ASTM A194 Grade 8FA specifies austenitic stainless steel nuts manufactured from AISI Type 303 (or Type 303Se). This grade is globally recognized as the "free-machining" variant of the 18/8 stainless family, allowing for high-speed CNC machining and exceptionally smooth thread profiles.
What sets Grade 8FA apart is the crucial "A" designation, which mandates that the fasteners undergo a final Carbide Solution Treatment (full annealing) after all forging, milling, and thread-tapping operations are completed. This vital thermal process eliminates residual cold-work stresses introduced during the free-machining process, ensuring a fully softened, strictly non-magnetic state (Max 90 HRB) that restores the maximum possible corrosion resistance to the alloy and drastically reduces susceptibility to Stress Corrosion Cracking (SCC).
| Material Classification | Free-Machining Austenitic Stainless Steel (Type 303 / 303Se - Annealed) |
|---|---|
| UNS Designation | S30300 (Sulfur) / S30323 (Selenium) |
| Magnetic State | Strictly Non-Magnetic (Annealed post-machining) |
| Density | ~8.00 g/cm³ (0.289 lb/in³) |
| Size Range | Metric: M6 to M100 | Imperial: 1/4" to 4" |
| Strengthening Method | Full Solution Annealing After Machining |
Contains detailed proof load thresholds, dimensional tolerances for ASME B18.2.2 standard and slotted hex profiles, and precise galling-prevention torque values for A193 B8FA assemblies.
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Grade 8FA utilizes an 18/8 austenitic base but significantly raises the allowable limits for Sulfur (S) or Selenium (Se) and Phosphorus (P). While this slightly reduces the alloy's resistance to severe pitting compared to Type 304 (Grade 8), it dramatically improves cutting speeds and thread surface finishes during production.
| Carbon (C) | Manganese (Mn) | Chromium (Cr) | Nickel (Ni) | Phosphorus (P) | Sulfur (S) OR Selenium (Se) | Silicon (Si) |
|---|---|---|---|---|---|---|
| 0.15 Max | 2.00 Max | 17.00 – 19.00 | 8.00 – 10.00 | 0.20 Max | 0.15 Min (S or Se) | 1.00 Max |
Grade 8FA is explicitly annealed after all machining, removing any localized cold-work hardening. Therefore, ASTM caps its maximum hardness to guarantee it remains fully softened.
| Mechanical Property | Grade 8FA (Annealed) Limits |
|---|---|
| Hardness Limit (Rockwell) | 60 - 90 HRB Maximum |
| Proof Load | Dependent on Size |
| Standard / System | Designation |
|---|---|
| ASME Standard | SA194 Grade 8FA |
| Common Trade Name | Type 303 Stainless Fasteners (Annealed) |
| European Equivalent | EN 1.4305 / X8CrNiS18-9 |
| Internal Threads (A194 Gr 8FA) | Mating Bolts (A193 Gr B8FA) | Mating Studs (A193 Gr B8FA) |
|---|---|---|
| Grade 8FA Heavy Hex Nuts (Annealed) | ASTM A193 Grade B8FA Bolts | A193 Grade B8FA Double End Studs |
| Nominal Diameter | Threads Per Inch (UNC) | Target Torque — Lubricated (ft-lbs) |
|---|---|---|
| 1/2" | 13 | 30 |
| 5/8" | 11 | 60 |
| 3/4" | 10 | 100 |
| 1" | 8 | 250 |
The "A" indicates that the component has been Annealed after all manufacturing and machining processes are complete. This removes all internal cold-work stresses generated by the free-machining process, ensuring the part is fully softened and strictly non-magnetic to maximize corrosion resistance.
Because the "A" specification is designed to eliminate stress through annealing, the maximum hardness is capped at 90 HRB. If a nut measures harder than this, it indicates residual cold-work stress remains from the machining process, and it would fail the 8FA specification.
No. Even when annealed, the high sulfur or selenium content makes Type 303 (Grade 8FA) highly prone to hot-cracking during welding. If heavy welding is required, standard Grade 8 (Type 304) or Grade 8C (Type 347) should be specified.
