Meta Description: Learn how to read stainless steel bolt markings! Understand A2 vs A4 grades, strength codes (70, 80), and ISO/DIN standards for proper fastener selection.
Stainless steel bolts are indispensable in construction, marine, and industrial applications—but their cryptic markings often leave users confused. Misinterpreting these codes can lead to catastrophic failures in corrosive or high-stress environments. This guide breaks down stainless steel bolt markings, empowering you to select the right fastener for your project’s demands.
Why Bolt Markings Matter
Stainless steel bolt markings provide essential data about:
- Corrosion Resistance (e.g., marine vs. indoor use)
- Mechanical Strength (load capacity under tension/shear)
- Compliance with international standards (ISO, DIN, ASTM)
- Traceability to manufacturers for quality assurance
Key Markings Explained
1. Material Grades: A2 vs A4
| Grade |
Equivalent Alloy |
Corrosion Resistance |
Common Applications |
| A2 |
304 stainless |
Good (indoor/atmospheric) |
Construction, machinery, appliances |
| A4 |
316 stainless |
Excellent (saltwater/chemicals) |
Marine, offshore, chemical plants |
Pro Tip: A4 bolts contain 2-3% molybdenum for enhanced pitting resistance.
2. Strength Classes (e.g., A2-70, A4-80)
The number after the hyphen indicates tensile strength in MPa (megapascals):
- A2-70: 700 MPa tensile strength (most common)
- A4-80: 800 MPa tensile strength (high-strength marine)
- A2-50: 500 MPa (low-stress indoor applications)
Critical Note: Higher numbers don’t always mean better—match strength to load requirements.
3. Manufacturer Codes
- Unique logos or initials (e.g., FINEX) identify the producer.
- Trace faulty bolts via manufacturer databases for recalls or warranties.
4. Size & Thread Specifications
- Metric Markings: M6 (6mm diameter), M12x50 (12mm dia × 50mm length)
- Thread Pitch: Fine (e.g., M8×1.0) vs coarse (e.g., M8×1.25) threads.
5. Standard Compliance
| Standard |
Scope |
Example Marking |
| DIN |
German industrial norms |
DIN 933 |
| ISO |
International standards |
ISO 3506 |
| ASTM |
U.S. material specs |
ASTM F593 |
How to Read Bolt Markings: A Step-by-Step Guide
- Identify the Grade: Look for A2/A4 near the head.
- Check Strength Class: Find the hyphenated number (e.g., -70).
- Verify Size: Match markings like M10 to your hole diameter.
- Confirm Standard: Ensure compliance with project specs (ISO/DIN).
- Trace the Maker: Use logos to confirm OEM authenticity.
Case Study: Why Markings Matter
Scenario: A coastal bridge used A2-70 bolts instead of A4-80.
Result: Severe pitting corrosion occurred within 18 months.
Solution: Replaced with A4-80 bolts—no issues after 5+ years.
FINEX Stainless Steel Bolts: Precision Marking Guaranteed
As a leading manufacturer, FINEX laser-etches bolts with:
- Clear Grade/Strength Markings: No guesswork for critical projects.
- ISO 3506 Compliance: Certified for marine (A4) and industrial (A2) use.
- Batch Traceability: QR codes link to material certificates and test reports.
FAQs: Bolt Marking Challenges Solved
Q: Can A2 bolts handle saltwater exposure?
A: No—use A4-80 bolts with 316 stainless for marine environments.
Q: What does “70” mean in A2-70?
A: 700 MPa tensile strength—sufficient for most structural loads.
Q: Are unmarked bolts safe to use?
A: Avoid them! Unmarked bolts may lack corrosion resistance or proper hardening.
SEO-Optimized Industry Solutions
- Marine Engineers: Search “A4-80 marine bolts” for offshore rigs.
- Construction Managers: Use “A2-70 DIN 933 bolts” for compliant builds.
- HVAC Technicians: Find “M8 A2-70 ISO 3506 bolts” for corrosion-resistant ducting.
Conclusion
Decoding stainless steel bolt markings isn’t just technical jargon—it’s a reliability safeguard. Whether you’re building a skyscraper or repairing a boat, matching A2/A4 grades, strength codes, and standards to your environment prevents costly failures. Partner with certified manufacturers like FINEX to ensure markings translate to performance.
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