A2 vs A4 Stainless Steel Fasteners: Which Grade to Specify

Ask a fastener supplier for "stainless steel bolts" and the first question back will be: A2 or A4? The two designations cover the vast majority of stainless fasteners sold into OEM and construction supply chains, and choosing the wrong one either wastes money or sets the assembly up for premature corrosion failure. This article explains what A2 and A4 actually mean under EN ISO 3506, how their corrosion resistance and mechanical properties differ, and how to decide which grade belongs on your drawing.

What A2 and A4 actually mean

A2 and A4 are material group designations from EN ISO 3506, the standard that governs stainless steel fasteners in the metric system (Parts 1 and 2 cover bolts, screws and studs; Parts 3 and 4 cover nuts). A2 corresponds broadly to the 304 family — around 18% chromium and 8–10% nickel, often called "18/8" stainless. A4 corresponds to the 316 family, which adds typically 2–3% molybdenum to the same base composition.

The designation continues with a property class number: A2-70, A2-80, A4-70 and A4-80 are the common combinations. The number is the minimum tensile strength in MPa divided by ten, so an A2-70 bolt has a minimum tensile strength of 700 MPa. A2-70 and A4-70 are the standard commercial offerings; the -80 classes are produced mainly in larger diameters and shorter lengths where the cold-working response allows it.

In North American inch-series supply, the same materials appear as 304 and 318 or 316 stainless under ASTM F593 and F594, so the underlying decision — with or without molybdenum — is identical regardless of the standard system your project uses.

Corrosion resistance: where the two grades part ways

For general atmospheric exposure — indoor, rural, urban and most light industrial environments — A2 and A4 perform comparably well. Both form a self-passivating chromium oxide layer that handles rain, humidity and ordinary handling without any coating. If your assembly lives indoors or in a benign outdoor climate, A2 is usually sufficient, and paying the A4 premium buys nothing visible.

The difference shows up in chloride-containing environments. Molybdenum significantly raises the pitting resistance of austenitic stainless, which is why A4 is the default specification for marine hardware, coastal construction within roughly a kilometre of the shoreline, swimming pool equipment, chemical processing, food and dairy installations exposed to saline wash-downs, and anywhere de-icing salt is present. In chloride service, A2 can develop pitting and crevice corrosion that penetrates the fastener cross-section even while the surrounding structure looks acceptable.

A practical way engineers compare grades is the pitting resistance equivalent number (PREN). A2 typically sits in the high teens, while A4 lands in the mid-twenties — the exact values depend on the specific heat analysis. The gap is modest on paper but decisive in practice under chlorides.

One related trap deserves mention: pairing stainless fasteners with a different structural metal can create a galvanic couple. Stainless bolts clamping aluminium, for example, will corrode the aluminium in a wet, conductive environment. The grade choice between A2 and A4 does not solve a galvanic problem — isolation, coating, or matching materials does, as covered in our guide to fastener surface coatings and galvanic compatibility.

Strength, temperature and galling behaviour

Both A2 and A4 are austenitic stainless steels, so they are non-magnetic in the annealed condition (a weak magnetic pull after cold forming is normal and not a defect), retain toughness at cryogenic temperatures, and cannot be hardened by heat treatment. Their strength comes entirely from cold working during manufacture.

That has two consequences for designers. First, the highest standard property class is -80, so stainless fasteners sit roughly between grade 5.6 and 8.8 carbon steel bolts in tensile terms — they are not a substitute for 10.9 or 12.9 high-strength bolts. If your joint design is strength-critical, review the grade logic in our bolt strength grade markings guide before defaulting to stainless. Second, elevated temperature warrants care: property classes above -70 are validated only up to ambient temperature ratings, and sustained service above roughly 300 °C can lead to sensitisation and reduced intergranular corrosion resistance, particularly for A2.

Galling is the characteristic failure mode of stainless threads. Austenitic stainless pairs have a tendency to cold-weld to themselves under high contact pressure, seizing the nut on the bolt before full preload is reached. Smaller diameters and fine pitches are the worst cases. Practical countermeasures include specifying a different nut material or a lubricated coating where allowed, keeping assembly speeds low, and avoiding repeated re-tightening. Note the tension between this and the corrosion logic: the coating that prevents galling may undercut the corrosion reason for choosing stainless in the first place, so the combination must be specified deliberately, not assembled by accident.

For fine-pitch threads and the strength implications of pitch selection, see our comparison of metric coarse and fine threads.

Cost, availability and the specification decision

A4 carries a price premium over A2 that varies with the nickel and molybdenum markets but is commonly in the range of 20–50% on the fastener itself. Availability is narrower too: A2-70 is stocked in the full range of standard sizes, while A4 deep inventories concentrate on the sizes used in marine, chemical and infrastructure work. Lead times for non-stocked A4 sizes, and especially for non-standard parts in A4 material, run longer than the A2 equivalent.

The specification logic therefore reduces to a short decision path. Specify A2 when the environment is atmospheric and chloride exposure is low or occasional. Specify A4 when the assembly faces seawater, salt spray, saline wash-down, chlorinated water or chloride-bearing chemicals, or when the maintenance cost of replacing a corroded fastener is high relative to the part cost — offshore-adjacent equipment and facade systems near the coast are typical examples. Escalate beyond stainless entirely when the joint requires 10.9-class strength, sustained high temperature, or when a coated carbon steel bolt delivers equivalent corrosion life at lower cost.

Because both grades fall under the same EN ISO 3506 framework, mixing A2 bolts with A4 nuts (or vice versa) is mechanically acceptable and sometimes used to save cost on the cheaper component — but in chloride service the joint is only as corrosion-resistant as its weakest part, so a mixed pair usually forfeits most of the A4 benefit. Designers specifying non-standard fasteners should state the grade and property class explicitly on the drawing rather than writing "stainless" alone; the additional drawing-level details worth including are the same ones that make any custom fastener RFQ efficient, as outlined in our article on standard versus non-standard fasteners.

Conclusion

A2 and A4 are the same family of austenitic stainless steels separated by one alloying element — molybdenum — and by the chloride environments that element protects against. A2-70 handles the broad middle of general engineering economically; A4-70 earns its premium in marine, coastal and chemical service. Neither grade reaches high-strength bolt territory, both demand attention to galling, and the choice should be written onto the drawing as a full designation, not a bare "stainless". Getting this single specification detail right is one of the cheapest forms of corrosion insurance available to a design team.

NXHD Manufacturing (contact@nxhdmfg.com) is a precision fastener and custom metal parts manufacturer with dual production bases in Handan and Datong, China. We supply standard and non-standard fasteners to OEM customers across Europe, the Americas and Asia.

Key takeaways

• A2 corresponds to the 304 (18/8) stainless family and A4 to the 316 family, which adds roughly 2–3% molybdenum for pitting resistance.
• For general atmospheric service, A2 and A4 corrode at comparable rates; A4 only justifies its premium under chlorides — seawater, salt spray, wash-down and chemical exposure.
• Property classes run to -80 at most, so stainless fasteners bracket roughly grade 5.6–8.8 strength and cannot replace 10.9 or 12.9 bolts in strength-critical joints.
• Austenitic stainless threads are prone to galling; control it through material pairing, lubrication where permitted, and low assembly speed rather than torque alone.
• Specify the full designation (for example A4-80, EN ISO 3506-1) on drawings — "stainless" alone is not a purchasable specification.

Frequently asked questions

What is the difference between A2 and A4 stainless steel fasteners?

A2 is the fastener designation for the 304 (18/8) austenitic stainless family, while A4 is the 316 family that adds typically 2–3% molybdenum. The molybdenum gives A4 substantially better resistance to pitting and crevice corrosion in chloride environments; in ordinary atmospheric service the two grades perform similarly.

Is A4 stainless steel stronger than A2?

No — at the same property class, such as A2-70 versus A4-70, the minimum tensile strength is identical at 700 MPa. Strength differences between individual lots come from cold working, not the grade. Both grades top out around the -80 class, so neither substitutes for grade 10.9 or 12.9 carbon steel bolts.

Can I use A2 stainless fasteners outdoors?

Yes, A2 performs well in general outdoor atmospheric exposure, including urban and rural environments, without any coating. It becomes a poor choice near the coast, on marine hardware, in swimming pool or saline wash-down environments, or wherever de-icing salt or chloride-bearing chemicals contact the fastener — those applications call for A4.

What do the numbers in A2-70 and A4-80 mean?

The number is the minimum tensile strength in MPa divided by ten, as defined in EN ISO 3506. A2-70 and A4-70 fasteners therefore have a minimum tensile strength of 700 MPa, and the -80 class indicates 800 MPa, available mainly in larger diameters. These designations appear on drawings and in the ASTM F593/F594 equivalents for inch-series supply.

Which standards govern A2 and A4 fasteners?

EN ISO 3506 is the governing standard: Parts 1 and 2 cover bolts, screws and studs, and Parts 3 and 4 cover nuts. It defines the A1–A5 material groups, the -50/-70/-80 property classes, and the marking requirements. Inch-series equivalents are covered by ASTM F593 and F594, and the underlying material compositions by EN 10088 and ASTM A240.

Do A4 stainless fasteners cost much more than A2, and does NXHD supply them?

A4 typically carries a premium in the 20–50% range over A2 depending on the nickel and molybdenum markets, with narrower size availability and longer lead times for non-stocked dimensions. NXHD Manufacturing supplies both A2 and A4 standard fasteners and can produce non-standard stainless parts to drawing, including riveting fasteners and special-shaped components, from its Handan and Datong production bases.