How to Select the SS304 Fastener

The SS304 fastener — bolts, nuts and studs in ASTM A193 Gr B8 stainless steel made to ANSI B18.2.1 — combines high strength with excellent corrosion resistance. On bolted joints it works with flange gaskets and other hardware across construction, manufacturing and industrial service.

SS304 Fastener Classes and Mechanical Data

ASTM A193 defines two strength classes of the same 304 (B8) stainless steel. Class 1 is the solution-annealed grade for ductile duty, while Class 2 is strain-hardened for higher loads. The table lists guaranteed values; 316 (B8M) is shown for comparison.

Guaranteed values per ASTM A193; Rockwell hardness where stated. 304 (B8) suits weakly corrosive media only.

Against plain carbon steel (tensile 400–550 MPa, yield 250–350 MPa, elongation 20–30%), Class 1 delivers far more ductility and corrosion resistance. Against alloy steel (tensile 800–1000 MPa, yield 600–800 MPa, elongation 10–15%), Class 2 carries the same load class and still stays stainless.

Application Scenarios

Low pressure, low temperature or vibrating duty belongs to Class 1; high pressure and high temperature duty belongs to Class 2. Bolted connections on pipe flanges, valves and pumps to ASME B16.5 are typical Class 2 work.

• Food processing — mixers, conveyors and storage tanks (Class 1).
• Building structures — steel bridges, curtain walls and pipe supports (Class 1).
• Vibrating equipment — fans and pumps, where ≥40% elongation absorbs micro-movement (Class 1).
• Boilers and heat exchangers — low-pressure bolted joints (Class 2).

Both classes resist air, freshwater, weak acids and weak alkalis, but high chloride environments force a change of grade.

316 (B8M) for Chloride Service

For seawater, de-icing salt and other high Cl⁻ duty the 304 grades run out of margin: their critical pitting temperature sits at 20–25 °C. The 316 alternative — ASTM A193 B8M — carries 2–3% Mo and lifts critical pitting temperature to 35–40 °C, with better oxidation resistance at high temperature.

• High Cl⁻ environments — seawater, de-icing salt exposure and coastal areas.
• Highly corrosive media — concentrated sulfuric acid, hydrochloric acid and chloride solutions.
• Trade-off — clearly better chloride and high-temperature resistance at a higher cost.

Selection Summary

Match the grade to the dominant threat — vibration and ductility, pressure and temperature, or chlorides — as summarized below.

• Low pressure, low temperature, vibration → SS304 Class 1: high ductility, cost-effective, not for high Cl⁻.
• High pressure, high temperature → SS304 Class 2: high strength and strain-hardened reliability, not for high Cl⁻.
• High Cl⁻ or highly corrosive media → 316 (B8M): chloride resistance and high-temperature performance at higher cost.

Frequently Asked Questions

What separates ASTM A193 B8 Class 1 from Class 2?

Class 1 is solution-annealed: tensile ≥515 MPa and elongation ≥40% for ductile duty. Class 2 is strain-hardened: tensile ≥860 MPa, yield ≥690 MPa and elongation ≥12% for high pressure and temperature.

Can SS304 fasteners run in seawater?

No — chlorides pit 304 grades above their 20–25 °C critical pitting temperature. Specify 316 (B8M) with 2–3% Mo instead.

What hardness limits apply to Class 2 studs?

Class 2 runs 248–352 HB (Rockwell C25–35), while Class 1 stays at or below 192 HB (Rockwell B90).

Which standards govern SS304 fasteners?

Material and classes follow ASTM A193 (B8, B8M), dimensions follow ANSI B18.2.1, and flange bolting duty follows ASME B16.5.