CN7M material is a corrosion-resistant cast stainless alloy widely used for industrial pumps, valves, and fittings that handle aggressive fluids. It belongs to the austenitic group of stainless-steel castings and is standardized under ASTM A743 and ASTM A744. Because of its balanced nickel, chromium, and molybdenum composition, CN7M offers excellent resistance to acids, chlorides, and oxidizing chemicals.
In chemical, petrochemical, and marine environments, CN7M Material is often selected when standard stainless grades such as CF8M or CF3M cannot withstand strong acids or high chloride concentrations. Engineers choose it for its combination of corrosion resistance, toughness, and stability across a wide temperature range.
| Property | Typical Value / Characteristic |
| Material Type | Austenitic stainless steel casting |
| Standard Specification | ASTM A743 / A744 Grade CN7M |
| Common Form | Pumps, valves, impellers, cast fittings |
| Corrosion Resistance | Excellent in acids and chlorides |
| Heat Treatment | Solution annealed around 1950°F (1065°C) |
| Magnetism | Non-magnetic in annealed condition |
1. Understanding CN7M Material Alloy
1.1 What is CN7M Material?
CN7M is a nickel-rich austenitic stainless-steel casting grade designed for exceptional resistance to both oxidizing and reducing acids. Its composition gives it a unique balance of corrosion resistance and strength compared to standard 300-series stainless steels.
Unlike martensitic or ferritic stainless grades, CN7M maintains an austenitic microstructure even after cooling, which means it stays non-magnetic and highly ductile. The alloy forms a protective chromium-oxide film on the surface, helping it resist general and localized corrosion.
1.2 Chemical Composition
| Element | Typical Range (Weight %) | Function |
| Chromium (Cr) | 19.0 – 21.0 | Forms passive film and general corrosion resistance |
| Nickel (Ni) | 32.0 – 36.0 | Stabilizes austenite, improves resistance to reducing acids |
| Molybdenum (Mo) | 2.0 – 3.0 | Improves resistance to pitting and crevice corrosion |
| Copper (Cu) | 3.0 – 4.0 | Increases resistance to sulfuric acid |
| Carbon (C) | ≤ 0.07 | Maintains toughness and weldability |
| Manganese (Mn) | ≤ 2.0 | Deoxidizer, aids hot workability |
| Silicon (Si) | ≤ 1.0 | Strengthens matrix, improves casting behavior |
| Iron (Fe) | Balance | Matrix element providing strength |
1.3 Microstructure and Physical Characteristics
CN7M solidifies as a fully austenitic structure, ensuring excellent toughness even at sub-zero temperatures. The high nickel and copper content reduce the risk of stress-corrosion cracking and improve performance in both oxidizing and reducing acid environments.
| Property | Value / Behavior |
| Density | ~7.9 g/cm³ |
| Melting Range | 1350–1400 °C |
| Magnetic Response | Essentially non-magnetic |
| Thermal Expansion | Similar to other austenitic steels |
2. Mechanical Properties
Although CN7M is primarily selected for corrosion resistance, its mechanical properties make it suitable for pressure-bearing components such as valve bodies and pump casings.
| Property | Typical Value (Annealed Condition) |
| Tensile Strength | 550 – 690 MPa |
| Yield Strength (0.2% offset) | 240 – 310 MPa |
| Elongation | 30 – 40 % |
| Hardness | 150 – 190 HB |
| Modulus of Elasticity | ~200 GPa |
CN7M retains strength at moderate temperatures and maintains ductility even when exposed to thermal cycling. The alloy cannot be hardened by heat treatment; it gains strength only through cold work.
3. Corrosion Resistance Behavior
3.1 General Corrosion
The combination of chromium, nickel, and molybdenum allows CN7M to resist general corrosion in a variety of acids, including sulfuric, phosphoric, and acetic acids. Its resistance to oxidation is also good in both air and aqueous solutions.
3.2 Pitting and Crevice Corrosion
Molybdenum in CN7M significantly reduces the susceptibility to pitting and crevice attack caused by chloride ions. The alloy is used where CF8M or CF3M components show premature pitting in seawater or brine.
| Environment | CN7M Performance |
| Seawater | Excellent resistance to pitting |
| Sulfuric acid (dilute to 50%) | Very good |
| Phosphoric acid | Excellent |
| Nitric acid | Good |
| Acetic acid | Excellent |
| Sodium chloride brine | Superior to CF8M and CF3M |
3.3 Stress-Corrosion Cracking
CN7M demonstrates improved resistance to stress-corrosion cracking (SCC) compared with lower-nickel stainless steels. Its high nickel and copper levels help stabilize the passive film even under tensile stress in chloride environments.
4. Heat Treatment and Fabrication
4.1 Solution Annealing
After casting, CN7M is typically solution annealed at 1950°F (1065°C) followed by rapid water quenching. This process dissolves carbides, homogenizes the microstructure, and restores corrosion resistance.
4.2 Welding
CN7M can be welded using common austenitic stainless techniques such as GTAW (TIG) and GMAW (MIG). Use matching filler wire or high-nickel consumables. Post-weld heat treatment is generally unnecessary, but cleaning and passivation of welds are critical to restore surface protection.
| Welding Parameter | Typical Practice |
| Preheat | Not required |
| Interpass Temperature | ≤150 °C |
| Filler Recommendation | Matching CN7M or nickel-based filler |
| Post-weld Treatment | Cleaning + passivation |
4.3 Machining and Casting
CN7M is more difficult to machine than standard stainless steels due to its work-hardening tendency and toughness. Sharp tools, slow speeds, and adequate cooling are recommended. The alloy casts cleanly with good dimensional stability, making it ideal for complex valve and pump geometries.
5. Industrial and Engineering Applications
CN7M material is widely applied across industries that require corrosion-resistant castings exposed to harsh chemicals or seawater.
| Industry | Typical Components | Service Conditions |
| Chemical Processing | Reactor vessels, pump casings, agitators | Sulfuric and phosphoric acid |
| Petrochemical | Valve bodies, fittings | Chloride and acid mixtures |
| Marine & Offshore | Pump impellers, seawater valves | Saltwater immersion |
| Power Generation | Cooling and scrubber systems | Hot condensate, flue-gas condensate |
| Pulp & Paper | Bleaching equipment | Chlorine-based chemicals |
In many chemical plants, CN7M castings extend service life by several years compared with CF8M, particularly where the fluid contains chlorides and reducing acids simultaneously.
6. Comparison: CN7M vs CF8M vs CF3M
Engineers often compare CN7M with other austenitic casting grades to evaluate cost and performance trade-offs.
| Property / Feature | CN7M | CF8M | CF3M |
| Composition (Cr–Ni–Mo) | 20–34–2.5 | 18–10–2.5 | 18–10–2.5 (low C) |
| Copper (Cu) | 3–4 % | None | None |
| Corrosion Resistance | Excellent in acids & chlorides | Very good general resistance | Very good, better weld corrosion |
| Stress-Corrosion Cracking | Low tendency | Moderate | Moderate |
| Mechanical Strength | Similar to CF8M | High | Slightly lower |
| Weldability | Good | Excellent | Excellent |
| Cost | Higher | Moderate | Moderate |
| Typical Use | Chemical & acid service | General purpose | Sanitary, welded structures |
Summary:
- CN7M is chosen for acidic and chloride environments where standard 300-series castings fail.
- CF8M (cast 316) is adequate for general service.
- CF3M (cast 316L) suits low-carbon, weld-intensive applications.
7. Advantages and Limitations
7.1 Advantages
| Feature | Benefit |
| High nickel and copper content | Excellent resistance to both oxidizing and reducing acids |
| Molybdenum addition | Superior resistance to chloride pitting |
| Fully austenitic microstructure | Non-magnetic and tough at all temperatures |
| Good castability | Complex shapes achievable |
| Proven service record | Long life in chemical and marine systems |
7.2 Limitations
| Limitation | Mitigation |
| Higher cost than CF8M | Use only where necessary |
| Difficult machining | Use carbide tools, low speed |
| Slightly lower yield strength | Design for thicker walls if needed |
| Not hardenable by heat | Relies on composition for strength |
8. Practical Selection Guide
8.1 When to Choose CN7M
Select CN7M material when:
- The component handles sulfuric, phosphoric, or organic acids.
- The system contains chlorides or brine at elevated temperatures.
- Standard 316 or 317 alloys fail due to localized corrosion.
- Long-term maintenance cost outweighs initial material price.
| Application | Risk | Recommended Material |
| Chemical reactor pump | Strong acids | CN7M |
| Seawater valve | High chloride | CN7M |
| Food-grade sanitary tank | Mild corrosion | CF3M |
| General water system | Low corrosion | CF8M |
8.2 Design and Maintenance Tips
- Always specify solution annealed CN7M castings.
- After welding, perform thorough cleaning and passivation.
- Avoid stagnant chloride solutions to maintain the passive film.
- Periodically inspect cast surfaces for early pitting or erosion.
9. Global Availability and Market Notes
As of 2025, CN7M material is produced by major foundries in the United States, Europe, and Asia. Supply depends on nickel pricing, since CN7M contains roughly one-third nickel by weight. Despite its higher price, demand remains strong in the chemical process equipment and marine pump sectors because of its long service life and reliability.
Fabricators often source CN7M castings under ASTM A743 Grade CN7M, ASME SA-743, or ASTM A744 standards, ensuring compatibility with most industrial specifications.
10. Conclusion
CN7M material stands out among austenitic cast stainless steels for its superior resistance to acids, chlorides, and corrosive environments. Its high nickel, chromium, molybdenum, and copper content make it a reliable choice for demanding industrial systems where ordinary stainless alloys fail.
In industrial design, the decision to use CN7M should consider both chemical environment and life-cycle cost. While the alloy’s price is higher than standard stainless grades, its ability to maintain integrity under aggressive service often delivers the lowest total cost of ownership over time.
FAQs
1. What is CN7M material?
CN7M is an austenitic stainless-steel casting grade defined in ASTM A743 and A744. It contains high nickel, chromium, molybdenum, and copper for excellent resistance to acids and chlorides.
2. What are the main alloying elements in CN7M?
Chromium (19–21%), nickel (32–36%), molybdenum (2–3%), and copper (3–4%) are the principal elements that provide its corrosion-resistant performance.
3. What is CN7M used for?
It’s used for industrial pumps, valves, and fittings that handle corrosive chemicals, seawater, and process fluids.
4. Is CN7M Material magnetic?
No. It is non-magnetic due to its austenitic structure, even after welding or cold work.
5. Can CN7M Material be welded?
Yes. It welds easily with matching or nickel-based filler metal. Post-weld cleaning and passivation are recommended.
6. What standard defines CN7M material?
The alloy is covered by ASTM A743 Grade CN7M and ASTM A744 Grade CN7M, used for corrosion-resistant castings.
7. How does CN7M Material compare to CF8M and CF3M?
CN7M provides better resistance to acids and chlorides, while CF8M and CF3M are lower-cost general-purpose stainless casting grades.
8. Can CN7M Material handle sulfuric acid?
Yes, it resists dilute and moderately concentrated sulfuric acid much better than standard 316 stainless steels.
9. What is the hardness of CN7M Material?
Typically 150–190 HB in solution-annealed condition; it cannot be hardened by heat treatment.
10. Why is CN7M Material more expensive?
Because it contains a high percentage of nickel and copper.
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