Complete Guide to Industrial Bolt Grades & Strength (8.8, 10.9, 12.9)

The Foundation of Fastener Strength

Bolt property classes (often called grades) are the standardized system used to identify the mechanical performance of a fastener. In the metric system, these are designated by two numbers separated by a dot.

#

Understanding the Property Class Notation

Take **Grade 8.8** as an example: - The first number (8) multiplied by 100 gives the **Nominal Tensile Strength** (800 N/mm²). - The second number (8) is 10% of the ratio between the yield strength and tensile strength. Multiplying the two numbers and then by 10 gives the **Yield Strength** (8 * 8 * 10 = 640 N/mm²).

#

Comparison of Common Grades

| Property Class | Tensile Strength | Yield Strength | Material | | :--- | :--- | :--- | :--- | | **4.6** | 400 N/mm² | 240 N/mm² | Low Carbon Steel | | **8.8** | 800 N/mm² | 640 N/mm² | Medium Carbon Steel (Quenched & Tempered) | | **10.9** | 1000 N/mm² | 900 N/mm² | Alloy Steel (Quenched & Tempered) | | **12.9** | 1200 N/mm² | 1080 N/mm² | Alloy Steel (Quenched & Tempered) |

Which Grade Should You Use?

Choosing the wrong grade can lead to catastrophic structural failure or unnecessary costs. For standard assembly, **4.6** is common. For structural steelwork, **8.8** is the industry workhorse. For heavy machinery and engine components, **10.9** or **12.9** are required.