ASTM A194 Grade 8MLNA specifies premium austenitic stainless steel nuts manufactured from AISI Type 316LN. This advanced alloy is engineered with an exceptionally low carbon profile (Max 0.030%) to prevent sensitization during high-temperature service or welding, combined with intentional additions of Molybdenum and Nitrogen. The molybdenum significantly enhances resistance to chloride pitting, while the nitrogen acts as a powerful solid-solution strengthening agent to compensate for the yield strength naturally lost when reducing carbon levels.
What sets Grade 8MLNA apart is the crucial "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 residual cold-work stresses, ensuring a fully softened, strictly non-magnetic state (60-90 HRB) that drastically minimizes susceptibility to Stress Corrosion Cracking (SCC).
| Material Classification | Nitrogen-Strengthened Austenitic Stainless Steel (Type 316LN - Annealed) |
|---|---|
| UNS Designation | S31653 |
| Microstructure | Austenitic (Low-Carbon, Nitrogen-Stabilized) |
| 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 precise galling-prevention torque values for A193 B8MLNA assemblies.
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| Carbon (C) | Manganese (Mn) | Chromium (Cr) | Nickel (Ni) | Molybdenum (Mo) | Phosphorus (P) | Sulfur (S) | Silicon (Si) |
|---|---|---|---|---|---|---|---|
| 0.030 Max | 2.00 Max | 16.0 – 18.0 | 10.0 – 13.0 | 2.00 – 3.00 | 0.045 Max | 0.030 Max | 1.00 Max |
Grade 8MLNA is explicitly annealed after all machining, removing any localized cold-work hardening. ASTM tightly controls the hardness to guarantee it remains fully softened and stress-free.
| Mechanical Property | Grade 8MLNA (Annealed) Limits |
|---|---|
| Hardness Limit (Rockwell) | 60 - 90 HRB Maximum |
| Proof Load | Dependent on Size |
| Standard / System | Designation |
|---|---|
| ASME Standard | SA194 Grade 8MLNA |
| Common Trade Name | Type 316LN Stainless Nuts (Annealed) |
| European Equivalent | EN 1.4406 / X2CrNiMoN17-11-2 |
| Internal Threads (A194 Gr 8MLNA) | Mating Bolts (A193 Gr B8MLNA) | Mating Studs (A193 Gr B8MLNA) |
|---|---|---|
| Grade 8MLNA Heavy Hex Nuts (Annealed) | ASTM A193 Grade B8MLNA Hex Bolts | A193 Grade B8MLNA Double End Studs |
Due to the fully austenitic and softened nature of 8MLNA, severe thread galling is a major risk under load. Premium anti-seize lubrication is absolutely mandatory for installation.
| 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 letters indicate the precise metallurgical profile of the nut: M indicates the presence of Molybdenum (for pitting resistance). L indicates Low Carbon (to prevent sensitization). N indicates Nitrogen addition (for yield strength enhancement). A indicates the part is fully Annealed after all machining.
Standard Grade 8M (Type 316) has a higher carbon limit and lacks intentional nitrogen strengthening. Grade 8MLNA (Type 316LN) reduces the carbon to improve stability in hot environments or during welding, and adds Nitrogen to keep the steel strong despite the low carbon content. Furthermore, the "A" ensures the nut is delivered completely stress-free.
Because the "A" specification is designed to eliminate internal stress through annealing, the hardness is restricted between 60 and 90 HRB. If a nut measures harder than 90 HRB, it strongly indicates that residual cold-work stress remains from the machining or threading process, meaning the part is not fully annealed and fails the specification.
