Quick Answer
Three questions decide it. Does the part see chlorides or aggressive chemicals? If yes, you need molybdenum — CF8M. Will it be welded after casting? If yes, you want low carbon — CF3 or CF3M. Neither? CF8 (cast 304) does the job at the lowest price. The grades form a simple matrix: CF8 = base, +M = molybdenum for corrosion, 3 instead of 8 = low carbon for welding.
The Matrix
| Grade | Wrought twin | C max | Mo | Choose when |
|---|---|---|---|---|
| CF8 | 304 | 0.08% | — | Fresh water, food, mild chemicals; no welding |
| CF3 | 304L | 0.03% | — | Same media, but welded assembly |
| CF8M | 316 | 0.08% | 2–3% | Chlorides, marine, chemical media; no welding |
| CF3M | 316L | 0.03% | 2–3% | Chlorides plus welding |
Why Carbon Matters Only If You Weld
At welding temperatures (roughly 450–850°C in the heat-affected zone), carbon grabs chromium and precipitates as carbides along grain boundaries — “sensitization”. The chromium-depleted zones then corrode first, sometimes dramatically. Two escapes exist: re-solution-anneal the whole welded assembly (often impractical), or start with so little carbon that sensitization stays harmless — the 0.03% limit of CF3/CF3M. If the casting ships as-cast and stays unwelded, sensitization never happens and CF8/CF8M’s extra carbon is irrelevant.
Strength: Practically a Tie
All four grades share the austenitic strength class — tensile minimum 485 MPa (CF3/CF3M: 450–485 MPa depending on spec edition), yield around 205 MPa. Nobody should pick between them for strength. Pick for chemistry and welding; if the application needs real strength, the conversation moves to 17-4 PH or duplex grades like CD4MCu.
Cost Ladder
- CF8: baseline
- CF3: +0–5% (tighter carbon control)
- CF8M: +10–20% over CF8 (molybdenum and nickel)
- CF3M: +15–25% over CF8
The classic over-spend is CF3M “to be safe” on a part that is neither welded nor chloride-exposed — paying two premiums for zero benefit. The classic under-spend is CF8 on a coastal or CIP-cleaned part, which returns as a corrosion claim within the year. Environment first, welding second, then the grade falls out.
Specifying and Verifying
- Name spec and grade: ASTM A351 (pressure) or A743/A744 (general) + CF8M etc.
- Require solution-anneal confirmation on the certificate — the treatment, not just the chemistry, delivers corrosion resistance
- Per-heat chemical analysis; add PMI (positive material identification) if your industry has substitution history
- For deeper detail on the flagship grade, see the CF8M material guide; for how these fit the wider spec family, the ASTM A351 grade overview
All four grades pour through our silica sol investment casting line with solution annealing and per-heat certification as standard.
FAQ
Can CF3M replace CF8M everywhere?
Technically yes — it does everything CF8M does plus weldability, at a small premium. Buy it when welding is possible-but-uncertain; skip the premium when the part definitively ships unwelded.
Is there a dual-certified option like wrought 316/316L?
Foundries can often melt to a chemistry meeting both CF8M and CF3M limits — ask, if your drawings mix both grades.
Do these grades harden by heat treatment?
No. Austenitic grades strengthen only by cold work, which castings don’t receive. Solution annealing is for corrosion, not strength.