Bolt Strength Classes: Choosing from 4.6 to 12.9

Markings on a bolt such as "8.8" or "10.9" are a code summarizing its mechanical properties. Choosing the wrong class leads either to unnecessary cost or to a safety risk. Here is a summary of the topic.

What do the two numbers say?

The number to the left of the dot is the minimum tensile strength divided by 100: 800 MPa for class 8.8, 1,000 MPa for 10.9, 1,200 MPa for 12.9. The number to the right is ten times the ratio of yield strength to tensile strength: 0.8 × 800 = 640 MPa yield for 8.8 and 0.9 × 1,000 = 900 MPa for 10.9.

Common classes and where they are used

• 4.6 – 5.8: Low-load general connections, timber, light sheet assembly
• 8.8: General-purpose in machinery, automotive and steel construction; the most common class
• 10.9: Highly stressed connections, engine and gearbox connections, structural HV sets
• 12.9: Machine tools, molds and precision socket-screw connections
• A2-70 / A4-80: Stainless steel; food, chemical, marine and outdoor applications

Matching nuts and washers

Even if the bolt class is high, the connection stays weak if the nut class is low. The matching rule: class 8 or 10 nuts with 8.8 bolts, class 10 nuts with 10.9 bolts, class 12 nuts with 12.9 bolts. Use 300 HV hardness washers in high-strength connections.

Is a higher class always better?

No. High-strength steels (10.9, 12.9) are more sensitive to hydrogen embrittlement and notch sensitivity. If electrolytic zinc plating is not baked, a brittle fracture risk arises; for this reason non-hydrogen-generating or mechanical coatings such as Geomet are preferred for these classes. As hardness increases, ductility also drops; under dynamic loads, fatigue strength depends more on coating and thread rolling quality.

Classes in stainless steel

A2-70 indicates 700 MPa tensile strength; "A2" is a 304-like alloy and "A4" a molybdenum-bearing 316-like alloy. A4 is more resistant to corrosion in chloride environments (marine, pools). Stainless bolts can gall because of friction; suitable grease or anti-seize should be used during assembly.

Three questions for selection

1. What load (tension, shear, dynamic) will the connection carry?
2. Is the environment corrosive or the temperature high?
3. Is a standard class enough, or is a design calculation needed?

For connections you are not sure about, you can get VDI 2230-based calculation support from our application engineers.