ (1.71)² ≈ 2.9

Implication: the pad cools ~3× slower → higher shrink risk and locally softer matrix.
Fixes that keep function: taper pad to 12–14 mm, add large fillets, bring a feeder closer or add a spot chill, or core a relief pocket under the pad.

Drawing notes you can copy

  • Material:EN-GJS-500-7 (ASTM A536 80-55-06 acceptable).”
  • Uniformity & transitions: “Adjacent wall ratio 0.7–1.3; thickness jumps ≤ 1.5× within 15 mm (0.6 in); fillets ≥ 0.5× wall (min 1.5 mm / 0.06 in).”
  • Feeding & chills: “Supplier may apply local chills and adjust riser modulus to ensure directional solidification; changes recorded in PPAP.”
  • Inspection: “Provide microstructure snapshots at hot vs cold locations; hardness map; CMM/3D scan on datum scheme.”
  • As-cast tolerance:ISO 8062-3 CT8 (mid-size) unless otherwise noted.”
    Internal reads to link: ISO 8062 CT grades /iso-8062-casting-tolerances-explained-ct-grades

What YB Metal delivers

YBmetal Solution includes modulus-based DFM at quote:

  • A V/A modulus map highlighting hot spots and suggested geometry tweaks.
  • A gating/risering concept that enforces directional solidification.
  • Chill & coating plan with risks (carbide/chill, shrink/leak) called out.
  • Evidence in pilots: hardness trend, micrographs, and CMM/3D scan overlays.

FAQs

Cooling too fast (low M) plus weak inoculation → carbides/chill. Balance M, strengthen inoculation and hold metal temperature.

You can reduce hot spots, but you still need directional feeding—riser position/sleeves/necks matter.

Solidification is the same. Post-HT adds small dimensional change (~+0.02–0.06% typical); verify with coupons.

Complex parts or mixed sections. Still, a quick hand modulus check up front catches most risks.

Try geometry first (taper, fillet, pocket). Use targeted chills if a hot spot survives or the gradient is weak.

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