A4-70 vs A4-80: Strength, Cold Working and Marine Service Limits

For marine stainless steel fasteners, the property classes A4-70 and A4-80 are defined in ISO 3506-1, covering corrosion-resistant stainless steels. Choosing between them is not simply "80 is stronger is better" — it depends on cold working, chloride exposure, and how the fastener is manufactured. This article explains what each class actually guarantees and when one is preferred for marine service.

What ISO 3506-1 Says

Per ISO 3506-1:2020, the materials group A4 designates austenitic stainless steel (typically 316-type) intended for corrosion-resistant service. The number after A4 is the property class, which states minimum mechanical properties: A4-70 has a minimum tensile strength of 700 MPa, while A4-80 has a minimum tensile strength of 800 MPa. The higher class is usually achieved through cold working (strain hardening) of the wire before thread rolling, rather than by a different alloy chemistry.

The Trade-Off: Strength vs Corrosion Resistance

Because A4-80 relies on cold working for its higher strength, it does not automatically offer better corrosion resistance than A4-70. In chloride-rich marine environments, higher strength levels can be more sensitive to stress-corrosion cracking (SCC) and crevice corrosion in some conditions. For ordinary seawater bolted joints where full A4-80 strength is not required by the design, A4-70 is commonly selected as a balanced, widely available choice for marine fasteners.

How to Specify

Always state the property class on the MTC and order: for example "A4-70 per ISO 3506-1" or "A4-80 per ISO 3506-1". Do not write only "316 bolt", because A4-70 and A4-80 both fall under the A4 group and the mechanical property class must be confirmed to match the design loads.

FAQ

Q1. Is A4-80 always better than A4-70 for seawater?
Not automatically. Higher cold-worked strength does not provide better corrosion resistance and can increase SCC sensitivity in some chloride conditions. Select by the required design loads and service environment, not by the 80 label alone.

Q2. Can I specify both A4-70 nut and A4-80 bolt?
Yes. Nuts and bolts are specified separately per ISO 3506-2 and ISO 3506-1; the assembly should be designed so the nut strength matches the bolt class and thread engagement.

Q3. What should appear on the MTC?
Material group (A4), property class (70 or 80), the governing standard ISO 3506-1, heat number and mill certificate reference for traceability.