A2-70 vs A4-80 Stainless Fasteners
A grade code that mixes chemistry and strength — and how to read it before it causes a corrosion failure on site.
A2-70 and A4-80 are the two most commonly specified stainless fastener grades in export projects, and they are regularly treated as if A4-80 were simply 'a stronger A2-70'. It is not. The letter group describes chemistry and the number describes mechanical property class. Choosing on strength alone is how a perfectly strong bolt fails from pitting corrosion eighteen months into service.
Our stainless fastener range covers both grades across DIN, ISO, ANSI and JIS standards.
What the Codes Mean
| Code | Steel | Key alloying | Tensile class | Typical use |
|---|---|---|---|---|
| A2-70 | AISI 304 / 1.4301 | ~18% Cr, 8% Ni | 700 MPa | General indoor and outdoor urban |
| A4-80 | AISI 316 / 1.4401 | ~17% Cr, 11% Ni, 2% Mo | 800 MPa | Marine, coastal, chemical, de-icing salts |
| A4-70 | AISI 316 / 1.4401 | Same as A4 | 700 MPa | Where corrosion resistance matters but 800 MPa is not needed |
The molybdenum in A4 is what raises resistance to chloride-induced pitting and crevice corrosion. It is also why A4 costs more. The 70/80 suffix refers to the property class under ISO 3506: 70 means a minimum tensile strength of 700 MPa, 80 means 800 MPa, achieved through cold working.
Choose by Environment, Not by Price
| Environment | Recommended | Why |
|---|---|---|
| Indoor, dry, non-critical | A2-70 | Lowest cost, adequate corrosion resistance |
| Outdoor urban, rain exposure | A2-70 or A4-70 | A2 is generally sufficient without chlorides |
| Coastal / marine splash zone | A4-80 | Chlorides cause pitting in 304 |
| Bridges, roads with de-icing salt | A4-80 | Salt exposure is aggressive and cyclic |
| Chemical processing, washdown | A4-80 or A4-70 | Consult on the specific medium and temperature |
| High load + chloride exposure | A4-80 | Combines higher strength with corrosion resistance |
For construction and infrastructure projects, the specification is usually set by the engineer, not the buyer. If the drawing says A4-80, substituting A2-70 to save cost is not an acceptable value-engineering proposal.
Standards, Marking and Interchangeability
Stainless fastener heads are normally marked with the steel group and property class, for example A4-80, together with the manufacturer identification. Under ISO 3506 the marking is part of traceability, so a batch without visible marking may be rejected at site inspection.
Also check the dimensional standard: DIN 933, ISO 4017 and ANSI hex bolts can share a nominal thread but differ in across-flats dimension, head height and thread length. State the standard on the purchase order, not just M10 × 40.
Galvanic Corrosion and Mating Materials
Stainless steel in contact with aluminum or carbon steel in a wet environment forms a galvanic couple. The less noble metal corrodes faster. Where fasteners join dissimilar metals, isolating washers, coatings or a change of fastener material may be needed. This is worth raising at the RFQ stage: tell us what the fastener mates with and we will flag the risk.
Documentation to Request
- EN 10204 3.1 material certificate with heat number
- Proof-load and tensile test results for the property class
- Salt-spray test report where the project specification requires it
- Certificate of conformity with standard, grade, size and lot number
- Carton labels matching the packing list, for site QA submission
See our inspection process for how these records are produced per lot.
Related Questions
Can A2-70 replace A4-80?
Not in chloride environments. A2 (304) lacks molybdenum and is prone to pitting near the coast or where de-icing salts are used.
Is A4-80 always stronger than A2-70?
Yes mechanically — 800 MPa vs 700 MPa minimum tensile — but strength is usually the less important difference. Corrosion resistance is the reason to specify A4.
What documents should I request for a construction project?
EN 10204 3.1 material certificate, proof-load results, certificate of conformity and carton labels tied to the lot number.
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