A2-70 vs A4-80: Stainless Bolt Grades and Markings

A2-70 vs A4-80 is not one step up a strength ladder. It is two independent facts printed as one code: the first block names the steel, the second block states a tensile strength. Change A2 to A4 and you change corrosion behaviour. Change 70 to 80 and you change the mechanical minimums. Read the pair as a single "quality level" and sooner or later the wrong bolt ships, and the argument that follows is always about what the head marking says. Here is what ISO 3506-1:2020 actually requires, table by table, plus the marking prefixes and suffixes that almost never reach an export spec sheet.

A2-70 vs A4-80: the full ISO 3506-1 property class table

In ISO 3506-1:2020, a stainless fastener designation is two blocks separated by a hyphen. The first block is the stainless steel grade — a letter for the group (A austenitic, C martensitic, F ferritic, D duplex) plus a digit for the composition range. The second block is the property class, a number equal to one tenth of the minimum tensile strength in megapascals. A2-70 therefore means an austenitic, work-hardened fastener with a minimum tensile strength of 700 MPa.

These are the mechanical minimums for bolts, screws and studs in the austenitic and duplex groups.

Stainless steel grade	Property class	Min tensile strength Rmf	Min stress at 0.2 % non-proportional elongation Rpf	Min elongation after fracture A
A1, A2, A3	50	500 MPa	210 MPa	0.6d
A1, A2, A3	70	700 MPa	450 MPa	0.4d
A1, A2, A3	80	800 MPa	600 MPa	0.3d
A4, A5	50	500 MPa	210 MPa	0.6d
A4, A5	70	700 MPa	450 MPa	0.4d
A4, A5	80	800 MPa	600 MPa	0.3d
A4, A5	100	1 000 MPa	800 MPa	0.2d
A8	70 / 80 / 100	700 / 800 / 1 000 MPa	450 / 600 / 800 MPa	0.4d / 0.3d / 0.2d
D2, D4, D6, D8 (duplex)	70 / 80 / 100	700 / 800 / 1 000 MPa	450 / 600 / 800 MPa	0.4d / 0.3d / 0.2d

Two things fall out of that table immediately. Property class 50 exists only for the austenitic grades A1 to A5. And elongation is specified in millimetres as a multiple of d, the nominal thread diameter, not as a percentage — that matters later.

The reading that saves the most argument: A2-70 and A4-70 are mechanically identical. Every number that differs between A2 and A4 is a chemistry number, not a strength number. The mechanical difference in A2-70 vs A4-80 lives entirely in the second block; the corrosion difference lives entirely in the first.

The first block is a composition range, not an alloy name

The shorthand "A2 is 304, A4 is 316" is a trade habit, not what the standard says. ISO 3506-1:2020 defines each grade as a window of cast analysis limits, and the windows are wider than the alloys people substitute into them.

Grade	C max	Cr	Ni	Mo	Usually supplied as
A1	0.12 %	16.0 – 19.0 %	5.0 – 10.0 %	0.70 % max	free-machining, high-sulfur type
A2	0.10 %	15.0 – 20.0 %	8.0 – 19.0 %	at the manufacturer's discretion	304-type
A3	0.08 %	17.0 – 19.0 %	9.0 – 12.0 %	at the manufacturer's discretion	stabilised with Ti or Nb
A4	0.08 %	16.0 – 18.5 %	10.0 – 15.0 %	2.00 – 3.00 %	316-type
A5	0.08 %	16.0 – 18.5 %	10.5 – 14.0 %	2.00 – 3.00 %	stabilised, Ti or Nb added
A8	0.030 %	19.0 – 22.0 %	17.5 – 26.0 %	6.0 – 7.0 %	super-austenitic

Three details in that table are worth reading twice.

A2 may legally contain molybdenum. The standard states that molybdenum may be present at the discretion of the manufacturer, and that if limiting it matters for your application, the purchaser has to say so at the time of ordering. A buyer who assumes A2 means "no Mo" and never writes it down has no claim.

A4 is a 5-point nickel window. Nickel from 10.0 % to 15.0 % and molybdenum from 2.00 % to 3.00 % covers 316 and 316L and several things that are neither. If the application needs a specific alloy, name the alloy in the purchase order; the grade letter alone will not get you there.

Low carbon gets its own letter. For austenitic steels with carbon not exceeding 0.030 %, the fastener may carry the letter L after the grade and before the hyphen — A4L-80. That is an addition to the designation, not a different property class.

Property class 80 is not grade 8.8

This is the substitution that shows up in real quotations, because 80 and 8.8 both trace back to 800 MPa. They are not equivalent on the number that actually governs a preloaded joint.

	A4-80 (ISO 3506-1:2020)	Steel property class 8.8 (ISO 898-1)
Minimum tensile strength	800 MPa	800 MPa up to M16, 830 MPa above M16
Minimum 0.2 % proof stress / yield	600 MPa	640 MPa up to M16, 660 MPa above M16
Minimum elongation	0.3d, in millimetres	12 %
Material condition	cold-worked austenitic stainless	quenched and tempered carbon or alloy steel

A4-80 sits about 6 % below class 8.8 on proof stress at M16 and below, and the gap widens above M16. The elongation requirements are not even expressed in the same unit, so they are not interchangeable acceptance criteria. If a drawing calls for 8.8 and someone offers A4-80 because "80 is the stainless version of 8.8", that is a specification change and it needs the designer's signature, not a sales decision.

Where A2-70 vs A4-80 actually differ in service

The mechanical tables say nothing about environment. That is deliberate: ISO 3506-1 specifies what a fastener must survive in a tensile test, while material selection for a given environment is handled in ISO 3506-6, a separate part of the same series.

Molybdenum is the whole story. A4 carries 2.00 – 3.00 % Mo, which is what buys pitting and crevice resistance in chloride environments — coastal installations, de-icing salt, pool halls, wash-down areas. A2 may contain none.
Mate like with like. For corrosion purposes the standard says to pair bolts, screws and studs with nuts and washers of the same grade. Other combinations are allowed, but the component with the lowest corrosion resistance governs the assembly.
Watch the galvanized interface. Where stainless fasteners meet non-stainless parts such as galvanized steel, the standard advises considering isolation components to avoid galvanic corrosion. An A4 bolt through a galvanized bracket is a bimetallic couple, not an upgrade.
Galling is a real failure mode. Lubrication is recommended to avoid galling during tightening, and the risk rises with thread damage, high preload and high tightening speed. Suppliers who ship dry stainless and let the customer discover this on site pay for it in replacements.

If you also list these SKUs on a marketplace rather than shipping them only against purchase orders, price the failure before it happens — a wrong-grade delivery costs the replacement plus freight both ways, and a return cost calculator turns that into a number you can put in front of a sales manager.

The prefixes and suffixes most export spec sheets never write down

Five marking elements carry real information, and four of them go missing on the average quotation.

Element	Where it sits	What it means
L, as in A4L-80	after the grade, before the hyphen	austenitic steel with carbon not exceeding 0.030 %
Lu, as in A4-80Lu	after the property class, in the designation and labelling	supplied with a lubricant or coating that gives a controlled torque-to-clamp-force relationship
P	after the property class symbol, on the package label	passivated in accordance with ISO 16048
Leading zero, as in 070 or 080	replaces the plain class symbol on the fastener	reduced loadability — head or shank geometry limits it, but it can still be tensile tested in the threaded shank
No property class at all	only the grade is marked	reduced loadability where thread length b is under 3d, so the fastener cannot be tensile tested and the class must not be referenced

That fourth row causes more rejected shipments than any chemistry dispute. A countersunk screw stamped A2 and 070 is not a mis-stamped A2-70. The zero is the standard telling you the head geometry, not the steel, limits what the fastener can carry — and once a product standard applies reduced loadability, the zero-prefixed symbol applies to every size in that standard, even sizes that would individually pass as fully loadable.

What has to be on the head, and who is allowed to put it there

Marking under ISO 3506-1:2020 consists of the stainless steel grade, the optional letter L, the hyphen, the property class symbol, and the manufacturer's identification mark. It has to be applied during manufacture, indented or embossed, and the height of embossed marking on the top of the head is not counted in the head height dimension.

Which fasteners must be marked: hexagon head bolts and screws, and hexagon socket or hexalobular socket bolts and screws, of nominal thread diameter 5 mm and above — for all grades and all property classes.
Where: preferably indented or embossed on the top of the head, or indented on the side. On flanged bolts and screws, on the flange when the process does not allow the top of the head.
Who counts as the manufacturer: a distributor who marks fasteners with its own identification mark is treated as the manufacturer under this document. That is a liability transfer, not a branding exercise.
When marking is permitted at all: only when both the chemical composition limits and the mechanical property requirements are met. Marking a bolt A4-80 on the strength of a mill certificate for the wire, without the finished-fastener testing, is outside the standard.

The same discipline applies to the paperwork around the head marking. Thread callouts get mixed up just as often as grades do, which is why a spec sheet that states the property class but leaves the thread ambiguous still generates questions — the split between metric vs imperial fastener threads is the other half of the same conversation, and hardness figures quoted alongside a property class need the scale named, since Rockwell, Brinell and Vickers hardness are not convertible without a conversion table and an assumption.

Torque is not part of the property class

ISO 3506-1:2020 does publish minimum breaking torque values, but only for austenitic grades, only for property classes 50, 70 and 80, and only for coarse pitch thread.

Thread	Class 50	Class 70	Class 80
M6	9.3 N·m	13 N·m	15 N·m
M8	23 N·m	32 N·m	37 N·m
M10	46 N·m	65 N·m	74 N·m
M12	80 N·m	110 N·m	130 N·m
M16	210 N·m	290 N·m	330 N·m

There are no published values for austenitic class 100, none for fine pitch thread, and none for martensitic, ferritic or duplex grades — in those cases minimum breaking torque has to be agreed between manufacturer and purchaser at the time of the order.

Two warnings sit on top of that table. Breaking torque is the torque at which the fastener fails, not a tightening torque. And the standard notes plainly that requirements for surface discontinuities and for torque-to-clamp-force properties are not specified in International Standards for stainless steel fasteners. That gap is exactly why the Lu suffix exists: if a customer needs a predictable relationship between the torque wrench and the clamp load, it has to be engineered into the finish and agreed at order time.

The designation line that ends the argument

Everything above collapses into one orderable line. Fill each field and the ambiguity has nowhere to hide.

Field	Example entry	Why it belongs
Product standard	Hexagon head bolt ISO 4014	fixes the geometry, and therefore the loadability question
Thread and length	M10 × 60	states the pitch series you mean
Grade and property class	A4-80	the two facts the head marking will carry
Low-carbon requirement	A4L-80 where carbon must not exceed 0.030 %	only meaningful if stated
Restricted elements	"A2, molybdenum not permitted" if that matters	the standard leaves Mo to the manufacturer otherwise
Finish	passivated to ISO 16048, label suffix P	passivated parts are not always bright
Lubrication	Lu, with the torque-to-clamp-force basis stated	the only way to make tightening repeatable
Mating parts	nuts and washers in the same grade	the weakest component governs corrosion
Marking	grade, class symbol and manufacturer's identification mark on the head	your inspection criterion at goods-in

Written out: Hexagon head bolt ISO 4014, M10 × 60, A4-80, passivated to ISO 16048, nuts and washers grade A4, head marked with grade, property class symbol and manufacturer's identification mark.

What a fastener spec image has to carry

Buyers overseas will not open a PDF to settle a question they think a photograph should answer. For fasteners that means the image has to do four jobs at once.

Show	Why the buyer needs it
The head marking, legible and in focus	it is the only field-checkable proof of grade and class
Width across flats and head height	drives the wrench and the counterbore
Thread designation, pitch and nominal length	length is measured differently for countersunk heads
Thread length b against 3d	it decides whether a property class may be marked at all

A drawing does this job for engineers. A photograph with the dimensions locked onto it does the job for everyone else — purchasing, warehouse, the customer's installer. The distinction that matters when you build those images is whether the numbers are measured or generated: software that snaps a dimension label to the real edge of the part and exports at the size a marketplace or catalogue expects gives you a number you can defend, while an image generator will happily letter "M10" onto a photograph of an M12 bolt because it is composing a caption rather than measuring an object. For a full product range the same set of callouts is what turns a folder of photos into an industrial spec diagram buyers can quote from.

Pre-shipment marking check

Head marking present on every hexagon head and socket head fastener of 5 mm and above
Marking reads grade, then property class symbol, plus the manufacturer's identification mark
Reduced-loadability parts carry the zero-prefixed symbol, or no class at all where thread length is under 3d
The L in A4L-80 is only used where carbon is confirmed at or below 0.030 %
Lu and P appear on the label whenever lubrication or passivation was ordered
Nuts and washers in the packing list match the bolt grade
Test certificate covers the finished fastener, not only the wire rod
Photographs of the head marking are on file for the shipment, keyed to the lot number

FAQ

Is A4-80 stronger than A2-70?

Yes, but only on the mechanical numbers, and the gap is smaller than the codes suggest. A4-80 has a minimum tensile strength of 800 MPa against 700 MPa for A2-70, and a minimum 0.2 % proof stress of 600 MPa against 450 MPa. The A4 part of the code says nothing about strength at all — A2-70 and A4-70 have identical mechanical minimums.

What does the 0 mean on a stainless bolt head marked A2 070?

It marks reduced loadability. Under ISO 3506-1:2020 a fastener whose head or shank geometry gives it less capacity than its thread — countersunk, raised countersunk and low-head screws are the usual cases — is marked with the property class symbol preceded by a zero. It is a correct marking, not a defect, and where the thread length is under three times the nominal diameter the fastener cannot be tensile tested at all, so the property class must not be referenced.

Is A2 the same as 304 and A4 the same as 316?

Not exactly. ISO 3506-1:2020 defines A2 and A4 as ranges of chemical composition, and both ranges are wider than the alloy people name. A2 allows chromium from 15.0 % to 20.0 % and nickel from 8.0 % to 19.0 %, with molybdenum left to the manufacturer's discretion. A4 allows nickel from 10.0 % to 15.0 % and molybdenum from 2.00 % to 3.00 %. If a specific alloy is required, name it in the purchase order.

Can I substitute A4-80 where the drawing calls for grade 8.8?

Not without approval. Both carry a minimum tensile strength of 800 MPa, but ISO 898-1 class 8.8 requires a minimum yield of 640 MPa up to M16 while A4-80 requires 600 MPa, and above M16 class 8.8 rises to 830 MPa tensile and 660 MPa yield while A4-80 does not move. The two also behave differently under torque, since stainless has no standardised torque-to-clamp-force requirement.

How do I show the bolt grade so an overseas buyer can verify it?

Photograph the head marking large enough to read the grade, the class symbol and the manufacturer's mark, then put the dimensional callouts on the same image rather than in a separate attachment — width across flats, thread and pitch, nominal length, thread length. Tools that measure against the actual object and lock the label to the real edge keep those numbers defensible when a customer compares your image against the part in their hand, which is the moment a spec dispute is either settled or started. Keep one marked image per lot number and goods-in disputes usually end in a single email.

A2-70 vs A4-80: What the Bolt Head Marking Really Says