A2 or A4? What the markings on a stainless fastener actually mean

Two of the most common stainless fastener mistakes come from the same misreading: treating the marking on the head as a single grade. It is not. It carries two independent decisions, and a buyer can get one right while getting the other wrong without noticing.

Reading the marking

Under ISO 3506-1, a stainless bolt is designated by a steel grade followed by a property class. Take A4-80:

Part	What it means	What it controls
A4	Austenitic steel group, molybdenum-bearing (the 316 family)	Corrosion resistance
80	Property class — tensile strength level	Mechanical strength

The "A" indicates an austenitic steel. A2 is the non-molybdenum austenitic group, broadly corresponding to 304-type material; A4 is the molybdenum-bearing group, broadly corresponding to 316-type. ISO 3506-1 also covers martensitic, ferritic and duplex grades, which use different letters.

What the molybdenum in A4 buys you

The functional difference is molybdenum — roughly 2–3% in the 316 family, essentially absent in 304. What it buys is resistance to chloride-induced pitting and crevice corrosion.

Pitting is a localised failure. Chloride ions concentrate at a point on the surface, break down the passive oxide layer, and open a small pit that keeps going inward. The fastener can look acceptable from outside while losing cross-section where it matters.

The usual way to compare grades on this axis is the pitting resistance equivalent number, calculated from the composition:

PREN = %Cr + 3.3 × %Mo + 16 × %N

Run that over the composition ranges permitted for each material and 304-type steel lands in the high teens to low twenties, while 316-type sits in the mid-twenties to around thirty. The gap comes almost entirely from the molybdenum term.

Two caveats worth stating, because PREN gets over-used. It is a comparative index, not a service prediction — it does not account for temperature, crevice geometry, surface finish or galvanic effects. And it says nothing about strength.

When A2 is genuinely enough

Upgrading everything to A4 is a common and expensive reflex. The question that actually decides it is simple: are chlorides present?

Environment	Typical choice
Indoor, dry, climate-controlled	A2
General outdoor, inland	A2
Coastal or marine exposure	A4
Swimming pools and adjacent structures	A4
Food processing, frequent chloride washdown	A4
De-icing salt exposure	A4

A useful discipline: decide by exposure, not by how important the assembly feels. A critical joint in a dry indoor cabinet does not need A4. A trivial bracket 200 metres from surf does.

The strength number is a separate decision

This is where the money gets wasted quietly. Moving from A2 to A4 changes corrosion behaviour and leaves strength where it was. A4-70 and A2-70 sit at the same property class. If the joint needed more clamp load, changing the letter did not deliver it.

The reverse error also happens: specifying a higher property class to "be safe" in a corrosive environment. A higher number does not resist chlorides.

So treat them as two questions, asked in this order:

1. What is the exposure? → picks the steel group (A2 / A4 / other)
2. What load does the joint carry? → picks the property class (-70 / -80 / other)

Answer them separately and the marking writes itself. Answer them together and you get the coin-flip results this page exists to prevent.

Why "18-8" is not a specification

"18-8" refers to a nominal composition — around 18% chromium and 8% nickel — and is used loosely for austenitic stainless in the 300 series. It is a description, not a grade, and it does not tell you whether molybdenum is present.

In practice, parts sold as 18-8 are usually 304-family material. If your application turns on chloride resistance, that assumption is the whole risk. Do not accept 18-8 on a document where you meant A4.

What to put on the order

Four lines remove nearly all of the ambiguity above:

1. The full designation, both parts — for example "ISO 3506-1, A4-80"
2. The material grade, if your drawing or approval requires a specific one
3. The certificate type you expect — see how to read a mill test certificate
4. The service environment, in one sentence

The fourth line looks redundant and is not. A supplier who knows the parts are going into a coastal installation can flag a mismatch before production. A supplier who only receives a part number cannot.

Frequently asked questions

Is A4 always better than A2?

It is more resistant to chloride pitting and crevice corrosion, which is not the same as better. In dry indoor service the two will behave the same for the life of the assembly, and A4 costs more. Specifying A4 everywhere is a way of avoiding the exposure question rather than answering it. The exception worth respecting is inventory simplification — some manufacturers standardise on A4 across the board to reduce the number of line items and the risk of the wrong box reaching the line, and that can be a rational trade even when the material is over-specified.

Does A4 mean 316?

Not exactly. A4 is a fastener steel group defined in ISO 3506-1 with its own permitted composition range; 316 is a material designation. They overlap heavily, which is why the two terms get used interchangeably in conversation, but they are defined by different documents. If your approval or drawing requires a specific material grade, name that grade on the order rather than relying on the fastener class to imply it.

Can I mix A2 and A4 in the same joint?

It is generally avoided. Beyond the practical risk of the wrong item being fitted during maintenance, joining dissimilar metals in the presence of an electrolyte introduces galvanic effects, and stainless grades are not immune to that just because both are stainless. Where a joint is exposed to chlorides, keeping the fastener, nut and washer in the same group avoids creating a problem that is difficult to diagnose later.

What temperature range do these fasteners cover?

ISO 3506-1 states that fasteners conforming to the document are used without restriction from −20 °C to +150 °C, and are also used outside that band, down to −196 °C and up to +300 °C. Applications at the extremes need more care than a property class alone conveys, because both strength and corrosion behaviour change with temperature — treat the wider range as "possible with engineering review" rather than as a blanket rating.

References

• ISO 3506-1:2020 - Fasteners — Mechanical properties of corrosion-resistant stainless steel fasteners — Part 1: Bolts, screws and studs with specified grades and property classes — the standard that defines the grades and property classes, and the stated temperature range