Guide
A4-70 or A4-80: Which ISO 3506 Property Class Should a Custom Marine Stud or Pin Meet?
A4-70 is a property class from ISO 3506-1 for austenitic stainless bolts, screws and studs: the A4 marks a molybdenum-bearing grade with corrosion resistance similar to 316, and the 70 marks a minimum tensile strength of 700 MPa per ISO 3506-1. Short answer: A4-70 and A4-80 are fastener property classes, not bar grades, so a stud machined from bar can only be called A4-80 if finished parts are tested to that class; for a custom stud or pin, specify the bar condition and a tensile test instead of the letters.
By Saola Manufacturing engineering team · Published · Updated
On this page
What do the A4 and the 70 or 80 actually mean?
The letter A means austenitic, the 4 places the steel in the molybdenum-bearing corrosion group, and the last two digits are the property class, one tenth of the minimum tensile strength in MPa.
The Steel Construction Institute's advisory note explains the code: the letter gives the steel group (austenitic, martensitic or ferritic), the next number from 1 to 5 reflects corrosion resistance, and the final two numbers denote the property class (SCI Advisory Desk AD 342). It says grade A4 steels contain molybdenum and have corrosion resistance equivalent to grade 1.4401, which is 316. The chemistry limits for A4 sit in the standard itself, so A4 is best read as similar to 316 in corrosion behavior, not as an exact synonym for it; check the composition table in your copy if the drawing needs 316 or 316L chemistry.
The standard in force is EN ISO 3506-1:2020, published in 2020 and covering the mechanical and physical properties of bolts, screws and studs tested at 10 to 35 °C (EVS record for EN ISO 3506-1:2020). Its tables are for the finished fastener, which is the point everything else in this guide turns on.
What do A4-50, A4-70 and A4-80 guarantee?
Per ISO 3506-1, A4-70 guarantees 700 MPa minimum tensile and 450 MPa proof stress, and A4-80 guarantees 800 MPa and 600 MPa, for finished bolts, screws and studs in the standard's size range.
The table repeats the tensile and 0.2 percent stress values from the BS EN ISO 3506 table in the SCI note; check them against the 2020 edition of ISO 3506-1 before they go on a drawing. The derived column divides proof stress by tensile strength: 450 divided by 700 is 0.64 for A4-70, and 600 divided by 800 is 0.75 for A4-80, so A4-80 is not just stronger but also has less margin between yielding and fracture.
Two limits on the scope are worth knowing. The standard covers coarse pitch threads from M1.6 to M39 and fine pitch threads from M8×1 to M39×3, tested at 10 to 35 °C, and it does not cover functional properties such as torque and clamp force, shear strength, fatigue resistance or weldability (EVS record for EN ISO 3506-1:2020), so a class number does not answer a shear or fatigue question for a pin.
| Property class | Tensile strength, minimum | 0.2% proof stress, minimum | Proof ÷ tensile (derived) |
|---|---|---|---|
| A4-50 | 500 MPa | 210 MPa | 0.42 |
| A4-70 | 700 MPa | 450 MPa | 0.64 |
| A4-80 | 800 MPa | 600 MPa | 0.75 |
Why can a stud machined from bar not simply claim A4-80?
Because the class describes a finished fastener made from cold-worked bar, and bar bought in the annealed condition has a much lower yield than A4-70 or A4-80 requires.
The SCI note says property class 70 fasteners are made from cold drawn bar, and property class 80 fasteners from severely hard cold drawn bar, with mechanical properties similar to carbon steel grade 8.8 bolts (SCI Advisory Desk AD 342). Atlas Steels grade data give a minimum 0.2% proof stress of 205 MPa for flat-rolled 316 per ASTM A240 (AZoM, 2001); dividing the A4-80 proof stress by it gives 600 divided by 205, about 2.9 (derived). That gap is closed by cold work in the bar, not by the machining.
ASTM A276 describes bar in Condition A (annealed), Condition S (strain hardened or relatively lightly cold worked) and Condition B (relatively severely cold worked) (ASTM A276/A276M-25), so the condition you order matters as much as the grade. Two things follow. A stud turned from annealed bar cannot be assumed to meet A4-70, let alone A4-80. And even from cold-worked bar, the stated properties belong to a tested finished part: the sources do not say how much cutting a thread removes from the strain-hardened surface, so the safe route is a tensile test on finished studs, not an assumption carried over from the bar.
When should the drawing call a standard A4-80 fastener, and when a custom machined stud?
Call a standard fastener to ISO 3506-1 when the thread form and length are standard and strength is the point; call a machined stud when the geometry is special, and then define the strength by bar condition and test.
ASTM F593 gives the North American parallel: it covers stainless bolts, hex cap screws and studs from 0.25 to 1.50 in. nominal diameter, with 316 and 316L in alloy Group 2 supplied cold worked (ASTM F593-22). A catalog fastener made to either system comes with a maker that heads, rolls and tests it; a custom stud does not, unless you add the test.
Custom studs and pins earn their place when a standard part cannot do the job: a stud-mount cleat stud with a shoulder or flange, a special length, a hex collar, a hinge pin, a clevis pin retained by a cotter. For those, the decision table below separates the cases.
| Situation | What to call out | Why |
|---|---|---|
| Standard thread, standard length, load-bearing joint | Standard A4-70 or A4-80 fastener to ISO 3506-1 from a fastener maker | The maker tests finished fasteners to the class |
| Special shoulder, flange or length, strength matters | Machined stud from Condition B bar, strength stated as numbers and a tensile test on finished studs | The class letters are for fasteners; a drawing note and a test make the claim honest |
| Lightly loaded hinge pin or clevis pin | Dimensions, bar grade and condition; no property class | Strength class adds cost and does not address wear or shear |
| Shear-loaded pin | Designer's shear check, not a class number | ISO 3506-1 does not cover shear strength |
| Need more strength with seawater resistance | Ask the designer to review other alloys | A4-80 is still a 316-type alloy; duplex bolts exist with class 80 properties per SCI AD 342 |
How should the drawing note read for a machined stud or pin?
Write the alloy, the bar specification and condition, the numeric strength you need and the test that proves it, and leave the A4 letters off unless the part is a standard fastener.
An example structure, with the numbers left for the designer to fill in from the load case: "Material UNS S31600 bar to [bar specification], Condition [A, S or B]. Minimum proof stress [value] and minimum tensile strength [value], both in MPa, verified by tensile test on [number] finished studs per lot, results reported." The test method and sample size are agreed with the supplier before the first order. If the stud is threaded at both ends, state which thread standard governs each end and whether the thread is cut or rolled.
Add the usual stainless callouts: heat number traceability, a material certificate, and the surface condition. Passivation is a partner-plant process, and the guide on passivation of machined stainless parts covers the callout. Saola machines to the drawing and does not assign or claim a property class for a custom stud; that claim stays with the designer or with the fastener maker who tests to ISO 3506-1. Standard threaded fasteners are best bought as fasteners, and custom pins and studs are quoted as CNC turning work, with CNC milling for flats and cross features.
Does a higher property class improve corrosion resistance or galling behavior?
No: the corrosion group (the 4) sets corrosion resistance, so A4-70 and A4-80 corrode alike, and the class affects strength and, in one respect, galling.
The SCI note says the number after the letter reflects corrosion resistance and the last two numbers describe mechanical properties (SCI Advisory Desk AD 342), so moving from A4-70 to A4-80 buys strength and no extra seawater resistance. Grade 316 itself is subject to pitting and crevice corrosion in warm chloride environments and is not resistant to warm seawater (AZoM, 2001), which is why a marine design still needs open, drained joints; see the guide on crevice corrosion design.
On galling, the same note says class 50 fasteners with machined threads may be more prone to thread galling than the higher classes. That favors the harder classes for threads that are repeatedly assembled, but it is not a cure, and the guide on stainless steel thread galling covers lubricants, thread fit and installation.
Frequently asked
Is A4-80 stronger than A4-70?
Yes. The BS EN ISO 3506 table in the SCI note gives a minimum tensile strength of 800 MPa against 700 MPa, and a minimum 0.2 percent stress of 600 MPa against 450 MPa (SCI Advisory Desk AD 342). Both are 316-type alloys with the same corrosion resistance.
Is A4 the same as 316?
Close, not identical. The SCI note gives A4 corrosion resistance equivalent to 1.4401 (316), but the A4 chemistry limits are set by ISO 3506-1, not by a 316 bar specification, so check the chemistry if the drawing requires 316 or 316L.
Can I machine a stud from 316 bar and mark it A4-70?
Not on the strength of the bar alone. The class belongs to a tested finished fastener. If you need the property, order cold-worked bar, state the numeric strength and test finished studs, and do not use the class marking without that test.
What is the ASTM equivalent of A4-70 and A4-80?
There is no one-to-one match. ASTM F593 covers stainless bolts, hex cap screws and studs, with 316 and 316L in Group 2 in the cold worked condition (ASTM F593-22), but it is a separate system with its own conditions and size range. Name one system on the drawing, not both.
Does a custom stud need to meet ISO 3506-1 to be used on a boat?
Only if your design or the boat builder says so. If the drawing does not call for a standard fastener, the drawing requirements govern. Class rules and builder specifications are the designer's responsibility.
Sources
- AD 342: Stainless steel fasteners, SCI Advisory Desk, The Steel Construction Institute, undated; refers to the 2009 revision of EN ISO 3506
- EVS-EN ISO 3506-1:2020, Fasteners - Mechanical properties of corrosion-resistant stainless steel fasteners - Part 1 (record page), Estonian standards body, 2020
- ASTM F593-22, Standard Specification for Stainless Steel Bolts, Hex Cap Screws, and Studs (store page), ASTM International, 2022
- ASTM A276/A276M-25, Standard Specification for Stainless Steel Bars and Shapes (store page), ASTM International, 2025
- Stainless Steel: Grade 316, supplied by Atlas Steels, AZoM, 2001