A995 4A vs 5A vs 6A
ASTM A995 Grades 4A, 5A, and 6A are the three most commonly specified duplex and super duplex stainless steel casting grades for oil and gas valve bodies. Grade 4A is standard duplex (cast 2205). Grades 5A and 6A are super duplex alloys with higher chromium, molybdenum, and nitrogen for severe chloride environments. The choice between them hinges on corrosion severity, temperature, and cost.
Head-to-Head Comparison
| Property | 4A (Duplex 2205) | 5A (Super Duplex 2507) | 6A (Zeron 100 Type) |
|---|---|---|---|
| UNS | J92205 | J93404 | J93380 |
| Chromium | 21.0-23.5% | 24.0-26.0% | 24.0-26.0% |
| Nickel | 4.5-6.5% | 6.0-8.0% | 6.0-8.0% |
| Molybdenum | 2.5-3.5% | 4.0-5.0% | 4.0-5.0% |
| Nitrogen | 0.10-0.30% | 0.10-0.30% | 0.10-0.30% |
| Copper | - | - | 0.50-1.00% |
| Tungsten | - | - | 0.50-1.00% |
| Tensile (min) | 620 MPa (90 ksi) | 690 MPa (100 ksi) | 690 MPa (100 ksi) |
| Yield (min) | 450 MPa (65 ksi) | 485 MPa (70 ksi) | 485 MPa (70 ksi) |
| Elongation (min) | 25% | 25% | 25% |
| PREN (typical) | 34-36 | 41-43 | 41-43 |
| Ferrite | 40-60% | 40-60% | 40-60% |
| Forged equivalent | A182 F51 | A182 F53 | A182 F55 |
| Relative cost | Baseline | +30-50% | +35-55% |
When to Use Each Grade
Grade 4A (Duplex 2205) covers the majority of offshore and chemical service applications. PREN above 34 provides reliable pitting resistance in chloride environments up to approximately 40°C seawater. It is the standard cast duplex grade for topside piping valves, fire water system valves, and process valves in moderate chloride service.
Grade 5A (Super Duplex 2507) is specified when 4A lacks sufficient pitting resistance: warm seawater above 40°C, high-chloride brines, and sour service with elevated chloride levels. Subsea valve bodies, HP/HT well completions, and aggressive chemical processing environments demand 5A.
Grade 6A (Zeron 100 type) adds copper and tungsten to the super duplex base. Copper improves resistance to sulfuric and phosphoric acids. Tungsten boosts PREN via the modified formula (PREN_W = %Cr + 3.3(%Mo + 0.5%W) + 16%N). Specify 6A for the most aggressive acid and chloride combinations.
PREN and Critical Pitting Temperature
The Pitting Resistance Equivalent Number (PREN = %Cr + 3.3 x %Mo + 16 x %N) predicts localized corrosion resistance. Higher PREN means higher critical pitting temperature (CPT):
| Grade | PREN Range | CPT (approx.) |
|---|---|---|
| CF8M (316) | 24-28 | 15-20°C |
| 4A (2205) | 34-36 | 35-45°C |
| 5A (2507) | 41-43 | 55-70°C |
| 6A (Zeron 100) | 41-43 | 55-70°C |
Quality Requirements
All ASTM A995 grades require solution annealing and water quenching, ferrite content verification (40-60%), and corrosion testing per ASTM A923 and/or ASTM G48. Mill test certificates must report full chemistry, mechanical properties, ferrite content, and corrosion test results. Radiographic and liquid penetrant examination per ASME B16.34, plus hydrostatic testing, are standard for duplex valve castings.
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