Intent: Vibration damping capacity and thermal conductivity (flake graphite) versus tensile ductility, elongation, and impact shock resistance (nodular graphite).
Engineering Verdict & When to Choose Each
Gray or ductile iron: use the actual loading and casting evidence
By Mat Grades Editorial Desk · Updated 2026-11-14
Consider gray iron where the approved design benefits from its casting and damping characteristics, and ductile iron where the required tensile and ductility performance supports that family. Neither choice can be approved from appearance or a generic workshop description.
For a housing, inspect local tensile stress around bosses, bolt loads and section transitions. For an arm or other impact-sensitive part, establish the relevant toughness evidence. A material upgrade may not correct porosity, a sharp corner or an overloaded fastening detail.
Read the standard’s coupon and section requirements with the foundry. A test bar result does not automatically describe every location in a complex casting. Specify the critical sections, machining allowance, permitted repairs and inspection before comparing quotations. An ASTM class and an EN grade may be useful candidates without being exact substitutes.
ASTM A48 Class 30 Gray Cast Iron: evidence to check
ASTM A48 Class 30 Gray Cast Iron has 9 cataloged property rows. Ultimate Tensile Strength is reported as minimum 207 MPa (minimum; As-Cast, 30.5 mm (1.2 in.) diameter Test Bar B). Modulus of Elasticity is reported as 100 GPa (typical; As-Cast). The record identifies ASTM A48 / A48M; check its scope and ordered edition. These are source-scoped examples rather than unrestricted design allowables. Read the full row context and certificate before using a value for acceptance; an absent value is not zero or a performance failure.
ASTM A536 Grade 65-45-12 Ductile Cast Iron: evidence to check
ASTM A536 Grade 65-45-12 Ductile Cast Iron has 11 cataloged property rows. Ultimate Tensile Strength is reported as minimum 448 MPa (minimum; As-Cast, Separately cast test coupon). 0.2% Offset Yield Strength is reported as minimum 310 MPa (minimum; As-Cast). The record identifies ASTM A536; check its scope and ordered edition. These are source-scoped examples rather than unrestricted design allowables. Read the full row context and certificate before using a value for acceptance; an absent value is not zero or a performance failure.
Four questions before approval
- Is the iron family positively identified?
- Are impact and local tensile loads reviewed?
- Does coupon evidence represent the relevant casting section?
- Are repair permission and inspection requirements explicit?
Check the linked material records for the complete source trail. Read the methodology before comparing minimum requirements with typical measurements. No direct substitution or qualified fabrication procedure is implied.
Material Comparison
Compare verified engineering materials side by side with condition-specific mechanical properties.
| Parameter | ASTM A48 Class 30 Gray Cast Iron | ASTM A536 Grade 65-45-12 Ductile Cast Iron |
|---|---|---|
| Material Family | Cast Iron | Cast Iron |
| Subfamily | Gray Cast Iron | Ductile (Nodular) Iron |
| ASTM Designation | A48 Class 30 | A536 Grade 65-45-12 |
| UNS Designation | F12101 | F33100 |
| SAE Designation | J431 G3000 | J434 D4512 |
| Element | ASTM A48 Class 30 Gray Cast Iron | ASTM A536 Grade 65-45-12 Ductile Cast Iron |
|---|---|---|
| C (Carbon) | 3.1 – 3.4% | 3.5 – 3.8% |
| Si (Silicon) | 1.9 – 2.3% | 2.2 – 2.8% |
| Mn (Manganese) | 0.6 – 0.9% | 0.2 – 0.5% |
| P (Phosphorus) | ≤ 0.12% | ≤ 0.05% |
| S (Sulfur) | ≤ 0.15% | ≤ 0.02% |
| Fe (Iron) | — | — |
| Mg (Magnesium) | — | 0.035 – 0.06% |
| Property | ASTM A48 Class 30 Gray Cast Iron | ASTM A536 Grade 65-45-12 Ductile Cast Iron |
|---|---|---|
| Density · TYPICALUnit: g/cm³ | 7.2 g/cm³ | 7.1 g/cm³ |
| Mechanical Parameter | ASTM A48 Class 30 Gray Cast IronAs-Cast (Standard Test Bar B, 30.5 mm diameter) | ASTM A536 Grade 65-45-12 Ductile Cast IronAs-Cast (Standard) |
|---|---|---|
| Poisson Ratio · TYPICALUnit: dimensionless | 0.26 dimensionless | 0.28 dimensionless |
| Property | ASTM A48 Class 30 Gray Cast Iron | ASTM A536 Grade 65-45-12 Ductile Cast Iron |
|---|---|---|
| Electrical Resistivity · TYPICALCategory: ELECTRICAL • Unit: nΩ·m | 680 nΩ·m | 560 nΩ·m |
| Thermal Conductivity · TYPICAL · 20 °CCategory: THERMAL • Unit: W/(m·K) | 48 W/(m·K) | — W/(m·K) |
| Thermal Conductivity · TYPICALCategory: THERMAL • Unit: W/(m·K) | — W/(m·K) | 35 W/(m·K) |
| Specific Heat Capacity · TYPICALCategory: THERMAL • Unit: J/(kg·K) | 480 J/(kg·K) | 470 J/(kg·K) |
| Coefficient of Thermal Expansion · TYPICALCategory: THERMAL • Unit: µm/(m·K) | 11 µm/(m·K) | 11.8 µm/(m·K) |
Machinability: Excellent machinability (~90% relative to C36000 free-cutting brass). Flake graphite acts as internal lubricant and natural chip breaker. Readily turned, milled, and drilled without burr formation.
Weldability: Poor weldability. Arc welding produces brittle martensitic heat-affected zones and crack risks. Recommended repair procedure requires 250°C–350°C (480°F–660°F) preheat, nickel-iron (ENiFe-CI) electrodes, short beads with peening, and slow insulation cooling.
Formability / Cold Working: Non-deformable. Gray cast iron is inherently brittle in tension and cannot be cold or hot deformed.
Heat Treatment & Annealing: Stress relief annealing at 540°C–595°C (1000°F–1100°F) for 1–2 hours eliminates residual casting stresses and prevents distortion. Flame or induction surface hardening can achieve 50–55 HRC on slideways.
Machinability: Very good machinability (~80% relative to C36000 baseline). Low cutting forces, good tool life, and smooth surface finishes.
Weldability: Moderately weldable with qualified procedures. Preheating to 200°C–250°C and nickel-iron (ENiFe-CI) filler wire are required to prevent hard heat-affected zones.
Formability / Cold Working: Limited localized forming (coining, staking) feasible due to 12% elongation.
Heat Treatment & Annealing: Typically utilized in the as-cast condition. Can be full annealed at 870°C for maximum ductility or normalized at 900°C for increased strength.