Bolt Grade Chart: 8.8 vs 10.9 vs 12.9 and A2-70/A4-80

Bolt grade is one of the first details a buyer should confirm before sending an RFQ. For carbon steel metric bolts, the most common strength classes are 8.8, 10.9, and 12.9. For stainless steel fasteners, buyers often see A2-70, A4-70, or A4-80 instead. These markings are not decorative labels; they tell the supplier what strength family, material direction, and application risk the order must satisfy.

A practical carbon steel bolt grade chart starts with three common classes. Class 8.8 is a general industrial strength choice for brackets, frames, machinery guards, and many non-critical assemblies. Class 10.9 is used when higher clamp load and fatigue resistance are needed, often with alloy steel and controlled heat treatment. Class 12.9 is a high-strength class for compact designs and demanding mechanical assemblies, but it also requires more care around brittleness, coating choice, and matching nuts and washers.

The first number in a metric property class relates to nominal tensile strength, while the second number relates to yield ratio. In buyer terms, 8.8, 10.9, and 12.9 are a strength ladder, not a simple quality ranking. Higher is not always better. A 12.9 bolt in the wrong coating, environment, or joint design can create avoidable risk. The correct grade depends on load, joint design, corrosion exposure, temperature, installation method, and the matching nut property class.

Stainless steel grades use a different marking logic. A2 usually points to 304-type stainless steel, which is common for indoor, architectural, food equipment, and general corrosion-resistant use. A4 usually points to 316-type stainless steel, which is preferred for marine, chemical, coastal, or chloride-exposed environments. The number after the dash, such as 70 or 80, describes strength class within that stainless family.

A2-70 is often selected when corrosion resistance matters more than maximum strength. A4-80 is usually considered when the buyer needs both better chloride resistance and a stronger stainless fastener. It is important not to compare A4-80 with carbon steel 10.9 as if they were interchangeable. Stainless and alloy steel behave differently in strength, galling, corrosion, magnetism, and price.

For export RFQs, the safest wording is specific. Instead of asking for 'high strength bolts', write the standard, size, material, grade, finish, quantity, and application. For example: hex head bolt DIN 933, M8 x 40, stainless steel A2-70, full thread, passivated, 20,000 pcs, packed for export cartons and pallets. For a higher-strength carbon steel order, write: hex bolt ISO 4017, M10 x 50, property class 10.9, zinc flake or black oxide, matched nut and washer required.

Matching components matter. A high-grade bolt should not be paired casually with a low-strength nut. Flat washers and spring washers also need material and finish alignment. If the assembly will be exposed outdoors, the finish and corrosion requirement can be more important than increasing the bolt grade. If the assembly is stainless, thread galling and lubrication should be discussed before bulk production.

Buyers should also ask for the right documents. For standard industrial orders, a certificate of conformity may be enough. For critical equipment, ask whether material test certificates, dimensional inspection, coating thickness reports, or sample approval can be supplied. The document requirement should be stated before quotation, because it affects lead time, inspection cost, and supplier selection.

Savin Industry can support RFQs that combine standard fasteners, material selection, and export packing requirements. Share the grade, size range, standard, material, finish, quantity, application, and target market so the quotation can separate what is technically required from what is only a preference.