CF8 vs CF8M vs CF3M
CF8, CF8M, and CF3M are austenitic stainless steel casting grades under ASTM A351. They are the cast equivalents of wrought Types 304, 316, and 316L, respectively, and are the most common body materials for stainless steel valves, pumps, and specialty fittings in the piping industry.
Comparison Table
| Property | CF8 | CF8M | CF3M |
|---|---|---|---|
| Wrought equivalent | Type 304 | Type 316 | Type 316L |
| ASTM specification | A351 | A351 | A351 |
| UNS | J92600 | J92900 | J92800 |
| Chromium | 18.0-21.0% | 18.0-21.0% | 17.0-21.0% |
| Nickel | 8.0-11.0% | 9.0-12.0% | 9.0-13.0% |
| Molybdenum | None | 2.0-3.0% | 2.0-3.0% |
| Carbon (max) | 0.08% | 0.08% | 0.03% |
| Tensile strength (min) | 485 MPa (70 ksi) | 485 MPa (70 ksi) | 485 MPa (70 ksi) |
| Yield strength (min) | 205 MPa (30 ksi) | 205 MPa (30 ksi) | 205 MPa (30 ksi) |
| Elongation (min) | 35% | 30% | 30% |
| PREN | ~18-20 | ~24-28 | ~24-28 |
| Sensitization risk | Moderate | Moderate | Low |
| Chloride resistance | Moderate | Good | Good |
| Common application | Valve bodies, pump casings | Valve bodies, pump casings | Welded valve bodies |
| Relative cost | Baseline | 10-15% premium | 12-18% premium |
Key Differences
CF8 vs CF8M: Molybdenum
The fundamental difference between CF8 and CF8M mirrors the wrought 304 vs 316 comparison. CF8M contains 2-3% molybdenum, which raises the PREN from approximately 19 to 26 and provides significantly better resistance to pitting and crevice corrosion in chloride-containing environments. CF8M is the default valve body material for chemical plants, offshore facilities, and any application involving seawater or chloride-bearing process fluids.
CF8 is adequate for clean water, steam, dilute organic chemicals, and non-chloride services where cost savings justify the lower corrosion margin.
CF8M vs CF3M: Carbon Content
CF3M is the low-carbon version of CF8M (0.03% C max vs. 0.08% max). The reduced carbon content prevents sensitization (chromium carbide precipitation at grain boundaries) during welding or prolonged exposure to 425-870°C. CF3M is specified when the casting will be welded in fabrication (such as weld-end valve bodies) or repaired by welding, and where intergranular corrosion resistance must be maintained.
In practice, most foundries produce CF8M heats with carbon below 0.03%, allowing dual certification as both CF8M and CF3M. This has made CF3M the de facto standard for new stainless steel valve procurement.
Casting vs Wrought Properties
Cast stainless steels contain 5-15% delta ferrite in the as-cast microstructure, whereas wrought equivalents are fully austenitic. This ferrite:
- Benefit: Improves resistance to hot cracking during solidification and welding
- Limitation: Reduces impact toughness at cryogenic temperatures and may be attacked in certain chemical environments
The ferrite content is controlled and reported on the mill test certificate as a Ferrite Number (FN), typically 5-20 FN for CF8M/CF3M.
Application Summary
| Application | Recommended Grade |
|---|---|
| Non-chloride, non-welded service | CF8 |
| Chloride service, general use | CF8M |
| Chloride service, welded construction | CF3M |
| Cryogenic service | CF8 or CF3 (no Mo) |
| High-temperature (above 538°C) | CF8 (stabilized grades preferred) |
The forging and casting processes for stainless steel valves are detailed in the dedicated guide.
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