CF8 vs CF8M: Cast Stainless Steel Grades
CF8 and CF8M are the two most common austenitic stainless steel casting grades per ASTM A351. CF8 is the cast equivalent of wrought Type 304. CF8M is the cast equivalent of wrought Type 316. The single-letter “M” suffix indicates the addition of 2-3% molybdenum, which fundamentally changes the material’s corrosion performance.
When to Use CF8 vs CF8M
CF8 (cast 304) is adequate for:
- Clean steam, condensate, and demineralized water systems
- Most organic chemicals, mild acids, and alkalis
- Food and beverage processing
- General-purpose corrosion resistance where chlorides are absent
CF8M (cast 316) is required for:
- Chloride-bearing environments (seawater, brine, brackish water)
- Sulfuric acid at moderate concentrations and temperatures
- Offshore and marine valve applications
- Pharmaceutical and high-purity systems
- Any service where pitting or crevice corrosion is a design concern
Specifications and Properties
| Property | CF8 (Cast 304) | CF8M (Cast 316) |
|---|---|---|
| ASTM specification | A351 CF8 | A351 CF8M |
| Wrought equivalent | Type 304 (UNS S30400) | Type 316 (UNS S31600) |
| Carbon | 0.08% max | 0.08% max |
| Chromium | 18.0-21.0% | 18.0-21.0% |
| Nickel | 8.0-11.0% | 9.0-12.0% |
| Molybdenum | None specified | 2.0-3.0% |
| Tensile strength (min) | 485 MPa (70 ksi) | 485 MPa (70 ksi) |
| Yield strength (min) | 205 MPa (30 ksi) | 205 MPa (30 ksi) |
| Elongation (min) | 35% | 30% |
| Pitting resistance (PREN) | ~18-20 | ~24-28 |
| Service temp (max) | 538°C (1000°F) | 538°C (1000°F) |
| Cryogenic rating | -254°C (-425°F) | -254°C (-425°F) |
| Relative cost | Lower | 15-25% premium |
The Molybdenum Factor
Molybdenum is the critical differentiator. The 2-3% Mo in CF8M raises the Pitting Resistance Equivalent Number (PREN = %Cr + 3.3 x %Mo + 16 x %N) from approximately 19 to 26. This translates to significantly better resistance to localized corrosion in chloride environments.
In practical terms: a CF8 valve body in seawater cooling service will develop pitting within months. CF8M will last years under the same conditions, though it is not immune to chloride stress corrosion cracking at elevated temperatures.
Low-Carbon Variants
Both grades have low-carbon counterparts:
- CF3 (cast 304L): 0.03% C max
- CF3M (cast 316L): 0.03% C max
The low-carbon variants resist sensitization (chromium carbide precipitation at grain boundaries) during welding and prolonged exposure to 425-870°C. Specify CF3M for welded assemblies or any service where intergranular corrosion is a risk.
Pairing with Wrought Materials
| Cast Grade | Forged Equivalent | Pipe Specification |
|---|---|---|
| CF8 | A182 F304 | A312 TP304 |
| CF8M | A182 F316 | A312 TP316 |
| CF3 | A182 F304L | A312 TP304L |
| CF3M | A182 F316L | A312 TP316L |
Both grades require solution annealing per ASTM A351 (min 1040°C, water quench). Mill test certificates must report ferrite content, full chemistry, and NDT results if specified. Hydrostatic testing follows the applicable valve or fitting standard. The casting process produces a dual-phase (austenite + ferrite) microstructure that differs from fully austenitic wrought 304/316.
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