ASTM A194 Grade 8R specifies premium, high-performance austenitic stainless steel nuts manufactured from UNS S20910, widely recognized in the industry by the commercial trade names Nitronic® 50 or XM-19. This highly advanced alloy utilizes a complex matrix of Chromium, Nickel, Molybdenum, and significantly elevated levels of Manganese (4.0% - 6.0%) and Nitrogen. This sophisticated metallurgical design provides a unique combination of exceptional corrosion resistance and mechanical strength that is vastly superior to standard 300-series stainless steels.
Grade 8R fasteners deliver nearly double the yield strength of standard Type 316 stainless steel, while simultaneously offering outstanding resistance to sulfide stress cracking, chloride pitting, and intergranular corrosion. Furthermore, unlike many other stainless alloys, Grade 8R remains remarkably non-magnetic even after undergoing severe cold-working processes, making it the premier choice for demanding offshore, marine, and highly specialized chemical applications.
| Material Classification | Nitrogen-Strengthened, High-Manganese Austenitic Stainless Steel (XM-19) |
|---|---|
| UNS Designation | S20910 |
| Microstructure | Austenitic (Stabilized by Manganese and Nitrogen) |
| Magnetic State | Strictly Non-Magnetic (Even after severe cold-working) |
| Density | ~7.88 g/cm³ (0.285 lb/in³) |
| Size Range | Metric: M6 to M100 | Imperial: 1/4" to 4" |
| Strengthening Method | Nitrogen and Manganese 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 for A193 B8R assemblies.
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The unique performance of Grade 8R is dictated by its distinct chemistry. The highly elevated Chromium (20.5-23.5%) and Molybdenum (1.50-3.00%) provide an exceptional shield against pitting and general corrosion. Meanwhile, the heavy Manganese content (4.0-6.0%) acts alongside Nitrogen to radically increase strength while maintaining phase stability.
| Carbon (C) | Manganese (Mn) | Chromium (Cr) | Nickel (Ni) | Molybdenum (Mo) | Phosphorus (P) | Sulfur (S) | Silicon (Si) |
|---|---|---|---|---|---|---|---|
| 0.06 Max | 4.0 – 6.0 | 20.5 – 23.5 | 11.5 – 13.5 | 1.50 – 3.00 | 0.045 Max | 0.030 Max | 1.00 Max |
*Note: The UNS S20910 specification also inherently includes strict tolerances for Nitrogen, Columbium, and Vanadium to achieve full material stabilization.
To ensure high ductility alongside its inherently high strength, ASTM strictly caps the maximum allowable hardness of Grade 8R nuts.
| Mechanical Property | Grade 8R Limits |
|---|---|
| Hardness Limit (Rockwell) | 25 HRC Maximum |
| Proof Load | Dependent on Size |
| Standard / System | Designation |
|---|---|
| ASME Standard | SA194 Grade 8R |
| Common Trade Name | Nitronic® 50, XM-19 Stainless, UNS S20910 |
| Internal Threads (A194 Gr 8R) | Mating Bolts (A193 Gr B8R) | Mating Studs (A193 Gr B8R) |
|---|---|---|
| Grade 8R Heavy Hex Nuts | ASTM A193 Grade B8R Hex Bolts | A193 Grade B8R Fully Threaded Studs |
Due to the extraordinarily tough, high-alloy austenitic nature of Grade 8R, thread galling is a prominent risk under clamping loads. Premium, heavy-duty anti-seize lubrication (such as nickel or molybdenum-disulfide pastes) is absolutely mandatory for safe installation.
| Nominal Diameter | Threads Per Inch (UNC) | Target Torque — Lubricated (ft-lbs) |
|---|---|---|
| 1/2" | 13 | 35 |
| 5/8" | 11 | 70 |
| 3/4" | 10 | 120 |
| 1" | 8 | 280 |
Grade 8R provides a massive upgrade in both strength and corrosion resistance compared to standard Type 316 (Grade 8M). Because of the added Nitrogen and Manganese, Grade 8R possesses nearly twice the yield strength of Type 316, while its highly elevated Chromium and Molybdenum levels offer vastly superior defense against pitting and crevice corrosion.
No. One of the unique properties of Grade 8R (XM-19) is that its heavily alloyed austenitic structure remains highly stable. While standard 304 or 316 stainless steels can become slightly magnetic after cold-working (such as thread rolling), Grade 8R remains strictly non-magnetic even after severe cold deformation.
Manganese (at a high 4.0% - 6.0%) acts as an austenite stabilizer. Because Nitrogen is heavily added to increase strength, the high Manganese content allows the steel to actually dissolve and absorb that Nitrogen safely into the matrix without causing defects, creating a final product that is incredibly strong yet ductile.
