ISO 898-1 Mechanical Properties of Bolts, Screws and Studs

ISO 898-1 is the standard behind the number stamped on a metric steel bolt head. It applies to bolts, screws and studs with ISO metric threads, made of carbon steel or alloy steel, tested at an ambient temperature of 10 to 35 °C. It is what turns property class 8.8 or 10.9 into a verifiable claim rather than a label: for each class it fixes the minimum tensile strength, the lower yield stress, elongation after fracture and a hardness window, together with the tests used to demonstrate them — tensile and wedge loading, proof load, hardness and impact.

Choose the class from the preload the joint needs, not from the strongest number on the shelf. 8.8 is the general engineering default, and the class where the ratio between yield and tensile strength still leaves useful ductility. 10.9 and 12.9 raise the yield strength but give up elongation: 12.9 reaches yield at 90 % of its ultimate strength, so a joint that overloads it fractures with little warning. Plated 10.9 and above need a baking schedule after electroplating to avoid hydrogen embrittlement — if that cannot be controlled in the supply chain, 8.8 is the safer specification. Stainless fasteners are not covered here at all; they belong to ISO 3506.

This standard fixes mechanical properties and test methods rather than dimensions, so there is no size table here. The head shapes and thread sizes it is applied to come from a separate product standard; the strength data itself — tensile, yield, hardness and the matching nut class — is on the class pages linked above.

The class number is arithmetic rather than a rank. The first digit is one hundredth of the nominal tensile strength in megapascals, and the second is ten times the ratio between the lower yield stress and that tensile strength. So 8.8 means a nominal 800 MPa with yield at 80 % of it, and 10.9 means 1000 MPa yielding at 90 %. Because the second figure rises with the first, the higher classes are proportionally less ductile — which is the whole reason a stronger bolt is not automatically a better one. Nuts are covered separately by ISO 898-2, and the established rule is that the nut should be the weaker part, so an overload strips the thread instead of breaking the bolt.