Learn how to read stainless steel bolt markings (A2-70, A4-80) and choose the right grade (304 vs 316) for your project. Master ISO, ASTM, and DIN standards with our expert guide.
30% of fastener failures stem from misread markings. This guide decodes stainless steel bolt identifiers, helping engineers and DIYers select corrosion-resistant, high-strength fasteners for marine, chemical, and structural applications.
Key Markings & Their Meanings
| Marking |
Material Equivalent |
Tensile Strength |
Best Applications |
| A2-70 |
304 Stainless |
700 MPa |
Indoor machinery, dry environments |
| A4-80 |
316 Stainless |
800 MPa |
Marine, chemical plants, coastal |
| B8M |
316 Stainless |
515 MPa |
High-temperature pipelines |
| ASTM F593 |
304/316 |
Varies by class |
General construction |
Pro Tip: A4-80 bolts withstand 5,000h salt spray vs A2-70’s 1,500h (ISO 9227).
Decoding International Standards
1. ISO 3506
- Class 70: 700 MPa tensile strength (e.g., A2-70)
- Class 80: 800 MPa tensile strength (e.g., A4-80)
- Certification: Requires full material traceability
2. ASTM Standards
- ASTM F593: Covers 304/316 stainless bolts
- ASTM A193 B8M: For high-temperature 316 applications
3. DIN/EN Standards
- DIN 912: Hex socket head bolts
- EN ISO 3506: Corrosion resistance certification
Material Comparison: 304 vs 316 Stainless
| Property |
304 (A2) |
316 (A4) |
| Corrosion Resistance |
Good (1,500h salt spray) |
Excellent (5,000h salt spray) |
| Molybdenum Content |
0% |
2–3% |
| Chloride Resistance |
Fails at >200ppm |
Handles 1,000ppm |
| Cost Premium |
Base Price |
+35–50% |
Case Study: Offshore oil rigs use A4-80 bolts to withstand 5% NaCl environments.
5-Step Bolt Selection Guide
-
Identify Environment
- Marine? Choose A4-80/316
- Chemical plant? Opt for Hastelloy-lined
-
Check Load Requirements
- General use: Class 70 (700 MPa)
- Structural: Class 80 (800 MPa)
-
Verify Markings
- Look for ISO 3506 or ASTM F593 compliance
-
Confirm Dimensions
- Metric (M6–M24) vs Imperial (1/4"–1")
-
Inspect Manufacturer Marks
- Reputable brands: FastenTech®, BUMAX®, UNBRAKO®
FAQs: Solving Common Confusions
Q: Can A2-70 bolts be used outdoors?
A: Only in mild climates – for coastal areas, upgrade to A4-80
Q: What does "B8M" mean on a bolt head?
A: ASTM A193 designation for 316 stainless with 515 MPa strength
Q: How to spot counterfeit bolts?
A: Check for complete markings + demand mill certificates
Why Proper Marking Matters
- Safety: A4-80 bolts have 15% higher shear strength than A2-70
- Compliance: ASME B31.3 requires full material traceability
- Cost Savings: Correct grading prevents premature replacements
Global Standards Cross-Reference
| Region |
Common Standard |
Equivalent Marking |
| USA |
ASTM F593 |
304 (CF8M)/316 (CF8M) |
| Europe |
ISO 3506 |
A2-70/A4-80 |
| Japan |
JIS B1057 |
SUS304/SUS316 |
Free Resource: Download Our Bolt Marking Cheat Sheet
Your message must be between 20-3,000 characters!