A2 and A4 stainless fasteners: what EN ISO 3506 actually specifies

A4 is not a stronger A2. The letter and the number after it describe two unrelated things: the steel group and the property class, and both classes are available in both groups. The part of the standard nobody quotes is part 6, which rules A4 out of swimming pools cleaned with chloride and out of many marine environments. Below is what each character in the designation means, where each group stops and what belongs in an enquiry.

Reading the designation: A4-80 in three pieces

A mark like A4-80 carries three separate pieces of information, and mixing them up is the source of most specification errors.

So A4-80 means an austenitic fastener of sub-group 4 with a minimum tensile strength of 800 MPa. A2-80 means an austenitic fastener of sub-group 2 with exactly the same minimum tensile strength. The corrosion behaviour differs; the strength does not.

The standard is seven parts, not one

EN ISO 3506 is a series, and quoting it without a part number leaves the reader guessing which requirements apply. The third edition of part 1 dates from 2020 and replaced the 2009 edition, not the 1997 one that many datasheets still echo.

Parts 3 and 4 are the trap in that list. They do not carry property classes at all, they carry hardness classes written with an H, so 12H means a minimum of 125 HV. A set screw marked A1-12H is not a weak A1-120; the number means something else entirely.

The 2020 revision matters for what it added. The earlier text knew neither the A8 group, nor duplex groups, nor property class 100. What it did already contain, contrary to a common claim, were the martensitic and ferritic groups: those go back to the 1997 edition. A datasheet that lists only A1 to A5 and classes 50, 70 and 80 is working from a document that was superseded.

Property classes: the same ladder for both groups

This is the point at which most comparisons go wrong. The property class is a separate axis from the steel group, and the standard sets out which classes each group can carry.

The numbers behind those classes are the same for A2 and A4:

Strength in austenitic stainless comes from cold work, not from heat treatment, which is why the numbers behave differently from carbon steel property classes such as 8.8. There is one asymmetry worth knowing: class 50 exists only for the austenitic groups A1 to A5, so A8 and the duplex groups start at 70.

A detail that catches people comparing datasheets: the standard states minimum elongation not as a percentage of a machined test piece but in millimetres, as a fraction of the thread diameter, measured on the actual screw. For class 70 that is 0,4d, so an M10 bolt has to stretch 4 mm before it parts.

The practical consequence: if a drawing calls for A4 because the designer wanted more strength, the drawing is asking for the wrong thing. A2-80 and A4-80 are equal on that axis. The only grain of truth in the belief is that class 100 is open to A4 and closed to A2, so the strongest standard A4 does beat the strongest standard A2. Grade against grade at equal class, it is a draw.

The diameter limit that is not in this standard

Distributor tables routinely print a cap on classes 70 and 80, usually at M20 or M24, and attribute it to ISO 3506. Read the standard and it is not there. Part 1 covers metric threads from M1,6 to M39, its load tables run classes 70, 80 and 100 all the way to M39, and the only diameter footnote in the mechanical tables applies to the ferritic group. The 2009 edition behaves the same way, so nothing changed in 2020 on this point.

The cap is real, but it comes from somewhere else. The Eurocode for stainless structures limits classes 70 and 80 to M24 in its own table of bolt strengths, with class 50 running to M39. That is a design rule for structural connections, not a manufacturing limit, and the difference matters: above M24 you can still buy a compliant A4-80 bolt, you simply cannot take its strength into a structural calculation under that Eurocode without agreement.

The M20 figure circulating in some widely copied tables has no basis we could find in either edition of the standard. If a supplier quotes a diameter cap, ask which document it comes from.

One more rule from the same Eurocode is worth carrying into design: high strength stainless bolts should not be used as preloaded bolts designed for a specific slip resistance. Stainless is not a drop-in substitute in a friction-grip connection.

What sits behind the letters

The groups are defined by composition ranges, not by a single grade. That is the reason the standard names groups rather than material numbers: several steels can satisfy an A2 or an A4 composition window, and the supplier chooses within it.

Look at the width of those windows. A2 is not a grade, it is a composition range, and chromium anywhere between 15,0 and 20,0 % satisfies it. Two bolts both correctly marked A2-70, from two different mills, can sit at opposite ends of that window and both be compliant. Ordering A4 does not pin the material number either; it pins the window.

Three details that datasheets get wrong. First, molybdenum in A2 is not prohibited, it is simply not required: the standard leaves it to the manufacturer, and adds that a purchaser who needs the molybdenum content limited has to say so at the time of ordering. Second, the allowance to raise carbon at larger diameters applies to A4, A5 and the martensitic groups, not to A2. Third, the stabilised grade often sold as a 316 variant is A5, not A4, so specifying A4 and expecting a titanium-stabilised steel will not get you one.

Why molybdenum changes the behaviour at all is the same mechanism that separates the sheet and bar grades, and it is set out in 304 and 316 stainless steel: grades, properties and how to choose.

Where A4 stops, and this is the part that is usually missing

Part 6 of the standard, the one on selection, is explicit about the limits of both groups. On A2 it is blunt: the grade is not suitable for non-oxidizing acids and agents, or for an environment with chloride, and it names swimming pools and marine environments as the examples. On A4 it softens but does not clear it: the grade may be used in some environments where chloride is present, but it is still not suitable in swimming pools using chloride as a cleaning agent, or many marine environments. For chlorinated pool environments it points instead to the A8 group, the one with around 6 % molybdenum.

There is a related instruction inside part 1 that deserves a wider audience. The table that classifies duplex grades carries a formula resembling a pitting index, and a footnote states that it is there solely to classify the steels and is not intended to be used as a selection criterion for corrosion resistance. The standard is telling you not to use its own number to pick a material.

That qualification is worth taking seriously rather than treating as small print. Chloride stress corrosion cracking in pool halls has a history of structural failures, and it attacks precisely the load-bearing fastener that looks perfect from the outside. A visual check does not find it.

So the widely repeated shorthand, A2 inland and A4 for coastal and marine, is a simplification that the standard itself qualifies. A4 is a step up in chloride resistance; it is not a marine-grade guarantee, and for a chlorinated pool hall it is the wrong answer. The general mechanism, and why a stainless fastener can still corrode, is covered in Why stainless steel rusts, and how to prevent it.

Choosing by environment, and the tool that does not fit

A common shortcut is to read the corrosivity category of the site and jump to A4 from C4 upwards. That rule is distributor practice, not a line in EN ISO 3506, and it rests on a classification built for other metals: the corrosivity categories are calibrated on carbon steel, zinc, copper and aluminium, not on stainless steel. Categories remain useful for framing the exposure, which is what Corrosivity categories C1 to CX: classifying the environment before choosing the material sets out, but they are not a stainless selection table.

The methodically correct route for structural stainless is the selection guidance in the Eurocode for stainless structures, which works from exposure and from the steel itself rather than from a category built for zinc. Where the decision carries consequence, the standard has a third rule that is easy to miss: consult an experienced metallurgist. That sentence exists because composition windows do not answer every site question.

One number we deliberately do not quote is a distance from the sea in kilometres. It circulates widely, the figures contradict each other, and we have not been able to ground any of them in a source worth citing.

Marking: what has to be on the head

Marking is mandatory on hexagon head bolts and on socket head and hexalobular drive screws from 5 mm nominal thread diameter upwards, for every grade and every class. The mark carries the grade, the property class and a manufacturer identification, applied during production either indented or embossed. Studs of the same size are a recommendation rather than a requirement, which is a change from the previous edition where they were mandatory.

The package label, by contrast, is required for every type and every size, and it carries more than the head does: identification of the manufacturer or distributor, the grade, the class, any passivation or lubrication suffix, and the production batch number. For a goods-in check that makes the label, not the bolt head, the document of record.

One clause has consequences for anyone in the supply chain: a distributor who puts their own identification mark on fasteners is treated by the standard as the manufacturer. Stamping a house mark is not branding, it is an assumption of responsibility.

Practical use: an unmarked bolt above the threshold is not evidence of a counterfeit, but it is a reason to ask for the inspection document rather than assume the class. Which document proves what is set out in EN 10204 material certificates: 2.1, 2.2, 3.1 and 3.2 explained.

Galling: the failure that no property class predicts

Austenitic stainless fasteners seize during tightening far more readily than carbon steel ones. The thread surfaces cold weld under pressure, the nut locks part way down and the joint has to be cut off. Nothing in the property class warns of this, because it is not a strength property.

The standard does address it, just not where people look. Part 1 recommends lubrication of stainless fasteners to avoid galling during tightening, and a note names three things that raise the risk: thread damage, high preload and high tightening speed. It also introduces a suffix, Lu, for fasteners supplied with a finish or lubricant that gives a controlled relationship between torque and clamp force. A4-80Lu is a legitimate designation, not a supplier invention.

Beyond those three factors the remedies are practice rather than requirement: clean threads free of swarf, and pairing components of dissimilar hardness rather than identical ones. It is worth planning for on site, because it turns a five-minute assembly into an angle grinder job.

A related trap sits in the same part of the standard. It contains a table of torque figures, and those are breaking torques used in a torsional test that replaces the tensile test on short fasteners; they are not tightening torques. The standard says outright that torque and clamp force properties are not among the requirements it specifies, and that such requirements are not set in international standards for stainless fasteners at all. Tightening torque is calculated from the target preload and the actual friction coefficients, which is why the same bolt carries different figures in different catalogues.

Stainless fasteners against aluminium and galvanised steel

Two contact pairings come up constantly in mounting work, and they behave differently.

The standard treats electrical isolation of dissimilar metals as its own consideration rather than as a step in the selection sequence, so an isolating washer is a design decision, not a substitute for choosing the right group. The wider comparison of the two protection philosophies is in Stainless steel or galvanised steel: which to specify, and why, and the fixing side of aluminium work in Joining and straightening aluminium profiles: methods that work.

What ULAMEX supplies

Fasteners and small mounting components sit in the PV Fasteners and Others category: hexagon and socket head bolts, hammer bolts for profile channels, nuts, sealing washers, threaded and mounting plates, earthing components and cable management. The standard material is A2 stainless, with A4 available on request, alongside anodised aluminium and hot-dip galvanised items where the application calls for them.

Where these parts sit in a complete array is covered in Solar PV mounting systems: components, materials and roof types.

ULAMEX is a metals wholesaler, in the business since 1988, that is 38 years. Dispatch across continental Europe typically takes 2-7 working days. A 3.1 inspection certificate is issued by the manufacturer of the material and we pass it on when you ask for it; our own structural fabrication is certified to EN 1090-1 EXC2 and EN ISO 3834-2, which is documented on Certificates.

What to put in the enquiry

Five lines prevent most of the back and forth:

• Group and property class together, written as A2-70 or A4-80, not simply as stainless.
• The fastener standard and the size, for example a hexagon head bolt to DIN 933 in M10 by 30 mm.
• The exposure, and in particular whether chlorides are present and whether the joint is in a pool hall, because that moves the answer past A4.
• The mating material, that is aluminium, galvanised steel or stainless, so the contact pairing can be checked.
• Whether you need a 3.1 inspection certificate, because that is an order requirement rather than something added afterwards.

If you are unsure whether a site justifies moving beyond A2, send the exposure and the joint detail through the contact form and we will come back with a recommendation and a non-binding quotation.

Frequently asked questions