Quick Answer
304 vs 316 stainless steel comes down to one element: molybdenum. 316 contains 2–3% Mo, which dramatically improves resistance to chlorides, salt water and many acids. 304 has none. For castings, the choice maps to the cast equivalents: CF8 is cast 304, CF8M is cast 316. If your part sees marine environments, de-icing salts, chemical media or chloride cleaning agents, specify 316/CF8M. If it lives indoors or in fresh water, 304/CF8 usually does the job at roughly 20–30% lower material cost.
Composition: The Only Difference That Matters
| Element | 304 (CF8 cast) | 316 (CF8M cast) |
|---|---|---|
| Chromium | 18–20% | 16–18% |
| Nickel | 8–10.5% | 10–14% |
| Molybdenum | — | 2–3% |
| Carbon (wrought) | ≤0.08% | ≤0.08% |
| Cast designation | CF8 (ASTM A351) | CF8M (ASTM A351) |
Everything else — strength, machinability, weldability — is close enough that it rarely decides the question. Both are austenitic, both are non-magnetic in the annealed condition (castings retain a small amount of ferrite, so a magnet may weakly stick — that is normal and not a defect), and both gain nothing from heat-treatment hardening.
Corrosion Resistance: Where 316 Earns Its Premium
The molybdenum in 316 specifically fights pitting and crevice corrosion caused by chlorides. Typical situations where 304 fails early and 316 survives:
- Coastal or offshore equipment exposed to salt spray
- Pump and valve parts handling brine, seawater or brackish water
- Food and pharma equipment cleaned with chlorinated sanitizers
- Parts near swimming pools or de-icing salt on roads
In clean indoor air, fresh water, and most food-contact applications without chloride cleaners, 304 performs essentially the same as 316. Paying for molybdenum you don’t need is the most common over-specification we see on buyer drawings.
Mechanical Properties (Cast, per ASTM A351)
| Property | CF8 (cast 304) | CF8M (cast 316) |
|---|---|---|
| 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% |
Identical strength on paper. Do not switch grades expecting a stronger part — switch for the environment.
Cost and Availability
316/CF8M typically runs 20–30% higher than 304/CF8 on casting price, driven by nickel and molybdenum content. Both grades cast equally well by investment casting; neither needs exotic foundry practice. At YCUMETAL we pour both through the silica sol lost wax casting process, which holds the surface finish and tolerances most valve, pump and food-machinery buyers require without heavy machining allowances.
How to Specify It on Your Drawing
- For castings, call out the cast grade: ASTM A351 Grade CF8 or CF8M — not just “304” or “316”. This removes ambiguity at the foundry.
- If welding is planned after casting, consider the low-carbon variants CF3 / CF3M to avoid sensitization at the weld zone.
- State the corrosion environment (media, temperature, chloride exposure) in the RFQ. A good foundry will flag over- or under-specification before quoting.
- Ask for material certification with chemical analysis per heat. Dimensional and material inspection should be standard — see our quality assurance process.
FAQ
Is 316 stronger than 304?
No. Minimum tensile and yield values are the same for the cast grades. 316’s advantage is corrosion resistance, not strength.
Why does a magnet stick slightly to my “non-magnetic” stainless casting?
Cast austenitic stainless contains a controlled amount of ferrite (typically 5–20%) to prevent hot cracking. Slight magnetism is inherent to the casting process and is not a sign of wrong material.
Can I substitute 304 where the drawing says 316?
Not without the designer’s approval. If the part sees chlorides, the substitution can fail in service within months.
What if I need better corrosion resistance than 316?
Step up to duplex grades like CD4MCu or nickel alloys like CN7M (Alloy 20), both castable by the same lost wax process.