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Strength Specifications of Metric Threaded Fasteners

The following table describes the strength specifications for the different property classes of metric steel bolts, screws and studs:

Property ClassNominal Size Range (mm)Proof Strength (MPa)Yield Strength, min(MPa)Tensile Strength, min(MPa)Material
4.65-100225240400Low or medium carbon steel
4.81.6-16310340420Low or medium carbon steel; fully or partially annealed
5.85-24380420520Low or medium carbon steel; cold worked
8.8Under 16 (incl.)660640800Medium carbon steel; quenched and tempered
8.817-72580660830Medium carbon steel; quenched and tempered
9.81.6-16650720900Medium carbon steel; quenched and tempered
10.95-1008309401040Alloy steel; quenched and tempered
12.91.6-10097011001220Alloy steel; quenched and tempered

The tensile strength or ultimate tensile strength can be defined as the maximum amount of tensile stress that a component can withstand before it fractures.

The proof strength can be defined as the maximum amount of tensile stress that a component can withstand before it begins to exhibit plastic or permanent deformation on release of the stress applied.

The yield strength can be defined as the amount of tensile stress that a component can withstand when it exhibits 0.2% plastic or permanent deformation.

The property class is normally stamped on the head of the bolt. The two figures indicate the tensile and yield strengths of the bolt or screw.

In the case of a 8.8 grade bolt the first figure signifies that the Tensile Strength is at least 800MPa. The second figure signifies that the fastener will begin to yield at 80% of the Ultimate Tensile Strength, i.e. at least 640MPa.


Strength Specifications of Stainless Steel Threaded Fasteners

The following table, taken from BS EN ISO 3506, describes the strength characteristics of different grades of austenitic stainless steel bolts, screws and studs:

Steel GradeProperty ClassNominal Size Range (mm)Tensile Strength, min(MPa)0.2% Proof Stress (MPa)
A1,A2,A3,A4,A550Under 39 (incl.)500210
70Under 24 (incl.)700450
80Under 24 (incl.)800600

Stainless steel fasteners do not exhibit a yield stress. In this case 0.2% proof stress is used instead, and can be defined as the tensile stress required for the component to exhibit elongation of 0.2%.