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.
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.