A4-50, A4-70 and A4-80 Stainless Metric Fasteners (ISO 3506 / ASTM F738M)
The metric 316 bolt: A4 is the molybdenum-bearing austenitic group for chloride and marine service, and A4-80 at 800 MPa strain hardened is the strongest standard stainless metric fastener.
What A4 stainless (A4-80) is.
A4 is the ISO 3506 group for 316 and 316L stainless. Like A2 it is offered at 500, 700 and 800 MPa, but the molybdenum in 316 gives it the pitting and crevice resistance that seawater, deicing salt and process chlorides demand. A4-70 is the usual marine and chemical-plant bolt; A4-80 is specified where strength close to Class 8.8 is needed without giving up stainless corrosion resistance. ASTM F738M carries the same classes with markings F738C, F738F and F738J.
We machine A4 bolts, screws, studs and nuts in M1.6 to M36 from certified 316 bar and supply them to the class ordered, passivated, with anti-seize or silver-plated nuts on request.
Chemical composition.
| Element | Symbol | Min % | Max % |
|---|---|---|---|
| Carbon | C | — | 0.08 |
| Manganese | Mn | — | 2.00 |
| Phosphorus | P | — | 0.045 |
| Sulfur | S | — | 0.030 |
| Silicon | Si | — | 1.00 |
| Chromium | Cr | 16.0 | 18.0 |
| Nickel | Ni | 10.0 | 14.0 |
| Molybdenum | Mo | 2.00 | 3.00 |
Values per ASTM F738M-02 Table 3 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team. 316L is listed in the same table with carbon 0.03 % max.
Mechanical properties.
| Property class · condition · size | Tensile strength, min (MPa) | Yield strength, min (MPa) | Elongation, min (%, machined specimen) | Hardness (Rockwell) | F738M marking |
|---|---|---|---|---|---|
| A4-50 · AF (annealed) · M6–M36 | 500 | 210 | 30 | B85 max | F738C |
| A4-70 · CW · M6–M20 | 700 | 450 | 20 | B96–C33 | F738F |
| A4-70 · CW · over M20–M36 | 550 | 300 | 25 | B83–C31 | F738F |
| A4-80 · SH · M6–M20 | 800 | 600 | 12 | C23–C36 | F738J |
| A4-80 · SH · over M20–M24 | 700 | 500 | 15 | B96–C33 | F738J |
| A4-80 · SH · over M24–M30 | 650 | 400 | 20 | B93–C30 | F738J |
| A4-80 · SH · over M30–M36 | 600 | 300 | 28 | B89–C28 | F738J |
Values per ASTM F738M-02 Table 2 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team. ISO 3506-1 gives the same class values; extension and torsional requirements are in the specification.
Specifications that apply.
How A4 stainless (A4-80) ships from Houston.
- Hex and heavy hex bolts and cap screws
- Studs and threaded rod
- Nuts and washers to match
- Machined specials to drawing
A4 stainless (A4-80) fasteners we make.
Every item is manufactured to your drawing or standard in A4 stainless (A4-80), imperial or metric, with an EN 10204 3.1 certificate.
A4-70 or A4-80?
Both are 316; the difference is cold work. A4-70 is cold worked to 700 MPa and is the everyday marine bolt in M6 to M20, dropping to 550 MPa above M20. A4-80 is strain hardened to 800 MPa with a 600 MPa yield in the same range, which puts it close to a Class 8.8 carbon-steel bolt, and it steps down through the larger sizes. Specify A4-80 where the joint load demands it; A4-70 is easier to source in the larger diameters.
Testing, certification and marking
- Tensile and hardness per lot; full-size testing to the F738M or ISO 3506 tables
- Property class marking, for example A4-80 (F738J under ASTM F738M), and manufacturer's identification on the head
- Certificate of conformance or test report with heat number; EN 10204 3.1 available
- Intergranular corrosion capability per ASTM A262 Practice E
Recent A4 stainless (A4-80) jobs.
A2-50, A2-70 and A2-80 Stainless Metric Fasteners (ISO 3506 / ASTM F738M)
The metric 304 bolt: A2 is the 18-8 austenitic group, and the number is the tensile strength in tens of MPa — A2-70 at 700 MPa cold worked is the standard stainless bolt on metric drawings.
ASTM A276 bar · ASTM F593 bolts316 Stainless Steel (UNS S31600)
The marine and chemical-plant stainless: 2 to 3 percent molybdenum for pitting and crevice resistance in chlorides, with the same strength tables as 304.
ASTM A193 / ASME SA-193ASTM A193 Grade B8M Class 1 (316 Stainless)
316 stainless bolting with molybdenum for chloride and pitting resistance — offshore, marine, chemical and coastal plant fasteners.