Quick Answer
CN7M is the cast form of Alloy 20 (Carpenter 20Cb-3 type), engineered specifically for sulfuric acid service. With roughly 28–30.5% nickel, 19–22% chromium, 2–3% molybdenum and a deliberate 3–4% copper addition, it survives the acid concentrations that destroy CF8M within months. If your pump, valve or fitting handles H₂SO₄ — especially in the treacherous 20–80% concentration range — CN7M is the standard first answer.
Composition (ASTM A351 / A743 CN7M)
| Element | Range | Why it’s there |
|---|---|---|
| Nickel | 27.5–30.5% | Stabilizes austenite, base resistance to reducing acids |
| Chromium | 19.0–22.0% | Passive film, oxidizing media resistance |
| Copper | 3.0–4.0% | The sulfuric acid key — suppresses active corrosion |
| Molybdenum | 2.0–3.0% | Pitting and crevice resistance |
| Carbon | ≤ 0.07% | Kept low; niobium stabilization common |
Mechanical Properties
| Property | Minimum |
|---|---|
| Tensile strength | 425 MPa (62 ksi) |
| Yield strength | 170 MPa (25 ksi) |
| Elongation | 35% |
Softer than CF8M on paper — nobody buys CN7M for strength. Its job is staying in one piece chemically. Design sections accordingly and lean on the excellent ductility.
Where CN7M Beats Everything Cheaper
- Sulfuric acid across wide concentration and temperature ranges — the defining application
- Phosphoric acid in fertilizer plants (wet-process acid with impurities)
- Chloride-bearing mixed acids where CF8M pits
- Hot organic acids (acetic, formic) in chemical processing
- Stress corrosion cracking resistance: the high nickel makes CN7M far more SCC-resistant than 300-series grades in hot chlorides
Typical Cast Parts
- Pump casings and impellers for acid transfer and dosing
- Valve bodies, plugs and trim in acid plants
- Mixer components, spargers and fittings
- Filter and heat-exchanger hardware in pickling and plating lines
Foundry and Specification Notes
- Supplied solution annealed (~1,100°C, water quench) — mandatory for corrosion performance; verify on the certificate
- High nickel makes the melt sluggish; experienced gating practice matters for soundness in thin hydraulic sections — a process question worth asking any bidder
- Weldable with matching filler (Alloy 20 type); niobium stabilization protects the HAZ
- 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
CN7M pours at YCUMETAL through silica sol investment casting with per-heat chemistry, solution-anneal records and NDT per order — see quality assurance for the inspection sequence.
FAQ
Is CN7M the same as Hastelloy?
No. Hastelloy grades are nickel-base (Ni > 50%) for even harsher media at higher cost. CN7M is an iron-base super-austenitic — the intermediate step that solves sulfuric acid without nickel-alloy prices.
Can CN7M handle hydrochloric acid?
Only dilute and cold, marginally. HCl service generally needs nickel-molybdenum alloys — ask before assuming.
Why did my CN7M part cost 3× the CF8M version?
Roughly 30% nickel plus copper vs 10% nickel — raw material dominates. If the media doesn’t demand it, step back down the grade ladder.