Bolt Property Classes: What 8.8, 10.9 and A4-80 Mean

Reading the number

On carbon and alloy steel bolts the class is written as two figures. The first is one hundredth of the nominal tensile strength in MPa; the second is ten times the ratio of yield strength to tensile strength. An 8.8 bolt is nominally 800 MPa tensile with yield at 80 per cent of that. The classes held here are 4.8, 5.8, 6.8, 8.8, 10.9 and 12.9.

Higher is not automatically better

Class 10.9 and 12.9 bolts are stronger but less ductile, and they are more sensitive to hydrogen embrittlement introduced by electroplating. Specify the class the connection was designed for, and tell us the class when you ask for a plated finish so the process can be handled correctly.

The stainless classes

A2-70, A2-80, A4-70 and A4-80. A2 is the chromium-nickel group, A4 the molybdenum-bearing group; 70 and 80 are the tensile classes.

The nut has to keep up

A nut of a lower class than its bolt will strip before the bolt reaches its design load, and the connection never develops the strength it was drawn for. Quote bolt, nut and washer together and in matching grade and finish.

Where structural bolts sit here

High tensile bolts in grades 8.8, 10.9 and 12.9 for steel construction and building work, M4–M30, in lengths from 6mm to 300mm.

Everything on this page describes the specification range Beyond Industry Groups Limited works to. Where a figure is a laboratory test result rather than a service life, it is described as such.