Bolts, screws, studs, nuts, washers and anchors made from austenitic, duplex or martensitic stainless steel. Commercial fasteners are graded by ISO 3506 (A2-70, A4-80); bolting for pressure joints follows ASTM A193 B8/B8M with A194 8/8M nuts. In the salty, humid Gulf climate, A4 (316) is the usual choice for anything outdoors or near the sea.
Stainless fasteners are used where a carbon steel bolt, even galvanized, would corrode — outdoors near the sea, in water and wastewater plant, in food processing, and wherever stainless sheet or sections are being joined (a carbon steel bolt in stainless becomes a sacrificial anode and rusts quickly). Most are cold headed from austenitic wire rod, with the thread rolled; large sizes are hot forged or machined from bar.
ISO 3506 sets the steel group and property class. The letter and figure before the dash give the steel: A2 is the 304 family (18Cr-8Ni), A4 the 316 family with molybdenum, A5 and A3 the stabilised variants, and the 2020 edition added duplex groups D2 to D8. The number after the dash is one tenth of the minimum tensile strength: A2-70 and A4-70 have 700 MPa minimum tensile strength, A4-80 has 800 MPa. The strength comes from cold work during heading, which is why large-diameter austenitic fasteners are often only class 50 or 70.
Bolting for flanges and pressure equipment follows ASTM A193 instead: grade B8 is 304, B8M is 316, both in Class 1 (solution treated, about 205 MPa yield) or Class 2 (strain hardened, much stronger in small diameters). The matching nuts are ASTM A194 grade 8 and 8M. Stud bolts for ASME B16.5 flanges are usually threaded rod with two heavy hex nuts.
Stainless threads gall: under load, the oxide film breaks and the two surfaces cold-weld, seizing the nut on the bolt. The usual cures are lower installation speed, an anti-seize lubricant, a nut of a different grade or hardness, or rolled rather than cut threads. Tightening torques published for carbon steel bolts do not apply — the friction depends heavily on lubrication, so torque should come from the fastener supplier's data for the actual lubricant. In the GCC, A4 is the default for facades, marine and coastal works and water plant; duplex and super duplex bolting is used in seawater and desalination service.
Metric fasteners are ordered by product standard, thread and length, and steel group and class — 'Hex screw ISO 4017 M12 × 40, A4-80'. Imperial bolting is ordered by ASME B18.2.1 or ASTM A193/A194 with UNC or 8UN threads.
| Region | Standard | What it covers |
|---|---|---|
| International | ISO 3506-1 ISO |
Mechanical properties of corrosion-resistant stainless steel fasteners — bolts, screws and studs; steel groups A1–A8, C1, C3, C4, F1 and D2–D8 and property classes |
| International | ISO 3506-2 ISO |
Mechanical properties of corrosion-resistant stainless steel fasteners — nuts |
| International | ISO 4017 ISO |
Hexagon head screws, fully threaded, product grades A and B |
| Germany | DIN 933 DIN replaced by ISO 4017 |
Hexagon head screws, fully threaded — the old German designation still used in trade |
| Germany | DIN 934 DIN replaced by ISO 4032 |
Hexagon nuts — the old German designation still used in trade |
| USA | ASTM F593 ASTM |
Stainless steel bolts, hex cap screws and studs — alloy groups and conditions |
| USA | ASTM F594 ASTM |
Stainless steel nuts |
| USA | ASTM A193/A193M ASTM |
Alloy and stainless steel bolting for high temperature or high pressure service — grades B8 and B8M among others |
| USA | ASTM A194/A194M ASTM |
Carbon, alloy and stainless steel nuts for bolts for high pressure or high temperature service — grades 8 and 8M |
| USA | ASME B18.2.1 ASME |
Square, hex, heavy hex and askew head bolts and hex, heavy hex, hex flange screws (inch series) |
| Japan | JIS B1054-1 JSA |
Mechanical properties of corrosion-resistant stainless steel fasteners — bolts, screws and studs |
| China | GB/T 3098.6 SAC |
Mechanical properties of fasteners — stainless steel bolts, screws and studs |
Grade equivalents and the standards bodies: Steel grades & standards
Stockist - Distributor · Fabrication · Contractors · Shanghai
Steel Mills - Agents & Traders · Stockist - Distributor · Other Steel Users · Sharjah
Steel Mills - Agents & Traders · Stockist - Distributor · Other Steel Users · Sharjah
Steel Mills - Agents & Traders · Fabrication · Design - Engineering · Amman
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