CN7M (Alloy 20) Castings: The Sulfuric Acid Specialist

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)

Mechanical Properties

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.

Engineering & Sourcing Tools: Use our ASTM vs EN/DIN vs JIS vs GB Casting Grade Cross-Reference Calculator and ISO 8062 CT4–CT12 Casting Tolerance & RMA Chart to verify specifications, or explore YCUMETAL’s precision investment casting, 3/4/5-axis CNC machining, automotive & drivetrain castings, and full foundry services (request a 24h DFM quote).

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.

For production DFM, tooling, and batch inspection on these components, explore our industrial hardware and fluid control castings capabilities at YCUMETAL’s 34,000㎡ Pingdu facility.