ASTM A193 stud bolts are widely used in high-temperature and high-pressure applications such as flanges, pressure vessels, and piping systems. Among them, B8, B8M, Class 1, and Class 2 are the most common grades—but also the most frequently misunderstood.
This article provides a clear and practical explanation of the differences between ASTM A193 B8 and B8M, as well as Class 1 and Class 2, covering material composition, mechanical properties, and real-world selection guidance.
Overview of ASTM A193
ASTM A193 is a standard specification for alloy steel and stainless steel bolting materials intended for high-temperature or high-pressure service. These stud bolts are typically supplied with ASTM A194 nuts and are widely used in:
- Oil and gas
- Petrochemical plants
- Power generation
- Pressure vessels and piping systems
Difference Between A193 B8 and B8M (Material Comparison)
Material Equivalents
| Grade | Stainless Steel Type | Main Chemical Elements | Characteristics |
| B8 | AISI 304 / 304L | 18Cr–8Ni | General-purpose stainless steel |
| B8M | AISI 316 / 316L | 16Cr–10Ni–2Mo | Molybdenum-added for superior corrosion resistance |
Key Material Differences
- B8M contains molybdenum (Mo), which significantly improves resistance to pitting and chloride corrosion
- B8M offers better performance in marine, chemical, and aggressive environments
Material Selection Tip
- Normal industrial environments → ASTM A193 B8
- Marine, chemical, or chloride-containing environments → ASTM A193 B8M
Difference Between Class 1 and Class 2 (Strength Levels)
Processing Condition
| Class | Processing Method | Strength Level | Description |
| Class 1 | Solution annealed | Medium strength | Standard condition |
| Class 2 | Cold worked (strain hardened) | Higher strength | Enhanced mechanical properties |
Important Note: The higher strength of Class 2 is achieved through cold working, not heat treatment.
Mechanical Properties (Minimum Tensile Strength)
| Class | Minimum Tensile Strength |
| Class 1 | ≥ 515 MPa |
| Class 2 | ≥ 690 MPa |
Class 2 provides higher strength, though with slightly reduced ductility compared to Class 1.
Common ASTM A193 B8 / B8M Combinations
| Standard Designation | Meaning |
| A193 B8 Class 1 | 304 stainless steel, standard strength |
| A193 B8 Class 2 | 304 stainless steel, high strength (cold worked) |
| A193 B8M Class 1 | 316 stainless steel, standard strength |
| A193 B8M Class 2 | 316 stainless steel, high strength and superior corrosion resistance |
Practical Selection Guide
Select material first, then strength class
- Corrosion resistance requirement → B8 or B8M
- Mechanical strength requirement → Class 1 or Class 2
Typical Application Recommendations
- High-temperature and high-pressure flange bolting → ASTM A193 B8M Class 2
- General stainless steel fastening → ASTM A193 B8 Class 1
- Chemical plants or chloride environments → ASTM A193 B8M (preferably Class 2)
Conclusion
- B8 vs B8M determines corrosion resistance
- Class 1 vs Class 2 determines strength level
- B8M Class 2 is one of the most commonly specified combinations due to its balanced strength and corrosion resistance
Choosing the correct ASTM A193 bolting grade ensures system safety, reliability, and extended service life.
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