The Short Answer
CF8M and CF3M are both corrosion-resistant cast stainless steel grades covered by ASTM A351, and they are close relatives of wrought 316 and 316L respectively. The practical difference is carbon content: CF8M has a max carbon around 0.08%, while CF3M is limited to 0.03%. Lower carbon in CF3M reduces the risk of sensitization and intergranular corrosion after welding, which is why CF3M is generally preferred for welded valve bodies in corrosive or chloride-bearing service.
What Each Designation Means
CF8M (316 Cast Equivalent)
CF8M is the cast equivalent of wrought 316 stainless steel, per ASTM A351. Its composition typically includes about 18-21% chromium and 9-12% nickel, with molybdenum added for pitting and crevice corrosion resistance. With a maximum carbon of about 0.08%, CF8M offers good strength and corrosion resistance for general process service and is one of the most widely specified cast valve body materials.
CF3M (316L Cast Equivalent)
CF3M is the low-carbon cast counterpart, equivalent to wrought 316L. The carbon limit of 0.03% minimizes carbide precipitation at grain boundaries during welding or elevated-temperature exposure. For valve bodies that are welded, or for services where the weld heat-affected zone could be sensitized, CF3M is typically the safer choice. The strength difference at room temperature is generally small.
Key Differences
| Property | CF8M | CF3M |
|---|---|---|
| Wrought equivalent | 316 | 316L |
| Max carbon | ~0.08% | ~0.03% |
| Standard | ASTM A351 | ASTM A351 |
| Sensitization resistance after welding | Moderate | Higher |
| Typical minimum tensile strength | ~70 ksi (data needs further verification for exact value per edition) | ~70 ksi (data needs further verification for exact value per edition) |
| Typical service | General process, non-welded or light welding | Welded bodies, chloride-bearing and corrosive service |
| Cost | Generally lower | Generally slightly higher |
Which to Choose
For valve bodies that are cast as a single piece with limited welding, either grade can work, and CF8M is the common default. For bodies fabricated with substantial weldments, or for services involving chlorides, seawater, or repeated thermal cycling, CF3M is generally the preferred choice because the low carbon content limits sensitization. Many pump and valve specifications for water and chemical service call out CF3M for this reason.
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Common Misconceptions
A frequent misunderstanding is that CF3M is simply “a better 316” in every way. In practice both grades share very similar corrosion resistance in many media, and the real difference appears mainly in welded or sensitizing conditions. Another misconception is that CF8M and CF3M are interchangeable in all applications; standards and project specifications often treat them as distinct materials, so substitution should follow the engineering specification rather than convenience.
Practical Recommendation
Check the project material specification before selecting the body casting grade. If welding is involved in fabrication or repair, prefer CF3M. If the service is non-welded, general-purpose, and cost-sensitive, CF8M is usually acceptable. When chloride resistance matters, confirm the molybdenum content and, where required, consider higher-alloy grades such as duplex or super duplex. For broader guidance on matching materials to service, see our valve selection guides and the materials section of the ball valve selection guide.
Conclusion
CF8M and CF3M differ mainly in carbon content and the resulting weldability and sensitization resistance. Choosing between them is a metallurgical decision that depends on fabrication method and service environment, not a simple upgrade path. When in doubt, the low-carbon CF3M is the conservative choice for welded service.
This article provides general educational knowledge only and is not professional engineering advice.
Sources & Further Reading
ASTM A351 (Standard Specification for Castings, Austenitic, for Pressure-Containing Parts); ASM International (cast stainless steel grades).
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Disclaimer: This article is provided for general educational and informational purposes only and does not constitute professional engineering advice. Valve applications vary with pressure, temperature, media, and applicable codes. Always verify specifications against the manufacturer’s data sheet and consult a qualified engineer before selecting, installing, or repairing any valve. Valve Knowledge Hub makes no warranties about the accuracy or completeness of the information on this site and assumes no liability for any use of it.