ASTM A194 Grade 8MLCuNA specifies premium super-austenitic stainless steel nuts, widely recognized as 254 SMO® or UNS S31254. This high-performance "6% Moly" grade is heavily alloyed with Molybdenum, Copper, and Nitrogen to provide extraordinary resistance to pitting, crevice corrosion, and chloride stress corrosion cracking in extreme marine and chemical environments.
What distinguishes Grade 8MLCuNA is the critical "A" designation, which mandates that the fasteners undergo a final Carbide Solution Treatment (full annealing) after all forging, machining, and thread-tapping operations are completed. This vital thermal process eliminates any residual cold-work stresses, ensuring a fully softened, strictly non-magnetic state (Max 90 HRB). This restoration of the matrix maximizes resistance to Stress Corrosion Cracking (SCC) in highly aggressive applications.
| Material Classification | Super Austenitic Stainless Steel (6% Moly Grade - Annealed) |
|---|---|
| UNS Designation | S31254 |
| Microstructure | Austenitic (Nitrogen-Strengthened, Post-Annealed) |
| 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 heavy hex profiles, and critical Pitting Resistance Equivalent Number (PREN) data.
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| Carbon (C) | Manganese (Mn) | Chromium (Cr) | Nickel (Ni) | Molybdenum (Mo) | Phosphorus (P) | Sulfur (S) | Silicon (Si) |
|---|---|---|---|---|---|---|---|
| 0.020 Max | 1.00 Max | 19.5 – 20.5 | 17.5 – 18.5 | 6.0 – 6.5 | 0.030 Max | 0.010 Max | 0.80 Max |
Because Grade 8MLCuNA is explicitly annealed after all machining to remove localized cold-work hardening, ASTM caps its maximum hardness to guarantee the matrix remains fully softened and stress-free.
| Mechanical Property | Grade 8MLCuNA (Annealed) Limits |
|---|---|
| Hardness Limit (Rockwell) | 60 - 90 HRB Maximum |
| Proof Load | Dependent on Size |
| Standard / System | Designation |
|---|---|
| ASME Standard | SA194 Grade 8MLCuNA |
| Common Trade Name | 254 SMO® (Annealed), 6% Moly Nuts, UNS S31254 |
| European Equivalent | EN 1.4547 / X1CrNiMoCuN20-18-7 |
| Internal Threads (A194 Gr 8MLCuNA) | Mating Bolts (A193 Gr B8MLCuNA) | Mating Studs (A193 Gr B8MLCuNA) |
|---|---|---|
| Grade 8MLCuNA Heavy Hex Nuts | ASTM A193 Grade B8MLCuNA Hex Bolts | A193 Grade B8MLCuNA Fully Threaded Studs |
Due to the incredibly high alloy content and the fully austenitic nature of 8MLCuNA, thread galling is a significant risk under high clamping loads. Premium, heavy-duty anti-seize lubrication (such as nickel or molybdenum-disulfide based pastes) is absolutely mandatory for installation.
| Nominal Diameter | Threads Per Inch (UNC) | Target Torque — Lubricated (ft-lbs) |
|---|---|---|
| 1/2" | 13 | 45 |
| 5/8" | 11 | 90 |
| 3/4" | 10 | 150 |
| 1" | 8 | 350 |
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 during machining, ensuring the part is fully softened and strictly non-magnetic to maximize resistance against Stress Corrosion Cracking.
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 manufacturing process, and it would fail the 8MLCuNA specification.
No, it is a fully Super Austenitic stainless steel. While its corrosion resistance is often compared to Super Duplex grades like 2507, its fully austenitic microstructure gives it superior ductility and toughness, especially at extremely low (cryogenic) temperatures.
