ASTM A194 Grade 8S specifies premium, high-performance austenitic stainless steel nuts manufactured from UNS S21800, commercially renowned as Nitronic® 60. This highly advanced alloy is meticulously engineered to provide world-class resistance to wear, galling, and fretting. By utilizing a highly specific metallurgical synergy of elevated Silicon (3.5% - 4.5%) and Manganese (7.0% - 9.0%), Grade 8S delivers an exceptionally resilient surface matrix that drastically reduces friction under heavy loads.
In standard 300-series stainless steels, stainless-on-stainless thread contact frequently leads to severe galling and cold-welding. Grade 8S overcomes this inherent flaw. It provides exceptional mechanical yield strength and a general corrosion resistance comparable to Type 304, but its standout feature is the ability to endure repeated assembly and disassembly in high-pressure, unlubricated environments without seizing.
| Material Classification | Galling-Resistant Austenitic Stainless Steel (Nitronic® 60) |
|---|---|
| UNS Designation | S21800 |
| Microstructure | Austenitic (High Silicon/Manganese Matrix) |
| Magnetic State | Non-Magnetic (Remains low permeability even after cold working) |
| Density | ~7.62 g/cm³ (0.275 lb/in³) |
| Size Range | Metric: M6 to M100 | Imperial: 1/4" to 4" |
| Strengthening Method | Solid-Solution Strengthening via Silicon, Manganese, and Nitrogen |
Contains detailed proof load thresholds, dimensional tolerances for ASME B18.2.2 standard and heavy hex profiles, and specialized friction coefficient data.
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The exceptional wear and anti-galling performance of Grade 8S is derived directly from its elevated Silicon (3.5% - 4.5%) and Manganese (7.0% - 9.0%) content. These elements create a highly wear-resistant oxide layer on the thread surface that acts as a solid-state barrier, preventing metal-to-metal adhesion during tightening.
| Carbon (C) | Manganese (Mn) | Silicon (Si) | Chromium (Cr) | Nickel (Ni) | Phosphorus (P) | Sulfur (S) |
|---|---|---|---|---|---|---|
| 0.10 Max | 7.0 – 9.0 | 3.5 – 4.5 | 16.0 – 18.0 | 8.0 – 9.0 | 0.060 Max | 0.030 Max |
*Note: The UNS S21800 specification inherently utilizes controlled Nitrogen additions to enhance yield strength.
| Mechanical Property | Grade 8S Limits |
|---|---|
| Hardness Limit (Rockwell) | 25 HRC Maximum |
| Proof Load | Dependent on Size |
| Standard / System | Designation |
|---|---|
| ASME Standard | SA194 Grade 8S |
| Common Trade Name | Nitronic® 60, Anti-Galling Stainless, UNS S21800 |
For ultimate galling resistance, it is actually standard practice to mate Grade 8S (Nitronic 60) nuts with standard 304/316 (B8/B8M) studs. The differing metallurgical hardness profiles prevent cold-welding.
| Internal Threads (A194 Gr 8S) | Typical Mating Bolts | Typical Mating Studs |
|---|---|---|
| Grade 8S Heavy Hex Nuts | ASTM A193 Grade B8 / B8M Hex Bolts | A193 Grade B8 / B8M Fully Threaded Studs |
While Grade 8S provides exceptional built-in resistance to thread galling, standard bolting practices still apply. When mating Grade 8S with standard austenitic studs (like B8M), smooth and predictable torque-tension relationships are easily achieved.
| Nominal Diameter | Threads Per Inch (UNC) | Target Torque — Standard (ft-lbs) |
|---|---|---|
| 1/2" | 13 | 35 |
| 5/8" | 11 | 70 |
| 3/4" | 10 | 120 |
| 1" | 8 | 280 |
Standard stainless steels have a tendency to microscopically tear and fuse together (cold weld) when sliding against each other under heavy clamping pressure. Grade 8S adds high levels of Silicon and Manganese, creating a tough, tightly adherent surface matrix that acts as a natural buffer, effectively preventing the microscopic tearing that leads to seized threads.
Yes. Grade 8S is often the go-to material for "clean" environments (like food processing, medical devices, or cryogenic chambers) where wet anti-seize lubricants cannot be used. However, if the environment permits lubrication, using it will still extend the overall life of the threads during repeated disassembly.
Grade 8S (Nitronic 60) offers general atmospheric and chemical corrosion resistance that falls squarely between Type 304 and Type 316 stainless steel. Its pitting and crevice corrosion resistance is slightly superior to standard 304 in most environments.
