ASTM A351 Grades: Choosing the Right Cast Stainless Steel
How ASTM A351 grade names work and how to choose between CF8, CF8M, CF3M, CN7M and the other cast austenitic stainless grades for pressure-containing parts.
Quick Answer
ASTM A351 covers austenitic stainless steel castings for pressure-containing parts — valve bodies, pump casings, fittings and flanges. The workhorse grades map to familiar wrought alloys: CF8 = cast 304, CF8M = cast 316, CF3/CF3M = the low-carbon versions, CN7M = Alloy 20 for sulfuric acid. Grade choice follows a simple logic chain: chloride exposure → add molybdenum (CF8M); welding after casting → drop carbon (CF3M); acids beyond 316’s reach → move to CN7M or duplex.
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Related Engineering References: Compare ASTM A351 grades with European EN 10213 (1.4308 / 1.4408) and JIS SCS13A/SCS14A in our ASTM vs EN/DIN vs JIS vs GB Casting Grade Cross-Reference Tool, or review martensitic alternatives in CA15 vs CA6NM Martensitic Stainless Steel Castings and dimensional limits in our ISO 8062 CT4–CT7 Investment Casting Tolerance Chart.
Decoding the Grade Names
The ACI naming system is compact once you see the pattern:
- First letter: C = corrosion-resistant service
- Second letter: rough nickel content position (F ≈ 19Cr/9Ni class, N = higher nickel)
- Number: maximum carbon × 100 (CF8 = 0.08% C max, CF3 = 0.03%)
- Suffix letters: alloy additions — M for molybdenum
So CF3M reads: corrosion class, 19/9 family, 0.03% carbon max, plus molybdenum — i.e., cast 316L.
The Main Grades at a Glance
| A351 grade | Wrought equivalent | Key feature | Typical service |
|---|---|---|---|
| CF8 | 304 | General corrosion resistance | Water, food, mild chemicals |
| CF3 | 304L | Low carbon, weld-safe | Welded assemblies, no chlorides |
| CF8M | 316 | Mo for chloride pitting resistance | Marine, chemical, chlorinated CIP |
| CF3M | 316L | Low carbon + Mo | Welded chloride service |
| CF8C | 347 | Niobium stabilized | Elevated temperature cycling |
| CN7M | Alloy 20 | 28–30% Ni, Cu addition | Sulfuric acid systems |
| CK3MCuN | 254 SMO class | 6% Mo super-austenitic | Seawater, severe pitting media |
Mechanical Properties
The austenitic A351 grades share a common strength class: tensile minimum 485 MPa (70 ksi) for CF8/CF8M, yield minimum 205 MPa (30 ksi), elongation 30–35%+. Strength rarely differentiates them — corrosion environment and weldability decide the grade. All ship solution annealed; that treatment, not chemistry alone, delivers the corrosion resistance, so its confirmation belongs on every certificate.
Selection Logic for Buyers
- Start with the media. Fresh water/food: CF8. Any chloride: CF8M. Sulfuric acid: CN7M. Seawater at temperature: consider CK3MCuN or duplex A995 grades.
- Will you weld it? Choose the 3-series carbon level (CF3/CF3M) and skip post-weld annealing headaches.
- Temperature cycling above ~425°C? Stabilized CF8C resists sensitization in service.
- Pressure class and code. A351 is the pressure-parts spec; the same alloys for general service can be bought to A743/A744, sometimes at lower cost.
Most A351 parts we pour at YCUMETAL run through silica sol investment casting — the process holds thin walls and smooth internal passages on valve trim and pump internals, cutting downstream machining on corrosion-critical surfaces. Deeper background on the flagship grade lives in our CF8M material guide.
FAQ
Is A351 only for valves?
No — any pressure-containing casting: pump casings, fittings, instrument bodies. Valves are simply the biggest consumer.
A351 CF8M vs A743 CF8M — what’s the difference?
Same alloy. A351 adds pressure-service requirements; A743 covers general corrosion-resistant use. Buy to the spec your code requires.
Why is my CF8M casting slightly magnetic?
Cast austenitics deliberately contain some ferrite for crack-free solidification — weak magnetism is normal and not a mix-up with a martensitic grade.
Which A351 grade for chloride + welding?
CF3M. Low carbon keeps the weld zone corrosion-resistant without re-annealing.
[…] Expect a significant price premium over CF8M — nickel and copper content plus tighter process control. Where the media allows, CF8M remains the economical choice; the comparison logic is in our A351 grade guide […]
[…] CA15 and CA6NM with austenitic and duplex stainless steels in our ASTM A351 Stainless Casting Grades Guide and Global Casting Grade Cross-Reference […]