ASTM A194 Grade B8ML4CuN specifies highly alloyed, premium austenitic stainless steel nuts designed for severe corrosive environments. This specific grade bridges the gap between standard Type 316 stainless and the extreme 6% Moly super-austenitic grades. By incorporating a robust 3.0% to 4.0% Molybdenum content, along with elevated Nickel (15.0% - 16.5%) and targeted Copper and Nitrogen additions, it provides a massive leap in localized pitting and crevice corrosion resistance compared to standard 300-series alloys.
The "L" designation indicates a strictly controlled low-carbon profile (Max 0.030%), which prevents carbide precipitation (sensitization) during high-temperature service or welding applications. Meanwhile, the Nitrogen addition serves as a powerful solid-solution strengthener, recovering the mechanical yield strength that is naturally reduced when carbon levels are lowered, ensuring structural integrity in high-pressure piping assemblies.
| Material Classification | Advanced Austenitic Stainless Steel (4% Moly, Cu/N Alloyed) |
|---|---|
| Microstructure | Austenitic (Low-Carbon, Nitrogen-Strengthened) |
| Magnetic State | Non-Magnetic |
| Density | ~8.00 g/cm³ (0.289 lb/in³) |
| Size Range | Metric: M6 to M100 | Imperial: 1/4" to 4" |
| Strengthening Method | Nitrogen Solid-Solution Strengthening |
Contains detailed proof load thresholds, dimensional tolerances for ASME B18.2.2 standard and heavy hex profiles, and precise galling-prevention torque values.
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The elevated Molybdenum (3.0% - 4.0%) provides exceptional resistance to chloride environments. The high Nickel content stabilizes the austenitic structure even at extreme low temperatures, while ultra-low sulfur (Max 0.010%) ensures a highly pure matrix resistant to localized attack.
| Carbon (C) | Manganese (Mn) | Chromium (Cr) | Nickel (Ni) | Molybdenum (Mo) | Phosphorus (P) | Sulfur (S) | Silicon (Si) |
|---|---|---|---|---|---|---|---|
| 0.030 Max | 2.00 Max | 17.0 – 19.0 | 15.0 – 16.5 | 3.0 – 4.0 | 0.040 Max | 0.010 Max | 1.00 Max |
| Mechanical Property | Grade B8ML4CuN Limits |
|---|---|
| Hardness Limit (Rockwell) | 32 HRC Maximum |
| Proof Load | Dependent on Size |
| Standard / System | Designation |
|---|---|
| ASME Standard | SA194 Grade B8ML4CuN |
| Common Trade Name | 4% Moly Stainless Nuts, Nitrogen/Copper Strengthened Alloy |
| Internal Threads (A194 Gr B8ML4CuN) | Mating Bolts (A193 Gr B8ML4CuN) | Mating Studs (A193 Gr B8ML4CuN) |
|---|---|---|
| Grade B8ML4CuN Heavy Hex Nuts | ASTM A193 Grade B8ML4CuN Hex Bolts | A193 Grade B8ML4CuN Fully Threaded Studs |
Due to the fully austenitic nature and high alloy content of this grade, severe thread galling is a major risk under high clamping loads. Premium high-performance anti-seize lubrication is absolutely mandatory for safe installation and future unbolting.
| 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 |
This complex nomenclature breaks down the exact metallurgical advantages of the alloy: B8M denotes the Molybdenum-bearing 316-family base. L stands for Low Carbon (preventing sensitization). 4 denotes the elevated ~4% Molybdenum content. Cu and N represent the strategic additions of Copper and Nitrogen for enhanced acid resistance and yield strength recovery.
Standard Grade 8M (Type 316) contains roughly 2.0% to 3.0% Molybdenum and is suitable for moderate chloride environments. B8ML4CuN pushes the Molybdenum content up to 4.0% while drastically increasing Nickel (up to 16.5%). This higher alloy content gives B8ML4CuN vastly superior resistance to pitting and crevice corrosion in hot seawater and aggressive acids compared to standard 316 stainless.
Fundamentally, this is a highly stable austenitic stainless steel, meaning it is nominally non-magnetic. Because of the extremely high Nickel content (15.0% - 16.5%), the austenitic matrix remains incredibly stable, meaning it resists becoming magnetic even when subjected to cold-working processes like thread rolling.
