Stainless steel bolt grades are often misunderstood in purchasing. Buyers may ask for “304 stainless bolts” or “316 stainless bolts,” but the actual fastener marking often appears as A2-70, A4-70, or A4-80.

These markings tell two things: the stainless steel group and the mechanical property class. Both matter.

What A2 and A4 Mean

Stainless Steel Group

A2 is commonly used for general corrosion-resistant stainless fasteners. It is often associated with 304-type stainless steel.

A4 is used where stronger corrosion resistance is required. It is commonly associated with 316-type stainless steel and is preferred in chloride, marine, chemical, or wet environments.

Buyers can review stainless steel fasteners when selecting between A2 and A4 materials.

What 70 and 80 Mean

Mechanical Property Class

The number after the dash indicates the property class. In practical purchasing terms, 70 means a minimum tensile strength level of about 700 MPa, while 80 means about 800 MPa.

For load-bearing applications, buyers should compare stainless grades with high-strength fasteners before replacing carbon steel bolts with stainless steel.

A2-70 vs A4-70

Corrosion Resistance Is the Main Difference

A2-70 is suitable for many general industrial uses. It performs well in dry indoor environments and moderate outdoor exposure.

A4-70 is better where general corrosion risk is higher. It is often selected for marine hardware, food processing equipment, coastal structures, and chemical plant applications.

For non-stainless coating alternatives, buyers can also review various coated fasteners.

A4-70 vs A4-80

Same Corrosion Family, Higher Strength Class

A4-80 provides higher tensile strength than A4-70, while still using the A4 stainless group. It is useful where both corrosion resistance and higher mechanical performance are required.

However, A4-80 should not be selected blindly. Confirm tightening method, nut matching, galling risk, thread fit, and inspection report requirements.

Common Buyer Mistakes

Avoid these issues:

1. Assuming stainless steel is always stronger than alloy steel.
2. Using A2 in strong chloride exposure.
3. Ordering “316 bolts” without property class.
4. Mixing A4 bolts with unsuitable nuts or washers.
5. Ignoring galling during stainless assembly.
6. Forgetting MTC and batch traceability for project orders.

For standard stainless bolts, start from standard fasteners. For special lengths, threads, or head styles, use custom non-standard fasteners.

Final Advice

A2-70, A4-70, and A4-80 are not just material names. They combine stainless steel group and strength class.

Choose A2-70 for general corrosion resistance, A4-70 for stronger corrosion resistance, and A4-80 where higher strength is also required. Always confirm size, thread, nut, washer, application environment, and test documents before orderng.