What is CN7M Stainless Steel?
Definition
CN7M is an austenitic stainless steel casting grade defined under ASTM A351 (pressure-containing castings), ASTM A743 (general corrosion-resistant castings), and ASTM A744 (severe corrosion-resistant castings).
It serves as the cast equivalent of wrought Alloy 20, a well-established nickel-iron-chromium alloy produced to meet the demanding quality requirements of the chemical processing and petrochemical industries.
Decoding the 4 Characters of “CN7M”
In the ASTM A351 designation system, each letter and digit reveals a specific metallurgical boundary; let’s break it down:
C: Corrosion Service
Designates CN7M as a corrosion-resistant grade within the ASTM casting specification system, distinguishing it from heat-resistant (“H”) or structural grades.
N: High Nickel Content
Denotes high nickel, with CN7M containing 27.5–30.5% nickel by weight. This places it among super-austenitic stainless steels rather than standard 300-series alloys.
7: Carbon Limit
Specifies a maximum carbon content of 0.07% —the number itself directly references this carbon cap. This low carbon level is a deliberate design feature of the alloy specification.
M: Molybdenum Addition
Indicates the presence of molybdenum, with CN7M containing 2.0–3.0% by weight, which is one of the key alloying elements that enhances resistance against localized pitting
CN7M Chemical Composition & Mechanical Properties
CN7M Chemical Composition
Below is the chemical composition of CN7M. These values define the alloy’s identity:
| Element | Content (%) |
| Carbon (C) | ≤ 0.07 |
| Silicon (Si) | ≤ 1.50 |
| Manganese (Mn) | ≤ 1.50 |
| Chromium (Cr) | 19.00 – 22.00 |
| Nickel (Ni) | 27.50 – 30.50 |
| Molybdenum (Mo) | 2.00 – 3.00 |
| Copper (Cu) | 3.00 – 4.00 |
| Phosphorus (P) | ≤ 0.04 |
| Sulfur (S) | ≤ 0.04 |
| Niobium (Nb) | ≥ 8×C% |
| Iron (Fe) | Balance |
CN7M Mechanical Properties
These mechanical properties tell you what loads and pressures your CN7M casting can handle—critical for your design calculations.
| Properties | Value |
| Tensile Strength | ≥ 425 MPa (62 ksi) |
| Yield Strength | ≥ 170 MPa (25 ksi) |
| Elongation | ≥ 35% |
| Hardness (Brinell) | ~137 HBW |
Key Features: Why Cu & Ni Make CN7M Unique
Copper (Cu, 3.0% – 4.0%)
Copper forms a protective passive film on the metal surface, enabling CN7M to resist corrosion in hot sulfuric acid at concentrations ranging from 20% to 80%—a level of resistance that conventional stainless steels cannot match.
High Nickel (Ni, 27.5% – 30.5%)
High nickel content stabilizes the pure austenitic microstructure, eliminating stress corrosion cracking (SCC) risks in warm, chloride-bearing, or acidic environments.
This is, arguably, the single most critical feature that makes CN7M the preferred choice for aggressive chemical service.
CN7M Material Equivalent Across Global Standards
Below are the international equivalents of CN7M. You’ll find these useful when communicating with overseas suppliers or reviewing non-US specifications.
| Standard | Equivalent Grade |
| ASTM (Cast) | ASTM A351 / A743 / A744 CN7M |
| UNS (Cast) | N08007 / J95150 |
| ASTM (Wrought) | ASTM B462 / B473 Alloy 20 |
| UNS (Wrought) | N08020 |
| DIN/EN (Wrought) | 2.4660 (NiCr20CuMo) |
| DIN/EN (Cast) | 1.4500 (GX7NiCrCuMo12-25) |
| JIS | SCS 23 |
Material Selection: CN7M vs. Alternative Alloys
CN7M vs. CF8M (316)
CF8M is the go-to cast stainless for general service, but it struggles in hot sulfuric acid. CN7M’s higher nickel (27.5% vs. ~12%) and copper addition (3–4%) provide significantly better corrosion resistance. For aggressive chemical environments where reliability matters, CN7M is the stronger choice.
CN7M (Cast) vs. Alloy 20 (Wrought)
CN7M and wrought Alloy 20 are essentially the same alloy—CN7M is simply the cast version. If your design requires complex-shaped components like valve bodies or pump casings, castings offer design flexibility and near-net shape production that wrought forms can’t match. For castings, you specify CN7M.
CN7M vs. Hastelloy C276 (CW12MW)
C276 offers outstanding resistance in extreme environments, but it comes at a significantly higher cost. For most hot sulfuric and phosphoric acid services, CN7M delivers all the corrosion resistance you need without the premium. C276 is best reserved for the most severe applications.
Common Applications for CN7M Custom Castings
Valves
If you’re specifying valves for sulfuric acid or aggressive chemical service, CN7M is a proven choice. Its corrosion resistance and castability allow you to produce complex valve geometries with confidence.
- Gate valves
- Globe valves
- Ball valves
- Butterfly valves
- Check valves
- Plug valves
Pumps & Impellers
For pump components handling corrosive fluids, CN7M gives you reliable protection against pitting and erosion. Custom casting also lets you achieve tight tolerances on complex impeller designs.
- Pump bodies
- Pump impellers
- Pump casings
- Volutes
- Wear rings
Fittings & Pressure Components
Beyond valves and pumps, CN7M castings are available for your piping systems and pressure-containing equipment where corrosion integrity is non-negotiable.
- Flanges
- Pipe fittings (elbows, tees, reducers)
- Valve trim
- Pressure vessel components
- Heat exchanger parts
How We Ensure Quality for Your Custom CN7M Castings
Casting Process Options
We produce your CN7M castings through both investment casting and sand casting.
- Investment casting delivers the dimensional precision you need for complex valve and impeller geometries.
- Sand casting is ideal for larger, heavier components where the superior corrosion resistance of CN7M is still required.
We’ll help you choose the right approach for your specific application.
Critical Process Controls
CN7M is prone to hot cracking due to its high copper and nickel content. To protect your castings, we strictly control two things:
- Trace Element Control (Sn< 0.01%): Restricting tin below 0.01% prevents hot shortness during solidification.
- Solution Annealing & Rapid Water Quenching: Heat treating at 1120°C–1170°C minimum followed by immediate water quenching fully restores corrosion resistance and dissolves grain-boundary carbides.
Quality Assurance & NDT Inspection
Every batch of your CN7M castings undergoes rigorous testing tailored to this alloy. Spectroscopic analysis verifies every heat to ensure Ni (27.5–30.5%) and Cu (3–4%) conform to ASTM A351, with Sn kept below 0.01%.
Non-Destructive Testing (NDT) includes Liquid Penetrant Inspection (PT) and Radiographic Testing (RT) for internal pressure integrity.
Conclusion
CN7M delivers the corrosion resistance your aggressive chemical service demands, but material alone isn’t enough; the casting quality determines real-world reliability. At YTD Casting, we produce CN7M castings with precision controls and rigorous inspection. Contact us for your custom casting needs.
FAQs
Is CN7M magnetic?
No. Its fully austenitic structure makes CN7M non-magnetic, even after welding.
What is the difference between CN7M and Alloy 20?
CN7M is the casting grade; Alloy 20 is the wrought equivalent of the same alloy.
What is CN7M mainly used for?
Valves, pumps, and fittings in sulfuric acid and aggressive chemical service.
How to prevent intergranular corrosion in CN7M castings?
Apply solution annealing (minimum 1120°C) after welding to prevent carbide precipitation.
Can CN7M castings be welded?
Yes, but welding is difficult due to hot shortness—preheat and low heat input are required.
What is the maximum service temperature for CN7M?
Approximately 400°C (750°F).
Why is CN7M resistant to sulfuric acid?
Copper builds a stable passive film in reducing acids, while molybdenum reinforces that film against localized breakdown. Together, they create a defense mechanism that standard stainless steels lack.
