Tensile Strengths

Every metric bolt carries its strength stamped on the head: 4.6, 8.8, 10.9. It is not a model number. Once you can read it, the number tells you what the steel will do before it stretches and before it breaks.

Reading the number

The two figures are a formula, not a ranking.

• The first number is the tensile strength — how hard you can pull before the bolt breaks — in hundreds of megapascals. Class 8.8 is around 800 MPa.
• The second number is the yield point as a tenth of that tensile figure. The .8 in 8.8 means the bolt starts to permanently stretch at about 80% of its breaking strength, roughly 640 MPa.

The class number is nominal; the standards set the actual minimums at or slightly above it. Class 10.9 is a good example — nominally 1000 MPa, but ISO 898-1 requires a minimum of 1040 MPa tensile and 940 MPa yield. The comparison chart below lists the figures MEGAFIX works to.

Yield matters more than people expect. A bolt stretched past its yield point does not spring back. It has lost clamping force while still looking perfectly serviceable, which is how over-tightening quietly ruins joints that never visibly fail.

Which grade for which job

• Class 4.6 — low carbon steel, not heat treated, 400 MPa. General fabrication, brackets, gates, fencing, light equipment mounting: holding things in place rather than carrying heavy load.
• Class 8.8 — medium carbon steel, quenched and tempered. 800 MPa up to and including M16, 830 MPa above M16. The structural and mechanical workhorse, and what most trade work is built around.
• Class 10.9 — quenched and tempered alloy steel, 1040 MPa. Heavy machinery, mining equipment, hydraulic and press work, and joints designed around a specific bolt tension.

Higher is not automatically better. High grade steel is harder but less ductile, so it tolerates shock loading, misalignment and over-tension far less gracefully than 8.8. And never substitute a higher grade into a structural connection that specifies 8.8 without an engineer's approval — those connections are designed around a known bolt behaviour, not the biggest available number.

Match the nut to the bolt

A bolt is only as strong as the nut on it. Nut property classes are numbered to match under ISO 898-2: class 8 nuts for class 8.8 bolts, class 10 for 10.9. Fit a low grade nut to a high grade bolt and the nut thread strips long before the bolt reaches capacity, throwing away everything the higher grade was bought for.

This guide is here to help you understand and sanity-check a specification, not to replace one. Structural and safety-critical connections should always be designed by a qualified engineer.