COMPREHENSIVE TECHNICAL ANALYSIS AND DATASHEET OF ASTM A194 GRADE 8S NUTS
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.
■ Product Overview & Specifications
| 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 |
■ Proprietary Datasheet Download (GATED)
ASTM A194 Grade 8S — Complete Technical Datasheet
Contains detailed proof load thresholds, dimensional tolerances for ASME B18.2.2 standard and heavy hex profiles, and specialized friction coefficient data.
⬇ DOWNLOAD DATASHEET■ MTC Integration Section
Material traceability is the backbone of safety in high-pressure and critical industrial environments. Every lot shipped by Ananka is accompanied by fully authenticated documentation mapping back to raw steel mill certificates.
- EN 10204 Type 3.1: Issued by our independent quality department, providing specific heat-traceable test results (actual chemical analysis verifying high Silicon/Manganese levels, mechanical test results, hardness limits, and proof load tests).
- EN 10204 Type 3.2: For critical commercial, offshore, or high-friction applications, we facilitate third-party witness testing.
■ Chemical Composition
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 Properties
| Mechanical Property | Grade 8S Limits |
|---|---|
| Hardness Limit (Rockwell) | 25 HRC Maximum |
| Proof Load | Dependent on Size |
■ Equivalent Grades & Designations
| Standard / System | Designation |
|---|---|
| ASME Standard | SA194 Grade 8S |
| Common Trade Name | Nitronic® 60, Anti-Galling Stainless, UNS S21800 |
■ Applications & Industries
- Valve and Pump Manufacturing: Valve stems, seating assemblies, and pump wear rings where repeated metal-on-metal sliding occurs without lubrication.
- Marine & Offshore Engineering: Turnbuckles, heavy rigging hardware, and structural fasting where stainless steel must endure heavy cyclic loading without seizing.
- Clean Room & Medical Environments: Specialized machinery requiring repeated disassembly where the use of liquid or paste lubricants (which can contaminate the environment) is strictly forbidden.
■ Compatible Fastener Assemblies
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 |
■ Torque & Installation Guidelines
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 |
■ Frequently Asked Questions
1. What makes Grade 8S (Nitronic 60) so resistant to galling?
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.
2. Can I use Grade 8S without anti-seize lubricant?
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.
3. How does its corrosion resistance compare to Type 304 or 316?
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.
