A182 F304 vs F316
F304 and F316 are austenitic stainless steel forging grades under ASTM A182. F304 is the forged equivalent of Type 304 (18Cr-8Ni). F316 adds 2-3% molybdenum (16Cr-10Ni-2Mo) for substantially better pitting and crevice corrosion resistance. The choice between them affects material cost, corrosion life, and project risk.
When to Use F304 vs F316
F304 handles most non-chloride corrosive services at lower cost. Suitable for clean process water, steam, dilute organic acids, and caustic solutions.
F316 is mandatory where chlorides, sulfuric acid, or marine atmospheres are present. Specify F316 for offshore platforms, seawater-cooled systems, chemical processing with halide contamination, and pharmaceutical/biotech applications requiring superior pitting resistance.
Specifications and Properties
| Property | F304 | F316 |
|---|---|---|
| UNS designation | S30400 | S31600 |
| Carbon | 0.08% max | 0.08% max |
| Chromium | 18.0-20.0% | 16.0-18.0% |
| Nickel | 8.0-11.0% | 10.0-14.0% |
| Molybdenum | None | 2.0-3.0% |
| Tensile strength (min) | 515 MPa (75 ksi) | 515 MPa (75 ksi) |
| Yield strength (min) | 205 MPa (30 ksi) | 205 MPa (30 ksi) |
| Elongation (min) | 30% | 30% |
| Hardness (max) | 183 HBW | 183 HBW |
| PREN | ~18-20 | ~24-28 |
| Heat treatment | Solution anneal 1040°C min | Solution anneal 1040°C min |
| Max service temp | 538°C (1000°F) | 538°C (1000°F) |
| Min service temp | -254°C (-425°F) | -254°C (-425°F) |
| Cast equivalent | A351 CF8 | A351 CF8M |
| Pipe specification | A312 TP304 | A312 TP316 |
| Relative cost | Baseline | 10-20% premium |
Comparison: Corrosion Performance
The Pitting Resistance Equivalent Number (PREN) quantifies localized corrosion resistance:
PREN = %Cr + 3.3 x %Mo + 16 x %N
F304 achieves a PREN of approximately 19. F316 reaches approximately 26. The 3.3x multiplier on molybdenum content is decisive. In chloride-containing environments above 60°C, F304 is vulnerable to pitting, crevice corrosion, and chloride stress corrosion cracking. F316 resists these mechanisms significantly longer, though it is not immune.
For even higher chloride resistance, consider duplex stainless (A182 F51, PREN ~35) or super duplex (A182 F53, PREN ~42).
Low-Carbon Variants
| Standard Grade | Low-Carbon Grade | Max Carbon | Key Advantage |
|---|---|---|---|
| F304 | F304L | 0.03% | Resists sensitization during welding |
| F316 | F316L | 0.03% | Resists sensitization + Mo corrosion resistance |
| F304H | - | 0.04-0.10% | Higher creep strength above 500°C |
| F316H | - | 0.04-0.10% | Higher creep strength + Mo |
Low-carbon “L” grades prevent chromium carbide precipitation at grain boundaries (sensitization) during welding or prolonged 425-870°C exposure. Most projects default to F304L/F316L for welded piping systems. The “H” grades provide higher creep strength for continuous service above 500°C.
Testing
Both grades require solution annealing, tensile testing, and hardness testing per ASTM A182. Mill test certificates (EN 10204 3.1 or 3.2) must report full chemistry and mechanical properties. Supplementary requirements cover intergranular corrosion testing per ASTM A262, ultrasonic and liquid penetrant examination, and hydrostatic testing. The forging process produces a fully austenitic microstructure with superior mechanical uniformity compared to castings.
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