A2-70 vs A4-80 Stainless Steel Bolts: 304 or 316?

Stainless steel bolts marked A2-70 or A4-80 — what do these designations mean? Choosing wrong can mean rust in six months.

What the Numbers Mean

• A2 = 304 austenitic stainless (18% Cr, 8% Ni)
• A4 = 316 austenitic stainless (18% Cr, 12% Ni, 2–3% Mo)
• 70 = minimum tensile strength 700 MPa
• 80 = minimum tensile strength 800 MPa

A2-70 = 304 cold-headed bolt, 700 MPa. A4-80 = 316 cold-headed bolt, 800 MPa.

Performance Comparison

A2-70 (304): Tensile 700 MPa, yield 450 MPa. Good corrosion resistance indoors/fresh water. Cost moderate. Use for indoor equipment, general outdoor, food machinery.

A4-80 (316): Tensile 800 MPa, yield 600 MPa. Molybdenum gives excellent chloride pitting resistance — no rust near coast. Costs 30–50% more. Use for coastal equipment, chemical, medical, marine.

Common Misconceptions

• "Stainless doesn't rust" — Wrong. 304 pits in chloride environments. Only 316 survives coastal exposure long-term.
• "A4 is always stronger" — A4-70 has same strength as A2-70; A4-80 is the higher-strength grade.
• "Stainless works at high temperature" — 304/316 lose strength above 500°C; use A-286 or Inconel for high temp.

Selection Guide

1. Indoor dry → A2-70 (304)

2. Outdoor, rain, chemical → A4-80 (316)

3. Coastal/marine → A4-80 minimum

4. Need above 800 MPa → don't use stainless; go to 12.9 alloy steel

Toprecision supplies A2-70 and A4-80 stainless bolts in metric and UNC/UNF sizes.