ASTM A743 CA6NM is a martensitic stainless steel casting grade specifically engineered to deliver high strength, corrosion resistance, and toughness in severe service environments.
With its 12–14% chromium and 3–4% nickel composition, CA6NM achieves a balanced microstructure that offers superior resistance to cavitation, erosion, and pitting while maintaining excellent weldability compared to other martensitic stainless steels.
This alloy has become a material of choice for hydroturbine runners, pump impellers, offshore platform components, and valve bodies, where a combination of structural reliability and environmental resilience is mandatory.
1. What is ASTM A743 CA6NM?
ASTM A743 CA6NM is a martensitic stainless steel casting grade designed for service in environments requiring high mechanical strength, good toughness, and moderate-to-high corrosion resistance.
The “CA” denotes a corrosion-resistant alloy in ASTM casting standards, “6” refers to the alloy series, and “NM” indicates the presence of nickel and molybdenum for enhanced corrosion resistance.
It is widely recognized for its balance of machinability, weldability, and resistance to environmental degradation, making it unique among martensitic grades.
2. Chemical Composition of CA6NM
CA6NM is a 12% chromium, 4% nickel, 0.5% molybdenum martensitic stainless steel developed to combine strength, toughness, and corrosion resistance in a single casting alloy.
Its composition is tightly controlled under ASTM A743/A743M to ensure consistent metallurgical performance.
Typical chemical composition limits (% by weight):
| Element | Specification Range (%) | Functional Role |
| Carbon (C) | ≤ 0.06 | Low carbon minimizes carbide precipitation, enhancing toughness and weldability. |
| Manganese (Mn) | ≤ 1.00 | Improves hot working characteristics and deoxidation during melting. |
| Silicon (Si) | ≤ 1.00 | Acts as a deoxidizer; excessive amounts may reduce toughness. |
| Chromium (Cr) | 11.5 – 14.0 | Primary element for passivation and corrosion resistance. |
| Nickel (Ni) | 3.50 – 4.50 | Stabilizes martensite, improves toughness, and enhances resistance to stress corrosion cracking. |
| Molybdenum (Mo) | 0.40 – 1.00 | Boosts pitting resistance, particularly in chloride-containing environments. |
| Phosphorus (P) | ≤ 0.04 | Kept low to prevent embrittlement. |
| Sulfur (S) | ≤ 0.03 | Low levels maintain toughness and corrosion resistance. |
| Iron (Fe) | Balance | Matrix element providing structural strength. |
3. Mechanical & Physical Properties of CA6NM
CA6NM is engineered to deliver a balanced combination of strength, ductility, and fracture toughness, even in large section castings.
Its properties are the result of its 12Cr–4Ni–Mo martensitic composition combined with controlled heat treatment.
Typical Mechanical Properties
(Values per ASTM A743/A743M requirements; actual results depend on section size, heat treatment, and test orientation)
| Property | Typical Value | Test Condition |
| Tensile Strength (Rm) | 655–795 MPa (95–115 ksi) | Room temperature, tempered martensite |
| Yield Strength (Rp0.2) | ≥ 450 MPa (65 ksi) | Same as above |
| Elongation | ≥ 15% | Gauge length = 50 mm |
| Reduction of Area | ≥ 35% | Room temperature |
| Charpy V-Notch Impact Energy | 40–80 J at –46°C (–50°F) | Longitudinal direction |
| Hardness | 207–255 HB (approx. 22–26 HRC) | After tempering |
| Fracture Toughness (K_IC) | ~110–130 MPa·√m | Room temperature, fine-grained condition |
Typical Physical Properties
| Property | Typical Value | Notes |
| Density | 7.74 g/cm³ (0.280 lb/in³) | Slightly lower than carbon steels due to alloying |
| Modulus of Elasticity | 200 GPa (29 × 10⁶ psi) | Comparable to other stainless steels |
| Thermal Conductivity | ~24 W/m·K at 100°C | Lower than carbon steels; affects heat dissipation |
| Specific Heat Capacity | 460 J/kg·K | At 20°C |
| Electrical Resistivity | 0.60 µΩ·m | Higher than carbon steels, beneficial for some erosion resistance |
| Coefficient of Thermal Expansion | 10.8 × 10⁻⁶ /°C (20–100°C) | Must be considered in multi-metal assemblies |
4. Heat Treatment & Microstructure Control
CA6NM derives its performance not only from its 12% chromium, 4% nickel, and molybdenum chemistry, but also from precise heat treatment sequences that transform its as-cast structure into a tough, tempered martensitic microstructure.
This transformation is essential to achieving the alloy’s targeted balance of strength, ductility, corrosion resistance, and dimensional stability.
Standard Heat Treatment Sequence
The typical heat treatment for CA6NM castings follows ASTM A743/A743M guidelines and is tailored to section thickness:
Solution Annealing (Austenitizing):
- Temperature: 1010–1050 °C (1850–1920 °F)
- Purpose: Dissolves carbides and homogenizes alloying elements. Produces a fully austenitic structure before quenching.
- Hold Time: ~1 hour per 25 mm (1 inch) of section thickness, minimum of 2 hours.
Quenching:
- Medium: Forced air or oil, depending on casting section size and desired cooling rate.
- Purpose: Transforms austenite to low-carbon martensite while minimizing distortion and residual stresses.
- Note: Nickel content in CA6NM lowers the martensite start (M_s) temperature, promoting uniform transformation.
Tempering:
- Temperature: 565–620 °C (1050–1150 °F) for standard balance of strength and toughness.
- Purpose: Relieves stresses, improves ductility, and adjusts hardness to 22–26 HRC.
- Effect of Temperature: Lower tempering temperatures yield higher strength but reduce impact toughness; higher temperatures improve toughness but slightly lower yield strength.
Microstructure Characteristics
A properly heat-treated CA6NM casting exhibits:
- Tempered Martensite Matrix: Provides high tensile and yield strength with good fracture toughness.
- Refined Grain Size: Nickel addition suppresses grain growth during austenitizing, aiding in high impact energy retention.
- Dispersed Carbides: Fine M₂₃C₆ carbides along lath boundaries improve wear resistance without severely impairing toughness.
- Minimal Retained Austenite (<5%): Excessive retained austenite can lower hardness and dimensional stability, so cooling rates and tempering cycles are carefully controlled.
5. Casting, Machining & Weldability
CA6NM’s value as a hydroturbine, valve, and pump alloy depends not only on its chemistry and heat treatment, but also on its castability, machinability, and repair weldability.
Casting Processes
CA6NM can be successfully produced using multiple foundry methods, allowing manufacturers to match process capabilities to part geometry, dimensional requirements, and production volume.
Sand Casting:
- Best suited for large, thick-walled components such as turbine casings, pump housings, and valve bodies in the 1–5 ton range.
- Typical tolerances: ±1 mm per 100 mm dimension.
- Surface finish: Ra 6.3–12.5 μm after shakeout.
- Advantages: High flexibility in size and shape; economical for low-to-medium volumes.
Investment Casting (Lost Wax):
- Ideal for intricate geometries like turbine blades, valve trims, and runner segments where smooth surfaces and fine detail are critical.
- Dimensional accuracy: ±0.1 mm.
- Surface finish: Ra 1.6–3.2 μm, reducing machining allowance and improving as-cast hydraulic efficiency.
Centrifugal Casting:
- Produces cylindrical or ring-shaped components such as pump sleeves, wear rings, and bearing shells.
- Ensures uniform density and minimal segregation—critical for high-pressure sealing surfaces.
- Often used for parts requiring concentricity tolerances within 0.25 mm.
Casting yield rates for CA6NM generally exceed 85% for simple geometries, while more complex shapes with deep pockets or thick-to-thin transitions may drop to 70–75% due to shrinkage cavity management and riser design limitations.
Machining Behavior
CA6NM is significantly easier to machine than fully hardened martensitic steels, especially in the tempered condition (22–26 HRC).
Key machining notes:
- Cutting Speeds: ~30–50 m/min with carbide tooling; up to 80 m/min with coated carbides in finishing passes.
- Tool Wear: Moderate—nickel improves toughness but can cause work hardening if feeds are too light.
- Coolant Use: Recommended for surface finish consistency and thermal stability.
- Dimensional Stability: Low retained austenite content means minimal distortion after rough machining.
- Machining Allowances: 3–6 mm is typical to remove surface scale and casting skin after heat treatment.
Weldability
CA6NM is more weldable than conventional 410 stainless due to:
- Low carbon content (≤0.06%)
- Nickel addition (~4%) stabilizing austenite during cooling
- Lower risk of hydrogen cracking when preheat and post-weld heat treatment are applied
Best practices for welding:
- Preheating: 150–250 °C (300–480 °F) to reduce thermal gradients and hydrogen cracking risk.
- Filler Metal Selection: Matching composition filler (e.g., AWS ER410NiMo for GTAW/GMAW or E410NiMo for SMAW) to maintain strength and corrosion resistance.
- Interpass Temperature: < 250 °C (480 °F) to avoid over-tempering adjacent heat-affected zones.
- Post-Weld Heat Treatment (PWHT): Local or full tempering at 565–620 °C (1050–1150 °F) to restore toughness and hardness uniformity.
Repair welding:
- Common in large hydroturbine runners or valve bodies to correct porosity or surface defects.
- Success depends on strict control of welding parameters, joint cleanliness, and PWHT application.
6. Corrosion Resistance: Tailored to Aqueous Environments
CA6NM’s corrosion resistance is engineered for freshwater, seawater, and mild chemical environments, making it far more resistant than carbon steel or low-alloy castings, and competitive with some austenitic grades in specific scenarios:
- Freshwater and Steam: The chromium oxide layer resists oxidation and pitting in freshwater (e.g., river water, coolant systems) with corrosion rates <0.02 mm/year.
It also withstands wet steam at 200–300°C, a key trait for power plant components. - Seawater: Molybdenum additions enhance resistance to chloride-induced pitting.
In seawater immersion tests, CA6NM exhibits a corrosion rate of 0.05–0.1 mm/year—superior to 410 stainless steel (0.2–0.3 mm/year) but slightly less than 316 (0.01–0.03 mm/year). - Mild Chemicals: Resists dilute acids (e.g., 5% sulfuric acid), alkalis (e.g., 10% sodium hydroxide), and petroleum products, making it suitable for oilfield valves and chemical processing pumps.
Limitations exist: CA6NM is not recommended for strong acids (e.g., 37% hydrochloric acid) or high-chloride environments (e.g., brines with >10% NaCl), where austenitic grades like CF8M (316 equivalent) perform better.
7. Typical Applications of CA6NM
ASTM A743 CA6NM’s high strength, excellent toughness at low temperatures, and resistance to corrosion, cavitation, and erosion make it the go-to material for critical hydraulic, marine, and energy sector components.
| Application Sector | Typical Components | Key Performance Requirements Met by CA6NM |
| Hydropower | Turbine runners (Kaplan, Francis, bulb), wicket gates, guide vanes, stay rings | High cavitation resistance, erosion resistance, toughness at low temperature |
| Marine & Offshore | Propeller blades, hubs, rudder stocks, pump shafts, seawater valve bodies | Seawater corrosion resistance, good fatigue strength, low magnetic permeability |
| Oil & Gas | Subsea pump impellers, sleeves, gate/globe/check valve trim, choke valves | Chloride stress corrosion resistance, erosion resistance, high strength |
| Industrial Pumping | Centrifugal pump impellers, wear rings, casings, diffuser plates | Wear resistance, corrosion resistance in brackish water and chemicals |
| Desalination Plants | High-pressure pump shafts, impellers, sealing rings | Resistance to chloride-induced pitting, dimensional stability |
| Tidal & Renewable Energy | Tidal turbine blades, hubs, shafts | Combined erosion and chloride corrosion resistance, long-term durability |
| Defense / Naval | Submarine propellers, shaft liners, steering gear components | Low magnetic signature, cavitation resistance, mechanical reliability |
8. Comparisons: CA6NM vs CA15 (410), 17-4PH, Duplex 2205
| Property / Feature | CA6NM (ASTM A743) | CA15 (410 SS) (ASTM A743) | 17-4PH (ASTM A747 CB7Cu-1) | Duplex 2205 (ASTM A890 Grade 4A) |
| Type / Microstructure | Martensitic (low C, 12Cr + Ni) | Martensitic (high C, 12Cr) | Precipitation-hardening martensitic | Ferritic-austenitic (duplex) |
| Typical Composition (wt%) | C ≤ 0.06, Cr 11.5–14, Ni 3.5–4.5, Mo 0.4–1.0 | Cr 11.5–14, Ni ≤ 1.0, C 0.15 | C ≤ 0.07, Cr 15–17, Ni 3–5, Cu 3–5 | C ≤ 0.03, Cr 21–23, Ni 4.5–6.5, Mo 2.5–3.5 |
| Tensile Strength (MPa) | 655–760 | 550–690 | 930–1,100 | 620–880 |
| Yield Strength (MPa) | 450–550 | 350–450 | 725–1,035 | 450–620 |
| Elongation (%) | 15–20 | 10–15 | 8–12 | 20–25 |
| Hardness (HB) | 200–240 | 180–230 | 300–360 | 220–270 |
| Toughness at 0°C (J) | Excellent (≥ 40) | Fair (10–20) | Moderate (20–30) | Excellent (≥ 60) |
| Corrosion Resistance | Good in fresh/seawater, resists cavitation | Fair, prone to pitting in chlorides | Good, but not for severe chloride environments | Excellent chloride and pitting resistance |
| Cavitation Resistance | High | Low | Medium | High |
| Heat Treatment | Solution anneal + temper | Tempering only | Solution + aging | Solution anneal only |
| Castability | Good, suitable for sand & investment casting | Good for sand casting | Moderate, more complex due to precipitation hardening | Moderate, requires precise control |
| Weldability | Good, but requires pre/post heat treatment | Moderate, prone to cracking | Good, but post weld aging required | Good, sensitive to intermetallics |
| Machinability | Moderate | Good | Fair | Moderate |
| Cost Level | Medium | Low | High | High |
| Typical Applications | Hydraulic turbines, pump impellers, marine propellers | General pump parts, low-duty valves | Aerospace, high-strength shafts | Offshore structures, desalination equipment |
9. Common Equivalents
CA6NM’s unique balance of strength, toughness, and corrosion resistance positions it among several related martensitic stainless steels. Its common equivalents in other standards or grades include:
- UNS J91660: Unified Numbering System designation for CA6NM.
- ASTM A297 Type CA6NM: An alternative ASTM designation for similar castings.
- EN 1.4528 / X12CrNiSi17-7: European equivalent martensitic stainless steel grade, used in casting or forging.
- JIS SUS630: Japanese equivalent precipitation hardening stainless steel, shares some similar applications though differing in microstructure.
- CA15 (ASTM A743 CA15): A higher carbon martensitic grade with similar chemistry but different mechanical and toughness profiles.
10. Conclusion
ASTM A743 CA6NM offers a proven balance of strength, corrosion resistance, and toughness that makes it indispensable in demanding rotating machinery and marine/offshore applications.
Its enhanced weldability and cavitation resistance allow for longer service life and reduced maintenance downtime—making it a cost-effective choice for severe environments.
FAQs
Is CA6NM magnetic?
Yes, it is martensitic and exhibits magnetic properties.
Is CA6NM suitable for seawater immersion?
No—its pitting corrosion rate (0.1–0.2 mm/year) makes it unsuitable for long-term seawater exposure. Use duplex 2205 instead.
What is the maximum temperature for CA6NM?
It retains useful strength up to 400°C. Above 500°C, oxidation and softening occur; use nickel-based alloys for higher temps.
Can CA6NM be used in food processing?
No—its moderate corrosion resistance and potential for pitting in acidic foods make austenitic grades (e.g., CF8) better.
How does CA6NM compare to 17-4PH in strength?
17-4PH offers higher tensile strength (860–1100 MPa) but is less castable; CA6NM is preferred for complex castings.
What is the typical lead time for CA6NM castings?
4–8 weeks for sand castings; 6–12 weeks for investment castings (due to mold making).
