ASTM A744 CN7M Stainless Steel | Alloy 20 Castings

ASTM A744 CN7M is a cast, high-nickel, molybdenum- and copper-bearing austenitic stainless alloy engineered for aggressive chemical service—notably sulfuric and other reducing acids, chloride-bearing process streams and mixed acid duties.

Its combination of high Ni, Cr, Mo and Cu yields superior resistance to localized corrosion, good ductility and reliable castability for complex geometries (pump bodies, valves, fittings).

This expanded guide provides in-depth metallurgy, design and fabrication guidance, inspection and procurement checklists, failure-mode analysis, and selection decision rules so engineers and procurement professionals can specify, buy and deploy CN7M castings with confidence.

1. What is ASTM A744 CN7M Stainless Steel

CN7M is a high-nickel, chromium–molybdenum, copper-bearing austenitic cast stainless steel belonging to the Alloy-20 family.

It is specifically engineered for severe chemical environments, particularly those involving sulfuric acid, mixed acids, and other reducing media where conventional 300-series stainless steels show rapid corrosion.

As a cast alloy specified under ASTM A744, CN7M is widely used for pressure-containing and corrosion-critical components such as pump casings, valve bodies, impellers, fittings, and reactor hardware.

Its high nickel content ensures a fully austenitic, non-magnetic structure with excellent toughness, while chromium promotes passive film stability.

Molybdenum improves resistance to pitting and crevice corrosion in chloride-containing environments, and copper significantly enhances performance in sulfuric acid and other reducing acids.

CN7M effectively bridges the performance gap between standard austenitic stainless steels (e.g., CF8M / 316 castings) and more expensive nickel-base alloys.

This balance of corrosion resistance, castability, mechanical integrity, and cost efficiency makes it a preferred material in chemical processing, petrochemical, fertilizer, pharmaceutical, and pulp-and-paper industries.

Standard designations & global equivalents

2. Typical chemical composition and metallurgical role

Values below are representative engineering ranges for CN7M castings supplied in the solution-annealed condition.

3. Microstructure and metallurgical behaviour — in depth

• Austenitic matrix: High Ni content ensures a fully austenitic γ-matrix at room temperature with excellent toughness and ductility. That microstructure is the base for CN7M’s mechanical and corrosion properties.
• Carbides and precipitation: Carbon is deliberately limited; however, improper casting, slow cooling or post-casting thermal exposures can precipitate chromium carbides at grain boundaries, locally depleting chromium and reducing corrosion resistance.
  A solution anneal dissolves such carbides.
• Intermetallic phases (sigma, chi): Long dwell times in the 600–900 °C range can precipitate sigma (σ) and associated phases in high-alloy austenitics.
  These phases embrittle and lower corrosion resistance. Avoid prolonged service in that temperature band or perform qualification testing if exposure is inevitable.
• Role of copper and molybdenum: Cu enhances resistance to sulfuric and other reducing acids by stabilizing surface chemistry under reducing conditions; Mo boosts local-attack resistance in chloride-bearing media.
  The synergistic effect produces an alloy that resists a broader set of chemistries than plain 316L.
• Cast microstructural heterogeneity: Cast components may show dendritic segregation and microsegregation at the microscopic scale.
  Good foundry practice—adequate melt treatment, filtration, homogenization and proper heat treatment—is required to minimize heterogeneities that compromise corrosion or mechanical integrity.

4. Mechanical properties — ASTM A744 CN7M (cast, solution-annealed)

The values below are representative engineering ranges for CN7M castings supplied solution-annealed and quenched.

Cast mechanical properties vary with section thickness, foundry practice, heat treatment and post-cast processing.

5. Corrosion Performance of CN7M Stainless Steel

Strengths

• Sulfuric and reducing acids: Superior performance relative to 300-series stainless due to Cu and Ni—CN7M is commonly selected where sulfuric acid contact is routine.
• Mixed acid and process chemistries: Good overall resistance to nitric, phosphoric and various organics with appropriate concentration/temperature limits.
• Improved pitting resistance: Mo provides raised pitting resistance compared with low-Mo austenitics; useful where chlorides are present at moderate levels.

Limitations & application boundaries

• Severe chloride immersion / splash zones: CN7M is better than 304 but in aggressive seawater immersion or splash zones duplex stainless steels or copper-nickel alloys may outperform CN7M in long-term service.
• SCC risk: In high tensile stress + chloride + elevated temperature combinations, stress-corrosion cracking remains a possibility; duplex or super-austenitics may be preferred for SCC-critical duties.
• High-temperature embrittlement: Avoid continuous service in the 600–900 °C band due to risk of sigma phase formation.

6. Casting Characteristics of CN7M Stainless Steel

Casting Processes

CN7M is primarily produced via sand casting and investment casting, with process parameters tailored to avoid segregation and defects:

• Sand casting: Used for large components (valve bodies, pump housings) with wall thickness ≥5 mm.
  Resin-coated sand (phenolic resin) is preferred for dimensional accuracy (tolerance ±0.2–0.5 mm) and surface finish (Ra 3.2–6.3 μm).
• Investment casting: For precision components (small valves, fittings) with thin walls (≥2 mm), achieving surface finish Ra 1.6–3.2 μm and tolerance ±0.1–0.3 mm.

Foundry Controls

• Melting & charge control: Use vacuum induction melting or controlled air/argon practice where possible to minimize dissolved gases and inclusion content. Strict control of alloy additions and deoxidation is essential.
• Filtration and gating: Ceramic filtration and well-designed gating minimize inclusions and porosity; small entrapped gases in pump impellers or valve seats are a common root cause of failure.
• Pouring temperature and solidification: Control pouring temperature to minimize shrinkage cavities and to promote directional solidification toward risers. Provide adequate risering for heavy sections.
• Heat treatment: Specify a solution anneal at the foundry-recommended temperature (typical cast austenitics heat to ≈1100–1120 °C, hold and quench) to dissolve segregated carbides and reset microstructure.
  Provide quench method (water/air/oil) per foundry recommendations to control distortion.

Hot isostatic pressing (HIP) and other densification options

• HIP uses: for the most critical pressure parts susceptible to shrinkage porosity or sub-surface inclusions, HIP can close internal porosity and improve fatigue life and corrosion integrity.
  HIP adds cost but is a valuable option for highly stressed or safety-critical components.
• Limitations: HIP requires that the part geometry and tolerances accommodate the process; subsequent heat treatment and machining may be necessary.

Machining allowance and dimensional control

• Machining allowance: specify realistic machining stock depending on casting finish and critical features: typical roughing allowance = 2–6 mm (0.08–0.25 in) for general surfaces;
  critical sealing faces / machined flanges = 0.5–2 mm after finish grinding as negotiated with the foundry. Thinner allowances may be specified for precision investment castings.
• Dimensional tolerances: castings have larger tolerances than forged/wrought parts; specify critical dimensions to be machined and provide true-position controls for features that must align. Use first-piece inspection and establish FAI criteria.

Surface finishing, cleaning and passivation

• Surface cleaning: remove sand, slag, scale and contaminants by shot-blast, pickling or mechanical cleaning before inspection and machining.
• Descale & pickling: for corrosion-sensitive applications, pickling removes discoloration and heat tint; follow with neutralization and passivation.
• Passivation: apply citric or nitric passivation processes per specification to restore the chromium-oxide passive film, especially on welded or pickled surfaces.
  Electropolishing can be used for sanitary applications to improve surface finish and reduce crevice sites.

7. Welding, joining and repair guidance

• Weldability: CN7M is weldable using matching or recommended filler metals engineered for high-Ni, Cu and Mo alloys. Follow qualified WPS/WPQ for each joint geometry and base-metal thickness.
• Filler metal selection: Use filler alloys with comparable corrosion performance—match Ni/Cr/Mo/Cu balance to avoid galvanic or metallurgical mismatch.
  Do not use generic 316 filler if process chemistry demands alloy-20-class corrosion resistance.
• Heat input control: Minimize excessive interpass temperatures and heat input to reduce grain growth and avoid local precipitation of deleterious phases in heat-affected zones (HAZ).
• Post-weld heat treatment (PWHT): If the weld is in a critical pressure-containing area or in severe corrosive service, consider solution anneal of the welded assembly if feasible—coordinate with design for distortion management.
  Alternatively, use CN7M/Alloy-20 compatible filler metal and limit heat so the HAZ retains acceptable corrosion resistance without PWHT.
• Weld inspection: Use dye-penetrant, MT/PT for surface defects and radiography/UT for volumetric assurance where required.

8. Industrial Applications of ASTM A744 CN7M Stainless Steel

CN7M’s unique combination of corrosion resistance, castability, and cost-effectiveness makes it indispensable in industries requiring reliable performance in harsh corrosive environments:

Chemical & Petrochemical Industry

Core applications: Sulfuric acid storage tanks, chemical reactors, heat exchangers, and piping for handling acids (H₂SO₄, H₃PO₄), organic solvents, and sour gas (H₂S).

Key advantage: Complies with NACE MR0175 for sour service, with a service life 3–5 times longer than 316L in acid environments.

Pump & Valve Manufacturing

Core applications: Valve bodies, trim, pump impellers, and casings for chemical process pumps and control valves.

Key advantage: Castability enables complex flow geometries; corrosion resistance minimizes wear and leakage in aggressive media.

Food & Pharmaceutical Industry

Core applications: Processing equipment for acidic foods (citrus, vinegar), pharmaceutical reactors, and cleanroom components.

Key advantage: Non-toxic, easy to clean, and resistant to food acids and sanitizing agents—complies with FDA 21 CFR Part 177 and ISO 10993.

Water Treatment & Desalination

Core applications: Reverse osmosis membranes, brine handling equipment, and wastewater treatment tanks.

Key advantage: Resistance to chloride-induced pitting and crevice corrosion in high-salinity environments.

Other Applications

• Power Generation: Flue gas desulfurization (FGD) systems, where resistance to sulfur dioxide and acidic condensates is critical.
• Marine Industry: Offshore platform components (valves, fittings) exposed to seawater and sour crude.
• Plastics & Rubber Manufacturing: Reactors for polymer synthesis, resistant to monomers and catalysts.

9. Advantages & Limitations

Core Advantages of ASTM A744 CN7M Stainless Steel

• Superior sulfuric acid resistance: Outperforms conventional stainless steels, reducing maintenance and replacement costs in acid service.
• Balanced corrosion protection: Resists oxidizing/reducing acids, chlorides, and SCC—versatile for mixed-corrosive environments.
• Excellent castability: Suitable for complex-shaped components (valves, pumps) that are difficult to fabricate via wrought processes.
• Cost-effectiveness: 30–40% cheaper than nickel-based alloys (e.g., Hastelloy C276) while offering comparable corrosion resistance in moderate environments.
• Nb stabilization: Eliminates IGC risk during welding/heat treatment, reducing post-processing costs.

Key Limitations of ASTM A744 CN7M Stainless Steel

• Higher cost than 316L: 2–3 times more expensive due to high Ni/Mo/Cu content, limiting use in non-critical applications.
• Moderate strength: Tensile strength (425–480 MPa) is lower than duplex stainless steels (e.g., 2205: 600–800 MPa), requiring thicker sections for structural loads.
• Work hardening: Prone to work hardening during machining, requiring specialized tools and slower cutting speeds.
• Limited high-temperature resistance: Not suitable for continuous service above 800°C (oxidation and NbC coarsening); use Hastelloy C276 for ultra-high temperatures.
• Residual element sensitivity: Trace Sn, Pb, or As can cause cracking, requiring strict raw material control.

10. Comparative Analysis: CN7M vs. Similar Alloys

11. Conclusion

ASTM A744 CN7M stainless steel stands as a premier super austenitic cast alloy, uniquely optimized for harsh corrosive environments—particularly sulfuric acid service.

Its balanced composition of high nickel, chromium, molybdenum, and copper, combined with niobium stabilization, delivers exceptional corrosion resistance, castability, and mechanical integrity, filling the performance-cost gap between conventional stainless steels and high-cost nickel-based alloys.

While CN7M faces limitations in strength, cost, and high-temperature service, ongoing innovations in microalloying, additive manufacturing, and green casting are expanding its application boundaries.

For engineers and material selectors, CN7M remains the optimal choice for cast components in chemical processing, pump/valve manufacturing, and acid-centric industries, where reliability and corrosion resistance are non-negotiable.

FAQs

Can CN7M stainless steel be welded without post-heat treatment?

Welding is possible, but solution annealing is recommended for critical corrosion service to restore the passive layer.

Is CN7M stainless steel suitable for chloride-rich environments?

Moderate performance; for high chloride SCC resistance, Duplex 2205 or nickel-base alloys may be preferred.

Can CN7M replace 316L stainless steel in sulfuric acid service?

Yes, CN7M outperforms 316L in sulfuric and reducing acid conditions, especially in cast components.

What are typical casting sizes and shapes for CN7M stainless steel?

Pumps, valves, impellers, and fittings with complex geometries, thin walls, and internal passages are common.