ASTM A48 Class 40 Gray Iron: High-Strength Flake Graphite
ASTM A48/A48M · Published: 2026-06-01 · Updated: 2026-06-02
ASTM A48 Class 40 is a high-strength gray cast iron with 276 MPa minimum tensile strength, achieved through a predominantly pearlitic matrix with Type A (random) flake graphite of controlled size (ASTM A247 Size 4-6). The higher strength compared...
ASTM A48 Class 40 is a high-strength gray cast iron with 276 MPa minimum tensile strength, achieved through a predominantly pearlitic matrix with Type A (random) flake graphite of controlled size (ASTM A247 Size 4-6). The higher strength compared to Class 30 (50% increase) enables thinner casting wall sections and higher load-bearing capacity. Used for heavy-duty machine tool beds (lathes, milling machines), large diesel engine blocks, compressor cylinders, and press frames where vibration damping and high compressive strength are critical. Requires inoculation practice during foundry processing to ensure Type A graphite formation and avoid chill (iron carbide) in thin sections. Section sensitivity is higher than Class 30—thinner sections produce harder, stronger material; thicker sections require alloying additions (Cr, Cu) to meet Class 40 minimums throughout.
Quick Facts
| Category | Cast Iron |
| Standard | ASTM A48/A48M |
| Density | 7.25 g/cm³ |
| Yield Strength | N/A (brittle) |
| Tensile Strength | 276 MPa (40 ksi) |
Global Equivalents & Cross-Reference
| Alternative Standard / Grade | Action |
|---|---|
| EN-GJL-300 | Compare |
| GG25 | Compare |
| FC300 | Compare |
| HT300 | Compare |
| ISO 185/JL/300 | Compare |
Related Materials
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Frequently Asked Questions
Is Class 40 gray iron machinable with carbide tooling?
Yes, though slower than Class 30. The higher pearlite content (95%+) and finer graphite flakes produce harder material (200-240 HB) that requires reduced cutting speeds—approximately 80% of Class 30 speeds for turning, 70% for milling. Use C-2 carbide grade with generous positive rake angles. The graphite still provides chip-breaking and lubricating benefits, so tool life remains good despite higher hardness. Hard spots (chill) at casting edges should be removed by grinding before machining.
References & International Standards
- ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
- International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
- American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
- European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu