The short answer: a stainless fastener marking is two separate pieces of information joined by a hyphen. The first block identifies the steel group — A2 is the 304 family, A4 the 316 family. The second block is the property class — the minimum tensile strength of the finished fastener in tens of megapascals, so 70 means 700 MPa. The marking tells you what the steel is and how strong the finished part is. It does not tell you the fatigue behavior, the torque to apply, or whether the grade suits your corrosion environment — those need separate decisions.
This decoding matters because two common substitutions are wrong: A2-70 is not "Type 304" as a material procurement spec, and it is not a stainless equivalent of property class 8.8. Both mistakes show up regularly on drawings and RFQs.
What do the letter and digit in the first block mean?
Under ISO 3506-1:2020, the letter identifies the steel group and the digit narrows the composition range within it.[1] The groups a buyer is likely to meet:
| Marking block | Steel group | Approximate alloy family |
|---|---|---|
| A1 | Austenitic, free-machining | ~303 (1.4305) |
| A2 | Austenitic | ~304 (1.4301) |
| A3 | Austenitic, stabilized | ~321 / 347 |
| A4 | Austenitic, molybdenum-alloyed | ~316 (1.4401) |
| A5 | Austenitic, stabilized with Mo | ~316Ti |
| A8 | Super-austenitic, ~6% Mo | ~1.4547 / 1.4529 |
| C1, C3, C4 | Martensitic | ~410, 431, 416 |
| F1 | Ferritic | ~430 |
| D2–D8 | Duplex (austenitic-ferritic) | lean duplex through super duplex |
Grade A8 and the duplex D grades entered the standard with the 2020 edition; earlier drawings and catalogs written against ISO 3506-1:2009 do not know them.[1] [2]
The letter group is a corrosion and chemistry statement, not a complete material specification. "A2" places the steel in the 304 family, but if the drawing needs a specific chemistry with traceability, state the material standard as well — for example 304 to ASTM A240. The fastener marking alone does not do that job. For the corrosion-side choice between the two common groups, see 304 vs 316 Stainless Steel Fasteners.
What does the number after the hyphen mean?
The property class is the minimum tensile strength of the finished bolt, screw or stud in tens of megapascals. The class number is one-tenth of that minimum. The same marking appears on nuts under ISO 3506-2:2020, where the class is tied to proof-load stress.[3]
| Property class | Manufacturing basis | Min. tensile strength | Min. 0.2% proof stress |
|---|---|---|---|
| 50 | Annealed | 500 MPa | 210 MPa |
| 70 | Cold worked (standard supply) | 700 MPa | 450 MPa |
| 80 | Severely cold worked | 800 MPa | 600 MPa |
| 100 | High-strength route, A4/A5/A8 and duplex only | 1000 MPa | 800 MPa |
Two practical consequences follow from the manufacturing basis:
- The strength comes from cold work, not heat treatment. Austenitic stainless cannot be quench-hardened. Class 70 and 80 fasteners are strong because the bar was cold drawn before heading and rolling. That is also why class-70 and class-80 parts can be slightly magnetic even though the steel is "non-magnetic" austenitic — the magnet is reading cold work, not grade.
- Size used to cap the class. The 2009 edition limited classes 70 and 80 to M24 and below. The 2020 edition raised that limit to M39, so A4-80 in larger diameters is now a standard callout rather than a special agreement.[1] [2]
Martensitic grades follow the same logic with heat treatment: C1-70 reaches 700 MPa, C1-110 reaches 1100 MPa quenched and tempered, and only martensitic and ferritic classes carry mandatory hardness ranges. Duplex grades start at class 70 — there is no soft duplex fastener in the standard.
What does the marking not tell you?
ISO 3506-1 states its own boundaries: it does not specify torque and clamp-force behavior, shear strength, fatigue resistance or weldability.[1] The unrestricted service range is −20 °C to +150 °C, with wider use down to −196 °C or up to +300 °C left to the user's responsibility.
So a complete specification still needs the items the marking omits:
- Corrosion environment. The letter block implies a corrosion family; it does not certify performance in your medium. A2-70 in a chloride washdown is still the wrong fastener.
- Friction and tightening. No part of ISO 3506 controls the friction coefficient or a torque table. Stainless threads have their own assembly hazard — galling — covered in Why Stainless Threads Seize. If a controlled torque-tension relationship matters, specify a lubricant condition or a tested torque, not just the class.
- Fatigue and shear. Neither is standardized by the class. If the joint is fatigue-critical, that requirement has to come from design calculation and project specification.
Is A2-70 a stainless version of class 8.8?
No, and the gap is in the number that matters for tightening. Class 8.8 under ISO 898-1 carries a 640 MPa minimum proof stress; A2-70 carries 450 MPa — about 30% lower, even though the tensile figures (800 vs 700 MPa) look close.[4] Preload, torque tables and joint design follow the proof stress, not the ultimate strength. A joint engineered around 8.8 preload values will be under-clamped if A2-70 is substituted by diameter, or over-stressed if tightened to the 8.8 torque.
Treat the swap as an engineering change, as outlined in How to Specify a Bolt: compare proof stress, head and thread dimensions, marking and service conditions before approving a substitution. If the joint needs more strength than class 80 offers, the answer is usually a coated alloy-steel fastener or a larger diameter — not a "stronger stainless." For the alloy-steel side, see Bolt Grades 8.8, 10.9 and 12.9.
What does A2-70 mean on an inch-series product?
Strictly speaking, nothing — formally. ISO 3506-1 covers ISO metric threads only.[1] An "A2-70" stamp on a 1/4-20 socket head cap screw is market shorthand, not a certifiable designation. Inch-series stainless fasteners have their own mechanical standards:
- ASTM F593-24 covers inch-series stainless bolts, hex cap screws and studs from 1/4 to 1-1/2 in, in seven alloy groups: Group 1 is the 304 family, Group 2 the 316 family, with martensitic, ferritic and precipitation-hardening groups beyond them.[5]
- ASTM F594-24 is the matching nut standard, using the same groups.[6]
- ASTM F837-20 covers stainless socket head cap screws specifically — the standard that actually governs an inch-series A2-70-style SHCS.[7]
Instead of property classes, F593 uses supply conditions. Condition CW — headed and rolled from annealed stock, picking up strength from cold work — is the default for Groups 1–3, with tensile minima around 100 ksi in small sizes and a step-down above 5/8 in. Condition SH (strain hardened) reaches 120 ksi minimum. That makes the informal mapping roughly: class 70 ≈ F593 CW, class 80 somewhere between CW and SH. Treat this as orientation, not equivalence — no standards body publishes a conversion, and the test methods and size breakpoints differ between the two systems.
When the drawing mixes systems — a Unified thread with an ISO-style marking — name one mechanical standard explicitly. "1/4-20 UNC-2A to ASTM F593 Group 1" is enforceable; "1/4-20, A2-70" is a description that two suppliers can interpret differently.
What about Chinese GB/T drawings?
The Chinese route mirrors the ISO system. GB/T 3098.6-2023, current since December 2023, adopts ISO 3506-1:2020 with modifications, so the A2-70 callout means the same thing on a GB drawing, with the national standard as the governing document.[8] Stainless nuts follow GB/T 3098.15-2023, which adopts ISO 3506-2:2020 the same way.[9] The rest of the specification — product standard, thread, finish — still comes from the GB/T product standards, exactly as for alloy steel; see the metric half of How to Specify a Bolt.
What to put on the RFQ
- Metric: product standard, full thread, and the marking with its governing standard — e.g. hex head bolt to ISO 4014, M12 × 1.75 × 60, A4-70 to ISO 3506-1.
- Inch: product standard, full thread, and the ASTM route — e.g. hex cap screw to ASME B18.2.1, 1/2-13 UNC-2A × 2, ASTM F593 Group 2, Condition CW.
- Nuts: same group as the bolt, same class — A4 bolt with A4 nut, or F593 Group 2 bolt with F594 Group 2 nut. A mixed-metal joint is only as corrosion-resistant as its weaker member.
- Environment: state the exposure (coastal air, washdown chemistry, immersion) so the group choice can be checked.
- Assembly: if the joint is tightened to a torque or preload, state the lubricant condition — dry, waxed or anti-seize — because the class says nothing about friction.
Specifying stainless fasteners? Send the size, thread, class or condition, and the exposure conditions with your quantity. We will confirm the grade route — metric, inch or GB/T — before quoting.
References
- ISO. ISO 3506-1:2020 — Mechanical properties of corrosion-resistant stainless steel fasteners, Part 1: Bolts, screws and studs with specified grades and property classes. Scope, designation system, property classes and stated exclusions (torque/clamp force, shear, fatigue, weldability).
- British Stainless Steel Association. Stainless steel fasteners. Grade and property-class tables transcribed from ISO 3506, including the 2020 edition changes (duplex grades, class 100, size limit raised to M39).
- ISO. ISO 3506-2:2020 — Mechanical properties of corrosion-resistant stainless steel fasteners, Part 2: Nuts with specified grades and property classes. Nut classes, styles and proof-load basis.
- ISO. ISO 898-1:2013 — Mechanical properties of fasteners made of carbon steel and alloy steel. Property class 8.8 minima used for the comparison.
- ASTM International. ASTM F593-24 — Stainless Steel Bolts, Hex Cap Screws, and Studs. Inch-series groups and conditions.
- ASTM International. ASTM F594-24 — Stainless Steel Nuts. Companion nut standard to F593.
- ASTM International. ASTM F837-20 — Stainless Steel Socket Head Cap Screws. Mechanical requirements for inch-series stainless SHCS.
- State Administration for Market Regulation, China. GB/T 3098.6-2023 — Mechanical properties of fasteners: Stainless steel bolts, screws and studs. Current edition; adopts ISO 3506-1:2020 (modified).
- State Administration for Market Regulation, China. GB/T 3098.15-2023 — Mechanical properties of fasteners: Stainless steel nuts. Current edition; adopts ISO 3506-2:2020 (modified).
From the Catalog
Products covered in this guide
